High-energy QCD asymptotic behavior of photon-photon collisions

Complex quasienergy and level width are calculated for a weakly bound atomic state in an intense circularly polarized monochromatic laser field using the method suggested by Zel’dovich for the regularization of divergent integrals with the Gamow wave function. It is demonstrated that this method converges, and the conditions for its applicability are indicated. These results are used to discuss the accuracy of the semiclassical approximation in the problems of ionization theory.     

High-energy hadron-hadron collisions and internal hadron structure

As shown by many authors, central particle emission in pp collisions at centre-of-mass energies ? 20 GeV can be described in terms of independent cluster production. In ref. [1] this process has been related to the quark-gluon picture of nucleon structure. The present paper carries this relation further and links the leading particle spectrum of non-diffractive hadron-hadron collisions to the x distribution of quarks in the hadron. Thus, the flat leading particle spectrum, as observed in non-diffractive pp collisions, is found to be naturally related to the steeply falling x distribution of quarks in the nucleon, as deduced from the structure functions measured in deep inelastic scattering of electrons and neutrinos on nucleons. Our analysis is extended to meson-nucleon collisions and leads to a prediction of the general shape of the quark x distribution in a meson.     

High-energy photons as a thermometer for ultra-relativistic nuclear collisions

I calculate the transverse momentum distribution for high-energy (p _T =1–3 GeV) photons produced in ultra-relativistic nuclear collisions. I assume a strong first-order deconfinement phase transition, and consider transition temperatures in the range 150–200 MeV. For simplicity, I also assume thermal and chemical equilibrium throughout the collision. I then fit the transverse momentum distribution in the range 1–2 GeV to a thermal distribution. The fitting temperature depends only on the transition temperature, so this provides an accurate (although theory-dependent) measure of the QCD transition temperature.     

Elliptic flow in central collisions of deformed nuclei

Nontrivial geometrical effects in relativistic central collisions of deformed nuclei are studied using a simple version of the optical Glauber model. For very small impact parameters, large centrality and eccentricity fluctuations are observed. In very high-multiplicity collisions of oblate nuclei, a significant fraction of events with nonzero elliptic-flow strength υ ₂ proportional to oblateness parameter −β ₂ is predicted. The text was submitted by the author in English.     

Scattering of particles by deformed non-rotating black holes

We study the excitation of axial quasi-normal modes of deformed non-rotating black holes by test particles and we compare the associated gravitational wave signal with that expected in general relativity from a Schwarzschild black hole. Deviations from standard predictions are quantified by an effective deformation parameter, which takes into account deviations from both the Schwarzschild metric and the Einstein equations. We show that, at least in the case of non-rotating black holes, it is possible to test the metric around the compact object, in the sense that the measurement of the gravitational wave spectrum can constrain possible deviations from the Schwarzschild solution.     

Evaporation of charged particles from highly deformed nucleus

Energy spectra of charged particles evaporated from compound nucleus have been of interest because current statistical-model codes do not reproduce the experimental results well. Deformation has been proposed as a cause of the obsered anomaly, but an adequate theoretical expression for the emission rate from deformed nucleus is as yet lacking. We present here a closed form expression for the emission width of charged particles from nuclei of arbitrary shape. In addition, we propose a new evaporation mechanism in which charged particles are emitted while leaving the residual nucleus in its fissionsaddle-point configuration. Applying this model to alpha-particle emission, we obtain a very low energy alpha-particle component which may have been observed experimentally.     

Emission of direct particles in heavy ions collisions

This paper presents a theoretical model for the emission of fast light particles derived from the PEP (Promptly Emitted Particles) model. This model is extended to the emission of fast alpha particles. With some restrictions, analytical formulae are derived for the production rate and the mean angular momentum associated with fast particles. Most of the qualitative features predicted by the analytical formulae are confirmed by the numerical results obtained from a more realistic calculation where the trajectories of the particles are followed during the whole collision. It is argued that PEP alpha particles emitted preferentially in low angular momentum collisions, could represent the major part of the fast alpha particles observed in incomplete fusion reactions.     

Emission of large-p T particles inp-nucleus and nucleus-nucleus collisions

The observed dependence of the yield of highpT particles on the atomic numberA of the target and the incident energy, inp-a, a-a andp-nucleus collisions, is explained in a coherent tube model.     

Multiplicity distribution of strange particles in heavy ion collisions

We study the multiplicity distribution of strange particles in a hadronic gas constrained by exact strangeness conservation. The multiplicity distribution obtained is narrower than both the Poisson and negative binomial distributions. Correlations among strange particles are also discussed. The results presented might be useful in determining the thermodynamic parameters (volume, temperature and baryon density) of a hadronic gas possibly formed in relativistic nuclear collisions.     

Electron-positron pair production by collisions of non-relativistic charged particles

The cross-section for (e ⁺,e ^?)-pair production in collisions of non-relativistic charged particles has been calculated to lowest order in the fine structure constant and leading power of the relative velocity of the colliding particles. Thus the deflection of colliding particles is included in one-photon-exchange approximation, while the Coulomb distortion of the (e ⁺,e ^?)-pair is omitted. We have laid particular emphasis on current conservation. The previously neglected contribution of the spatial part of the current turns out to be dominant.     

Formation of dense groups of particles in intermediate-energy nucleus-nucleus collisions

The formation of dense groups (fluctuations) of particles produced in the central CCu and MgMg collisions at the projectile momenta of, respectively, 4.5 and 4.3 GeV/c per nucleon is analyzed. The distributions of the maximum densities and of the centers of charged-particle fluctuations in pseudorapidity space are studied in searches for dynamical multiparticle correlations. The distributions of the centers show two peaks above the statistical background with a structure similar to that which is expected in the model of coherent gluon emission and which was observed in hadronic interactions. The charge independence of the distributions in question and an azimuthal isotropy of events involving pseudorapidity fluctuations are observed. The distributions of events with respect to the maximum density of fluctuations are governed primarily by the statistical contribution, although the behavior of the distributions in CCu collisions is in qualitative agreement with the prediction of the one-dimensional intermittency model. It is found that the resulting distributions are of a non-Poisson character both in CCu and in MgMg collisions. The results of this study indicate that, in describing local dynamical fluctuations in multiparticle production processes, the coherent and the stochastic approach supplement each other. The procedure employed in the analysis described here makes it possible to draw a direct comparison of the present results with the results of similar investigations of different reactions.