Transverse momentum spectra and elliptic flow in ideal hydrodynamics and geometric scaling

In an ideal hydrodynamic model, with an equation of state where the confinement-deconfinement transition is a cross-over at $T_{co}=196 MeV$, we have simulated $\sqrt{s}$=200 GeV Au+Au collisions. Simultaneous description of the experimental charged particle's $p_T$ spectra and elliptic flow require that in central (0-10%) Au+Au collisions, initial energy density scales with the binary collision number density. In less central collisions, experimental data demand scaling with the participant density. Simulation studies also indicate that in central collisions viscous effects are minimal.Comment: 4 pages, 3 figures

Direct photon production from viscous QGP

We simulate direct photon production in evolution of viscous QGP medium. Photons from Compton and annihilation processes are considered. Viscous effect on photon production is very strong and reliable simulation is possible only in a limited $p_T$ range. For minimally viscous fluid $\eta/s$=0.08), direct photons can be reliably computed only up to $p_T \leq$ 1.3 GeV. With reduced viscosity ($\eta/s$=0.04), the limit increases to $p_T \leq$2GeV.Comment: 6 pages, 5 figure

Di-jet hadron pair correlation in a hydrodynamical model with a quenching jet

In jet quenching, a hard QCD parton, before fragmenting into a jet of hadrons, deposits a fraction of its energy in the medium, leading to suppressed production of high-$p_T$ hadrons. Assuming that the deposited energy quickly thermalizes, we simulate the subsequent hydrodynamic evolution of the QGP fluid. Hydrodynamic evolution and subsequent particle emission depend on the jet trajectories. Azimuthal distribution of excess $\pi^-$ due to quenching jet, averaged over all the trajectories, reasonably well reproduce the di-hadron correlation as measured by the STAR and PHENIX collaboration in central and in peripheral Au+Au collisions.Comment: 5 pages, 4 figures. Some minor corrections are made in the revised manuscrip

Blunting the Spike: the CV Minimum Period

The standard picture of CV secular evolution predicts a spike in the CV distribution near the observed short-period cutoff P_0 ~ 78 min, which is not observed. We show that an intrinsic spread in minimum (`bounce') periods P_b resulting from a genuine difference in some parameter controlling the evolution can remove the spike without smearing the sharpness of the cutoff. The most probable second parameter is different admixtures of magnetic stellar wind braking (at up to 5 times the GR rate) in a small tail of systems, perhaps implying that the donor magnetic field strength at formation is a second parameter specifying CV evolution. We suggest that magnetic braking resumes below the gap with a wide range, being well below the GR rate in most CVs, but significantly above it in a small tail.Comment: 5 pages, 4 figures; accepted for publication in MNRA

Particle-Antiparticle Asymmetry Due to Non-Renormalizable Effective Interactions

We consider a model for generating a particle-antiparticle asymmetry through out-of-equilibrium decays of a massive particle due to non-renormalizable, effective interactions.Comment: preliminary version, 38 pages; LaTeX source, epsf.sty and EPS files included in tar archiv

Nonthermal Supermassive Dark Matter

We discuss several cosmological production mechanisms for nonthermal supermassive dark matter and argue that dark matter may be elementary particles of mass much greater than the weak scale. Searches for dark matter should not be limited to weakly interacting particles with mass of the order of the weak scale, but should extend into the supermassive range as well.Comment: 11 page LaTeX file. No major changes. Version accepted by PR

An Open Inflationary Model for Dimensional Reduction and its Effects on the Observable Parameters of the Universe

Assuming that higher dimensions existed in the early stages of the universe where the evolution was inflationary, we construct an open, singularity-free, spatially homogeneous and isotropic cosmological model to study the effects of dimensional reduction that may have taken place during the early stages of the universe. We consider dimensional reduction to take place in a stepwise manner and interpret each step as a phase transition. By imposing suitable boundary conditions we trace their effects on the present day parameters of the universe.Comment: 5 pages, accepted for publication in Int. J. of Mod. Phys.

Dissipative hydrodynamics in 2+1 dimension

In 2+1 dimension, we have simulated the hydrodynamic evolution of QGP fluid with dissipation due to shear viscosity. Comparison of evolution of ideal and viscous fluid, both initialised under the same conditions e.g. same equilibration time, energy density and velocity profile, reveal that the dissipative fluid evolves slowly, cooling at a slower rate. Cooling get still slower for higher viscosity. The fluid velocities on the otherhand evolve faster in a dissipative fluid than in an ideal fluid. The transverse expansion is also enhanced in dissipative evolution. For the same decoupling temperature, freeze-out surface for a dissipative fluid is more extended than an ideal fluid. Dissipation produces entropy as a result of which particle production is increased. Particle production is increased due to (i) extension of the freeze-out surface and (ii) change of the equilibrium distribution function to a non-equilibrium one, the last effect being prominent at large transverse momentum. Compared to ideal fluid, transverse momentum distribution of pion production is considerably enhanced. Enhancement is more at high $p_T$ than at low $p_T$. Pion production also increases with viscosity, larger the viscosity, more is the pion production. Dissipation also modifies the elliptic flow. Elliptic flow is reduced in viscous dynamics. Also, contrary to ideal dynamics where elliptic flow continues to increase with transverse momentum, in viscous dynamics, elliptic flow tends to saturate at large transverse momentum. The analysis suggest that initial conditions of the hot, dense matter produced in Au+Au collisions at RHIC, as extracted from ideal fluid analysis can be changed significantly if the QGP fluid is viscous.Comment: 11 pages, 10 figures (revised). In the revised version, calculations are redone with ADS/CFT and perurbative estimate of viscosity. Comments on the unphysical effects like early reheating of the fluid, in 1st order dissipative theories are added. The particle spectra calculations are redone with modified programm

Dissipative hydrodynamics for viscous relativistic fluids

Explicit equations are given for describing the space-time evolution of non-ideal (viscous) relativistic fluids undergoing boost-invariant longitudinal and arbitrary transverse expansion. The equations are derived from the second-order Israel-Stewart approach which ensures causal evolution. Both azimuthally symmetric (1+1)-dimensional and non-symmetric (2+1)-dimensional transverse expansion are discussed. The latter provides the formal basis for the hydrodynamic computation of elliptic flow in relativistic heavy-ion collisions including dissipative effects.Comment: 12 pages, no figures. Submitted to Physical Review

Hydrodynamical model for J/ψJ/\psi suppression and elliptic flow

In a hydrodynamic model, we have studied $J/\psi$ suppression and elliptic flow in Au+Au collisions at RHIC energy $\sqrt{s}$=200 GeV. At the initial time, $J/\psi$'s are randomly distributed in the fluid. As the fluid evolve in time, the free streaming $J/\psi$'s are dissolved if the local fluid temperature exceeds a melting temperature $T_{J/\psi}$. Sequential melting of charmonium states ($\chi_c$, $\psi\prime$ and $J/\psi$), with melting temperatures $T_{\chi_c}=T_{\psi\prime} \approx 1.2T_c$, $T_{J/\psi} \approx2T_c$ and feed-down fraction $F\approx 0.3$, is consistent with the PHENIX data on $J/\psi$ suppression and near zero elliptic flow for $J/\psi$'s. It is also shown that the model will require substantial regeneration of charmoniums, if the charmonium states dissolve at temperature close to the critical temperature, $T_{\chi_c}=T_{\psi\prime} \leq T_c$, $T_{J/\psi}\approx1.2T_c$. The regenerated charmoniums will have positive elliptic flow.Comment: 4 pages, 2 figures, to be published in Phys Rev.