Abstract
The NA60 experiment at the CERN SPS has studied light vector meson production in In-In collisions at 158A GeV. The ϕ meson was detected via both the K + K − and the μ + μ − decay channels. The yields and inverse slope parameters of the m T 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. In the invariant mass region 0.2 M μμ M μμ π + π → ρ → μ + μ − . For M μμ > 1 GeV, the excess is found to be prompt, with remarkable differences with respect to the Drell-Yan process. The T eff slope parameter, extracted from the transverse mass spectra, shows a rise with mass up to the ρ , followed by a sudden decline for higher masses. The former is consistent with radial flow of a hadronic source, while the latter suggests a dominantly partonic emission source.
Highlights
The theory of Quantum Chromo-Dynamics (QCD) predicts that matter under extreme conditions of temperature and energy density undergoes a phase transition from hadronic matter to a plasma of decon¿ned quarks and gluons (QGP)
The yields and inverse slope parameters of the mT 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
The results presented in this paper refer to the 4000 A data sample, which
Summary
The theory of Quantum Chromo-Dynamics (QCD) predicts that matter under extreme conditions of temperature and energy density undergoes a phase transition from hadronic matter to a plasma of decon¿ned quarks and gluons (QGP). The ρ meson is important in the study of the hot and dense state formed in high-energy heavy ion collisions, due to its strong coupling to the ππ channel and the short lifetime (∼ 1.3 fm/c), which is much shorter than the lifetime of the ¿reball. Due to these characteristics, its in-medium properties, like mass and width, have long been considered as an ideal signature for the restoration of chiral symmetry [15,16,17], which is associated to the phase transition between hadronic matter and the QGP. The most important results on the excess dimuons detected by NA60 are summarized
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