Laboratoire de Physique
Theorique d'Orsay

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Univ. Paris-Sud
Université Paris-Saclay
91405 Orsay Cedex
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Agenda > Séminaires / Seminars > Particules Dernier ajout : jeudi 2 octobre 2014.

Séminaires Particules 2008-2009

Les séminaires se tiennent en salle 114 au 1er étage du bâtiment 210, généralement le jeudi à 16h30, sauf indication contraire.

Contacts pour les séminaires : Yann Mambrini et Mario Gravina.

-  Vendredi 10 Juillet à 14h30, salle 114, Bâtiment 210

James Barnard (IPPP - Durham) : Chiral SUSY Breaking

Recent work by Intriligator, Seiberg and Shih has provided a simple method for SUSY breaking in which the SUSY breaking vacuum is not the true groundstate. We ask whether their method can be extended to stable, rather than metastable, SUSY breaking and show that models with some degree of chirality are good candidates.

ATTENTION : changement d’heure du séminaire !

-  Jeudi 9 Juillet à 17h, salle 114, Bâtiment 210

Prof. Dr. Kei-Ichi Kondo (Univ. of Tokyo/Chiba Univ., Japan) : Gribov-Zwanziger horizon condition, ghost and gluon propagators and Kugo-Ojima confinement criterion

We discuss how the restriction to the Gribov region restricts the possible value for the Kugo-Ojima parameter for color confinement and ghost and gluon dressing functions. Within the Gribov-Zwanziger theory, (1) we prove that the Kugo-Ojima color confinement criterion u(0)=-1 is not satisfied in a naive form, rather u(0)=-2/3, (2) we prove that the ghost propagator behaves like free in the deep infrared regime, (3) we give a general field theoretical argument supporting that the gluon propagator is non-vanishing at low momenta, in harmony with recent lattice results and decoupling solution of the Schwinger-Dyson equation. We also discuss a nilpotent BRST like symmetry in the Gribov-Zwanziger theory.

-  Jeudi 18 Juin à 16h30, salle 114, Bâtiment 210

Al Mueller : Ads/CFT and Heavy Ion Collisions

Essential features of data from heavy ion collisions are used to motivate the interest in viewing the quark gluon plasma as a strongly interacting system. The successes, and difficulties, of such a picture are reviewed, and possible tests of the strongly coupled picture are proposed using lattice gauge theory.

Séminaire informel

-  Mardi 2 Juin à 16h30, salle 114, Bâtiment 210

Mathieu Segond (Institut fur theoretische Physik Leipzig Universität) : Resummation of small x contributions in Catani-Hautmann k_T factorisation

-  Jeudi 28 Mai à 16h30, salle 114, Bâtiment 210

Dimitri Colferai (Florence) : A matrix formulation for small-x parton densities

Precise predictions for hadronic parton densities at small values of Bjorken x are important for phenomenology at LHC and high-energy colliders. More generally, high-energy QCD dynamics is still an open theoretical problem in many aspects. The most promising approaches for a quantitative description of partons densities are based on resummation methods which generalize DGLAP and BFKL equations.

After reviewing the basic framework of QCD evolution equations, I present a recent kt-factorized matrix formulation in which quarks and gluons are treated on the same ground and exact NLO and NLLx calculations are incorporated. I then show results for the resummed eigenvalue functions and the splitting function matrix which show an overall gentle matching of resummation effects to fixed order quantities. The shallow dip occurring in previous treatments of P_gg is confirmed, and found in P_gq also.

-  Jeudi 14 Mai à 16h30, salle 114, Bâtiment 210

Hirotaka Sugawara (KEK) : Photon-photon collider Higgs factory as a precusor to ILC

Low energy photon-photon collider is proposed as a precursor to ILC when the mass of Higgs is as low as the suggestion of theoretical models. The theories include Standard model, MSSM, NMSSM etc.. The cost of the machine will be less than half of the currently conceived ILC. The necessary laser technology is still marginal. We need, therefore, to push it in a world-wide collaboration.

ATTENTION : Séminaire annulé

-  Mercredi 15 Avril à 11h, salle 114, Bâtiment 210

Diego Guadagnoli (TU Munich) : Physics beyond the Standard Model in the light of data

Established experimental data from electroweak and flavor observables have not yet uncovered significant discrepancies with respect to the Standard Model (SM) expectations. Yet, they do give us important qualitative information on the form allowed to SM extensions, that should guide us in future searches. One such piece of information is that the physics generating and stabilizing the electroweak scale may have litttle to do with the physics deciding the observed form of the SM Yukawa couplings — i.e. that Fermi-scale new physics is likely to be minimally flavour violating (MFV). I will review —with specific models as well as in a model-independent fashion— various realizations of the hypthesis of MFV, all inspired by agreement with collider data. For each of them I will show that clear signatures at future colliders do exist, and that they may be discriminating ones.

-  Jeudi 9 Avril à 16h30, salle 114, Bâtiment 210

Lydia Pieri (IAP, Paris) : Dark Matter structures and their fingerprints on the indirect detection of gamma-rays

According to the Lambda Cold Dark Matter paradigm, the dark matter halos today were formed through successive mergers of smaller mass halos. Part of these merging halos survived without being disrupted in the merging event, and form the structure of galaxies and clusters today. We implement different models describing the properties of a population of substructures inside the Milky Way as well as at cosmological distances, based on recent N-body simulation. We also study the properties of the observed satellites of our Milky Way such as the dwarf galaxies, based on an hybrid modeling derived from astronomical data and numerical simulations. We present our predictions for the indirect detection of dark matter through the observation of gamma-rays produced in the annihilation process, in the case when the dark matter particle is the lightest supersymmetric particle. We study both the detectability of subhalos and their angular signatures. We also include the effect of the Sommerfeld enhancement of the annihilation cross section in the "slow", small mass galactic and extragalactic halos. We make predictions for both the Fermi telescope and for the Air Cherenkov Telescopes and show comparisons with the already available data.

-  Mardi 7 Avril à 16h30, salle 114, Bâtiment 210

Nicolas Garron (Universidad Autonoma de Madrid) : Non perturbative HQET at the 1/m order

A natural framework to study B-mesons is provided by heavy quark effective theory (HQET). I will remind the audience how HQET can be implemented non perturbatively on the lattice, trough a finite volume matching to QCD. In particular this can be done beyond the static approximation (including the 1/m effects). I will then show how this method can be applied to the computations of physical quantities like the b-quark mass, the heavy-light decay constant F_B_s, or the mass splitting mB*-mB. I will present some results in the quenched approximation, and I will report on the situation of our simulations with nf=2 flavors of dynamical quarks.

ATTENTION : Suite à la journée du 2 avril ’LABO MORT’, le séminaire de Lydia Pieri est reporté au 9 avril

-  Mardi 31 Mars à 16h30, salle 114, Bâtiment 210

Jean-Philippe Lansberg (SLAC National Accelerator Laboratory, Stanford U.) : Perspectives de production de J/\psi et \Upsilon au LHC

Je discuterai les différents mécanismes potentiellement impliqués dans la production de quarkonium lourd dans les collisions pp à la lumière des récents calculs des corrections QCD au NLO et NNLO pour les transitions singulet et octet de couleur. Je discuterai également les contributions venant de la coupure dans la voie s dans les transitions singulet de couleurs. A chaque fois, je confronterai les résultats obtenus avec les mesures expérimentales venant du Tevatron et de RHIC et présenterai les prédictions pour le LHC. Par ailleurs, je discuterai l’utilité et la faisabilité au LHC de mesures de nouvelles observables ayant pour but de mieux discriminer les différents mécanismes de production possibles.

ATTENTION : changement d’heure

-  Vendredi 27 Mars à 11h, salle 114, Bâtiment 210

Mauro Papinutto (LPSC Grenoble) : Non-perturbative Heavy Quark Effective Theory and B-physics

We briefly review the strategy to perform non-perturbative heavy quark effective theory computations and we consider some phenomenological applications relevant for B-physics.

-  Jeudi 26 Mars à 16h30, salle 114, Bâtiment 210

Carla Biggio : Neutrino NSI & non-unitarity : which constraints ?

In this seminar we describe neutrino non-standard interactions (NSI) focusing on the way they can be generated from new physics beyond the Standard Model. In particular we analize their connection to the non-unitarity of the leptonic mixing matrix that arises, for example, in type-I and type-III seesaw models. We then review present constraints on seesaw parameters, that translate into bounds for this type of NSI, and other existing constraints on generic NSI. Finally we focus on loop bounds showing that, contrary to what previously claimed in the literature, they are model-dependent.

ATTENTION : Séminaire reporté pour cause de grève le 19 mars

-  Vendredi 20 Mars à 11h, salle 114, Bâtiment 210

Cyrille Marquet : Heavy-quark energy loss and thermalization in a strongly coupled plasma

I will discuss the problem of heavy-quark energy loss and pT-broadening in a strongly-coupled N=4 SYM plasma. Using the AdS/CFT correspondence, the strong coupling problem has been reduced by Herzog et al. to solving the classical dynamics of a trailing string in AdS space. I will propose an interpretation of the results in terms of the saturation momentum Qs at strong coupling. Interestingly, the energy loss is parametrically the same than for a weakly-coupled QCD plasma, when expressed in terms of Qs while pT-broadening is radiation dominated in SYM theory and multiple scattering dominated in pQCD. Finally, I will consider the situation where the heavy quark is slowing down in the medium and estimate the thermalization time.

-  Mardi 17 Mars à 16h30, salle 114, Bâtiment 210

Daniel Hernandez (UAM/CSIC) : Gauge Invariant Non Standard Neutrino Interactions

I tackle non standard neutrino interactions (NSIs) from the perspective of effective field theories. A general overview of the theoretical aspects of the subject will be given along with current experimental bounds. Our recent work on the constraints implied by the Standard Model gauge symmetries on possible NSI models is then discussed and some examples are provided. Finally I briefly mention future experimental and theoretical prospects.

-  Jeudi 12 Mars à 16h30, ATTENTION Amphi 2, Bâtiment 210

Adam Falkowki (CERN) : Unhiggs

The idea of unparticle Higgs (Unhiggs) is to replace an elementary Higgs field with a more complicated non-local object with a continuous mass spectrum. This leads to a fully consistent scenario for electroweak symmetry breaking with distinct physical properties. I will discuss the construction and some phenomenological properties of the Unhiggs.

-  Jeudi 5 Mars à 16h30, salle 114, Bâtiment 210

Florian Schwennsen (LPT Orsay, CPhT X) : Hard Pomeron-Odderon interference in production of \pi^+\pi^- pairs in \gamma\gamma collisions at the LHC

The C-odd exchange in hadronic reactions with rapidity gaps is described by the so-called Odderon but its experimental evidence is still weak since the C-even exchange (the Pomeron) dominates. In this talk I will present a possibility to observe the Odderon by interference effects with the Pomeron in charge asymmetries in the production of two pion pairs. After the discussion of the theoretical framework for our calculation based on high energy factorization and the necessary phenomenological input, the calculation of the charge asymmetry and its result will be presented. I will conclude with a discussion of experimental rates to be expected in ultra-peripheral collisions at the LHC.

Séminaire annulé et reporté au 3 mars à 16h30, AMPHI 1, Bâtiment 210

-  Jeudi 26 Février à 16h30, salle 114, Bâtiment 210

Michele Trapletti (LPT, Orsay) : Strings & GUTS

We present the state of the art of model building in the context of heterotic string compactifications on orbifolds. We show the details of a specific model having the MSSM chiral spectrum, with SU(5) unification at an energy scale set by the compactification scale. We also show how to link orbifold constructions to smooth manifold compactifications, and why this is crucial in the study of the phenomenological properties of a model.

-  Jeudi 12 Février à 16h30, salle 114, Bâtiment 210

Carlos Yaguna (Universidad Autonoma de Madrid) : The singlet scalar model of dark matter

A simple extension of the Standard Model that can explain the dark matter is the addition of a singlet scalar that is odd under a new Z_2 symmetry. In this talk I will discuss the main features of this model. First, an updated analysis of its parameter space will be presented. By enforcing the relic density constraint from the very beginning, the viable parameter space gets reduced to just two variables : the singlet mass and the higgs mass. Then we calculate the direct detection rate and show that constraints from current experiments require a singlet mass larger than 50 GeV. Finally, we compute the gamma ray signal and show that it lies, over most of the viable region, within the sensitivity of GLAST.

-  Mercredi 4 Février à 16h30, salle 114, Bâtiment 210

Svjetlana Fajfer (Institut J.Stefan, Ljubljana, Slovenia) : Signatures of Nnew Physics models in top FCNC decay t —> c(u) l+ l-

Many scenarios of new physics allow top quark FCNC decays. Using most general model independent Lagrangian of new physics we investigate possible experimental signals of new physics in t —> c(u) l+ l- FCNC top decays. We find that measurement of two possible asymmetries might give very important and interesting information of NP. It is particularly interesting to use them to discriminate among variety on NP scenarios. We consider also contributions of the interference between scalar and vector mediators of the FCNC decays.

-  Jeudi 22 Janvier à 16h30, salle 114, Bâtiment 210

Samuel Wallon (LPT, Orsay) : QCD factorization beyond leading twist in exclusive processes : \rho_{T} -meson production

Exclusive processes in hard electroproduction with high \gamma^* p center of mass energy is one of the best place for understanding QCD in the perturbative Regge limit. The HERA experiment recently provided precise data for \rho electroproduction, including all spin density matrix elements. From QCD, it is expected that such a process should factorize between a hard (calculable) coefficient function, and hadronic (P and \rho) matrix elements. Such a factorization is up to now only proven for a longitudinaly polarized \rho. Within the k_t-factorization approach (valid at large s_{\gamma^* p}), we evaluate the impact factor of the transition \gamma^{*} \to \rho_{T} taking into account the twist 3 contributions. We show that a gauge invariant expression is obtained with the help of QCD equations of motion.

More generally, relying on these equations and on the gauge invariance of the factorized amplitude, the involved non-perturbative Distribution Amplitudes can be reduced to a minimal set. This opens the way to a consistent treatment of factorization for exclusive processes with a transversally polarized vector meson.


-  Les lundis 19 et 26 janvier, 2 et 9 février à 11h

Grégory Korchemsky : Wilson loop/scattering amplitudes duality in Yang-Mills theories

Série de cours introductifs (une heure chacun)

-  Jeudi 8 Janvier à 16h30, salle 114, Bâtiment 210

Florian Bonnet (LPT, Orsay) : Low energy effects of neutrino masses

We consider the three different realisations of the seesaw mechanism (whether the neutrino masses arise from the exchange of fermionic singlets, fermionic triplets or scalar triplets) and study their phenomenolgy. We use the effective theory approach. While all models lead to the same dimension five operator, which conserves lepton number, the dimension six operators induced at low energy conserve lepton number and differ depending on the high energy model. We derive these operators for each seesaw realisation. We will see that the effects resulting from the dimension six operators can be large compare to the one resulting from the dimension five operator (neutrino masses) if their coefficients are decoupled along a certain pattern which is common to all models. Finally we discuss some peculiarities of the phenomenology of the fermionic triplet seesaw.

-  Mardi 6 Janvier à 16h30, salle 114, Bâtiment 210

Giovanni Chirilli (Jefferson Lab) : NLO evolution of color dipoles in QCD and {\cal N}=4 SYM

The small-x_B deep inelastic scattering in the saturation region is governed by the non-linear evolution of Wilson-line operators. In the leading logarithmic approximation it is given by the BK equation for the evolution of color dipoles. I discuss recent calculation of the next-to-leading order evolution of color dipoles in QCD and {\cal N}=4 SYM. I will also present recent result of the conformal NLO BFKL kernel obtained from the requirement of Mobius invariance of {\cal N}=4 SYM amplitudes in the Regge limit.

-  Jeudi 18 Décembre à 16h30, salle 114, Bâtiment 210

Alberto Romagnoni (LPT, Orsay) : Gauge versus Gravity Mediation

Among different mechanisms for supersymmetry breaking mediation to the Standard Model, gauge mediation (GMSB) is an interesting possibility due to its model-independent experimental signatures and to the absence of the flavour-changing neutral currents problem. I will show that in stringy inspired setups, in the presence of an anomalous U(1) gauge group, it is actually difficult to achieve a scenario of pure gauge mediation, once the stabilisation of the Kahler moduli is required. Indeed, in the largest region of the parameter space, the phenomenology at low energy is the result of the interplay between three different sources : standard GMSB, non-standard GMSB and gravity. I will discuss the different possibilities that this kind of model gives for the soft masses spectrum and the dark matter candidates.

-   Jeudi 27 Novembre à 16h30, salle 114

Séminaire conjoint de cosmologie et de physique des particules

Fernando Quevedo (University of Cambridge) : The LARGE volume string scenario and closed moduli inflation

An overview will be presented of the LARGE volume scenario of moduli stabilisation in IIB string compactifications. Potential implications to achieve cosmological inflation from the geometric (closed string) moduli in this scenario will be reviewed.

-  Mercredi 26 Novembre à 14h30

D. I. Kazakov (Bogoliubov Lab. of Theoretical Physics, JINR, Dubna, Russia) : Conformal Invariance in Deformed N=4 SYM Theory

We discuss a class of deformations of N=4 SYM theory and look for conditions under which the theory would be conformally invariant and finite. Applying the algorithm of perturbative adjustments of the couplings we construct the family of theories which are conformal up to 3 loops in the non-planar case and up to 4 loops in the planar one. We found particular solutions in the planar case when the conformal condition seems to be exhausted in the one loop order and present the arguments that these solutions might be valid in any loop order.

-  Jeudi 20 Novembre à 16h30

Pablo Roig (IFIC, Univ. Valencia) : Monte Carlo Generators for Low-Energy Physics : Tau decays

Any measurement of a physical quantity in particle physics is a complicated process involving a Monte Carlo simulation of the detector response to a physical signal. With the accuracy sought nowadays, the relation between a theory and a measurement can be established at high precision only by providing the experimental groups with Monte Carlo event generators simulating the measured processes.

I will give a short overview on the work undertaken by the Working Group on Radiative Corrections and MC Generators for Low Energies, that covers fours main areas : R measurement with direct scan, R measurement with Initial State Radiation, Luminosiy and Tau.

After that, I will focus on our contributions to the hadronic matrix elements for tau decays, that can improve this part of TAUOLA. Along the same lines, we plan to tackle the study of hadronic final states in e+e- annihilation, which may be of use for PHOKHARA.

-  Jeudi 6 Novembre à 16h30

Ana Teixeira (LPT Orsay) : Why the NMSSM ? Low-energy phenomenology and possible signatures at the LHC

We briefly motivate the next-to-minimal supersymmetric extension of the Standard Model (NMSSM). In the NMSSM, a richer Higgs and neutralino spectrum allow for many interesting phenomena that are are not present in the minimal supersymmetric extension of the Standard Model.

We propose a supergravity inspired version of the NMSSM. After considering the different constraints on the parameter space (such as LEP and dark matter constraints), we discuss the phenomenology of the model, the implications regarding dark matter detection, and the prospects for the LHC (sparticle searches and displaced vertices).

-  Mardi 4 Novembre à 14h30

J.F. Kamenik (Laboratori Nazionali di Frascati) : Signatures of Minimal Flavour Violation at Large Tan Beta

We review and update constraints on dimension-six effective operators in models respecting the MFV hypothesis, both in the one-Higgs doublet case and in the two-Higgs doublet scenario with large tan beta. The constraints are derived mainly from FCNC and helicity suppressed observables measured at the kaon and B meson factories. The implications of these bounds in view of improved measurements in exclusive and inclusive observables in b -> s and s -> d transitions are discussed.

-  Jeudi 23 Octobre à 16h30

Dmitri Melikhov (IINP, Moscow State University & HEPHY, Vienna) : Accuracy of bound-state parameters obtained from dispersive sum rules

Making use of exactly solvable quantum-mechanical model, the possibility to control the accuracy of hadron parameters extracted from different versions of sum rules is studied. I discuss three examples : (i) decay constant from two-point vacuum correlator, (ii) form factor from three-point vacuum correlator, and (iii) form factor from two-point hadron-to-vacuum correlator.

-  Jeudi 9 Octobre à 16h30

Maxim Chernodub (LMPT Tours) : Electric-magnetic asymmetry of the dimension-2 condensate and the phases of Yang-Mills theory

We study the finite-temperature behavior of the A^2 condensate in the Landau gauge of SU(2) Yang-Mills theory on the lattice in a wide range of temperatures. The asymmetry between the electric (temporal) and magnetic (spatial) components of this unconventional dimension-2 condensate is a convenient ultraviolet-finite quantity which possesses, as we demonstrate, unexpected properties. The low-temperature behavior of the condensate asymmetry suggests that the mass of the lowest thermal excitation in the condensate is unexpectedly low, about 200 MeV, which is much smaller than the glueball mass. The asymmetry is peaked at the phase transition being a monotonically decreasing function in the deconfinement phase. A symmetric point is reached in the deconfinement phase at a temperature approximately equal twice the critical temperature. The behavior of the electric-magnetic asymmetry of the condensate separates the phase diagram of Yang-Mills theory into three regions. We suggest that these regions are associated with the condensed, liquid and gaseous states of the confining gluonic objects, the Abelian monopoles.


-  Mercredis 8, 15 et 22 Octobre et vendredis 10, 17 et 24 Octobre de 15h à 16h, Amphi I, Bat 210

Mikhail SHIFMAN (University of Minnesota (USA) et Laboratoire de Physique Théorique (Orsay)) : Polyakov’s Confinement in Three and Four Dimensions

This lecture course begins with a pedagogical presentation of the Polyakov Abelian confinement in three dimensions. The crucial theoretical ingredients are dualization and instantons-monopoles whose contribution is responsible for dual photon mass generation. Dual photon in this theory is a (pseudo)scalar phase field \varphi which lives on S_1. Confining strings are equivalent to domain lines, with vortices of the \varphi field at the endpoints. In the original formulation (before dualization) the vortices are electric charges. Confinement of electric charges is explicit and occurs at weak coupling which makes it fully calculable. Introduction of massless quarks suppresses Polyakov’s confinement.

In the second part of the lecture course I discuss recent developments which may have serious implications for four-dimensional QCD-like theories. If we formulate QCD-like theories on R_3\times S_1 with a small radius of the compactified dimension r(S_1) and stabilize the center symmetry, then we deal with a weakly coupled gauge theory, with SU(N)_{\rm gauge} broken down to U(1)^{N-1}. The low-energy theory confines à la Polyakov, even in spite of massless quarks that can be present in the low-energy spectrum. Moreover, with one quark flavor, a discrete chiral symmetry of the theory is spontaneously broken. One can argue that the transition to large r(S_1) (i.e. to four-dimensional one flavor QCD) is smooth. If so, one can treat qualitatively, and, perhaps, even semi-quantitatively, basic regularities of four-dimensional QCD.

With two or more flavors we get a remarkable example of of a dynamical gauge theory with confinement but without chiral symmetry breaking.

-  Mardi 16 Septembre à 16h30

Prof. Rahul Sinha (Université de New Delhi) : Clean signals of New Physics in B decays

We review some of the signals of New Physics in B decays. We show that some of the signals are free from hadronic uncertainties to a very good approximation. Such signals would play a critical role in the hunt for New Physics at future super flavor factories.