14,691 research outputs found
Optimized Constant Pressure Stochastic Dynamics
A recently proposed method for computer simulations in the
isothermal-isobaric (NPT) ensemble, based on Langevin-type equations of motion
for the particle coordinates and the ``piston'' degree of freedom, is
re-derived by straightforward application of the standard Kramers-Moyal
formalism. An integration scheme is developed which reduces to a
time-reversible symplectic integrator in the limit of vanishing friction. This
algorithm is hence expected to be quite stable for small friction, allowing for
a large time step. We discuss the optimal choice of parameters, and present
some numerical test results.Comment: 16 pages, 2 figures, submitted to J. Chem. Phy
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.
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
Dilaton dominance in the early Universe dilutes Dark Matter relic abundances
The role of the dilaton field and its coupling to matter may result to a
dilution of Dark Matter (DM) relic densities. This is to be contrasted with
quintessence scenarios in which relic densities are augmented, due to
modification of the expansion rate, since Universe is not radiation dominated
at DM decoupling. Dilaton field, besides this, affects relic densities through
its coupling to dust which tends to decrease relic abundances. Thus two
separate mechanisms compete each other resulting, in general, to a decrease of
the relic density. This feature may be welcome and can rescue the situation if
Direct Dark Matter experiments point towards small neutralino-nucleon cross
sections, implying small neutralino annihilation rates and hence large relic
densities, at least in the popular supersymmetric scenarios. In the presence of
a diluting mechanism both experimental constraints can be met. The role of the
dilaton for this mechanism has been studied in the context of the non-critical
string theory but in this work we follow a rather general approach assuming
that the dilaton dominates only at early eras long before Big Bang
Nucleosynthesis.Comment: 11 pages, Latex, 4 figures: Comments and references added, version to
appear in Phys. Rev.
Pre-Inflation in the Presence of Conformal Coupling
We consider a massless scalar field, conformally coupled to the Ricci scalar
curvature, in the pre-inflation era of a closed FLRW Universe. The scalar field
potential can be of the form of the Coleman-Weinberg one-loop potential, which
is flat at the origin and drives the inflationary evolution. For positive
values of the conformal parameter \xi, less than the critical value xi_c=(1/6),
the model admits exact solutions with non-zero scale factor and zero initial
Hubble parameter. Thus these solutions can be matched smoothly to the so called
Pre-Big-Bang models. At the end of this pre-inflation era one can match
inflationary solutions by specifying the form of the potential and the whole
solution is of the class C^(1).Comment: 11 pages, 5 figures, LaTeX, Accepted for publication in MPL
Non-critical String Cosmologies
Non-critical String Cosmologies are offered as an alternative to Standard Big
Bang Cosmology. The new features encompassed within the dilaton dependent
non-critical terms affect the dynamics of the Universe\'s evolution in an
unconventional manner being in agreement with the cosmological data.
Non-criticality is responsible for a late transition to acceleration at
redshifts z=0.2. The role of the uncoupled rolling dilaton to relic abundance
calculations is discussed. The uncoupled rolling dilaton dilutes the neutralino
relic densities in supersymmetric theories by factors of ten, relaxing
considerably the severe WMAP Dark Matter constraints, while at the same time
leaves almost unaffected the baryon density in agreement with primordial
Nucleosynthesis.Comment: 16 pages, 7 figures, conference tal
Analyzing Correlation Functions with Tesseral and Cartesian Spherical Harmonics
The dependence of inter-particle correlations on the orientation of particle
relative-momentum can yield unique information on the space-time features of
emission in reactions with multiparticle final states. In the present paper,
the benefits of a representation and analysis of the three-dimensional
correlation information in terms of surface spherical harmonics is presented.
The harmonics include the standard complex tesseral harmonics and the real
cartesian harmonics. Mathematical properties of the lesser-known cartesian
harmonics are illuminated. The physical content of different angular harmonic
components in a correlation is described. The resolving power of different
final-state effects with regarding to determining angular features of emission
regions is investigated. The considered final-state effects include identity
interference and strong and Coulomb interactions. The correlation analysis in
terms of spherical harmonics is illustrated with the cases of gaussian and
blast-wave sources for proton-charged meson and baryon-baryon pairs.Comment: 32 pages 10 figure
Self-intersecting marginally outer trapped surfaces
We have shown previously that a merger of marginally outer trapped surfaces (MOTSs) occurs in a binary black hole merger and that there is a continuous sequence of MOTSs which connects the initial two black holes to the final one. In this paper, we confirm this scenario numerically and we detail further improvements in the numerical methods for locating MOTSs. With these improvements, we confirm the merger scenario and demonstrate the existence of self-intersecting MOTSs formed in the immediate aftermath of the merger. These results will allow us to track physical quantities across the non-linear merger process and to potentially infer properties of the merger from gravitational wave observations
Cosmology of biased discrete symmetry breaking
The cosmological consequences of spontaneous breaking of an approximate discrete symmetry are studied. The breaking leads to formation of proto-domains of false and true vacuum separated by domain walls of thickness determined by the mass scale of the model. The cosmological evolution of the walls is extremely sensitive to the magnitude of the biasing; several scenarios are possible, depending on the interplay between the surface tension on the walls and the volume pressure from the biasing. Walls may disappear almost immediately after they form, or may live long enough to dominate the energy density of the Universe and cause power-law inflation. Limits are obtained on the biasing that characterizes each possible scenario
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