R. Tieulent

First measurement of the ϱ spectral function in nuclear collisions

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158xa0Au2009GeV 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.

NA60 results on p(T) spectra and the rho 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

Study of the electromagnetic transition form-factors in η→μ+μ−γ and ω→μ+μ−π0 decays with NA60

rho

Study of the electromagnetic transition form-factors in η→μ+μ−γη→μ+μ−γ and ω→μ+μ−π0ω→μ+μ−π0 decays with NA60

). 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.

{phi} Production in Proton-Nucleus and Indium-Indium Collisions at the CERN SPS

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 the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions - NA60 Collaboration

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.

J/psi production in indium-indium collisions at SPS energies

The quality of the dimuon measurements made by NA60, in proton‐nucleus and heavy‐ion collisions, is much better than that reached by previous experiments, such as NA38 and NA50. The most important improvement is due to the use of a radiation‐tolerant silicon vertex telescope, placed immediately downstream of the target. This allows NA60 to do a high quality measurement of φ meson yields and pT distributions.This paper presents results obtained in p‐Be, p‐In and p‐Pb collisions at 400 GeV, from data collected in 2002, and in In‐In collisions at 158 AGeV, as a function of centrality, from the 2003 running period. In particular, we show that the inverse mT slope measured in In‐In collisions, in the φ → μμ decay channel, increases with the number of nucleons participating in the collisions, rather than following a flat trend as seen in the NA50 data collected in the same decay channel but restricted to high pT values. We also show that our measurements seem to agree with the values previously measured by NA49...

Results from NA60 experiment at the CERN SPS

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.

Dimuon production in p-A and In-In collisions

The NA60 experiment collected data on dimuon production in indium‐indium collisions at 158 GeV/c per incident nucleon, in year 2003, to contribute to the clarification of several questions raised by previous experiments studying high‐energy heavy‐ion physics at the CERN SPS in search of the quark gluon plasma. Among these previous results stands the observation, by NA50, that the production yield of J/ψ mesons is suppressed in central Pb‐Pb collisions beyond the normal nuclear absorption defined by proton‐nucleus data. By comparing the centrality dependence of the suppression pattern between different colliding systems, S‐U, Pb‐Pb and In‐In, we should be able to identify the corresponding scaling variable, and the physics mechanism driving the suppression. In this paper, we will present the ratio of J/ψ and Drell‐Yan production cross‐sections in indium‐indium collisions, in three centrality bins, and how these values compare to previous measurements. We will also present a study of the transverse momentum...

Anomalous J/ψ suppression in In-In collisions at 158 GeV/nucleon

The NA60 experiment studies open charm and prompt dimuon production in proton‐nucleus and nucleus‐nucleus collisions at the CERN SPS. During 2003 the experiment collected data in Indium‐Indium collisions at 158 GeV per nucleon. In this paper the first results on low mass dimuons, intermediate mass dimuons and J/ψ suppression are presented.