243 research outputs found
Pion damping width from SU(2) x SU(2) NJL model
Within the framework of the NJL model, we investigate the modification of the
pion damping width in a hot pion gas for temperatures ranging from 0 to 180
MeV. The pion is found to broaden noticeably at T > 60 MeV. Near the chiral
phase transition T ~ 180 MeV, the pion width is saturated and amounts to 70
MeV. The main contribution to the width comes from pion-pion collisions. Other
contributions are found negligibly small.Comment: LaTeX2e, 13 pages, 2 figure
Recent results from Pb-Au collisions at 158 GeV/c per nucleon obtained with the CERES spectrometer
During the 1996 lead run time, CERES has accumulated 42 million events,
corresponding to a factor of 5 more statistics than in 1995 and 2.5 million
events of a special photon-run. We report on the results of the low-mass
ee-pair analysis. Since the most critical item is the poor
signal-to-background ratio we also discuss the understanding of this
background, in absolute terms, with the help of a detailed Monte Carlo
simulation. We show preliminary results of the photon analysis and summarize
the results of the hadron analysis preliminarily reported on already at QM'97Comment: 10 pages, 9 figures, Proceedings of the XIV Int. Conf. on
Nucleus-Nucleus Collisions,Quark Matter 99, Torino, Italy, May 10 - 15, 199
What does the rho-meson do? In-medium mass shift scenarios versus hadronic model calculations
The NA60 experiment has studied low-mass muon pair production in In-In
collisions at with unprecedented precision. With these results
there is hope that the in-medium modifications of the vector meson spectral
function can be constrained more thoroughly than before. We investigate in
particular what can be learned about collisional broadening by a hot and dense
medium and what constrains the experimental results put on in-medium mass shift
scenarios. The data show a clear indication of considerable in-medium
broadening effects but disfavor mass shift scenarios where the -meson
mass scales with the square root of the chiral condensate. Scaling scenarios
which predict at finite density a dropping of the -meson mass that is
stronger than that of the quark condensate are clearly ruled out since they are
also accompanied by a sharpening of the spectral function.Comment: Proceeding contribution, Talk given by J. Ruppert at Workshop for
Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus
Collisions (Hot Quarks 2006), Villasimius, Sardinia, Italy, 15-20 May 2006.
To appear in EPJ
Lepton-pair production in nuclear collisions - past, present, future
The key results on lepton-pair production in ultra-relativistic nuclear
collisions are shortly reviewed, starting at the roots of pp collisions in the
seventies, and ending at the perspectives of the colliders RHIC and LHC. The
presence is dominated by the recent precision results from NA60 at the CERN
SPS, culminating in the first measurement of the in-medium rho spectral
function and the transverse flow of the associated thermal radiation. The
seeming cut-off of the flow above the rho may well be the first direct hint for
thermal radiation of partonic origin in nuclear collisions. The major
milestones in the theoretical developments are also covered.Comment: Invited talk at INPC07, Tokyo, June 3-8, 200
New method for measuring azimuthal distributions in nucleus-nucleus collisions
The methods currently used to measure azimuthal distributions of particles in
heavy ion collisions assume that all azimuthal correlations between particles
result from their correlation with the reaction plane. However, other
correlations exist, and it is safe to neglect them only if azimuthal
anisotropies are much larger than 1/sqrt(N), with N the total number of
particles emitted in the collision. This condition is not satisfied at
ultrarelativistic energies. We propose a new method, based on a cumulant
expansion of multiparticle azimuthal correlations, which allows to measure much
smaller values of azimuthal anisotropies, down to 1/N. It is simple to
implement and can be used to measure both integrated and differential flow.
Furthermore, this method automatically eliminates the major systematic errors,
which are due to azimuthal asymmetries in the detector acceptance.Comment: final version (misprints corrected), to be published in Phys.Rev.
Semi-Hard Scattering Unraveled from Collective Dynamics by Two-Pion Azimuthal Correlations in 158 A GeV/c Pb + Au Collisions
Elliptic flow and two-particle azimuthal correlations of charged hadrons and
high- pions ( 1 GeV/) have been measured close to mid-rapidity in
158A GeV/ Pb+Au collisions by the CERES experiment. Elliptic flow ()
rises linearly with to a value of about 10% at 2 GeV/. Beyond
1.5 GeV/, the slope decreases considerably, possibly indicating
a saturation of at high . Two-pion azimuthal anisotropies for
1.2 GeV/ exceed the elliptic flow values by about 60% in mid-central
collisions. These non-flow contributions are attributed to near-side and
back-to-back jet-like correlations, the latter exhibiting centrality dependent
broadening.Comment: Submitted to Phys. Rev. Letters, 4 pages, 5 figure
Thermal rho and sigma mesons from chiral symmetry and unitarity
We study the temperature evolution of the rho and sigma mass and width, using
a unitary chiral approach. The one-loop pion-pion scattering amplitude in
Chiral Perturbation Theory at finite temperature is unitarized via the Inverse
Amplitude Method. Our results predict a clear increase with T of both the rho
and sigma widths. The masses decrease slightly for high T, while the
rho-pion-pion coupling increases. The rho behavior seems to be favored by
experimental results. In the sigma case, it signals chiral symmetry
restoration.Comment: 5 pages, 5 figures, revtex. References and brief comments added.
Final version to appear in Phys. Rev.
Dileptons from hot heavy static photons
We compute the production rate of lepton pair by static photons at finite
temperature at two-loop order. We treat the infrared region of the gluon phase
space carefully by using a hard thermal loop gluon propagator. The result is
free of infrared and collinear divergences and exhibits an enhancement which
produces a result of order instead of as would be
expected from ordinary perturbation theory.Comment: 14 pages, 2 figure
Event-by-event fluctuations at SPS
Results on event-by-event fluctuations of the mean transverse momentum and
net charge in Pb-Au collisions, measured by the CERES Collaboration at
CERN-SPS, are presented. We discuss the centrality and beam energy dependence
and compare our data to cascade calculations.Comment: 4 pages, 4 figures, proceedings to INPC2004 Goteborg, Swede
e+e--pair production in Pb-Au collisions at 158 GeV per nucleon
We present the combined results on electron-pair production in 158 GeV/n
{Pb-Au} (= 17.2 GeV) collisions taken at the CERN SPS in 1995 and
1996, and give a detailed account of the data analysis. The enhancement over
the reference of neutral meson decays amounts to a factor of 2.31 for semi-central collisions (28%
) when yields are integrated over 200 MeV/ in
invariant mass. The measured yield, its stronger-than-linear scaling with
, and the dominance of low pair strongly suggest an
interpretation as {\it thermal radiation} from pion annihilation in the
hadronic fireball. The shape of the excess centring at 500
MeV/, however, cannot be described without strong medium modifications of
the meson. The results are put into perspective by comparison to
predictions from Brown-Rho scaling governed by chiral symmetry restoration, and
from the spectral-function many-body treatment in which the approach to the
phase boundary is less explicit.Comment: 39 pages, 40 figures, to appear in Eur.Phys.J.C. (2005
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