Production of quark pairs from classical gluon fields

We compute by numerical integration of the Dirac equation the number of quark-antiquark pairs produced in the classical color fields of colliding ultrarelativistic nuclei. Results for the dependence of the number of quarks on the strength of the background field, the quark mass and time are presented. We also perform several tests of our numerical method. While the number of qqbar pairs is parametrically suppressed in the coupling constant, we find that in this classical field model it could even be compatible with the thermal ratio to the number of gluons.Comment: 8 pages, 12 figures, contribution to the proceedings of the Quark Gluon Plasma Thermalization Workshop, Vienna, August 200

Quark-antiquark production from classical fields and chemical equilibration

We compute by numerical integration of the Dirac equation the number of quark-antiquark pairs produced in the classical color fields of colliding ultrarelativistic nuclei. The backreaction of the created pairs on the color fields is not taken into account. While the number of qqbar pairs is parametrically suppressed in the coupling constant, we find that in this classical field model it could even be compatible with the thermal ratio to the number of gluons. After isotropization one could thus have quark-gluon plasma in chemical equilibrium.Comment: 4 pages, 6 figures, contribution to proceedings of Quark Matter 2005, Budapest, Aug 4-

Ground-based variability surveys towards Centaurus A: worthwhile or not?

Context: Difference imaging has proven to be a powerful technique for detecting and monitoring the variability of unresolved stellar sources in M 31. Using this technique in surveys of galaxies outside the Local Group could have many interesting applications. Aims: The goal of this paper is to test difference imaging photometry on Centaurus A, the nearest giant elliptical galaxy, at a distance of 4 Mpc. Methods: We obtained deep photometric data with the Wide Field Imager at the ESO/MPG 2.2m at La Silla spread over almost two months. Applying the difference imaging photometry package DIFIMPHOT, we produced high-quality difference images and detected variable sources. The sensitivity of the current observational setup was determined through artificial residual tests. Results: In the resulting high-quality difference images, we detect 271 variable stars. We find a difference flux detection limit corresponding to m_R~24.5. Based on a simple model of the halo of Centaurus A, we estimate that a ground-based microlensing survey would detect in the order of 4 microlensing events per year due to lenses in the halo. Conclusions: Difference imaging photometry works very well at the distance of Centaurus A and promises to be a useful tool for detecting and studying variable stars in galaxies outside the local group. For microlensing surveys, a higher sensitivity is needed than achieved here, which would be possible with a large ground-based telescope or space observatory with wide-field imaging capabilities.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and Astrophysic

A light-fronts approach to electron-positron pair production in ultrarelativistic heavy-ion collisions

We perform a gauge-transformation on the time-dependent Dirac equation describing the evolution of an electron in a heavy-ion collision to remove the explicit dependence on the long-range part of the interaction. We solve, in an ultra-relativistic limit, the gauged-transformed Dirac equation using light-front variables and a light-fronts representation, obtaining non-perturbative results for the free pair-creation amplitudes in the collider frame. Our result reproduces the result of second-order perturbation theory in the small charge limit while non-perturbative effects arise for realistic charges of the ions.Comment: 39 pages, Revtex, 7 figures, submitted to PR

Coherent Vector Meson Photoproduction with Nuclear Breakup in Relativistic Heavy Ion Collisions

Relativistic heavy ions are copious sources of virtual photons. The large photon flux gives rise to a substantial photonuclear interaction probability at impact parameters where no hadronic interactions can occur. Multiple photonuclear interactions in a single collision are possible. In this letter, we use mutual Coulomb excitation of both nuclei as a tag for moderate impact parameter collisions. We calculate the cross section for coherent vector meson production accompanied by mutual excitation, and show that the median impact parameter is much smaller than for untagged production. The vector meson rapidity and transverse momentum distribution are very different from untagged exclusive vector meson production.Comment: 14 pages, including 4 figure

Asymptotic channels and gauge transformations of the time-dependent Dirac equation for extremely relativistic heavy-ion collisions

We discuss the two-center, time-dependent Dirac equation describing the dynamics of an electron during a peripheral, relativistic heavy-ion collision at extreme energies. We derive a factored form, which is exact in the high-energy limit, for the asymptotic channel solutions of the Dirac equation, and elucidate their close connection with gauge transformations which transform the dynamics into a representation in which the interaction between the electron and a distant ion is of short range. We describe the implications of this relationship for solving the time-dependent Dirac equation for extremely relativistic collisions.Comment: 12 pages, RevTeX, 2 figures, submitted to PR

Strong suppression of Coulomb corrections to the cross section of e+e- pair production in ultrarelativistic nuclear collisions

The Coulomb corrections to the cross section of $e^+e^-$ pair production in ultrarelativistic nuclear collisions are calculated in the next-to-leading approximation with respect to the parameter $L=\ln \gamma_A\gamma_B$ ($\gamma_{A,B}$ are the Lorentz factors of colliding nuclei). We found considerable reduction of the Coulomb corrections even for large $\gamma_A\gamma_B$ due to the suppression of the production of $e^+e^-$ pair with the total energy of the order of a few electron masses in the rest frame of one of the nuclei. Our result explains why the deviation from the Born result were not observed in the experiment at SPS.Comment: 4 pages, RevTe

Solar Neutrinos and the Eclipse Effect

The solar neutrino counting rate in a real time detector like Super--Kamiokanda, SNO, or Borexino is enhanced due to neutrino oscillations in the Moon during a partial or total solar eclipse. The enhancement is calculated as a function of the neutrino parameters in the case of three flavor mixing. This enhancement, if seen, can further help to determine the neutrino parameters.Comment: 24 Pages Revtex, 8 figures as one ps file. To appear in Phys. Rev. D; Some typos corrected and a reference adde