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Séminaires 2013 (physique de particules)

Jeudi 12 Décembre 2013
Towards a quantum treatment of leptogenesis
Mathias GARNY (DESY, Hambourg)
Two of the most profound open questions within particle physics arise from cosmological observations : the origin of the baryon asymmetry of the Universe and the identity of dark matter. Both observations can be accommodated by extending the Standard Model of particle physics. In this talk, I discuss leptogenesis which is an attractive mechanism for explaining the baryon asymmetry, and provides interesting links between cosmology and neutrino physics. However, the predictions depend on an accurate calculation of the generated asymmetry in terms of the fundamental masses and couplings. I present progress towards this goal based on a first-principle approach, that is applicable to the phenomenologically interesting case of resonant leptogenesis.
Ecole Polytechnique, CPHT, Salle des Conférences

Mardi 3 Décembre 2013
The proton spin decomposition : some news about the controversy>
Cédric LORCÉ (IFPA, Universit\é de Liège)
The controversy about the proton spin decomposition has recently been revived by a new approach proposed by Chen et al. based on the separation of physical degrees of freedom from pure-gauge degrees of freedom. We propose a summary of the present situation and focus on the issues of gauge invariance, path dependence and measurability.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 21 Novembre 2013
Vortex states of charged particles : recent progress and physics opportunities
Igor IVANOV (IFPA, Université de Liège )
Vortex states refer to freely propagating wavepackets with helical wavefronts, which carry a non-zero orbital angular momentum with respect to the propagation direction. In optics, photon vortex states have been known and routinely used since decades. Recently, several groups experimentally created vortex electrons and started to explore the physics opportunities they offer. Producing vortex states of protons and other particles should also be feasible. I will briefly review the recent experimental progress, and then give examples of physical problems, both within particle physics and beyond, where the vortex states of charged particles can have an impact.
Ce séminaire aura lieu à 11h, Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 7 Novembre 2013
Acoustic spectrum of vortex lattice in magnetic-field-induced superconducting phase of QCD
Maxim N. CHERNODUB (CNRS, LMPT, Université François Rabelais, Tours)
In the background of a sufficiently strong magnetic field the QCD x QED vacuum was suggested to become an ideal electric conductor (highly anisotropic superconductor) due to an interplay between the strong and electromagnetic forces. The superconducting ground state resembles the Abrikosov lattice state in an ordinary type—II superconductor : it is an inhomogeneous structure made of a (charged vector) quark-antiquark condensate which is pierced by vortices parallel to the magnetic field axis. In this talk the acoustic (phonon) vibrational modes of the vortex lattice at zero temperature are discussed. We show that in the infrared limit the transverse acoustic vibrations of the vortex lattice possess a quadratic dispersion relation corresponding to type—II Nambu— Goldstone modes, while the longitudinal modes are always linear in momentum similarly to type—I Nambu—Goldstone bosons. The phonon fluctuations may melt the vortex lattice into a vortex liquid. [pdf].
IPN, Orsay, Bât. 100, Salle A201

Mardi 29 Octobre 2013
Effective field theories for non-relativistic particles in a thermal bath
Antonio VAIRO (Technische Universität München, Garching)
We discuss recently proposed effective field theories for non-relativistic particles in a thermal bath on the examples of (a) heavy Majorana neutrinos colliding with a hot plasma of Standard Model particles in the early universe and (b) quarkonia produced in heavy-ion collisions dissociating in a quark-gluon plasma [pdf].
IPN, Orsay, Bât. 100, Salle A201

Jeudi 17 Octobre 2013
Generalized parton distributions of the photon with helicity flip
Sreeraj NAIR (Department of Physics, Indian Institute of Technology, Bombay)
We present a calculation of the generalized parton distributions (GPDs) of the photon when the helicity of the initial photon is different from that of the final photon. We calculate the GPDs using overlaps of photon light-front wave functions (LFWFs) at leading order in electromagnetic coupling α and at zeroth order in the strong coupling αs, when the momentum transfer is purely in the transverse direction. These involve a contribution of orbital angular momentum of two units in the LFWFs. We express these GPDs in the impact parameter space.
Ecole Polytechnique, CPHT

Jeudi 3 Octobre 2013
New aspects of QCD dynamics at high parton density : Jet evolution in the quark-gluon-plasma
Yacine MEHTAR-TANI (IPhT, CEA/Saclay)
QCD jets are being extensively investigated in heavy ion collisions at the LHC : firstly, as a probe of the dense partonic medium (often referred to as the quark-gluon-plasma (QGP)) known to form in such high energy collisions ; secondly, as they provide us with a new laboratory for the study of QCD dynamics in a regime of high parton density. In this talk, we show that in the presence of a dense medium the large angle component of a high energy jet can be described as a classical branching process thanks to the large separation between the time scale of the branching of a parton into two and that involved in the classical propagation of a parton in the medium, which is typically of the order of the length L of the medium. In this kinematic limit, the branching process is quasi-instantaneous and can occur anywhere along the medium with constant rate. A master equation for the generating functional of multi-parton distributions is derived.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 26 Septembre 2013
Weak gauge boson scattering at the era of post-Higgs boson discovery
TZU-CHIANG YUAN (Institute of Physics, Academia Sinica, Taiwan)
More evidences have now been collected at the Large Hadron Collider suggesting the new 125 - 126 GeV boson is likely the long sought Higgs boson in the Standard Model. One pressing question continued being asked by theorists is whether this Higgs boson is a lone player responsible for the full electroweak symmetry breaking. Current data still allow room for additional Higgs bosons or some other UV physics that may play a partial role in electroweak symmetry breaking as well. We use the WW scattering to investigate such a possibility, using the two-Higgs-doublet model as a prototype. The WW scattering becomes strong when the extra Higgs bosons are very heavy. We study the sensitivity of the partially strong WW scattering signals at the 13 TeV Large Hadron Collider.
Ecole Polytechnique, CPHT, Salle des Conférences

Vendredi 5 Juillet 2013
Hadron melting and QCD thermodynamics
Antal JAKOVAC (Eötvös University, Budapest )
There are several direct and indirect hints from Monte Carlo measurements that beyond the QCD would-be-critical temperature T_c=156 MeV the hadronic excitations survive. This makes it probable that hadrons gradually melt. In the talk I would like to study field theoretical models of quasiparticle melting, concentrating in particular on the partial pressure. The findings will be applied to QCD thermodynamics, and see, how the hadron melting combined with hadron resonance gas ideas lead to a good description of the measured thermodynamical quantities in QCD. The success of this description leads to a new physical picture of QCD phase transition, where at T=156 MeV only the hadron melting process starts, quarks and gluons appearing only later, at T=200-250 MeV.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 4 Juillet 2013
The puzzle of eta(1405) and eta(1475)
Qiang ZHAO (IHEP, Theory Division, Beijing)
We raise the issue on the nature of the eta(1405) and eta(1475). We will show that so far there is no strong evidence for their being individual states. In particular, there exists a so-called "triangle singularity" mechanism which will alter the lineshape and shift the mass of a state eta(1440) in its decays into K bar K pi, eta pi pi, and 3pi.
IPN, Orsay, Bât. 100, Salle A201

Lundi 17 Juin 2013
Baryon—to—meson transition distribution amplitudes and the QCD description of hard exclusive reactions
K. SEMENOV-TIAN-SHANSKY (IFPA, Université de Liège)
Baryon—to—meson Transition Distribution Amplitudes (TDAs) appear as building blocks in the collinear factorized description of amplitudes for a class of hard exclusive reactions prominent examples being hard exclusive pion electroproduction off a nucleon in the backward region and baryon—antibaryon annihilation into a pion and a lepton pair. Baryon—to—meson TDAs extend the concept of generalized parton distributions (GPDs) and encode valuable complementary information on the hadronic structure. In this talk we will review the basic properties of baryon—to—meson TDAs and discuss the perspectives of the experimental access. In particular, we will present the application of the formalism to the description of the associated production of charmonium and a pion in nucleon—antinucleon annihilation for the PANDA at GSI—FAIR conditions.
IPN, Orsay

Jeudi 13 Juin 2013
Hidden charm molecules in a finite volume
Miguel ALBALADEJO (IFIC, Universidad de Valencia - CSIC )
We address the question of the interaction of pairs of charmed mesons with hidden charm in a finite box. We use the interaction from a recent model based on heavy quark spin symmetry that predicts molecules of hidden charm in the infinite volume. The energy levels in the box are generated within this model and several methods for the analysis of these levels (’inverse problem’) are investigated.
IPN, Orsay, Bât. 100

Jeudi 30 Mai 2013
Heavy-quarkonium suppression in p-A collisions from parton energy loss in cold QCD matter
François ARLEO (LLR, Ecole Polytechnique, Palaiseau )
The effects of parton energy loss in cold nuclear matter on heavy-quarkonium suppression in p-A collisions are studied. It is shown from first principles that at large quarkonium energy E and small production angle in the nucleus rest frame, the medium-induced energy loss scales as E. Using this result, a phenomenological model depending on a single free parameter is able to reproduce J/psi and Upsilon suppression data in a broad x_F-range and at various center-of-mass energies. These results strongly support energy loss as the dominant effect in heavy-quarkonium suppression in p-A collisions. Predictions for J/psi and Upsilon suppression in p-Pb collisions at the LHC are made. It is argued that parton energy loss scaling as E should generally apply to hadron production in p-A collisions, such as light hadron or open charm production.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 16 Mai 2013
Experimental tests of chiral symmery:entering a new era
A.M. BERNSTEIN (MIT, Cambridge, USA)
We are entering a new era in which we can test confinement scale QCD in some well chosen reactions such as pi pi scattering, pion properties, pi N scattering and electromagnetic pion production from the nucleon. Based on the spontaneous breaking of chiral symmetry, chiral perturbation theory (ChPT) is believed to approximate confinement scale QCD as demonstrated by increasingly accurate lattice calculations, particularly in pi pi scattering. At present there is good agreement between experiment and ChPT calculations, including the pi pi and pi N $s$ wave scattering lengths and the pi^0 lifetime. Recent, accurate pionic atom data are in agreement with chiral calculations once isospin breaking effects due to the mass difference of the up and down quarks are taken into account, as was required to extract the pi pi scattering lengths. In addition to tests of the theory, comparisons between pi pi and pi N interactions based on general chiral principles will be discussed. Increasingly accurate experiments in electromagnetic pion production experiments from the proton which test ChPT calculations (and their energy region of validity) will be presented. These experiments are also beginning to measure the final state pi N interaction.
IPN, Orsay, Bât. 100, Salle A201

Mardi 30 Avril 2013
A theoretical framework for hard exclusive processes
Samuel WALLON (UPMC, Univ. Paris 06 et LPT, Orsay)
Hard exclusive processes are central in order to get access to the three-dimensional structure of hadrons in terms of quarks and gluons. We review their modern theoretical background. The collinear factorization approach is presented and illustrated, including deep virtual Compton scattering (DVCS), non-exotic and exotic meson electroproduction. We focus in particular on threshold resummation effects involved in the DVCS coefficient functions. We then provide a short survey of k_T-factorization in the limit of asymptotical center-of-mass energies. Special emphasis is devoted to the power corrections to the scattering amplitude. This is further illustrated by the study of saturation effects in exclusive vector meson electroproduction.
IPN, Orsay, Bât. 100, Salle A201

Jeudi 25 Avril 2013
Broken Chiral Symmetry on a Null Plane
Silas R. BEANE (Helmholtz-Institut, Universität Bonn)
The realization of chiral symmetry breaking on a null-plane (=light-front) is rather peculiar and has long puzzled theorists. Intuitively, this is not surprising since one usually thinks about spontaneous chiral symmetry breaking as a vacuum phenomenon, and it’s hard to visualize how the vacuum can affect a system which is, in some sense, moving relative to the vacuum at or close to the speed of light. In this talk, I will discuss various essential aspects of chiral symmetry breaking on a null plane ; a general proof of Goldstone’s theorem on a null-plane will be presented, and the interesting manner in which the Gell-Mann-Oakes-Renner relation, a statement about quark condensation in Quantum Chromodynamics (QCD), is recovered in null-plane quantized QCD will be demonstrated. Finally, I will show that the spin-flavor symmetry group of the non-relativistic quark model is contained as a non-trivial solution of the algebra that mixes the internal and spacetime symmetries on a null-plane.
IPN, Orsay, Bât. 100, Salle A201

Jeudi 18 Avril 2013
Di-gluon (de)coherence at the LHC
Julien LAIDET (IPhT, CEA/DSM/Saclay)
For a few months the LHC is gathering data of its first p-Pb runs. This is the first time that we expect to clearly probe dense QCD matter. Saturated media in QCD seem to be properly described by the CGC effective field theory. On this basis we worked out the di-gluon production inclusive cross-section which is the dominant process at the LHC and the simplest evidence of saturation through the so called decoherence effect. My aim is first to give a detalled explanation of the importance for studying the di-gluon production at the LHC. Then I sketch the computation and emphasize the meaning of the structure of the cross-section. At the end I push up the strongly correlated regime in which analyticcalculations can be easily continued.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 11 Avril 2013
Relation between transverse angular momentum and generalized parton distributions and some open puzzles
Elliot LEADER (Imperial College, London)
I derive a rigorous relation between the expectation value of the transverse component of the Belinfante version of the angular momentum < JTbel > of a quark in a transversely polarized nucleon in terms of the Generalized Parton Distributions H and E, namely
< JTbel > (quark)=1/2M [ P_0 int-1,1 dx xE_q(x,0,0) + M int-1,1 dx x H_q(x,0,0) ] ,
where P_0 is the energy of the nucleon and where "quark" implies the sum of quark and antiquark of a given flavor. A similar relation holds for gluons. The result is remarkably similar to Ji’s relation for the case of longitudinal polarization, but which he now claims is NOT a relation for longitudinal polarization ! I draw attention to some very tricky issues and ambiguities in dealing with the angular momentum in a quantized gauge theory
IPN, Orsay, Bât. 100, Salle A201

Jeudi 14 Mars 2013
Understanding isotropization in scalar field theories
Thomas EPELBAUM (IPhT, CEA/DSM/Saclay, Gif-sur-Yvette )
In recent years, the problem of thermalization in Heavy Ion Collision has received much attention, but has yet to be solved. The issue is the following : on the one hand, viscous hydrodynamics suggests that the matter produced in these collisions behaves like a nearly perfect fluid, and should do so very shortly after the collision (around 1 fm/c). On the other hand, all theoretical models predict that this matter is initially very anisotropic, and thus that viscous effects should be large. One of the approaches developed to study this nonperturbative problem in QCD is a resummation scheme that amounts to averaging over classical fields with random initial conditions. Its numerical implementation is presented here for the case of a scalar field theory that shares some important features with QCD. We will show that this resummation captures the physics essential for istropisation and possibly thermalization.
Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 28 Février 2013
40 years of nonperturbative treatment of soft pion physics
TRAN N. TRUONG (CPHT, École Polytechnique, Palaiseau)
Effective Range Theory of Bethe, Landau and Schwinger is reviewed. Chiral Perturbation Theory can be supplemented with this notion to make it satisfying unitarity. Work in this direction at the Centre de Physique Théeorique, École Polytechnique, is reviewed ; in particular, nonperturbative calculation on the pion form factor is presented.
École Polytechnique, CPHT

Jeudi 21 Février 2013
Chiral anomaly in the context of flow equations
Benjamin LÉVÊQUE (CPHT, Ecole Polytechnique, Palaiseau)
We analyze the chiral (Adler) anomaly in abelian gauge theories. The anomaly is closely linked to the infrared behaviour of the theory. With the aid of the renormalization group flow equation, we prove UV and IR bounds on the euclidean perturbative 1-PI functions of massless theories. We then prove the absence of anomalies for a class of chiral abelian gauge theories. At the end, we comment on the standard model.
Ecole Polytechnique, CPHT

Jeudi 31 Janvier 2013
The cosmological constant problem
Jérôme MARTIN (Institut d’Astrophysique de Paris)
In this talk, I will review and discuss what is the celebrated cosmological constant problem.[ppt]
IPN, Orsay, Bât. 100, Salle A201

Jeudi 24 Janvier 2013
Nucleon-nucleon effective field theory with dibaryon fields
Joan SOTO (Departament d’Estructura i Constituents de la Matèria and Institut de Ciències del Cosmos, Universitat de Barcelona))
I will introduce a non-relativistic effective field theory for low energy nucleons that includes explicit dibaryon fields in addition to nucleons and pions. The calculations can be carried out in dimensional regularization and standard minimal subtraction scheme, and hence the power counting becomes explicit. I will present results at NLO and NNLO for the nucleon-nucleon phase shifts in the 1S0 and 3S1-3D1 channels. The outcome is analogous to that of the KSW approach. I will finally show how this effective theory facilitates the resummation of a certain class of diagrams, involving the so called radiation pions, that may give rise to large contributions to the scattering lengths. Recent results on chiral extrapolation formulas for the latter will be presented. [pdf]
IPN, Orsay, Bât. 100, Salle A201



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