2,903 research outputs found
Effective description of dark matter as a viscous fluid
Treating dark matter at large scales as an effectively viscous fluid provides
an improved framework for the calculation of the density and velocity power
spectra compared to the standard assumption of an ideal pressureless fluid. We
discuss how this framework can be made concrete through an appropriate
coarse-graining procedure. We also review results that demonstrate that it
improves the convergence of cosmological perturbation theory.Comment: 8 pages, 3 figures, talk by N. Tetradis at Quarks-2016, includes
unpublished materia
Symmetry constraints for the emission angle dependence of Hanbury Brown--Twiss radii
We discuss symmetry constraints on the azimuthal oscillations of two-particle
correlation (Hanbury Brown--Twiss interferometry) radii for non-central
collisions between equal spherical nuclei. We also propose a new method for
correcting in a model-independent way the emission angle dependent correlation
function for finite event plane resolution and angular binning effects.Comment: 8 pages revtex4, 2 tables, no figures. Short Section VI added and
correction algorithm in Section VII made more explicit. Submitted to Physical
Review
Jet quenching via jet collimation
The strong modifications of dijet properties in heavy ion collisions measured
by ATLAS and CMS provide important constraints on the dynamical mechanisms
underlying jet quenching. In this work, we show that the transport of soft
gluons away from the jet cone - jet collimation - can account for the observed
dijet asymmetry with values of that lie in the expected order of
magnitude. Further, we show that the energy loss attained through this
mechanism results in a very mild distortion of the azimuthal angle dijet
distribution.Comment: 4 pages, 2 figures; Proceedings of the "Quark Matter 2011" conferenc
The complex environment of the bright carbon star TX Psc as probed by spectro-astrometry
Context: Stars on the asymptotic giant branch (AGB) show broad evidence of
inhomogeneous atmospheres and circumstellar envelopes. These have been studied
by a variety of methods on various angular scales. In this paper we explore the
envelope of the well-studied carbon star TX Psc by the technique of
spectro-astrometry. Aims: We explore the potential of this method for detecting
asymmetries around AGB stars. Methods:We obtained CRIRES observations of
several CO v=1 lines near 4.6 m and HCN lines near 3 m in
2010 and 2013. These were then searched for spectro-astrometric signatures. For
the interpretation of the results, we used simple simulated observations.
Results: Several lines show significant photocentre shifts with a clear
dependence on position angle. In all cases, tilde-shaped signatures are found
where the positive and negative shifts (at PA 0deg) are associated with blue
and weaker red components of the lines. The shifts can be modelled with a
bright blob 70 mas to 210 mas south of the star with a flux of several percent
of the photospheric flux. We estimate a lower limit of the blob temperature of
1000 K. The blob may be related to a mass ejection as found for AGB stars or
red supergiants. We also consider the scenario of a companion object.
Conclusions: Although there is clear spectro-astrometric evidence of a rather
prominent structure near TX Psc, it does not seem to relate to the other
evidence of asymmetries, so no definite explanation can be given. Our data thus
underline the very complex structure of the environment of this star, but
further observations that sample the angular scales out to a few hundred
milli-arcseconds are needed to get a clearer picture
Dissipative Hydrodynamics and Heavy Ion Collisions
Recent discussions of RHIC data emphasized the exciting possibility that the
matter produced in nucleus-nucleus collisions shows properties of a
near-perfect fluid. Here, we aim at delineating the applicability of fluid
dynamics, which is needed to quantify the size of corresponding dissipative
effects. We start from the equations for dissipative fluid dynamics, which we
derive from kinetic theory up to second order (Israel-Stewart theory) in a
systematic gradient expansion. In model studies, we then establish that for too
early initialization of the hydrodynamic evolution (\tau_0 \lsim 1 fm/c) or
for too high transverse momentum (p_T \gsim 1 GeV) in the final state, the
expected dissipative corrections are too large for a fluid description to be
reliable. Moreover, viscosity-induced modifications of hadronic transverse
momentum spectra can be accommodated to a significant degree in an ideal fluid
description by modifications of the decoupling stage. We argue that these
conclusions, drawn from model studies, can also be expected to arise in
significantly more complex, realistic fluid dynamics simulations of heavy ion
collisions.Comment: 18 pages, 5 figures, uses revtex4; v2: references added, typos
correcte
Two-particle interferometry for non-central heavy-ion collisions
In non-central heavy ion collisions, identical two particle
Hanbury-Brown/Twiss (HBT) correlations C(K,q) depend on the azimuthal direction
of the pair momentum K. We investigate the consequences for a harmonic analysis
of the corresponding HBT radius parameters. Our discussion includes both, a
model- independent analysis of these parameters in the Gaussian approximation,
and the study of a class of hydrodynamical models which mimic essential
geometrical and dynamical properties of peripheral heavy ion collisions. Also,
we discuss the additional geometrical and dynamical information contained in
the harmonic coefficients of these HBT radius parameters. The leading
contribution of their first and second harmonics are found to satisfy simple
constraints. This allows for a minimal, azimuthally sensitive parametrization
of all first and second harmonic coefficients in terms of only two additional
fit parameters. We determine to what extent these parameters can be extracted
from experimental data despite finite multiplicity fluctuations and the
resulting uncertainty in the reconstruction of the reaction plane.Comment: 14 pages, RevTeX, 7 eps-figures include
Cyclic mutually unbiased bases, Fibonacci polynomials and Wiedemann's conjecture
We relate the construction of a complete set of cyclic mutually unbiased
bases, i. e., mutually unbiased bases generated by a single unitary operator,
in power-of-two dimensions to the problem of finding a symmetric matrix over
F_2 with an irreducible characteristic polynomial that has a given Fibonacci
index. For dimensions of the form 2^(2^k) we present a solution that shows an
analogy to an open conjecture of Wiedemann in finite field theory. Finally, we
discuss the equivalence of mutually unbiased bases.Comment: 11 pages, added chapter on equivalenc
Turbulent fluctuations around Bjorken flow
We study the evolution of local event-by-event deviations from smooth average
fluid dynamic fields, as they can arise in heavy ion collisions from the
propagation of fluctuating initial conditions. Local fluctuations around
Bjorken flow are found to be governed by non-linear equations whose solutions
can be characterized qualitatively in terms of Reynolds numbers. Perturbations
at different rapidities decouple quickly, and satisfy (after suitable
coordinate transformations) an effectively two-dimensional Navier-Stokes
equation of non-relativistic form. We discuss the conditions under which
non-linearities in these equations cannot be neglected and turbulent behavior
is expected to set in.Comment: 4 pages, 2 figures - To appear in the conference proceedings for
Quark Matter 2011, May 23 - May 28, Annecy, Franc
Jet coherence in QCD media: the antenna radiation spectrum
We study the radiation of a highly energetic partonic antenna in a colored
state traversing a dense QCD medium. Resumming multiple scatterings of all
involved constituents with the medium we derive the general gluon spectrum
which encompasses both longitudinal color coherence between scattering centers
in the medium, responsible for the well known Landau-Pomeranchuk-Migdal (LPM)
effect, and transverse color coherence between partons inside a jet, leading,
in vacuum, to angular ordering of the parton shower. We discuss shortly the
onset of transverse decoherence which is reached in opaque media. In this
regime, the spectrum consists of independent radiation off the antenna
constituents.Comment: 15 pages, 2 figures, paper shortened and partly rewritten, references
added, results unchange
Identified Hadrons and Jet Chemistry for p+p and Au+Au Collisions at RHIC
The study of hadron spectra at high in p+p collisions provides a good
test of perturbative quantum chromo-dynamic calculations (pQCD) and baseline
for measurements of nuclear modification factors in Au+Au collisions. Using
events triggered by the Barrel Electro-Magnetic Calorimeter, identified charged
hadron transverse momentum () spectra are measured up to 15 GeV/ at
mid-rapidity ( 0.5) and neutral kaon spectra up to 12
GeV/ in p + p collisions at = 200 GeV. The particle ratios
of , and / in p + p
collisions are shown and compared with next-to-leading order pQCD calculations.
In central Au+Au collisions, we report nuclear modification factors ()
for pion, kaon, proton and and discuss several model calculations:
color-charge dependence of jet quenching and jet conversion. Finally,
centrality dependence of at high ( 5.5 GeV/c) for kaon are
compared with that of pion in Au + Au collisions at 200 GeV.Comment: 6 pages, 4 figures, SQM 2009 contributio
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