Indications of early thermalization in relativistic heavy-ion collisions

The directed flow of particles emitted from the fireball created in a heavy-ion collision is shown to be a very sensitive measure of the pressure equilibration in the first 1 fm/c of the evolution. Performing a 3+1 dimensional relativistic hydrodynamic calculation with nonequilibrated longitudinal and transverse pressures, we show that the directed flow is strongly reduced if the pressure imbalance survives for even a short time. Transverse momentum spectra, elliptic flow and interferometry correlation radii are not very sensitive to this early pressure anisotropy. Comparison with the data points toward a short equilibration time of the order of $0.25$fm/c or less

Directed flow in ultrarelativistic heavy-ion collisions

We study the generation of directed flow in the hydrodynamic expansion of the hot matter formed in ultrarelativistic heavy-ion collisions at 200GeV. The experimentally observed negative directed flow in a wide range of central pseudorapidities isreproduced assuming that the fireball is tilted away from the collision axis. The tilt of the source is consistent with a preferential emission in the forward/backward hemisphere from forward/backward participating nucleons. The model reproduces the experimentally observed scaling of the directed flow when going from Au-Au to Cu-Cu systems.Comment: figure adde

A comparison of different methods in the study of dynamical fluctuations in high energy e+e- collisions

Different methods in the study of anomalous scaling of factorial moments in high energy e+e- collisions are examined in some detail. It is shown that the horizontal and vertical factorial moments are equivalent only when they are used in combination with the cummulant variables. The influence of different reference frames and that of phase space restrictions is also discussed.Comment: 5 pages, 6 figure

Intermittency in Branching Processes

We study the intermittency properties of two branching processes, one with a uniform and another with a singular splitting kernel. The asymptotic intermittency indices, as well as the leading corrections to the asymptotic linear regime are explicitly computed in an analytic framework. Both models are found to possess a monofractal spectrum with $\varphi_{q}=q-1$. Relations with previous results are discussed.Comment: 20 pages, UCLA93/TEP/2

π+−π−\pi^+ - \pi^- Asymmetry and the Neutron Skin in Heavy Nuclei

In heavy nuclei the spatial distribution of protons and neutrons is different. At CERN SPS energies production of $\pi^+$ and $\pi^-$ differs for $pp$, $pn$, $np$ and $nn$ scattering. These two facts lead to an impact parameter dependence of the $\pi^+$ to $\pi^-$ ratio in $^{208}Pb + ^{208}Pb$ collisions. A recent experiment at CERN seems to confirm qualitatively these predictions. It may open a possibility for determination of neutron density distribution in nuclei.Comment: 6 pages and 2 figures, a talk by A.Szczurek at the international conference MESON2004, June 4-8, Cracow, Polan

On the possible space-time fractality of the emitting source

Using simple space-time implementation of the random cascade model we investigate numerically a conjecture made some time ago which was joining the intermittent behaviour of spectra of emitted particles with the possible fractal structure of the emitting source. We demonstrate that such details are seen, as expected, in the Bose-Einstein correlations between identical particles. \\Comment: Thoroughly rewritten and modify version, to be published in Phys. Rev.

Search for Scaling Dimensions for Random Surfaces with c=1

We study numerically the fractal structure of the intrinsic geometry of random surfaces coupled to matter fields with $c=1$. Using baby universe surgery it was possible to simulate randomly triangulated surfaces made of 260.000 triangles. Our results are consistent with the theoretical prediction $d_H = 2+\sqrt{2}$ for the intrinsic Hausdorff dimension.Comment: 10 pages, (csh will uudecode and uncompress ps-file), NBI-HE-94-3

Multiplicity Distributions and Rapidity Gaps

I examine the phenomenology of particle multiplicity distributions, with special emphasis on the low multiplicities that are a background in the study of rapidity gaps. In particular, I analyze the multiplicity distribution in a rapidity interval between two jets, using the HERWIG QCD simulation with some necessary modifications. The distribution is not of the negative binomial form, and displays an anomalous enhancement at zero multiplicity. Some useful mathematical tools for working with multiplicity distributions are presented. It is demonstrated that ignoring particles with pt<0.2 has theoretical advantages, in addition to being convenient experimentally.Comment: 24 pages, LaTeX, MSUHEP/94071

Anomalous radial expansion in central heavy-ion reactions

The expansion velocity profile in central heavy-ion reactions in the Fermi energy domain is examined. The radial expansion is non-hubblean and in the surface region it scales proportional to a higher exponent ($\alpha > 1$) of the radius. The anomalous expansion velocity profile is accompanied by a power law nucleon density profile in the surface region. Both these features of central heavy-ion reactions disappear at higher energies, and the system follows a uniform Hubble expansion ($\alpha \simeq 1$)