9 research outputs found
In-medium vector mesons and low mass lepton pairs from heavy ion collisions
The rho and omega meson self-energy at finite temperature and baryon density
have been analysed for an exhaustive set of mesonic and baryonic loops in the
real time formulation of thermal field theory. The large enhancement of
spectral strength below the nominal rho mass is seen to cause a substantial
enhancement in dilepton pair yield in this mass region. The integrated yield
after space-time evolution using relativistic hydrodynamics with quark gluon
plasma in the initial state leads to a very good agreement with the
experimental data from In-In collisions obtained by the NA60 collaboration.Comment: Invited Talk at the DAE-BRNS Workshop on Hadron Physics, Bhabha
Atomic Research Centre, Mumbai, India, October 31-November 4, 201
NA60 results on spectra and the spectral function in In-In collisions
The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158
AGeV In-In collisions. A strong excess of pairs is observed above the yield
expected from neutral meson decays. The unprecedented sample size of close to
400K events and the good mass resolution of about 2% have made it possible to
isolate the excess by subtraction of the decay sources (keeping the ).
The shape of the resulting mass spectrum exhibits considerable broadening, but
essentially no shift in mass. The acceptance-corrected transverse-momentum
spectra have a shape atypical for radial flow and show a significant mass
dependence, pointing to different sources in different mass regions.Comment: 4 pages, 4 figures, Quark Matter 2006 conference proceeding
The Vector Probe in Heavy-Ion Reactions
We review essential elements in using the channel as a probe for
hot and dense matter as produced in (ultra-) relativistic collisions of heavy
nuclei. The uniqueness of the vector channel resides in the fact that it
directly couples to photons, both real and virtual (dileptons), enabling the
study of thermal radiation and in-medium effects on both light () and heavy () vector mesons. We emphasize the importance
of interrelations between photons and dileptons, and characterize relevant
energy/mass regimes through connections to Quark-Gluon-Plasma emission and
chiral symmetry restoration. Based on critical analysis of our current
understanding of data from fixed-target energies, we identify open key
questions to be addressed.Comment: Invited Talk at the Hot Quarks 2004 Workshop, July 18-24, 2004 (Taos
Valley, NM, USA), 15 pages latex incl 14 figs and iop style files, to appear
in the proceeding
Low Mass Dimuon Production in Indium-Indium Collisions at the CERN SPS
NA60 is a fixed-target experiment at the CERN SPS which measured dimuon
production in nucleus-nucleus and proton-nucleus collisions. In this paper we
report on a precision measurement of low-mass muon pairs in 158 AGeV
indium-indium collisions. A significant excess of pairs is observed above the
yield expected from neutral meson decays. The excess can be isolated by
subtraction of expected sources, thanks to the excellent mass resolution and
large sample size.Comment: 4 pages, 3 figures, Contribution at XLIst Rencontres de Moriond, "QCD
and High Energy Hadronic Interactions
J/ suppression in In-In collisions at 158 GeV/nucleon
The NA60 experiment has studied J/ production in Indium-Indium
collisions at 158 AGeV. In this paper we present an updated set of
results obtained with the complete set of available statistics and an improved
alignment of the vertex tracker. The centrality dependence of the J/
production, obtained with an analysis technique based only on the J/
sample, indicates that a suppression beyond that induced by nuclear absorption
is present in In-In collisions, setting in at 80 participant nucleons. A
first study of the systematic errors related with this measurement is
discussed. We also present preliminary results on the J/ azimuthal
distributions.Comment: 8 pages, 3 figures, proceedings of Hard Probes 2006 International
Conferenc
A comparative measurement of and in In-In collisions at the CERN SPS
The NA60 experiment at the CERN SPS has studied meson production in
In-In collisions at 158A GeV via both the and the decay
channels. The yields and inverse slope parameters of the spectra observed
in the two channels are compatible within errors, different from the large
discrepancies seen in Pb-Pb collisions between the hadronic (NA49) and dimuon
(NA50) decay channels. Possible physics implications are discussed
Evidence for the production of thermal-like muon pairs with masses above 1 GeV/c^2 in 158A GeV Indium-Indium Collisions
The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c^2 produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of {\em prompt} muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c^2, rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c^2 to be of largely partonic origin, when radial flow has not yet built up.The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c^2 produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of {\em prompt} muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c^2, rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c^2 to be of largely partonic origin, when radial flow has not yet built up
First results on angular distributions of thermal dileptons in nuclear collisions
The NA60 experiment at the CERN SPS has studied dimuon production in 158A GeV
In-In collisions. The strong excess of pairs above the known sources found in
the complete mass region 0.2<M<2.6 GeV has previously been interpreted as
thermal radiation. We now present first results on the associated angular
distributions. Using the Collins-Soper reference frame, the structure function
parameters \lambda, \mu and \nu are measured to be zero, and the projected
distributions in polar and azimuth angles are found to be uniform. The absence
of any polarization is consistent with the interpretation of the excess dimuons
as thermal radiation from a randomized system.Comment: Submitted to Physical Review Letter