Critical exponents and phase transition in gold nuclei fragmentation at energies 10.6 and 4.0 GeV/nucleon

An attempt to extract critical exponents gamma, beta and tau from data on gold nuclei fragmentation due to interactions with nuclear emulsion at energies 4.0 A GeV and 10.6 A GeV is presented. Based on analysis of Campi's 2nd charge moments, two subsets of data at each energy are selected from the inclusive data, corresponding to 'liquid' and 'gas' phases. The extracted values of critical exponents from the selected data sets are in agreement with predictions of 'liquid-gas' model of phase transition.Comment: 21 pages, 15 figure

Spin-polarized transport through a single-level quantum dot in the Kondo regime

Nonequilibrium electronic transport through a quantum dot coupled to ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium Green function technique. The system is described by the Anderson model with arbitrary correlation parameter $U$. Exchange interaction between the dot and ferromagnetic electrodes is taken into account {\it via} an effective molecular field. The following situations are analyzed numerically: (i) the dot is symmetrically coupled to two ferromagnetic leads, (ii) one of the two ferromagnetic leads is half-metallic with almost total spin polarization of electron states at the Fermi level, and (iii) one of the two electrodes is nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak in the density of states (DOS) becomes spin-split when the total exchange field acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads to splitting and suppression of the corresponding zero bias anomaly in the differential conductance.Comment: 9 pages, 7 figure

On a possible photon origin of the most-energetic AGASA events

In this work the ultra high energy cosmic ray events recorded by the AGASA experiment are analysed. With detailed simulations of the extensive air showers initiated by photons, the probabilities are determined of the photonic origin of the 6 AGASA events for which the muon densities were measured and the reconstructed energies exceeded 10^20 eV. On this basis a new, preliminary upper limit on the photon fraction in cosmic rays above 10^20 eV is derived and compared to the predictions of exemplary top-down cosmic-ray origin models.Comment: 3 pages, 1 figure, 2 tables; presented at XIII ISVHECRI, Pylos, Greec

Simulation of Ultra-High Energy Photon Propagation in the Geomagnetic Field

The identification of primary photons or specifying stringent limits on the photon flux is of major importance for understanding the origin of ultra-high energy (UHE) cosmic rays. We present a new Monte Carlo program allowing detailed studies of conversion and cascading of UHE photons in the geomagnetic field. The program named PRESHOWER can be used both as an independent tool or together with a shower simulation code. With the stand-alone version of the code it is possible to investigate various properties of the particle cascade induced by UHE photons interacting in the Earth's magnetic field before entering the Earth's atmosphere. Combining this program with an extensive air shower simulation code such as CORSIKA offers the possibility of investigating signatures of photon-initiated showers. In particular, features can be studied that help to discern such showers from the ones induced by hadrons. As an illustration, calculations for the conditions of the southern part of the Pierre Auger Observatory are presented.Comment: 41 pages, 9 figures, added references in introduction, corrected energy in row 1 of Table 3, extended caption of Table

Atmospheric multiple scattering of fluorescence and Cherenkov light emitted by extensive air showers

Atmospheric scattering of light emitted by an air shower not only attenuates direct fluorescence light from the shower, but also contributes to the observed shower light. So far only direct and singly-scattered Cherenkov photons have been taken into account in routine analyses of the observed optical image of air showers. In this paper a Monte Carlo method of evaluating the contribution of multiply scattered light to the optical air shower image is presented, as well as results of simulations and a parameterization of scattered light contribution to measured shower signal.Comment: 27 pages, 18 figures, accepted for publication in NIM

Strongly damped nuclear collisions: zero or first sound ?

The relaxation of the collective quadrupole motion in the initial stage of a central heavy ion collision at beam energies $E_{lab}=5\div20$ AMeV is studied within a microscopic kinetic transport model. The damping rate is shown to be a non-monotonic function of E_{lab} for a given pair of colliding nuclei. This fact is interpreted as a manifestation of the zero-to-first sound transition in a finite nuclear system.Comment: 15 pages, 4 figure

Characteristics of geomagnetic cascading of ultra-high energy photons at the southern and northern sites of the Pierre Auger Observatory

Cosmic-ray photons above 10^19 eV can convert in the geomagnetic field and initiate a preshower, i.e. a particle cascade before entering the atmosphere. We compare the preshower characteristics at the southern and northern sites of the Pierre Auger Observatory. In addition to a shift of the preshower patterns on the sky due to the different pointing of the local magnetic field vectors, the fact that the northern Auger site is closer to the geomagnetic pole results in a different energy dependence of the preshower effect: photon conversion can start at smaller energies, but large conversion probabilitites (>90%) are reached for the whole sky at higher energies compared to the southern Auger site. We show how the complementary preshower features at the two sites can be used to search for ultra-high energy photons among cosmic rays. In particular, the different preshower characteristics at the northern Auger site may provide an elegant and unambiguous confirmation if a photon signal is detected at the southern site.Comment: 25 pages, 14 figures, minor changes, conclusions unchanged, Appendix A replaced, accepted by Astroparticle Physic

Upper limit on the photon fraction in highest-energy cosmic rays from AGASA data

A new method to derive an upper limit on photon primaries from small data sets of air showers is developed which accounts for shower properties varying with the primary energy and arrival direction. Applying this method to the highest-energy showers recorded by the AGASA experiment, an upper limit on the photon fraction of 51% (67%) at a confidence level of 90% (95%) for primary energies above 1.25 * 10^20 eV is set. This new limit on the photon fraction above the GZK cutoff energy constrains the Z-burst model of the origin of highest-energy cosmic rays.Comment: 4 pages, 3 figures. Analysis extended to account for primary energy resolution; conclusions unchanged. Accepted by Phys. Rev. Let