M. Keil

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

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.

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

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.

First results on angular distributions of thermal dileptons in 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.

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.

Intermediate mass dimuon production in indium-indium collisions at the CERN SPS

R. Arnaldi 10, R. Averbeck 9, K. Banicz 2,4, J. Castor 3, B. Chaurand 7, C. Cicalò 1, A. Colla 10, P. Cortese 10, S. Damjanovic 4, A. David 2,5, A. De Falco 1, A. Devaux 3, A. Drees 9, L. Ducroux 6, H. En’yo 8, A. Ferretti 10, M. Floris 1, P. Force 3, A. ,Förster 2, N. Guettet 2,3, A. Guichard 6, H. Gulkanian 11, J. Heuser 8, M. Keil 2,5, L. Kluberg 2,7, C. Lourenço 2, J. Lozano 5, F. Manso 3, A. Masoni 1, P. Martins 2,5, A. Neves 5, H. Ohnishi 8, C. Oppedisano 10, P. Parracho 2, P. Pillot 6, G. Puddu 1, E. Radermacher 2, P. Ramalhete 2, P. Rosinsky 2, E. Scomparin 10, J. Seixas 2,5, S. Serci 1, P. Sonderegger 5, H.J. Specht 4, R. Tieulent 6, G. Usai 1, R. Veenhof 2,5, H.K. Wöhri 2,5

First results and future perspectives of the NA60 experiment

The NA60 experiment at the CERN SPS studies the production of open charm and prompt dimuons in collisions induced by proton and heavy ion beams on nuclear targets. The experimental setup includes a silicon vertex telescope, placed inside a dipole magnetic field, to match charged particle tracks with the muon tracks measured by a muon spectrometer. In p-A collisions the vertex telescope is mostly made of silicon microstrip planes, whereas in A-A interactions the higher multiplicity imposes the use of high granularity silicon pixel detectors. The high interaction rate required for the study of rare processes imposes the use of radiation tolerant pixel detectors. NA60 took its first data in the year 2002, with 400 GeV protons and with Pb ions of 20 and 30 GeV per nucleon. The proton data have been used for the study of dimuon production, while the heavy ion data were devoted to the commissioning of the pixel detectors and to the study of charged particle pseudorapidity densities. After an overview of the detector concept, this paper presents the first results obtained with the 2002 data.