NA60 Experiment: Dimuon Production in Nuclear Collisions

1. Dimuon production in p-nucleus and nucleus-nucleus collisionsThe NA60 experiment • Outline: • Reminder of the physics motivation • A glimpse of the detector concept • First results from the Indium data m m Roberta Arnaldi – INFN Torinofor the NA60 Collaboration

2. QCD phase transition → nuclear collisions → dilepton production • QCD predicts that, above a critical temperature or energy density, strongly interacting matter undergoes a phase transition to a new state where the quarks and gluons are no longer confined in hadrons, and chiral symmetry is restored • The search for such a phase transition, from hadrons to quark-gluon matter, motivated many people to probe high-energy nuclear collisions at the CERN SPS, since 1986, guided by theory-driven “signatures” • Some of these require measuring dileptons and justified NA38, CERES and NA50: • the production of thermal dimuons directly emitted from the new phase, if in thermal equilibrium • changes in the r spectral function (mass shifts, broadening, disappearance) when chiral symmetry restoration is approached • the suppression of strongly bound heavy quarkonia states dissolved when certain critical thresholds are exceeded

3. NA50Pb-Pb 158 GeV CERESPb-Au 158 GeV DY charm central collisions M(GeV) _ DY DD Interesting observations made by previous experiments NA38/NA50 • The yield of intermediate mass dimuons seen in heavy-ion collisions (S-U, Pb-Pb) exceeds the sum of Drell-Yan and D meson decays, which describes the proton data • The low mass dielectron data collected in heavy-ion collisions (S-Au, Pb-Au) exceeds the expected sum of light meson decays, which describes the proton data • The J/y and y’ production is suppressed in heavy-ion collisions (S-U, Pb-Pb) with respect to the yields extrapolated from proton-nucleus data CERES NA38/NA50

4. Specific questions that remain open • What is the origin of the low mass dilepton excess?  Get much more statistics, better signal to background ratio and mass resolution; resolve the w peak; study the signal versus pT and collision centrality • Is the intermediate mass excess due to thermal dimuons from a quark-gluon plasma? • What is the open charm yield in nucleus-nucleus collisions? Measure secondary vertices with ~ 50 µm precision; separate prompt dimuons from D meson decays • What is the physics variable driving the J/y suppression? L, Npart, energy density? • Are the charmonium states broken by deconfined quarks and gluons?  Measure the J/y pattern in Indium-Indium and compare it with Pb-Pb • What is the impact of the cc feed-down on the observed J/y suppression pattern?  Study the nuclear dependence ofcc production in p-A collisions NA60 New and accurate measurements are needed

5. CERN Heidelberg Bern Palaiseau BNL Riken Yerevan Stony Brook Torino Lisbon Cagliari Clermont Lyon The NA60 Experiment http://cern.ch/na60 Place a high granularity and radiation-hard silicon tracking telescope in the vertex regionto measure the muons before they suffer multiple scattering and energy loss in the absorber ~ 60 people 13 institutes8 countries R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen,B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanović, A. David, A. de Falco, N. de Marco,A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A. Grigorian, J.Y. Grossiord,N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço,J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, G. Usai, H. Vardanyan, R. Veenhof, D. Walker and H. Wöhri

6. muon trigger and tracking magnetic field ZDC hadron absorber Origin of muons can be accurately determined Improved dimuon mass resolution prompt dimuon or muon pair fromdisplaced vertices Measuring dimuons in the target region pixel telescopein a 2.5 T dipole beam tracker targets Matching in coordinate and momentum space ZDC: dimuon studies vs. collision centrality

7. The NA60 target region: reality 2.5 T dipole magnet Beam Tracker Pixel detectors 10 cm

8. Broad centrality coveragemeasured by the ZDC and by the number of tracksseen in the pixel telescope Indium beam 158 A GeV Transverse vertexingwith 20 µm accuracy(and < 200 µm in Z) Indiumtargets A first look at the Indium data • 5-week long run in Oct.–Nov. 2003 • Indium beam of 158 GeV/nucleon • ~ 4 ×1012 ions delivered in total • ~ 230 million dimuon triggers on tape 4000 A (40% of collected statistics) 6500 A Opposite-sign dimuon massdistributions before quality cutsand muon track matching(two spectrometer settings) M (GeV)

9. less than 1 % of total statistics no centrality selection S/B ~ 0.75 opposite-sign signal combinatorial background Low mass dimuon production in Indium-Indium collisions from a preliminary analysisof a very small event sample … mass resolution :20–25 MeV at M ~ 1 GeV w f • With respect to the Pb-Au CERES data: • Higher statistics by factor ~200 Signal / background better by factor ~10factor ~ 2000higher effective statistics • Mass resolution ~2%, better by a factor 2 • Full information on associated track multiplicity • Completely different systematic uncertainties The combinatorial background resulting from p and K decays is estimated through a mixed-event technique, using like-sign muon pairs. The normalization is still preliminary, and fake matches are not yet included at all

10. pT distributions of low mass dimuons pT distribution of f→mm events Good pT coverage down tothe lowest dimuon masses after applying a first orderacceptance correction no centrality selection raw data NA60 will certainly solve the long standing f puzzle between NA49 and NA50 > 100 000 f→mm decays in the full data samplef→K+K-decays alsounder analysis

11. all events after rejecting beam pile-up& non-interacting beam ions all events after rejecting beam pile-up after muon quality cuts & in dimuon phase space window & non-interacting Indium ions EZDC distributions and data selection for high mass dimuon analysis Minimum bias (ZDC) trigger Dimuon trigger • Beam pile-up is rejected using Beam Tracker timing information • Non-interacting beam ions are rejected using Interaction Counter • Severe quality cuts have been used in this preliminary analysis(statistics will increase in the future, especially for peripheral collisions) • Dimuon data analysis performed for events with EZDC < 15 TeV • and in the phase space window: 0 < ycms < 1 ; |cos CS| < 0.5

12. DY yield = 162± 131302 ± 104in range 2.9–4.5 GeV J/y yield = 23532 ± 298 Understanding the opposite-sign dimuon mass distribution Dimuon data from the 6500 A event sample No muon track matching used in this analysis Mass resolution at the J/y : ~107 MeV Combinatorial background from  and K decays estimated from the measured like-sign pairs Signal mass shapes from Monte Carlo:PYTHIA and MRS A (Low Q2) parton densitiesGEANT 3.21 for detector simulation reconstructed as the measured data Acceptances from Monte Carlo simulation:  for J/y : 12.4 %  for DY : 13.4 % (in mass window 2.9–4.5 GeV) Background J/y Charm y’ DY A multi-step fit (max likelihood) is performed:a) M > 4.2 GeV : normalise the DY b) 2.2<M<2.5 GeV: normalise the charm (with DY fixed) c) 2.9<M<4.2 GeV: get the J/y yield (with DY & charm fixed)

13. Projectile J/y L Target J/y / Drell-Yan in Indium-Indium collisions preliminary B s(J/y) / s(DY) = 19.5 ± 1.6  0.87 ± 0.07 w.r.t. the absorption curve In-In collisions of EZDC < 15 TeV L = 7.0 fm(from Glauber fit to the minimum bias EZDC distribution) and Npart = 133 all data rescaled to 158 GeV Stability checks: Background increase by 10% : less than 3% changeDifferent event selection or fitting procedure : less than 8% change Using GRV parton densities instead of MRS : 0.87 ± 0.07  0.93 ± 0.08

14. Summary and outlook • Harvest from the 5-week long Indium run in Oct.–Nov. 2003: • more than 100 000 reconstructed J/y events (before track matching) • ~ 1 million signal low mass dimuons (after track matching) • mass resolution ~ 20–25 MeV at the w and f masses • signal to background ratio around 1:1 or 1:2 depending on collision centrality (a factor 4 better than before muon track matching) To understand the heavy-ion results we need a solid reference baseline from p-A data NA60 will take ~ 70 days of 400 GeV protons in 2004, with 7 different nuclear targets,at high beam intensities (~ 2 × 109 p/burst), to study: the impact of cc production on the J/y suppression  the nuclear dependence of open charm production  the intermediate mass prompt dimuons  the low mass dimuons with unprecedented accuracy • Together with the proton run of 2004, NA60 should be able to : • study the production of low mass dimuons, including the r, w and f resonances • clarify the cause of the excess of intermediate mass dimuons in heavy-ion collisions • improve the understanding of the production and suppression of charmonium states