arXiv subjects

Bjoern Schenke

Publications and source records attributed to Bjoern Schenke.

At least 73 records · Page 4Linked to original sources

arXiv2012

Event-by-event gluon multiplicity, energy density and eccentricities at RHIC and LHC

The event-by-event multiplicity distribution, the energy densities and energy density weighted eccentricity moments epsilon_n (up to n=6) at early times in heavy-ion collisions at both RHIC (root-s=200 GeV) and LHC (root-s=2.76 TeV) are computed in the IP-Glasma model. This framework combines the impact parameter dependent saturation model (IP-Sat) for nucleon parton distributions (constrained by HERA deeply inelastic scattering data) with an event-by-event classical Yang-Mills description of early-time gluon fields in heavy-ion collisions. The model produces multiplicity distributions that are convolutions of negative binomial distributions without further assumptions or parameters. The eccentricity moments are compared to the MC-KLN model; a noteworthy feature is that fluctuation dominated odd moments are consistently larger than in the MC-KLN model.

hep-ph↗

arXiv2012

Fluctuating Glasma initial conditions and flow in heavy ion collisions

We compute initial conditions in heavy-ion collisions within the Color Glass Condensate (CGC) framework by combining the impact parameter dependent saturation model (IP-Sat) with the classical Yang-Mills description of initial Glasma fields. In addition to fluctuations of nucleon positions, this IP-Glasma description includes quantum fluctuations of color charges on the length-scale determined by the inverse nuclear saturation scale Q_s. The model naturally produces initial energy fluctuations that are described by a negative binomial distribution. The ratio of triangularity to eccentricity is close to that in a model tuned to reproduce experimental flow data. We compare transverse momentum spectra and v_(2,3,4)(p_T) of pions from different models of initial conditions using relativistic viscous hydrodynamic evolution.

nucl-th↗

arXiv2011

Viscous photons in relativistic heavy ion collisions

Theoretical studies of the production of real thermal photons in relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) are performed. The space-time evolution of the colliding system is modelled using MUSIC, a 3+1D relativistic hydrodynamic simulation, using both its ideal and viscous versions. The inclusive spectrum and its azimuthal angular anisotropy are studied separately, and the relative contributions of the different photon sources are highlighted. It is shown that the photon v2 coefficient is especially sensitive to the details of the microscopic dynamics like the equation of state, the ratio of shear viscosity over entropy density, eta/s, and to the morphology of the initial state.

hep-ph↗

arXiv2011

MARTINI for heavy quarks: initialization, parton evolution, and hadronization with event generation

We present additions to the MARTINI event generator for examining heavy quarks and quarkonia in heavy-ion collisions. All stages of a heavy-ion collision affect the observables associated with heavy quarks: the initial phase space of the heavy quarks are sampled with PYTHIA8.1, the heavy quarks are evolved using Langevin dynamics and a 3+1-dimensional hydrodynamical description of the heavy-ion collision, and are fragmented and hadronized using a modified version of the color evaporation model that takes into account non-trivial evolution in position space, as well as the possibility of recombinant quarkonium production in heavy-ion collisions. We use this to re-examine the production of quarkonium at RHIC, and anticipating vertex detection we predict yields of B_c mesons at RHIC and the LHC.

nucl-th↗

arXiv2011

Higher flow harmonics from (3+1)D event-by-event viscous hydrodynamics

We present event-by-event viscous hydrodynamic calculations of the anisotropic flow coefficients v_2 to v_5 for heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC). We study the dependence of different flow harmonics on shear viscosity and the morphology of the initial state. v_3 and higher flow harmonics exhibit a particularly strong dependence on both the initial granularity and shear viscosity. We argue that a combined analysis of all available flow harmonics will allow to determine eta/s of the quark gluon plasma precisely. Presented results strongly hint at a value (eta/s)_QGP < 2/4pi at RHIC. Furthermore, we demonstrate the effect of shear viscosity on pseudo-rapidity spectra and the mean transverse momentum as a function of rapidity.

hep-ph↗

arXiv2011

Monte-Carlo simulation of jets in heavy-ion collisions

We present Monte-Carlo simulations of jet evolution in lead-lead collisions at the Large Hadron Collider (LHC) at CERN focusing on the dijet asymmetry measured by the ATLAS and CMS collaborations. In the simulation, hard partons are interacting with the hydrodynamical background medium, undergoing radiative and collisional processes. The measured dijet asymmetry is well described by the simulation and is hence consistent with partonic energy loss in a hot, strongly-interacting medium.

hep-ph↗

arXiv2011

Renormalization group evolution of multi-gluon correlators in high energy QCD

Many-body QCD in leading high energy Regge asymptotics is described by the Balitsky-JIMWLK hierarchy of renormalization group equations for the x evolution of multi-point Wilson line correlators. These correlators are universal and ubiquitous in final states in deeply inelastic scattering and hadronic collisions. For instance, recently measured di-hadron correlations at forward rapidity in deuteron-gold collisions at the Relativistic Heavy Ion Collider (RHIC) are sensitive to four and six point correlators of Wilson lines in the small x color fields of the dense nuclear target. We evaluate these correlators numerically by solving the functional Langevin equation that describes the Balitsky-JIMWLK hierarchy. We compare the results to mean-field Gaussian and large N_c approximations used in previous phenomenological studies. We comment on the implications of our results for quantitative studies of multi-gluon final states in high energy QCD.

hep-ph↗

arXiv2011

Photons at RHIC: The role of viscosity and of initial state fluctuations

We study real photons produced in heavy ion collisions at RHIC, and we calculate their spectrum and its azimuthal momentum anisotropy. The photons from a variety of sources are included, and the interplay and the time-evolution of those sources are modelled in a full 3D hydrodynamic simulation. We quantify the v_2 of thermal photons produced in ideal and viscous fluids, and the consequences of using different initial conditions are explored.

hep-ph↗

arXiv2011

Flow in heavy-ion collisions - Theory Perspective

I review recent developments in the field of relativistic hydrodynamics and its application to the bulk dynamics in heavy-ion collisions at the Relativistic Heavy- Ion Collider (RHIC) and the Large Hadron Collider (LHC). In particular, I report on progress in going beyond second order relativistic viscous hydrodynamics for conformal fluids, including temperature dependent shear viscosity to entropy density ratios, as well as coupling hydrodynamic calculations to microscopic hadronic rescattering models. I describe event-by-event hydrodynamic simulations and their ability to compute higher harmonic flow coefficients. Combined comparisons of all harmonics to recent experimental data from both RHIC and LHC will potentially allow to determine the desired details of the initial state and the medium properties of the quark-gluon plasma produced in heavy-ion collisions.

nucl-th↗

arXiv2011

Anisotropic flow in sqrt(s)=2.76 TeV Pb+Pb collisions at the LHC

The results on elliptic flow in sqrt(s)=2.76 TeV Pb+Pb collisions at the Large Hadron Collider (LHC) reported by the ALICE collaboration are remarkably similar to those for sqrt(s)=200 GeV gold-gold collisions at the Relativistic Heavy Ion Collider (RHIC). This result is surprising, given the expected longer lifetime of the system at the higher collision energies. We show that it is nevertheless consistent with 3+1 dimensional viscous event-by-event hydrodynamic calculations, and demonstrate that elliptic flow at both RHIC and LHC is built up mostly within the first 5 fm/c of the evolution. We conclude that an "almost perfect liquid" is produced in heavy-ion collisions at the LHC. Furthermore, we present predictions for triangular flow as a function of transverse momentum for different centralities.

hep-ph↗

arXiv2011

Hydrodynamic evolution and jet energy loss in Cu+Cu collisions

We present results from a hybrid description of Cu+Cu collisions using 3+1 dimensional hydrodynamics (MUSIC) for the bulk evolution and a Monte-Carlo simulation (MARTINI) for the evolution of high momentum partons in the hydrodynamical background. We explore the limits of this description by going to small system sizes and determine the dependence on different fractions of wounded nucleon and binary collisions scaling of the initial energy density. We find that Cu+Cu collisions are well described by the hybrid description at least up to 20% central collisions.

hep-ph↗

arXiv2010

Elliptic and triangular flow in event-by-event (3+1)D viscous hydrodynamics

We present results for the elliptic and triangular flow coefficients in Au+Au collisions at root-s=200 AGeV using event-by-event (3+1)D viscous hydrodynamic simulations. We study the effect of initial state fluctuations and finite viscosities on the flow coefficients v_2 and v_3 as functions of transverse momentum and pseudo-rapidity. Fluctuations are essential to reproduce the measured centrality dependence of elliptic flow. We argue that simultaneous measurements of v_2 and v_3 can determine eta/s more precisely.

hep-ph↗

arXiv2010

3+1D hydrodynamic simulation of relativistic heavy-ion collisions

We present MUSIC, an implementation of the Kurganov-Tadmor algorithm for relativistic 3+1 dimensional fluid dynamics in heavy-ion collision scenarios. This Riemann-solver-free, second-order, high-resolution scheme is characterized by a very small numerical viscosity and its ability to treat shocks and discontinuities very well. We also incorporate a sophisticated algorithm for the determination of the freeze-out surface using a three dimensional triangulation of the hyper-surface. Implementing a recent lattice based equation of state, we compute p_T-spectra and pseudorapidity distributions for Au+Au collisions at root s = 200 GeV and present results for the anisotropic flow coefficients v_2 and v_4 as a function of both p_T and pseudorapidity. We were able to determine v_4 with high numerical precision, finding that it does not strongly depend on the choice of initial condition or equation of state.

hep-ph↗

arXiv2009

MARTINI - Monte Carlo simulation of jet evolution

We present the Modular Algorithm for Relativistic Treatment of heavy IoN Interactions (MARTINI), an event generator for the hard and penetrating probes in high energy nucleus-nucleus collisions. The simulation consists of a time evolution model for the soft background, such as hydrodynamics, PYTHIA 8.1 to generate and hadronize the hard partons after the medium evolution, which is based on the McGill-AMY formalism and includes both radiative and elastic processes. MARTINI allows for the generation of full event configurations in the high transverse momentum region. We present results for the neutral pion and photon nuclear modification factor in Au+Au collisions at RHIC.

hep-ph↗

arXiv2009

MARTINI: An event generator for relativistic heavy-ion collisions

We introduce the Modular Algorithm for Relativistic Treatment of heavy IoN Interactions (MARTINI), a comprehensive event generator for the hard and penetrating probes in high energy nucleus-nucleus collisions. Its main components are a time evolution model for the soft background, PYTHIA 8.1 and the McGill-AMY parton evolution scheme including radiative as well as elastic processes. This allows us to generate full event configurations in the high p_T region that take into account thermal QCD and QED effects as well as effects of the evolving medium. We present results for the neutral pion nuclear modification factor in Au+Au collisions at RHIC as a function of p_T for different centralities, and also as a function of the angle with respect to the reaction plane for non-central collisions. Furthermore, we study the production of high transverse momentum photons incorporating a complete set of photon-production channels.

hep-ph↗

arXiv2009

Jet evolution in Yang-Mills-Wong simulations

We present results for collisional energy loss and momentum broadening of high momentum partons in a hot and dense non-Abelian plasma obtained by solving the coupled system of Yang-Mills-Wong equations on a lattice in real time. Including hard elastic collisions among the particles we obtain cutoff independent results for the collisional energy loss dE/dx and the transport coefficient q-hat. The latter is found to receive a sizable contribution from a power-law tail in the transverse momentum distribution of high-momentum partons. We further argue that the effect of instabilities on jet broadening should be accessible by experiment when employing jet cones with elliptical bases or studying correlations within the cone in full jet reconstruction methods.

hep-ph↗

arXiv2009

The evolving distribution of hard partons traversing a hot strongly interacting plasma

We present results for the evolution of the momentum distribution of a hard parton traversing a brick of quark-gluon plasma, considering both bremsstrahlung and collisional energy loss. The complete leading order transition rates are included. We find a significant modification of the evolving momentum distribution compared to results obtained by employing an approximate implementation of the collisional energy loss that uses the mean energy loss, together with momentum diffusion.

hep-ph↗

arXiv2009

Transverse momentum diffusion and jet energy loss in non-Abelian plasmas

We consider momentum broadening and energy loss of high momentum partons in a hot non-Abelian plasma due to collisions. We solve the coupled system of Wong-Yang-Mills equations on a lattice in real time, including binary hard elastic collisions among the partons. The collision kernel is constructed such that the total collisional energy loss and momentum broadening are lattice spacing independent. We find that the transport coefficient $\hat{q}$ corresponding to transverse momentum broadening receives sizable contributions from a power-law tail in the $p_\perp$-distribution of high-momentum partons. We establish the scaling of $\hat{q}$ and of $dE/dx$ with density, temperature and energy in the weak-coupling regime. We also estimate the nuclear modification factor $R_{AA}$ due to elastic energy loss of a jet in a classical Yang-Mills field.

hep-ph↗