H.J. Specht

First Measurement of the

We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360,000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi- -->rho -->mu+mu- annihilation. The associated space-time averaged spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.

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The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.

Spectral Function in High-Energy Nuclear Collisions

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158Axa0GeV158Axa0GeV In–In collisions. The mass and pTpT spectra associated with peripheral collisions can quantitatively be described by the known neutral meson decays. The high data quality has allowed to remeasure the electromagnetic transition form factors of the Dalitz decays η→μ+μ−γη→μ+μ−γ and ω→μ+μ−π0ω→μ+μ−π0. Using the usual pole approximation F=(1−M2/Λ2)−1F=(1−M2/Λ2)−1 for the form factors, we find Λ−2Λ−2 (in GeV−2GeV−2) to be 1.95±0.17(stat.)±0.05(syst.)1.95±0.17(stat.)±0.05(syst.) for the η and 2.24±0.06(stat.)±0.02(syst.)2.24±0.06(stat.)±0.02(syst.) for the ω. While the values agree with previous results from the Lepton-G experiment, the errors are greatly improved, confirming now on the level of 10σ the strong enhancement of the ω form factor beyond the expectation from vector meson dominance. An improved value of the branching ratio BR(ω→μ+μ−π0)=[1.73±0.25(stat.)±0.14(syst.)]×10−4BR(ω→μ+μ−π0)=[1.73±0.25(stat.)±0.14(syst.)]×10−4 has been obtained as a byproduct.

Evidence for radial flow of thermal dileptons in high-energy nuclear collisions

The yield of muon pairs in the invariant mass region 1<M<2.5xa0GeV/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 Au2009GeV 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 190xa0MeV. The values are independent of mass and significantly lower than those found at masses below 1xa0GeV/c2, rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1xa0GeV/c2 to be of largely partonic origin, when radial flow has not yet built up.

Study of the electromagnetic transition form-factors in eta ---> mu+ mu- gamma and omega ---> mu+ mu- pi0 decays with NA60

Abstract The NA60 experiment has studied J / ψ production in p–A collisions at 158 and 400 GeV, at the CERN SPS. Nuclear effects on the J / ψ yield have been estimated from the A -dependence of the production cross section ratios σ J / ψ A / σ J / ψ Be (Axa0=xa0Al, Cu, In, W, Pb, U). We observe a significant nuclear suppression of the J / ψ yield per nucleon–nucleon collision, with a larger effect at lower incident energy, and we compare this result with previous observations by other fixed-target experiments. An attempt to disentangle the different contributions to the observed suppression has been carried out by studying the dependence of nuclear effects on x 2 , the fraction of the nucleon momentum carried by the interacting parton in the target nucleus.

Evidence for the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions

The NA60 experiment has measured muon pair production in In–In collisions at 158 AGeV at the CERN SPS. This paper presents a high statistics measurement of φ→μμ meson production. Differential spectra, yields, mass and width are measured as a function of centrality and compared to previous measurements in other colliding systems at the same energy. The width of the rapidity distribution is found to be constant as a function of centrality, compatible with previous results. The decay muon polar angle distribution is measured in several reference frames. No evidence of polarization is found as a function of transverse momentum and centrality. The analysis of the pT spectra shows that the φ has a small radial flow, implying a weak coupling to the medium. The Teff parameter measured in In–In collisions suggests that the high value observed in Pb–Pb in the kaon channel is difficult to reconcile with radial flow alone. The absolute yield is compared to results in Pb–Pb collisions: though significantly smaller than measured by NA50 in the muon channel, it is found to exceed the NA49 and CERES data in the kaon channel at any centrality. The mass and width are found to be compatible with the PDG values at any centrality and at any pT: no evidence for in-medium modifications is observed.

J/ψ production in proton-nucleus collisions at 158 and 400 GeV

The NA60 experiment at the CERN Super Proton Synchrotron 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.

phi Production in In-In Collisions at 158-A-GeV

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

First results on angular distributions of thermal dileptons in nuclear collisions

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NA60 results on p(T) spectra and the rho spectral function in In-In collisions

). 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.