The quark gluon plasma produced in ultrarelativistic 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 multiparton scatterings, 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 ultrarelativistic 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.
Experimental data measured in 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 ( ) for full heavy-ion collisions, which numerically solves the 3 + 1D Boltzmann equation based on as well as inelastic 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.
The partonic transport approach BAMPS (Boltzmann Approach to Multi-Parton Scatterings) aims to describe microscopically the partonic phase of ultra-relativistic heavy-ion collisions by perturbative ...quantum chromodynamics. To this end, it numerically solves the 3+1D Boltzmann equation by allowing interactions among all parton species: both elastic 2→2 and inelastic 2↔3 collisions for gluons as well as quarks. For the inelastic collisions, we employ the improved Gunion–Bertsch matrix element, which cures problems of the original Gunion–Bertsch result in characteristic regions of the phase space. Based on extensive numerical calculations, the improved matrix element agrees well with the exact pQCD calculation. By employing the improved GB matrix element together with a running coupling evaluated at the microscopic scale, we present our recent results for the suppression of high pt particles as well as the collectivity of the bulk medium and compare them to LHC data of the nuclear modification factor RAA and the elliptic flow v2. Furthermore, we present studies of RAA for different hadron species and the momentum imbalance AJ of reconstructed jets.
Focusing on the simultaneous investigation of the hard and the soft regime of ultra-relativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) at the Brookhaven National ...Laboratory (BNL) and the Large Hadron Collider (LHC) at CERN, we present our recent results on the nuclear modification factor RAA and the elliptic flow v2 within the partonic transport model BAMPS (Boltzmann Approach to Multi-Parton Scatterings). While using interactions provided by perturbative QCD, BAMPS allows the full 3+1D simulation of the quark–gluon plasma (QGP) at the microscopic level by solving the relativistic Boltzmann equation for quarks and gluons. The microscopic interactions include both elastic 2→2 collisions, calculated by screened leading order pQCD matrix elements, and inelastic 2↔3 processes, calculated within an improved Gunion–Bertsch approximation. Furthermore, for all partonic processes a microscopic, running coupling is explicitly taken into account. We show that when fixing the parameter XLPM resulting from the effective modeling of the Landau–Pomeranchuk–Migdal effect, we find not only a good description of RAA at both RHIC and LHC, but also a sizable elliptic flow is built up within the partonic phase.
The measured data on the nuclear modification factor for pions and reconstructed jets as well as on the high-pT elliptic flow at RHIC and LHC energies are compared to results from a linear pQCD and a ...highly non-linear hybrid AdS holographic model of jet-energy loss. We find that the high-pT ellitic flow requires to include realistic medium transverse flow fields and a jet-medium coupling including the effects of the energy of the jet, the temperature of the bulk medium, and non-equilibrium effects close to the phase transition. We extend our jet-energy loss model that is coupled to state-of-the-art hydrodynamic prescriptions to backgrounds generated by the parton cascade BAMPS. We demonstrate that the results for the hydrodynamic and the parton-cascade backgrounds show a remarkable similarity. Unfortunately, the results for both the pion and a parton-jet nuclear modification factor are insensitive to the jet-path dependence of the models considered.
Experiments have measured an elliptic flow of direct photons comparable to the flow of charged particles. Employing an on-shell partonic cascade, we investigate the microscopic processes that might ...lead to this unexpected large flow of photons. Compton scattering and quark-antiquark annihilation are important photon production processes in the QGP, which we implement in the cascade and subsequently show the agreement of their spectra in thermal equilibrium with analytic results. In order to be comparable with state-of-the art photon production rates at leading order, we make progress towards bremsstrahlung diagrams by computing their differential and total cross sections. Running the cascade in heavy-ion collision setups for RHIC and LHC, we find a very small photonic elliptic flow from the quark-gluon plasma phase.
The early stage of a high multiplicity pp, pA and AA collisions 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, a first-order phase transition into a confined Hagedorn glueball fluid. This 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 the Large Hadron Collider and the Relativistic Heavy-Ion Collider.
After the spherical theta pinch had been developed at the Institute of Applied Physics in Frankfurt, considerable progress in creating high efficiency and long lifetime plasma sources had been made. ...Several devices have been built to study the characteristics of the spherically confined plasma. Scaling rules and investigated setups are presented, showing devices with electron densities of up to some 10 23 m -3 , electron temperatures of several eV and confinement times of ~10 μs. Typical dimensions are: discharge vessel radius R = 10 cm, radius of the pinched plasma r = 2 cm, initial gas pressures of 1-200 Pa at currents of up to 35 kA and resonance frequencies of 10-40 kHz. During the harmonic discharge of the stored energy, the plasma periodically pinches and expands again as long as the current rise rate creates sufficient electrical field strengths for ionization, which allows for pinching sequences in the millisecond range. The principles, improvements, theoretical approaches, and applications including pulsed ion source, plasma ion stripping, and vacuum ultraviolet flash lighting are discussed.
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