### ATTENTION : Séminaire de dernière minute !

**Lundi 23 avril à 14h**, salle 114

Léa Gauthier (CEA Saclay) : **Studies in the same-sign dilepton channel of the BSM top**

The same sign dilepton channel is a good probe for BSM physics at LHC due
to its very low background in the SM and this final state is expected in a
wide variety of new physics model. Moreover, it wasn’t very studied by
precision measurement at LEP or research of resonance at Tevatron.

Two analysis are presented here :

— a theoretical analysis with 4 tops final state : fourtop production is
a spectacular final state and a sensitive probe of new physics. Examples
are models where the top quark is composite or where a new heavy particle
couples strongly or exclusively to top quarks.

— an experimental analysis with an extradimentional model which includes
top partner particles. The ATLAS analysis is presented here as well the
mass limit of the fourth generation quark.

**Lundi 19 mars à 14h**, salle 114

Antonin Coutant (LPT) : **Hawking radiation for dummies**

Since the work of Bekenstein, it is well understood than black hole are
thermodynamical objects that posses their own entropy. One of the main
confirmations is obtained when taking into account quantum effects, and
concluding that they radiate as black body at definite temperature.
I will present the effect of black hole radiation. For pedagogical
reasons, I will follow the derivation of Unruh, presented right after the
historical discovery of Hawking. I will try to avoid technicalities in
order to focus on physical concepts, and provide a physically motivated
reason for this phenomenon. If time does not flow too fast, I will discuss
some open issues concerning the validity of this calculation.

**Lundi 12 mars à 14h**, salle 114

Cédric Lorcé (LPT/IPNO) : **Transverse charge densities and hadron internal structure**

According to textbooks, electromagnetic form factors are related to
charge densities via a three-dimensional Fourier transform. This
connection is however only valid in the non-relativistic limit. A
relativistic interpretation of the form factors can however be obtained
in frames where the target is moving with almost the speed of light (and
therefore looks like a pancake) via a two-dimensional Fourier transform.
I will present the transverse charge densities of some light hadrons and
discuss their structure in terms of quark motion. Then I will show that,
in absence of any internal structure, natural values for the
electromagnetic multipole moments arise, providing important contraints
on any theory of elementary higher-spin particles coupled to
electromagnetism.

**Lundi 27 Février à 14h**, salle 114

Fabien Nugier (LPTENS) : **Gauge invariant light-cone averaging of the luminosity-redshift relation in inhomogeneous cosmology**

One theoretical problem in cosmology which has gained more importance this
last decade, as cosmological observations have grown more precise, is the
averaging problem. The description of the Universe as a homogeneous and
isotropic model (FLRW) now needs to be reconsidered in order to unveil
more refined (and possibly measurable) phenomena affecting the
determination of cosmological parameters.
In this talk I will first recall a recent proposal for a gauge invariant
averaging procedure on lightlike hypersurfaces (in particular on the past
light-cone of a geodesic observer) together with that of adapted
“geodesic light-cone” coordinates. I will then give a concrete application
of this formalism to the luminosity-distance redshift relation in the
presence of a stochastic spectrum of inhomogeneities of inflationary
origin. I will then conclude on the current limitations and prospects
offered by this work and briefly discuss the link to observations.

**Lundi 20 Février à 14h**, salle 114

Cedric Weiland (LPT) : **Lepton Flavour Violation in the Supersymmetric Inverse Seesaw Model**

Neutrino oscillations have provided indisputable evidence for flavour
violation in the neutral lepton sector and non-zero neutrino masses. Many
mechanisms exist to generate these masses that differ by the type of
neutrinos (Dirac or Majorana) and the effective operators they produce at
low energy. In the absence of any fundamental principle that prevents
charged lepton flavour violation, one expects that extensions of the
Standard Model (SM) accommodating neutrino masses and mixings should also
allow for lepton flavour violation (LFV) in the charged lepton sector
with processes such as radiative (e.g. μ → e γ) and three-body lepton
decays (for instance τ → μμμ). These decays generally arise from higher
order processes, and so their branching ratios (Brs) are expected to be
small, making them difficult to observe. Thus, any cLFV signal would
provide clear evidence for new physics : mixings in the lepton sector and
probably the presence of new particles, possibly shedding light on the
origin of neutrino mass generation.

**Lundi 6 Février à 14h**, salle 114

Yannis Bardoux (LPT) : **Junction conditions in General Relativity**

You are certainly familiar with the junction conditions for
electromagnetism across any surface. During this seminar we propose to
establish the analog conditions in General Relativity : considering a
spacetime composed of two regions seperated by a hypersurface, what
conditions must be put on the metrics to joined smoothly these regions ?

First of all, General Relativity will be presented in a nutshell. Then a
toolkit in order to describe a timelike hypersurface will be given by
introducing the notions of induced metric and extrinsic curvature. In this
way we will be able to derive the junction conditions. Finally we will
present two nice applications of this formalism : the gravitational
collapse to a black hole given by Oppenheimer and Snyder in 1939 and the
collapse of a thin spherical shell of matter.

Les relations de passage en électromagnétisme à travers une surface vous
sont certainement familières. Durant ce séminaire, nous proposons
d’établir les conditions analogues en Relativité Générale : considérant un
espace-temps composé de deux régions séparées par une hyper-surface,
quelles conditions doit-on imposer sur les métriques pour recoller ces
régions ?
Tout d’abord, la Relativité Générale sera présentée en quelques mots. Puis
les outils nécessaires à la description d’une hypersurface de genre temps
seront donnés en introduisant les notions de métrique induite et de
courbure extrinsèque. De cette manière, nous serons en mesure d’établir
les conditions de jonctions. Finalement nous présenterons deux belles
applications de ce formalisme : l’effondrement gravitationnel en un trou
noir donné par Oppenheimer et Snyder en 1939 et l’effondrement d’une fine
coquille sphérique de matière.

**Vendredi 27 Janvier à 11h**, salle 114

Axel Orgogozo (LPTENS) : **Higgsless models of strong EWSB and EWPT Technicolor vs Conformal Technicolor**

I will describe a generic Higgsless model of strong EWSB. I will study the effect of spin-1 resonances, arising from the strong dynamics, on the S and T parameters. For this, I will build an effective field theory including the first axial and vector resonances and will assume Vector Meson Dominance. I will constraint the parameter space by requiring a good UV behaviour of a set of form factors and by imposing elastic unitarity in WW scattering. Those constraints will allow me to go beyond NDA estimates of the S and T parameters.
In this framework, I will explain how one can recover the fact that minimal Technicolor is in disagreement with EWPT. Finally, I will show how the situation change if the strong dynamics reach a strongly coupled fixed point (Conformal Technicolor). In this case one can show that one of the constraints, the 2nd Weinberg sum rule, doesn’t hold anymore, leaving some freedom in the parameter space. I will show that this freedom allows to get a good fit to EWPT.

**Vendredi 13 Janvier à 11h**, salle 114

Vincent Durand (APC/CEA Irfu) : **The Double Chooz reactor neutrino experiment**

The Double Chooz experiment consists of two identical detectors,
measuring the flux of electron anti-neutrinos coming from the nuclear
reactors of Chooz in the French Ardennes. The goal is to measure the
unknown leptonic parameter theta13. One of the two detectors, the
farthest, sensitive to theta13, is already taking data since April 2011.
The other detector, the nearest, aiming at monitoring the reactor
anti-neutrino flux, is under construction and is expected to be
delivered in 2013.
This talk will present the experiment, its concept and detection method
as well as its first results on theta13.

**Vendredi 16 décembre à 11h**, salle 114

Gabriel Charles (Irfu, SPhN) : **Micromegas detectors and particle physics, nuclear physics, astrophysics...**

After a presentation of the gaseous particle detectors, we will particularly study the Micromegas (micro mesh gaseous detector). Thus we will see
that the R&D carried out on Micromegas is of major interest in various fields such as research for dark matter, neutrinos oscillation or forest fire detection. Then, following a concret example we will detail the steps needed for a theoretical result to become a result.

**Vendredi 25 novembre à 11h**, salle 114

Julien Sabin ( Laboratoire de Mathématiques Analyse Géométrie Modélisation, Université de Cergy Pontoise) : **Création statique de paires électron/positron dans un modèle non-
linéaire relativiste**

Je vais présenter un modèle à plusieurs corps relativiste qui est une
approximation de champ moyen de l’Electrodynamique Quantique, indépendante du
temps et négligeant les photons. Ce modèle permet notamment de construire
rigoureusement le vide relativiste polarisé par un champ électrique extérieur.
Dans ce cadre, je présenterai une estimation de la probabilité de créer une
paire électron/positron par un champ électrique extérieur fort, qui peut être
vue comme une version statique de l’effet Schwinger.

**Jeudi 17 novembre à 11h**, salle 114

Caroline Barbara Bräuninger (TU München) : **Flavour in Warped Extra Dimensions**

The idea that space-time might have more than the four dimensions we
experience in our daily lives dates back to the work of Kaluza & Klein in
the 1920s. Fallen into oblivion, this idea was revived in the 1970s with
the advent of String Theory which predicts more than four space-time
dimensions. In phenomenology, extra dimensions experienced a second
revival with the seminal papers of Randall & Sundrum and Arkani-Hamed,
Dimopoulos & Dvali in the late 1990s.
We will focus here on the model of Randall & Sundrum, based on a
ﬁve-dimensional Anti-de-Sitter space-time wedged in between two
four-dimensional branes of ﬂat space-time. In the original model, only the
graviton can propagate into the extra-dimension while all other particles
are conﬁned to one of the 4d branes, the so-called IR-brane. Due to the
AdS-metric all energy scales of the ﬁve-dimensional theory are "warped
down" on the IR-brane that we live on. This solves the hierarchy problem,
i.e. the question of why the fundamental scale of gravity, the Planck
scale, is so much bigger than the scale of electroweak symmetry breaking
that is explored at the LHC.
Since higher dimensional operators are also suppressed by a warped down
energy scale only, the original model suffers from proton decay. This
problem can be solved by letting fermions and gauge bosons propagate into
the extra-dimension as well. Moreover, the hierarchies of fermion masses
and mixing angles can then be explained by their wave functions’
localisation in the extra dimension. In the seminar we will see how this
works exactly. Unfortunately, the rates of ﬂavour changing neutral
currents (FCNCs) are in conﬂict with experiment in these models. In our
recent work, my colleagues and me developed a model where FCNCs are
sufﬁciently suppressed.

**Vendredi 28 octobre à 11h**, salle 114

Julien Serreau (APC) : **Nonequilibrium quantum field theory : Theory and applications**

I review the description of nonequilibrium quantum field theory (QFT)
based on the use of two-particle-irreducible (2PI) functional techniques.
The latter resum the secular divergences of real-time perturbation theory
and have provided an important breakthrough in describing the
time-evolution of nonequilibrium quantum fields. After a general
presentation of nonequilibrium QFT and 2PI techniques, I describe the most
recent theoretical developments concerning e.g. 2PI renormalization, or
the formulation of 2PI techniques for gauge theories. I also review
various applications such as the study of thermalization and, more
recently, of quantum decoherence in QFT, or the application of these
techniques to QFT in expanding geometries.

**Vendredi 21 octobre à 11h**, salle 114

Adrien Renaud (LAL) : **Search for scalar gluons at the LHC**

Scalar gluons - or sgluons - are color octet scalars without electroweak
charges. They occur in
supersymmetric models of Dirac gauginos as the scalar partners of the
gluino and carry Standard-
Model type R charge. This allows them to interact with ordinary matter
and to be produced at the
LHC, singly as well as in pairs. Sgluons dominantly decay into gluons,
top pairs, squark pairs or
gluino pairs depending on the mass hierarchy. This talk presents a
dedicated search in the four jets
final state with the ATLAS detector focusing on the data-driven
background estimation and the limit
setting procedure.

**Vendredi 7 octobre à 11h15**, salle 114

Wahb Ettoumi (LPP) : **Stochastic Treatment of finite-N fluctuations in the approach towards
equilibrium for mean field models**

The lifetime of metastable states is a topical issue in out-of-equilibrium
statistical mechanics. The intricate interplay between non-integrable
Hamiltonian dynamics and equilibrium statistical ensemble predictions has
given rise to numerous studies aiming to characterize the length of such
states, mainly depending on the number of degrees of freedom. Only
approximate scalings have been suggested so far, but none of them is
actually derived from a rigorous calculation or a proper modeling. We
confront here the results of a novel stochastic approach overtaking the
non-integrability with numerical simulations. Our method successfully
predicts the behaviour of non-trivial quantities, leading to proper
estimations of metastable states’ lifetimes, which improve the
already-known heuristic expectations.

**Vendredi 16 septembre à 16h30**, salle 114

Christophe Hugon, CENBG : **Surprise Sinje**

**Vendredi 23 septembre à 11h**, salle 114

Julien Baglio, LPT Orsay : **Looking for the Higgs at the hadron colliders : from the Standard Model to SuperSYmmetry**

The Higgs boson, relic of the spontaneous electroweak symmetry
breaking, is one of the most important searches at current hadron colliders. This seminar [thesis] wishes to present theoretical predictions for the production and decay of the Higgs boson(s), either in the context of the Standard Model (SM) or in its minimal supersymmetric extension (MSSM) where five Higgs bosons are present. We will focus on the two current hadron colliders, the Fermilab Tevatron collider and the CERN Large Hadron Collider (LHC) which is running at 7 TeV. We will focus on the dominant production and decay channels and conduct a thorough analyzis of the theoretical uncertainties affecting these predictions : the scale uncertainties which probe our ignorance of the higher—order terms in a fixed order perturbative calculation, the parton distribution functions (PDF) uncertainties and its related uncertainties from the value of the strong coupling constant, and the uncertainties coming from the use of an effective field theory to simplify the hard calculation. We will give the implications on experimental searches and in particular revisit the inferred limits on SM Higgs boson mass and on the MSSM parameter space. Some perspectives for futur work will be given in the end.