The quark gluon plasma produced in ultra-relativistic heavy-ion collisions exhibits remarkable features. It behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio and leads to the quenching of highly energetic particles. We show that both effects can be understood for the first time within one common framework. Employing the parton cascade Boltzmann Approach to Multi-Parton Scatterings (BAMPS), the microscopic interactions and the space-time evolution of the quark gluon plasma are calculated by solving the relativistic Boltzmann equation. Based on cross sections obtained from perturbative QCD with explicitly taking the running coupling into account, we calculate the nuclear modification factor and elliptic flow in ultra-relativistic heavy-ion collisions. With only one single parameter associated with coherence effects of medium-induced gluon radiation, the experimental data of both observables can be understood on a microscopic level. Furthermore, we show that perturbative QCD interactions with a running coupling lead to a sufficiently small shear viscosity to entropy density ratio of the quark gluon plasma, which provides a microscopic explanation for the observations stated by hydrodynamic calculations.In ultra-relativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN a hot and dense medium is created that consists of quarks and gluons. Experimental data shows that this quark gluon plasma (QGP) possesses a strong collective behavior and that high-energy partons deposit a sizeable amount of their energy in this medium [1,2].The collective behavior is often quantified by the elliptic flow coefficient v 2 , which is the second harmonic of the Fourier decomposition of the azimuthal angle distribution of particle yields. Comparisons to hydrodynamic calculations reveal that the QGP behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio [3]. However, the microscopic reason for this small ratio is currently not understood.Experimental data of the nuclear modification factor R AA , which is defined as the yield in heavy-ion (A+A) collisions divided by the yield in proton-proton (p+p) collisions scaled with the number of binary collisions,and the momentum imbalance of fully reconstructed jets indicate that high-energy particles are quenched by the created medium and lose lots of their energy [1, 2]. Several calculations based on perturbative QCD (pQCD) energy loss in the QGP can describe the experimental data [4][5][6][7][8][9][10][11]. A simultaneous understanding of collective bulk phenomena and jet quenching on the microscopic level remains a challenge, although several partonic transport models [12][13][14][15][16][17] have been developed to address this issue. In this paper we will present new results on both observables obtained with the partonic transport model Boltzmann Approach to Multi-Parton Scatterings (BAMPS). Based on cross sections calculated in pQCD, soft and ha...

We discuss the implementation of leading-order photon production in nonequilibrium partonic transport simulations. In this framework photons are produced by microscopic scatterings, where we include the exact matrix elements of Compton scattering, quark-antiquark annihilation, and bremsstrahlung processes. We show how the hard-thermal loop inspired screening of propagators has to be modified such that the microscopic production rate agrees well with the analytically known resummed leading-order rate. We model the complete quark-gluon plasma phase of heavy-ion collisions by using the partonic transport approach called the Boltzmann approach to multiparton scatterings (BAMPS), which solves the ultrarelativistic Boltzmann equation with Monte Carlo methods. We show photon spectra and elliptic flow of photons from BAMPS and discuss nonequilibrium effects. Due to the slow quark chemical equilibration in BAMPS, the yield is lower than the results from other groups; in turn we see a strong effect from scatterings of energetic jet-like partons with the medium. This nonequilibrium photon production can dominate the thermal emission, such that the spectra are harder and the photonic elliptic flow of the quark-gluon plasma becomes negative.Comment: 20 pages, 24 figures. Revised version as accepted by PR

The early stage of high multiplicity pp, pA and AA collider is represented by a nearly quarkless, hot, deconfined pure gluon plasma. According to pure Yang -Mills Lattice Gauge Theory, this hot pure glue matter undergoes, at a high temperature, Tc = 270 MeV, a first order phase transition into a confined Hagedorn-GlueBall fluid. These new scenario should be characterized by a suppression of high pT photons and dileptons, baryon suppression and enhanced strange meson production. We propose to observe this newly predicted class of events at LHC and RHIC. 12.38.Mh, 24.85.+p The proper understanding of the initial and the early stage of ultra-relativistic pp-, pA-and heavy ion AAcollisions is a topic of great importance for our understanding of hot and dense QCD matter formed in the laboratory and its phase structure.At present, the community favors a paradigm of an extremely rapid (t eq less than 0.3 fm/c) thermalization and chemical saturation of soft gluons and light quarks, their masses and momenta emerging from the decay of coherent massive color flux tubes of strings, which are formed in the primary hadron-hadron collisions.However, for a long time also another scenario has been discussed, namely the hot glue scenario, where the initial stage is dominated by gluons [5,57,68,105].We ask the question whether due to initial state color coherence fluctuations two quite distinct classes of events may exist in collider experiments, or in ultra high energy cosmic ray events, UHECR events. They could be experimentally distinguished in a high statistics analysis of the collider data at RHIC, LHC, and the FCC, from UHE-CRs, or from high intensity fixed target experiments at FAIR [2-4, 29, 38-40, 42, 43, 47, 48, 50, 58, 61, 62, 64, 89-99], NICA [56] and J-Parc.Do soft particles at midrapidity in pp-, pA-, and AAcollider experiments develop from an initially quark-free color glass condensate, CGC, through a pre-equilibrium Glasma-stage into a rapidly chemically saturated, thermalized quarkless pure gluon plasma [110] (see Fig. 1(a)) [36]?The CGC model predicts that the early Glasma is strongly overpopulated -that means that a 'simple' thermally equilibrated Bose-Einstein distribution can NOT exist, as it can not accommodate the overabundant gluons.Hence, dynamically a temporary gluon condensate [19,111] may be formed in order to accomodate those excess gluons, at least transiently, see for example Fig. 1(b).The surprising finding is that only very few soft quarks are present in this early stage according to modern transport calculations [9,10,18,35,84,107] (see, however, also [85,86], where opposite conclusion of quark equilibration is drawn, mostly due to that they put massive gluons there which can easier produce the lighter quarks, while considering Debye screening and other non-perturbative arXiv:1509.00160v1 [hep-ph] 1 Sep 2015

The early stage of high multiplicity nuclear collisions is represented by a nearly quarkless, hot, deconfined pure gluon plasma. These new scenario should be characterized by a suppression of high p T photons and dileptons as well as by reduced baryon to meson ratios. We present the numerical results for central Pb+Pb collisions at the LHC energies by using the ideal Bjorken hydrodynamics with time-dependent quark fugacity. It is shown that about 25 % of final total entropy is generated during the hydrodynamic evolution of chemically undersaturated quark-gluon plasma.

Experimental data measured in √ s = 2.76 TeV Pb + Pb collisions at the LHC show a significant enhancement of events with an unbalanced pair of reconstructed jet momenta in comparison with p + p collisions. This enhancement of momentum imbalance is supposed to be caused by the different momentum loss of the initial back-to-back di-partons by scatterings within the created dense medium. For investigating the underlying partonic momentum loss we employ the on-shell transport model Bamps (Boltzmann Approach for Multi-Parton Scattering) for full heavy-ion collisions, which numerically solves the 3+1D Boltzmann equation based on 2 → 2 as well as inelastic 2 ↔ 3 scattering processes, together with Pythia initial state conditions for the parton showers. Due to the employed test-particle approach jet reconstruction within Bamps events is not trivial. We introduce a method that nevertheless allows the microscopic simulation of the full evolution of the shower particles, recoiled medium particles, and the underlying bulk medium in one common microscopic framework. With this method it is possible to investigate the role of the medium recoil for the momentum imbalance AJ while using well-established background subtraction algorithms. Due to the available particle information in configuration as well as momentum space within Bamps, it is additionally possible to reproduce the entire evolution of the reconstructed jets within the medium. With this information we investigate the sensitivity of the jet momentum loss from the difference in the partonic in-medium path lengths.