12,546 research outputs found
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 , we
have simulated =200 GeV Au+Au collisions. Simultaneous description of
the experimental charged particle's 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 range. For minimally viscous fluid =0.08), direct
photons can be reliably computed only up to 1.3 GeV. With reduced
viscosity (=0.04), the limit increases to 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- 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 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 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
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 than at
low . 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
Constraints on radiative decay of the 17-keV neutrino from COBE Measurements
It is shown that, for a nontrivial radiative decay channel of the 17-keV
neutrino, the photons would distort the microwave background radiation through
ionization of the universe. The constraint on the branching ratio of such
decays from COBE measurements is found to be more stringent than that from
other considerations. The limit on the branching ratio in terms of the Compton
parameter is for an
universe.Comment: 7 pages. (figures will be sent on request) (To appear in Phys. Rev.
D.
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