Laboratoire de Physique Theorique d'Orsay
 Bâtiment 210 Univ. Paris-Sud 11 91405 Orsay Cedex France T. 01 69 15 63 53 F. 01 69 15 82 87

 Agenda > Séminaires / Seminars > Particules Dernier ajout : mercredi 2 août 2017.

Séminaires Particules 2011-2012

 Les séminaires se tiennent en salle 114 au 1er étage du bâtiment 210, généralement le jeudi à 16h, sauf indication contraire. Seminars are held in room 114, 1st floor of Bdg 210, generally on Thursdays at 4 PM, unless stated otherwise. Contact : Benoît Blossier.   Jeudi 12 juillet 2012 à 16 heures, salle 114 Jernej Kamenik (Jozef Stefan Institute) : Hunting New Physics in Top Quark Pair Production Experimental results on the forward-backward asymmetry in top quark pair production from the Tevatron persistently exhibit intriguing deviations from expectations within the standard model. On the other hand current LHC measurements of the related charge asymmetry are well consistent with QCD predictions. I will show that the two observables are in general not tightly correlated. They can even have opposite signs, if the underlying new physics model is general enough. I will demonstrate this using two examples : a light pseudo-axigluon, and a vector that is a color octet and electroweak triplet. I will further identify observables where significant deviations are still expected at the Tevatron and the LHC : the b bbar production forward-backward asymmetry and spin polarizations of the pair-produced tops and anti-tops.   Jeudi 28 juin 2012 à 16 heures, salle 114 Kei-Ichi Kondo (Shiba University) : Physical unitarity of a massive Yang-Mills theory without the Higgs field : Implications for low-energy QCD In this talk, we propose a massive Yang-Mills model blessed with both physical unitarity and renormalizability without the Higgs field, from a viewpoint of low-energy QCD. We show that a physical and massive vector field can be constructed from the renormalizable Curci-Ferrari model so that it is invariant under an extended BRST transformation and it has the correct physical degrees of freedom as a spin-one massive boson. We discuss the implications for the low-energy QCD in relation to color confinement, BRST-invariant dimension-two condensate and glueball mass.   Mercredi 13 juin 2012 à 16 heures, salle 114 Jure Zupan (University of Cincinnati) : Recent anomalies in processes with heavy quarks I will show several results regarding recent anomalies with heavy quarks : the forward-backward asymmetry in ttbar production, the direct CP asymmetry in two body D decays and lepton universality violations in semileptonic B decays. The forward-backward asymmetry in ttbar production can in principle be checked using charge asymmetry measurements at the LHC, however, as I will show for general enough new physics models the correlation between the two observables are lost. I will discuss how one can experimentally check whether the observed direct CP asymmetry is due to new physics (and why we believe it is due to the standard model). Finally, I will show that at least in effective field theory a consistent picture of lepton universality violation recently hinted at by BaBar is possible.   Jeudi 31 mai 2012 à 16 heures, salle 114 Pasquale di Bari (Université de Southampton) : The interplay between lepton and heavy neutrino flavours in leptogenesis In the talk I will discuss how a full description of the interplay between lepton and heavy neutrino flavour effects in leptogenesis has a few non trivial consequences that open new scenarios and perspectives. For example, it implies a new solution to the problem of the independence of the initial conditions and it gives rise to new terms in the final asymmetry, so called phantom terms, that escape the wash-out at the production. At the same time this interplay increases the importance of a calculation of the final asymmetry within a density matrix formalism, a more general kinetic description than Boltzmann equations accounting for quantum decoherence. I will discuss how within the density matrix formalism it is possible to work out systematically, in the proper limits, the correct sets of Boltzmann equations for all different heavy neutrino mass patterns accounting for all fore-mentioned new effects, such as the appearance of phantom terms in the final asymmetry.   Mardi 15 mai 2012 à 10 heures, salle 101 du Bâtiment 200 (séminaire commun LAL/LPT) Valeri Khoze (IPPP, University of Durham, UK) : Central exclusive production at hadron colliders Central exclusive production (CEP) processes in high-energy hadron collisionsprovide an especially clean environment in which to study the nature and quantum numbers of the new resonance states. In this talk we discuss selected topics on the CEP, from the so-called diffractive Higgs to heavy resonace production within the formalism developed by the Durham group. We present the new results on the CEP of heavy quarkonia, diphotons and dimesons. We compare the theoretical expectations with the existing experimental results from the Tevatron and the LHC and present further predictions for the CEP processes at the RHIC and LHC colliders.   Jeudi 3 mai 2012 à 16 heures, salle 114 Vladimir Braun (Université de Regensburg) : Finite-t and target-mass corrections to hard exclusive processes Huge progress in experimental techniques has made possible an observation of (rare) processes where all contributing hadrons are identified and a large momentum is transferred to a hadronic system. The hopes are that in this way one can get access to "holographic" images of the proton as two-dimensional slices in transverse plane for a given momentum fraction One of the theory challenges in this program appears to be to calculate the contributions to correlation functions of suitable currents of all operators that are related to the leading-twist ones by adding total derivatives. I explain how this problem is solved using conformal symmetry.   Mercredi 18 avril 2012 à 16 heures, salle 114 Chris Sachrajda (Université de Southampton) : Lattice computation of nonleptonic kaon decay amplitudes I will review the status of the RBC-UKQCD collaborations’s programme of the study of nonleptonic kaon decays. For the amplitude A_2, where the subscript 2 indicates that the two pions are in an isospin 2 state, we have completed a calculation at (almost) physical kinematics, finding a value of Re(A_2) which agrees with the experimental result and for the first time determining Im(A_2). Combining the results with the known experimental values of Re(A_0) and epsilon’/epsilon allows us also to determine Im(A_0). I will also present the results of a direct exploratory study of A_0. The published material can be found in arXiv:1111.1699 and arXiv:1106.2714.   Jeudi 29 mars 2012 à 16 heures, salle 114 Veronica Sanz (York University) : Non-Standard Electroweak Symmetry Scenarios In this talk we review electroweak spontaneous symmetry breaking in the Standard Model, and describe scenarios with no fundamental scalar : composite Higgs and Higgsless scenarios. We will discuss their basic phenomenology and ways to characterize them.   Mardi 20 mars 2012 à 16 heures 30, salle 114 Cyrille Marquet (CERN) : The Electron-Ion Collider (EIC) : the next high-energy nuclear physics facility ? As Brookhaven and Jefferson lab scientists are preparing to make their case that the EIC - a high-energy e+A and polarized e+p collider - is the next must-have facility for QCD and nuclear physics studies in the US, I will review selected topics of its physics program. On the one hand, high-energy e+A deep inelastic scattering is ideal to explore the QCD dynamics of small-x gluons in nuclei, as many different measurements allow to extract important information about the nuclear gluon distribution and correlations. On the other hand, the goal of polarized e+p collisions is to shed light on the 3-dimensional partonic structure of nucleons, and in particular to study the orbital motion of the quarks and gluons, known to contribute to about half the nucleon spin.   Jeudi 15 mars 2012 à 16 heures, salle 114 Gregorio Herdoiza (Université de Madrid) : Constraints on the Standard Model from Lattice QCD Many processes currently being studied in particle physics experiments depend on large non-perturbative QCD contributions. Accurate determinations of the associated hadronic matrix elements is needed to constrain the parameters of the Standard Model (SM) and to help in unravelling the presence of physics beyond the SM. I will review the considerable progress of recent lattice QCD calculations and present results related to the flavour sector of the SM and to the direct detection of Dark Matter.   Jeudi 8 mars 2012 à 16 heures, salle 114 Alberto Romagnoni (Université de Madrid) : Low scale supersymmetry breaking and signatures at the LHC from Higgs decay I will discuss how the usual MSSM Higgs sector is modified when supersymmetry is assumed to be spontaneously broken at very low energies. In such scenarios, it is possible to raise the tree level masses of all the Higgs particles above the experimental bounds and moreover, due to the presence of the goldstino fermion as well as a CP-even and a CP-odd singlet (sgoldstino) scalar in the neutral Higgs sector, non-standard Higgs decays can take place and give rise to characteristic signatures at the LHC.   Mardi 28 février 2012 à 14 heures 30, salle 114 Michael Baker (IFIC, Université de Valence) : Investigating the Rank-One Rotating Mass Matrix After discussing some motivation for a rank-one rotating mass matrix, we will describe a natural solution to the strong CP problem. We will show consistency of the scheme with experimental data and look at some implications for Higgs decay.   Jeudi 23 février 2012 à 16 heures, salle 114 Asmita Mukherjee (IIT Bombay) : Generalized Parton Distributions of the Photon We present a calculation of the generalized parton distributions of the photon when there is non-zero momentum transfer both in the transverse and longitudinal directions. We consider only the contributions when the photon helicity is not flipped and calculate those at leading order in electromagnetic coupling $\alpha$ and zeroth order in the strong coupling $\alpha_s$. We keep the leading logarithmic terms as well as the quark mass terms in the vertex. By taking Fourier transforms of the GPDs with respect to the transverse and longitudinal momentum transfer, we obtain the parton distributions of the photon in position space.   Mardi 14 février 2012 à 16 heures, salle 114 Christopher Smith (IPN Lyon) : Top, Higgs, and New Physics In this talk, I will discuss three New Physics scenarios of relevance for current LHC analyses. Firstly, baryon number violation would immediately signal non-standard physics. Enforcing low-energy constraints through minimal flavor violation, such interactions could show up in peculiar top production channels. Further, they may now be much welcome in a supersymmetric context, since they alleviate the already tight constraints on sparticle masses. Secondly, the presence of a fourth generation will be reappraised by pointing out the consequences of a mismatch between the number of QCD colors and flavors. Finally, the motivations and possible signals of a new light weakly interacting particle in Higgs boson decays will be detailed.   Jeudi 9 février 2012 à 16 heures, salle 114 Svjetlana Fajfer (Department of Physics, University of Ljubljana and J. Stefan Institute) : Colored scalars and proton destabilization Most general baryon number violating effective operators induced by the scalar exchange are discussed. The scalars should interact with standard model fermions. We investigate bounds on the appropriate couplings coming from the current SuperKamiokande results. Relatively light states might explain observed discrepancy of the forward-backward asymmetries in ttbar-production observed at Tevatron. The baryon number violating effective operators can be searched in top quark decays at LHC. We investigate the role of these light scalar states in the proton destabilization. We question the role of baryon number violating dimension nine tree level amplitude, as well as box diagram induced dimension six amplitude.   Jeudi 2 février 2012 à 16 heures, salle 114 Igor Ivanov (Université de Liège) : Scalar sectors in multi-Higgs-doublet models Models with several Higgs doublets are a popular framework to go beyond the Standard model in electroweak symmetry breaking. Usually, only simple scalar sectors in such models are considered, and no systematic exploration of all the possibilities with N doublets exists so far. The main reason is that a general scalar potential with more than one doublet just cannot be minimized explicitly. I will present a geometric approach to the analysis of the most general scalar sector in multi-doublet model, which allows one to study many properties of the model without explicit minimization of the potential. In the case of two doublets, the full phase diagram of the scalar sector can be constructed in the tree-level approximation, and several (astro)particle issues can be addressed. For more than two doublets, only partial success has been achieved so far.   Mardi 24 janvier 2012 à 11 heures, salle 114 J. A. Oller (Université de Murcie) : Meson scattering in U(3) CHPT. Insight on scalar spectrum from large N_C. Scalar, pseudoscalar Weinberg-like sum rules. In this work, the complete one loop calculation of meson-meson scattering amplitudes within U(3)\otimes U(3) Chiral Perturbation Theory (CHPT) with explicit resonance states is carried out for the first time. Partial waves are unitarized from the perturbative calculation employing a non-perturbative approach based on the N/D method, generally called Unitary CHPT (UCHPT). Once experimental data are reproduced in a satisfactory way we then study the resonance properties, such as the pole positions, corresponding residues and their N_C behaviors. The resulting N_C dependence is the first one in the literature that takes into account the fact that the \eta_1 becomes the ninth Goldstone boson in the chiral limit for large N_C. Within this scheme the vector resonances studied, \rho(770), K^*(892) and \phi(1020), follow an N_C trajectory in agreement with their standard \barqq interpretation. The scalars f_0(1370), a_0(1450) and K^*(1430) also have for large N_C a \barqq pole position trajectory and all of them tend to a bare octet of scalar resonances around 1.4 GeV. The f_0(980) tends asymptotically to the bare pole position of a singlet scalar resonance around 1 GeV. The \sigma, \kappa and a_0(980) scalar resonances have a very different N_C behavior. The case of the \sigma resonance is analyzed with special detail. Making use of these non-perturbative techniques we also evaluate different combinations of the \baruu, \bardd and \barss scalar form factors and the pseudoscalar ones. We implement them in Weinberg-like scalar and pseudoscalar sum rules that establishes interesting non-trivial constraints on the combined scalar and pseudoscalar spectroscopy.   Mardi 10 janvier 2012 à 16 heures, salle 114 Silas Beane (New Hampshire University) : Hadron-hadron and multi-hadron interactions from lattice QCD I will first review the essential methodology for extracting information about the hadronic S-matrix —and multi-hadron interactions— from a lattice QCD calculation. Recent results for meson-meson scattering, and few-body systems will then be presented, and a road-map for the near future will be discussed.   Jeudi 15 décembre 2011 à 16 heures, salle 114 Nicolas Garron (Université d’Edinburgh) : Kaon physics from the lattice. In the last years, enormous progress have been made by the lattice community, in particular in the kaon sector. The simulations have become realistic and new techniques have been developed, allowing us to have a better control on the systematic, but also to access new quantities. I will present a selection of recent lattice results and their phenomenological impact in the quest for new physics. In particular I will report on the computation of K-> pi pi decays achieved by the RBC-UKQCD collaborations, which is relevant for the study of direct and indirect CP violation. The DeltaI=3/2 part of this computation has been performed with a realistic setup, and represents the first "ab initio" calculation of a hadronic weak decay.   Jeudi 8 décembre 2011 à 16 heures, salle 114 Andreas Goudelis (DESY Hamburg) : Higgs-induced Lepton Flavor Violation We shall examine the consequences of considering that the Yukawa couplings depend on the Higgs field through operators of mass dimension greater than 4, focusing mainly on the lepton sector. The motivation for this approach is to reproduce the hierarchy of the fermion masses, which in the Standard Model remains unexplained. Due to the smallness especially of the lepton Yukawa couplings, these operators can have a significant contribution to the lepton masses. In such a case, the lepton mass matrix and the matrix of the Higgs-lepton couplings can be misaligned leading to Lepton Flavor Violation (LFV) mediated by the Standard Model Higgs boson. We shall show model-independent bounds on the Higgs LFV couplings and quantify LFV in radiative decays of leptons and electric dipole moments for a class of lepton-Higgs operators contributing to lepton masses.   Jeudi 1er décembre 2011 à 16 heures, salle 114 Christoph Bobeth (Technical University of Munich) : Towards a global analysis of $b \to s \ell^+ \ell^-$ Flavour changing neutral current decays mediated by $b \to s \ell^+ \ell^-$ (and $b \to s \gamma$) are indirect probes of new physics at or beyond the electroweak scale. They have been under investigation at the $B$-factories and the Tevatron during the last decade and are investigated currently at the LHC. Especially LHCb is expected to provide unprecedented accuracy and the measurement of new observables for the first time until the end of 2012. The theoretical predictions of inclusive and exclusive decays have also been refined allowing the first global analysis of these decays. In particular the angular distribution of $B\to K^* (\to K \pi)\ell^+\ell^-$ offers observables with reduced hadronic uncertainties whose potential role in a global anaylsis is studied. First results of model-independent fits are discussed.   Jeudi 24 novembre 2011 à 16 heures, salle 114 Ulrich Ellwanger (LPT Orsay) : Results of Higgs Searches, and its Interpretation in the SM and its SUSY Extensions The status of searches for a Standard-Model-like Higgs boson at the LHC and the Tevatron (as of September 2011) is reviewed, and confronted to expectations within the Standard Model and its supersymmetric extensions.   Mardi 22 novembre 2011 à 16 heures, salle 114 Lorenzo Calibbi (Max Planck Institute of Munich) : Light Neutralino Dark Matter in the MSSM We investigate the constraints to the light neutralino dark matter scenario in the minimal supersymmetric standard model from available experimental observations such as decays of B and K meson, relic dark matter abundance, and the search for neutralino and Higgs production at colliders. We find that two regions of the MSSM parameter space fulfill all the constraints : a fine-tuned strip with large tan beta where the lightest neutralino can be as light as 8 GeV, and a low tan beta region providing a neutralino mass larger than 16 GeV. The large tan beta strip, which can be compatible with recently reported signals from direct detection experiments, can be fully tested by means of low-energy observables and, in particular, by B_s -> mu mu and Higgs bosons searches at the LHC within the upcoming months.   Jeudi 17 novembre 2011 à 16 heures, salle 114 Nils Offen (Université de Ratisbonne) : Meson form factors and the BaBar-puzzle I will give a short introduction to the general theory of transition form factors for pseudoscalar mesons to two photons and to the so called BaBar-puzzle. Subsequently some of the possible resolutions are discussed and an account of a recent calculation using light cone sum rules is given. Several still open questions are discussed without giving definite answers.   Jeudi 10 novembre 2011 à 16 heures, salle 114 Luca Panizzi (IPN Lyon) : Heavy Vector-like Top Partners at the LHC and Flavour Constraints Vector-like fermions appear at the electroweak scale in many new physics extensions of the SM. The phenomenology of a heavy non-standard quark doublet which couples with all SM generations through Yukawa interactions will be considered. After discussing the constraints coming from flavour physics at tree level and loop level, and the implications of a generalized CKM matrix, the phenomenology of the new quarks at the LHC will be presented, focusing mostly on the single production and decay of a heavy vector-like top partner.   Jeudi 27 octobre 2011 à 16 heures, salle 114 Chan Beom Park (Université de Madrid) : Reducing combinatorial uncertainties using kinematic variables Reducing combinatorial uncertainties in hadron collider events is of great importance when reconstructing the event topology to probe particle properties such as mass and spin. Based on the kinematic variables, we propose a new method to resolve combinatorial ambiguities of correctly assigning visible particles to the event topology with two invisible particles in the final state.   Jeudi 20 octobre 2011 à 16 heures, Amphi I Dirk Pleiter (Université de Regensburg) : Dirac and Pauli form factors from Nf = 2 Clover-fermion simulations In this talk we present recent results from the QCDSF collaboration on the electromagnetic form factors of the nucleon. With statistical errors becoming sufficiently small we start to have better control on the parametrization of the Q^2 dependence. Using results obtained on an extensive set of lattice gauge field ensembles we explore different sources for systematic errors. The lattice results are compared with results from effective field theories.   Mardi 11 octobre 2011 à 16 heures, salle 114 Antonio Pich (Université de Valence) : Phenomenological constraints on multi-Higgs-doublet models : Flavour Alignment Alternative scenarios of electroweak spontaneous symmetry breaking should be considered in view of the recent LHC limits on the Higgs boson. Additional scalar doublets are the simplest extension of the Standard Model, compatible with electroweak precision tests. The absence of flavour-changing neutral currents requires the flavour alignment of the different Yukawa structures. The phenomenology of the Aligned Two-Higgs-Doublet Model will be discussed, stressing the presence of new sources of CP violation and its possible signatures at high-energy colliders.   Mardi 4 octobre 2011 à 11 heures, Auditorium Pierre Lehmann Michael E. Peskin (SLAC) : Antenna Approach to Parton Showers In a conventional parton shower, the basic move is a 1->2 splitting radiating a quark or gluon in accord with Altarelli-Parisi evolution. An antenna parton shower is a parton shower in which the basic moveis a 2->3 splitting in which a color dipole coherently radiates a quark or gluon. In this talk, I will describe the formalism of antenna showers, including inclusion of spin and of massive particles, and show some results of the implementation.