Bjoern Schenke

Event-by-event anisotropic flow in heavy-ion collisions from combined Yang-Mills and viscous fluid dynamics

Anisotropic flow coefficients v(1)-v(5) in heavy ion collisions are computed by combining a classical Yang-Mills description of the early time Glasma flow with the subsequent relativistic viscous hydrodynamic evolution of matter through the quark-gluon plasma and hadron gas phases. The Glasma dynamics, as realized in the impact parameter dependent Glasma (IP-Glasma) model, takes into account event-by-event geometric fluctuations in nucleon positions and intrinsic subnucleon scale color charge fluctuations; the preequilibrium flow of matter is then matched to the music algorithm describing viscous hydrodynamic flow and particle production at freeze-out. The IP-Glasma+MUSIC model describes well both transverse momentum dependent and integrated v(n) data measured at the Large Hadron Collider and the Relativistic Heavy Ion Collider. The model also reproduces the event-by-event distributions of v(2), v(3) and v(4) measured by the ATLAS Collaboration. The implications of our results for better understanding of the dynamics of the Glasma and for the extraction of transport properties of the quark-gluon plasma are outlined.

Elliptic and triangular flow in event-by-event D=3+1 viscous hydrodynamics.

We present results for the elliptic and triangular flow coefficients v(2) and v(3) in Au+Au collisions at √s=200  AGeV using event-by-event D=3+1 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 pseudorapidity. 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 η/s more precisely.

Fluctuating Glasma initial conditions and flow in heavy ion collisions

We compute initial conditions in heavy ion collisions within the color glass condensate framework by combining the impact parameter dependent saturation model with the classical Yang-Mills description of initial Glasma fields. In addition to fluctuations of nucleon positions, this impact parameter dependent 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 ε(3)/ε(2) 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.

Hydrodynamic Modeling of Heavy-Ion Collisions

We review progress in the hydrodynamic description of heavy-ion collisions, focusing on recent developments in modeling the fluctuating initial state and event-by-event viscous hydrodynamic simulations. We discuss how hydrodynamics can be used to extract information on fundamental properties of quantum chromodynamics from experimental data, and review successes and challenges of the hydrodynamic framework.

Initial-state geometry and the role of hydrodynamics in proton-proton, proton-nucleus, and deuteron-nucleus collisions

We apply the successful Monte Carlo Glauber and IP-Glasma initial state models of heavy ion collisions to the much smaller size systems produced in proton-proton, proton-nucleus and deuteron- nucleus collisions. We observe a significantly greater sensitivity of the initial state geometry to details of multi-particle production in these models compared to nucleus-nucleus collisions. In particular, we find that the size of the system produced in p+A collisions is very similar to the one produced in p+p collisions, and predict comparable Hanbury-Brown-Twiss radii in the absence of flow in both systems. Differences in the eccentricities computed in the models are large, while differences amongst the generated flow coefficients v_2 and v_3 are smaller. For a large number of participants in proton-lead collisions, the v_2 generated in the IP-Glasma model is comparable to the value obtained in proton-proton collisions. Viscous corrections to flow are large over characteristic lifetimes in the smaller size systems. In contrast, viscous contributions are significantly diminished over the longer space-time evolution of a heavy ion collision.

Event-by-event gluon multiplicity, energy density, and eccentricities in ultrarelativistic heavy-ion collisions.

The event-by-event multiplicity distribution, the energy densities and energy density weighted eccentricity moments ǫn (up to n = 6) at early times in heavy-ion collisions at both RHIC ( √ s = 200GeV) and LHC ( √ s = 2.76TeV) 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 YangMills 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.

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.

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

Abstract 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.

Eccentric protons? Sensitivity of flow to system size and shape in p+p, p+Pb, and Pb+Pb collisions.

We determine the transverse system size of the initial nonequilibrium Glasma state and of the hydrodynamically evolving fireball as a function of produced charged particles in p+p, p+Pb, and Pb+Pb collisions at the Large Hadron Collider. Our results show features similar to those of recent measurements of Hanbury Brown-Twiss (HBT) radii by the ALICE Collaboration. Azimuthal anisotropy coefficients vn generated by combining the early time Glasma dynamics with viscous fluid dynamics in Pb+Pb collisions are in excellent agreement with experimental data for a wide range of centralities. In particular, event-by-event distributions of the vn values agree with the experimental data out to fairly peripheral centrality bins. In striking contrast, our results for p+Pb collisions significantly underestimate the magnitude and do not reproduce the centrality dependence of data for v2 and v3 coefficients. We argue that the measured vn data and HBT radii strongly constrain the shapes of initial parton distributions across system sizes that would be compatible with a flow interpretation in p+Pb collisions. Alternately, additional sources of correlations may be required to describe the systematics of long-range rapidity correlations in p+p and p+Pb collisions.

Jet broadening in unstable non-Abelian plasmas

We perform numerical simulations of the SU(2) Boltzmann-Vlasov equation including both hard elastic particle collisions and soft interactions mediated by classical Yang-Mills fields. Using this technique we calculate the momentum-space broadening of high-energy jets in real time for both locally isotropic and anisotropic plasmas. In both cases we introduce a separation scale that separates hard and soft interactions and demonstrate that our results for jet broadening are independent of the precise separation scale chosen. For an isotropic plasma this allows us to calculate the jet transport coefficient q-circumflex including hard and soft nonequilibrium dynamics. For an anisotropic plasma the jet transport coefficient becomes a tensor with q-circumflex{sub L}{ne}q-circumflex{sub perpendicular}. We find that for weakly coupled anisotropic plasmas the fields develop unstable modes, forming configurations where B{sub perpendicular}>E{sub perpendicular} and E{sub z}>B{sub z}, which lead to q-circumflex{sub L}>q-circumflex{sub perpendicular}. We study whether the effect is strong enough to explain the experimental observation that high-energy jets traversing the plasma perpendicular to the beam axis experience much stronger broadening in rapidity, {delta}{eta}, than in azimuth, {delta}{phi}.