WEDNESDAY FEBRUARY 13, 2012

Title: Custodial Leptons and Higgs Decays

Abstract: We study the effects of extended fermion sectors, respecting custodial symmetry, on Higgs production and decay. The resulting protection for the Z->b_L b_L and Z->tau_Rtau_R decays allows for potentially interesting signals in Higgs physics, while maintaining the good agreement of the Standard Model with precision tests, without significant fine-tuning. Although being viable setups on their own, the models we study can particularly be motivated as the low energy effective theories of the composite Higgs models MCHM_5 and MCHM_10 or the corresponding gauge-Higgs unification models. The spectra can be identified with the light custodians present in these theories. These describe the relevant physics in the fermion sectors of the models in a simplified and transparent way. In contrast to previous studies of composite models, we consider the impact of a realistic lepton sector on the Higgs decays. We find significant modifications in the decays to tau leptons and photons due to the new leptonic resonances. While from a pure low energy perspective an enhancement of the channel pp->h->gammagamma turns out to be possible, if one considers constraints on the parameters from the full structure of the composite models, the decay mode into photons is always reduced. We also demonstrate that taking into account the non-linearity of the Higgs sector does not change the qualitative picture for the decays into tau-leptons or photons in the case of the dominant Higgs production mechanism.

Title: New physics in top decay

Abstract: After a short introduction to effective field theories, most of their features are illustrated using the top decay. The effects of heavy new physics on the top decay are computed and the constraints on the coefficients of the dimension-six operators are derived from the available measurements.

Title: Possible indication for non-zero neutrino mass and additional neutrino species from cosmological observations

Abstract: The constraints on total neutrino mass and effective number of neutrino species based on CMB anisotropy power spectrum, Hubble constant, baryon acoustic oscillations and galaxy cluster mass function data are presented. It is shown that discrepancies between various cosmological data in Hubble constant and density fluctuation amplitude, measured in standard LCDM cosmological model, can be eliminated if more than standard effective number of neutrino species and non-zero total neutrino mass are considered. This extension of LCDM model appears to be approx 3 sigma significant when all cosmological data are used. The model with approximately one additional neutrino type, N_effapprox4, and with non-zero total neutrino mass, approx0.5eV, provide the best fit to the data. In the model with only one massive neutrino the upper limits on neutrino mass are slightly relaxed. It is shown that these deviations from LCDM model appear mainly due to the usage of recent data on the observations of baryon acoustic oscillations. The larger than standard number of neutrino species is measured mainly due to the comparison of the BAO data with direct measurements of Hubble constant, which was already noticed earlier. As it is shown below, the data on galaxy cluster mass function in this case give the measurement of non-zero neutrino mass.

Title: SUSY vs LHC

Abstract: In light of the discovery of the new particle at 125GeV and the strong lower limits on the masses of superparticles from LHC, we discuss a possible picture of weak scale supersymmetry.

Title: Probing exotic Higgs sectors from the precise measurement of Higgs boson couplings

Abstract: We study coupling constants of the standard model like Higgs boson with the gauge bosons $hZZ$ and $hWW$ and fermions $hfbar{f}$ in the general Higgs sector which contains higher isospin representations with arbitrary hypercharge. In Higgs sectors with exotic Higgs representations, the $hZZ$ and $hWW$ coupling constants can be larger than those in the standard model. We calculate deviations in the Higgs boson couplings from standard model values in the model with a real or complex triplet field, the Georgi-Machacek model and the model with a septet scalar field. We also study deviations in the event rates of $hto ZZ^*$, $hto WW^*$, $hto gammagamma$, $hto bbar{b}$ and $hto tautau$ channels.

Title: New bounds on neutrino electric millicharge from limits on neutrino magnetic moment

Abstract: In various extensions of the Standard Model a neutrino can have nontrivial electromagnetic properties. In particular, a neutrino can be an electrically millicharged particle. The corresponding non-standard electromagnetic interactions generates additional contributions to the neutrino magnetic moment. New limit on the neutrino magnetic moment recently obtained by the GEMMA experiment on measurements of the reactor antineutrino scattering off electrons allows us to get a new upper bound on the neutrino electric millicharge $mid q_{nu} mid < 5.8 times 10^{-19} e_0$ that exceed most of the available in literature bounds on the neutrino millicharge. We also obtain the bound, $mid q_{nu} mid < 6 times 10^{-20}e_0$, from the astrophysical limit on the neutrino magnetic moment that is much stronger than all known astrophysical bounds on the millicharge. end{abstract}

Title: Renormalization Group Scaling of Higgs Operators and Γ(h -> γ γ)

Abstract: We compute the renormalization of dimension six Higgs-gauge boson operators that can modify Gamma(h -> gamma gamma) at tree-level. Operator mixing is shown to lead to an important modification of new physics effects which has been neglected in past calculations. We also find that the usual formula for the S oblique parameter contribution of these Higgs-gauge boson operators needs additional terms to be consistent with renormalization group evolution. We study the implications of our results for Higgs phenomenology and for new physics models which attempt to explain a deviation in Gamma(h -> gamma gamma). We derive a new relation between the S parameter and the Gamma(h -> gamma gamma) and Gamma(h ->Z gamma) decay rates.

WEDNESDAY FEBRUARY 6, 2013

Title: Searches for New Vector Like Quarks: Higgs Channels

Abstract: New vector-like quarks can mix sizably with first generation Standard Model quarks without conflicting with current experimental constraints. Searches for such new quarks have been performed in pair production and electroweak single production channels with subsequent decays into electroweak gauge bosons. To fully explore the underlying structure of the theory the channels with heavy quark decays into Higgs bosons are crucial and in this article we consider for the first time the LHC reach for such channels. The two main production mechanisms involve single production of new quarks through the fusion of a vector boson and the Higgs and single production in association with a Higgs boson. We show that both channels have promising reach at the LHC and that they complement the current direct searches involving decays into electroweak gauge bosons.

WEDNESDAY JANUARY 23, 2013

Title: How to test for mass degenerate Higgs resonances

Abstract: The Higgs-like signal observed at the LHC could be due to several mass degenerate resonances. We show that the number of resonances is related to the rank of a "production and decay" matrix, $R_{if}$. Each entry in this matrix contains the observed rate in a particular production mode i and final state f. In the case of $N$ non-interfering resonances, the rank of R is, at most, N. If interference plays a role, the maximum rank is N^2 or in the CP limit N(N+1)/2. As an illustration we use the present experimental data to constrain the rank of the corresponding matrix. We estimate the LHC reach of probing two and three resonances under various speculations on future measurements and uncertainties.

WEDNESDAY DECEMBER 12, 2012

Title: Open windows for a light axigluon explanation of the top forward-backward asymmetry

Abstract: The top forward-backward asymmetry (tAFB) measured at the Tevatron remains one of the most puzzling outstanding collider anomalies. After two years of LHC running, however, few models for tAFB remain consistent with LHC data. In this paper we take a detailed look at the most promising surviving class of models, namely light (m_G' <~ 450 GeV), broad axigluons. We show which models simultaneously satisfy constraints from Tevatron and LHC top measurements, hadronic resonance searches, and LEP precision electroweak (PEW) observables. We consider three flavor structures: flavor-universal; down-type nonuniversal, designed to ease constraints from LHC charge asymmetry measurements; and top-type nonuniversal, designed to ameliorate constraints from PEW. We compute contributions to the PEW observables from states in the minimal UV completion of the axigluon model and demonstrate that new heavy fermions make the constraints universally more stringent, while related contributions from new scalars are much smaller, but act to relax the constraints. Paired dijet searches from ATLAS and CMS rule out all narrow axiglue models, while the LHC charge asymmetry measurement is less constraining than expected due to the high central value measured by ATLAS. Excepting the tension with the CMS charge asymmetry measurement, a broad axigluon is consistent with all data over the entire mass range we consider (50 GeV <~m_G' <~450 GeV) in the flavor-universal and top-type nonuniversal models, while it is consistent for m_G' >~200 GeV in the down-type non-universal model. The LHC charge asymmetry remains the best avenue for excluding, or observing, these models.

Title: Light axigluon explanation of the Tevatron ttbar asymmetry and multijet signals at the LHC

Abstract: The ttbar asymmetry measured at the Tevatron continues to disagree with Standard Model predictions at the 3 sigma level. We update the status of the phenomenological light axigluon model in explaining the asymmetry data, taking into account constraints from the charge asymmetry at the LHC and the ttbar cross section at both Tevatron and LHC. We find that an axigluon with a mass between 100 and 400 GeV provides an excellent fit to the data. Recent searches by ATLAS and CMS for pair production of heavy resonances which decay to dijets rule out axigluons with large branching fractions to dijets. However axigluons which predominantly decay to multijets via intermediate resonances are still a possibility. We outline four distinct scenarios which cover the most important decay topologies and discuss how one might exclude or discover axigluons as multijet resonances at the LHC. MadGraph implementations for each of the scenarios are provided.

Title: Is the 125 GeV Higgs the superpartner of a neutrino?

Abstract: Recent LHC searches have provided strong evidence for the Higgs, a boson whose gauge quantum numbers coincide with those of a SM fermion, the neutrino. This raises the mandatory question of whether Higgs and neutrino can be related by supersymmetry. We study this possibility in a model in which an approximate R-symmetry acts as a lepton number. We show that Higgs physics resembles that of the SM-Higgs with the exception of a novel invisible decay into Goldstino and neutrino with a branching fraction that can be as large as ~10%. Based on naturalness criteria, only stops and sbottoms are required to be lighter than the TeV with a phenomenology dictated by the R-symmetry. They have novel decays into quarks+leptons that could be seen at the LHC, allowing to distinguish these scenarios from the ordinary MSSM.

WEDNESDAY NOVEMBER 21, 2012

Title: (Un)determined finite regularization dependent quantum corrections: the Higgs decay into two photons and the two photon scattering examples

Abstract: We investigate the appearance of arbitrary, regularization dependent parameters introduced by divergent integrals in two a priori finite but superficially divergent amplitudes: the Higgs decay into two photons and the two photon scattering. We use a general parametrization of ultraviolet divergences which explicitates such ambiguities. Thus we separate in a consistent way using Implicit Regularization the divergent, finite and regularization dependent parts of the amplitudes which in turn are written as surface terms. We find that, although finite, these amplitudes are ambiguous before the imposition of physical conditions namely momentum routing invariance in the loops of Feynman diagrams. In the examples we study momentum routing invariance turns out to be equivalent to gauge invariance. We also discuss the results obtained by different regularizations and show how they can be reproduced within our framework allowing for a clear view on the origin of regularization ambiguities.

Title: A four-dimensional approach to quantum field theories

Abstract: I present a novel Four-Dimensional Regularization/Renormalization approach (FDR) to ultraviolet divergences in field theories which can be interpreted as a natural separation between physical and non physical degrees of freedom. Based on the observation that, in some cases, infinities can be reabsorbed into the vacuum expectation value of the fields, a new type of four-dimensional, gauge invariant and cutoff independent loop integral is introduced, which reproduces the correct ABJ anomaly and does not require changes in the definition of gamma_5. Finally, I comment on a possible interpretation of non-renormalizable theories in the context of the proposed procedure, and show how FDR can also be used to regularize infrared and collinear divergences.

THURSDAY NOVEMBER 15, 2012

Title: Search for narrow resonances and quantum black holes in inclusive and b-tagged dijet mass spectra from pp collisions at sqrt(s)=7 TeV

Abstract: A search for narrow resonances and quantum black holes is performed in inclusive and b-tagged dijet mass spectra measured with the CMS detector at the LHC. The data set corresponds to 5 inverse femtobarns of integrated luminosity collected in pp collisions at sqrt(s) = 7 TeV. No narrow resonances or quantum black holes are observed. Model-independent upper limits at the 95% confidence level are obtained on the product of the cross section, branching fraction into dijets, and acceptance for three scenarios: decay into quark-quark, quark-gluon, and gluon-gluon pairs. Specific lower limits are set on the mass of string resonances (4.31 TeV), excited quarks (3.32 TeV), axi-gluons and colorons (3.36 TeV), scalar color-octet resonances (2.07 TeV), E(6) diquarks (3.75 TeV), and on the masses of W' (1.92 TeV) and Z' (1.47 TeV) bosons. The limits on the minimum mass of quantum black holes range from 4 to 5.3 TeV. In addition, b-quark tagging is applied to the two leading jets and upper limits are set on the production of narrow dijet resonances in a model-independent fashion as a function of the branching fraction to b-jet pairs.

Title: Warm Inflection

Abstract: While ubiquitous in supersymmetric and string theory models, inflationary scenarios near an inflection point in the scalar potential generically require a severe fine-tuning of a priori unrelated supersymmetry breaking effects. We show that this can be significantly alleviated by the inclusion of dissipative effects that damp the inflaton's motion and produce a nearly-thermal radiation bath. We focus on the case where the slow-rolling inflaton directly excites heavy virtual modes that then decay into light degrees of freedom, although our main qualitative results should apply in other regimes. Furthermore, our analysis shows that the minimum amount of dissipation required to keep the temperature of the radiation bath above the Hubble rate during inflation is largely independent of the degree of flatness of the potential, although it depends on the field value at the inflection point. We then discuss the relevance of this result to warm inflation model building.

WEDNESDAY OCTOBER 31, 2012

Title: Production and propagation of heavy hadrons in air-shower simulators

Abstract: Very energetic charm and bottom hadrons may be produced in the upper atmosphere when a primary cosmic ray or the leading hadron in an extensive air shower collide with a nucleon. At $Eapprox 10^8$ GeV their decay length becomes of the order of 10 km, implying that they tend to interact in the air instead of decaying. Since the inelasticity in these collisions is much smaller than the one in proton and pion collisions, there could be rare events where a heavy-hadron component transports a significant amount of energy deep into the atmosphere. We have developed a module for the detailed simulation of these processes and have included it in a new version of the air shower simulator AIRES. We study the frequency, the energy distribution and the depth of charm and bottom production, as well as the depth and the energy distribution of these quarks when they decay. As an illustration, we consider the production and decay of tau leptons (from $D_s$ decays) and the lepton flux at PeV energies from a 30 EeV proton primary. The proper inclusion of charm and bottom hadrons in AIRES opens the possibility to search for air-shower observables that are sensitive to heavy quark effects.

Title: On the Tuning and the Mass of the Composite Higgs

Abstract: We analyze quantitatively the tuning of composite Higgs models with partial compositeness and its interplay with the predicted Higgs mass. In this respect we identify three classes of models, characterized by different quantum numbers of the fermionic colored resonances associated with the top quark, the so-called top partners. The main result of this classification is that in all models with moderate tuning a light Higgs, of 125 GeV mass, requires the presence of light top partners, around 1 TeV. The minimal tuning is comparable to the one of the most attractive supersymmetric models in particular the ones realizing Natural SUSY. This gives further support to an extensive program of top partners searches at the LHC that can already probe the natural region of composite Higgs models.

WEDNESDAY OCTOBER 24, 2012

WEDNESDAY MARCH 7, 2012

Title: Momentum reconstruction at the LHC for probing CP-violation in the stop sector

Abstract: We study the potential to observe CP-violating effects in SUSY stop cascade decay chains at the LHC. Asymmetries composed by triple products of momenta of the final state particles are sensitive to CP-violating effects. Due to large boosts that dilute the asymmetries, these can be difficult to observe. If all particle masses in a cascade decay are known, it may be possible to reconstruct all momenta in the decay chains on an event-by-event basis even when we have missing momentum due to a stable LSP. After the reconstruction, the non-diluted CP-violating signal can be recovered and gets significantly enhanced so that an observation may become feasible. A fully hadronic study has been completed to define the areas of the mSUGRA parameter space that may yield a 3-sigma observation with 500 fb^(-1) at the LHC.

Title: New Physics in B-Bbar mixing in the light of recent LHCb data

Abstract: We perform model-independent statistical analyses of three scenarios accommodating New Physics (NP) in Delta F=2 flavour-changing neutral current amplitudes. In a scenario in which NP in B_d-B_d-bar and B_s-B_s-bar is uncorrelated, we find the parameter point representing the Standard-Model disfavoured by 2.7 standard deviations. However, recent LHCb data on B_s neutral-meson mixing forbid a good accommodation of the D0 data on the semileptonic CP asymmetry A_SL. We introduce a fourth scenario with NP in both M_12^d,s and Gamma_12^d,s, which can accommodate all data. We discuss the viability of this possibility and emphasise the importance of separate measurements of the CP asymmetries in semileptonic B_d and B_s decays. All results have been obtained with the CKMfitter analysis package, featuring the frequentist statistical approach and using Rfit to handle theoretical uncertainties.

Title: Discriminating spin through quantum interference

Abstract: Many of the proposed solutions to the hierarchy and naturalness problems postulate new `partner' fields to the standard model particles. Determining the spins of these new particles will be critical in distinguishing among the various possible SM extensions, yet proposed methods rely on the underlying models. We propose a new model-independent method for spin measurements which takes advantage of quantum interference among helicity states. We demonstrate that this method will be able to discriminate scalar particles from higher spin states at the ILC, and discuss application to higher spins and possible uses at the LHC.

Title: Measurement of CP asymmetries in neutralino production at the ILC

Abstract: We study the prospects to measure the CP-sensitive triple-product asymmetries in neutralino production e+e- -> ~chi^0_i ~chi^0_1 and subsequent leptonic two-body decays ~chi^0_i -> ~l_R l, ~l_R -> ~chi^0_1 l, for l=e, mu, within the Minimal Supersymmetric Standard Model. We include a full detector simulation of the International Large Detector for the International Linear Collider. The simulation was performed at a center-of-mass energy of sqrt{s}=500 GeV, including the relevant Standard Model background processes, a realistic beam energy spectrum, beam backgrounds and a beam polarization of 80% and -60% for the electron and positron beams, respectively. In order to effectively disentangle different signal samples and reduce SM and SUSY backgrounds we apply a method of kinematic reconstruction. Assuming an integrated luminosity of 500 fb^-1 collected by the experiment and the performance of the current ILD detector, we arrive at a relative measurement accuracy of 10% for the CP-sensitive asymmetry in our scenario. We demonstrate that our method of signal selection using kinematic reconstruction can be applied to a broad class of scenarios and it allows disentangling processes with similar kinematic properties.

Title: Constraints on the pMSSM from searches for squarks and gluinos by ATLAS

Abstract: We study the impact of the jets and missing transverse momentum SUSY analyses of the ATLAS experiment on the phenomenological MSSM (pMSSM). We investigate sets of SUSY models with a flat and logarithmic prior in the SUSY mass scale and a mass range up to 1 and 3 TeV, respectively. These models were found previously in the study 'Supersymmetry without Prejudice'. Removing models with long-lived SUSY particles, we show that 99% of 20000 randomly generated pMSSM model points with a flat prior and 87% for a logarithmic prior are excluded by the ATLAS results. For models with squarks and gluinos below 600 GeV all models of the pMSSM grid are excluded. We identify SUSY spectra where the current ATLAS search strategy is less sensitive and propose extensions to the inclusive jets search channel.

WEDNESDAY FEBRUARY 29, 2012

Title: Interpreting LHC Higgs Results from Natural New Physics Perspective

Abstract: We analyze the 2011 LHC Higgs data in the context of simplified new physics models addressing the naturalness problem. These models are expected to contain new particles with sizable couplings to the Higgs boson, which can easily modify the Higgs production cross sections and branching fractions. We focus on searches in the Higgs to 4 leptons and Higgs to diphoton channels, in the latter case including the vector boson fusion production mode. Combining the available ATLAS and CMS data in these channels, we derive constraints on an effective low-energy theory of the Higgs boson. We then map several simplified scenarios to the effective theory, capturing numerous natural new physics models such as supersymmetry and Little Higgs, and extract the constraints on the corresponding parameter space. We show that simple models where one fermionic or one scalar partner is responsible for stabilizing the Higgs potential are already constrained in a non-trivial way by LHC Higgs data.

Title: Model-Independent Bounds on a Light Higgs

Abstract: We present up-to-date constraints on a generic Higgs parameter space. An accurate assessment of these exclusions must take into account statistical, and potentially signal, fluctuations in the data currently taken at the LHC. For this, we have constructed a straightforward statistical method for making full use of the data that is publicly available. We show that, using the expected and observed exclusions which are quoted for each search channel, we can fully reconstruct likelihood profiles under very reasonable and simple assumptions. Even working with this somewhat limited information, we show that our method is sufficiently accurate to warrant its study and advocate its use over more naive prescriptions. Using this method, we can begin to narrow in on the remaining viable parameter space for a Higgs-like scalar state, and to ascertain the nature of any hints of new physics---Higgs or otherwise---appearing in the data.

Title: Fingerprinting Higgs Suspects at the LHC

Abstract: We outline a method for characterizing deviations from the properties of a Standard Model (SM) Higgs boson. We apply it to current data in order to characterize up to which degree the SM Higgs boson interpretation is consistent with experiment. We find that the SM Higgs boson is consistent with the current data set at the 82 % confidence level, based on data of excess events reported by CMS and ATLAS, which are interpreted to be related to the mass scale mh = 124-126 GeV and on published CL_s exclusion regions. We perform a global fit in terms of two parameters characterizing the deviation from the SM value in the gauge and fermion couplings of a Higgs. We find two degenerate minima in the global fit and identify observables that can remove this degeneracy.

WEDNESDAY FEBRUARY 22, 2012

Title: Warm natural inflation

Abstract: In warm inflation models there is the requirement of generating large dissipative couplings of the inflaton with radiation, while at the same time, not de-stabilising the flatness of the inflaton potential due to radiative corrections. One way to achieve this without fine tuning unrelated couplings is by supersymmetry. In this paper we show that if the inflaton and other light fields are Pseudo-Nambu-Goldstone Bosons then the radiative corrections to the potential are suppressed and the thermal corrections are small as long as the temperature is below the symmetry breaking scale. In such models it is possible to fulfill the contrary requirements of an inflaton potential which is stable under radiative corrections and the generation of a large dissipative coupling of the inflaton field with other light fields. We construct a warm inflation model which gives the observed CMB-anisotropy amplitude and spectral index where the symmetry breaking is at the GUT scale.

Title: Meissner effect, diamagnetism, and classical physics - a review

Abstract: We review the literature on what classical physics has to say about the Meissner effect and the London equations. We first discuss the relevance of the Bohr-van Leeuwen theorem for the perfect diamagnetism of superconductors. The conclusion is that the theorem is based on assumptions that are not valid. We also point out results in the literature which prove that the magnetic flux expulsion from a sample cooled to superconductivity can be simply understood as an approach to the magnetostatic energy minimum. These results have been published several times but still many textbooks on magnetism claim that there is no classical diamagnetism, and virtually all books on superconductivity repeat Meissner's 1933 statement that flux expulsion has no classical explanation.

WEDNESDAY FEBRUARY 15, 2012

Title: What if the Higgs couplings to W and Z bosons are larger than in the Standard Model?

Abstract: We derive a general sum rule relating the Higgs coupling to W and Z bosons to the total cross section of longitudinal gauge boson scattering in I=0,1,2 isospin channels. The Higgs coupling larger than in the Standard Model implies enhancement of the I=2 cross section. Such an enhancement could arise if the Higgs sector is extended by an isospin-2 scalar multiplet including a doubly charged, singly charged, and another neutral Higgs.

Title: Has a fermiophobic Higgs boson been detected at the LHC ?

Abstract: We show that, in the present searches for the Higgs boson at the LHC, a fermiophobic Higgs mimics the standard-model-like Higgs if its mass is around 125 GeV. For that mass the order-of-magnitude reduction of fermiophobic Higgs production cross sections is compensated by a corresponding increase in the Higgs branching fraction into gammagamma, while the WW*, ZZ*, Zgamma, signals are predicted to be somewhat smaller. The excess seen in the CMS exclusive vector-boson-fusion analysis suggests that the LHC sees a fermiophobic instead of a standard-model-like Higgs boson. Due to the different kinematics, a dedicated LHC analysis should be able to discriminate between the two scenarios. If the Higgs boson turns out to be fermiophobic, many of our present ideas of new physics should be revised.

Title: Higgs Phenomenology of Minimal Universal Extra Dimensions

Abstract: The minimal model of Universal Extra Dimensions (MUED) is briefly reviewed. We explain how the cross-sections for Higgs production via gluon fusion and decay into two photons are modified, relative the the Standard Model (SM) values, by KK particles running in loops, leading to an enhancement of the gg to h to two photons and gg to h to W+W- cross-sections. ATLAS and CMS searches for the SM Higgs in these channels are reinterpreted in the context of MUED and used to place new limits on the MUED parameter space. Only a small region of between 1 and 3 GeV around mh = 125 GeV for 500 GeV < 1/R < 1600 GeV remains open at the 95 % confidence level.

WEDNESDAY NOVEMBER 30, 2011

Title: Measurement of CP--violating asymmetries in D0 --> pi+ pi- and D0 --> K+K- decays at CDF

Abstract: We report on a measurement of CP-violating asymmetries (Acp) in the Cabibbo-suppressed D0 --> pi+ pi- and D0 --> K+K- decays reconstructed in a data sample corresponding to 5.9 fb-1 of integrated luminosity collected by the upgraded Collider Detector at Fermilab. We use the strong decay D*+ --> D0 pi+ to identify the flavor of the charmed meson at production and exploit CP-conserving strong c-cbar pair-production in p-pbar collisions. High-statistics samples of Cabibbo-favored D0 --> K- p+ decays with and without a Docencia tag are used to correct for instrumental effects and significantly reduce systematic uncertainties. We measure Acp(D0 --> pi+ pi-) = (+0.22 +- 0.24 (stat) +- 0.11 (syst))% and Acp(D 0 --> K+ K-) = (-0.24 +- 0.22 (stat) +- 0.09 (syst))%, in agreement with CP conservation. These are the most precise determinations from a single experiment to date. Under the assumption of negligible direct CP violation in D0 --> pi+ pi- and D0 --> K+K- decays, the results provide an upper limit to the CP-violating asymmetry in D0 mixing, |Acp^{ind}(D0)|< 0.13% at the 90% confidence level.

Title: Eliminating the η-problem in SUGRA Hybrid Inflation with Vector Backreaction

Abstract: It is shown that, when the inflaton field modulates the gauge kinetic function of the gauge fields in supergravity realisations of inflation, the dynamic backreaction leads to a new inflationary attractor solution, in which the inflaton's variation suffers additional impedance. As a result, slow-roll inflation can naturally occur along directions of the scalar potential which would be too steep and curved to support it otherwise. This provides a generic solution to the infamous eta-problem of inflation in supergravity. Moreover, it is shown that, in the new inflationary attractor, the spectral index of the generated curvature perturbations is kept mildly red despite eta of order unity. The above findings are applied to a model of hybrid inflation is supergravity with a generic K"{a}hler potential. The spectral index of the generated curvature perturbations is found to be 0.97 - 0.98, in excellent agreement with observations. The gauge field can play the role of the vector curvaton after inflation but observable statistical anisotropy requires substantial tuning of the gauge coupling.

WEDNESDAY NOVEMBER 23, 2011

Title: On the size of direct CP violation in singly Cabibbo-suppressed D decays

Abstract: The first experimental evidence for direct CP violation in charm-quark decays has recently been presented by the LHCb collaboration in the difference between the D -> K^+ K^- and D -> pi^+ pi^- time-integrated CP asymmetries. We estimate the size of the effects that can be expected within the Standard Model and find that at leading order in 1/m_c they are an order of magnitude smaller. However, tree-level annihilation type amplitudes are known to be large experimentally. This implies that certain formally 1/m_c suppressed penguin amplitudes could plausibly account for the LHCb measurement.

Title: Implications of the LHCb Evidence for Charm CP Violation

Abstract: The LHCb collaboration recently announced preliminary evidence for CP violation in D meson decays. We discuss this result in the context of the standard model (SM), as well as its extensions. In the absence of reliable methods to evaluate the hadronic matrix elements involved, we can only estimate qualitatively the magnitude of the non-SM tree level operators required to generate the observed central value. In the context of an effective theory, we list the operators that can give rise to the measured CP violation and investigate constraints on them from other processes.

Title: Beyond the Minimal Composite Higgs Model

Abstract: The Higgs spectrum of the minimal composite Higgs model, based on the SO(5)/SO(4) coset, consists of a unique Higgs doublet whose phenomenology does not differ greatly from the Standard Model (SM). Nevertheless, extensions beyond this minimal coset structure exhibit a richer Higgs spectrum and therefore very different Higgs physics. We explore one of these extensions, the SO(6)/SO(5) model, whose Higgs spectrum contains a CP-odd singlet scalar, eta, in addition to the Higgs doublet. Due to the pseudo-Nambu-Goldstone nature of these Higgs bosons, their physical properties can be derived from symmetry considerations alone. We find that the mass of eta can be naturally light, opening up the possibility that the SM Higgs decays predominantly to the singlet, and therefore lowering the LEP bound on its mass to 86 GeV. We also show that eta can have interesting consequences in flavour-violating processes, as well as induce spontaneous CP-violation in the Higgs sector. The model can also have anomalies, giving rise to interactions between the SM gauge bosons and eta which, if measured at the LHC, would give quantitative information about the structure of the high energy theory.

WEDNESDAY OCTOBER 26, 2011

Title: Off-shell OPERA neutrinos

Abstract: In the OPERA experiment, superluminal propagation of neutrinos can occur if one of the neutrino masses is extremely small. However the effect only has appreciable amplitude at energies of order this mass and thus has negligible overlap with the multi-GeV scale of the experiment.

Title: Superluminal neutrinos and extra dimensions: constraints from the null energy condition

Abstract: In light of the recent results from the OPERA collaboration, indicating that neutrinos can travel superluminally, I review a simple extra-dimensional strategy for accommodating such behavior; and I also explain why it is hard in this strategy to avoid violating the null energy condition somewhere in the extra dimensions.

Title: New Constraints on Neutrino Velocities

Abstract: The OPERA collaboration has claimed that muon neutrinos with mean energy of 17.5 GeV travel 730 km from CERN to the Gran Sasso at a speed exceeding that of light by about 7.5 km/s or 25 ppm. However, we show that such superluminal neutrinos would lose energy rapidly via the bremsstrahlung of electron-positron pairs ($nurightarrow nu+e^-+e^+$). For the claimed superluminal neutrino velocity and at the stated mean neutrino energy, we find that most of the neutrinos would have suffered several pair emissions en route, causing the beam to be depleted of higher energy neutrinos. Thus we refute the superluminal interpretation of the OPERA result. Furthermore, we appeal to Super-Kamiokande and IceCube data to establish strong new limits on the superluminal propagation of high-energy neutrinos.

Title: Interpreting OPERA results on superluminal neutrino

Abstract: OPERA has claimed the discovery of superluminal propagation of neutrinos. We analyze the consistency of this claim with previous tests of special relativity. We find that reconciling the OPERA measurement with information from SN1987a and from neutrino oscillations requires stringent conditions. The superluminal limit velocity of neutrinos must be nearly flavor independent, must decrease steeply in the low-energy domain, and its energy dependence must depart from a simple power law. We construct illustrative models that satisfy these conditions, by introducing Lorentz violation in a sector with light sterile neutrinos. We point out that, quite generically, electroweak quantum corrections transfer the information of superluminal neutrino properties into Lorentz violations in the electron and muon sector, in apparent conflict with experimental data.

Wednesday October 19, 2011

Title: Asteroseismology of the Kepler field DBV White Dwarf - It's a hot one!

Abstract: We present an asteroseismic analysis of the helium atmosphere white dwarf (a DBV) recently found in the field of view of the Kepler satellite. We analyze the 5-mode pulsation spectrum that was produced based on one month of high cadence Kepler data. The pulsational characteristics of the star and the asteroseismic analysis strongly suggest that the star is hotter (29200 K) than the 24900 K suggested by model fits to the low S/N survey spectrum of the object. This result has profound and exciting implications for tests of the Standard Model of particle physics. Hot DBVs are expected to lose over half of their energy through the emission of plasmon neutrinos. Continuous monitoring of the star with the Kepler satellite over the course of 3 to 5 years is not only very likely to yield more modes to help constrain the asteroseismic fits, but also allow us to obtain a rate of change of any stable mode and therefore measure the emission of plasmon neutrinos.

Title: A Solution to the Flavor Problem of Warped Extra-Dimension Models

Abstract: A minimal solution to the flavor problem of warped extra-dimension models, i.e. the excessive mixed-chirality contribution to CP violation in K-Kbar mixing arising from Kaluza-Klein (KK) gluon exchange, is proposed. Extending the strong-interaction gauge group in the bulk by an additional SU(3), and breaking this symmetry to QCD via boundary conditions, the constraints arising from the epsilon_K parameter are significantly relaxed. As a result, KK scales M_KK ~ 2 TeV are consistent with all flavor observables without significant fine-tuning. The model predicts the existence of an extended Higgs sector featuring massive color-octet scalars and a tower of KK pseudo-axial gluon resonances, whose existence is not in conflict with recent LHC dijet bounds.

Title: Flavor Phenomenology in General 5D Warped Spaces

Abstract: We have considered a general 5D warped model with SM fields propagating in the bulk and computed explicit expressions for oblique and non-oblique electroweak observables as well as for flavor and CP violating effective four-fermion operators. We have compared the resulting lower bounds on the Kaluza-Klein (KK) scale in the RS model and a recently proposed model with a metric modified towards the IR brane, which is consistent with oblique parameters without the need for a custodial symmetry. We have randomly generated 40,000 sets of O(1) 5D Yukawa couplings and made a fit of the quark masses and CKM matrix elements in both models. This method allows to identify the percentage of points consistent with a given KK mass, which in turn provides us with a measure for the required fine-tuning. Comparison with current experimental data on Rb, FCNC and CP violating operators exhibits an improved behavior of our model with respect to the RS model. In particular, allowing 10% fine-tuning the combined results point towards upper bounds on the KK gauge boson masses around 3.3 TeV in our model as compared with 13 TeV in the RS model. One reason for this improvement is that fermions in our model are shifted, with respect to fermions in the RS model, towards the UV brane thus decreasing the strength of the modifications of electroweak observables.

TUESDAY APRIL 12, 2011

Title: Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in ppbar Collisions at sqrt(s) = 1.96 TeV

Abstract: We report a study of the invariant mass distribution of jet pairs produced in association with a W boson using data collected with the CDF detector which correspond to an integrated luminosity of 4.3 fb^-1. The observed distribution has an excess in the 120-160 GeV/c^2 mass range which is not described by current theoretical predictions within the statistical and systematic uncertainties. In this letter we report studies of the properties of this excess.

TUESDAY APRIL 5, 2011

Title: Probing the Tevatron t tbar asymmetry at LHC

Abstract: We use an effective operator framework to study the contributions to the Tevatron t tbar asymmetry from arbitrary vector bosons and scalars, and compare with their effect on the t tbar tail at LHC. Our study shows, for example, that models reproducing the t tbar asymmetry by exchange of Z' and W' bosons or colour-triplet scalars lead to a large enhancement in the t tbar tail at LHC. This fact can be used to exclude these models as the sole explanation for the asymmetry, using the data already collected by CMS and ATLAS. Our analysis is model independent in the sense that we scan over all possible extra particles contributing to the asymmetry, and allow for general couplings. We also explore a class of Standard Model extensions which can accommodate the Tevatron asymmetry without contributing to the total t tbar cross section at first order, so that the enhancement of the tail at Tevatron and LHC is moderate.

Title: A new global fit of the $L^r_i$ at next-to-next-to-leading order in Chiral Perturbation Theory

Abstract: A new fit is done to obtain numerical values for the order $p^4$ low-energy-constants $L_i^r$ in Chiral Perturbation Theory. This includes both new data and new calculated observables. We take into account masses, decay constants, $K_{ell4}$, $pipi$ and $pi K$ scattering lengths and slopes and the slope of the pion scalar formfactor. We compare in detail where the changes w.r.t. to the 10 year old "fit 10" come from. We discuss several scenarios for estimating the order $p^6$ constants $C_i^r$ and search for possible values of them that provide a good convergence for the ChPT series. We present two such sets. One big change is that the fits do not have the expected behaviour in the limit of large $N_c$ as well as before.

TUESDAY MARCH 15, 2011

Title: Top quark spin correlations at the Tevatron and the LHC

Abstract: Spin correlations of top quarks produced in hadron collisions have not been observed experimentally with large significance. In this Letter, we propose a new variable that may enable demonstration of the existence of spin correlations with 3-4 sigma significance using just a few hundred dilepton events both at the Tevatron and the LHC. Such number of dilepton events has been observed at the Tevatron. At the LHC, it will become available once integrated luminosity of a few hundred inverse picobarns is collected.

Title: Automation of one-loop QCD corrections

Abstract: We present the complete automation of the computation of one-loop QCD corrections, including UV renormalization, to an arbitrary scattering process in the Standard Model. This is achieved by embedding the OPP integrand reduction technique, as implemented in CutTools, into the MadGraph framework. By interfacing the tool so constructed, which we dub MadLoop, with MadFKS, the fully automatic computation of any infrared-safe observable at the next-to-leading order in QCD is attained. We demonstrate the flexibility and the reach of our method by calculating the production rates for a variety of processes at the 7 TeV LHC.

TUESDAY FEBRUARY 15, 2011

Title: Implications of the Measurement of Ultra-Massive Boosted Jets at CDF

Abstract: The CDF collaboration reported a 3.44 sigma deviation from the estimated background in events with two ultra-massive boosted jets. We discuss the interpretation of the measurement and its fundamental implications. In case new physics is involved, the most naive contribution is from a new particle produced with a cross section that is a few times higher than that of the top quark, and a sizable hadronic branching ratio. We quantify the resulting tension of a possible larger top pair cross section with the absence of excess found in events with one massive boosted jet and missing energy. The measured planar flow distribution shows deviation from CDF's Pythia QCD prediction at high planarity, while we find a somewhat smaller deviation when comparing with other Monte Carlo tools. As a simple toy model, we analyze the case of a light gluino with R-parity violation, and show that it can be made consistent with the data.

TUESDAY FEBRUARY 8, 2011

Title: Two-Higgs Leptonic Minimal Flavour Violation

Abstract: We construct extensions of the Standard Model with two Higgs doublets, where there are flavour changing neutral currents both in the quark and leptonic sectors, with their strength fixed by the fermion mixing matrices $V_{CKM}$ and $V_{PMNS}$. These models are an extension to the leptonic sector of the class of models previously considered by Branco, Grimus and Lavoura, for the quark sector. We consider both the cases of Dirac and Majorana neutrinos and identify the minimal discrete symmetry required in order to implement the models in a natural way.

TUESDAY DECEMBER 21, 2010

Title: Higgs Discovery through Top-Partners using Jet Substructure

Abstract: Top-partners -- vector-like quarks which mix predominantly with the top quark -- are simple extensions of the standard model present in many theories of new physics such as little Higgs models, topcolor models, and extra dimensions. Through renormalizable mixing with the top quark, these top-partners inherit couplings to the Higgs boson. Higgs bosons produced from the decay of top-partners are often highly boosted and ideal candidates for analyses based on jet substructure. Using substructure methods, we show that light Higgs bosons decaying to b b-bar can be discovered at the 14 TeV LHC with less than 10 inverse fb for top-partner masses up to 1 TeV.

Title: A Natural Hierarchy and a low New Physics scale from a Bulk Higgs

Abstract: We show that a bulk Higgs with a mass saturating the Breitenlohner-Freedman bound can naturally generate and stabilize an exponential hierarchy on a nearly AdS background. The physical Higgs boson in this class of models emerges as the lightest eigenstate of the Higgs/radion system and has a mass strictly lighter than the Kaluza-Klein scale. These theories are dual to strongly coupled CFTs deformed by a marginally relevant Higgs mass operator. On the 5D side, the marginally relevant nature of the Higgs mass operator implies that the Higgs VEV is maximally spread in the bulk. This feature significantly decreases the lower bound on the new physics scale in models that address the SM flavor problem. The collider phenomenology interpolates between Randall-Sundrum scenarios with a heavy Higgs and a light radion, and composite Higgs models.

Tuesday December 14, 2010

Title: A search for concentric circles in the 7-year WMAP temperature sky maps

Abstract: In a recent analysis of the 7-year WMAP temperature sky maps, Gurzadyan and Penrose claim to find evidence for violent pre-Big Bang activity in the form of concentric low-variance circles at high statistical significance. In this paper, we perform an independent search for such concentric low-variance circles, employing both chi^2 statistics and matched filters, and compare the results obtained from the 7-year WMAP temperature sky maps with those obtained from LCDM simulations. Our main findings are the following: We do reproduce the claimed ring structures observed in the WMAP data as presented by Gurzadyan and Penrose, thereby verifying their computational procedures. However, the results from our simulations do not agree with those presented by Gurzadyan and Penrose. On the contrary we obtain a substantially larger variance in our simulations, to the extent that the observed WMAP sky maps are fully consistent with the LCDM model as measured by these statistics.

Title: No evidence for anomalously low variance circles on the sky

Abstract: In a recent paper, Gurzadyan & Penrose claim to have found directions on the sky centred on which are circles of anomalously low variance in the cosmic microwave background (CMB). These features are presented as evidence for a particular picture of the very early Universe. We attempted to repeat the analysis of these authors, and we can indeed confirm that such variations do exist in the temperature variance for annuli around points in the data. However, we find that this variation is entirely expected in a sky which contains the usual CMB anisotropies. In other words, properly simulated Gaussian CMB data contain just the sorts of variations claimed. Gurzadyan & Penrose have not found evidence for pre-Big Bang phenomena, but have simply re-discovered that the CMB contains structure.

Title: More on the low variance circles in CMB sky

Abstract: Two groups [3,4] have confirmed the results of our paper concerning the actual existence of low variance circles in the cosmic microwave background (CMB) sky. They also point out that the effect does not contradict the LCDM model - a matter which is not in dispute. We point out two discrepancies between their treatment and ours, however, one technical, the other having to do with the very understanding of what constitutes a Gaussian random signal. Both groups simulate maps using the CMB power spectrum for LCDM, while we simulate a pure Gaussian sky plus the WMAP's noise, which points out the contradiction with a common statement [3] that "CMB signal is random noise of Gaussian nature". For as it was shown in [5], the random component is a minor one in the CMB signal, namely, about 0.2. Accordingly, the circles we saw are a real structure of the CMB sky and they are not of a random Gaussian nature. Although the structures studied certainly cannot contradict the power spectrum, which is well fitted by LCDM model, we particularly emphasize that the low variance circles occur in concentric families, and this key fact cannot be explained as a purely random effect. It is, however a clear prediction of conformal cyclic cosmology.

Title: Concentric circles in WMAP data may provide evidence of violent pre-Big-Bang activity

Abstract: Conformal cyclic cosmology (CCC) posits the existence of an aeon preceding our Big Bang 'B', whose conformal infinity 'I' is identified, conformally, with 'B', now regarded as a spacelike 3-surface. Black-hole encounters, within bound galactic clusters in that previous aeon, would have the observable effect, in our CMB sky, of families of concentric circles over which the temperature variance is anomalously low, the centre of each such family representing the point of 'I' at which the cluster converges. These centres appear as fairly randomly distributed fixed points in our CMB sky. The analysis of Wilkinson Microwave Background Probe's (WMAP) cosmic microwave background 7-year maps does indeed reveal such concentric circles, of up to 6{sigma} significance. This is confirmed when the same analysis is applied to BOOMERanG98 data, eliminating the possibility of an instrumental cause for the effects. These observational predictions of CCC would not be easily explained within standard inflationary cosmology.

TUESDAY NOVEMBER 2, 2010

Title: Schizophrenic Neutrinos and $nu$-less Double Beta Decay

Abstract: We point out a novel possibility for neutrinos where all neutrino flavors can be part Dirac and part Majorana. Our primary motivation for this model comes from an attempt to use supersymmetric see-saw models to tie inflation, baryon asymmetry of the Universe and dark matter to the neutrino sector. The idea however could stand on its own, with or without supersymmetry. We present a realization of this possibility within an $S_3$ family symmetry for neutrino masses, where we obtain tri-bi-maximal mixing for neutrinos to the leading order. The model predicts that for the case of inverted hierarchy, the lower limit on the neutrino mass measured in neutrinoless double beta decay experiments is about a factor of two larger than the usual case.

Title: UV-Completion by Classicalization

Abstract: We suggest a novel approach to UV-completion of a class of non-renormalizable theories, according to which the high-energy scattering amplitudes get unitarized by production of extended classical objects (classicalons), playing a role analogous to black holes, in the case of non-gravitational theories. The key property of classicalization is the existence of a classicalizer field that couples to energy-momentum sources. Such localized sources are excited in high-energy scattering processes and lead to the formation of classicalons. Two kinds of natural classicalizers are Nambu-Goldstone bosons (or, equivalently, longitudinal polarizations of massive gauge fields) and scalars coupled to energy-momentum type sources. Classicalization has interesting phenomenological applications for the UV-completion of the Standard Model both with or without the Higgs. In the Higgless Standard Model the high-energy scattering amplitudes of longitudinal $W$-bosons self-unitarize via classicalization, without the help of any new weakly-coupled physics. Alternatively, in the presence of a Higgs boson, classicalization could explain the stabilization of the hierarchy. In both scenarios the high-energy scatterings are dominated by the formation of classicalons, which subsequently decay into many particle states. The experimental signatures at the LHC are quite distinctive, with sharp differences in the two cases.

TUESDAY MAY 25, 2010

Title: Electroweak Limits on General New Vector Bosons

Abstract: We study extensions of the Standard Model with general new vector bosons. The full Standard Model gauge symmetry is used to classify the extra vectors and constrain their couplings. We derive the corresponding effective Lagrangian, valid at energies lower than the mass of the extra vectors, and use it to extract limits from electroweak precision observables, including LEP 2 data. We consider both universal and nonuniversal couplings to fermions. We study the interplay of several extra vectors, which can have the effect of opening new regions in parameter space. In particular, it allows to explain the anomaly in the bottom forward-backward asymmetry with perturbative couplings. Finally, we analyze quantitatively the implications for the Higgs mass.

Title: CP violation in sbottom decays

Abstract: We study CP asymmetries in two-body decays of bottom squarks into charginos and tops. These asymmetries probe the SUSY CP phases of the sbottom and the chargino sector in the Minimal Supersymmetric Standard Model. We identify the MSSM parameter space where the CP asymmetries are sizeable, and analyze the feasibility of their observation at the LHC. As a result, potentially detectable CP asymmetries in sbottom decays are found, which motivates further detailed experimental studies for probing the SUSY CP phases.

Title: Normal tau polarisation as a sensitive probe of CP violation in chargino decay

Abstract: CP violation in the spin-spin correlations in chargino production and subsequent two-body decay into a tau and a tau-sneutrino is studied at the ILC.
From the normal polarisation of the tau, an asymmetry is defined to test the CP-violating phase of the higgsino mass parameter mu. Asymmetries of more than pm70% are obtained, also in scenarios with heavy first and second generation sfermions. Bounds on the statistical significances of the CP asymmetries are estimated. As a result, the normal tau polarisation in the chargino decay is one of the most sensitive probes to constrain or measure the phase phi_mu at the ILC, motivating further detailed experimental studies.

Tuesday April 27, 2010

Title: T-odd correlations from CP violating anomalous top-quark couplings revisited

Abstract: We revisit the effect of CP violating anomalous top-quark couplings in $tbar{t}$ production and decay. We consider $tbar{t}$ production through gluon fusion (and light $q{bar q}$ annihilation) followed by top-quark decay into $bW$ or $bellnu$. We find explicit analytic expressions for all the triple products generated by the anomalous couplings that fully incorporate all spin correlations. Our results serve as a starting point for numerical simulations for the LHC.

Title: Composite Higgs Search at the LHC

Abstract: The Higgs boson production cross-sections and decay rates depend, within the Standard Model (SM), on a single unknown parameter, the Higgs mass. In composite Higgs models where the Higgs boson emerges as a pseudo-Goldstone boson from a strongly-interacting sector, additional parameters control the Higgs properties which then deviate from the SM ones. These deviations modify the LEP and Tevatron exclusion bounds and significantly affect the searches for the Higgs boson at the LHC. In some cases, all the Higgs couplings are reduced, which results in deterioration of the Higgs searches but the deviations of the Higgs couplings can also allow for an enhancement of the gluon-fusion production channel, leading to higher statistical significances. The search in the H to gamma gamma channel can also be substantially improved due to an enhancement of the branching fraction for the decay of the Higgs boson into a pair of photons.

TUESDAY APRIL 20, 2010

Title: Limits on a strongly interacting Higgs sector

Abstract: Using the classical argument about tree level unitarity breakdown in combination with the precision electroweak data, it is shown, that if part of the Higgs sector is strongly interacting, this part is small and is out of range of the LHC.

Title: Warped Penguins

Abstract: We present an analysis of the loop-induced magnetic dipole operator in the Randall-Sundrum model of a warped extra dimension with anarchic bulk fermions and an IR brane localized Higgs. We find that these operators are finite and explicitly calculate the branching ratio for mu to e gamma using the mixed position/momentum space formalism. This class of models typically exhibit a tension between the experimentally preferred parameters for loop- and tree-level processes. In particular, the upper bound on the Yukawa set by mu to e gamma conflicts with the lower bound set by mu to e conversion in nuclei and mu to 3e. We find that this tension is still present for generic models, though it can be somewhat relaxed by imposing an additional custodial protection in the leptonic sector. Finally, we remark on the importance of matching the total number of KK modes to the cutoff scale when performing loop integrals in the KK-decomposed theory.

TUESDAY APRIL 6, 2010

Title: Zt, gamma t and t production at hadron colliders via strong flavour-changing neutral couplings

Abstract: We consider gq -> Zt, gq -> gamma t and gq -> t production (q=u,c) mediated by strong flavour-changing neutral interactions within an effective operator framework. We provide total cross sections for Tevatron and LHC, showing explicitly that the six processes can be described in full generality in terms of only two parameters (anomalous couplings) for q=u plus two for q=c. In our work we take into account and study in detail the effects top quark decay. For gamma t, the inclusion of the top quark decay in the matrix element reveals an striking result: the largest contribution to the final state, e.g. gamma l nu b, with l = e, mu, tau, does not result from gq -> gamma t -> gamma l nu b but from on-shell gq -> t production with t -> gamma l nu b, being the photon radiated off the top decay products. This contribution, missed in previous literature, increases the signal cross sections by factors ranging between 3 and 6.5.

Title: Power suppression from disparate mass scales in effective scalar field theories of inflation and quintessence

Abstract: A scalar potential coupled to other fields of large disparate masses will exhibit power suppression of the quantum loop corrections from these massive fields. Quintessence fields in the dark energy regime and inflaton fields during inflation often have a very large background field value. Thus any other field with its mass dependent on the quintessence/inflaton background field value through a moderate coupling will become very massive during the dark energy/inflation phase and its quantum corrections to the scalar effective potential will be suppressed. This concept is developed in this paper using the decoupling theorem. The problem then reduces to a quantitative question of the size of suppression effects within the parameter space of coupling constants, scalar field background value and renormalization scale. Some numerical examples are presented both for inflation and quintessence, but the approach is general and can be applied to any scalar field effective potential. The consequences to dark energy of the decoupling effect developed here is that the quintessence field need not just be an incredibly weakly interacting field, often added as simply an add-on to generate dark energy and have no other purpose. Instead, this quintessence field could play a central role in the particle physics dynamics at early times and then simply decouple at late times before the onset of the dark energy phase. For inflation a consequence is coupling of the inflaton to other fields can be much larger in certain models, without needing supersymmetry to control quantum corrections.

Title: An A4 flavor model for quarks and leptons in warped geometry

Abstract: We propose a spontaneous A4 flavor symmetry breaking scheme implemented in a warped extra dimensional setup to explain the observed pattern of quark and lepton masses and mixings. The main advantages of this choice are the explanation of fermion mass hierarchies by wave function overlaps, the emergence of tribimaximal neutrino mixing and zero quark mixing at the leading order and the absence of tree-level gauge mediated flavor violations. Quark mixing is induced by the presence of bulk flavons, which allow for cross-brane interactions and a cross-talk between the quark and neutrino sectors, realizing the spontaneous symmetry breaking pattern A4 --> nothing first proposed in [X.G.,He, Y.Y.,Keum, R.R.,Volkas, JHEP{0604}, 039 (2006)]. We show that the observed quark mixing pattern can be explained in a rather economical way, including the CP violating phase, with leading order cross-interactions, while the observed difference between the smallest CKM entries V_{ub} and V_{td} must arise from higher order corrections. We briefly discuss bounds on the Kaluza-Klein scale implied by flavor changing neutral current processes in our model and show that the residual little CP problem is milder than in flavor anarchic models.

Tuesday March 16, 2010

Title: A Unified Approach to Cosmic Acceleration

Abstract: We present a unified framework for the study of late time cosmic acceleration. Using methods of effective field theory, we show that existing proposals for late time acceleration can be subsumed in a single framework, rather than many compartmentalized theories. We construct the most general action consistent with symmetry principles, derive the back- ground and perturbation evolution equations, and demonstrate that for special choices of our parameters we can reproduce results already existing in the literature. Lastly, we lay the foundation for future work placing phenomenological constraints on the parameters of the effective theory. Although in this paper we focus on late time acceleration, our construction also generalizes the effective field theory of inflation to the scalar-tensor and multi-field case.

Title: Electroweak Symmetry Breaking From Monopole Condensation

Abstract: We examine models where massless chiral fermions with both "electric" and "magnetic" hypercharges could form condensates. When some of the fermions are also electroweak doublets such condensates can break the electroweak gauge symmetry down to electromagnetism in the correct way. Since ordinary hypercharge is weakly coupled at the TeV scale, magnetic hypercharge is strongly coupled and can potentially drive the condensation. Such models are similar to technicolor, but with hypercharge playing the role of the technicolor gauge group, so the standard model gauge group breaks itself. A heavy top mass can be generated via the Rubakov-Callan effect and could thus decouple the scale of flavor physics from the electroweak scale.

TUESDAY MARCH 9, 2010

Title: Higgs in Space!

Abstract: We consider the possibility that the Higgs can be produced in dark matter annihilations, appearing as a line in the spectrum of gamma rays at an energy determined by the masses of the WIMP and the Higgs itself. We argue that this phenomenon occurs generally in models in which the the dark sector has large couplings to the most massive states of the SM and provide a simple example inspired by the Randall-Sundrum vision of dark matter, whose 4d dual corresponds to electroweak symmetry-breaking by strong dynamics which respect global symmetries that guarantee a stable WIMP. The dark matter is a Dirac fermion that couples to a Z' acting as a portal to the Standard Model through its strong coupling to top quarks. Annihilation into light standard model degrees of freedom is suppressed and generates a feeble continuum spectrum of gamma rays. Loops of top quarks mediate annihilation into gamma Z, gamma h, and gamma Z', providing a forest of lines in the spectrum. Such models can be probed by the Fermi/GLAST satellite and ground-based Air Cherenkov telescopes.

Title: Measurement of the Depth of Maximum of Extensive Air Showers above 10^18 eV

Abstract: We describe the measurement of the depth of maximum, Xmax, of the longitudinal development of air showers induced by cosmic rays. Almost four thousand events above 10^18 eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/- 0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.

Title: Have Cherenkov telescopes detected a new light boson?

Abstract: Recent observations by H.E.S.S. and MAGIC strongly suggest that the Universe is more transparent to very-high-energy gamma rays than previously thought. We show that this fact can be reconciled with standard blazar emission models provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect indeed explains the observed data and in particular the spectrum of blazar 3C279.

Wednesday March 3, 2010

Title: Suppressing Lepton Flavour Violation in a Soft-Wall Extra Dimension

Abstract: A soft-wall warped extra dimension allows one to relax the tight constraints imposed by electroweak data in conventional Randall-Sundrum models. We investigate a setup, where the lepton flavour structure of the Standard Model is realised by split fermion locations. Bulk fermions with general locations are not analytically tractable in a soft-wall background, so we follow a numerical approach to perform the Kaluza-Klein reduction. Lepton flavour violation is induced by the exchange of Kaluza-Klein gauge bosons. We find that rates for processes such as muon-electron conversion are significantly reduced compared to hard-wall models, allowing for a Kaluza-Klein scale as low as 2 TeV. Accommodating small neutrino masses forces one to introduce a large hierarchy of scales into the model, making pressing the question of a suitable stabilisation mechanism.

Title: Using single top rapidity to measure V_td, V_ts, V_tb at hadron colliders

Abstract: Single top production processes are usually regarded as the ones in which V_tb can be directly measured at hadron colliders. We show that the analysis of the single top rapidity distribution in t-channel and tW production can also set direct limits on V_td. At LHC with 10 fb^-1 at 14 TeV the combined limits on V_td may be reduced by almost a factor of two when the top rapidity distribution is used. This also implies that the limits on V_tb can also be reduced by 15%, since both parameters as well as V_ts must be simultaneously obtained from a global fit to data. At Tevatron the full explotation of this distribution would require very high statistics. Still, with the luminosity achievable in the near future the analysis of the single top rapidity might improve the limits on V_td and V_tb up to 10%.

Title: Comment on Higgs Inflation and Naturalness

Abstract: We rebut the recent claim (arXiv:0912.5463) that Einstein-frame scattering in the Higgs inflation model is unitary above the cut-off energy Lambda ~ Mp/xi. We show explicitly how unitarity problems arise in both the Einstein and Jordan frames of the theory. In a covariant gauge they arise from non-minimal Higgs self-couplings, which cannot be removed by field redefinitions because the target space is not flat. In unitary gauge, where there is only a single scalar which can be redefined to achieve canonical kinetic terms, the unitarity problems arise through non-minimal Higgs-gauge couplings.

Title: On Inflation with Non-minimal Coupling

Abstract: A simple realization of inflation consists of adding the following operators to the Einstein-Hilbert action: (partial phi)^2, lambda phi^4, and xi phi^2 R, with xi a large non-minimal coupling. Recently there has been much discussion as to whether such theories are well behaved quantum mechanically and if the inflaton phi can also be the Standard Model Higgs. In this note we answer these questions. Firstly, for a single scalar phi, we show that the quantum field theory is well behaved at high scales in the sense that the quantum generated corrections are small (however it enters a regime in which perturbation theory breaks down in describing many particle scattering.) Secondly, we show that the theory changes dramatically for multiple scalars phi with non-minimal coupling xi phi dot phi R, since this introduces qualitatively new interactions which manifestly generate large quantum corrections, spoiling the theory for energies > m_pl / xi. Since the Higgs doublet of the Standard Model includes the Higgs boson and 3 Goldstone bosons, it falls into the latter category and therefore its validity is spoiled. We show that these conclusions hold in both the Jordan and Einstein frames. We describe an intuitive analogy in the form of the pion Lagrangian and discuss the effects of higher spin fields, as well as curvature-squared models. Our work appears to go beyond the recent discussions.

WEDNESDAY FEBRUARY 24, 2010

Title: Graviton collider effects in one and more large extra dimensions

Abstract: Astrophysical bounds severely limit the possibility of observing collider signals of gravity with less than 3 flat extra dimensions. However, small distortions of the compactified space can lift the masses of the lightest graviton excitations, evading astrophysical bounds without affecting collider signals of quantum gravity. Following this procedure we reconsider theories with one large extra dimension. A slight space warping gives a model which is safe in the infrared against astrophysical and observational bounds, and which has the ultraviolet properties of gravity with a single flat extra dimension. We extend collider studies to the case of one extra dimension, pointing out its peculiarities. Finally, for a generic number of extra dimensions, we compare different channels in LHC searches for quantum gravity, introducing an ultraviolet cutoff as an additional parameter besides the Planck mass.

Title: Low-Energy Probes of a Warped Extra Dimension

Abstract: We investigate a natural realization of a light Abelian hidden sector in an extended Randall-Sundrum (RS) model. In addition to the usual RS bulk we consider a second warped space containing a bulk U(1)_x gauge theory with a characteristic IR scale of order a GeV. This Abelian hidden sector can couple to the standard model via gauge kinetic mixing on a common UV brane. We show that if such a coupling induces significant mixing between the lightest U(1)_x gauge mode and the standard model photon and Z, it can also induce significant mixing with the heavier U(1)_x Kaluza-Klein (KK) modes. As a result it might be possible to probe several KK modes in upcoming fixed-target experiments and meson factories, thereby offering a new way to investigate the structure of an extra spacetime dimension.

Title: Revealing Randall-Sundrum Hidden Valleys

Abstract: We study 5D gauge symmetries in the Randall-Sundrum geometry that are hidden from the standard model through either small 5D gauge coupling, or through vanishing quantum numbers for the standard model fields. Geometric warping of 5D gravity creates a TeV scale bridge from the standard model to the hidden sector gauge fields. We apply these concepts to a revival of the electroweak axion model, in which the dynamics of Peccei-Quinn symmetry breaking occur at the TeV scale.

Title: A Warped Model of Dark Matter

Abstract: We present a model of dark matter in a warped extra dimension in which the dark sector mass scales are naturally generated without supersymmetry. The dark force, responsible for dark matter annihilating predominantly into leptons, is mediated by dark photons that naturally obtain a mass in the GeV range via a dilaton coupling. As well as solving the gauge hierarchy problem, our model predicts dark matter in the TeV range, including naturally tiny mass splittings between pseudo-Dirac states. By the AdS/CFT correspondence both the dark photon and dark matter are interpreted as composite states of the strongly-coupled dual 4d theory. Thus, in our model the dark sector emerges at the TeV scale from the dynamics of a new strong force.

Wednesday February 17, 2010

Title: Constraints on the Unhiggs Model from Top Quark Decay

Abstract: We compute the top quark decay rate in the Unhiggs model. In this model, the longitudinally polarized W's are unparticles, owing to their Goldstone boson nature, while the transversely polarized W's are not. Thus the fraction of decays with a longitudinal W emitted is different than in the Standard Model. Comparing this calculation to CDF data, we are able to rule out some of the Unhiggs model parameter space. We also use the expected increased accuracy of top decay measurements at the LHC to anticipate further constraints on the Unhiggs.

Title: The Top Triangle Moose

Abstract: We introduce a deconstructed model that incorporates both Higgsless and top-color mechanisms. The model alleviates the typical tension in Higgsless models between obtaining the correct top quark mass and keeping delta-rho small. It does so by singling out the top quark mass generation as arising from a Yukawa coupling to an effective top-Higgs which develops a small vacuum expectation value, while electroweak symmetry breaking results largely from a Higgsless mechanism. As a result, the heavy partners of the SM fermions can be light enough to be seen at the LHC.