4,899 research outputs found
Cosmic Evolution with Early and Late Acceleration Inspired by Dual Nature of the Ricci Scalar Curvature
In the present paper, it is found that dark energy emerges spontaneously from
the modified gravity. According to cosmological scenario, obtained here, the
universe inflates for sec. in the beginning and late universe
accelerates after 8.58 Gyrs. During the long intermediate period, it
decelerates driven by radiation and subsequently by matter. Emerged
gravitational dark energy mimics quintessence and its density falls by 115
orders from its initial value to its current
value .Comment: 40 pages. To appearin Int. J. Mod. Phys.
Empirical determination of charm quark energy loss and its consequences for azimuthal anisotropy
We propose an empirical model to determine the form of energy loss of charm
quarks due to multiple scatterings in quark gluon plasma by demanding a good
description of production of D mesons and non-photonic electrons in
relativistic collision of heavy nuclei at RHIC and LHC energies. Best results
are obtained when we approximate the momentum loss per collision , where is a constant depending on the
centrality and the centre of mass energy. Comparing our results with those
obtained earlier for drag coefficients estimated using Langevin equation for
heavy quarks we find that up to half of the energy loss of charm quarks at top
RHIC energy could be due to collisions while that at LHC energy at 2760 GeV/A
the collisional energy loss could be about one third of the total. Estimates
are obtained for azimuthal anisotropy in momentum spectra of heavy mesons, due
to this energy loss. We further suggest that energy loss of charm quarks may
lead to an enhanced production of D-mesons and single electrons at low in
AA collisions.Comment: 11 pages, 3 figures, Typographical errors corrected, Key-words and
PACS indices added, sequence of figures corrected, references added in
section 3, discussions expande
Evolution of mechanism of parton energy loss with transverse momentum at RHIC and LHC in relativistic collision of heavy nuclei
We analyze the suppression of particle production at large transverse momenta
in ( most) central collisions of gold nuclei at
200 GeV and lead nuclei at 2.76 TeV. Full
next-to-leading order radiative corrections at , and
nuclear effects like shadowing and parton energy loss are included. The parton
energy loss is implemented in a simple multiple scattering model, where the
partons lose an energy per collision, where
is their mean free path. We take for a treatment
which is suggestive of the Bethe Heitler (BH) mechanism of incoherent
scatterings, for LPM mechanism, and
constant for a mechanism which suggests that the rate of energy loss ()
of the partons is proportional to total path length () of the parton in the
plasma, as the formation time of the radiated gluon becomes much larger than
.
We find that while the BH mechanism describes the nuclear modification factor
for 5 GeV/ (especially at RHIC energy), the LPM
and more so the constant mechanism provides a good description at
larger . This confirms the earlier expectation that the energy loss
mechanism for partons changes from BH to LPM for ,
where 1 fm and 1 GeV is the average
transverse kick-squared received by the parton per collision. The energy loss
per collision at the =2.76 TeV is found to be about
twice of that at 0.2 TeV.Comment: Discussion expanded, additional references added, 14 pages, 6
figures, To appear in Journal of Physics
Are direct photons a clean signal of a thermalized quark gluon plasma?
Direct photon production from a quark gluon plasma (QGP) in thermal
equilibrium is studied directly in real time. In contrast to the usual S-matrix
calculations, the real time approach is valid for a QGP that formed and reached
LTE a short time after a collision and of finite lifetime ( as expected at RHIC or LHC). We point out that during such
finite QGP lifetime the spectrum of emitted photons carries information on the
initial state. There is an inherent ambiguity in separating the virtual from
the observable photons during the transient evolution of the QGP. We propose a
real time formulation to extract the photon yield which includes the initial
stage of formation of the QGP parametrized by an effective time scale of
formation . This formulation coincides with the S-matrix approach
in the infinite lifetime limit. It allows to separate the virtual cloud as well
as the observable photons emitted during the pre- equilibrium stage from the
yield during the QGP lifetime. We find that the lowest order contribution
which does \emph{not} contribute to the S-matrix
approach, is of the same order of or larger than the S-matrix contribution
during the lifetime of the QGP for a typical formation time . The yield for momenta features a
power law fall-off and is larger than that obtained
with the S-matrix for momenta . We provide a
comprehensive numerical comparison between the real time and S-matrix yields
and study the dynamics of the build-up of the photon cloud and the different
contributions to the radiative energy loss. The reliability of the current
estimates on photon emission is discussed.Comment: 31 pages, 12 eps figures, version to appear in PR
System size dependence of nuclear modification and azimuthal anisotropy of jet quenching
We investigate the system size dependence of jet-quenching by analyzing
transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at
=200 GeV for different centralities. The fast partons
are assumed to lose energy by radiating gluons as they traverse the plasma and
undergo multiple collisions. The energy loss per collision, , is
taken as proportional to (where is the energy of the parton),
proportional to , or a constant depending on whether the formation
time of the gluon is less than the mean path, greater than the mean free path
but less than the path length, or greater than the path length of the partons,
respectively. NLO pQCD is used to evaluate pion production by modifying the
fragmentation function to account for the energy loss. We reproduce the nuclear
modification factor by treating as the only free
parameter, depending on the centrality and the mechanism of energy loss. These
values are seen to explain the nuclear modification of prompt photons, caused
by the energy lost by final state quarks before they fragment into photons.
These also reproduce the azimuthal asymmetry of transverse momentum
distribution for pions within a factor of two and for prompt photons in a fair
agreement with experimental data.Comment: 26 pages, 17 figures. One more figure added. Discussion expanded.
Typographical corrections done, several references added. To appear in
Journal of Physics
Quasilocal formalism and thermodynamics of asymptotically flat black objects
We study the properties of 5-dimensional black objects by using the
renormalized boundary stress-tensor for locally asymptotically flat spacetimes.
This provides a more refined form of the quasilocal formalism which is useful
for a holographic interpretation of asymptotically flat gravity. We apply this
technique to examine the thermodynamic properties of black holes, black rings,
and black strings. The advantage of using this method is that we can go beyond
the `thin ring' approximation and compute the boundary stress tensor for any
general (thin or fat) black ring solution. We argue that the boundary stress
tensor encodes the necessarily information to distinguish between black objects
with different horizon topologies in the bulk. We also study in detail the susy
black ring and clarify the relation between the asymptotic charges and the
charges defined at the horizon. Furthermore, we obtain the balance condition
for `thin' dipole black rings.Comment: v2 clarifications on the advantage of using quasilocal formalism for
black rings added, CQG versio
Semiclassical mechanics of a non-integrable spin cluster
We study detailed classical-quantum correspondence for a cluster system of
three spins with single-axis anisotropic exchange coupling. With autoregressive
spectral estimation, we find oscillating terms in the quantum density of states
caused by classical periodic orbits: in the slowly varying part of the density
of states we see signs of nontrivial topology changes happening to the energy
surface as the energy is varied. Also, we can explain the hierarchy of quantum
energy levels near the ferromagnetic and antiferromagnetic states with EKB
quantization to explain large structures and tunneling to explain small
structures.Comment: 9 pages. For related works see
"http://www.msc.cornell.edu/~clh/clh.html
Multiwavelength Observations of Supersonic Plasma Blob Triggered by Reconnection Generated Velocity Pulse in AR10808
Using multi-wavelength observations of Solar and Heliospheric Observatory
(SoHO)/Michelson Doppler Imager (MDI), Transition Region and Coronal Explorer
(TRACE) 171 \AA, and H from Culgoora Solar Observatory at Narrabri,
Australia, we present a unique observational signature of a propagating
supersonic plasma blob before an M6.2 class solar flare in AR10808 on 9th
September 2005. The blob was observed between 05:27 UT to 05:32 UT with almost
a constant shape for the first 2-3 minutes, and thereafter it quickly vanished
in the corona. The observed lower bound speed of the blob is estimated as
215 km s in its dynamical phase. The evidence of the blob with
almost similar shape and velocity concurrent in H and TRACE 171 \AA\
supports its formation by multi-temperature plasma. The energy release by a
recurrent 3-D reconnection process via the separator dome below the magnetic
null point, between the emerging flux and pre-existing field lines in the lower
solar atmosphere, is found to be the driver of a radial velocity pulse outwards
that accelerates this plasma blob in the solar atmosphere. In support of
identification of the possible driver of the observed eruption, we solve the
two-dimensional ideal magnetohydrodynamic equations numerically to simulate the
observed supersonic plasma blob. The numerical modelling closely match the
observed velocity, evolution of multi-temperature plasma, and quick vanishing
of the blob found in the observations. Under typical coronal conditions, such
blobs may also carry an energy flux of 7.0 ergs cm
s to re-balance the coronal losses above active regions.Comment: Solar Physics; 22 Pages; 8 Figure
Two Loop Scalar Self-Mass during Inflation
We work in the locally de Sitter background of an inflating universe and
consider a massless, minimally coupled scalar with a quartic self-interaction.
We use dimensional regularization to compute the fully renormalized scalar
self-mass-squared at one and two loop order for a state which is released in
Bunch-Davies vacuum at t=0. Although the field strength and coupling constant
renormalizations are identical to those of lfat space, the geometry induces a
non-zero mass renormalization. The finite part also shows a sort of growing
mass that competes with the classical force in eventually turning off this
system's super-acceleration.Comment: 31 pages, 5 figures, revtex4, revised for publication with extended
list of reference
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