Accueil du site > Activités scientifiques et techniques > Théorie > Séminaires > Séminaires 2012 (physique de particules)

Jeudi 13 Décembre 2012

Effective range corrections to universal tetramers properties

Tobias FREDERICO (Instituto Tecnologico de Aeronautica, Sao Jose dos Campos, Brasil)

The universal properties of tetramers, weakly bound close to the scaling limit, are investigated by solving a subtracted set of Faddeev-Yakubovsky (FY) equations for identical bosons with a zero-range interaction. The solution demands a four-body scale independence of the trimer properties. Furthermore, the effect of a finite effective range is introduced in the FY equations, which we show produces results that are distinct from the three- and four-body short range scale variations. The contribution of the effective range to the universal scaling functions for the tetramers are given. In particular, the correction due to the effective range for the correlation between the positions of the four-atom recombination peaks is shown. In this case, we found a shift in the scaling function, which, for cesium atoms, allows one to extract the effective range from the existing experimental data for broad Feshbach resonances.

IPN, Orsay, Bât. 100, Salle A201

Mardi 11 Décembre 2012

Light-by-light scattering sum rules and meson transition form factors

Vladyslav PAUK (Johannes Gutenberg Universität, Mainz )

In the seminar a set of exact sum rules for the light-by-light fusion process will be discussed. These sum rules involve energy weighted integrals of light-by-light fusion cross sections, which can be measured at e+e- colliders and allow one to constrain the energy behavior of these response functions. The implication of the sum rules within a perturbative quantum field theory framework will be shown. Specifically, electrodynamics of spin-0, spin-1/2, and spin-1 fields will be considered at tree level and beyond. The sum rules will then be applied to the production of mesons by a virtual photon and a real photon. It will be demonstrated that these sum rules imply non-trivial new relations between the two-photon decay widths of mesons and the gamma*gamma transition form factors for (pseudo-) scalar, axial-vector and tensor mesons. The phenomenological implications of these results for mesons both in the light quark sector and in the charm quark sector will be discussed. The seminar will also give an outlook on the application of these new results to constrain the hadronic uncertainties due to the light-by-light contribution to the muon’s anomalous magnetic moment.

Ecole Polytechnique

Lundi 3 Décembre 2012

Solving the Bloch-Nordsieck Model

Peter MATI (Institute of Physics, Budapest University of Technology and Economics)

The Bloch-Nordsieck Model was invented to describe the low energy regime of QED in 1937. Its main feature : the full fermion propagator can be calculated exactly (by path integral method), the IR singularities are summed up. In the seminar I will show how these exact results can be used to benchmark various techniques of quantum field theory, such as Perturbation Theory, 2PI resummation and Dyson-Schwinger equations. The finite temperature case will be discussed as well.

Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 29 Novembre 2012

Roy-Steiner equations for pion-nucleon scattering

Martin HOFERICHTER (HISK and BCTP, Universität Bonn and ITP, Universität Bern)

Pion-nucleon scattering is one of the basic processes of low-energy QCD. Its parameters serve as input to more complicated reactions such as nucleon-nucleon scattering, while the amplitudes for the crossed process can be related to basic properties of the nucleon. Relying on analyticity, unitarity, and crossing symmetry, I will present a system of dispersion relations that allows for a model-independent analysis of low-energy pion-nucleon scattering. In particular, the solution of the crossed-channel equations as well as the relation to the scalar form factor of the nucleon and the pion-nucleon sigma term will be discussed. [pdf]

IPN, Orsay, Bât. 100, Salle A201

Mardi 23 Octobre 2012

A covariant formalism for the N* electroproduction

Teresa PEÑA (IST, Lisboa)

Baryons were at the root of the development of the quark model. Constituent quark models are one approach, between several others, to deal with the excitation of N*, each one corresponding to a different choice of degrees of freedom, and therefore to a different resolution or momentum transfer domain. These different pictures can be matched at the limits of their validity range, for mutual benefit and more insight. For example, in this talk I will connect a constituent quark model to LQCD data. In doing so we gain information that would not be directly accessed by any of the two approaches separately.

IPN, Orsay, Bât. 100, Auditorium

Mercredi 25 Juillet 2012

Strong fields and QED as function of the g-factor

Lance LABUN (University of Arizona)

Precision QED experiments (muon g-2 and Lamb shift require understanding of QED with arbitrary gyromagnetic ratio g>2. We obtain the nonperturbative effective action of QED within Schwinger proper time method in arbitrarily strong quasi-constant external electromagnetic fields as a function of g. The expression is divergent for |g|>2, given the magnetic instability of the vacuum. A generalized form of the effective action valid for all g is obtained by recognizing the periodicity of the spectrum of Landau eigenenergies as a function. We show the presence of a cusp at the periodic points g=... -6,-2,2,6 .... As a result, the QED beta function and light-by-light scattering differ from perturbative computation near to g=2 and an asymptotically free domain of g for QED arises.

IPN, Orsay, Bât. 100, Salle A201

Jeudi 12 Juillet 2012

On a relation between production processes and total cross sections

Stéphane MUNIER (CPHT, Ecole Polytechnique, Palaiseau)

Perturbative QCD is the appropriate tool to describe many important properties of the inclusive observables measured at electron-proton (or ion) colliders, such as the energy dependence of the total cross sections in well-chosen kinematical regions. This is because the electron may effectively be replaced by its cloud of photons, whose virtualities provide a hard scale.

At hadron colliders instead, there is no hard scale in the initial state. Therefore, the observables one may compute perturbatively involve the production of jets, and thus belong to a quite different class of observables.

However, it turns out that there is a formal relation between production processes and total cross sections, enabling one to apply calculations of the latter to the former. We will review this relation, and present our recent proof that it holds at next-to-leading order.

Ecole Polytechnique, CPHT, Salle des Conférences

Lundi 9 Juillet 2012

Pion-nucleon scattering in covariant chiral perturbation theory

Deliang YAO (Institute of Theoretical Physics, Peking University)

In order to study the pion-nucleon scattering covariantly, we work within relativistic chiral perturbation theory using extended-on-mass-shell renormalization scheme. The amplitude, valid up to O(p)^3, possesses satisfactory analytic, convergent and scale-independent properties. The resultant description of existing pi N phase-shift data is successful for c.m. energies up to 1.13 GeV. Including the Born-term contribution of Delta1232 explicitly, we can extend the energy domain up to 1.2 GeV. Values of low-energy constants are determined and used for predicting important quantities, such as the sigma-term and the violation of the Goldberger-Treiman relation. [pdf]

IPN, Orsay, Bât. 100, Salle A201

Jeudi 7 Juin 2012

eta —> 3 pi and light quark masses

Emilie PASSEMAR (Theoretical Division, Los Alamos National Laboratory)

Recently, several experimental collaborations have invested considerable effort into new and more precise measurements of the eta —> 3 pi decays. These experimental advances require revisiting the corresponding theoretical analyses. In this talk, we present a new calculation of the eta —> 3 pi decay amplitude relying on dispersive methods. We show how the study of this decay allows one to extract a fundamental parameter of the Standard Model, namely the quark mass ratio (m_d^2-m_u^2)/(m_s^2-\hatm^2), with good precision. We then discuss the possibility of extracting the individual light quark masses.

IPN, Orsay, Bât. 100, Salle A201

Jeudi 24 mai 2012

Anomalous magnetic moments of charged leptons (a guided tour)

Marc KNECHT (Centre de Physique Théorique, Luminy, Marseille)

The aim of this presentation is to review the theoretical aspects related to the predictions of the anomalous magnetic moments of the charged leptons in the standard model, with emphasis on the electron and on the muon. The experimental situation as well as future perspectives will also be briefly addressed. [pdf]

IPN, Orsay, Bât. 100, Salle A201

Jeudi 10 Mai 2012

Pion properties from QCD sum rules

Dmitri MELIKHOV (SINP, Moscow and HEPHY, Vienna)

We analyze elastic and meson-photon transition form factors for light pseudoscalar mesons within the local-duality (LD) version of QCD sum rules. To probe the accuracy of this method, we consider, in parallel to QCD, a potential model : in this case, the exact form factors may be calculated from the solutions of the Schrödinger equation and confronted with the results from the quantum-mechanical LD sum rule. On the basis of this analysis we conclude that the LD sum rule is expected to give reliable predictions for the form factors in the region of momentum transfers Q^2 ≥ 5-6 GeV^2. Moreover, the accuracy of the method improves rather fast with growing Q^2 in this region. For the pion elastic form factor, the data at small Q^2 indicate that the LD limit may be reached already at relatively low values of momentum transfers, Q^2 \approx 4-8 GeV^2 ; we therefore conclude that large deviations from LD in the region Q^2= 20-50 GeV^2 reported in some recent theoretical analyses seem unlikely. The data on the meson-photon transition form factors for eta and eta’ mesons meet the expectations from the LD model. Surprisingly, the BaBar results for the pi —> gamma gamma^* form factor imply a violation of LD growing with Q^2 even at Q^2 \approx 40$ GeV^2, at odds with the eta and eta’ cases and the experience from quantum mechanics. [pdf]

IPN, Orsay, Bât. 100, Salle A201

Lundi 23 Avril 2012

Spacelike pion form factor from analytic continuation and the onset of perturbative QCD

I. SENTITEMSU IMSONG (CHEP, Indian Institute of Science, Bangalore)

The factorization theorem for exclusive processes in perturbative QCD predicts the behaviour of the pion electromagnetic form factor F(t) at asymptotic spacelike momenta t(=-Q^2)<0. We address the question of the onset energy using a suitable mathematical framework of analytic continuation, which uses as input the phase of the form factor below the first inelastic threshold, known with great precision through the Fermi- Watson theorem from pi pi elastic scattering, and the modulus measured from threshold up to 3 GeV by the BaBar collaboration. The method leads to almost model-independent upper and lower bounds on the spacelike form factor. Further inclusion of the value of the charge radius and the experimental value at -2.45 GeV^2 measured at JLab considerably increases the strength of the bounds in the region Q^2\le 10 GeV^2, excluding the onset of the asymptotic perturbative QCD regime for Q^2<7 GeV^2. We also compare the bounds with available experimental data and with several theoretical models proposed for the low and intermediate spacelike region.

IPN, Orsay, Bât. 100, Salle A201

Jeudi 5 Avril 2012

The search for gluonic excitations in light mesons with the GlueX experiment

Igor SENDEROVICH (University of Connecticut)

One of the present challenges of nuclear and particle physics is quantitative understanding of soft quantum chromodynamics (QCD). Efforts to compute QCD bound states on the lattice are progressing, with calculation regimes getting close to the physical pion mass. New experimental programs in light meson spectroscopy are underway to test the theoretical results. The status of these projects will be reviewed with focus on the GlueX experiment. This experiment will employ the high-rate 12GeV electron beam from Jefferson Lab’s upgraded CEBAF accelerator to mount a search for ``exotic mesons’’. Identification of these represents an unambiguous measurement of the predicted gluonic degrees of freedom in a meson. Amplitude Analysis of meson decays in the hermetic GlueX detector will determine the resonance quantum numbers and select the exotic states. The status of the facility’s construction and the current efforts in Amplitude Analysis of simulated exotic states will be presented.

Ecole Polytechnique, CPHT, Salle des Conférences

Jeudi 8 Mars 2012

A new determination of alpha_S from tau decays

Diogo BOITO (RWTH - Aachen University)

We will present a new framework for the extraction of the strong coupling constant alpha_S from hadronic tau decays. Emphasis is given to the nonperturabative part. We employ a quantitative model in order to account for violations of quark-hadron duality and enforce a consistent treatment of the higher dimensional contributions in the operator product expansions. We will discuss the value thus obtained for the strong coupling constant based on an analysis of the OPAL data.

IPN, Orsay, Bât. 100, salle A015

Jeudi 23 Février 2012

New Measurements of Muon g-2 and EDM with Ultra-Cold Muon Beam at J-PARC

Naohito SAITO (KEK, Tsukuba / J-PARC)

We have proposed a new measurement of muon anomalous magnetic moment and electric dipole moment at J-PARC. The new measurement utilizes ultra-cold muon beam. The transverse dispersion of the beam is much less than its longitudinal momentum, i.e. Delta pT/pL ˜ 10-5. Such a muon beam can be stored in a static magnetic field with an effectively-zero focusing field. With this technique, the storage field can be much smaller than in previous experiments. The proposed experiment will employ 3 T high precision field to store 300 MeV/c muon beam in the circular orbit of 33 cm in radius. Such smaller dimensions would be beneficial in the precision control as well as geometrical acceptance of the decay positron detector.

Our first goal is to improve the precision of muon g-2 down to 0.1 ppm, which is a factor of 5 improvement, and the search for the permanent electric dipole moment at the level of 10-21 e cm, which is a two orders of magnitudes better sensitivity compared to the last measurement. Further improvements of both $g-2$ and EDM are also possible with a future extension.

We discuss the current status and the future potential of the project.

IPN, Orsay, Bât. 100, Salle A201

Jeudi 2 Février 2012

Using vortex beams in high-energy physics

Igor P. IVANOV (IFPA, Université de Liège et Sobolev Institute of Mathematics, Novosibirsk)

Optical vortex beams (laser beams with helical wave-fronts) are well-known and routinely used in optics. Each photon in this beam carries a well-defined non-zero orbital angular momentum (OAM) with respect to the beam axis. Electron vortex beams have also been created recently, and there exist suggestions how to create high-energy vortex beams of protons and other particles. Thus, the OAM of incoming or outgoing particles can become a new degree of freedom in high-energy collisions, and one can ask what novel insights it can bring. I will show that collision of vortex beams can probe quantities which are inaccessible in the usual plane wave collisions, and illustrate the general formalism with examples from hadronic physics.

IPN, Orsay, Bât. 100, Salle A201

Jeudi 12 Janvier 2012

Exclusive resummation in dihadron production at NLL (+NNLL)

Leandro G. ALMEIDA (IPhT, CEA-Saclay)

I will discuss the resummation of large logarithmic perturbative corrections to the partonic cross sections relevant for dihadron production in hadronic collisions, H1H2 —> h1h2X, at high invariant mass of the produced hadron pair. These corrections arise near the threshold for the partonic reaction and are associated with soft-gluon emission. We perform the resummation to next-to-leading logarithmic (NLL) accuracy and the current work in extending it to NNLL order, and show how to incorporate consistently cuts in rapidity and transverse momentum of the observed particles. We present numerical results for fixed-target and ISR regimes and find enhancements over the next-to-leading order cross section, which significantly improve the agreement between theoretical predictions and data.

Ecole Polytechnique, CPHT, Salle des Conférence

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