Heavy flavour production in the semi-muonic channel in pp and Pb-Pb collisions measured with the ALICE experiment

International audienceThe properties of the high-density medium formed in heavy-ion collisions at LHC can be investigated through heavy-quark production. These quarks are created in the initial hard collision processes with short formation time and are expected to lose energy while passing through the high-density strongly-interacting system. A complete understanding of heavy flavour production mechanisms in heavy-ion collisions requires the study of their production in proton-proton collisions. We will describe the ALICE muon spectrometer and then present the results on the production of single muons from heavy flavour decays at forward pseudo-rapidity (2.5 < eta < 4) in proton-proton collisions at s^1/2 = 7 TeV and Pb-Pb collisions at s^1/2_NN = 2.76 TeV. In particular, we will show the pt-differential production cross-section in proton-proton collisions and compare this to the perturbative QCD predictions as well as the nuclear modification factors R_AA and R_CP in Pb-Pb collisions

RbR_b in supergravity models

We compute the supersymmetric contribution to $R_{b}\equiv \Gamma (Z\to b{\bar b})/\Gamma (Z\to {\rm hadrons})$ in a variety of supergravity models. We find R^{\rm susy}_b\lsim0.0004, which does not shift significantly the Standard Model prediction ($R^{\rm SM}_b=0.2162$ for m_t=160\GeV). An improvement in experimental precision by a factor of four would be required to be sensitive to such an effect.Comment: 5 pages, LaTeX, 1 figure (included)

Electronic and optical properties of InAs(110)

The electronic and optical properties of the cleavage InAs(110) surface are studied using a semi-empirical tight-binding method which employs an extended atomic-like basis set. We describe and discuss the electronic character of the surface electronic states and we compare with other theoretical approaches, and with experimental observations. We calculate the surface electronic band structure and the Reflectance Anisotropy Spectrum, which are described and discussed in terms of the surface electronic states and the atomic structure.Comment: 15 pages + 4 figures, submitted to PR

On the Correlation between the Magnetic Activity Levels, the Metallicities and the Radii of Low-Mass Stars

The recent burst in the number of radii measurements of very low-mass stars from eclipsing binaries and interferometry of single stars has opened more questions about what can be causing the discrepancy between the observed radii and the ones predicted by the models. The two main explanations being proposed are a correlation between the radius of the stars and their activity levels or their metallicities. This paper presents a study of such correlations using all the data published to date. The study also investigates correlations between the radii deviation from the models and the masses of the stars. There is no clear correlation between activity level and radii for the single stars in the sample. Those single stars are slow rotators with typical velocities v_rot sini < 3.0 km s^-1. A clear correlation however exists in the case of the faster rotating members of binaries. This result is based on the of X-ray emission levels of the stars. There also appears to be an increase in the deviation of the radii of single stars from the models as a function of metallicity, as previously indicated by Berger et al. (2006). The stars in binaries do not seem to follow the same trend. Finally, the Baraffe et al. (1998) models reproduce well the radius observations below 0.30-0.35Msun, where the stars become fully convective, although this result is preliminary since almost all the sample stars in that mass range are slow rotators and metallicities have not been measured for most of them. The results in this paper indicate that stellar activity and metallicity play an important role on the determination of the radius of very low-mass stars, at least above 0.35Msun.Comment: 22 pages, 4 figures. Accepted for publication on Ap