Guido Barbiellini

Observations of MilkyWay Dwarf Spheroidal galaxies with the Fermi-LAT detector and

We report on the observations of 14 dwarf spheroidal galaxies with the Fermi Gamma-Ray Space Telescope taken during the first 11 months of survey mode operations. The Fermi telescope provides a new opportunity to test particle dark matter models through the expected gamma-ray emission produced by pair annihilation of weakly interacting massive particles (WIMPs). Local Group dwarf spheroidal galaxies, the largest galactic substructures predicted by the cold dark matter scenario, are attractive targets for such indirect searches for dark matter because they are nearby and among the most extreme dark matter dominated environments. No significant gamma-ray emission was detected above 100 MeV from the candidate dwarf galaxies. We determine upper limits to the gamma-ray flux assuming both power-law spectra and representative spectra from WIMP annihilation. The resulting integral flux above 100 MeV is constrained to be at a level below around 10^-9 photons cm^-2 s^-1. Using recent stellar kinematic data, the gamma-ray flux limits are combined with improved determinations of the dark matter density profile in 8 of the 14 candidate dwarfs to place limits on the pair annihilation cross-section of WIMPs in several widely studied extensions of the standard model. With the present data, we are able to rule out large parts of the parameter space where the thermal relic density is below the observed cosmological dark matter density and WIMPs (neutralinos here) are dominantly produced non-thermally, e.g. in models where supersymmetry breaking occurs via anomaly mediation. The gamma-ray limits presented here also constrain some WIMP models proposed to explain the Fermi and PAMELA e^+e^- data, including low-mass wino-like neutralinos and models with TeV masses pair-annihilating into muon-antimuon pairs. (Abridged)

Electromagnetic shower development in uranium and tungsten: A comparison of data from a silicon sampling calorimeter

Abstract Longitudinal electromagnetic shower development has been studied in uranium and tungsten using a sandwich calorimeter with silicon as the active medium. Data were taken with incoming electron energies of 2, 4 and 6 GeV. The silicon detectors were operated with depletion layers of both 200 and 70 μm. The energy resolution of the calorimeter was reduced by less than 10–15% when the detectors had depleted layer widths of 70 μm.

The local hardening effect on electromagnetic showers. A way for signal equalization in Si/high-Z hadron calorimeters

Abstract The condition for obtaining the linear response of a calorimeter to hadronic showers and an energy resolution improving as the incident energy increases is the equalization between the electromagnetic and the hadronic signals. This equalization is obtained within a new approach exploiting a local hardening effect that is realized by inserting low-Z absorbers next to the silicon readout detectors. In this way, the calorimeter response to the electromagnetic component of the hadronic shower is reduced. A systematic investigation of the visible energy response for electromagnetic showers in Si/U and Si/W calorimeters has been carried out for incoming electron energies of 2, 4, and 6 GeV. The insertion of low-Z material (G10 plates) in front or at the rear of the silicon detectors allows a fine tuning of the calorimeter response.

Gamma-Light: High-Energy Astrophysics above 10 MeV

The energy range between 10 and 50 MeV is an experimentally very difficult range and remained uncovered since the time of COMPTEL. Here we propose a possible mission to cover this energy range.

The AGILE Instrument

AGILE is an ASI gamma-ray astrophysics space Mission which will operate in the 30 MeV - 50 GeV range with imaging capabilities also in the 10 - 40 keV range. Primary scientific goals include the study of AGNs, gamma-ray bursts, Galactic sources, unidentified gamma-ray sources, diffuse Galactic and extragalactic gamma-ray emission, high-precision timing studies, and Quantum Gravity testing. The AGILE scientific instrument is based on an innovative design of three detecting systems: (1) a Silicon Tracker, (2) a Mini-Calorimeter, and (3) an ultralight coded mask system with Si-detectors (Super-AGILE). AGILE is designed to provide: (1) excellent imaging in the energy bands 30 MeV-50 GeV (5-10 arcmin for intense sources) and 10-40 keV (1-3 arcmin); (2) optimal timing capabilities, with independent readout systems and minimal deadtimes for the Silicon Tracker, Super-AGILE and Mini-Calorimeter; (3) large field of view for the gamma-ray imaging detector (~3 sr) and Super-AGILE (~1 sr). AGILE will be the only Mission entirely dedicated to source detection above 30 MeV during the period 2004-2006.

The next generation of high-energy gamma-ray detectors for satellites: The AGILE silicon tracker

AGILE (Light Imaging Detector for Gamma Astronomy) is a satellite for the detection of gamma-ray sources in the energy range 30 MeV–50 GeV within a large field of view (∼1/5 of the sky) and it is planned to fly in the years 2003–2006, a period in which no other mission entirely dedicated to photon detection above 30 MeV is planned. AGILE is made of a Tungsten-Silicon Tracker, a CsI Minicalorimeter, an anticoincidence system and a X-ray detector sensitive in the 10–40 keV range. The Tracker consists of 14 planes, each one made of 2 layers of 16 single-sided, AC coupled, 410 μm thick, 9.5×9.5 cm2 Silicon detectors with a readout pitch of 242 μm and a floating strip. The AGILE trigger is generated by the Silicon strips which are readout by the TAAI, a low noise, self triggering ASIC used in a very low power configuration (∼400 μW/channel) with analog readout. The number of Tracker readout channels is 43000. We present a description of the Tracker and the performance of the detector (position resolution, clus...

Experimental results from a sandwich calorimeter using U absorbers and 0.25 m2 of Si active area

Abstract The electromagnetic section of a hadronic calorimeter, consisting of uranium absorbers and of silicon sampling units with an active area of 0.25 m2, was investigated. The overall performance of the silicon detectors and especially developed associated electronics, seems to be stable and reliable. During a four-week run at the t9, CERN-PS (Proton Synchrotron) electron beam (energies of 2 to 6 GeV), no variation of energy calibration of the calorimeter was observed. The energy resolution for electromagnetic showers was found to be about 19.8% ( τ E ) , where E is the energy of the incoming electron and τ is the number of radiation lengths of passive material interspaced between two active samplers, for a calorimeter depth of 15.6 X0 (radiation lengths), with Si samplers depleted to 200 μm. The fiberglass supports of the silicon mosaics cause a reduction of energy response to electromagnetic showers. It can be exploited to equalize the response of a Si/U hadronic calorimeter to incoming electrons and hadrons.

A measurement of the bb forward-backward asymmetry using the semileptonic decay into muons

The forward-backward asymmetry of bottom quarks is measured with statistics of approximately 80 000 hadronic Z0 decays produced in e+e− collisions at a centre of mass energy of √s≈Mz. The tagging of b quark events has been performed using the semileptonic decay channel b→X+μ. Because the asymmetry depends on the weak coupling, this leads to a precise measurement of the electroweak mixing angle sin2θw. The experimental result is AFBb = 0.115±0.043(stat.)±0.013(syst.). After correcting the value for the B0B0 mixing this becomes AFBb=0.161±0.060(stat.)±0.021(syst.) corresponding to sin2θWMS=0.221±0.011(stat.)±0.004(syst.).

The connection between gamma-ray bursts and supernovae Ib/c

Context. It has been established that Gamma-Ray Bursts (GRBs) are connected to Supernovae (SNe) explosions of type Ib/c. Aims. We intend to test whether the hypothesis of type Ib/c SNe from different massive progenitors can reproduce the local GRB rate as well as the GRB rate as a function of redshift. We aim to predict the GRB rate at very high redshift under different assumptions about galaxy formation and star formation histories in galaxies. Methods. We assume different star formation histories in galaxies of different morphological types: ellipticals, spirals and irregulars, which have already been tested in self-consistent galaxy models reproducing both chemical and photometrical properties of galaxies. We explore different hypotheses concerning the progenitors of type Ib/c SNe: i) single massive stars (M > 25 M� , Wolf-Rayet stars), ii) massive close binaries (12–20 M� ), and iii) both Wolf-Rayet stars and massive binaries. We conclude that the mixed scenario (iii) is preferable to reproduce the local type Ib/c SN rates in galaxies and we adopt this scenario for comparison with the GRB rates. Results. We find an excellent agreement between the observed GRB local rate and the predicted type Ib/c SN rate in irregular galaxies,

The energy response dependence of a silicon sampling calorimeter on the silicon depleted layer width

Abstract The dependence of the energy response of a silicon sampling calorimeter on the depletion depth of silicon detectors has been studied with tungsten as absorber and for incoming electrons with an energy of 4 GeV. The detectors were operated with depletion depths of 40, 70, 100, 125, 150 and 200 μm (by adjusting the reverse bias voltage). The total deposited energy varies linearly with the depleted layer width. However, at large depths (greater than 18 radiation lengths), where the deposited energy is small compared to the total energy, a deviation from linearity is observed.