Hadronization via quark coalescence of the quark gluon plasma at ultrarelativistic heavy-ion collisions

Abstract

Quantum Chromodynamics (QCD) is the fundamental theory that governs the dynamics of strongly interacting particles and has quarks and gluons as elementary constituents, they represent the fundamental degrees of freedom of the theory carrying the "color" charge. The QCD have two peculiar features: colour confinement and asymptotic freedom. The asymptotic freedom allow us to study the strong interaction in a perturbative regime for sufficiently high energy processes, and implies that under particular condition of temperature or density the strong interaction that confines quarks and gluons becomes smaller enough to release them. Hence a new state of matter can exist in which the colour charges are deconfined in a Quark Gluon Plasma (QGP), the predictions of Lattice QCD indicate that the critical temperature in which the nuclear matter experiences a phase transition is Tc~160 MeV . Heavy Ion Collisions (HIC) at ultrarelativistic energy can be used to probe the properties of nuclear matter under extreme condition. In the studies of the QGP created in HIC is necessary to consider that partonic behaviour in QGP is not directly projected on the observables measured in experiments. This happens because the quark and gluon constituents must combine into hadrons. Thus the choice of the model for Hadronization process is a crucial point in order to have a comparison with experimental data. We are interested on an approach with the problem of hadronization of QGP that takes care about microscopic mechanism of hadronization. In the fragmentation scheme each parton fragments into a jet of hadrons which carries a fraction of initial parton momentum. Instead coalescence model describes the recombination of two or three quarks adding their momenta to form mesons or baryons. The coalescence model can predict the enhancement of baryon to meson ratio and the scaling of elliptic flow (v2) according to the constituent quark number that are observed at RHIC. We have implemented an hadronization model based on a quark coalescence mechanism. Our first purpose has been to reproduce the spectra and the particle ratio at RHIC and LHC implementating a coalescence model applied on a static medium. At RHIC and LHC we obtain a good description of spectrum in the whole pt range. Furthermore our model reproduces experimental data for both proton to pion ratio and Lambda to kaons ratio that was one component of the so-called "baryonic puzzle". We can see that the ratio is quite well predicted from its rise at low pt up to the peak region and then the falling-down behaviour. However in both cases it is clear that in the region high pt there is a lack of baryon yield and one can say that it seems that the spectrum from fragmentation appears too flat. Then we have studied the hadronization effect with a coalescence model applied in the heavy quark sector. Several theoretical efforts show some difficulties to describe simultaneously the Raa and the v2. Coalescence modify the relation between these two observables and is fundamental to reproduce the experimental data. In the final part we have presented a more realistic implementation of coalescence model, in which we have developed a model self-consistently applied to the freeze-out hypersurface of a Boltzmann Transport equation. Comparing the spectra of pions, kaons and protons with the experimental data at RHIC and LHC we find a really good agreement in the intermediate pt region. While for higher pt we slightly underestimate the experimental data, because the partonic spectrum results over-suppressed in the high pt region. Finally we have studied the v2 for pions and we have obtained that the coalescence overestimate the v2 observed experimentally for pt>2 GeV, on the other hand the fragmentation v2 is about two times smaller than the experimental data. But an approach that take in account both coalescence and fragmentation is able to reasonably describe the v2 behaviour in a large pt range.