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Prompt dimuons and D meson production in Heavy Ion collisions at the SPS

OUTLINE Physics motivation Experimental apparatus Physics performance capability Summary. Prompt dimuons and D meson production in Heavy Ion collisions at the SPS. Bjorn Lenkeit, CERN-EP on behalf of the NA60 Coll. Moriond, March 2001.

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Prompt dimuons and D meson production in Heavy Ion collisions at the SPS

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  1. OUTLINE Physics motivation Experimental apparatus Physics performance capability Summary Prompt dimuons and D meson productionin Heavy Ion collisions at the SPS Bjorn Lenkeit, CERN-EPon behalf of the NA60 Coll.Moriond, March 2001

  2. Intermediate mass dimuons in p-A collisions _ • The p-A data is properly described by a superposition of Drell-Yan and DD decays • The required charm cross-section is consistent with previous direct measurements NA50

  3. Excess production of intermediate mass dimuons NA38+NA50 In heavy ion collisions, S-U and Pb-Pb, the yield of produced dimuons exceeds the superposition of the expected sources, Drell-Yan and D meson decays • Signal / background ~ 1/20 ; unsubtracted background ? Many tests exclude this hypothesis; excess and background have different shapes

  4. Charm enhancement ? The measured yields can be reproduced by scaling up the charm contribution But can the charm yield be enhanced by a factor 3 ?  A much better measurement is needed L. Capelli, NA50, at QM2001

  5. Thermal dimuons production ? L. Capelli, NA50, at QM2001 • mm yield based on qq annihilation rate • integration over space-time history • central collisions only • parameters : • fireball lifetime : 14 fm/c • initial temperature : Ti = 192 MeV • explicit introduction of a QGP phase • critical temperature : Tc = 175 MeV • no free parameters Rapp and Shuryak, PLB473(2000) 13. the IMR excess can be well accounted for by thermal radiation when combined with DY and open charm

  6. Low mass dilepton production • The p-Be and p-Au data measured by CERES are properly described by the standard cocktail of hadronic decays but there is an excess in the Pb-Au data ! • The excess increases with the square of the charged particle multiplicity and ismore pronounced at low pT • Degeneracy of  and a1 spectral functions ? Chiral symmetry restoration ? • Better statistics and resolution needed : 2000 pairs out of 40 million events !

  7. Physics Motivation • The present SPS results are very interesting but important issues remain unclear : • Is the  meson modified by the medium ?  measure the  ! Is there a threshold in the dilepton enhancement ? • What is the origin of the intermediate mass dimuon excess ? Thermal dimuons ? • Is the open charm yield enhanced in nucleus-nucleus collisions ? How does it compare to the suppression pattern of bound charm states ? • What is the variable that rules the onset of ’, c and J/ suppression ? • What is the physical origin of the ’ suppression ? If it is Debye screening, what is its melting temperature ( value of Tc) ? • What fraction of J/ come from c decays ? Does it change from p-Be to p-Pb ?

  8. From NA50 to NA60 (1996 - 2000) Let’s add silicon detectors to track the muons before they traverse the hadron absorber Improved measurement of prompt dimuon production and open charm in heavy ion collisions

  9. m vertex p, K m { offset D m Detector concept • 3-dimensional track matching with the muon spectrometer • Improved mass resolution • Rejection of muons from p and K decays • Improved low mass dimuon acceptance • Muon track offset measurement • Separate charm from prompt (thermal) dimuons

  10. The NA60 silicon vertex spectrometer Beamscope Silicon pixel telescope • 2 x-y stations of m-strip Si detectors at T = 130 K • ~ 20 mm resolution on the transverse coordinates of the beam ions 2.5 T dipole field • 10 planes • 88 pixel readout chips • 720 000 channels • pixel size : 50425 mm2

  11. Physics performance capability • Detector setup : • The whole detector geometry is described using GEANT • Event generation : • Soft signals with Genesis code (thermal distributions) • Hard processes with PYTHIA • Underlying hadronic background with VENUS

  12. Dimuon mass resolution : simulation NA50 • Clear improvement in mass resolution and signal / background ratio M at M = 1 GeV : 70 MeV in NA50; 20 MeV in NA60 without pixels ’ Vertex spectrometer NA60 J/  with pixels

  13. Dimuon mass resolution : data • few hours at ~ 10 8 protons / burst, 450 GeV, 10 mm Be target ( 5 % lint ) • half acceptance, bump-bonding, radiation damage  low detector efficiency without pixels with pixels sM = 20 MeV sM = 70 MeV

  14. Low mass dimuons • Good sensitivity on the shape of the r • Very good statistics and pT coverage for the r,  and  mesons • A few 100 events expected for very low pT at M ~ 500 MeV r / total signal statistical uncertainty

  15. Measurement of the muon track offset Determination of the interaction vertex Impact parameter of the muon tracks D+ : ct = 317 mm D0 : ct = 124 mm D

  16. Muon track offset for different sources • Full tracking and vertex reconstruction • Signal embedded on top of Venus events • select offset windows for • prompt and charm samples

  17. IMR : charm or thermal dimuons ? • Charm selection : events with muon track offset in the range 90 800 mm and muons > 180 mm away from each other in the transverse plane at zv • Prompt dimuons selection : events with muon track offset < 90 mm

  18. IMR dimuons : pT distributions • Charm selection • Prompt dimuons selection

  19. Summary • SPS is still alive  NA60 : third generation experiment to separately study the production of prompt dimuons and of muons coming from the decay of charmed mesons • NA60 will clarify the origin of the excess IM dimuon production and measure the yield of charmed mesons produced in heavy ion collisions • Low mass dimuon data with good statistics, mass resolution and signal to background ratio will allow to study the production of r,  and  mesons • NA60 will certainly contribute to the understanding of the present results concerning the production of a deconfined state of matter in high energy heavy ion collisions at the SPS

  20. The NA60 Collaboration R. Arnaldi, A. Baldit, K. Banicz, K. Borer, L. Casagrande, J. Castor, B. Chaurand, W. Chen, B. Cheynis, P. Chochula, C. Cicalò, M.P. Comets, P. Cortese, V. Danielyan, A. David, A. De Falco, N. De Marco, A. Devaux, B. Dezillie, L. Ducroux, B. Espagnon, P. Force, E. Gangler, V. Granata, A. Grigorian, S. Grigorian, J.Y. Grossiord, A. Guichard, H. Gulkanian, R. Hakobyan, E. Heijne, M. Hess, P. Jarron, D. Jouan, L. Kluberg, Y. Le Bornec, B. Lenkeit, Z. Li, C. Lourenço, M.P. Macciotta, M. Mac Cormick, F. Manso, D. Marras, A. Masoni, S. Mehrabyan, H. Muller, A. Musso, A. Neves, B. Pes, S. Popescu, G. Puddu, P. Ramalhete, P. Rosinsky, P. Saturnini, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, E. Siddi, P. Sonderegger, G. Usai, G. Vandoni, H. Vardanyan, N. Willis, H. Woehri and M. Zagiba CERN Bern Brookhaven Orsay Bratislava Torino Lisbon Yerevan Clermont Lyon Cagliari

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