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Séminaires 2014 (physique de particules)

Jeudi 26 Juin 2014
Are bottom PDFs needed at the LHC ?
Maria UBIALI (DAMTP, University of Cambridge, UK)
Several key processes at the LHC in the Standard Model and beyond involve bottom quarks : single-top, Higgs, and weak vector boson associated production. They can be described in QCD either in a 4-flavor or 5-flavor scheme. In the former, bottom quarks appear only in the final state and are typically considered massive final state particles. In 5-flavor schemes, calculations include bottom quarks in the initial state, they are simpler and allow the resummation of possibly large initial state logarithms of the type ln(Q/m_b) into the bottom parton distribution function (PDF), Q being the typical scale of the hard process. An analogous classification can be applied to processes involving bottom quarks in the final state, in which the b fragmentation acts like the b PDF. In this talk, I describe a method for assessing the impact of resummation of collinear logarithms and provide an explanation of why and how a substantial agreement between total cross sections can be obtained in the two schemes, as far as the initial state is concerned. Finally I discuss several generalizations of the proposed prescription and future perspectives.
Ecole Polytechnique, CPHT, Salle des Conférences

Mardi 24 Juin 2014
Bounds on neutral and charged Higgs from the LHC
Victor ILISIE (IFIC, Universitat de Valencia)
After the recent discovery of a boson with mass around 125 GeV with Standard Model-like properties, the possibility of an enlarged scalar sector arises as a natural question. The discovery of an extra scalar would be the ultimate proof of this scenario. Keeping the generic Yukawa structure of the Aligned Two-Higgs Doublet Model framework, we study the implications of the LHC data on the allowed scalar spectrum. [pdf]
IPN, Orsay, Bât. 100, Salle A201

Jeudi 19 Juin 2014
Quark-hadron phase transition in the PNJL with mesonic and baryonic excitations
Kanako YAMAZAKI (Institute of Physics, University of Tokyo )
We study the QCD phase transition by a Nambu-Jona-Lasinio model extended with the Polyakov loop (PNJL model). The Nambu-Jona-Lasinio model describes the chiral transition, while the Polyakov loop works as an order parameter of the de-confinement transition. The aim of this work is to describe the change of degrees of freedom from hadrons to quarks through the transition region. For this purpose, we calculate an equation of state by an approach beyond the mean field approximation to take thermal excitations of hadrons into account. In this method, mesons can be introduced as auxiliary fields, and their excitations are described as thermal fluctuations of the latter. I will present an equation of state concerning mesonic excitations at zero quark chemical potential in the two- and three-flavor PNJL model for interacting quarks. In addition, baryonic correlation is introduced as a three body system of quarks at finite chemical potential. I will also consider the way of introducing interactions in three quark systems.
IPN, Orsay, Bât. 100, Salle A201

Jeudi 12 Juin 2014
Convergent perturbation theory on the lattice
Vasily K. SAZONOV (University of Graz)
Traditional perturbation theory in quantum field theory and lattice models lead to asymptotic expansions. An appropriate regularization of the path and lattice integrals allows one to get convergent series at any values of the expansion parameters. I will present the method of the approximate calculation of the multi-dimensional integrals and construct converging perturbation theory for lattice QED.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 5 Juin 2014
Gluon radiation by heavy quarks
Thierry GOUSSET (Subatech, Nantes)
In order to study the transport properties of heavy quarks propagating through the medium created in high-energy heavy-ion collisions, the radiation phenomenon that accompanies the heavy quark diffusion will be discussed, as well as the resulting energy loss.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 15 Mai 2014
The non-linear evolution of jet quenching
Edmond IANCU (Institut de Physique Théorique, CEA/Saclay)
Within perturbative QCD, we consider the high-energy evolution of phenomena like the transverse momentum broadening and the radiative energy loss, which accompany the propagation of an energetic parton through a dense QCD medium, and which are globally known as "jet quenching". When the medium size L is sufficiently large, say L >> 1/T for a slice of quark-gluon plasma with temperature T, we find large one-loop corrections, which are enhanced by single or even double powers of the logarithm ln(LT). Such corrections, which are associated with soft gluon emissions (bremsstrahlung) by the energetic projectile, represent only the first step in a quantum evolution, which is generally non-linear, due to multiple scattering in the medium. We construct the equations describing this non-linear evolution to leading logarithmic accuracy in pQCD. These equations may be viewed as a generalization, going beyond the eikonal approximation, of the BK-JIMWLK equations for the evolution of the gluon distribution in a large nucleus. An important prediction of these equations is a rapid growth of the jet quenching parameter with increasing medium size L.
Ecole Polytechnique, CPHT, Salle des Conférences

Lundi 14 Avril 2014
Infrared logs in EFT and chiral inflation of the pion radius
Maxim V. POLYAKOV (Institute for Theoretical Physics II, Ruhr-University, Bochum)
We show how the leading IR logs can be calculated to any loop order in an effective field theory, As an application, we derive, in a model independent way, the behaviour of the pion radius, at small Bjorken x. It turns out that due to spontaneous chiral symmetry breaking the pion radius grows exponentially with the rapidity. This phenomenon is at variance with the Gribov diffusion. Possible experimental checks of the phenomenon are discussed. [pdf]
IPN, Orsay, Bât. 100

Jeudi 10 Avril 2014
The pion pole in hard exclusive vector-meson leptoproduction
Peter KROLL (Fachbereich Physik, Universität Wuppertal)
It will be reported on a study of the omega spin density matrix elements recently measured by the HERMES collaboration. In this study the helicity amplitudes for the process gamma^* p —> omega p are calculated within the handbag approach by exploiting a set of generalized parton distributions derived from analyses of hard exclusive leptoproduction of rho^0, phi and pi^+ mesons. The importantrole of the pion pole in omega production will be demonstrated. As an intermediate step of the analysis, the pi omega transition form factor can be extracted for photon virtualities less than 4 GeV^2. The results on this form factor will be compared with the present experimental and theoretical knowledge on it. The role of the pion pole in exclusive rho^0 and phi leptoproduction will be discussed too.
Ecole Polytechnique, CPHT, Salle des Conférences

Mardi 11 mars 2014
Chiral-Scale perturbation theory about an infrared fixed point
Lewis TUNSTALL (ITP, University of Bern)
It has been known for quite some time that the low-energy expansion of chiral SU(3) x SU(3) perturbation theory (ChPT3) is afflicted with a peculiar malady : it typically diverges for amplitudes involving both the scalar-isoscalar channel and O(mK) extrapolations in momenta. In this talk, I will review some of the classic examples where this phenomenon occurs and comment on the origin of the large corrections in the chiral series. I will present a proposal to obtain a numerically convergent series which involves the replacement of ChPT3 with a chiral-scale effective theory (ChPTsigma) based on a low-energy expansion about an infrared fixed point in the strong coupling of 3-flavour QCD. At the fixed point, the quark condensate induces nine Nambu-Goldstone bosons : pi, K, eta and a QCD dilaton sigma which is identified with the f0(500) resonance. I will discuss the construction of the ChPTsigma Lagrangian and its implications for meson phenomenology at low energies. The main results which arise from this picture include a simple explanation for the Delta I = 1/2 rule in K-decays and an estimate for the Drell-Yan ratio in the infrared limit. [pdf]
IPN, Orsay

Jeudi 6 Mars 2014
Entropy in an effective model for the quark-hadron transition
Tetsuo MATSUI (University of Tokyo)
Entropy is sensitive to the effective degrees of freedom thermally excited in the system. I discuss how the entropy changes through a quark-hadron phase transition in the PNJL model for interacting quarks which describes, with inclusion of the mesonic correlations, the change of the effective degrees of freedom of thermal excitations from those of mesons at low temperatures to those of quarks at high temperatures.
IPN, Orsay, Bât. 100

Jeudi 27 Février 2014
Next-to-eikonal corrections to the CGC : gluon production in pA
Guillaume BEUF (DFP et IGFAE, Universidade de Santiago de Compostela)
In the high energy limit, scattering processes are usually described within the eikonal approximation, neglecting contributions which are power-suppressed. The Color Glass Condensate effective theory (CGC) is one of the most convenient formalisms based on the eikonal approximation which is valid for processes involving a large nuclear target, like pA or AA collisions. Successful phenomenological studies have been performed in this framework for LHC and RHIC in particular. However, little is known about power-suppressed corrections and thus about the validity range of the eikonal approximation. I will present a method to expand systematically the gluon propagator in a strong background field beyond the eikonal approximation. Generically, this allows one to calculate a subset of corrections beyond the eikonal approximation for high-energy processes : the corrections associated with the finite longitudinal width of the target (or with the time evolution of the projectile during the collision). As a first example, I will discuss the application of that method to single-inclusive gluon production at central rapidity in pA collisions.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 6 Février 2014
Covariant gauges without Gribov ambiguities in Yang-Mills theories
Julien SERREAU (APC, Université Denis Diderot, Paris)
We propose a formulation of a certain class of (nonlinear) covariant gauges as an extremization procedure that can be implemented on the lattice. This generalizes the usual Landau gauge and allows for continuous deviations from it in terms of a real parameter. Ignoring the issue of Gribov ambiguities in the ultraviolet, this is equivalent to the Curci-Ferrari-Delbourgo-Jarvis gauges. We further propose a continuum formulation in terms of a local action which is free of Gribov ambiguities and avoids the Neuberger zero problem of the standard Faddeev-Popov construction. This involves an averaging over Gribov copies with a nonuniform weight, which introduces a new gauge-fixing parameter with mass dimension two. We show that the obtained gauge-fixed action is perturbatively renormalizable in four dimensions. Remarkably, in the case of the Landau gauge, our proposal is perturbatively equivalent to the Curci-Ferrari model for the calculation of gluon and ghost correlators. Explicit one-loop calculations of the two-point correlators in the vacuum show remarkable agreement with existing lattice data in four dimensions. Away from the Landau gauge, our proposal differs from the Curci-Ferrari model and involves new dynamical fields. We discuss various perspectives of the present proposal for the study of Yang-Mills correlators.
IPN, Orsay, Bât. 100

Jeudi 30 Janvier 2014
Functional renormalization group equations and analytic continuation
Functional renormalization group equations provide a powerful approach to non-perturbative properties of quantum field theory. I will review the main ideas and key elements of the formalism and illustrate how they can be applied to non-perturbative problems on the example of bound state formation. A recent development is the analytic continuation of flow equations from Euclidean to Minkowski space-time. This allows the determination of many additional quantities, for example decay widths, viscosities or conductivities and other features of quantum field theories that are hard to access non-perturbatively, so far.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 16 Janvier 2014
Confinement from correlation functions
Leonard FISTER (Institut de Physique Théorique, CEA/Saclay)
We study confinement of static quarks via the Polyakov loop potential, which can be computed in non-perturbative functional continuum methods from low-order correlation functions. This links confinement of quarks to thermal correlation functions of pure gauge theory. In terms of the (shape of the) propagators only we derive a confinement criterion, which can even be applied to other theories. For the numerical study of the confinement-deconfinement phase transition we study the gluon and ghost propagators in detail, taking both non-trivial momentum and temperature dependence into account. We obtain good agreement with results from lattice gauge theory in both gauge groups considered, SU(2) and SU(3), for the propagators, general properties of the phase transition and for the critical temperatures.
Ecole Polytechnique, CPHT, Salle des Conférences



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