14,291 research outputs found
Transverse momentum spectra and elliptic flow in ideal hydrodynamics and geometric scaling
In an ideal hydrodynamic model, with an equation of state where the
confinement-deconfinement transition is a cross-over at , we
have simulated =200 GeV Au+Au collisions. Simultaneous description of
the experimental charged particle's spectra and elliptic flow require
that in central (0-10%) Au+Au collisions, initial energy density scales with
the binary collision number density. In less central collisions, experimental
data demand scaling with the participant density. Simulation studies also
indicate that in central collisions viscous effects are minimal.Comment: 4 pages, 3 figures
Dark matter and Higgs boson physics
A vector-like colorless fermion doublet and a singlet added to the Standard
Model allow a consistent interpretation of dark matter in terms of the lightest
neutral particle, as they may help in obtaining successful gauge coupling
unification. We analyze in detail the mass range of the lightest neutral
particle below the W mass, i.e. in a range of the parameters where the physics
of the Standard Model Higgs boson may be substantially affected either directly
or indirectly.Comment: 17 pages, 7 figures. v3: published version (small corrections
Direct photon production from viscous QGP
We simulate direct photon production in evolution of viscous QGP medium.
Photons from Compton and annihilation processes are considered. Viscous effect
on photon production is very strong and reliable simulation is possible only in
a limited range. For minimally viscous fluid =0.08), direct
photons can be reliably computed only up to 1.3 GeV. With reduced
viscosity (=0.04), the limit increases to 2GeV.Comment: 6 pages, 5 figure
Di-jet hadron pair correlation in a hydrodynamical model with a quenching jet
In jet quenching, a hard QCD parton, before fragmenting into a jet of
hadrons, deposits a fraction of its energy in the medium, leading to suppressed
production of high- hadrons. Assuming that the deposited energy quickly
thermalizes, we simulate the subsequent hydrodynamic evolution of the QGP
fluid. Hydrodynamic evolution and subsequent particle emission depend on the
jet trajectories. Azimuthal distribution of excess due to quenching
jet, averaged over all the trajectories, reasonably well reproduce the
di-hadron correlation as measured by the STAR and PHENIX collaboration in
central and in peripheral Au+Au collisions.Comment: 5 pages, 4 figures. Some minor corrections are made in the revised
manuscrip
v4: A small, but sensitive observable for heavy ion collisions
Higher order Fourier coefficients of the azimuthally dependent single
particle spectra resulting from noncentral heavy ion collisions are
investigated. For intermediate to large transverse momenta, these anisotropies
are expected to become as large as 5 %, and should be clearly measurable. The
physics content of these observables is discussed from two different extreme
but complementary viewpoints, hydrodynamics and the geometric limit with
extreme energy loss.Comment: as published: typos corrected, Fig. 3 slightly improved in numerics
and presentatio
Revised Canonical Quantum Gravity via the Frame Fixing
We present a new reformulation of the canonical quantum geometrodynamics,
which allows to overcome the fundamental problem of the frozen formalism and,
therefore, to construct an appropriate Hilbert space associate to the solution
of the restated dynamics. More precisely, to remove the ambiguity contained in
the Wheeler-DeWitt approach, with respect to the possibility of a (3 +
1)-splitting when the space-time is in a quantum regime, we fix the reference
frame (i.e. the lapse function and the shift vector) by introducing the
so-called kinematical action; as a consequence the new super-Hamiltonian
constraint becomes a parabolic one and we arrive to a Schroedinger-like
approach for the quantum dynamics. In the semiclassical limit our theory
provides General Relativity in the presence of an additional energy-momentum
density contribution coming from no longer zero eigenvalues of the Hamiltonian
constraints; the interpretation of these new contributions comes out in natural
way as soon as it is recognized that the kinematical action can be recasted in
such a way it describes a pressureless, but, in general, non geodesic perfect
fluid.Comment: 24 pages, 0 figures, to appear on Int. Jour. Mod. Phys.
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
Blunting the Spike: the CV Minimum Period
The standard picture of CV secular evolution predicts a spike in the CV
distribution near the observed short-period cutoff P_0 ~ 78 min, which is not
observed. We show that an intrinsic spread in minimum (`bounce') periods P_b
resulting from a genuine difference in some parameter controlling the evolution
can remove the spike without smearing the sharpness of the cutoff. The most
probable second parameter is different admixtures of magnetic stellar wind
braking (at up to 5 times the GR rate) in a small tail of systems, perhaps
implying that the donor magnetic field strength at formation is a second
parameter specifying CV evolution. We suggest that magnetic braking resumes
below the gap with a wide range, being well below the GR rate in most CVs, but
significantly above it in a small tail.Comment: 5 pages, 4 figures; accepted for publication in MNRA
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