No Arabic abstract
Strings at T ~ T_c are known to be subject to the so-called Hagedorn phenomenon, in which a strings entropy (times T) and energy cancel each other and result in the evolution of the string into highly excited states, or string balls. Intrinsic attractive interaction of strings -- gravitational for fundamental strings or in the context of holographic models of the AdS/QCD type, or sigma exchanges for QCD strings -- can significantly modify properties of the string balls. If heavy enough, those start approaching properties of the black holes. We generate self-interacting string balls numerically, in a thermal string lattice model. We found that in a certain range of the interaction coupling constants they morph into a new phase, the entropy-rich string balls. These objects can appear in the so-called mixed phase of hadronic matter, produced in heavy ion collisions, as well as possibly in the high multiplicity proton-proton or proton-nucleus collisions. Among discussed applications are jet quenching in the mixed phase and also the study of angular deformations of the string balls.
We investigate the implications of Nambu-Goto (NG), Luscher-Weisz (LW) and Polyakov-Kleinert (PK) string actions for the Casimir energy of the QCD flux-tube at one and two loop order at finite temperature. We perform our numerical study on the 4-dim pure SU(3) Yang-Mills lattice gauge theory at finite temperature $beta=6.0$. The static quark-antiquark potential is calculated using link-integrated Polyakov loop correlators. At a high temperature-close to the critical point- We find that the rigidity and self-interactions effects of the QCD string to become detectable. The remarkable feature of this model is that it retrieves a correct dependency of the renormalized string tension on the temperature. Good fit to static potential data at source separations $R ge 0.5$ fm is obtained when including additional two-boundary terms of (LW) action. On the other-hand, at a lower temperature-near the QCD plateau- We detect signatures of two boundary terms of the Luscher-Weisz (LW) string action. The (LW) string with boundary action is yielding a static potential which is in a good agreement with the lattice data, however, for color source separation as short as $R=0.3$ fm.
The string breaking phenomenon in QCD can be studied using the gauge/string duality. In this approach, one can make estimates of some of the string breaking distances at non-zero temperature and baryon chemical potential. These point towards the enhancement of baryon production in strong decays of heavy mesons in dense baryonic medium.
We consider a non-critical five dimensional string setup which could provide a dual description of QCD in the limit of large number of colors and flavors. The model corresponds to N_c color D3-branes and N_f D4/anti D4-brane pairs supporting flavor degrees of freedom. The matching of the string model spectrum with the dual field theory one is considered. We discuss the consequences of the possible matching of the gravity modes with the light glueballs and the interpretation of the brane spectrum in Yang-Mills and QCD.
Making use of the gauge/string duality, it is possible to study some aspects of the string breaking phenomenon in the three quark system. Our results point out that the string breaking distance is not universal and depends on quark geometry. The estimates of the ratio of the string breaking distance in the three quark system to that in the quark-antiquark system would range approximately from $frac{2}{3}$ to $1$. In addition, it is shown that there are special geometries which allow more than one breaking distance.
We consider the string breaking phenomenon within effective string models which purport to mimic QCD with two light flavors, with a special attention to baryon modes. We make some estimates of the string breaking distances at zero and non-zero baryon chemical potentials. Our estimates point towards the enhancement of baryon production in strong decays of heavy mesons in dense baryonic matter. We also suggest that the enhanced production of $Lambda_c^+$ baryons in PbPb collisions is mainly due to larger values of chemical potential.