19,936 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
Communication: Resonance reaction in diffusion-influenced bimolecular reactions
We investigate the influence of a stochastically fluctuating step-barrier potential on bimolecular reaction rates by exact analytical theory and stochastic simulations. We demonstrate that the system exhibits a new "resonant reaction" behavior with rate enhancement if an appropriately defined fluctuation decay length is of the order of the system size. Importantly, we find that in the proximity of resonance, the standard reciprocal additivity law for diffusion and surface reaction rates is violated due to the dynamical coupling of multiple kinetic processes. Together, these findings may have important repercussions on the correct interpretation of various kinetic reaction problems in complex systems, as, e.g., in biomolecular association or catalysis
The minimum period problem in cataclysmic variables
We investigate if consequential angular momentum losses (CAML) or an
intrinsic deformation of the donor star in CVs could increase the CV bounce
period from the canonical theoretical value ~65 min to the observed value
min, and if a variation of these effects in a CV population
could wash out the theoretically predicted accumulation of systems near the
minimum period (the period spike). We are able to construct suitably mixed CV
model populations that a statisticial test cannot rule out as the parent
population of the observed CV sample. However, the goodness of fit is never
convincing, and always slightly worse than for a simple, flat period
distribution. Generally, the goodness of fit is much improved if all CVs are
assumed to form at long orbital periods. The weighting suggested by King,
Schenker & Hameury (2002) does not constitute an improvment if a realistically
shaped input period distribution is used. Put your abstract here.Comment: 10 pages, Latex, 13 postscript figures, Accepted for publication in
MNRA
Reorganization of a dense granular assembly: the `unjamming response function'
We investigate the mechanical properties of a static dense granular assembly
in response to a local forcing. To this end, a small cyclic displacement is
applied on a grain in the bulk of a 2D disordered packing under gravity and the
displacement fields are monitored. We evidence a dominant long range radial
response in the upper half part above the sollicitation and after a large
number of cycles the response is `quasi-reversible' with a remanent dissipation
field exhibiting long range streams and vortex-like symmetry.Comment: 5 pages, 5 figures, accepted version for publication in Phys. Rev.
Dark matter axions
The hypothesis of an `invisible' axion was made by Misha Shifman and others,
approximately thirty years ago. It has turned out to be an unusually fruitful
idea, crossing boundaries between particle physics, astrophysics and cosmology.
An axion with mass of order eV (with large uncertainties) is one of
the leading candidates for the dark matter of the universe. It was found
recently that dark matter axions thermalize and form a Bose-Einstein condensate
(BEC). Because they form a BEC, axions differ from ordinary cold dark matter
(CDM) in the non-linear regime of structure formation and upon entering the
horizon. Axion BEC provides a mechanism for the production of net overall
rotation in dark matter halos, and for the alignment of cosmic microwave
anisotropy multipoles. Because there is evidence for these phenomena,
unexplained with ordinary CDM, an argument can be made that the dark matter is
axions.Comment: 12 pages, invited talk at the Workshop `Crossing the Boundaries:
Gauge Dynamics at Strong Coupling' in honor of Misha Shifman's 60th birthday,
March 14-17, 2009, in Minneapolis, M
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.
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