A. Neves

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 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.

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

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.

A new ultra radiation hard cryogenic silicon tracker for heavy ion beams

AbstractWe have designed and built a new hodoscope for heavy ion beams consisting of two tracking stations equippedwith silicon microstrip detectors operated at 130K. Preliminary results from tests performed in the high intensityCERN-SPSPbionbeamshowthatwewereabletodetectandidentifyeachincomingionuptoabeamintensityofB10 7 ions/s.Thebeamprofilecouldbemonitoredon-linewith50mmresolution.Thedetectorperformedsatisfactoryevenafteradoseof90740Grad.r 2002ElsevierScienceB.V.Allrightsreserved. PACS:07.77. n;85.30.ZKeywords:Cryogenicsilicondetectors;Heavyions 1. IntroductionIn the framework of the CERN NA60 [1] andRD39Collaborations [2], we have developed anew cryogenic beam hodoscope, hereafter namedBeamscope, based on silicon microstrip detectorsoperated at 130K. This detector represents thefirst application in a high energy physics experi-mentofthecryogenicsilicontechnology,triggeredbythediscoveryofthe‘‘Lazaruseffect’’[3]in1998anddevelopedbyRD39.Operationofsiliconatcryogenictemperaturesisanattractivetechnologyindeedforapplicationinharsh environments. It has been shown [4,5] thatheavily irradiated silicon detectors operatedaround 130K have charge collection efficiency

Latest results from NA60

The NA60 experiment has measured the production of muon pairs and charged particles in In+In collisions at a beam energy of 158 A GeV. For invariant dimuon masses below the , the spacetime averaged ρ spectral function was isolated by a novel procedure. It shows a strong broadening but essentially no shift in mass. The production of J/ψ was measured as a function of the collision centrality. As in previous experiments studying Pb+Pb collisions an anomalous suppression is observed, setting in at approximately 90 participant nucleons. Using the charged particles, the reaction plane was reconstructed. The elliptic flow of charged particles increases with pt showing a saturation for pt > 2 GeV/c. For the first time, azimuthal distributions for J/ψ are shown.

Low mass dimuon production in proton-nucleus and indium-indium collisions

The NA60 experiment studied low-mass dimuon production in proton-nucleus and indium-indium collisions at the CERN SPS. While the p-A data can be well described by a superposition of light neutral meson decays, an excess is observed in the heavy-ion data, increasing with collision centrality, with respect to these expected sources. Thanks to the high statistics and good quality of our measurement, we can subtract the muon pairs due to h, h0, w and f decays from the signal dimuon mass distribution. The resulting spectra are compatible with a r spectral function broadening from peripheral to central collisions, with no change in the pole mass.

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

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...