7,103 research outputs found
Observational tests for oscillating expansion rate of the Universe
We investigate the observational constraints on the oscillating scalar field
model using data from type Ia supernovae, cosmic microwave background
anisotropies, and baryon acoustic oscillations. According to a Fourier
analysis, the galaxy number count from redshift data indicates that
galaxies have preferred periodic redshift spacings. We fix the mass of the
scalar field as such that the scalar
field model can account for the redshift spacings, and we constrain the other
basic parameters by comparing the model with accurate observational data. We
obtain the following constraints: (95% C.L.),
(95% C.L.) (in the range
). The best fit values of the energy density parameter of the scalar
field and the coupling constant are and ,
respectively. The value of is close to but not equal to .
Hence, in the scalar field model, the amplitude of the galaxy number count
cannot be large. However, because the best fit values of and
are not , the scalar field model has the possibility of accounting for
the periodic structure in the -- relation of galaxies. The variation of
the effective gravitational constant in the scalar field model is not
inconsistent with the bound from observation.Comment: 9 pages, 11 figures, 1 table, Accepted for publication in Physical
Review
The rapidity dependence of the proton-to-pion ratio in Au+Au and p+p collisions at sqrt(sNN) = 62.4 and 200 GeV
The BRAHMS measured proton-to-pion ratios in Au+Au and p+p collisions at
\rootsnn{62.4} and \rootsnn{200} are presented as a function of transverse
momentum and collision centrality within the pseudo-rapidity range 0 < eta <
3.8 The results for Au+Au at sqrt(sNN) = 200 GeV are compared with predictions
from models which incorporate hydro-dynamics, hadron rescattering and jet
production, in the eta interval covered. In Au+Au collisions at sqrt(sNN) = 200
GeV, eta ~ 2.2, and at sqrt(sNN) = 62.4 GeV, eta = 0, the bulk medium can be
characterized by the common value of \mi_B ~ 65 MeV. The p/pi ratios measured
for these two selections display a striking agreement in the pT range covered
(up to 2.2 GeV/c). At a collision energy of 62.4 GeV and at forward
pseudo-rapidity we found a crossing point of p/pi+ ratios measured in central
and semi-peripheral Au+Au and in p+p reactions. The crossing occurs in the
narrow eta bin around value of 3.2, simultaneously in the whole covered pT
range (0.3 GeV/c < pT < 2 GeV/c).Comment: 4 pages, 4 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee Included
corrections indicated in internal review report. Minor correction to Fig.
Can transport peak explain the low-mass enhancement of dileptons at RHIC?
We propose a novel relation between the low-mass enhancement of dielectrons
observed at PHENIX and transport coefficients of QGP such as the charge
diffusion constant and the relaxation time . We parameterize
the transport peak in the spectral function using the second-order relativistic
dissipative hydrodynamics by Israel and Stewart. Combining the spectral
function and the full (3+1)-dimensional hydrodynamical evolution with the
lattice EoS, theoretical dielectron spectra and the experimental data are
compared. Detailed analysis suggests that the low-mass dilepton enhancement
originates mainly from the high-temperature QGP phase where there is a large
electric charge fluctuation as obtained from lattice QCD simulations.Comment: To appear in the conference proceedings for Quark Matter 2011, May 23
- May 28, Annecy, Franc
Mechanism of phase transitions and the electronic density of states in (La,Sm)FeAsOF from ab initio calculations
The structure and electronic density of states in layered
LnFeAsOF (Ln=La,Sm; =0.0, 0.125, 0.25) are investigated using
density functional theory. For the =0.0 system we predict a complex
potential energy surface, formed by close-lying single-well and double-well
potentials, which gives rise to the tetragonal-to-orthorhombic structural
transition, appearance of the magnetic order, and an anomaly in the specific
heat capacity observed experimentally at temperatures below 140--160 K.
We propose a mechanism for these transitions and suggest that these phenomena
are generic to all compounds containing FeAs layers. For 0.0 we demonstrate
that transition temperatures to the superconducting state and their dependence
on correlate well with the calculated magnitude of the electronic density
of states at the Fermi energy.Comment: 4 pages, 3 figures, 1 tabl
Effects of fluctuations on the initial eccentricity from the Color Glass Condensate in heavy ion collisions
We introduce a modified form of the Kharzeev-Levin-Nardi (KLN) approach for
nuclear collisions. The new ansatz for the unintegrated gluon distribution
function preserves factorization, and the saturation scale is bound from below
by that for a single nucleon. It also reproduces the correct scaling with the
number of collisions at high transverse momentum. The corresponding Monte Carlo
implementation allows us to account for fluctuations of the hard sources
(nucleons) in the transverse plane. We compute various definitions of the
eccentricity within the new approach, which are relevant for the interpretation
of the elliptic flow. Our approach predicts breaking of the scaling of the
eccentricity with the Glauber eccentricity at the level of about 30%.Comment: 9 pages, 10 figures, Updated version as accepted by Phys.Rev.
Jet quenching and direct photon production
Jet quenching effect has been investigated in the direct photon production,
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. Our resultant \pt spectra agree
with recent PHENIX data in a broad \pt range. Parton energy loss in the
plasma eventually effect significantly direct photon production from
fragmentation and jet photon conversion, similar to hadron suppression in
central heavy ion collisions. But this only causes about 40% decrease in the
total production of direct photons, due to the mixture with other direct photon
sources.Comment: 6 pages and 3 figures, To appear in the proceedings of the
International Conference on Strangeness in Quark matter (SQM2008), Beijing,
China, Oct 6-10, 200
CGC, Hydrodynamics, and the Parton Energy Loss
Hadron spectra in Au+Au collisions at RHIC are calculated by hydrodynamics
with initial conditions from the Color Glass Condensate (CGC). Minijet
components with parton energy loss in medium are also taken into account by
using parton density obtained from hydrodynamical simulations. We found that
CGC provides a good initial condition for hydrodynamics in Au+Au collisions at
RHIC.Comment: Quark Matter 2004 contribution, 4 pages, 2 figure
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