7,188 research outputs found
Early thermalization at RHIC
It is shown that recent RHIC data on hadron spectra and elliptic flow can be
excellently reproduced within a hydrodynamic description of the collision
dynamics, and that this provides strong evidence for rapid thermalization while
the system is still in the quark-gluon plasma phase. But even though the
hydrodynamic approach provides an impressive description of the single-particle
momentum distributions, it fails to describe the two-particle momentum
correlation (HBT) data for central Au+Au collisions at RHIC. We suggest that
this is not likely to be repaired by further improvements in our understanding
of the early collision stages, but probably requires a better modelling of the
freeze-out process. We close with a prediction of the phases of the azimuthal
oscillations of the HBT radii in noncentral collisions at RHIC.Comment: 12 pages, including 6 figures. Invited talk at the International
Conference on "Statistical QCD", Bielefeld, August 26-30, 2001, to appear in
the proceedings (F. Karsch and H. Satz, eds.) in Nucl. Phys.
Evolution of pion HBT radii from RHIC to LHC -- Predictions from ideal hydrodynamics
We present hydrodynamic predictions for the charged pion HBT radii for a
range of initial conditions covering those presumably reached in Pb+Pb
collisions at the LHC. We study central (b=0) and semi-central (b=7fm)
collisions and show the expected increase of the HBT radii and their azimuthal
oscillations. The predicted trends in the oscillation amplitudes reflect a
change of the final source shape from out-of-plane to in-plane deformation as
the initial entropy density is increased.Comment: 6 pages, incl. 5 figures. Contribution to the CERN Theory Institute
Workshop "Heavy Ion Collisions at the LHC -- Last Call for Predictions",
CERN, 14 May - 8 June 2007, to appear in J. Phys.
Whitening of the Quark-Gluon Plasma
Parton-parton collisions do not neutralize local color charges in the
quark-gluon plasma as they only redistribute the charges among momentum modes.
We discuss color diffusion and color conductivity as the processes responsible
for the neutralization of the plasma. For this purpose, we first compute the
conductivity and diffusion coefficients in the plasma that is significantly
colorful. Then, the time evolution of the color density due to the conductivity
and diffusion is studied. The conductivity is shown to be much more efficient
than the diffusion in neutralizing the plasma at the scale longer than the
screening length. Estimates of the characteristic time scales, which are based
on close to global equilibrium computations, suggest that first the plasma
becomes white and then the momentum degrees of freedom thermalize.Comment: 9 pages, revised, to appear in Phys. Rev.
Photon HBT interferometry for non-central heavy-ion collisions
Currently, the only known way to obtain experimental information about the
space-time structure of a heavy-ion collision is through 2-particle momentum
correlations. Azimuthally sensitive HBT interferometry (Hanbury Brown-Twiss
intensity interferometry) can complement elliptic flow measurements by
constraining the spatial deformation of the source and its time evolution.
Performing these measurements on photons allows us to access the fireball
evolution at earlier times than with hadrons. Using ideal hydrodynamics to
model the space-time evolution of the collision fireball, we explore
theoretically various aspects of 2-photon intensity interferometry with
transverse momenta up to 2 GeV, in particular the azimuthal angle dependence of
the HBT radii in non-central collisions. We highlight the dual nature of
thermal photon emission, in both central and non-central collisions, resulting
from the superposition of QGP and hadron resonance gas photon production. This
signature is present in both the thermal photon source function and the HBT
radii extracted from Gaussian fits of the 2-photon correlation function.Comment: 18 pages, 12 figure
Radial flow afterburner for event generators and the baryon puzzle
A simple afterburner including radial flow to the randomized transverse
momentum obtained from event generators, Pythia and Hijing, has been
implemented to calculate the ratios and compare them with available
data. A coherent trend of qualitative agreement has been obtained in
collisions and in for various centralities. Those results indicate that
the radial flow does play an important role in the so called baryon puzzle
anomaly.Comment: 11 pages, 5 figures. To appear in Journal of Physics
Hydrodynamic emission of strange and non-strange particles at RHIC and LHC
The hydrodynamic model is used to describe the single-particle spectra and
elliptic flow of hadrons at RHIC and to predict the emission angle dependence
of HBT correlations at RHIC and LHC energies.Comment: 6 pages LaTeX, 3 postscript figures. Proceedings for the conference
"Strange Quark Matter 2003", Atlantic Beach, NC, March 12-17, 2003, to appear
in J. Phys.
Snell's law for surface electrons: Refraction of an electron gas imaged in real space
On NaCl(100)/Cu(111) an interface state band is observed that descends from
the surface-state band of the clean copper surface. This band exhibits a
Moire-pattern-induced one-dimensional band gap, which is accompanied by strong
standing-wave patterns, as revealed in low-temperature scanning tunneling
microscopy images. At NaCl island step edges, one can directly see the
refraction of these standing waves, which obey Snell's refraction law.Comment: 4 pages, 4 figure
Evidence for a parsec scale X-ray jet from the accreting neutron star Circinus X-1
We analyzed the zero-order image of a 50 ks Chandra gratings observation of
Circinus X-1, taken in 2005 during the source's low-flux state. Circinus X-1 is
an accreting neutron star that exhibits ultra-relativistic arcsecond-scale
radio jets and diffuse arcminute-scale radio jets and lobes. The image shows a
clear excess along the general direction of the north-western counter-jet,
coincident with the radio emission, suggesting that it originates either in the
jet itself or in the shock the jet is driving into its environment. This makes
Circinus X-1 the first neutron star for which an extended X-ray jet has been
detected. The kinetic jet power we infer is significantly larger than the
minimum power required for the jet to inflate the large scale radio nebula.Comment: Added journal reference, corrected on reference and typo in labels
for Fig. 1; 5 pages, 3 figures, ApJ Letter, in pres
What is the meaning of the statistical hadronization model?
The statistical model of hadronization succeeds in reproducing particle
abundances and transverse momentum spectra in high energy collisions of
elementary particles as well as of heavy ions. Despite its apparent success,
the interpretation of these results is controversial and the validity of the
approach very often questioned. In this paper, we would like to summarize the
whole issue by first outlining a basic formulation of the model and then
comment on the main criticisms and different kinds of interpretations, with
special emphasis on the so-called "phase space dominance". While the ultimate
answer to the question why the statistical model works should certainly be
pursued, we stress that it is a priority to confirm or disprove the fundamental
scheme of the statistical model by performing some detailed tests on the rates
of exclusive channels at lower energy.Comment: 14 pages, to be published in the Proceedings of the International
workshop "Focus on multiplicity", Bari (Italy) June 17-19 200
- …