Centre de Physique Théorique


Mars 2019

Mercredi 6 mars 14:00-15:00, Amphi 5 du CPT

Social Interactions and Collective States in Fish Schools

Clément Sire (LPT, Toulouse)


The flexible coordination of individuals’ movements ensures rapid and coherent
changes in direction of travel of fish schools for instance as a reaction to a
predator detected in the neighbourhood. However the ’microscopic level’
interaction rules involved in the coordination of fish movements and the
adapted collective response of a school still remain to a large extent
unknown. Knowing such interaction rules could offer new sources of inspiration
to design distributed control algorithms for swarms of drones. Here we present
a systematic methodology to measure and analyze social interactions
controlling the collective motion of animal groups. Contrary to classical
forces between physical objects, social interactions between individuals
explicitly depend on their relative headings and are affected by their
anisotropic and asymmetric perception of their environment. Hence they
strongly break the Newtonian’s law of action-reaction. When applied to fish
groups, this approach leads to the quantitative measurement of the spontaneous
behaviour of a fish, of its avoidance interaction with the tank walls, and of
its attraction and alignment interaction with another fish. We use the results
of this analysis to build an explicit and faithful model that convincingly
reproduces quantitative and qualitative features of the actual fish dynamics.
We also show that the type of models derived from such analysis reproduces the
main collective states observed in actual fish schools, when one varies the
intensity of the alignment and attraction interactions between fish.

Jeudi 7 mars 14:00-15:00, Amphi 5 du CPT

N-body chaos, noise, and instabilities and the continuum limit in self-gravitating systems

Pierfrancesco Di Cintio (CNR, Florence)


In this talk I will revisit the concept of N-body chaos in the continuum
limit of self-gravitating systems described by the collisionless
Boltzmann-Poisson equations. In particular, with the aid of N-body
simulations I will question the use of frozen N-body realizations to obtain
qualitative information on the N-body chaos in particle systems with large
N. Moreover, I will present some recent results concerning the so-called
Radial-orbit instability (ROI) that rule out N-body chaos as a source of
such process relevant for the dynamical evolution of elliptical galaxies.
Finally, I will discuss the effects of externally imposed noise and
friction on the onset of ROI.
The results presented here do not apply only to the case of
self-gravitating systems but can be extended also to other long-range
interacting systems such as for example non-neutral plasmas and charged
particle beams.

Vendredi 8 mars 14:00-15:00, Amphi 5 CPT


Debasish Banerjee

Mercredi 13 mars 14:00-15:00, Amphi 5 du CPT

Les systèmes en interactions répétées et la thermodynamique hors-équilibre

Jean-François Bougron (Institut Fourier, Grenoble)


Les systèmes interactions répétées ou SIR sont un cas particulier de système quantique ouvert qui interagissent successivement avec les maillons d’une chaîne de systèmes quantiques : ils servent à modéliser, entre autres, l’expérience du maser à un atome dans lequel un champ électromagnétique piégé dans une cavité est traversé par un faisceau d’atomes. L’étude mathématique des SIR initiée notamment par S. Attal, L. Bruneau, A. Joye, M. Merkli, Y. Pautrat et C.-A. Pillet depuis les années 2000, consiste à déterminer les propriétés d’un semigroupe discret d’applications complètement positives appliquées à une matrice densité qui modélise l’état initial du champ électromagnétique, dans le cas idéal où les atomes ont tous la même température et où l’interaction entre champ électromagnétique et atomes est identique pour chaque atome.

Dans le cas non-idéal, notamment où les températures sont différentes, on peut continuer à utiliser des dynamiques en semigroupe, modulo quelques hypothèses. Dans un article en préparation avec L. Bruneau, on montre que des résultats attendus de thermodynamique hors-équilibre sont vérifiés sous une certaine forme pour ces SIR non idéaux : on montre notamment une formule de Green-Kubo, une forme généralisée des relations de réciprocité d’Onsager et un théorème de fluctuations entropiques. Il s’agit d’une adaptation de la démarche décrite par Jaksic, Pillet et Westrich dans un article de 2014 sur les semigroupes dynamiques quantiques modélisant des systèmes quantiques ouverts faiblement couplés à plusieurs réservoirs thermiques. Une généralisation de ces résultats est en cours dans un travail en collaboration avec A. Joye et C.-A. Pillet.

Dans cet exposé, je tâcherai de rappeler un peu de formalisme quantique adapté aux SIR, avant d’expliciter les modèles étudiés et de présenter une partie des résultats obtenus.

Vendredi 15 mars 14:00-15:00, Amphi 5 CPT

Flavour physics in the D_(s) and B_(s) meson system

Tobias Tsang (U. of Edinburgh)

After a brief motivation, I will present two studies of our (domain wall fermion) charm and bottom physics program. In the first part I will focus on our recent computation (arXiv:1812.08791) of SU(3) breaking ratios in the $D_s$ and $B_s$ meson systems and the ratios of CKM matrix elements $V_cd/V_cs$ and $V_td/V_ts$.

In the second part I will describe our ongoing efforts to compute semi-leptonic $B_(s)$ decays (arXiv:1903.02100) before concluding by summarising RBC/UKQCD’s wider efforts in charm and bottom phenomenology.

Lundi 18 mars 14:00-15:00, CPPM Auditorium

Non-equilibrium effects in the evolution of Dark Matter

Andrzej Hryczuk (National Centre for Nuclear Research, Warsaw, Poland)

This is a joint CPT-CPPM seminar !

Among viable dark matter (DM) production mechanisms the thermal freeze-out stands out as the most natural and best motivated one. In the usual theoretical approach to the determination of thermal relic abundance the assumption of local thermal equilibrium is made. But is this assumption always justified ? In this talk I will first introduce the topic and then address this question. I will discuss more accurate treatments, one relying on the inclusion of higher moments of the Boltzmann equation and the second on solving the evolution of the phase space distribution function fully numerically. Examples where such improved treatment is necessary will be discussed, including DM annihilation below threshold, semi- or resonant annihilation.

Mercredi 20 mars 11:00-12:00, Amphi 5 du CPT

Effective field theories for phases of matter and cosmology

Alberto Nicolis (Columbia University)

I will review some modern applications of effective field theories outside their traditional particle physics domain. In particular, I will focus on spontaneous symmetry breaking for spacetime symmetries. The effective theories for the associated Goldstone excitations capture the low-energy/long-distance dynamics of a number of physical systems, from ordinary macroscopic media (solids, fluids, superfluids, supersolids) to more exotic cosmological ones

Mercredi 20 mars 14:00-15:00, Amphi 5 du CPT

Absence of Singular Spectrum for the Particles Bounded to an Infinite Planar Curve

J. Dittrich (Nuclear Physics Institute CAS, Rez, Czech Republic)


Non-relativistic quantum particles bounded to an infinite curve in the plane by
the attractive contact delta-interaction are considered. The interval between
the energy of the transversal bound state and zero is shown to belong
to the absolutely continuous spectrum, with possible embedded eigenvalues.
The curve is assumed smooth and non-intersecting, asymptotically
approaching two different half-lines. The proof is inspired by the proof
of Agmon-Kato-Kuroda theorem.