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 2005-2006

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 : Abdelhak Djouadi et Dominique Schiff.

-  Jeudi 22 juin à 16h 30

Redamy Perez-Ramos (LPTHE Jussieu) : MLLA hadronic inclusive distributions and 2-particle correlations inside one jet at small x

Distributions and 2-particle correlations inside one jet are analytically calculated at MLLA in the small x approximation by exactly solving approximate (MLLA) evolution equations. We describe the procedure, give predictions for LHC, Tevatron and LEP, compare them with previous (DLA) calculations and with existing experimental results. The domain of applicability of this approximation is studied, together with the reliability of the Local Parton Hadron Duality for inclusive hadronic variables. We will introduce a second method to calculate 2-particle correlations, which uses the steepest descent method.

-  Jeudi 15 juin à 16h 30

Achille Stocchi (LAL, Orsay - IN2P3/CNRS) : Le Triangle d’Unitarité et la violation de CP. Nouveautés et prémices du printemps 2006

Dans ce séminaire on discutera la connaissance des paramètres du Triangle d’Unitarité dans le cadre du Modèle Standard et au delà en incluant les résultats les plus récents des usines à B, du Tevatron et également les progrès théoriques les plus importants, notamment dans le domaine de la QCD sur réseau. A ce propos on montrera l’importance cruciale de la détermination précise des paramètres hadroniques. Nous montrerons ensuite que, même en laissant libre de la façon la plus générale, des contributions de nouvelle physique, la forme du Triangle d’Unitarité reste très proche de celle du Modèle Standard. Finalement, on présentera brièvement l’importance d’améliorer les déterminations actuelles afin de pouvoir poursuivre la chasse à la nouvelle physique de façon complémentaire au LHC et en étudier la structure une fois qu’elle sera découverte...

-  Jeudi 1er juin à 16h 30

Ian Balitsky (Theory group, JLAB, USA) : Quark contribution to the evolution of the color dipole

The small-$x$ deep inelastic scattering in the saturation region is governed by the non-linear evolution equation for Wilson-lines operators. In the leading logarithmic approximation it is given by the BK equation for the evolution of color dipoles. In the NLO the nonlinear equation gets contributions from quark and gluon loops. I present the result of the calculation of the quark-loop contribution to the small-x evolution of Wilson lines in the NLO and discuss the argument of coupling constant in the BK evolution.

-  Jeudi 11 mai à 16h 30

DP. Roy (TIFR Bombay and CERN) : Probing the Higgsino LSP Model in Collider & Dark Matter Experiments

After a quick introduction to SUSY I shall discuss how the Higgsino can be the lightest superparticle (LSP) in the minimal SUGRA model. An important part of the mSUGRA parameter space, consistent with the observed dark matter relic density, will be seen to correspond to a 1 TeV Higgsino LSP. I shall discuss the signature of this Higgsino LSP in the future linear collider (CLIC). I shall also discuss its signature in dark matter experiments as 1 TeV gamma ray lines coming from the galactic centre. We shall see that these signatures hold equally well for the Higgsino LSP scenario in nonuniversal SUSY models.

-  Jeudi 27 avril à 16h 30

Yuri Dokshitzer (LPTHE, Universite P. & M. Curie) : Relating Bjorken and Feynman x

DIS and e+e- annihilation processes were always known to be related. I will describe the ongoing project, with Pino Marchesini and Gavin Salam, of deepening our knowledge about this inter-connection. It aims, in particular, at better understanding and significant simplification of the results of multi-loop QCD calculations.

-  Mercredi 26 avril à 14h 30

Nicolas Chatillon (Syracuse University) : Brane worlds and supersymmetry breaking

Brane world models assume that some of the fundamental fields, typically gravity, propagate in a higher dimensional spacetime, possibly highly curved, while the other are confined on a 4d membrane. This structure allows one to propose new solutions to the naturalness problems of the Standard Model of particle physics in a geometrical framework, and may have exotic gravity signatures. I will discuss in particular warped brane worlds and their applications to supersymmetry breaking.

-  Mardi 25 avril à 14h 30

Marc Thormeier (Spht Saclay) : What is the discrete gauge symmetry of the MSSM ?

I present an analysis of discrete symmetries in supersymmetry (which are useful in order to avoid unobserved processes like proton decay) :

  • how do they arise from a high-energy theory,
  • which ones are possible ?

A particular emphasis is one a newly found discrete symmetry, so-called proton hexality. I’ll finish with a concrete example on how to embed proton hexality in an underlying model.

-  Mardi 25 avril à 11h

Jamal Jalilian-Marian (University of Washington, Seattle) : Color Glass Condensate at RHIC and LHC

After a brief introduction to the concept of Color Glass Condensate, I consider its signatures at RHIC and LHC and argue that we may have seen a glimpse of it in the forward rapidity region of RHIC and discuss further probes of the Color Glass Condensate at RHIC and LHC.

-  Lundi 24 avril à 11h

Nicolai Uraltsev (Université de St Petersbourg) : Nonperturbative effects in jet bremsstrahlung and the OPE

The Operator Product Expansion, where applicable, allows to treat nonperturbative effects based on the first principles of QCD. Its applications are most precise and informative in inclusive B decays. However, the decays of a heavy quark into a light quark have a peculiarity : radiation of a highly collinear gluon can be nonperturbative regardless of its energy. Nevertheless, the principal OPE relations for the moments of the jet mass distributions remain unaffected. Nonperturbatively the moments are affected only by soft, but not by collinear modes.

-  Jeudi 20 avril à 16h 30 au LPT - Séminaire commun LPT, IPN/X, Jussieu

Dieter Müller (Arizona State Univ.) : A view inside the proton

With the Stanford Linear Accelerator a new era in particle physics started in 1966 yielding a new view of the world of elementary particles. It was realized that the proton with a radius of about $10^-15$ m is not elementary rather it is made of so-called partons. These particles were identified as quarks, hypothetically introduced by Gell-Mann and Zweig. The dynamics of quarks is described by Quantum Chromodynamics, which allows to analyze experimental measurements in terms of partonic degrees of freedom. In the last four decades both experimental and theoretical efforts lead to an impressive quantitative improvement of measurements, e.g., providing a precise knowledge of the longitudinal distribution of partons. More recently, the new concept of generalized parton distribution has been introduced and developed. In this framework the proton can be explored from a new perspective and the unsolved problem of the proton spin decomposition and the three dimensional distribution of partons in the proton can be addressed.

-  Jeudi 30 mars à 16h 30

A.B. Kaidalov (ITEP, Moscou) : Double Diffractive Production of particles : from hadrons to Higgs Bosons

A review of hadronic production in exclusive double diffractive processes will be given. It will be shown that transverse momenta distributions of protons in these processes can serve as spin-parity analyzers of produced particles. Central exclusive diffractive Higgs production will be discussed in details and estimates of the cross sections at very high energies will be calculated in QCD. It will be emphasized that proton tagging may significantly enlarge the potential of studying Higgs sector at LHC. The situation in the MSSM model can be favorable for double diffractive production of Higgs bosons.

-  Jeudi 23 mars à 16h 30

R. Baier (Bielefeld, Germany) : Dissipative Hydrodynamics and heavy Collisions

Is hydrodynamics able to describe collective effects in relativistic heavy ion collisions ? Recent discussions of RHIC data emphasize the possibility that the produced matter shows properties of a near-perfect fluid. The eqautions for dissipative fluid dynamics are derived from kinetic theory (Israel-Stewart theory), and in model studies their applicability is discussed.

-  Jeudi 9 mars à 16h 30

B. Ermolaev (Ioffe Phys. Tech. Inst) : Description of the Polarized Deep-Inelastic Scattering at small x

The standard instrument for theoretical studying of the polarized DIS is DGLAP. Although this approach allows to explain available experimental data even in the small-x region, it is not satisfactory from the theoretical point of view. Indeed, DGLAP accounts for logarithms of Q2 to all orders inas and neglects the total resummation of lx. These contributions become very important in the small-x region and should be accounted for. Resummation of them leads to new expressions for the coefficient functions and anomalous dimensions for the structure function g1 and leads to the conclusion that DGLAP should not have worked work at x < 0.01. However it is known that DGLAP works successfully at the small-x region. The reason is that the standard DGLAP fits for the initial parton densities always include the phenomenological factors singular becoming when x®0. They mimic the total resummation of logarithms of x and ensure the rise of g1 at small x. When the lx- resummation is accounted for, these singular factors can be dropped and as a consequence, the rather complicated structure of the DGLAP fits can be considerably simplified.

-  Mardi 7 mars à 16h 30 (JOUR EXCEPTIONNEL)

E.G. Ferreiro (Univ. de Santiago de Compostela) : J/psi Suppression, Elliptic Flow and Fixed pT suppression in a Final State Interaction Model

I review the problem of the J/psi suppression pattern in a comovers-partons and hadrons- interaction model that takes into account shadowing in the initial state of the collision. I compare our results with the ones from a deconfining scenario at RHIC energies. I will also show how this final state interaction model, used to describe J/psi suppression, can also describe the fixed pT suppression of the pi0 yield at all values of pT, with a final state interaction cross-section close to one milibarn. We propose an extension of the model to the pi0 motion in the transverse plane — which introduces a dependence of the suppression on the azimuthal angle theta. Using the same value of the cross-section, we obtain values of the elliptic flow v2 close to the experimental ones, for all values of pT, including the soft pT region.

-  Jeudi 23 Février à 16h 30

Damir Becirevic (LPT, Orsay) : New Puzzles with Charm

I make a short and comprehensive review of the new experimentally detected charmed states that are hardly compatible (or completely incompatible) with predictions. This is followed by a critical account of the theoretical approaches used to tackle these puzzles. A special attention is devoted to the puzzles concerning the scalar heavy-light mesons

-  Jeudi 9 Février à 16h 30

Alexander Khodjamirian (Siegen) : Light-Cone Sum Rules and Exclusive Decays of B-Mesons

Exclusive decays of B-mesons represent a complicated mixture of short-distance electroweak and quark-gluon interactions with long-distance hadronic effects. Accurate QCD estimates of the relevant hadronic amplitudes are needed in order to determine the fundamental quark-flavour mixing parameters from the measurements of these decays. The method of QCD light-cone sum rules provides a possibility to calculate hadronic amplitudes for B decays into energetic light mesons and /or photons. I will introduce the basic elements of the method : the correlation function, operator-product expansion near the light-cone and hadronic dispersion relation. Applications of light-cone sum rules to charmless B decays will be discussed, as well as the recent development of the method : the use of correlation functions with B-meson distribution amplitudes.

-  Jeudi 26 Janvier à 16h 30

G. Salam (LPTHE Jussieu) : Résumé de l’atelier de la Fédération de Recherche Interactions Fondamentales sur les corrections non perturbatives dans la physique des jets (12-14 janvier 2006 à Jussieu).

-  Jeudi 19 Janvier à 16h 30

F. Richard (LAL Orsay) : Comment construire ILC ?

L’effort mondial mis en place en vue d’un collisionneur linéaire e+e- au TeV sera décrit. En particulier seront évoqués les choix en cours et l’échéance d’un projet chiffré à la fin de 2006.

-  Mardi 10 Janvier à 16h 30 (JOUR EXCEPTIONNEL)

E. Gardi (Cambridge) : Inclusive Decay Spectra by Dressed Gluon Exponentiation

I present a new approach to inclusive B-decay spectra that relies on the resummed calculation of the on-shell heavy-quark decay and involves non-perturbative contributions associated with the meson structure only through power corrections. This stands in contrast with the standard approach, where conventional Sudakov-resummed perturbation theory describing the decay of an on-shell heavy quark yields a divergent expansion. Detailed understanding of this divergence in terms of infrared renormalons has paved the way for making quantitative predictions. This way the moments of the \barB\to X(s) gamma spectrum with experimentally-accessible cuts - which had been so far just parametrized - were computed by perturbative means and found to be in good agreement with new data from BaBar. Very recently the method has been applied to the triple differential spectrum in charmless semileptonic decays \barB -> X_u l \bar\nu, which is considered the best observable for determining |V_ub|.

-  Jeudi 15 Décembre à 16h 30

N. Borghini (CERN) : Indices d’une thermalisation incomplète dans les données de RHIC

Les distributions en impulsion transverse mesurées au RHIC en collisions or-or présentent une importante anisotropie, qui est souvent mise en avant comme preuve décisive de la formation d’un plasma de quarks et de gluons interagissant fortement. L’argument sous-jacent est que la valeur mesurée de l’anisotropie est aussi grande que celle prédite par les modèles hydrodynamiques, qui font l’hypothèse d’une thermalisation complète et rapide. Je tenterai de montrer que cette argumentation n’est pas réellement justifiée, et qu’un examen détaillé des divers résultats expérimentaux tend plutôt à indiquer que le système créé dans les collisions est hors d’équilibre.

-  Mardi 13 Décembre à 16h 30 (JOUR EXCEPTIONNEL)

D. Dietrich (Niels Bohr Institute) : Strong electroweak symmetry breaking

In the first part of the talk, an introduction into electroweak symmetry breaking by new strong interactions (technicolour) is given. Thereafter, in the second part, a special technicolour model with techniquarks in a higher dimensional representation of the gauge group is discussed. It is found to be quasi-conformal, consistent with electroweak precision data, and to feature a light composite Higgs.

-  Jeudi 8 Décembre à 16h 30 (Séminaire doctorants)

C. Papineau (doctorante LPT, Orsay et CPhT, X) : Flowing from 6 to 4 dimensions

Nous allons présenter un modèle de champ scalaire en codimension 2 avec potentiel localisé sur la brane. La régularisation de l’épaisseur de la brane conduit à des équations de groupe de renormalisation pour les constantes de couplage localisées. En particulier, un terme de masse tachyonique croît dans l’infra-rouge jusqu’à atteindre un pôle de Landau. Une valeur particulière du couplage nu apparaît alors naturellement dans la théorie : celle pour laquelle le couplage explose exactement à l’échelle de compactification 1/R. Au-delà de cette valeur critique, un mécanisme de Higgs peut être réalisé.

R. Zegers (doctorant LPT, Orsay) : Higher codimension branes and Lovelock gravity

After introducing Lovelock gravity as the natural extension of General Relativity to higher dimensional space-times, I will discuss recent results concerning self-gravitating branes with more than one extra dimension. In particular, I will present the matching conditions (which generalize to higher codimensions the Israel junction conditions holding for self-gravitating hypersurfaces) in the case of a 3-brane with two extra dimensions. Finally, I will talk about the arbitrary even codimension cases, showing how induced gravity terms can appear quite naturally.

-  Mardi 6 Décembre à 16h 30 (JOUR EXCEPTIONNEL)

R. Ferrari (Université de Milan) : Taming the infinities in the nonlinear sigma model (D=4)

A procedure is proposed for the symmetric renormalization of the nonlinear sigma model in D=4. Examples at one loop and two loops are provided.

-  Jeudi 1er Décembre à 16h 30

C. Grojean (SPhT Saclay) : Dimensions supplémentaires et brisure de symétrie électrofaible

Une impulsion le long d’une dimension supplémentaire est équivalente à une masse à quatre dimensions. Ceci ouvre la perspective de générer les masses des particules élémentaires en imposant des conditions aux bords aux extrémités d’une cinquième dimension compacte. L’amplitude de diffusion des bosons de jauge massifs est unitarisée par l’échange de résonances vectorielles et non plus par l’intermédiaire d’un champ scalaire à basse énergie. La construction d’un modèle réaliste n’est cependant pas sans obstacle : corrections aux observables électrofaibles, masses des fermions, non-universalité des couplages. Je montrerai comment ces obstacles peuvent être contournés pour obtenir un modèle réaliste de brisure de symétrie électrofaible sans boson de Higgs.

-  Jeudi 24 Novembre à 16h 30

S. Vempati (CPhT Polytechnique) : Classical running of neutrino masses from six dimensions

We discuss a six dimensional mass generation for the neutrinos. Active neutrinos live on a three-brane and interact via a brane localized mass term with a bulk six-dimensional standard model singlet (sterile) Weyl fermion, the two dimensions being transverse to the three-brane. We derive the physical neutrino mass spectrum and show that the active neutrino mass and Kaluza-Klein masses have a logarithmic cutoff divergence related to the zero-size limit of the three-brane in the transverse space. This translates into a renormalisation group running of the neutrino masses above the Kaluza-Klein compactification scale coming from classical effects, without any new non-singlet particles in the spectrum. For compact radii in the eV—MeV range, relevant for neutrino physics, this scenario predicts running neutrino masses which could affect, in particular, neutrinoless double beta decay experiments.

-  Jeudi 17 Novembre à 16h 30 (Séminaire commun LPT/LAL)

I. Bigi (Notre-Dame) : Lessons on QCD’s nonperturbative dynamics — news from the heavy quark expansion

Heavy Quark Symmetry coupled with $1/m_Q$ expansions allows to treat QCD’s nonperturbative dynamics with an accuracy that would have seemed unattainable even a few years ago. I demonstrate the present status with particular emphasis on semileptonic and radiative $B$ decays. These findings not only improve our predictions on CP asymmetries and other observables in $B$ decays, but teach us also important lessons on QCD and create accurate validation challenges for lattice QCD.

-  Jeudi 3 Novembre à 16h (Séminaire doctorants)

B. Blossier (doctorant LPT, Orsay) : Physique du B et QCD sur réseau

Après une brève introduction de la théorie effective des quarks lourds et un court rappel sur le calcul non perturbatif des fonctions de corrélations de la théorie de jauge SU3 discrétisée, on présentera quelques résultats concernant la spectroscopie du méson B et le facteur de forme hadronique B_B_s paramétrant le mélange B_s-\barB_s.

Y. Mehtar-Tani (doctorant LPT, Orsay) : La saturation (le "Color Glass Condensate") a-t-elle été observée dans les collisions Deuteron-Or à RHIC ?

Dans le cadre du modèle de saturation, nous examinons d’une manière systématique les prédictions pour les spectres de particules à grand moment transverse mesurés dans les collisions deuteron-or à RHIC. Il apparaît que le modèle de saturation n’est pas nécessairement en accord avec les données à mi-rapidité. Le régime de grande rapidité permet de déterminer la forme de la distribution non intégrée du gluon dans le noyau en testant différentes paramétrisations inspirées par l’équation de Balitsky-Kovchegov. Un accord avec les données est obtenu avec une échelle caractéristique en accord avec la saturation.

-  Jeudi 27 Octobre à 16 h 30

E. Sokatchev (LAPTH Annecy) : A method for computing anomalous dimensions in conformal field theory

We describe a method for computing the anomalous dimension of certain classes of composite operators in conformal field theories such as N=4 super-Yang-Mills. These operators are close to the unitarity bound of the continuous series of representations of the (super)conformal algebra. As a consequence, in the free theory they satisfy generalised conservation conditions which becomes anomalous in the presence of interaction. Exploiting this anomaly in the quantum theory, we are able to determine the anomalous dimension of the operators at n loops by performing perturbative calculations at n-1 loops. We present several examples relevant to the current search for integrability in conformal field theory and to the AdS/CFT correspondence.

-  Jeudi 20 Octobre à 16 h 30

M. Quiros (Instituto de Fisica de Altas Energias Barcelone) : Electroweak Baryogenesis and Dark Matter

Prospects for low energy theories where electroweak baryogenesis (EWBG) can be accomplished with realistic Dark Matter (DM) candidates will be reviewed and their experimental signatures discussed.