Séminaires Particules 2013-2014

  Vendredi 27 juin à 11h, salle 114

Bertrand Ducloué (LPT Orsay) : Confronting BFKL dynamics with experimental studies of Mueller-Navelet jets at the LHC

In the high energy limit of QCD, the smallness of the strong coupling due to the presence of a hard scale can be compensated by large logarithms of the center of mass energy. All these logarithmically-enhanced contributions can be resummed by the Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation. To investigate these high energy dynamics, Mueller and Navelet proposed to study the production of two forward jets with a large interval of rapidity at hadron colliders. In this talk, we will present the results of a study of this process at next-to-leading logarithmic accuracy and show how the use of the Brodsky-Lepage-Mackenzie procedure to fix the renormalization scale leads to a very good description of recent CMS data on the azimuthal correlations of these jets.

  Jeudi 26 juin à 15h, salle 114

Axel Weber (Universidad Michacana, Mexique) : Callan-Symanzik approach to Yang-Mills theory in the Landau gauge

The main tool so far for the determination of the correlation functions of Landau gauge Yang-Mills theory in the continuum, in particular in the infrared regime, have been the Dyson-Schwinger equations of the theory. I shall argue that the use of Callan-Symanzik renormalization group equations has distinctive advantages over the Dyson-Schwinger equations, particularly for the vertex functions. I shall present a generalization of the infrared safe renormalization scheme proposed by Tissier and Wschebor in 2011. The renormalization scheme dependence can be used to improve the matching to the existing lattice data for the gluon and ghost propagators.

  Jeudi 19 juin à 15h, Amphi 1

Jérémie Quévillon : Soutenance de thèse de doctorat : Higgs Physics Beyond the Standard Model

Abstract : On the 4th of July 2012, the discovery of a new scalar particle with a mass of order 125 GeV was announced by the ATLAS and CMS collaborations. This closes the first era of the probing of the mechanism that triggers the breaking of the electroweak symmetry and generates the fundamental particle masses. An important era is now opening : the precise determination of the properties of the produced particle. This is of extreme importance in order to establish that this particle is indeed the relic of the mechanism responsible for the electroweak symmetry breaking and to pin down effects of new physics, if additional ingredients beyond those of the Standard Model are involved in the symmetry breaking mechanism. In this thesis we have tried to understand and characterize to which extent this new scalar field is the Standard Model Higgs Boson and set the implications of this discovery in the context of supersymmetric theories and dark matter models.

Résumé : Le 4 Juillet 2012, la découverte d’une nouvelle particule scalaire avec une masse de l’ordre de 125 GeV a été annoncée par les collaborations ATLAS et CMS. Une nouvelle ère s’annonce : celle au cours de laquelle il faudra déterminer précisément les propriétés de cette nouvelle particule. Cela est crucial afin d’établir si cette particule est bien la trace du mécanisme responsable de la brisure de la symétrie du secteur électro-faible. Cela permettrait aussi de repérer tout élément susceptible d’être associé à une "nouvelle physique" dans le cas où le mécanisme de brisure ferait intervenir des ingrédients autres que ceux prédits par le Modèle Standard. Dans cette thèse, nous avons essayé de comprendre et de caractériser jusqu’à quel point ce nouveau champ scalaire est le boson de Higgs prédit par le Modèle Standard de la physique des particules. Nous avons établi les implications d’une telle découverte dans le contexte de théories supersymétriques et de modèles décrivant la matière noire.

Un "pot de thèse" sera organisé après la soutenance dans la cafétéria.

  Jeudi 12 juin à 16h, salle 114

Brando Bellazzini (CEA Saclay) : Universal Sum Rules for EFTs

I present universal sum rules for scattering amplitudes in theories invariant under an arbitrary symmetry group. The sum rules relate the coefficients of the low-energy expansion of the scattering amplitudes in the IR to total cross sections integrated all the way up to the UV. For EFTs, the amplitudes in the IR are calculable and hence the sum rules set non-trivial constraints on the parameters of the effective Lagrangian. I present explicit examples that are relevant for WW-scattering, composite Higgs models, and chiral perturbation theory. Certain sum rules based on custodial symmetry, and its extensions, provide constraints on the Higgs boson coupling to the electroweak gauge bosons.

  Jeudi 5 juin à 16h, salle 114

Roberto Vega (Southern Methodist University) : A Non-minimal Supersymmetric Model with Custodial Higgs Triplets

The measurement of a Higgs mass at around 126 GEV has renewed interest on non-minimal supersymmetric models. This work pertains in particular to models with Higgs triplets. Generally, in these models a Higgs mass heavier than the Z-mass is easily accommodated without the need of large radiative corrections. However, models with a single hypercharge Higgs triplet violate the custodial SU(2) symmetry and thus suffer from fine-tuning of the $\rho$ parameter at tree level. In this work we consider a supersymmetric extension of the Georgi-Machachek (GM) model , a custodially invariant model with two Higgs triplet representations with no fine tuning at tree level. However, the GM model suffers from quadratic divergences at the one loop level. We find that in the supersymmetric extension of the GM model (SGM) the quadratic divergent corrections to $\rho$ do indeed cancel. Unlike the GM model, the model naturally provides a natural scale at which to impose custodial invariance. Furthermore, as the model is evolved via RGE’s from the SUSY breaking scale down to the EW scale we reproduce EW symmetry breaking and agreement with the measured value of the Higgs mass for a wide range of input parameters. In particular the SUSY breaking scale can be as low as 1 TeV and as high as 500 TeV and still satisfy the Higgs mass constraints.

SOUTENANCE DE HDR

  Jeudi 3 avril à 14h30, Amphi 1

Benoît Blossier (LPT Orsay) : On the first-principles determination of the Standard Model fundamental parameters in the quark sector

The 2-years old observation at LHC of a new boson, with a mass of 126 GeV, is a great achievement. Its interpretation as a Brout-Englert-Higgs boson is very plausible and appealing to complete the zoology of fundamental particles in the Standard Model. The interplay between theorists and experimentalists that we have witnessed has come with a huge work to determine with enough precision the parameters of the Standard Model : couplings, masses, mixing angles. Among the various tools developed by physicists, lattice QCD is particularly suitable to know those parameters in the quark sector. Indeed, the confinement of quarks in hadrons makes the analytical approaches, though very educative, intrinsically limited in a proper estimate of systematic errors from their uncontrolled treatment of long-distance dynamics of QCD, that is involved in low-energy processes like hadrons decays or hadron to hadron transitions. In this talk I will discuss the lattice measurement of Standard Model fundamental parameters that are closely related to Higgs boson : its main production mode is the gluon-gluon fusion, whose the magnitude is governed by the strong coupling constant, while its most favored decay channel, $H \to b \barb$, has a coupling proportional to the $b$ quark mass. I will outline the improvements brought by the lattice community : simulations with $N_f=2+1+1$ dynamical quarks are crucial to study how much the charm quark impacts the strong coupling constant calculation. A non perturbative matching between Heavy Quark Effective Theory and QCD is welcome to handle in an appropriate way the $b$ quark physics. An extensive methodological exploration is necessary to get rid of contribution from excited states in correlation functions computed to extract hadron masses and decay constants.

  Mardi 1er avril à 16h, salle 110 (salle de cosmologie)

Ivan Nisandzic (Institute "Jozef Stefan" Ljubljana, Slovenia) : Searching Beyond the Standard Model in $b\to c l \nu$ transitions

There is a two years old BaBar’s hint of disagreement with the Standard Model predictions for branching ratios in the decays $B \to D^(*) \tau \nu$. We study, in a model independent way, the angular distributions that could reveal the effects not seen in the total decay rates. We also note some of the possible interpretations of the disagreement between theory and experiments in terms of particular New Physics models.

  Jeudi 27 mars à 14h30, salle 114

Giancarlo Rossi (Université de Rome 2) : Dynamical Fermion Mass Generation

Taking inspiration from lattice QCD data, we argue that a finite non-perturbative mass contribution for quarks is generated as a consequence of the dynamical phenomenon of spontaneous chiral symmetry breaking, in turn triggered by the explicitly breaking of chiral symmetry induced by the critical Wilson term in the action. In pure lattice QCD this mass term cannot be separated from the unavoidably associated linearly divergent contribution. However, if QCD is enlarged to a theory where also a scalar field is present, coupled to a doublet of SU(2) fermions via a Yukawa and a Wilson-like term, then in the phase where the scalar field takes a non-vanishing expectation value, a dynamically generated and "naturally" light fermion mass (numerically unrelated to the expectation value of the scalar field) is conjectured to emerge at a critical value of the Yukawa coupling where the symmetry of the model is maximally enhanced. Masses dynamically generated in this way display a natural hierarchy according to which the stronger is the strongest of the interactions the fermion is subjected to the larger is its mass.

  Mardi 25 mars à 16h, salle 114

Francesco SANFILIPPO (University of Southampton) : Recent Lattice QCD results relevant for flavour physics phenomenology

Forty years after their introductions, lattice Quantum Chromodynamics (QCD) methods are becoming more and more reliable tools to study the nonperturbative regime of strong interactions, whose features are particularly important in many sectors of the flavour phenomenology of the Standard Model (SM). After long and painstaking efforts, thanks to striking hardware and algorithmically evolution the methods entered an era in which all the main systematics effects affecting typical calculation can be kept under control. The increasing reliability of lattice QCD results make it a more and more powerful tool to make precision tests of flavour sector of SM. In the present talk I will illustrate how recent improvements are helping in providing tighten tests of flavour sector of SM, with particular attention to processes involving heavy hadrons, such as $D\toK\ell\nu$, $J/\psi\to e^+e^-$ and the radiative decays of charmonia system, discussing their relevance for the research of New Physics beyond the SM, both in the contest of the Unitarity Triangle analysis and besides of it.

  Jeudi 20 mars à 16h, salle 114

Aoife Bharucha (TUM Munich) : Electroweakinos, dark matter and the LHC

The LHC has been tremendously successful in pushing the limits on SUSY particle (sparticle) masses towards the TeV scale, particularly for coloured sparticles. As the production of electroweak sparticles (electroweakinos) is suppressed, the LHC is in principle less sensitive to them. However, as the bounds on the coloured sector become stronger, these channels become increasingly interesting. Moreover, constraints on the SUSY electroweak sector are particularly important for the study of dark matter (DM), as the lightest neutralino is a potential DM candidate. I will first present NLO results for chargino-neutralino production at the LHC, and discuss how the recent experimental limits from ATLAS and CMS, which are presented in terms of simplified models, can require careful interpretation. Monojet+missing transverse energy searches have attracted a lot of attention recently due to the close connection to DM, and I will further present results for the projected limits on the mass of the neutralino from LHC14.

  Jeudi 6 mars à 16h, salle 114

Paolo Panci (IAP Paris) : New Directions in Direct Dark Matter Searches

Direct searches for Dark Matter (DM) aim at detecting the nuclear recoils arising from a scattering between DM particles and target nuclei in underground detectors. Since the physics that describes the collision between DM particles and target nuclei is deeply non-relativistic, in the first part of this seminar I’ll review a different and more general approach to study signal in direct DM searches based on the formalism of non-relativistic operators. Then I’ll present the main observables and the experimental landscape pointing out all the uncertainties that enter in this field. Finally, since the underlying relativistic theory that describes both the DM and standard model fields is unknown, in the last part of the seminar I’ll be focussed on the uncertainties coming from the nature of the interaction. On a more specific level, it will be divided in two parts : i) a model independent part, in which I’m going to present a new method and a self-contained set of numerical tools to the derive the bounds from some current direct detection experiments in a completely model independent way ; ii) a model dependent part, in which I’ll review the different phenomenology of “non-standard" DM models characterized by a long-range DM-nucleus interaction.

  Jeudi 27 février à 16h, salle 114

Robert Ziegler (LPTHE Jussieu) : Relating the SUSY Flavor Problem to the SM Flavor Puzzle

The absence of any significant deviations from the SM in flavor observables requires new physics at the TeV scale to have a highly non-trivial flavor structure. This suggests a relation to the SM flavor puzzle, i.e. the hierarchical structure of fermion masses and mixing. After a short introduction to the SUSY flavor problem I will discuss two possible solutions that were presented in arXiv:1304.1453 and arXiv:1308.1090. In the first case a deformation of gauge mediation with new messenger-matter couplings controlled by a simple flavor symmetry leads to strong suppression of sfermion flavor violation. In the second case a SU(2)xU(1) flavor model allows to neatly explain the quark Yukawa structure and gives rise to a natural SUSY spectrum in which only the sfermions are light that are required by Naturalness.

  Jeudi 20 février à 16h, salle 114

Giuliano Panico (CERN) : Light generations partners at the LHC

I discuss the phenomenological implications of a large degree of compositeness for the light generation quarks in minimal composite Higgs models. In particular I will focus my attention on the scenarios where the right-handed up-type quarks have a sizable mixing with the composite dynamics. Depending on the quantum numbers of the partners interesting collider signatures are expected in channels with EW bosons, Higgses and jets in the final state. The current ALTAS and CMS analyses can be used to put some direct constraints on the mass of the partners. These bounds are particularly stringent in "universal" flavor scenarios but can be significantly relaxed if the partners of the various generations are non-degenerate.

  Jeudi 13 février à 16h, salle 114

Lorenzo Calibbi (Brussels University) : Probing light Neutralino Dark Matter at the LHC

I will discuss the current status of the light neutralino dark matter scenario within the minimal supersymmetric standard model (MSSM) taking into account latest results from the LHC. Constraints from the dark matter relic abundance lead us to a manageable simplified model defined by a subset of MSSM parameters, within which we could reinterpret recent searches for electroweak supersymmetric particle production based on events with multi-tau plus missing transverse momentum, performed by the ATLAS collaboration. In this way we could derive stringent constraints on the light neutralino parameter space. In combination with further experimental information from the LHC, we obtained a lower bound on the lightest neutralino that is stronger than any current limit set by underground direct dark matter searches or indirect detection experiments.

  Jeudi 6 février à 16h, salle 114

Filippo Sala (IPhT Saclay) : Higgs and flavour physics implications for almost natural models

I will first discuss how the Higgs couplings measurements affect the phenomenology of the next-to-minimal supersymmetric standard model (NMSSM), with an emphasis on the searches for the extra CP-even scalar bosons. The MSSM will also be considered for comparison. Then I will discuss how the many flavour data impact on U(3)^3 and U(2)^3 flavour symmetric models, with a focus on their supersymmetric realizations and on the interplay of collider and flavour searches.

  Jeudi 30 janvier à 16h, salle 114

Michel Fontannaz (LPT Orsay) : Prompt Photon Physics

This talk is a review of the prompt photon physics from the seventies to the LHC. I will discuss NLO and NNLO (for diphoton pairs) calculations and the detailed comparisons between theory and data. The recent LHC data are impressive and we can consider that the goal to test QCD in large-pt photon reactions is achieved, at least at the NLO accuracy. Prompt photon pairs are now a background for new physics signals.

  Jeudi 23 janvier à 16h, Amphi 1

Emilie Passemar (Los Alamos) : Probing lepton flavour violation in the Higgs sector with hadronic tau decays

With the discovery of a new boson with a mass around 125 GeV so far compatible with the standard model Higgs, we have entered a new era in the understanding of the electroweak symmetry breaking mechanism. Searches for lepton flavour violating (LFV) effects at the LHC associated to the Higgs sector offer an interesting possibility to test for new physics scenarios. Low-energy constraints still allow for sizable flavour-changing decay rates of the 125 GeV boson into leptons, h -> l (l = e, mu). In this talk, we discuss the special role played by hadronic tau-lepton decays in this quest and in particular by tau -> l pi pi decays. A remarkable feature is that at low energy the effective Higgs coupling to gluons induced by heavy quarks contributes to hadronic tau-decays establishing a direct connection with the relevant process at the LHC, pp(gg) -> h -> l nu. Finally, we will explore the discriminating power of hadronic tau decays on probing LFV in models with an extended Higgs sector.

  Jeudi 16 janvier à 16h30, salle 114

Sylvain Fichet (UFRN Natal) : Anomalous gauge couplings from composite Higgs and warped extra dimensions

We examine trilinear and quartic anomalous gauge couplings (AGCs) generated in composite Higgs models and models with warped extra dimensions. We first revisit the SU(2)L x U(1)Y effective Lagrangian and derive the charged and two-photon neutral AGCs. We derive the general perturbative contributions to the pure field-strength operators from spin 0, 1/2, 1 resonances by means of the heat kernel method. In the composite Higgs framework, we derive the pattern of expected deviations from typical SO(N) embeddings of the light composite top partner. We then study a generic warped extra dimension framework with AdS5 background, recasting in few parameters the features of models relevant for AGCs. We also present a detailed study of the latest bounds from electroweak and Higgs precision observables, with and without brane kinetic terms. For vanishing brane kinetic terms, we find that the S and T parameters exclude KK gauge modes of the RS custodial [non-custodial] scenario below 7.7 [14.7] TeV, for a brane Higgs and below 6.6 [8.1] TeV for a Pseudo Nambu-Goldstone Higgs, at 95% CL. These constraints can be relaxed in presence of brane kinetic terms. The leading AGCs are probing the KK gravitons and the KK modes of bulk gauge fields in parts of the parameter space. In these scenarios, the future ATLAS forward proton detectors could be sensitive to the effect of KK gravitons in the multi-TeV mass range.

  Jeudi 5 décembre à 16h, salle 114

Daniel Stolarski (CERN) : Stop Searches and Natural SUSY

Searches at the LHC for flavor blind supersymmetry have placed very stringent bounds on such models. In this talk, I will explore the concept of natural supersymmetry where only the superpartners required for naturalness are at the TeV scale. This motivates a direct search for stops. I will then discuss different strategies to search for stops including the one outlined in arXiv:1205.5816, which proposes searching for direct stop production using jet substructure. Finally, I will review current LHC bounds and give projections for future runs of the LHC and higher energy colliders as outlined in arXiv:1309.1514

  Jeudi 14 novembre à 16h, salle 114

Mark Goodsell (LPTHE Jussieu) : Exploring Dirac Gaugino GUTs

I will discuss how a constrained minimal Dirac gaugino GUT model can be constructed, discuss the constraints upon it, its predictions and show the results of a first exploration of its parameter space.

  Jeudi 31 octobre à 16h, salle 114

Sébastian Descotes-Genon (LPT Orsay) : Résultats récents autour de la désintégration B\rightarrow K^\ast \mu^+\mu^-

La désintégration B\rightarrow K^\ast \mu^+\mu^- constitue un processus particulièrement intéressant. Ce courant neutre changeant la saveur est source d’une phénoménologie particulièrement riche, avec de nombreuses observables accessibles (en particulier) auprès de l’expérience LHCb. De plus, ce processus peut être analysé dans le cadre indépendant de modèle du Hamiltonien effectif, permettant une séparation élégante entre processus de courte distance et contributions hadroniques. Des résultats récents obtenus à LHCb suggèrent que certaines observables dévient par rapport aux attentes du Modèle Standard dans la région de grand recul du K^\ast. Après avoir rappelé l’intérêt de ces observables et les éléments nécessaires à leur analyse (analyse angulaire, facteurs de forme, factorisation), je discuterai ces déviations en terme de contributions de Nouvelle Physique. La combinaison de ces résultats avec les désintégrations radiatives b\rightarrow s \gamma suggère de manière cohérente une contribution importante de Nouvelle Physique pour l’opérateur effectif O_9. Je discuterai aussi les contributions du Modèle Standard susceptibles d’imiter un tel phénomène.

  Jeudi 17 octobre à 16h, salle 114

Marina Marinkovic (University of Southampton) : Computation of the strong coupling in N_f =4 quantum chromodynamics

Lattice discretisation and numerical simulations are well established tools for the computation of input parameters of perturbative QCD in terms of low energy hadronic observables. Using the Schroedinger functional strategy for these computations enables us to overcome the multi-scale problem in QCD and keep full control over the systematic errors. In this talk we will present a non-perturbative computation of the running of N_f=4 QCD coupling with four flavours of O(a) improved Wilson fermions and Wilson plaquette action. We check that cutoff effects in the step scaling function are weak by taking one more resolution compared to a previous computation. Finally, we shall show an improved result of the Lambda parameter in four flavour QCD in the units of hadronic scale Lmax.

  Jeudi 3 octobre à 16h, salle 114

Roberto Vega Morales (LPT Orsay) : Extracting Effective Higgs Couplings in the Golden Channel at the LHC

In this talk I will present an overview of the Higgs Golden Channel and a strategy for using it to extract the effective Higgs couplings to neutral electroweak bosons. In particular I will discuss how we construct the full 8 dimensional likelihood, based on analytic expressions of the fully differential cross section, which utilizes the full kinematic information available in the Higgs decay and includes all the relevant detector effects.