12,982 research outputs found
Towards an understanding of the RHIC single electron data
High transverse momentum () single non-photonic electrons which have
been measured in the RHIC experiments come dominantly from heavy meson decay.
The ratio of their spectra in pp and AA collisions ()
reveals the energy loss of heavy quarks in the environment created by AA
collisions. Using a fixed coupling constant and the Debye mass () as infrared regulator perturbative QCD (pQCD) calculations are not able to
reproduce the data, neither the energy loss nor the azimuthal
distribution. Employing a running coupling constant and replacing the Debye
mass by a more realistic hard thermal loop (HTL) calculation we find a
substantial increase of the collisional energy loss which brings the
distribution as well as to values close to the experimental ones
without excluding a contribution from radiative energy loss.Comment: Accepted for publication in Physical Review
A comprehensive population synthesis study of post-common envelope binaries
We apply population synthesis techniques to calculate the present day
population of post-common envelope binaries (PCEBs) for a range of theoretical
models describing the common envelope (CE) phase. Adopting the canonical energy
budget approach we consider models where the ejection efficiency,
\alpha_{\rmn{CE}} is either a constant, or a function of the secondary mass.
We obtain the envelope binding energy from detailed stellar models of the
progenitor primary, with and without the thermal and ionization energy, but we
also test a commonly used analytical scaling. We also employ the alternative
angular momentum budget approach, known as the -algorithm. We find that
a constant, global value of \alpha_{\rmn{CE}} \ga 0.1 can adequately account
for the observed population of PCEBs with late spectral-type secondaries.
However, this prescription fails to reproduce IK Pegasi, which has a secondary
with spectral type A8. We can account for IK Pegasi if we include thermal and
ionization energy of the giant's envelope, or if we use the -algorithm.
However, the -algorithm predicts local space densities that are 1 to 2
orders of magnitude greater than estimates from observations. In contrast, the
canonical energy budget prescription with an initial mass ratio distribution
that favours unequal initial mass ratios gives a local space density which is
in good agreement with observations, and best reproduces the observed
distribution of PCEBs. Finally, all models fail to reproduce the sharp decline
for orbital periods, P_{\rmn{orb}} \ga 1 d in the orbital period distribution
of observed PCEBs, even if we take into account selection effects against
systems with long orbital periods and early spectral-type secondaries.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society. 18 pages, 10 figures. Work concerning the
reconstruction of the common envelope phase presented in the previous version
will now be submitted in a separate paper in the near futur
Consequences of energy conservation in relativistic heavy-ion collisions
Complete characterization of particle production and emission in relativistic
heavy-ion collisions is in general not feasible experimentally. This work
demonstrates, however, that the availability of essentially complete
pseudorapidity distributions for charged particles allows for a reliable
estimate of the average transverse momenta and energy of emitted particles by
requiring energy conservation in the process. The results of such an analysis
for Au+Au collisions at sqrt{s_{NN}}= 130 and 200 GeV are compared with
measurements of mean-p_T and mean-E_T in regions where such measurements are
available. The mean-p_T dependence on pseudorapidity for Au+Au collisions at
130 and 200 GeV is given for different collision centralities.Comment: 8 pages, 8 figures, Submitted to Phys. Rev.
Thermodynamics of (2+1)-flavor QCD: Confronting Models with Lattice Studies
The Polyakov-quark-meson (PQM) model, which combines chiral as well as
deconfinement aspects of strongly interacting matter is introduced for three
light quark flavors. An analysis of the chiral and deconfinement phase
transition of the model and its thermodynamics at finite temperatures is given.
Three different forms of the effective Polyakov loop potential are considered.
The findings of the (2+1)-flavor model investigations are confronted to
corresponding recent QCD lattice simulations of the RBC-Bielefeld, HotQCD and
Wuppertal-Budapest collaborations. The influence of the heavier quark masses,
which are used in the lattice calculations, is taken into account. In the
transition region the bulk thermodynamics of the PQM model agrees well with the
lattice data.Comment: 13 pages, 7 figures, 3 tables; minor changes, final version to appear
in Phys. Rev.
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.
Uses of a small field value which falls from a metastable maximum over cosmological times
We consider a small, metastable maximum vacuum expectation value of
order of a few eV, for a pseudoscalar Goldstone-like field, which is related to
the scalar inflaton field in an idealized model of a cosmological,
spontaneously-broken chiral symmetry. The b field allows for relating
semi-quantitatively three distinct quantities in a cosmological context.
(1) A very small, residual vacuum energy density or effective cosmological
constant of ~ lambda b_0^4 ~ 2.7 x 10^{-47}GeV^4, for lambda ~ 3 x 10^{-14},
the same as an empirical inflaton self-coupling.
(2) A tiny neutrino mass, less then b_0.
(3) A possible small variation downward of the proton to electron mass ratio
over cosmological time. The latter arises from the motion downward of the
field over cosmological time, toward a nonzero limiting value as . Such behavior is consistent with an equation of motion.
We argue that hypothetical b quanta, potentially inducing new long-range
forces, are absent, because of negative, effective squared mass in an equation
of motion for -field fluctuations.Comment: version accepted for publication in Mod.Phys.Lett.
Gravitational waves from an early matter era
We investigate the generation of gravitational waves due to the gravitational
instability of primordial density perturbations in an early matter-dominated
era which could be detectable by experiments such as LIGO and LISA. We use
relativistic perturbation theory to give analytic estimates of the tensor
perturbations generated at second order by linear density perturbations. We
find that large enhancement factors with respect to the naive second-order
estimate are possible due to the growth of density perturbations on sub-Hubble
scales. However very large enhancement factors coincide with a breakdown of
linear theory for density perturbations on small scales. To produce a
primordial gravitational wave background that would be detectable with LIGO or
LISA from density perturbations in the linear regime requires primordial
comoving curvature perturbations on small scales of order 0.02 for Advanced
LIGO or 0.005 for LISA, otherwise numerical calculations of the non-linear
evolution on sub-Hubble scales are required.Comment: 23 pages, 2 figure
Scattering of Dirac and Majorana Fermions off Domain Walls
We investigate the interaction of fermions having both Dirac and left-handed
and right-handed Majorana mass terms with vacuum domain walls. By solving the
equations of motion in thin-wall approximation, we calculate the reflection and
transmission coefficients for the scattering of fermions off walls.Comment: 6 pages, 1 figure, some typos corrected, one reference added, major
revisions, title changed, version accepted for publication in Phys. Rev.
Large-amplitude isothermal fluctuations and high-density dark-matter clumps
Large-amplitude isothermal fluctuations in the dark matter energy density,
parameterized by \Phi\equiv\delta\rhodm/\rhodm, are studied within the
framework of a spherical collapse model. For \Phi \ga 1, a fluctuation
collapses in the radiation-dominated epoch and produces a dense dark-matter
object. The final density of the virialized object is found to be \rho_F
\approx 140\, \Phi^3 (\Phi+1) \rhoeq, where \rhoeq is the matter density at
equal matter and radiation energy density. This expression is valid for the
entire range of possible values of , both for and . Some astrophysical consequences of high-density dark-matter clumps are
discussed.Comment: 15 pages plus 3 figures (included at the end as a uuencoded
postscript file), LaTeX, FNAL--PUB--94/055--
Interpretation of the variability of the <i>β</i> Cephei star <i>λ</i> Scorpii. I. The multiple character
We derive accurate values of the orbital parameters of the close binary β Cephei star λ Scorpii. Moreover, we present the first determination of the properties of the triple system to which λ Scorpii belongs. Our analysis is based on a time series of 815 high-resolution spectra, covering a timespan of 14 years. We find a close orbit of 5d.9525days (e=0.26) and a wide orbit of approximately 1082d days (e=0.23). The orbital parameters of the triple star and a spectrum synthesis lead us to conclude that the system is composed of two early-type B stars and a low-mass pre-main-sequence star rather than containing an ultra-massive white dwarf as claimed before. Our proposed configuration is compatible with population synthesis. The radial velocity variations of the primary allow us to confirm the presence of at least one pulsation mode with frequency 4.679410 c d-1 which is subject to the light-time effect in the triple system. A detailed analysis of the complex line-profile variations is described in a subsequent paper
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