Jeudi 16 avril à 14h, salle 114
Noel Martin (IPNO, Université Paris-Sud 11) : Collectives modes in the neutron star inner crust
Neutron stars are relevant macroscopic laboratories for nuclear physics. Actually they are dense because of their large mass - about the mass of the sun - and small diameter - about 10 km. The density is high enough to strongly displace the beta-equilibrium to the neutron production. The nuclei become too much neutron rich to keep all nucleons bound, thus the excess is liberated in the medium as a dilute gas. Now nuclear clusters are supposed to be deformed and immersed in a neutron gas. Many works expect exotic phases called : pasta phases with spaghetti and lasagna, respectively cylinders and planes of bound nucleons. Electrons, neutrons and clusters vibrate and contribute to the star cooling mechanism. Where temperature is one of the rare neutron star properties observable from earth.
Jeudi 5 mars à 14h, salle 114
Chieh-An Lin (SAp, CEA Saclay) : Cosmology with weak gravitational lensing
Abstract : Weak gravitational lensing is the deflection effect by gravitational potentials while light travels from distanced sources to Earth. This deflection is a powerful tool to probe the matter distribution and to constrain the physics of dark energy. Cosmologists expect that lensing studies can help us improve the knowledge about our Universe, in particular its history and composition.
I will start my presentation with an introduction on modern cosmology for people who are not familiar with it. Then I will talk about weak lensing observables and their formalism. The presentation will go on with challenges and difficulties that one might encounter for lensing studies. And finally, I will present some state-of-art techniques and the perspectives on lensing for next 10 years.
Mardi 10 février à 14h, salle 114
Martino Borsato (LAL, Univ. Paris-Sud 11) : Photon polarization in b ?s ? from an
angular analysis of B0 ?K*0 e+e- in the low-q² region
Abstract : The B0 —> K*0 e+e- decay is a flavour changing neutral current process
that is mediated by electroweak box and loop diagrams in the Standard Model (SM).
Its angular distribution is particularly sensitive to contributions from non-SM
physics (NP). In particular, in the region of large recoil (low q²), some
theoretical uncertainties from long distance contributions are greatly reduced
and the contribution from the virtual photon coupling to the lepton pair
dominates (C7 Wilson coefficient). This allows a measurement of the helicity of
the photon in b ?s ? transitions, which is predominantly left-handed in the SM, but
receives large right handed contributions in many NP models.
I will present the latest result on this subject from the LHCb experiment
(arXiv:1501.03038) which sets new stringent limits on this kind of NP
contributions. I will cover in detail the experimental methods that were used to
carry out an angular analysis of this rare decay channel in the difficult hadron
collider environment of LHC.
Mardi 27 janvier à 14h, salle 114
Pierre Ronceray (LPTMS, Univ. Paris-Sud 11) : How active stress emerges in
biological elastic media
In contrast with ordinary materials, living matter drives its own
motion by generating active contractile internal stresses. These stresses
typically originate from localized active elements embedded in an elastic
medium, such as molecular motors inside the cell or contractile cells in a
tissue. I will present a new exact result connecting the distribution of
active forces to the emergent macroscopic stress in linear elastic media.
This very general result reveals two distinct sources for macroscopic
stress : local forces and material non-linearity. I will present an
application of these results to the contractility of the actomyosin cortex
beneath the cell membrane, revealing the key role of buckling
Mardi 09 décembre à 14h, salle 114
Frederic Dreyer (LPTHE) : Jet Physics and the Small-R Limit
As hadron collider physics continues to push the boundaries of precision, it becomes increasingly important to have methods for predicting properties of jets across a broad range of jet radius values R, and in particular for small R.
In this presentation we will start with a brief review of jet physics at hadron colliders, and introduce a method to resum all leading logarithmic terms, αs·log(R), in the limit of small R, for a wide variety of observables. These include the inclusive jet spectrum, jet vetoes for Higgs physics and jet substructure tools. We will examine and comment on the underlying order-by-order convergence of the perturbative series for different R values.
Vendredi 28 novembre à 14h, salle 114
Lucien Heurtier (CPHT, Ecole Polytechnique) : Introduction to the Physics of
Inflation, concepts and phenomenology
In this presentation I will cover basic aspects of Inflation theories. I
will start from the ground by motivating the concept from experimental and
theoretical puzzles and build step by step the most simple inflationary
scenarios. This will include discussion about the recent claims of the
BICEP2 collaboration. Main issues and present prospects of the theory of
inflation will also be exposed.