Electromagnetic Dissociation of Nuclei in Heavy-Ion Collisions

Large discrepancies have been observed between measured Electromagnetic Dissociation(ED) cross sections and the predictions of the semiclassical Weiz\"acker-Williams-Fermi(WWF) method. In this paper, the validity of the semiclassical approximation is examined. The total cross section for electromagnetic excitation of a nuclear target by a spinless projectile is calculated in first Born approximation, neglecting recoil. The final result is expressed in terms of correlation functions and convoluted densities in configuration space. The result agrees with the WWF approximation to leading order(unretarded electric dipole approximation), but the method allows an analytic evaluation of the cutoff, which is determined by the details of the electric dipole transition charge density. Using the Goldhaber-Teller model of that density, and uniform charge densities for both projectile and target, the cutoff is determined for the total cross section in the nonrelativistic limit, and found to be smaller than values currently used for ED calculations. In addition, cross sections are calculated using a phenomenological momentum space cutoff designed to model final state interactions. For moderate projectile energies, the calculated ED cross section is found to be smaller than the semiclassical result, in qualitative agreement with experiment.Comment: 28 page

Measurement of inclusive electron cross section in γγ\gamma \gamma collisions at TRISTAN

We have studied open charm production in $\gamma \gamma$ collisions with the TOPAZ detector at the TRISTAN $e^{+}e^{-}$ collider. In this study, charm quarks were identified by electrons (and positrons) from semi-leptonic decays of charmed hadrons. The data corresponded to an integrated luminosity of 95.3 pb$^{-1}$ at a center-of-mass energy of 58 GeV. The results are presented as the cross sections of inclusive electron production in $\gamma \gamma$ collisions with an anti-tag condition, as well as the subprocess cross sections, which correspond to resolved-photon processes. The latter were measured by using a sub-sample with remnant jets. A comparison with various theoretical predictions based on direct and resolved-photon processes showed that our data prefer that with relatively large gluon contents in a photon at small $x (x \le 0.1)$, with the next-to-leading order correction, and with a charm-quark mass of 1.3 GeV.Comment: 26 pages, Latex format (article), 5 figures included, to be published in Phys. Lett.

B_c meson production in \gamma\gamma collisions and charm content of the photon

Based on photon structure function formalism we have calculated $B_c(B_c^{*})$ meson production cross section in $\gamma\gamma$ collisions via partonic subprocess $c\gamma\to B_c(B_c^{*}) b$. It was shown that this approach gives the same results for $B_c^*(B_c)$ meson cross sections as the direct leading order QCD calculation of the subprocess $\gamma\gamma\to B_c(B_c^{*})b\bar c$ at small energy. Howere, opposite to the direct QCD calculation we have obtained the increase of the $B_c(B_c^{*})$ meson cross section at large energies. We have found also additional contribution to the $B_c$ meson production via the nonperturbative fluctuations $\gamma\leftrightarrow J/\psi$.Comment: LaTeX, 7 pages and 6 figures available upon reques