7 research outputs found
Distance traveled by random walkers before absorption in a random medium
We consider the penetration length of random walkers diffusing in a
medium of perfect or imperfect absorbers of number density . We solve
this problem on a lattice and in the continuum in all dimensions , by means
of a mean-field renormalization group. For a homogeneous system in , we
find that , where is the absorber density
correlation length. The cases of D=1 and D=2 are also treated. In the presence
of long-range correlations, we estimate the temporal decay of the density of
random walkers not yet absorbed. These results are illustrated by exactly
solvable toy models, and extensive numerical simulations on directed
percolation, where the absorbers are the active sites. Finally, we discuss the
implications of our results for diffusion limited aggregation (DLA), and we
propose a more effective method to measure in DLA clusters.Comment: Final version: also considers the case of imperfect absorber
Self-gravitating Brownian systems and bacterial populations with two or more types of particles
We study the thermodynamical properties of a self-gravitating gas with two or
more types of particles. Using the method of linear series of equilibria, we
determine the structure and stability of statistical equilibrium states in both
microcanonical and canonical ensembles. We show how the critical temperature
(Jeans instability) and the critical energy (Antonov instability) depend on the
relative mass of the particles and on the dimension of space. We then study the
dynamical evolution of a multi-components gas of self-gravitating Brownian
particles in the canonical ensemble. Self-similar solutions describing the
collapse below the critical temperature are obtained analytically. We find
particle segregation, with the scaling profile of the slowest collapsing
particles decaying with a non universal exponent that we compute perturbatively
in different limits. These results are compared with numerical simulations of
the two-species Smoluchowski-Poisson system. Our model of self-attracting
Brownian particles also describes the chemotactic aggregation of a
multi-species system of bacteria in biology
Phase transitions in self-gravitating systems and bacterial populations surrounding a central body
International audienceWe study the nature of phase transitions in a self-gravitating classical gas in the presence of a central body. The central body can mimic a black hole at the center of a galaxy or a rocky core (protoplanet) in the context of planetary formation. In the chemotaxis of bacterial populations, sharing formal analogies with self-gravitating systems, the central body can be a supply of ``food'' (chemoattractant). We consider both microcanonical (fixed energy) and canonical (fixed temperature) descriptions and study the inequivalence of statistical ensembles. At high energies (resp. high temperatures), the system is in a ``gaseous'' phase and at low energies (resp. low temperatures) it is in a condensed phase with a ``cusp-halo'' structure, where the cusp corresponds to the rapid increase of the density of the gas at the contact with the central body. For a fixed density of the central body, we show the existence of two critical points in the phase diagram, one in each ensemble, depending on the core radius : for small radii , there is no phase transition at all. We study how the nature of these phase transitions changes as a function of the dimension of space. We also discuss the analogies and the differences with phase transitions in the self-gravitating Fermi gas [P.H. Chavanis, Phys. Rev. E 65, 056123 (2002)]
Contribution of germline deleterious variants in the RAD51 paralogs to breast and ovarian cancers
International audienceRAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3) have recently been involved in breast and ovarian cancer predisposition: RAD51B, RAD51C, and RAD51D in ovarian cancer, RAD51B and XRCC2 in breast cancer. The aim of this study was to estimate the contribution of deleterious variants in the five RAD51 paralogs to breast and ovarian cancers. The five RAD51 paralog genes were analyzed by next-generation sequencing technologies in germline DNA from 2649 consecutive patients diagnosed with breast and/or ovarian cancer. Twenty-one different deleterious variants were identified in the RAD51 paralogs in 30 patients: RAD51B (nâ=â4), RAD51C (nâ=â12), RAD51D (nâ=â7), XRCC2 (nâ=â2), and XRCC3 (nâ=â5). The overall deleterious variant rate was 1.13% (95% confidence interval (CI): 0.72â1.55%) (30/2649), including 15 variants in breast cancer only cases (15/2063; 0.73% (95% CI: 0.34â1.11%)) and 15 variants in cases with at least one ovarian cancer (15/570; 2.63% (95% CI: 1.24â4.02%)). This study is the first evaluation of the five RAD51 paralogs in breast and ovarian cancer predisposition and it demonstrates that deleterious variants can be present in breast cancer only cases. Moreover, this is the first time that XRCC3 deleterious variants have been identified in breast and ovarian cancer cases