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The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158 A GeV In–In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2< M<2.6 GeV. The mass spectrum for M<1 GeV is consistent with a dominant contribution from π+ π−→ ρ→ μ+ μ− annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass. For M>1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell–Yan pairs. The slope parameter Teff associated with the transverse momentum spectra rises with mass up to the ρ, followed by a sudden decline above. The rise for M<1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for M>1 GeV and its relation to parton–hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low p T and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on ω in-medium effects. [ABSTRACT FROM AUTHOR]

The J/ ψ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/ ψ mesons at SPS energies. Hence, the measurement of J/ ψ elliptic anisotropy, quantified by the Fourier coefficient v2 of the J/ ψ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J/ ψ suppression observed in Pb-Pb collisions. We present the measured J/ ψ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v2 as a function of the collision centrality and of the J/ ψ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or pT sub-samples. The extracted v2 values are significantly larger than zero for non-central collisions and are seen to increase with pT. [ABSTRACT FROM AUTHOR]

The yield of muon pairs in the invariant mass region 1< M<2.5 GeV/ c2 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 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/ c2, rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/ c2 to be of largely partonic origin, when radial flow has not yet built up. [ABSTRACT FROM AUTHOR]

ψ′ production is studied in Pb–Pb collisions at 158 GeV/c per nucleon incident momentum. Absolute cross-sections are measured and production rates are investigated as a function of the centrality of the collision. The results are compared with those obtained for lighter colliding systems and also for the J/ψ meson produced under identical conditions. [ABSTRACT FROM AUTHOR]

NA60 is a fixed-target experiment at the CERN SPS which measured dimuon production in nucleus–nucleus and proton–nucleus collisions. The experiment collected muon pair samples of unprecedented quality in heavy-ion experiments. This paper presents a high quality measurement of the pT distribution of the φ meson, covering a broad pT window. The data were collected in 2003 in In-In collisions at 158 GeV per nucleon. The results, presented as a function of centrality, were studied against several possible sources of systematic effects and proved to be fairly stable. We show that the inverse mT slope measured in In-In collisions, in the φ→μμ decay channel, depends significantly on the range used to perform the fit. When the fit is performed at low transverse momentum, the effective inverse slope increases from peripheral to central collisions, as measured by other experiments. We finally show that our measurement for In-In is compatible with the overall systematics of T slope versus mass, measured in different collision systems by the NA49 experiment [ABSTRACT FROM AUTHOR]

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158 A GeV In–In collisions. A strong excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size close to 400000 events and the good mass resolution of about 2% made it possible to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum shows some non-trivial centrality dependence, but is largely consistent with a dominant contribution from π+π-→ϱ→μ+μ- annihilation. The associated ϱ spectral function exhibits considerable broadening, but essentially no shift in mass. The pT-differential mass spectra show the excess to be much stronger at low pT than at high pT. The results are compared to theoretical model predictions; they tend to rule out models linking hadron masses directly to the chiral condensate. [ABSTRACT FROM AUTHOR]

We report a new measurement of J/ψ, ψ′ and Drell–Yan cross-sections, in the kinematical domain -0.425cm<0.575 and -0.5CS<0.5, performed at the CERN-SPS using 400 GeV/c incident protons on Be, Al, Cu, Ag, W and Pb targets. The dependence of the charmonia production cross-sections on the size of the target nucleus allows to quantify the so-called normal nuclear absorption. In the framework of the Glauber model, this new measurement is combined with results previously obtained with the same apparatus, under different experimental conditions, and leads to a precise determination of the J/ψ and ψ′ absorption cross-sections in the surrounding nuclear matter. [ABSTRACT FROM AUTHOR]

NA60 is an experiment at the CERN-SPS devoted to the study of dimuon production in heavy-ion and proton-nucleus collisions. The main topics under study are low mass vector meson production, J/ $\psi$ production and suppression, and the sources of the dimuon continuum in the mass range 1.2-2.7 GeV/ c2. In 2003, NA60 collected $\sim 230$ million dimuon events from Indium-Indium collisions. We present preliminary results of the analysis of this data sample in view of measuring the open charm contribution to the dimuon spectrum. Although we are still working on the final background subtraction procedure, we can already demonstrate that the detector performance is good enough to allow the separation of prompt dimuons from muon pairs originating in distant D $\overline{\rm D}$ decays. [ABSTRACT FROM AUTHOR]

We present a new measurement ofproduction in Pb-Pb collisions at 158 GeV/nucleon, from the data sample collected in year 2000 by the NA50 Collaboration, under improved experimental conditions with respect to previous years. With the target system placed in vacuum, the setup was better adapted to study, in particular, the most peripheral nuclear collisions with unprecedented accuracy. The analysis of this data sample shows that thecross-sections ratio measured in the most peripheral Pb-Pb interactions is in good agreement with the nuclear absorption pattern extrapolated from the studies of proton-nucleus collisions. Furthermore, this new measurement confirms our previous observation that thecross-sections ratio departs from the normal nuclear absorption pattern for semi-central Pb-Pb collisions and that this ratio persistently decreases up to the most central collisions. [ABSTRACT FROM AUTHOR]

The production of J/Ψand &Psi′ charmonium states in proton-nucleus interactions has been investigated by the NA50 experiment, at the CERN SPS. High statistics data sets were collected with collisions induced by 450 GeV protons incident on Be, Al, Cu, Ag and W targets. The J/ψand ψ production cross-sections have been determined for each p-A system and their dependences on the nucleus size have been studied, leading to the so-called normal nuclear absorption. Comparing the two patterns we see that the nuclear absorption is stronger for the &Psi′ than for the J/Ψ. Given the high statistics of the data samples, the xF (or rapidity) differential cross-sections of the J/Ψ and &Psi′ states have also been studied, for each of the target nuclei. [ABSTRACT FROM AUTHOR]

The NA60 experiment has studied low-mass muon pair production in proton–nucleus collisions with a system of Be, Cu, In, W, Pb and U targets, using a 400 GeV proton beam at the CERN SPS. The transverse momentum spectra of the ρ / ω and ϕ mesons are measured in the full p T range accessible, from p T = 0 up to 2 GeV/c . The nuclear dependence of the production cross sections of the η ω and ϕ mesons has been found to be consistent with the power law σ pA ∝ A α , with the α parameter increasing as a function of p T for all the particles, and an approximate hierarchy α η ≈ α ϕ > α ω . The cross section ratios σ η / σ ω σ ρ / σ ω and σ ϕ / σ ω have been studied as a function of the size A of the production target, and an increase of the η and ϕ yields relative to the ω is observed from p–Be to p–U collisions. [ABSTRACT FROM AUTHOR]