Using data collected at the Pierre Auger Observatory during the past 3.7 years, we demonstrated a correlation between the arrival directions of cosmic rays with energy above 6 x 10(19) electron volts and the positions of active galactic nuclei (AGN) lying within approximately 75 megaparsecs. We rejected the hypothesis of an isotropic distribution of these cosmic rays with at least a 99% confidence level from a prescribed a priori test. The correlation we observed is compatible with the hypothesis that the highest-energy particles originate from nearby extragalactic sources whose flux has not been substantially reduced by interaction with the cosmic background radiation. AGN or objects having a similar spatial distribution are possible sources.

20 pages, 3 figures Submitted to Phys.Rev.Lett.International audienceThe surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from ντ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of ντ at EeV energies. Assuming an Eν-2 differential energy spectrum the limit set at 90% C.L. is Eν2dNντ/dEν<1.3×10-7GeVcm-2s-1sr-1 in the energy range 2×1017e

Collisions induced by 9;10;11 Be on a 64 Zn target at the same c.m. energy were studied. For the first time, strong effects of the 11 Be halo structure on elastic-scattering and reaction mechanisms at energies near the Coulomb barrier are evidenced experimentally. The elastic-scattering cross section of the 11 Be halo nucleus shows unusual behavior in the Coulomb-nuclear interference peak angular region. The extracted total-reaction cross section for the 11 Be collision is more than double the ones measured in the collisions induced by 9;10 Be. It is shown that such a strong enhancement of the total-reaction cross section with 11 Be is due to transfer and breakup processes. A hundred years after Rutherford's scattering experiment [1], heavy-ion-elastic-scattering angular distributions (AD) are usually plotted as a ratio of the Rutherford cross section (i.e., pure Coulomb scattering). Such representation usually shows a decrease of the elastic cross section with the angle due to absorption at small impact parameters by nonelastic processes, and an oscillatory behavior. The latter, using the language of optics, is described in terms of refraction by nonabsorbing lenses (Coulomb rainbow model) or diffraction by sharp-edged, nonrefracting apertures (Fraunhofer or Fresnel diffraction model). However, the refraction or diffraction descriptions are oversimplifications of the realistic process; rather, the nucleus behaves as a ''cloudy crystal ball.'' The elasticscattering AD may show a peak resulting from the interference between the Coulomb and nuclear amplitudes (Coulomb-nuclear interference peak) [2], which, in analogy with the Coulomb rainbow model, is sometimes called the rainbow peak. Since elastic scattering is a peripheral process, it does not give information on the interior region of nuclei. It probes the tail of the wave function, and hence one can learn about surface properties, such as size of nuclei and surface diffuseness. Therefore, elastic scattering is an ideal tool to study peculiar nuclear structures as, for example, the nuclear halo. Such structure originates when very weakly bound nucleon(s) can tunnel into the classically forbidden region, giving rise to a diffuse tail surrounding a well-bound core. The behavior of the system in nuclear reactions is mostly determined by the tail of the wave function [3]. The reaction mechanisms may also be affected by the weak binding: at energies around the Coulomb barrier, couplings between the entrance channel and the continuum [4][5][6][7][8], as well as to the various reaction channels [9][10][11][12], are expected to be very important. Direct reactions, such as breakup or transfer, may be favored owing to the low binding energy, the extended tail of halo nuclei, and the large Q values for selected transfer channels.Almost all elastic-scattering and reaction mechanism studies around the barrier with halo nuclei have been performed with the 2n halo nucleus 6 He. All authors agree that, due to the 6 He structure, one has an enhancement of the total-reaction (T...

In this paper details of the experimental procedure and data analysis of the collision of 11 Be+ 64 Zn around the Coulomb barrier are described and discussed in the framework of different theoretical approaches. In a previous work [A. Di Pietro et al., Phys. Rev. Lett. 105, 022701 (2010).], the elastic scattering angular distribution of the collisions 9,10 Be+ 64 Zn as well as the angular distribution for the quasielastic scattering and transfer/breakup cross sections for the 11 Be+ 64 Zn reaction were briefly reported. The suppression of the quasielastic angular distribution in the Coulomb-nuclear interference angular region observed in the collision of the 11 Be halo nucleus with respect to the other two beryllium isotopes was interpreted as being caused by a long-range absorption owing to the long decay length of the 11 Be wave function. In this paper, new continuum-discretized coupledchannel calculations of the 11 Be+ 64 Zn reaction are reported in the attempt to interpret the effect of coupling with the breakup channels on the measured cross sections. The calculations show that the observed suppression of the Coulomb-nuclear interference peak is caused by a combined effect of Coulomb and nuclear couplings to the breakup channels.

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The sub-barrier fusion excitation function of 40Ca + 96Zr has been measured down to cross sections ≃2.4 μb, i.e. two orders of magnitude smaller than obtained in a previous experiment, where the subbarrier fusion of this system was found to be greatly enhanced with respect to 40Ca + 90Zr, and the need of coupling to transfer channels was suggested relying on coupled-channels calculations. The purpose of this work has been to investigate the behavior of 40Ca + 96Zr fusion far below the barrier, thereby disentangling the elusive interplay of effects due to inelastic couplings, transfer couplings and, possibly, the appearance of the fusion hindrance. The smooth trend of the excitation function has been found to continue, and the logarithmic slope increases very slowly. No indication of hindrance shows up, and a comparison with 40Ca + 96 is illuminating in this respect. A new CC analysis of the complete excitation function has been performed, including explicitly one- and two-nucleon Q > 0 transfer channels. Such transfer couplings bring significant cross section enhancements, even at the level of a few μb. Locating the hindrance threshold, if any, in 40Ca + 96 would require challenging measurements of cross sections in the sub-μb range

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The article describes the main achievements of the NUMEN project together with an updated and detailed overview of the related R&D activities and theoretical developments. NUMEN proposes an innovative technique to access the nuclear matrix elements entering the expression of the lifetime of the double beta decay by cross section measurements of heavy-ion induced Double Charge Exchange (DCE) reactions. Despite the two processes, namely neutrinoless double beta decay and DCE reactions, are triggered by the weak and strong interaction respectively, important analogies are suggested. The basic point is the coincidence of the initial and final state many-body wave-functions in the two types of processes and the formal similarity of the transition operators. First experimental results obtained at the INFN-LNS laboratory for the 40 Ca( 18 O, 18 Ne) 40 Ar reaction at 270 MeV, give encouraging indication on the capability of the proposed technique to access relevant quantitative information.The two major aspects for this project are the K800 Superconducting Cyclotron and MAGNEX spectrometer. The former is used for the acceleration of the required high resolution and low emittance heavy ion beams and the latter is the large acceptance magnetic spectrometer for the detection of the ejectiles. The use of the high-order trajectory reconstruction technique, implemented in MAGNEX, allows to reach the experimental resolution and sensitivity required for the accurate measurement of the DCE cross sections at forward angles. However, the tiny values of such cross sections and the resolution requirements demand beam intensities much larger than manageable with the present facility. The on-going upgrade of the INFN-LNS facilities in this perspective is part of the NUMEN project and will be discussed in the article.3