Laboratoire de Physique
Theorique d'Orsay

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Bât. 210
Univ. Paris-Sud
Université Paris-Saclay
91405 Orsay Cedex
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Agenda > Séminaires / Seminars > Particules Dernier ajout : jeudi 6 novembre 2014.

Séminaires Particules 2012-2013

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 : Adam Falkowski.

-  Vendredi 5 juillet à 11h, salle 114

Martin Hirsch (IFIC Valencia) : Double beta decay and the LHC

Neutrinoless double beta decay (0nbb) is mostly known as a sensitive probe for Majorana neutrino masses. However, since double beta decay is a process which violates lepton number, in principle all extensions of the standard model of particle physics which include lepton number violation are constrained by limits from double beta decay. Examples are left-right symmetric models or R-parity violating supersymmetry. After some fairly general introduction to neutrinoless double beta decay, this talks discusses the general decomposition of the d=9 0nbb decay operator and possible tests of the short-range part of the 0nbb decay amplitude at the LHC.

Soutenance de Thèse

-  Jeudi 4 juillet à 14h, Amphi 2

Cédric Weiland (LPT Orsay) : Effects of fermionic singlet neutrinos on high- and low-energy observables

In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios $R_K$ and $R_\pi$ provide new, additional constraints on the inverse seesaw. We have also c onsidered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and Z bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.

Soutenance de Thèse

-  Mardi 2 juillet à 14h, Amphi 1

Adrien Besse (LPT Orsay) : Helicity amplitudes of the diffractive exclusive leptoproduction of ρ meson in the high energy limit beyond the leading twist

One of the important challenge to unravel the structure of nucleons is to understand exclusive processes from the first principles of QCD. I discuss the high-energy exclusive leptoproduction of rho mesons, for both longitudinal and transversal polarizations. The theoretical approach presented in this thesis first relies on $k_T$-factorization which allows to factorize the $\gamma^* \to \rho$ transition, the so-called impact factor, which is then computed in perturbative QCD using the collinear factorization procedure. One of the first new result of this thesis is the demonstration how in the impact parameter space this approach is consistent with the dipole model picture at leading twist for the $\gamma^*L \to \rho L$ and at twist 3 for the $\gamma^*T \to \rho T$ transitions. Next, on the basis of above results, I propose a new phenomenological model for the polarized cross-sections, based on models for the dipole cross-section and a model for higher twist distribution amplitudes of the rho meson. The obtained predictions are then compared successfully to HERA data.

Attention : Séminaire à 16h30 et non 16h

-  Jeudi 27 juin 2013 à 16h30, salle 114

Giorgio Arcadi (University of Göttingen) : Minimal decaying DM and the LHC

I will discuss the prospects of correlation between DM Indirect detection and collider signals in the case of decaying dark matter. I will consider a very simple particle physics framework in which the SM is extended with a gauge singlet Majorana fermion, which is the DM candidate, and scalar field charged, at least partially, under the SM group, without imposing any symmetry for stabilizing the DM. The correct dark matter relic density can be achieved in two scenarios, corresponding to FIMP/SuperWIMP and WIMP generation mechanisms. I will identify, within these two scenarios,the regions of parameter space which can account for an Indirect DM signal in next future experiments and discuss possible collider signatures pointing out possible cases in which a combined detection, namely ID and collider, can occur.

-  Jeudi 13 juin 2013 à 16 heures, salle 114

Adrien Besse (LPT Orsay) : A model for high energy rho meson leptoproduction based on collinear factorization and dipole models

One of the important challenge to understand the structure of nucleons is to understand exclusive processes from the first principles of QCD. We discuss the high-energy exclusive leptoproduction of rho mesons, for both longitudinal and transversal polarizations. Our theoretical approach first relies on kT-factorization which allows to factorize the gamma*->rho transition, so-called impact factor, which is then computed in perturbative QCD using the collinear factorization procedure. We show how in the impact parameter space this approach is consistent with the dipole model picture at leading twist for the gamma*L->rhoL and twist 3 for the gamma*T->rhoT transitions. Finally, we present a phenomenological model for the polarized cross-sections based on models for the dipole cross-section and a model for higher twist distribution amplitudes of the rho meson. We compare our predictions with HERA data and discuss our results.

-  Jeudi 6 juin 2013 à 16 heures, salle 114

Vittorio LUBICZ (Università Roma Tre) : Isospin breaking effects on the lattice

Isospin symmetry is an almost exact property of the strong interactions which is only violated by the small difference between the up and down quark masses and by their different electric charges. Nowadays, with the increasing precision of the experimental determinations of many physical quantities, and in some cases with the improvement of the theoretical predictions, the control over isospin breaking effects is becoming phenomenologically relevant. In this talk, I present a method to evaluate on the lattice the leading isospin breaking effects due to both quark masses and QED interactions. Results are obtained for several quantities, including a determination of the up and down quark masses, the charged/neutral pion and kaon mass splittings, the meson decay constants, the difference between the neutron and the proton mass.

-  Jeudi 30 mai 2013 à 16 heures, salle 114

Jose Ramon Espinosa (IFAE Barcelona) : Stability of the Electroweak Vacuum after LHC

The discovery of the Higgs boson, and especially the determination of its mass around 125 GeV, at the LHC together with the absence of any trace of new physics make it conceivable that we live in a metastable (but long-lived) electroweak vacuum. I will describe the state-of-the-art calculation that leads to this conclusion and elaborate on possible implications as well as cures of this instability of the Higgs potential.

-  Jeudi 16 mai 2013 à 16 heures, salle 114

Cédric Weiland (LPT Orsay) : Impact of fermionic singlets on lepton universality tests

A fundamental consequence of the gauge structure of the Standard Model (SM) is the universality of the coupling constants. The high experimental precision achieved by lepton universality tests offers a unique opportunity to probe deviations from the SM.  We consider a tree-level enhancement to the violation of lepton flavour universality in meson decays arising from modified W\ell \nu couplings in the Standard Model minimally extended by fermionic singlets (present in many SM extensions). Due to the presence of additional mixings between the active neutrinos and the new sterile states, the deviation from unitarity of the leptonic mixing matrix might lead to a tree-level enhancement of, for instance, R_{P} = \Gamma (P \to e \nu) / \Gamma (P \to \mu \nu), with P=K, \pi. We illustrate these enhancements in the case of the inverse seesaw model, showing that one can saturate the current experimental bounds on \Delta r_{K} (and \Delta r_{\pi}), while complying with different experimental and observational constraints.

-  Jeudi 2 mai 2013 à 16 heures, salle 114

Siavash Neshatpour (LPC Clermont-Ferrand) : B \to K^* l l implications on the CMSSM

The rare B \to K^* l l decay provides a large number of observables for probing New Physics. In this talk we will present an overview of the theoretical framework for this decay. We then address the implications of recent LHCb results for B \to K^* l l observables. In the absence of significant deviations from the Standard Model predictions, the constraining power of different observables on the Constrained Minimal Supersymmetric Standard Model (CMSSM) is shown.

-  Jeudi 25 avril 2013 à 16 heures, salle 114

Bartjan van Tent (LPT Orsay) : Cosmological results from Planck in 2013 : overview of main results and special focus on non-Gaussianity

The Planck satellite was launched in 2009 to observe the cosmic microwave background (CMB) temperature and polarization anisotropies. Regarding temperature it was designed to be the ultimate full-sky CMB experiment : limited only by cosmic variance and astrophysical foregrounds, not by detector noise or systematic effects. Recently Planck released the first 15 months of temperature data and many accompanying papers with analyses and results. Even with just 15 months of data and the current status of the analysis, Planck has already reached or even surpassed many of its design goals. In this talk I will discuss the Planck space mission and several of its main results. In the second part of the talk I will focus in particular on the Planck non-Gaussianity results.

-  Jeudi 11 avril 2013 à 16 heures, salle 114

Francesco Riva (EPFL Lausanne) : Higgs Compositeness Tests

In this talk I review models in which the Higgs arises as a composite scalar from a strongly interacting sector. The observed value of the Higgs mass suggests that, if the Higgs is composite, it is a naturally light Nambu-Goldstone boson of a broken global symmetry (similar to QCD pions). I’ll show how this implies measurable modification in the Higgs couplings. Furthermore, simple arguments based on sum-rules can be used to connect the observed value of the Higgs mass with the mass of the lightest resonances of the strong sector. I’ll review searches for these resonances which, thanks to this connection, represent the ultimate test for compositeness.

-  Jeudi 4 avril 2013 à 16 heures, salle 114

Nicolás Wschebor (Universidad de la República, Montevideo) : Do gluons have a mass ?

In recent years a big effort have been done both with Monte-Carlo simulations and with (semi) analytical methods in order to understand non gauge invariant correlation functions in Quantum Cromodynamics (QCD). It will be explained why such objects have been studied despite of their unphysical nature. After that, the main results for such correlation functions in Landau gauge will be presented. It will be presented the surprising property that such correlation functions show a mass-like behavior for gluons. It will be shown how this feature can be exploited in order to develop a successful perturbation theory for QCD that reproduces correctly those correlation functions even in the low momentum regime.

-  Jeudi 28 mars 2013 à 16 heures, salle 114

Thomas Schwetz (MPI Heidelberg) : Dark Matter direct detection : phenomenology, status, and astrophysics-independent methods

The search for galactic Dark Matter particles scattering off nuclei in underground detectors is a very active field of research. I review the phenomenology of Dark Matter direct detection and give an overview of the experimental status. I will argue that the current and upcoming generation of experiments is probing a region highly motivated by the WIMP hypothesis. In the second half of the talk I will discuss the ambiguities in the interpretation of possible signals due to uncertainties in the local Dark Matter distribution. I will present methods to compare results of different experiments largely independent of such uncertainties and apply it comparing the DAMA/LIBRA signal for annual modulation to the bounds from other experiments.

-  Mardi 26 mars 2013 à 16 heures, salle 114

Jack Gunion : Diagnosing the Nature of the 125-126 GeV LHC Higgs-like signal

-  Jeudi 21 mars 2013 à 16 heures, salle 114

Christoph Luhn (IPPP Durham) : Abelian and non-Abelian discrete symmetries in models of flavor

Discrete symmetries play a crucial role in physics beyond the Standard Model. Focusing on supersymmetric models which aim at explaining the family structure of the quarks and leptons, I first discuss how discrete Abelian symmetries such as e.g. R-parity can emerge from an underlying U(1) Froggatt-Nielsen family symmetry. The observation of large and very peculiar mixing angles in the neutrino sector motivates the idea a non-Abelian discrete family symmetry. I review their implementation in supersymmetric model building, and discuss the implications of the recent measurement of theta_13 of about 9 degrees by the Daya Bay and RENO collaborations.

-  Jeudi 21 février 2013 à 16 heures, salle 114

Marco Cirelli (IPhT Saclay) :Dark Matter Indirect Searches : hints and constraints

I present a phenomenological, data-centered and data-driven assessment of the status of indirect searches for particle Dark Matter, and of the main implications for DM model building. Tantalizing hints and stringent constraints seem to continuosly chase each other, as a number of well performing experiments keep delivering their data. This year may be the one in which we finally see the resolution of these dark puzzles, or it may not.

-  Jeudi 14 février 2013 à 16 heures, salle 114

David Straub (THEP, Mainz) : Flavour and electroweak constraints on composite Higgs models

Composite Higgs models with partial compositeness lead to tree-level corrections to electroweak precision observables and flavour-changing neutral currents. I will present an overview of electroweak, flavour and collider constraints on this class of models, considering several choices for the electroweak representations, comparing flavour-symmetric models to flavour-anarchic ones and identifying the least fine-tuned cases and their prospects.
-  Jeudi 7 février 2013 à 16 heures, salle 114

Svjetlana Fajfer (J. Stefan Institute, Ljubljana) : Colored scalars and Higgs physics

Colored scalars might appear in extensions of the Standard model. These states can modify Higgs physics. Current experimental constraints on the rate of h → γγ require such colored states to have an order one coupling to the Standard Model Higgs and a mass below 300 GeV. The best fit values are used to predict the correlated effect in h → Zγ and di-Higgs production. These states appear in simple SU(5) setups to show that these two full-fledged models not only accommodate a light color octet state but correlate its mass with observable partial proton decay lifetimes.

-  Jeudi 31 janvier 2013 à 16 heures, salle 114

Roberto Salerno (LLR) : Higgs searches at CMS

Results are summarized from searches for the standard model Higgs boson in pp collisions at sqrts = 7 and 8 TeV in the CMS experiment at the LHC. The measurements of mass, cross section, and main properties of the new boson recently observed at a mass near 125 GeV are reported. In view of the presented results the next steps are outlined.

-  Jeudi 24 janvier 2013 à 16 heures, salle 114

Sylvain Fichet (LPSC, Grenoble) : 5d mass RG flow and radiative localization

We report on a study about quantum corrections to scalar and fermion 5d masses in flat compact spaces in a Wilsonian approach. We set up a formalism in the mixed 5d position/4d momentum space. We calculate the generic corrections induced by spin 0, 1/2, 1 fields on 5d masses. Various quantum features might be useful to build mechanisms for beyond-Standard-Model physics. Among other things, we focus on the phenomenon of radiative localization of light modes.

-  Vendredi 21 décembre à 11h

Séminaire commun LAL/LPT, auditorium Pierre Lehmann au LAL

Jean-Baptiste de Vivie : Higgs boson searches at the LHC : From discovery to property measurements

-  Jeudi 13 décembre 2012 à 16 heures, salle 114

Ruggero Ferrari (Milano) : Metamorphosis versus Decoupling in Nonabelian Gauge Theories at Very High Energies

Massive Yang-Mills gauge theory can be achieved by Higgs mechanism, but also through a nonlinear realization of the gauge group. I compare the two mechanisms for very large energy. For the Higgs mechanism there is a metamorphosis of the longitudinal modes into the "former" Goldstone bosons, which are upgraded to physical modes (equivalence theorem). In the nonlinear realization a phase transition line in the parameter space (g,M) is envisaged. By crossing the line the longitudinal modes decouple and perturbation theory in extreme kinematical configurations is a massless gauge theory. A lattice gauge model supports this scenario.

-  Jeudi 29 novembre 2012 à 16 heures, salle 114

Subhadip Mitra (LPT Orsay) : Color octet electrons at the LHC

In this talk I shall discuss the phenomenology of color octet electrons at the LHC. Color octet electrons are a type of leptogluon that appears in the quark-lepton compositeness models. I shall discuss various production processes of these particles at the LHC and present some estimations of the discovery potential of a generic color octet electron at the LHC.

-  Jeudi 15 novembre 2012 à 16 heures, salle 114

Christ Wymant (IPPP Durham) : Hitting a Natural Higgs in Susy

In supersymmetric models a large average stop mass is well known to both boost the lightest Higgs boson mass and also make radiative electroweak-symmetry breaking unnaturally tuned. The case of ’maximal mixing’ of the stops allows them to be as light as possible for a given Higgs mass. Here we make the distinction between minimal stop mass and optimal naturalness, and calculate the latter. The splitting between the two stop mass eigenvalues is shown to be unconstrained by naturalness considerations. We introduce the NMSSM and explain its potential utility and limitations for boosting the Higgs mass naturally. We touch upon exotic Higgs decays and their searches in the NMSSM and beyond.

-  Jeudi 8 novembre 2012 à 16 heures, salle 114

Nuria Carrasco Vela (Universitat de Valencia) :

K^0-\bar K^0, D^0-\bar D^0, and B^0-\bar B^0 oscillations from lattice QCD

Flavour Changing Neutral Currents (FCNC) and CP violation may provide relevant information on the impact of beyond the Standard Model (BSM). The lattice computation of the relevant matrix elements appearing in K^0-\bar{K}^0 and B^0-\bar{B}^0 mixing in combination with the experimental value of \epsilon_K, \Delta M_{Bd} and \Delta M_{Bs} offers the opportunity to constrain the BSM model parameters. In contrast, D^0-\overline{D}^0, at variance with K^0-\bar{K}^0 and B^0-\bar{B}^0 mixing, involves up-type quarks so it is sensitive to a different sector of NP. In this seminar I will motivate phenomenologically the computation on the lattice of the bag parameters controlling the neutral meson mixing. Since discretization errors on current lattices are expected to be large at the physical value of b-quark mass, our strategy in the B-sector is based on the ratio method approach which will also be discussed.

-  Jeudi 18 octobre 2012 à 16 heures, salle 114

Avelino Vicente (LPT) : Supersymmetric seesaw models after the Higgs discovery

Motivated by the discovery of a Higgs-like boson with a mass around 125 GeV, and by the need of explaining neutrino masses, we analyze the three canonical SUSY versions of the seesaw mechanism (type I, II and III) with CMSSM boundary conditions. In type II and III cases, SUSY particles are lighter than in the CMSSM (or the constrained type I seesaw), for the same set of input parameters at the universality scale. Thus, to explain mh0 125 GeV at low energies, one is forced into regions of parameter space with very large values of m0 , M1/2 or A0. We compare the squark and gluino masses allowed by the ATLAS and CMS ranges for mh0 and discuss the possibility of distinguishing seesaw models in view of future results on SUSY searches. In particular, we briefly comment on the discovery potential of LHC upgrades, for squark/gluino mass ranges required by present Higgs mass constraints. We also take into account the MEG upper limit on Br(μ → e γ) and show that, in some cases, this may help to restrict the SUSY parameter space, as well as to set complementary limits on the seesaw scale.

-  Jeudi 11 octobre 2012 à 16 heures, salle 114

Roberto Barceló (LPT) : t-tbar Forward-Backward Asymmetry, Massive Gluons and Heavy Quarks

The large coupling of the top quark to the EW symmetry breaking sector implies that the new physics stabilizing the latter could also appear in top-quark observables. This argument makes the 2-3 sigma deviation versus the standard value in the Tevatron FB asymmetry specially interesting. Even if it is not statistically significant at the level of discovery, the consistency among different CDF and D0 measurements strengthens the case for new physics. However, any candidate responsible for the asymmetry has to be carefully disguised, as its large contribution there should not translate into any significant departure from the SM in other related observables. In particular, the tt total cross section, its invariant-mass distribution, dijet production, or the tt charge asymmetry at the LHC are observables where correlated anomalies could be expected. In this talk I will start reviewing the status of the Tevatron asymmetry. Then I will motivate the framework that we propose to explain the asymmetry : a 700-900 GeV gluon of very large width caused by new decay channels. In particular, heavy quarks strongly coupled to the gluon will be introduced in the model. Finally, I will show a complete analysis of the model in order to reveal the best strategy for its observation at the LHC.

-  Jeudi 27 septembre 2012 à 16 heures, salle 114

Grégory Soyez (IPhT Saclay) : Pile-up subtraction for jet pt, mass and shapes

After a brief introduction on jets and how they are defined at the LHC, I will focus on the effects of soft contamination from pile-up, namely, multiple pp interactions during a single bunch crossing. I will show how jet-area-based techniques, currently used at the LHC, allow to correct for that contamination in a wide number of cases : the jet transverse momentum and more recently, the jet mass and jet shapes. I will also discuss briefly the case of heavy-ion collisions where similar techniques apply.

-  Jeudi 6 septembre 2012 à 16 heures, salle 114

Abhishek Iyer (Indian Institute of Science, Bangalore) : Lepton Masses and Flavour Violation in Randall Sundrum Model

Lepton masses and mixing angles via localization of 5D fields in the bulk are revisited in the context of Randall-Sundrum models. The Higgs is assumed to be localized on the IR brane. Three cases for neutrino masses are considered : (a) The higher dimensional LH.LH operator (b) Dirac masses (c) Type I see-saw with bulk Majorana mass terms. Neutrino masses and mixing as well as charged lepton masses are fit in the first two cases using chi-square minimisation for the bulk mass parameters, while varying the O(1) Yukawa couplings between 0.1 and 4. Lepton flavour violation is studied for all the three cases. It is shown that large negative bulk mass parameters are required for the right handed fields to fit the data in the LHLH case. This case is characterized by a very large Kaluza-Klein (KK) spectrum and relatively weak flavour violating constraints at leading order. The zero modes for the charged singlets are composite in this case and their corresponding effective 4-D Yukawa couplings to the KK modes could be large. For the Dirac case, good fits can be obtained for the bulk mass parameters, c , lying between 0 and 1. However, most of the "best fit regions" are ruled out from flavour violating constraints. In the bulk Majorana terms case, we have solved the profile equations numerically. We give example points for inverted hierarchy and normal hierarchy of neutrino masses. Lepton flavor violating rates are large for these points. We then discuss various minimal flavor violation (MFV) schemes for Dirac and bulk Majorana cases. In the Dirac case with MFV hypothesis, it is possible to simultaneously fit leptonic masses and mixing angles and alleviate lepton flavor violating constraints for Kaluza-Klein modes with masses of around 3 TeV. Similar examples are also provided in the Majorana case.