CERN Document Server: Theoretical Physics (TH)
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CERN Document Server latest documents in Theoretical Physics (TH)enSat, 20 Dec 2014 17:58:54 GMTInvenio 1.1.3.1106-62468cds.support@cern.ch36013946125http://cds.cern.ch/img/site_logo_rss.pngCERN Document Server
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Automated next-to-leading order predictions for colored scalar production at the LHC
http://cds.cern.ch/record/1978043
We explore scenarios beyond the Standard Model where new colored scalar particles can be pair produced in hadron collisions. Using simplified models to describe the new field interactions with the Standard Model, we present precision predictions for the LHC by automatically matching next-to-leading order matrix elements with parton showers within the MadGraph5 aMC@NLO framework. We illustrate in this way, for the first time in our field, the full automation of Monte Carlo event generation at the next-to-leading order accuracy in QCD for new physics theories with an extended colored sector, the sole inputs being the model Lagrangian and the process of interest.Degrande, CélineSat, 20 Dec 2014 08:55:34 GMThttp://cds.cern.ch/record/1978043Robustness of cosmic neutrino background detection in the cosmic microwave background
http://cds.cern.ch/record/1978003
The existence of a cosmic neutrino background can be probed indirectly by CMB experiments, not only by measuring the background density of radiation in the universe, but also by searching for the typical signatures of the fluctuations of free-streaming species in the temperature and polarisation power spectrum. Previous studies have already proposed a rather generic parametrisation of these fluctuations, that could help to discriminate between the signature of ordinary free-streaming neutrinos, or of more exotic dark radiation models. Current data are compatible with standard values of these parameters, which seems to bring further evidence for the existence of a cosmic neutrino background. In this work, we investigate the robustness of this conclusion under various assumptions. We generalise the definition of an effective sound speed and viscosity speed to the case of massive neutrinos or other dark radiation components experiencing a non-relativistic transition. We show that current bounds on these effective parameters do not vary significantly when considering an arbitrary value of the particle mass, or extended cosmological models with a free effective neutrino number, dynamical dark energy or a running of the primordial spectrum tilt. We conclude that it is possible to make a robust statement about the detection of the cosmic neutrino background by CMB experiments.Audren, BenjaminSat, 20 Dec 2014 07:06:17 GMThttp://cds.cern.ch/record/1978003Gauged R-symmetry and its anomalies in 4D N=1 supergravity and phenomenological implications
http://cds.cern.ch/record/1977668
We consider a class of models with gauged U(1)_R symmetry in 4D N=1 supergravity that have, at the classical level, a metastable ground state, an infinitesimally small (tunable) positive cosmological constant and a TeV gravitino mass. We analyse if these properties are maintained under the addition of visible sector (MSSM-like) and hidden sector state(s), where the latter may be needed for quantum consistency. We then discuss the anomaly cancellation conditions in supergravity as derived by Freedman, Elvang and K\"ors and apply their results to the special case of a U(1)_R symmetry, in the presence of the Fayet-Iliopoulos term ($\xi$) and Green-Schwarz mechanism(s). We investigate the relation of these anomaly cancellation conditions to the "naive" field theory approach in global SUSY, in which case U(1)_R cannot even be gauged. We show the two approaches give similar conditions. Their induced constraints at the phenomenological level, on the above models, remain strong even if one lifted the GUT-like conditions for the MSSM gauge couplings. In an anomaly-free model, a tunable, TeV-scale gravitino mass may remain possible provided that the U(1)_R charges of additional hidden sector fermions (constrained by the cubic anomaly alone) do not conflict with the related values of U(1)_R charges of their scalar superpartners, constrained by existence of a stable ground state. This issue may be bypassed by tuning instead the coefficients of the Kahler connection anomalies (b_K, b_{CK}).Antoniadis, IThu, 18 Dec 2014 07:36:02 GMThttp://cds.cern.ch/record/1977668Instantaneous stochastic perturbation theory
http://cds.cern.ch/record/1977661
A form of stochastic perturbation theory is described, where the representative stochastic fields are generated instantaneously rather than through a Markov process. The correctness of the procedure is established to all orders of the expansion and for a wide class of field theories that includes all common formulations of lattice QCD.Lüscher, MartinThu, 18 Dec 2014 07:35:45 GMThttp://cds.cern.ch/record/1977661Phenomenology of theories of gravity without Lorentz invariance: the preferred frame case
http://cds.cern.ch/record/1977484
Theories of gravitation without Lorentz invariance are candidates of low-energy descriptions of quantum gravity. In this review we will describe the phenomenological consequences of the candidates associated to the existence of a preferred time directionBlas, DiegoThu, 18 Dec 2014 07:28:38 GMThttp://cds.cern.ch/record/1977484A Symbol of Uniqueness: The Cluster Bootstrap for the 3-Loop MHV Heptagon
http://cds.cern.ch/record/1976326
Seven-particle scattering amplitudes in planar super-Yang-Mills theory are believed to belong to a special class of generalised polylogarithm functions called heptagon functions. These are functions with physical branch cuts whose symbols may be written in terms of the 42 cluster A-coordinates on Gr(4,7). Motivated by the success of the hexagon bootstrap programme for constructing six-particle amplitudes we initiate the systematic study of the symbols of heptagon functions. We find that there is exactly one such symbol of weight six which satisfies the MHV last-entry condition and is finite in the $7 \parallel 6$ collinear limit. This unique symbol is both dihedral and parity-symmetric, and remarkably its collinear limit is exactly the symbol of the three-loop six-particle MHV amplitude, although none of these properties were assumed a priori. It must therefore be the symbol of the three-loop seven-particle MHV amplitude. The simplicity of its construction suggests that the n-gon bootstrap may be surprisingly powerful for n>6.Drummond, James MFri, 12 Dec 2014 07:09:25 GMThttp://cds.cern.ch/record/1976326Flavoured Dark Matter Beyond MFV
http://cds.cern.ch/record/1976306
We review a model of quark flavoured dark matter with new flavour violating interactions. This simplified model describes Dirac fermionic dark matter that is charged under a new U(3) flavour symmetry and couples to right-handed down quarks via a scalar mediator. The corresponding coupling matrix is assumed to be the only new source of flavour violation, which we refer to as the Dark Minimal Flavour Violation (DMFV) hypothesis. This ansatz ensures the stability of dark matter. We discuss the phenomenology of the simplest DMFV model in flavour violating observables, LHC searches, and direct dark matter detection experiments. Especially interesting is the non-trivial interplay between the constraints from the different sectors.Blanke, MonikaFri, 12 Dec 2014 07:09:07 GMThttp://cds.cern.ch/record/1976306Hamiltonian Truncation Study of the Phi^4 Theory in Two Dimensions
http://cds.cern.ch/record/1976298
We defend the Fock-space Hamiltonian truncation method, which allows to calculate numerically the spectrum of strongly coupled quantum field theories, by putting them in a finite volume and imposing a UV cutoff. The accuracy of the method is improved via an analytic renormalization procedure inspired by the usual effective field theory. As an application, we study the two-dimensional Phi^4 theory for a wide range of couplings. The theory exhibits a quantum phase transition between the symmetry-preserving and symmetry-breaking phases. We extract quantitative predictions for the spectrum and the critical coupling and make contact with previous results from the literature. Future directions to further improve the accuracy of the method and enlarge its scope of applications are outlined.Rychkov, SlavaFri, 12 Dec 2014 07:08:55 GMThttp://cds.cern.ch/record/1976298The Quark Condensate in Multi-Flavour QCD - Planar Equivalence Confronting Lattice Simulations
http://cds.cern.ch/record/1976072
Planar equivalence between the large $N$ limits of ${\cal N}=1$ Super Yang-Mills (SYM) theory and a variant of QCD with fermions in the antisymmetric representation is a powerful tool to obtain analytic non-perturbative results in QCD itself. In particular, it allows the quark condensate for $N=3$ QCD with quarks in the fundamental representation to be inferred from exact calculations of the gluino condensate in ${\cal N}=1$ SYM. In this paper, we review and refine our earlier predictions for the quark condensate in QCD with a general number $n_f$ of flavours and confront these with lattice results.Armoni, AdiThu, 11 Dec 2014 06:31:50 GMThttp://cds.cern.ch/record/1976072Generalized Born--Infeld Actions and Projective Cubic Curves
http://cds.cern.ch/record/1976061
We investigate $U(1)^{\,n}$ supersymmetric Born-Infeld Lagrangians with a second non-linearly realized supersymmetry. The resulting non-linear structure is more complex than the square root present in the standard Born-Infeld action, and nonetheless the quadratic constraints determining these models can be solved exactly in all cases containing three vector multiplets. The corresponding models are classified by cubic holomorphic prepotentials. Their symmetry structures are associated to projective cubic varieties.Ferrara, SThu, 11 Dec 2014 06:31:04 GMThttp://cds.cern.ch/record/1976061Softened Gravity and the Extension of the Standard Model up to Infinite Energy
http://cds.cern.ch/record/1975745
Attempts to solve naturalness by having the weak scale as the only breaking of classical scale invariance have to deal with two severe difficulties: gravity and the absence of Landau poles. We show that solutions to the first problem require premature modifications of gravity at scales no larger than $10^{11}$ GeV, while the second problem calls for many new particles at the weak scale. To build models that fulfil these properties, we classify 4-dimensional Quantum Field Theories that satisfy Total Asymptotic Freedom (TAF): the theory holds up to infinite energy, where all coupling constants flow to zero. We develop a technique to identify such theories and determine their low-energy predictions. Since the Standard Model turns out to be asymptotically free only under the unphysical conditions $g_1 = 0$, $M_t = 186$ GeV, $M_\tau = 0$, $M_h = 163$ GeV, we explore some of its weak-scale extensions that satisfy the requirements for TAF.Giudice, Gian FWed, 10 Dec 2014 06:46:07 GMThttp://cds.cern.ch/record/1975745The Beautiful Physics of LHC Run 2
http://cds.cern.ch/record/1975705
Run 2 of the LHC offers some beautiful prospects for new physics, including flavour physics as well as more detailed studies of the Higgs boson and searches for new physics beyond the Standard Model (BSM). One of the possibilities for BSM physics is supersymmetry, and flavour physics plays various important r\^oles in constraining supersymmetric models.Ellis, JohnWed, 10 Dec 2014 06:44:31 GMThttp://cds.cern.ch/record/1975705Charged Higgs: Interpretation of $B$-physics results
http://cds.cern.ch/record/1975700
In these proceedings we review the impact of additional Higgs bosons on $B$ physics observables. For this purpose, we consider first the 2HDM of type II which respects natural flavour conservation. Afterwards, we study a 2HDM with generic flavour structure (of type III). While constraints from $B_{s,d}\to\mu^+\mu^+$ impose stringent bounds on non-minimal flavour violation in the down sector, $b\to s,d\gamma$ can also constrain flavour violation in the up sector. Despite these stringent constraints and the recent CMS bounds from $A^0\to\tau^+\tau^-$ it is shown that still large effects in tauonic $B$ decays are possible, such that the BABAR measurements of these decays can be explained. The matching of the Yukawa sector of the MSSM on the 2HDM is discussed.Crivellin, AndreasWed, 10 Dec 2014 06:44:28 GMThttp://cds.cern.ch/record/1975700Opportunities with top quarks at future circular colliders
http://cds.cern.ch/record/1974547
We describe various studies relevant for top physics at future circular collider projects currently under discussion. We show how highly-massive top-antitop systems produced in proton-proton collisions at a center-of-mass energy of 100 TeV could be observed and employed for constraining top dipole moments, investigate the reach of future proton-proton and electron-positron machines to top flavor-changing neutral interactions, and discuss top parton densities.Fuks, BenjaminMon, 08 Dec 2014 11:42:51 GMThttp://cds.cern.ch/record/1974547The $SU(\infty)$ twisted gradient flow running coupling
http://cds.cern.ch/record/1973884
We measure the running of the $SU(\infty)$ 't Hooft coupling by performing a step scaling analysis of the Twisted Eguchi-Kawai (TEK) model, the SU($N$) gauge theory on a single site lattice with twisted boundary conditions. The computation relies on the conjecture that finite volume effects for SU(N) gauge theories defined on a 4-dimensional twisted torus are controlled by an effective size parameter $\tilde l = l \sqrt{N}$, with $l$ the torus period. We set the scale for the running coupling in terms of $\tilde l$ and use the gradient flow to define a renormalized 't Hooft coupling $\lambda(\tilde l)$. In the TEK model, this idea allows the determination of the running of the coupling through a step scaling procedure that uses the rank of the group as a size parameter. The continuum renormalized coupling constant is extracted in the zero lattice spacing limit, which in the TEK model corresponds to the large $N$ limit taken at fixed value of $\lambda(\tilde l)$. The coupling constant is thus expected to coincide with that of the ordinary pure gauge theory at $N =\infty$. The idea is shown to work and permits us to follow the evolution of the coupling over a wide range of scales. At weak coupling we find a remarkable agreement with the perturbative two-loop formula for the running coupling.Pérez, Margarita GarcíaThu, 04 Dec 2014 07:04:03 GMThttp://cds.cern.ch/record/1973884Flavour Physics Beyond the Standard Model: Recent Developments and Future Perspectives
http://cds.cern.ch/record/1973867
Flavour physics plays a crucial role in the search for physics beyond the Standard Model (SM). While $B$ physics offers many observables to look for deviations from the SM, the highest new physics sensitivity can be obtained in the rare kaon decays $K\to\pi\nu\bar\nu$. Of particular interest are correlations between various flavour violating observables that allow to test the symmetries and the operator structure of the new physics flavour sector. New physics searches in rare meson decays are complemented by searches for new flavour violating interactions in the production and decay of new particles at the LHC and in dark matter phenomenology.Blanke, MonikaThu, 04 Dec 2014 07:02:55 GMThttp://cds.cern.ch/record/1973867Flipped GUT Inflation
http://cds.cern.ch/record/1973839
We analyse the prospects for constructing hybrid models of inflation that provide a dynamical realisation of the apparent closeness between the supersymmetric GUT scale and the possible scale of cosmological inflation. In the first place, we consider models based on the flipped SU(5)$\times$U(1) gauge group, which has no magnetic monopoles. In one model, the inflaton is identified with a sneutrino field, and in the other model it is a gauge singlet. In both cases we find regions of the model parameter spaces that are compatible with the experimental magnitudes of the scalar perturbations, $A_s$, and the tilt in the scalar perturbation spectrum, $n_s$, as well as with an indicative upper limit on the tensor-to-scalar perturbation ratio, $r$. We also discuss embeddings of these models into SO(10), which is broken at a higher scale so that its monopoles are inflated away.Ellis, JohnThu, 04 Dec 2014 07:01:00 GMThttp://cds.cern.ch/record/1973839On the applicability of approximations used in calculation of spectrum of Dark Matter particles produced in particle decays
http://cds.cern.ch/record/1973821
For the Warm Dark Matter candidates the momentum distribution of particles becomes important, since it can be probed with observations of Lyman-$\alpha$ forest structures. We recall the calculation by Kaplinghat (2005) of the spectrum in case of dark matter nonthermal production in decays of heavy particles emphasizing on the inherited applicability conditions, which are rather restrictive and sometimes ignored in literature.Bezrukov, FedorThu, 04 Dec 2014 07:00:58 GMThttp://cds.cern.ch/record/1973821Collider limits on leptophilic interactions
http://cds.cern.ch/record/1972795
In order to observe leptophilic interactions at the LHC we must require at least four leptons, which may be in particular neutrinos, in the final state. If they are mediated by a resonance in the di-leptonic channel, this must be of spin 1 to be produced through a renormalizable coupling. We study the LHC reach for such a vector boson allowing for arbitrary couplings, what in practice means for arbitrary couplings to muons because lepton flavor violating couplings are constrained by rare processes, couplings to electrons by LEP and LHC is not sensitive to final states involving taus in this case. This makes the ILC complementary as it will provide the best limits on $Z'$ couplings to tau leptons. A prominent example is the case of the anomaly-free $Z'$ coupling to the muon minus tau lepton number ${\rm L}_\mu - {\rm L}_\tau$. If no departure from the Standard Model is observed at the LHC, the most stringent bounds on this vector boson are provided from events with only three charged leptons plus missing energy. Masses of the order of 1 TeV can be probed at the high-luminosity phase of the LHC for $Z^\prime$ couplings of order one. Generic four-lepton operators parametrizing leptophilic interactions can be also restricted using three and four (or two at the ILC) charged lepton samples, but the corresponding limits are marginal, if meaningful, because the resonant behavior appears to be essential for the signal to be significant.del Aguila, FranciscoMon, 01 Dec 2014 06:21:31 GMThttp://cds.cern.ch/record/1972795On O(a^2) effects in gradient flow observables
http://cds.cern.ch/record/1972219
In lattice gauge theories, the gradient flow has been used extensively both, for scale setting and for defining finite volume renormalization schemes for the gauge coupling. Unfortunately, rather large cutoff effects have been observed in some cases. We here investigate these effects to leading order in perturbation theory, considering various definitions of the lattice observable, the lattice flow equation and the Yang Mills lattice action. These considerations suggest an improved set- up for which we perform a scaling test in the pure SU(3) gauge theory, demonstrating strongly reduced cutoff effects. We then attempt to obtain a more complete understanding of the structure of O(a^2) effects by applying Symanzik's effective theory approach to the 4+1 dimensional local field theory with flow time as the fifth dimension. From these considerations we are led to a fully O(a^2) improved set-up the study of which is left to future work.Ramos, AlbertoThu, 27 Nov 2014 07:07:54 GMThttp://cds.cern.ch/record/1972219New 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/1969230