Yan Zhu

Medium Modification of

Two puzzling features in the experimental study of jet quenching in central Pb+Pb collisions at the LHC are explained within a linearized Boltzmann transport model for jet propagation. A γ-tagged jet is found to lose about 15% of its initial energy while its azimuthal angle remains almost unchanged due to rapid cooling of the medium. The reconstructed jet fragmentation function is found to have some modest enhancement at both small and large fractional momenta as compared to that in the vacuum because of the increased contribution of leading particles to the reconstructed jet energy and induced gluon radiation and recoiled partons. A γ-tagged jet fragmentation function is proposed that is more sensitive to jet-medium interaction and the jet transport parameter in the medium. The effects of recoiled medium partons on the reconstructed jets are also discussed.

\gamma

Motivated by applications in thermal QCD and cosmology, we elaborate on a general method for computing next-to-leading order spectral functions for composite operators at vanishing spatial momentum, accounting for real, virtual as well as thermal corrections. As an example, we compute these functions (together with the corresponding imaginary-time correlators which can be compared with lattice simulations) for scalar and pseudoscalar densities in pure Yang-Mills theory. Our results may turn out to be helpful in non-perturbative estimates of the corresponding transport coefficients, which are the bulk viscosity in the scalar channel and the rate of anomalous chirality violation in the pseudoscalar channel. We also mention links to cosmology, although the most useful results in that context may come from a future generalization of our methods to other correlators.

-jets in High-energy Heavy-ion Collisions

Medium excitation by jet shower propagation inside a quark-gluon plasma is studied within a linear Boltzmann transport and a multiphase transport model. Contrary to the naive expectation, it is the deflection of both the jet shower and the Mach-cone-like excitation in an expanding medium that is found to give rise to a double-peak azimuthal particle distribution with respect to the initial jet direction. Such a deflection is the strongest for hadron-triggered jets which are often produced close to the surface of a dense medium due to trigger bias and travel against or tangential to the radial flow. Without such trigger bias, the effect of deflection on γ-jet showers and their medium excitation is weaker. Comparative study of hadron and γ-triggered particle correlations can therefore reveal the dynamics of jet-induced medium excitation in high-energy heavy-ion collisions.

Next-to-leading order thermal spectral functions in the perturbative domain

A bstractWe determine a next-to-leading order result for the shear channel thermal spectral function in SU(N) Yang-Mills theory, working in the limit of vanishing external three-momentum. The result is subsequently applied to the evaluation of the corresponding imaginary time correlator, and its use in the context of sum rules is discussed. Our hope is that the calculation will eventually find use in the nonperturbative determination of the shear viscosity of the theory.

Mach Cone Induced by gamma-Triggered Jets in High-Energy Heavy-Ion Collisions

We determine a next-to-leading order result for the correlator of the shear stress operator in high-temperature Yang-Mills theory. The computation is performed via an ultraviolet expansion, valid in the limit of small distances or large momenta, and the result is used for writing operator product expansions for the Euclidean momentum and co-ordinate space correlators as well as for the Minkowskian spectral density. In addition, our results enable us to confirm and refine a shear sum rule originally derived by Romatschke, Son and Meyer.

The shear channel spectral function in hot Yang-Mills theory

We calculate the centrality dependence of transverse momentum (p{sub t}) spectra for direct photons in Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC) energy {radical}(s{sub NN})=200 GeV, based on a realistic data-constrained (3+1)-dimensional hydrodynamic description of the expanding hot and dense matter, a reasonable treatment of the propagation of partons and their energy loss in the fluid, and a systematic study of the main sources of direct photons. The resultant p{sub t} spectra agree with recent PHENIX data in a broad p{sub t} range. The competition among the different direct photon sources is investigated at various centralities. Parton energy loss in the plasma is considered for photons from fragmentation and jet-photon conversion, which causes about 40% decrease in the total contribution. In the high p{sub t} region, the observed R{sub AA} of photons is centrality independent at the accuracy of 5% based on a realistic treatment of energy loss. We also link the different behavior of R{sub AA} for central and peripheral collisions, in the low p{sub t} region, to the fact that the plasma in central collisions is hotter than that in peripheral ones.

The Ultraviolet limit and sum rule for the shear correlator in hot Yang-Mills theory

The transverse momentum (p t ) dependence, the centrality dependence, and the rapidity dependence of the elliptic flow of thermal photons in Au + Au collisions at √S NN = 200 GeV are predicted on the basis of a three-dimensional ideal hydrodynamic description of the hot and dense matter. The elliptic flow parameter ν 2 , i.e., the second Fourier coefficient of azimuthal distribution, of thermal photons first increases with p t and then decreases for p t > 2 GeV/c, because of the weak transverse flow at the early stage. The p t -integrated v 2 first increases with centrality, reaches a maximum at about 50% centrality, and then decreases. The rapidity dependence of the elliptic flow v 2 (y) of direct photons (mainly thermal photons) is very sensitive to the initial energy density distribution along the longitudinal direction, which provides a useful tool to extract the realistic initial condition from measurements.

Centrality-dependent direct photon p{sub t} spectra in Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC) energy {radical}(s{sub NN})=200 GeV

We calculate the centrality-dependence of transverse momentum (pt) spectra for direct photons in Au+Au collisions at the RHIC energy, based on a realistic data-constrained (3+1) dimensional hydrodynamic description of the expanding hot and dense matter, a reasonable treatment of the propagation of partons and their energy loss in the fluid, and a systematic study of the main sources of direct photons. The resultant pt spectra agree with recent PHENIX data in a broad pt range. The competition among the different direct photon sources is investigated at various centralities. Parton energy loss in the plasma is considered for photons from fragmentation and jet photon conversion, which causes about 40% decrease in the total contribution. In the high pt region, the observed RAA of photons is centrality independent at the accuracy of 5% based on a realistic treatment of energy loss. We also link the different behavior of RAA for central and peripheral collisions, in the low pt region, to the fact that the plasma in central collisions is hotter compared to peripheral ones. PACS numbers:

Elliptic flow of thermal photons inAu+Aucollisions atsNN=200GeV

Abstract The elliptic flow v 2 of thermal photons at midrapidity in Au+Au collisions at S N N = 200 GeV is predicted, based on three-dimensional ideal hydrodynamics. Because of the interplay between the asymmetry and the strength of the transverse flow, the thermal photon v 2 reaches a maximum at p T ∼ 2 GeV/ c and the p T -integrated v 2 reaches a maximum at about 50% centrality. The p T -integrated v 2 is very sensitive to the lower limit of the integral but not sensitive to the upper limit due to the rapid decrease in the spectrum of the transverse momentum.

Centrality-dependent Direct Photon pt spectra in Au+Au Collisions at RHIC

We calculate the centrality-dependence of transverse momentum (pt) spectra for direct photons in Au+Au collisions at the RHIC energy, based on a realistic data-constrained (3+1) dimensional hydrodynamic description of the expanding hot and dense matter, a reasonable treatment of the propagation of partons and their energy loss in the fluid, and a systematic study of the main sources of direct photons. The resultant pt spectra agree with recent PHENIX data in a broad pt range. The competition among the different direct photon sources is investigated at various centralities. Parton energy loss in the plasma is considered for photons from fragmentation and jet photon conversion, which causes about 40% decrease in the total contribution. In the high pt region, the observed RAA of photons is centrality independent at the accuracy of 5% based on a realistic treatment of energy loss. We also link the different behavior of RAA for central and peripheral collisions, in the low pt region, to the fact that the plasma in central collisions is hotter compared to peripheral ones. PACS numbers: