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CERN Document Server latest documents in Theoretical Physics (TH)enTue, 25 Nov 2014 20:35:02 GMTInvenio 1.1.3.1106-62468cds.support@cern.ch36013926125http://cds.cern.ch/img/site_logo_rss.pngCERN Document Server
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New Higgs Inflation in a No-Scale Supersymmetric SU(5) GUT
http://cds.cern.ch/record/1971489
Higgs inflation is attractive because it identifies the inflaton with the electroweak Higgs boson. In this work, we construct a new class of supersymmetric Higgs inflationary models in the no-scale supergravity with an SU(5) GUT group. Extending the no-scale Kahler potential and SU(5) GUT superpotential, we derive a generic potential for Higgs inflation that includes the quadratic monomial potential and a Starobinsky-type potential as special limits. This type of models can accommodate a wide range of the tensor-to-scalar ratio $r = O(10^{-3}-10^{-1})$, as well as a scalar spectral index $n_s \sim 0.96$.Ellis, JohnTue, 25 Nov 2014 11:23:37 GMThttp://cds.cern.ch/record/1971489Small-radius jets to all orders in QCD
http://cds.cern.ch/record/1971030
As hadron collider physics continues to push the boundaries of precision, it becomes increasingly important to have methods for predicting properties of jets across a broad range of jet radius values R, and in particular for small R. In this paper we resum all leading logarithmic terms, $\alpha_s^n \ln^n R$, in the limit of small R, for a wide variety of observables. These include the inclusive jet spectrum, jet vetoes for Higgs physics and jet substructure tools. Some of the quantities that we consider are relevant also for heavy-ion collisions. Furthermore, we examine and comment on the underlying order-by-order convergence of the perturbative series for different R values. Our results indicate that small-R effects can be substantial. Phenomenological studies will appear in a forthcoming companion paper.Dasgupta, MrinalThu, 20 Nov 2014 07:20:31 GMThttp://cds.cern.ch/record/1971030N=2 Born-Infeld Attractors
http://cds.cern.ch/record/1970659
We derive new types of $U(1)^n$ Born-Infeld actions based on N=2 special geometry in four dimensions. As in the single vector multiplet (n=1) case, the non--linear actions originate, in a particular limit, from quadratic expressions in the Maxwell fields. The dynamics is encoded in a set of coefficients $d_{ABC}$ related to the third derivative of the holomorphic prepotential and in an SU(2) triplet of N=2 Fayet-Iliopoulos charges, which must be suitably chosen to preserve a residual N=1 supersymmetry.Ferrara, SWed, 19 Nov 2014 08:18:16 GMThttp://cds.cern.ch/record/1970659On sgoldstino-less supergravity models of inflation
http://cds.cern.ch/record/1969255
We go a step further in the search for a consistent and realistic supergravity model of large-field inflation by building a class of models with the following features: during slow-roll, all the scalar fields other than the inflaton are frozen by large inflaton-dependent masses or removed from the spectrum; at the end of inflation, supersymmetry is spontaneously broken with naturally vanishing classical vacuum energy. We achieve this by combining some geometrical properties of the Kaehler potential with the consistent use of a single nilpotent chiral superfield, in one-to-one correspondence with the supersymmetry-breaking direction in field space at the vacuum.Dall'Agata, GianguidoThu, 13 Nov 2014 06:53:05 GMThttp://cds.cern.ch/record/1969255Nonlinear evolution of density and flow perturbations on a Bjorken background
http://cds.cern.ch/record/1969230
Density perturbations and their dynamic evolution from early to late times can be used for an improved understanding of interesting physical phenomena both in cosmology and in the context of heavy-ion collisions. We discuss the spectrum and bispectrum of these perturbations around a longitudinally expanding fireball after a heavy-ion collision. The time-evolution equations couple the spectrum and bispectrum to each other, as well as to higher-order correlation functions through nonlinear terms. A non-trivial bispectrum is thus always generated, even if absent initially. For initial conditions corresponding to a model of independent sources, we discuss the linear and nonlinear evolution is detail. We show that, if the initial conditions are sufficiently smooth for fluid dynamics to be applicable, the nonlinear effects are relatively small.Brouzakis, NikolaosThu, 13 Nov 2014 06:52:35 GMThttp://cds.cern.ch/record/1969230Signs for new physics in the recent LHCb data?
http://cds.cern.ch/record/1969223
We comment on some tensions with the Standard Model predictions in the recent LHCb data.Hurth, TThu, 13 Nov 2014 06:52:33 GMThttp://cds.cern.ch/record/1969223Accelerating cosmological expansion from shear viscosity
http://cds.cern.ch/record/1969160
The dissipation of energy from local velocity perturbations in the cosmological fluid affects the time evolution of spatially averaged fluid dynamic fields and the cosmological solution of Einstein's field equations. We show how this backreaction effect depends on shear viscosity and other material properties of the dark sector, as well as the spectrum of perturbations. If sufficiently large, this effect could account for the acceleration of the cosmological expansion.Floerchinger, StefanThu, 13 Nov 2014 06:49:20 GMThttp://cds.cern.ch/record/1969160WIMPs at the Galactic Center
http://cds.cern.ch/record/1969059
Simple models of weakly interacting massive particles (WIMPs) predict dark matter annihilations into pairs of electroweak gauge bosons, Higgses or tops, which through their subsequent cascade decays produce a spectrum of gamma rays. Intriguingly, an excess in gamma rays coming from near the Galactic center has been consistently observed in Fermi data. A recent analysis by the Fermi collaboration confirms these earlier results. Taking into account the systematic uncertainties in the modelling of the gamma ray backgrounds, we show for the first time that this excess can be well fit by these final states. In particular, for annihilations to $(WW, ZZ, hh, tt)$, dark matter with mass between threshold and approximately (165, 190, 280, 310) GeV gives an acceptable fit. The fit range for $b\bar{b}$ is also enlarged to 35 GeV$\lesssim m_\chi \lesssim$ 165 GeV. These are to be compared to previous fits that concluded only much lighter dark matter annihilating into $b$, $\tau$, and light quark final states could describe the excess. We demonstrate that simple, well-motivated models of WIMP dark matter including a thermal-relic neutralino of the MSSM, Higgs portal models, as well as other simplified models can explain the excess.Agrawal, PrateekThu, 13 Nov 2014 06:49:10 GMThttp://cds.cern.ch/record/1969059Complementarity of direct and indirect searches in the pMSSM
http://cds.cern.ch/record/1968368
We explore the pMSSM parameter space in view of the constraints from SUSY and monojet searches at the LHC, from Higgs data and flavour physics observables, as well as from dark matter searches. We show that whilst the simplest SUSY scenarios are already ruled out, there are still many possibilities left over in the pMSSM. We discuss the complementarity between different searches and consistency checks which are essential in probing the pMSSM and will be even more important in the near future with the next round of data becoming available.Mahmoudi, FTue, 11 Nov 2014 07:12:11 GMThttp://cds.cern.ch/record/1968368Why should we care about the top quark Yukawa coupling?
http://cds.cern.ch/record/1968356
We give an answer to the question formulated in the title.Bezrukov, FedorTue, 11 Nov 2014 07:12:08 GMThttp://cds.cern.ch/record/1968356The Elusive Gluon
http://cds.cern.ch/record/1968309
We study the phenomenology of vector resonances in the context of natural composite Higgs models. A mild hierarchy between the fermionic partners and the vector resonances can be expected in these models based on the following arguments. Both direct and indirect (electroweak and flavor precision) constraints on fermionic partners are milder than the ones on spin one resonances. Also the naturalness pressure coming from the top partners is stronger than that induced by the gauge partners. This observation implies that the search strategy for vector resonances at the LHC needs to be modified. In particular, we point out the importance of heavy gluon decays (or other vector resonances) to top partner pairs that were overlooked in previous experimental searches at the LHC. These searches focused on simplified benchmark models in which the only new particle beyond the Standard Model was the heavy gluon. It turns out that, when kinematically allowed, such heavy-heavy decays make the heavy gluon elusive, and the bounds on its mass can be up to 2 TeV milder than in the simpler models considered so far for the LHC14. We discuss the origin of this difference and prospects for dedicated searches.Chala, MikaelTue, 11 Nov 2014 07:10:42 GMThttp://cds.cern.ch/record/1968309Approach to equilibrium in weakly coupled nonabelian plasmas
http://cds.cern.ch/record/1967245
We follow the time evolution of nonabelian gauge bosons from far-from-equilibrium initial conditions to thermal equilibrium by numerically solving an effective kinetic equation that becomes accurate in the weak coupling limit. We consider initial conditions that are either highly overoccupied or underoccupied. We find that overoccupied systems thermalize through a turbulent cascade reaching equilibrium in multiples of a thermalization time $t\approx 72./ (1-0.12\log \lambda)/\lambda^2 T$, whereas underoccupied systems undergo a "bottom-up" thermalization in a time $t\approx (34. +21. \ln(Q/T))/ (1-0.037\log \lambda)(Q/T)^{1/2}/\lambda^2 T$, where $Q$ is the characteristic momentum scale of the initial condition. We apply this result to model initial stages of heavy-ion collisions and find rapid thermalization roughly in a time $Qt \lesssim 10$ or $t\lesssim 1$ fm/c.Kurkela, AleksiThu, 06 Nov 2014 07:31:59 GMThttp://cds.cern.ch/record/1967245Physical and cut-off effects of heavy sea quarks
http://cds.cern.ch/record/1967242
We simulate a theory with two dynamical O($a$) improved Wilson quarks whose mass $M$ ranges from a factor eight up to a factor two below the charm quark mass and at three values of the lattice spacing ranging from 0.066 to 0.034 fm. This theory is a prototype to study the decoupling of heavy quarks. We measure the mass and cut-off dependence of ratios of gluonic observables defined from the Wilson flow or the static potential. The size of the 1/$M$ corrections can be determined and disentangled from the lattice artifacts. The difference with the pure gauge theory is at the percent level when two quarks with a mass of the charm quark are present.Knechtli, FrancescoThu, 06 Nov 2014 07:31:44 GMThttp://cds.cern.ch/record/1967242Two-Loop Higgs mass calculations in supersymmetric models beyond the MSSM with SARAH and SPheno
http://cds.cern.ch/record/1967210
We present an extension to the Mathematica package SARAH which allows for Higgs mass calculations at the two-loop level in a wide range of supersymmetric models beyond the MSSM. These calculations are based on the effective potential approach and include all two-loop corrections which are independent of electroweak gauge couplings. For the numerical evaluation Fortran code for SPheno is generated by SARAH. This allows the prediction of the Higgs mass in more complicated SUSY models with the same precision that most state-of-the-art spectrum generators provide for the MSSM.Goodsell, Mark DThu, 06 Nov 2014 07:30:53 GMThttp://cds.cern.ch/record/1967210Invisible dark gauge boson search in top decays using a kinematic method
http://cds.cern.ch/record/1967208
We discuss the discovery potential of a dark force carrier ($Z'$) of very light mass, $m_{Z'} \lesssim {\cal O}(1-10)$ GeV, at hadron colliders via rare top quark decays, especially when it decays invisibly in typical search schemes. We emphasize that the top sector is promising for the discovery of new particles because top quark pairs are copiously produced at the Large Hadron Collider. The signal process is initiated by a rare top decay into a bottom quark and a charged Higgs boson ($H^\pm$) decaying subsequently into a $W$ and one or multiple $Z'$s. The light $Z'$ can be invisible in collider searches in various scenarios, and it would be hard to distinguish the relevant collider signature from the regular $t\bar{t}$ process in the Standard Model. We suggest a search strategy using the recently proposed on-shell constrained $M_2$ variables. Our signal process is featured by an $\textit{asymmetric}$ event topology, while the $t\bar{t}$ is $\textit{symmetric}$. The essence behind the strategy is to evoke some contradiction in the relevant observables by applying the kinematic variables designed under the assumption of the $t\bar{t}$ event topology. To see the viability of the proposed technique, we perform Monte Carlo simulations including realistic effects such as cuts, backgrounds, detector resolution, and so on at the LHC of $\sqrt{s}=14$ TeV.Kim, DoojinThu, 06 Nov 2014 07:30:52 GMThttp://cds.cern.ch/record/1967208TEK twisted gradient flow running coupling
http://cds.cern.ch/record/1966814
We measure the running of the twisted gradient flow coupling in the Twisted Eguchi-Kawai (TEK) model, the SU(N) gauge theory on a single site lattice with twisted boundary conditions in the large N limit.Pérez, Margarita GarcíaTue, 04 Nov 2014 06:37:44 GMThttp://cds.cern.ch/record/1966814Parton distributions for the LHC Run II
http://cds.cern.ch/record/1966481
We present NNPDF3.0, the first set of parton distribution functions (PDFs) determined with a methodology validated by a closure test. NNPDF3.0 uses a global dataset including HERA-II deep-inelastic inclusive cross-sections, the combined HERA charm data, jet production from ATLAS and CMS, vector boson rapidity and transverse momentum distributions from ATLAS, CMS and LHCb, W+c data from CMS and top quark pair production total cross sections from ATLAS and CMS. Results are based on LO, NLO and NNLO QCD theory and also include electroweak corrections. To validate our methodology, we show that PDFs determined from pseudo-data generated from a known underlying law correctly reproduce the statistical distributions expected on the basis of the assumed experimental uncertainties. This closure test ensures that our methodological uncertainties are negligible in comparison to the generic theoretical and experimental uncertainties of PDF determination. This enables us to determine with confidence PDFs at different perturbative orders and using a variety of experimental datasets ranging from HERA-only up to a global set including the latest LHC results, all using precisely the same validated methodology. We explore some of the phenomenological implications of our results for the upcoming 13 TeV Run of the LHC, in particular for Higgs production cross-sections.Ball, Richard DMon, 03 Nov 2014 06:41:04 GMThttp://cds.cern.ch/record/1966481A naturally light Higgs without light Top Partners
http://cds.cern.ch/record/1966470
We demonstrate that the inclusion of a realistic lepton sector can relax significantly the upper bound on top partner masses in minimal composite Higgs models, induced by the lightness of the Higgs boson. To that extend, we present a comprehensive survey of the impact of different realizations of the fermion sectors on the Higgs potential, with a special emphasis on the role of the leptons. The non-negligible compositeness of the $\tau_R$ in a general class of models that address the flavor structure of the lepton sector and the smallness of the corresponding FCNCs, can have a significant effect on the potential. We find that, with the $\tau_R$ in the symmetric representation of $SO(5)$, an increase in the maximally allowed mass of the lightest top partner of $\gtrsim 1$ TeV is possible for minimal quark setups like the MCHM$_{5,10}$, without increasing the tuning. A light Higgs boson $m_H \sim(100-200)$ GeV is a natural prediction of such models, which thus provide a new setup that can evade ultra-light top partners without ad-hoc tuning in the Higgs mass. Moreover, we advocate a more minimal realization of the lepton sector than generally used in the literature, which still can avoid light partners due to its contributions to the Higgs mass in a different and very natural way, triggered by the seesaw mechanism. This allows to construct the most economical $SO(5)/SO(4)$ composite Higgs models possible. Using both a transparent 4D approach, as well as presenting numerical results in the 5D holographic description, we demonstrate that, including leptons, minimality and naturalness do not imply light partners. Leptonic effects, not considered before, could hence be crucial for the viability of composite models.Carmona, AdrianMon, 03 Nov 2014 06:41:02 GMThttp://cds.cern.ch/record/1966470The Effective Planck Mass and the Scale of Inflation
http://cds.cern.ch/record/1966457
Observable quantities in cosmology are dimensionless, and therefore independent of the units in which they are measured. This is true of all physical quantities associated with the primordial perturbations that source cosmic microwave background anisotropies such as their amplitude and spectral properties. However, if one were to try and \textit{infer} an absolute energy scale for inflation-- a priori, one of the more immediate corollaries of detecting primordial tensor modes-- one necessarily makes reference to a particular choice of units, the natural choice for which is Planck units. In this note, we discuss various aspects of how inferring the energy scale of inflation is complicated by the fact that the effective Planck mass as seen by inflationary quanta necessarily differs from the effective Planck mass as seen by gravitational experiments at presently accessible scales. The uncertainty in the former relative to the latter has to do with the unknown spectrum of universally coupled particles between laboratory scales and the putative scale of inflation. These intermediate particles could be in hidden as well as visible sectors or could also be associated with Kaluza-Klein resonances associated with a compactification scale below the scale of inflation. We discuss various implications for cosmological observables.Antoniadis, IgnatiosMon, 03 Nov 2014 06:40:25 GMThttp://cds.cern.ch/record/1966457Parity Violation by a Dark Gauge Boson
http://cds.cern.ch/record/1966315
We overview the dark parity violation, which means the parity violation induced by a dark gauge boson of very small mass and coupling. When a dark gauge boson has an axial coupling, as in dark Z model, it can change the effective Weinberg angle in the low-energy experiments such as the atomic parity violation and the low-Q^2 polarized electron scatterings. Such low-energy parity tests are an excellent probe of the dark force.Lee, Hye-SungSat, 01 Nov 2014 07:00:51 GMThttp://cds.cern.ch/record/1966315The Effective Standard Model after LHC Run I
http://cds.cern.ch/record/1958019
We treat the Standard Model as the low-energy limit of an effective field theory that incorporates higher-dimensional operators to capture the effects of decoupled new physics. We consider the constraints imposed on the coefficients of dimension-6 operators by electroweak precision tests (EWPTs), applying a framework for the effects of dimension-6 operators on electroweak precision tests that is more general than the standard $S,T$ formalism, and use measurements of Higgs couplings and the kinematics of associated Higgs production at the Tevatron and LHC, as well as triple-gauge couplings at the LHC. We highlight the complementarity between EWPTs, Tevatron and LHC measurements in obtaining model-independent limits on the effective Standard Model after LHC Run~1. We illustrate the combined constraints with the example of the two-Higgs doublet model.Ellis, JohnWed, 29 Oct 2014 07:07:14 GMThttp://cds.cern.ch/record/1958019Cosmological constraints on deviations from Lorentz invariance in gravity and dark matter
http://cds.cern.ch/record/1957430
We consider a scenario where local Lorentz invariance is violated by the existence of a preferred time direction at every space-time point. This scenario can arise in the context of quantum gravity and its description at low energies contains a unit time-like vector field which parameterizes the preferred direction. The particle physics tests of Lorentz invariance preclude a direct coupling of this vector to the fields of the Standard Model, but do not bear implications for dark matter. We discuss how the presence of this vector and its possible coupling to dark matter affect the evolution of the Universe. At the level of homogeneous cosmology the only effect of Lorentz invariance violation is a rescaling of the expansion rate. The physics is richer at the level of perturbations. We identify three effects crucial for observations: the rescaling of the matter contribution to the Poisson equation, the appearance of an extra contribution to the anisotropic stress and the scale-dependent enhancement of dark matter clustering. These effects result in distinctive features in the power spectra of the CMB and density fluctuations. Making use of the data from Planck and WiggleZ we obtain the most stringent cosmological constraints to date on departures from Lorentz symmetry. Our analysis provides the first direct bounds on deviations from Lorentz invariance in the dark matter sector.Audren, BMon, 27 Oct 2014 06:46:56 GMThttp://cds.cern.ch/record/1957430Holography beyond conformal invariance and AdS isometry?
http://cds.cern.ch/record/1957270
We suggest that the principle of holographic duality can be extended beyond conformal invariance and AdS isometry. Such an extension is based on a special relation between functional determinants of the operators acting in the bulk and on its boundary, provided that the boundary operator represents the inverse propagators of the theory induced on the boundary by the Dirichlet boundary value problem from the bulk spacetime. This relation holds for operators of general spin-tensor structure on generic manifolds with boundaries irrespective of their background geometry and conformal invariance, and it apparently underlies numerous $O(N^0)$ tests of AdS/CFT correspondence, based on direct calculation of the bulk and boundary partition functions, Casimir energies and conformal anomalies. The generalized holographic duality is discussed within the concept of the "double-trace" deformation of the boundary theory, which is responsible in the case of large $N$ CFT coupled to the tower of higher spin gauge fields for the renormalization group flow between infrared and ultraviolet fixed points. Potential extension of this method beyond one-loop order is also briefly discussed.Barvinsky, A OSat, 25 Oct 2014 05:38:35 GMThttp://cds.cern.ch/record/1957270Matching NLO with parton shower in Monte Carlo scheme
http://cds.cern.ch/record/1957165
A new method of including NLO QCD corrections to the hard process in the LO Monte Carlo (MC) shower is discussed. The method is based on a recently proposed MC factorization scheme, which dramatically simplifies the NLO coefficient functions. The NLO corrections are introduced by simple reweighing of the events produced by the LO shower with a single, positive MC weight. A practical implementation of the method is presented for the case of electro-weak boson production in the hadron-hadron collision, and the results are compared with well established approaches to NLO+PS matching.Sapeta, SebastianFri, 24 Oct 2014 10:33:29 GMThttp://cds.cern.ch/record/1957165Resurgent Analysis of Localizable Observables in Supersymmetric Gauge Theories
http://cds.cern.ch/record/1956995
Localization methods have recently led to a plethora of new exact results in supersymmetric gauge theories, as certain observables may be computed in terms of matrix integrals. These can then be evaluated by making use of standard large N techniques, or else via perturbative expansions in the gauge coupling. Either approximation often leads to observables given in terms of asymptotic series, which need to be properly defined in order to obtain nonperturbative results. At the same time, resurgent analysis has recently been successfully applied to several problems, e.g., in quantum, field and string theories, precisely to overcome this issue and construct nonperturbative answers out of asymptotic perturbative expansions. The present work uses exact results from supersymmetric localization to address the resurgent structure of the free energy and partition function of Chern-Simons and ABJM gauge theories in three dimensions, and of N=2 supersymmetric Yang-Mills theories in four dimensions. For each case, the complete structure of Borel singularities is exactly determined, and the relation of these singularities with the large-order behavior of (multi-instanton) perturbative expansions is made fully precise.Aniceto, InêsThu, 23 Oct 2014 05:58:55 GMThttp://cds.cern.ch/record/1956995