Transport Coefficients of Bulk Viscous Pressure in the 14-moment approximation

We compute the transport coefficients that appear in the fluid-dynamical equations for the bulk viscous pressure and shear-stress tensor using the 14-moment approximation in the limit of small, but finite, masses. In this limit, we are able to express all these coefficients in terms of known thermodynamic quantities, such as the thermodynamic pressure, energy density, and the velocity of sound. We explicitly demonstrate that the ratio of bulk viscosity to bulk relaxation time behaves very differently, as a function of temperature, than the ratio of shear viscosity to shear relaxation time. We further explicitly compute, for the first time, the transport coefficients that couple the bulk viscous pressure to the shear-stress tensor and vice versa. The coefficient that couples bulk viscous pressure to shear-stress tensor is found to be orders of magnitude larger than the bulk viscosity itself, suggesting that bulk viscous pressure production owes more to this coupling than to the expansion rate of the system.Comment: 12 pages, 2 figure

Thermal photon radiation in high multiplicity p+Pb collisions at the Large Hadron Collider

The collective behaviour of hadronic particles has been observed in high multiplicity proton-lead collisions at the Large Hadron Collider (LHC), as well as in deuteron-gold collisions at the Relativistic Heavy-Ion Collider (RHIC). In this work we present the first calculation, in the hydrodynamic framework, of thermal photon radiation from such small collision systems. Owing to their compact size, these systems can reach temperatures comparable to those in central nucleus-nucleus collisions. The thermal photons can thus shine over the prompt background, and increase the low $p_T$ direct photon spectrum by a factor of 2-3 in 0-1% p+Pb collisions at 5.02 TeV. This thermal photon enhancement can therefore serve as a clean signature of the existence of a hot quark-gluon plasma during the evolution of these small collision systems, as well as validate hydrodynamic behavior in small systems.Comment: 6 pages, 4 figure

Electroweak Penguin and Leptophobic Z′Z^\prime model

We consider the leptophobic $Z^\prime$ model which can appear naturally in the flipped SU(5) or string-inspired $E_6$ models. This model can be constrained by measurements of the $B\to M \nu\bar{\nu}$ decays and $\Delta m_s$. We find that although the latter give much stronger constraints on the coupling than the former, they are complementary to each other.Comment: Talk given at Summer Institute 2006, APCTP Pohang Korea, August 23-30, 2006, 5 pages, 4 figures, Late

B_s^0 - \bar{B}_s^0 Mixing in Leptophobic Z^\prime Model

Leptophobic Z^\prime gauge boson appears naturally in many grand unified theories, such as flipped SU(5) or string-inspired E_6 models. This elusive particle easily escapes the direct/indirect detections because it does not couple to charged leptons. However, it can generate flavor changing neutral current at tree level. In this letter, we show that the recently measured mass difference, \Delta m_s, in the B_s^0 -\bar{B}_s^0 system improves the previous bound of flavor changing effective coupling by about one order of magnitude, i.e irrespective of its phase, |U_{sb}^{Z^\prime}| \leq 0.036 for M_{Z^\prime} = 700 GeV, and |U_{sb}^{Z^\prime}| \leq 0.051 for M_{Z^\prime} = 1 TeV.Comment: 13 pages, 8 figures, added one reference, Final version to be published in PL

Thermally Fluctuating Second-Order Viscous Hydrodynamics and Heavy-Ion Collisions

The fluctuation-dissipation theorem requires the presence of thermal noise in viscous fluids. The time and length scales of heavy ion collisions are small enough so that the thermal noise can have a measurable effect on observables. Thermal noise is included in numerical simulations of high energy lead-lead collisions, increasing average values of the momentum eccentricity and contributing to its event by event fluctuations.Comment: 13 pages, 4 figure

Nuclear Suppression of Jets and R_AA at the LHC

The nuclear modification factor R_AA for charged hadron production at the LHC is predicted from jet energy loss induced by gluon bremsstrahlung. The Arnold, Moore, and Yaffe formalism is used, together with an ideal hydrodynamical model.Comment: 2 pages, 1 figure, contributed to Workshop on Heavy Ion Collisions at the LHC: Last Call for Predictions, Geneva, Switzerland, 14 May - 8 Jun 200

The importance of the bulk viscosity of QCD in ultrarelativistic heavy-ion collisions

We investigate the consequences of a nonzero bulk viscosity coefficient on the transverse momentum spectra, azimuthal momentum anisotropy, and multiplicity of charged hadrons produced in heavy ion collisions at LHC energies. The agreement between a realistic 3D hybrid simulation and the experimentally measured data considerably improves with the addition of a bulk viscosity coefficient for strongly interacting matter. This paves the way for an eventual quantitative determination of several QCD transport coefficients from the experimental heavy ion and hadron-nucleus collision programs.Comment: 5 pages, 3 figures. Light modifications to text and figures. To be published in PR

Dilepton production in high energy heavy ion collisions with 3+1D relativistic viscous hydrodynamics

We present a first calculation of the dilepton yield and elliptic flow done with 3+1D viscous hydrodynamical simulations of relativistic heavy ion collisions at the top RHIC energy. A comparison with recent experimental data from the STAR collaboration is made.Comment: 4 pages, 3 figures. Talk (GV) given at Quark Matter 201

Effect of initial-state nucleon-nucleon correlations on collective flow in ultra-central heavy-ion collisions

We investigate the effect of nucleon-nucleon correlations on the initial condition of ultra-central heavy ion collisions at LHC energies. We calculate the eccentricities of the MC-Glauber and IP-Glasma models in the 0--1% centrality class and show that they are considerably affected by the inclusion of such type of correlations. For an IP-Glasma initial condition, we further demonstrate that this effect survives the fluid-dynamical evolution of the system and can be observed in its final state azimuthal momentum anisotropy.Comment: 6 pages, 4 figure

(Multi)Strangeness Production in Pb+Pb collisions at LHC. HIJING/BBar v2.0 predictions

Strangeness and multi-strangeness particles production can be used to explore the initial transient field fluctuations in heavy ion collisions. We emphasize the role played by Junction anti-Junction (JJbar) loops and strong color electric fields (SCF) in these collisions. Transient field fluctuations of SCF on the baryon production in central (0-5%) Pb+Pb collisions at nucleon-nucleon centre of mass energy of 5.5 TeV will be discussed in the framework of HIJING/BBbar v2.0 model, looking in particular to the predicted evolution of nuclear modification factors R_AA from RHIC to LHC energies. Our results indicate the importance of a good description of the baseline elementary p+p collisions at this energy.Comment: Comments: 3 pages, 2 figures, submitted to the CERN Theory Institute Workshop, Heavy Ion Collisions at the LHC. Last Call Predictions, May 14 - June 8, 200