Cosmic voids detection without density measurements

Cosmic voids are effective cosmological probes to discriminate among competing world models. Their identification is generally based on density or geometry criteria that, because of their very nature, are prone to shot noise. We propose two void finders that are based on dynamical criterion to select voids in Lagrangian coordinates and minimise the impact of sparse sampling. The first approach exploits the Zel'dovich approximation to trace back in time the orbits of galaxies located in voids and their surroundings, the second uses the observed galaxy-galaxy correlation function to relax the objects' spatial distribution to homogeneity and isotropy. In both cases voids are defined as regions of the negative velocity divergence, that can be regarded as sinks of the back-in-time streamlines of the mass tracers. To assess the performance of our methods we used a dark matter halo mock catalogue CoDECS, and compared the results with those obtained with the ZOBOV void finder. We find that the void divergence profiles are less scattered than the density ones and, therefore, their stacking constitutes a more accurate cosmological probe. The significance of the divergence signal in the central part of voids obtained from both our finders is 60% higher than for overdensity profiles in the ZOBOV case. The ellipticity of the stacked void measured in the divergence field is closer to unity, as expected, than what is found when using halo positions. Therefore our void finders are complementary to the existing methods, that should contribute to improve the accuracy of void-based cosmological tests.Comment: 12 pages, 18 figures, accepted for publication in MNRA

Neutron irradiation test on ATLAS MDT chambers

Abstract The Monitored Drift Tubes (MDT) chambers of the ATLAS muon spectrometer are crucial for the identification of high-momentum final-state muons, which represent very promising and robust signatures of physics at the LHC. They will operate in a high rate and high background environment and therefore their performances should not significantly degrade for the whole ATLAS data taking. The maximum expected total flux, mainly consisting of neutrons and photons in the MeV range, is of the order of 5 kHz/cm 2 for the barrel MDTs, while at SLHC, with machine working at higher luminosity, fluxes can be 10 times higher. To test detector robustness, a MDT test chamber was exposed to intensive neutron irradiation at the TAPIRO ENEA-Casaccia Research Center facility

The relation between Lyman-alpha absorbers and gas--rich galaxies in the local universe

We use high-resolution hydrodynamical simulations to investigate the spatial correlation between weak (N(HI) <10^{15} cm^{-2}) Ly-alpha absorbers and gas-rich galaxies in the local universe. We confirm that Ly-alpha absorbers are preferentially expected near gas-rich galaxies and that the degree of correlation increases with the column density of the absorber. The real-space galaxy auto-correlation is stronger than the cross-correlation (correlation lengths r_{0,gg}=3.1 \pm 0.1 Mpc/h and r_{0,ag}=1.4 \pm 0.1 Mpc/h, respectively), in contrast with the recent results of Ryan-Weber (2006, RW06), and the auto-correlation of absorbers is very weak. These results are robust to the presence of strong galactic winds in the hydrodynamical simulations. In redshift-space a further mismatch arises since at small separations the distortion pattern of the simulated galaxy-absorber cross-correlation function is different from the one measured by RW06. However, when sampling the intergalactic medium along a limited number of lines-of-sight, as in the real data, uncertainties in the cross correlation estimates are large enough to account for these discrepancies. Our analysis suggests that the statistical significance of difference between the cross-correlation and auto-correlation signal in current datasets is ~ 1-sigma only.Comment: 11+ pages, 9 figures. MNRAS in pres

Dark Matter Annihilation around Intermediate Mass Black Holes: an update

The formation and evolution of Black Holes inevitably affects the distribution of dark and baryonic matter in the neighborhood of the Black Hole. These effects may be particularly relevant around Supermassive and Intermediate Mass Black Holes (IMBHs), the formation of which can lead to large Dark Matter overdensities, called {\em spikes} and {\em mini-spikes} respectively. Despite being larger and more dense, spikes evolve at the very centers of galactic halos, in regions where numerous dynamical effects tend to destroy them. Mini-spikes may be more likely to survive, and they have been proposed as worthwhile targets for indirect Dark Matter searches. We review here the formation scenarios and the prospects for detection of mini-spikes, and we present new estimates for the abundances of mini-spikes to illustrate the sensitivity of such predictions to cosmological parameters and uncertainties regarding the astrophysics of Black Hole formation at high redshift. We also connect the IMBHs scenario to the recent measurements of cosmic-ray electron and positron spectra by the PAMELA, ATIC, H.E.S.S., and Fermi collaborations.Comment: 12 pages, 7 figures. Invited contribution to NJP Focus Issue on "Dark Matter and Particle Physics

Measurement of Gamma(phi -> eta' gamma)/Gamma(phi -> eta gamma) and the pseudoscalar mixing angle

We have measured the radiative decays phi -> eta gamma, phi ->etaprime gamma selecting pi+ pi- gamma gamma gamma final state in a sample of about 5 times 10^7 phi mesons produced at the Frascati phi factory DAFNE. We obtain Gamma(phi -> etaprime gamma)/Gamma(phi -> eta gamma)=(4.70 +- 0.47 +- 0.31) times 10^-3. From this result we derive new accurate values for the branching ratio BR(phi ->etaprime gamma) = (6.10 +- 0.61 +- 0.43) times 10^-5, and the mixing angle of pseudoscalar mesons in the flavour basis phi_P=(41.8 +1.9 -1.6) degrees.Comment: Submitted to Phys. Lett.

Study of the Decay phi --> eta pi0 gamma with the KLOE detector

In a sample of 5.3x10^7 phi-decays observed with the KLOE detector at the Frascati phi-factory Dafne we find 605 eta pi0 gamma events with eta --> gamma\gamma and 197 eta pi0 gamma events with eta --> pi+ pi- pi0. The decay phi --> eta pi0 gamma is dominated by the process phi --> a0 gamma. From a fit to the eta pi0 mass spectrum we find BR(phi --> ao(980) gamma)= (7.4 +- 0.7)x10^-5.Comment: 12 pages, 6 figures, submitted to Phys.Lett.

Search for composite and exotic fermions at LEP 2

A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb^{-1} at an e^+e^- centre-of-mass energy of 183 GeV and about 20 pb^{-1} equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes 95% confidence level mass limits in the region between 70 GeV/c^2 and 90 GeV/c^2, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio of the coupling of the excited fermio