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Register a new userHBT interferometry and the parton-hadron phase transition
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Sven Soff
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We discuss predictions for the pion and kaon interferometry measurements in relativistic heavy ion collisions at SPS and RHIC energies. In particular, we confront relativistic transport model calculations that include explicitly a first-order phase transition from a thermalized quark-gluon plasma to a hadron gas with recent data from the RHIC experiments. We critically examine the HBT-puzzle both from the theoretical as well as from the experimental point of view. Alternative scenarios are briefly explained.
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In the late stage of the evolution of a pion system in high-energy heavy-ion collisions when pions undergo multiple scatterings, the quantum transport of the interfering pair of identical pions plays an important role in determining the characteristics of the Hanbury-Brown-Twiss (HBT) interference. We study the quantum transport of the interfering pair using the path-integral method, in which the evolution of the bulk matter is described by relativistic hydrodynamics while the paths of the two interfering pions by test particles following the fluid positions and velocity fields. We investigate in addition the effects of secondary pion sources from particle decays, for nuclear collisions at AGS and RHIC energies. We find that quantum transport of the interfering pair leads to HBT radii close to those for the chemical freeze-out configuration. Particle decays however lead to HBT radii greater than those for the chemical freeze-out configuration. As a consequence, the combined effects give rise to HBT radii between those extracted from the chemical freeze-out configuration and the thermal freeze-out configuration. Proper quantum treatments of the interfering pairs in HBT calculations at the pion multiple scattering stage are important for our understanding of the characteristics of HBT interferometry in heavy-ion collisions.
We study the nucleation of a quark gluon plasma (QGP) phase in a hadron gas at low temperatures and high baryon densities. This kind of process will presumably happen very often in nuclear collisions at FAIR and NICA. When the appropriate energy densities (or baryon densities) and temperatures are reached the conversion of one phase into another is not instantaneous. It is a complex process, which involves the nucleation of bubbles of the new phase. One important element of this transition process is the rate of growth of a QGP bubble. In order to estimate it we solve the Relativistic Rayleigh$-$Plesset equation which governs the dynamics of a relativistic spherical bubble in a strongly interacting medium. The baryon rich hadron gas is represented by the nonlinear Walecka model and the QGP is described by the MIT bag model and also by a mean field model of QCD.
In this work we present the features of the hadron-quark phase transition diagrams in which the pions are included in the system. To construct such diagrams we use two different models in the description of the hadronic and quark sectors. At the quark level, we consider two distinct parametrizations of the Polyakov-Nambu-Jona-Lasinio (PNJL) models. In the hadronic side, we use a well known relativistic mean-field (RMF) nonlinear Walecka model. We show that the effect of the pions on the hadron-quark phase diagrams is to move the critical end point (CEP) of the transitions lines. Such an effect also depends on the value of the critical temperature (T_0) in the pure gauge sector used to parametrize the PNJL models. Here we treat the phase transitions using two values for T_0, namely, T_0 = 270 MeV and T_0 = 190 MeV. The last value is used to reproduce lattice QCD data for the transition temperature at zero chemical potential.
We investigate the process of phase conversion in a thermally-driven {it weakly} first-order quark-hadron transition. This scenario is physically appealing even if the nature of this transition in equilibrium proves to be a smooth crossover for vanishing baryonic chemical potential. We construct an effective potential by combining the equation of state obtained within Lattice QCD for the partonic sector with that of a gas of resonances in the hadronic phase, and present numerical results on bubble profiles, nucleation rates and time evolution, including the effects from reheating on the dynamics for different expansion scenarios. Our findings confirm the standard picture of a cosmological first-order transition, in which the process of phase conversion is entirely dominated by nucleation, also in the case of a weakly first-order transition. On the other hand, we show that, even for expansion rates much lower than those expected in high-energy heavy ion collisions, nucleation is very unlikely, indicating that the main mechanism of phase conversion is spinodal decomposition. Our results are compared to those obtained for a strongly first-order transition, as the one provided by the MIT bag model.
Lev Lipatov was a giant in the field of strong interactions and a dominant force in high-energy QCD for many decades. His work deeply influenced both how we think about QCD and how we perform calculations in the theory. Below we describe the work in two related research directions: the physics of parton saturation and proton spin at small $x$. Both developments would have been impossible without Lipatovs groundbreaking work. Saturation physics would not have happened without the Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation. The recent progress in our theoretical understanding of the proton spin contribution coming from small-$x$ partons started with the seminal paper by Kirschner and Lipatov resumming double logarithms of energy in the Reggeon evolution.
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