arXiv subjects
Publications and source records attributed to D. Franco.
Borexino is an experiment designed to detect in real-time low energy solar neutrinos. It is installed at the Gran Sasso Underground Laboratory and has started taking data in May 2007. We report the direct measurement of the 7Be solar neutrino signal rate after 1 year of data taking. Implications and perspectives are discussed.
The time evolution of the electromagnetic emission from blazars, in particular high frequency peaked sources (HBLs), displays irregular activity not yet understood. In this work we report a methodology capable of characterizing the time behavior of these variable objects. The Maximum Likelihood Blocks (MLBs) is a model-independent estimator which sub-divides the light curve into time blocks, whose length and amplitude are compatible with states of constant emission rate of the observed source. The MLBs yields the statistical significance in the rate variations and strongly suppresses the noise fluctuations in the light curves. We apply the MLBs for the first time on the long term X-ray light curves (RXTE/ASM) of Mkn~421,Mkn~501, 1ES 1959+650 and 1ES 2155-304, which consist of more than 10 years of observational data (1996-2007). Using the MLBs interpretation of RXTE/ASM data, the integrated time flux distribution is determined for each single source considered. We identify in these distributions the characteristic level as well as the flaring states of the blazars. All the distributions show a significant component at negative flux values, most probably caused by an uncertainty in the background subtraction and by intrinsic fluctuations of RXTE/ASM. This effect interests in particular short time observations. In order to quantify the probability that the intrinsic fluctuations give rise to a false identification of a flare, we study a population of very faint sources and their integrated time flux distribution. We determine duty cycle or fraction of time a source spent in the flaring state of the source Mkn~421, Mkn~501, 1ES 1959+650 and 1ES 2155-304. Moreover, we study the random coincidences between flares and generic sporadic events such as high energy neutrinos or flares in other wavelengths.
We present a study of the magnetic properties of Gd2/3Ca1/3MnO3 single crystals at low temperatures. We show that this material behave as an inhomogeneous ferrimagnet. In addition to small saturation magnetization at 5 K, we have found history dependent effects in the magnetization and the presence of exchange bias. These features are compatible with microscopic phase separation in the clean Gd2/3Ca1/3MnO3 system studied.
The double polarization (beam-recoil) observables Ox and Oz have been measured for the reaction gamma p -> K Lambda from threshold production to Egamma = 1500 MeV. The data were obtained with the linearly polarized beam of the GRAAL facility. Values for the target asymmetry T could also be extracted despite the use of an unpolarized target. Analyses of our results by two isobar models tend to confirm the necessity to include new or poorly known resonances in the 1900 MeV mass region.
Beam asymmetry and differential cross section for the reaction gamma+p->eta+p were measured from production threshold to 1500 MeV photon laboratory energy. The two dominant neutral decay modes of the eta meson, eta->2g and eta->3pi0, were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found.
The gamma n -> eta n quasi-free cross section reveals a resonant structure at W ~ 1.675 GeV. This structure may be a manifestation of a baryon resonance. A priori its properties, the possibly narrow width and the strong photocoupling to the neutron, look surprising. This structure may also signal the existence of a narrow state.
New (preliminary) data on eta photoproduction off the neutron are presented. These data reveal a resonant structure at W=1.67 GeV.
Several possible background sources determine the detectability of pep and CNO solar neutrinos in organic liquid scintillator detectors. Among such sources, the cosmogenic 11C nuclide plays a central role. 11C is produced underground in reactions induced by the residual cosmic muon flux. Experimental data available for the effective cross section for 11C by muons indicate that 11C will be the dominant source of background for the observation of pep and CNO neutrinos. 11C decays are expected to total a rate 2.5 (20) times higher than the combined rate of pep and CNO neutrinos in Borexino (KamLAND) in the energy window preferred for the pep measurement, between 0.8 and 1.3 MeV. This study examines the production mechanism of 11C by muon-induced showers in organic liquid scintillators with a novel approach: for the first time, we perform a detailed ab initio calculation of the production of a cosmogenic nuclide, 11C, taking into consideration all relevant production channels. Results of the calculation are compared with the effective cross sections measured by target experiments in muon beams. This paper also discusses a technique for reduction of background from 11C in organic liquid scintillator detectors, which allows to identify on a one-by-one basis and remove from the data set a large fraction of 11C decays. The background reduction technique hinges on an idea proposed by Martin Deutsch, who suggested that a neutron must be ejected in every interaction producing a 11C nuclide from 12C. 11C events are tagged by a three-fold coincidence with the parent muon track and the subsequent neutron capture on protons.
We prove that a smooth, subcanonical surface of P^4 (projective space over an algebraically closed field of characteristic zero) is complete intersection if it is contained in a quartic hypersurface.
We prove an effective bound for the degree of a smooth divisor of a hypersurface of P^n, n>4 (projective space over an algebraically closed field of characteristic zero). Our result follows from a strong (since the degree of the divisor is not involved) generalization of the "Speciality theorem" of Gruson-Peskine, which we prove to hold for any smooth, subcanonical, codimension two, projective verieties of dimension at least three.
We prove that a smooth surface, non of general type, in projective four-space, which lies on a quartic hypersurface with isolated singularities has degree at most 27 (in fact we prove a slightly more general result).
We show, among other things, that in most cases the gonality of a subcanonical curve in projective-three space is computable by multisecants.