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R. Fabbri

Publications and source records attributed to R. Fabbri.

At least 37 records · Page 2Linked to original sources
arXiv2010

Exclusive Leptoproduction of Real Photons on a Longitudinally Polarised Hydrogen Target

Polarisation asymmetries are measured for the hard exclusive leptoproduction of real photons from a longitudinally polarised hydrogen target. These asymmetries arise from the deeply virtual Compton scattering and Bethe-Heitler processes. From the data are extracted two asymmetries in the azimuthal distribution of produced real photons about the direction of the exchanged virtual photon: A_UL with respect to the target polarisation and A_LL with respect to the product of the beam and target polarisations. Results for both asymmetries are compared to the predictions from a generalised parton distribution model. The sin(phi) and cos(0*phi) amplitudes observed respectively for the A_UL and A_LL asymmetries are compatible with the sizeable predictions from the model. Unexpectedly, a sin(2*phi) modulation in the A_UL asymmetry with a magnitude similar to that of the sin(phi) modulation is observed.

hep-ex↗
arXiv2010

Construction and Commissioning of the CALICE Analog Hadron Calorimeter Prototype

An analog hadron calorimeter (AHCAL) prototype of 5.3 nuclear interaction lengths thickness has been constructed by members of the CALICE Collaboration. The AHCAL prototype consists of a 38-layer sandwich structure of steel plates and highly-segmented scintillator tiles that are read out by wavelength-shifting fibers coupled to SiPMs. The signal is amplified and shaped with a custom-designed ASIC. A calibration/monitoring system based on LED light was developed to monitor the SiPM gain and to measure the full SiPM response curve in order to correct for non-linearity. Ultimately, the physics goals are the study of hadron shower shapes and testing the concept of particle flow. The technical goal consists of measuring the performance and reliability of 7608 SiPMs. The AHCAL was commissioned in test beams at DESY and CERN. The entire prototype was completed in 2007 and recorded hadron showers, electron showers and muons at different energies and incident angles in test beams at CERN and Fermilab.

physics.ins-det↗
arXiv2009

Overview of Studies on the SPIROC Chip Characterisation

This note reports the results of extensive measurements of the SPIROC ASIC charaterisation. This chip is forseen to be used in the next generation of the Analogue Hadronic Calorimeter (AHCAL) prototype for the International Linear Collider.

physics.ins-det↗
arXiv2005

The synchrotron foreground and CMB temperature-polarization cross correlation power spectrum from the first year WMAP data

We analyse the temperature-polarization cross-correlation in the Galactic synchrotron template that we have recently developed, and between the template and CMB temperature maps derived from WMAP data. Since the polarized synchrotron template itself uses WMAP data, we can estimate residual synchrotron contamination in the CMB $C_\ell^{TE}$ angular spectrum. While $C_2^{TE}$ appears to be contamined by synchrotron, no evidence for contamination is found in the multipole range which is most relevant for the fit of the cosmological optical depth.

astro-ph↗
arXiv2004

Cosmic Microwave Background Polarization and reionization: constraining models with a double reionization

Neutral hydrogen around high-z QSO and an optical depth tau ~ 0.17 can be reconciled if reionization is more complex than a single transition at z ~ 6-8. Tracing its details could shed a new light on the first sources of radiation. Here we discuss how far such details can be inspected through planned experiments on CMB large-scale anisotropy and polarization, by simulating an actual data analysis. By considering a set of double reionization histories of Cen (2003) type, a relevant class of models not yet considered by previous works, we confirm that large angle experiments rival high resolution ones in reconstructing the reionization history. We also confirm that reionization histories, studied with the prior of a single and sharp reionization, yield a biased tau, showing that this bias is generic. We further find a monotonic trend in the bias for the models that we consider, and propose an explanation of the trend, as well as the overall bias. We also show that in long-lived experiments such a trend can be used to discriminate between single and double reionization patterns.

astro-ph↗
arXiv2004

A polarized synchrotron template for CMBP experiments after WMAP data

We build template maps for the polarized Galactic--synchrotron emission on large angular scales (FWHM =~7$^\circ$), in the 20-90 GHz microwave range, by using WMAP data. The method, presented in a recent work, requires a synchrotron total intensity survey and the {\it polarization horizon} to model the polarized intensity and a starlight polarization map to model polarization angles. The basic template is obtained directly at 23 GHz with about 94% sky--coverage by using the synchrotron map released by the WMAP team. Extrapolations to 32, 60 and 90 GHz are performed by computing a synchrotron spectral index map, which strongly reduces previous uncertainties in passing from low (1.4 GHz) to microwave frequencies. Differing from low frequency data, none of our templates presents relevant structures out of the Galactic Plane. Our map at 90 GHz suggests that the synchrotron emission at high Galactic latitudes is low enough to allow a robust detection of the $E$--mode component of the cosmological signal on large--scale, even in models with low--reionization ($τ= 0.05$). Detection of the weaker $B$--mode on the largest scales ($\ell < 10$) might be jeopardized unless the value $τ= 0.17$ found by WMAP is confirmed, and $T/S > 0.1$. For lower levels of the gravitational--wave background the $B$--mode seems to be accessible only at the $\ell \sim 100$ peak and in selected low--synchrotron emission areas.

astro-ph↗
arXiv2004

The Sky Polarization Observatory

SPOrt is an ASI-funded experiment specifically designed to measure the sky polarization at 22, 32 and 90 GHz, which was selected in 1997 by ESA to be flown on the International Space Station. Starting in 2006 and for at least 18 months, it will be taking direct and simultaneous measurements of the Stokes parameters Q and U at 660 sky pixels, with FWHM=7 degrees. Due to development efforts over the past few years, the design specifications have been significantly improved with respect to the first proposal. Here we present an up-to-date description of the instrument, which now warrants a pixel sensitivity of 1.7 microK for the polarization of the cosmic background radiation, assuming two years of observations. We discuss SPOrt scientific goals in the light of WMAP results, in particular in connection with the emerging double-reionization cosmological scenario.

astro-ph↗
arXiv2003

An iterative destriping technique for diffuse background polarization data

We describe a simple but effective iterative procedure specifically designed to destripe Q and U Stokes parameter data as those collected by the SPOrt experiment onboard the International Space Station (ISS). The method is general enough to be useful for other experiments, both in polarization and total intensity. The only requirement for the algorithm to work properly is that the receiver knee frequency must be lower than the signal modulation frequency, corresponding in our case to the ISS orbit period. Detailed performances of the technique are presented in the context of the SPOrt experiment, both in terms of added rms noise and residual correlated noise.

astro-ph↗
arXiv2003

A New approach for a Galactic Synchrotron Polarized Emission Template in the Microwave Range

We present a new approach in modelling the polarized Galactic synchrotron emission in the microwave range (20-100 GHz), where this radiation is expected to play the leading role in contaminating the Cosmic Microwave Background (CMB) data. Our method is based on real surveys and aims at providing the real spatial distributions of both polarized intensity and polarization angles. Its main features are the modelling of a polarization horizon to determine the polarized intensity and the use of starlight optical data to model the polarization angle pattern. Our results are consistent with several existing data, and our template is virtually free from Faraday rotation effects as required at frequencies in the cosmological window.

astro-ph↗
arXiv2003

The BaR-SPOrt Experiment

BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation Observations) is an experiment to measure the linearly polarized emission of sky patches at 32 and 90 GHz with sub-degree angular resolution. It is equipped with high sensitivity correlation polarimeters for simultaneous detection of both the U and Q stokes parameters of the incident radiation. On-axis telescope is used to observe angular scales where the expected polarization of the Cosmic Microwave Background (CMBP) peaks. This project shares most of the know-how and sophisticated technology developed for the SPOrt experiment onboard the International Space Station. The payload is designed to flight onboard long duration stratospheric balloons both in the Northern and Southern hemispheres where low foreground emission sky patches are accessible. Due to the weakness of the expected CMBP signal (in the range of microK), much care has been spent to optimize the instrument design with respect to the systematics generation, observing time efficiency and long term stability. In this contribution we present the instrument design, and first tests on some components of the 32 GHz radiometer.

astro-ph↗
arXiv2002

SPOrt: an Experiment Aimed at Measuring the Large Scale Cosmic Microwave Background Polarization

SPOrt (Sky Polarization Observatory) is a space experiment to be flown on the International Space Station during Early Utilization Phase aimed at measuring the microwave polarized emission with FWHM = 7deg, in the frequency range 22-90 GHz. The Galactic polarized emission can be observed at the lower frequencies and the polarization of Cosmic Microwave Background (CMB) at 90 GHz, where contaminants are expected to be less important. The extremely low level of the CMB Polarization signal (< 1 uK) calls for intrinsically stable radiometers. The SPOrt instrument is expressly devoted to CMB polarization measurements and the whole design has been optimized for minimizing instrumental polarization effects. In this contribution we present the receiver architecture based on correlation techniques, the analysis showing its intrinsic stability and the custom hardware development carried out to detect such a low signal.

astro-ph↗
arXiv2002

Hard Exclusive Electroproduction of Two Pions off Proton and Deuteron at HERMES

Exclusive electroproduction of $π^+π^-$ pairs off hydrogen and deuterium targets has been studied with the HERMES experiment. The angular distribution of the $π^+$ in the $π^+π^-$ rest system has been studied in the invariant mass range $0.3 < m_{ππ} <1.5$ GeV. Theoretical models derived in the framework of the Generalized Parton Distributions show that this angular distribution receives only contributions from the interference between the isoscalar channel I=0 and the isovector channel I=1.

hep-ex↗
arXiv2002

Polarization angular spectra of Galactic synchrotron emission on arcminute scales

We study the angular power spectra of the polarized component of the Galactic synchrotron emission in the 28-deg^2 Test Region of the Southern Galactic Plane Survey at 1.4 GHz. These data were obtained by the Australia Telescope Compact Array and allow us to investigate angular power spectra down to arcminute scales. We find that, at this frequency, the polarization spectra for E- and B-modes seem to be affected by Faraday rotation produced in compact foreground screens. A different behavior is shown by the angular spectrum of the polarized intensity PI=\sqrt{Q^2+U^2}. This is well fitted by a power law with slope \~1.7, which agrees with higher frequency results and can probably be more confidently extrapolated to the cosmological window.

astro-ph↗
arXiv2000

The angular power spectra of polarized Galactic synchrotron

We derive the angular power spectra of intensity and polarization of Galactic synchrotron emission in the range 36 < l < 10^3 from the Parkes survey mapping the southern Galactic plane at 2.4 GHz. The polarization spectra of both electric and magnetic parity up to l \simeq 10^3 are approximated very well by power laws with slope coefficients \simeq 1.4, quite different from the CMB spectra. We show that no problem should arise from Galactic synchrotron for measurements of CMB polarization in the cosmological window.

astro-ph↗
arXiv1999

CMB Anisotropies due to Feedback-Regulated Inhomogeneous Reionization

We calculate the secondary anisotropies in the CMB produced by inhomogeneous reionization from simulations in which the effects of radiative and stellar feedback effects on galaxy formation have been included. This allows to self-consistently determine the beginning ($z_i\approx 30$), the duration ($ δz\approx 20$) and the (nonlinear) evolution of the reionization process for a critical density CDM model. In addition, from the simulated spatial distribution of ionized regions, we are able to calculate the evolution of the two-point ionization correlation function, $C_χ$, and obtain the power spectrum of the anisotropies, $C_\ell$, in the range $5000 < \ell < 10^6$. The power spectrum has a broad maximum around $\ell \approx 30000$, where it reaches the value $2\times 10^{-12}$. We also show that the angular correlation function $C(θ)$ is not Gaussian, but at separation angles $% θ\lower.5ex\hbox{\ltsima} 10^{-4}$ rad it can be approximated by a modified Lorentzian shape; at larger separations an anticorrelation signal is predicted. Detection of signals as above will be possible with future mm-wavelength interferometers like ALMA, which appears as an optimum instrument to search for signatures of inhomogeneous reionization.

astro-ph↗
arXiv1999

The SPOrt Project: an Experimental Overview

The Sky Polarization Observatory (SPOrt) is presented as a project aimed to measure the diffuse sky polarized emission, from the International Space Station, in the frequency range 20-90 GHz with 7 degrees of HPBW. The SPOrt experimental configuration is described with emphasis on the aspects that make SPOrt the first European scientific payload operating at microwave wavelengths.

astro-ph↗