A Statistical Model to Explain the Mendel--Fisher Controversy

In 1866 Gregor Mendel published a seminal paper containing the foundations of modern genetics. In 1936 Ronald Fisher published a statistical analysis of Mendel's data concluding that "the data of most, if not all, of the experiments have been falsified so as to agree closely with Mendel's expectations." The accusation gave rise to a controversy which has reached the present time. There are reasonable grounds to assume that a certain unconscious bias was systematically introduced in Mendel's experimentation. Based on this assumption, a probability model that fits Mendel's data and does not offend Fisher's analysis is given. This reconciliation model may well be the end of the Mendel--Fisher controversy.Comment: Published in at http://dx.doi.org/10.1214/10-STS342 the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

Optimal diffusion in ecological dynamics with Allee effect in a metapopulation

How diffusion impacts on ecological dynamics under the Allee effect and spatial constraints? That is the question we address. Employing a microscopic minimal model in a metapopulation (without imposing nonlinear birth and death rates) we evince --- both numerically and analitically --- the emergence of an optimal diffusion that maximises the survival probability. Even though, at first such result seems counter-intuitive, it has empirical support from recent experiments with engineered bacteria. Moreover, we show that this optimal diffusion disappears for loose spatial constraints.Comment: 16 pages; 6 figure

Invisible Z decay width bounds on active-sterile neutrino mixing in the (3+1) and (3+2) models

In this work we consider the standard model extended with singlet sterile neutrinos with mass in the eV range and mixed with the active neutrinos. The active-sterile neutrino mixing renders new contributions to the invisible Z decay width which, in the case of light sterile neutrinos, depends on the active-sterile mixing matrix elements only. We then use the current experimental value of the invisible Z decay width to obtain bounds on these mixing matrix elements for both (3+1) and (3+2) models.Comment: 10 pages, 5 figure

Spinwave damping in the two-dimensional ferromagnetic XY model

The effect of damping of spinwaves in a two-dimensional classical ferromagnetic XY model is considered. The damping rate $\Gamma_{q}$ is calculated using the leading diagrams due to the quartic-order deviations from the harmonic spin Hamiltonian. The resulting four-dimensional integrals are evaluated by extending the techniques developed by Gilat and others for spectral density types of integrals. $\Gamma_{q}$ is included into the memory function formalism due to Reiter and Solander, and Menezes, to determine the dynamic structure function $S(q,\omega)$. For the infinite sized system, the memory function approach is found to give non-divergent spinwave peaks, and a smooth nonzero background intensity (``plateau'' or distributed intensity) for the whole range of frequencies below the spinwave peak. The background amplitude relative to the spinwave peak rises with temperature, and eventually becomes higher than the spinwave peak, where it appears as a central peak. For finite-sized systems, there are multiple sequences of weak peaks on both sides of the spinwave peaks whose number and positions depend on the system size and wavevector in integer units of $2\pi/L$. These dynamical finite size effects are explained in the memory function analysis as due to either spinwave difference processes below the spinwave peak or sum processes above the spinwave peak. These features are also found in classical Monte Carlo -- Spin-Dynamics simulations.Comment: 20 two-column page

Searching for new thermally emitting isolated neutron stars in the 2XMMp catalogue - Discovery of a promising candidate

The group of 7 thermally emitting and radio-quiet isolated neutron stars (INSs) discovered by ROSAT constitutes a nearby population which locally appears to be as numerous as that of the classical radio pulsars. So far, attempts to enlarge this particular group of INSs finding more remote objects failed to confirm any candidate. We found in the 2XMMp catalogue a handful of sources with no catalogued counterparts and with X-ray spectra similar to those of the ROSAT discovered INSs, but seen at larger distances and thus undergoing higher interstellar absorptions. In order to rule out alternative identifications such as an AGN or a CV, we obtained deep ESO-VLT and SOAR optical imaging for the X-ray brightest candidates. We report here on the current status of our search and discuss the possible nature of our candidates. We focus particularly on the X-ray brightest source of our sample, 2XMM J104608.7-594306, observed serendipitously over more than four years by the XMM-Newton Observatory. A lower limit on the X-ray to optical flux ratio of ~ 300 together with a stable flux and soft X-ray spectrum make it the most promising thermally emitting INS candidate. Beyond the finding of new members, our study aims at constraining the space density of this population at large distances and at determining whether their apparently high local density is an anomaly or not.Comment: 3 pages, 2 figures, proceedings of the conference "40 Years of Pulsars", 12-17 August 2007, Montreal, Canad