Hydrogen-like Atoms from Ultrarelativistic Nuclear Collisions

The number of hydrogen-like atoms produced when heavy nuclei collide is estimated for central collisions at the Relativistic Heavy Ion Collider using the sudden approximation of Baym et al. As first suggested by Schwartz, a simultaneous measurement of the hydrogen and hadron spectra will allow an inference of the electron or muon spectra at low momentum where a direct experimental measurement is not feasible.Comment: 6 pages, 4 figure

Prompt Photon and Inclusive π0\pi^0 Production at RHIC and LHC

We present results for prompt photon and inclusive $\pi^0$ production in p-p and A-A collisions at RHIC and LHC energies. We include the full next-to-leading order radiative corrections and nuclear effects, such as nuclear shadowing and parton energy loss. We find the next-to-leading order corrections to be large and $p_T$ dependent. We show how measurements of $\pi^0$ production at RHIC and LHC, at large $p_T$, can provide valuable information about the nature of parton energy loss. We calculate the ratio of prompt photons to neutral pions and show that at RHIC energies this ratio increases with $p_T$ approaching one at $p_T \sim 10$ GeV, due to the large suppression of $\pi^0$ production. We show that at the LHC, this ratio has steep $p_T$ dependence and approaches 10% effect at $p_T \sim 20$ GeV.Comment: Talk presented by I. Sarcevic, to appear in the Proceedings of Quark Matter 2002; 4 pages including 4 color figure

Multiplicity Fluctuations in Au+Au Collisions at RHIC

The preliminary data of the PHENIX collaboration for the scaled variances of charged hadron multiplicity fluctuations in Au+Au at $\sqrt{s}=200$ GeV are analyzed within the model of independent sources. We use the HSD transport model to calculate the participant number fluctuations and the number of charged hadrons per nucleon participant in different centrality bins. This combined picture leads to a good agreement with the PHENIX data and suggests that the measured multiplicity fluctuations result dominantly from participant number fluctuations. The role of centrality selection and acceptance is discussed separately.Comment: 7 pages, 3 figures, submitted to Phys. Rev. C (Rapid Communication

Particle number fluctuations in nuclear collisions within excluded volume hadron gas model

The multiplicity fluctuations are studied in the van der Waals excluded volume hadron-resonance gas model. The calculations are done in the grand canonical ensemble within the Boltzmann statistics approximation. The scaled variances for positive, negative and all charged hadrons are calculated along the chemical freeze-out line of nucleus-nucleus collisions at different collision energies. The multiplicity fluctuations are found to be suppressed in the van der Waals gas. The numerical calculations are presented for two values of hard-core hadron radius, $r=0.3$ fm and 0.5 fm, as well as for the upper limit of the excluded volume suppression effects.Comment: 19 pages, 4 figure

Condensation for a fixed number of independent random variables

A family of m independent identically distributed random variables indexed by a chemical potential \phi\in[0,\gamma] represents piles of particles. As \phi increases to \gamma, the mean number of particles per site converges to a maximal density \rho_c<\infty. The distribution of particles conditioned on the total number of particles equal to n does not depend on \phi (canonical ensemble). For fixed m, as n goes to infinity the canonical ensemble measure behave as follows: removing the site with the maximal number of particles, the distribution of particles in the remaining sites converges to the grand canonical measure with density \rho_c; the remaining particles concentrate (condensate) on a single site.Comment: 6 page

Multiplicity Fluctuations in Nucleus-Nucleus Collisions: Dependence on Energy and Atomic Number

Event-by-event multiplicity fluctuations in central C+C, S+S, In+In, and Pb+Pb as well as p+p collisions at bombarding energies from 10 to 160 AGeV are studied within the HSD and UrQMD microscopic transport approaches. Our investigation is directly related to the future experimental program of the NA61 Collaboration at the SPS for a search of the QCD critical point. The dependence on energy and atomic mass number of the scaled variances for negative, positive, and all charged hadrons is presented and compared to the results of the model of independent sources. Furthermore, the nucleus-nucleus results from the transport calculations are compared to inelastic proton-proton collisions for reference. We find a dominant role of the participant number fluctuations in nucleus-nucleus reactions at finite impact parameter $b$. In order to reduce the influence of the participant numbers fluctuations on the charged particle multiplicity fluctuations only the most central events have to be selected. Accordingly, the samples of the 1% most central nucleus-nucleus collisions with the largest numbers of the projectile participants are studied. The results are compared with those for collisions at zero impact parameter. A strong influence of the centrality selection criteria on the multiplicity fluctuations is pointed out. Our findings are essential for an optimal choice of colliding nuclei and bombarding energies for the experimental search of the QCD critical point.Comment: 26 pages, 12 figures, extended version, to be published in Phys. Rev.

Dissipation in equations of motion of scalar fields

The methods of non-equilibrium quantum field theory are used to investigate the possibility of representing dissipation in the equation of motion for the expectation value of a scalar field by a friction term, such as is commonly included in phenomenological inflaton equations of motion. A sequence of approximations is exhibited which reduces the non-equilibrium theory to a set of local evolution equations. However, the adiabatic solution to these evolution equations which is needed to obtain a local equation of motion for the expectation value is not well defined; nor, therefore, is the friction coefficient. Thus, a non-equilibrium treatment is essential, even for a system that remains close to thermal equilibrium, and the formalism developed here provides one means of achieving this numerically.Comment: 17 pages, 5 figure