The topography of the environment alters the optimal search strategy for active particles

In environments with scarce resources, adopting the right search strategy can make the difference between succeeding and failing, even between life and death. At different scales, this applies to molecular encounters in the cell cytoplasm, to animals looking for food or mates in natural landscapes, to rescuers during search-and-rescue operations in disaster zones, as well as to genetic computer algorithms exploring parameter spaces. When looking for sparse targets in a homogeneous environment, a combination of ballistic and diffusive steps is considered optimal; in particular, more ballistic L\'evy flights with exponent {\alpha} <= 1 are generally believed to optimize the search process. However, most search spaces present complex topographies, with boundaries, barriers and obstacles. What is the best search strategy in these more realistic scenarios? Here we show that the topography of the environment significantly alters the optimal search strategy towards less ballistic and more Brownian strategies. We consider an active particle performing a blind search in a two-dimensional space with steps drawn from a L\'evy distribution with exponent varying from {\alpha} = 1 to {\alpha} = 2 (Brownian). We demonstrate that the optimal search strategy depends on the topography of the environment, with {\alpha} assuming intermediate values in the whole range under consideration. We interpret these findings in terms of a simple theoretical model, and discuss their robustness to the addition of Brownian diffusion to the searcher's motion. Our results are relevant for search problems at different length scales, from animal and human foraging to microswimmers' taxis, to biochemical rates of reaction

Brownian Motion in a Speckle Light Field: Tunable Anomalous Diffusion and Deterministic Optical Manipulation

The motion of particles in random potentials occurs in several natural phenomena ranging from the mobility of organelles within a biological cell to the diffusion of stars within a galaxy. A Brownian particle moving in the random optical potential associated to a speckle, i.e., a complex interference pattern generated by the scattering of coherent light by a random medium, provides an ideal mesoscopic model system to study such phenomena. Here, we derive a theory for the motion of a Brownian particle in a speckle and, in particular, we identify its universal characteristic timescale levering on the universal properties of speckles. This theoretical insight permits us to identify several interesting unexplored phenomena and applications. As an example of the former, we show the possibility of tuning anomalous diffusion continuously from subdiffusion to superdiffusion. As an example of the latter, we show the possibility of harnessing the speckle memory effect to perform some basic deterministic optical manipulation tasks such as guiding and sorting by employing random speckles, which might broaden the perspectives of optical manipulation for real-life applications by providing a simple and cost-effective technique

Longterm Influence of Inertia on the Diffusion of a Brownian Particle

We demonstrate experimentally that a Brownian particle is subject to inertial effects at long time scales. By using a blinking optical tweezers, we extend the range of previous experiments by several orders of magnitude up to a few seconds. The measured mean square displacement of a freely diffusing Brownian particle in a liquid shows a deviation from the Einstein-Smoluchowsky theory that diverges with time. These results are consistent with a generalized theory that takes into account not only the particle inertia but also the inertia of the fluid surrounding the particle. This can lead to a bias in the estimation of the diffusion coefficient from finite-time measurements. We show that the decay of the relative error is polynomial and not exponential and, therefore, can have significant effects at time scales relevant for experiments.Comment: 5 pages, 4 figure

Search for CP violation in the lepton sector

One of the major open issues in neutrino physics is the possible existence of CP violation in the neutrino sector. Such an observation would have an important impact in various domains of physics, from high energy physics to cosmology. Its search requires future accelerator neutrino facilities producing intense and pure neutrino beams such as "beta-beams". Here we review the different beta-beam scenarios proposed so far and discuss the present status, with a particular emphasis on the original baseline scenario and its feasibility. Alternative strategies for the CP violation search are to be pursued as well. A possibility is to search for CP violation effects in astrophysical environments. Here we present recent analytical and numerical results obtained in the context of core-collapse supernovae. In particular, we point out the conditions under which there can be CP violating effects in dense media and show numerical results on the supernova (anti-)neutrino fluxes and on the electron fraction, relevant for the r-process nucleosynthesis.Comment: 12 page

Recent advances in neutrino astrophysics

Neutrinos are produced by a variety of sources that comprise our Sun, explosive environments such as core-collapse supernovae, the Earth and the Early Universe. The precise origin of the recently discovered ultra-high energy neutrinos is to be determined yet. These weakly interacting particles give us information on their sources, although the neutrino fluxes can be modified when neutrinos traverse an astrophysical environment. Here we highlight recent advances in neutrino astrophysics and emphasise the important progress in our understanding of neutrino flavour conversion in media.Comment: Proceedings for the Symposium "Frontiers of Fundamental Physics 2014", July 15-18, Marseille, 8 pages, 1 figur

Beta-beams

Beta-beams is a new concept for the production of intense and pure neutrino beams. It is at the basis of a proposed neutrino facility, whose main goal is to explore the possible existence of CP violation in the lepton sector. Here we briefly review the original scenario and the low energy beta-beam. This option would offer a unique opportunity to perform neutrino interaction studies of interest for particle physics, astrophysics and nuclear physics. Other proposed scenarios for the search of CP violation are mentioned.Comment: 8 pages, 1 table, 5 figures, Proceedings of "13th Lomonosov Conference on Elementary Particle Physics

Neutrino flavour conversion and supernovae

We summarize the recent developments in our understanding of neutrino flavour conversion in core-collapse supernovae and discuss open questions.Comment: Proceedings to the "Eleventh Conference on the Intersections of Particle and Nuclear Physics (CIPANP2012)", May 29 to June 3, Florida, 8 pages, 2 figure