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Anomalous J/ y suppression in Indium-Indium collisions at 158 GeV/nucleon

Anomalous J/ y suppression in Indium-Indium collisions at 158 GeV/nucleon. Study of the centrality dependence of the J/ y suppression, using different normalization techniques Comparison with theoretical models. Roberta Arnaldi INFN Torino (Italy)

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Anomalous J/ y suppression in Indium-Indium collisions at 158 GeV/nucleon

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  1. Anomalous J/y suppression in Indium-Indium collisions at 158 GeV/nucleon Study of the centrality dependence of the J/ysuppression, using different normalization techniques Comparison with theoretical models Roberta Arnaldi INFN Torino (Italy) on behalf of the NA60 Collaboration Quark Matter 2005 XVIII International Conference on Nucleus-Nucleus Collisions August 4-9, Budapest, Hungary

  2. Light systems and peripheral Pb-Pb collisions: J/y is subject to nuclear absorption and the scaling variable for this process is L (length of nuclear matter crossed by the J/y), • Central Pb-Pb collisions: the L scaling is broken and an anomalous suppression sets in ~ J/y suppression from p-A to Pb-Pb collisions J/y suppression is generally considered one of the most direct signatures of QGP formation (Matsui-Satz 1986) J/y production has been extensively studied inp-A, S-UandPb-Pbcollisions by the NA38 and NA50 experiments at the CERN SPS Projectile J/y L Target J/y normal nuclear absorption curve

  3. S-U Pb-Pb L (fm) In-In pure Glauber calculation Npart New and accurate measurements are needed to answer these questions Specific questions that remain open Is the anomalous suppression also present in lighter nuclear systems? Study collisions between other systems, such as Indium-Indium Which is the variable driving the suppression? Study the J/ suppression pattern as a function of different centrality variables, including data from different collision systems What is the normal nuclear absorption cross-section at the energy of the heavy ion data? Study J/ production in p-A collisions at 158 GeV What is the impact of the cc feed-down on the observed J/y suppression pattern? Study the nuclear dependence of cc production in p-A collisions

  4. MWPC’s m ~ 1m Muon Spectrometer Iron wall Hadron absorber Toroidal Magnet Target area m beam Trigger Hodoscopes Dipole field2.5 T ZDC TARGET BOX MUON FILTER Matching in coordinate and in momentum space BEAM BEAMTRACKER VERTEX TELESCOPE IC  not to scale • Origin of muons can be accurately determined • Improved dimuon mass resolution allows studies vs. collision centrality  ZDC NA60’s detector concept Idea: 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

  5. J/y production in Indium-Indium: event selection A clean sample of events is obtained with the following requirements: Beam pile-up removed (in ±12 ns window) Matching between muon spectrometer and vertex telescope tracks is not mandatory for J/y studies. Requiring the matching: Interaction in one of the 7 targets (Z-vertex of the collision determined with ~ 200 µm accuracy) • J/y mass resolution improves (from ~105 MeV to ~70 MeV) • combinatorial background is reduced (from ~ 3% to <1% in  1 σ at the J/y peak) • dimuon vertex is used to select only dimuons produced in Indium-Indium collisions • statistics is reduced (dimuon matching e: ~ 70% at the J/y) Phase space window: -0.5 < cosCS < 0.5 & 2.92 < yLAB < 3.92 • Available statistics for J/y studies vs. centrality (before matching): • ~ 43000 J/y • ~ 300 Drell-Yan (Mass >4.2 GeV)

  6. Projectile Target EZDC (GeV) Projectile Target Centrality estimate NA60 can estimate the centrality of the collisions from the energy released in the Zero Degree Calorimeter (ZDC) taking into account: the small contribution of secondary particles emitted in the ZDC angular acceptance (η > 6.3) the smearing due to the ZDC experimental resolution (~9% at the peak) The link between NPart and other centrality variables is established within the Glauber model 1 < EZDC < 2 TeV 2 < EZDC < 3 TeV 3 < EZDC < 4 TeV …  For each EZDC range we can estimate the corresponding NPart, L, NColl … Number of participants

  7. J/ production analyses methods Two different analyses have been performed to investigate the centrality dependence of the J/ production. They correspond to two different ways of normalizing the J/ yield: 1)DY normalization (J/ / DY standard analysis) 2)analysis of the J/ sample standalone

  8. The J/ / DY standard analysis Background without matching 6500 data set no centrality selection • Combinatorial background from  and K decays estimated from the measured like-sign pairs (<3% contribution under the J/y) • Signal mass shapes from Monte Carlo: • PYTHIA and GRV 94 LO parton densities • GEANT 3.21 for detector simulation • reconstructed as the measured data • Acceptances from Monte Carlo simulation: • for J/y : 12.4 % (6500 A); 13.8 % (4000 A) • for DY : 13.2 % (6500 A); 14.1 % (4000 A) • (in mass window 2.9–4.5 GeV) J/y Charm y’ DY A multi-step fit (max likelihood) is performed: a) M > 4.2 GeV : normalize the DY b) 2.2 < M < 2.5 GeV: normalize the charm (with DY fixed) c) 2.9 < M < 4.2 GeV: get the J/y yield (with DY & charm fixed)

  9. J/y/DY: advantages & drawbacks Advantages: DY is a hard process. Its production cross section scales as the number of binary collisions and does not suffer sizeable final state effects J/ / DY is a ratio of events collected with the same trigger (i.e. dimuon trigger). event selection is the same for both event samples inefficiencies or experimental biases (affecting both J/ and DY) cancel out Easy comparison with the results previously obtained by the NA50 experiment Drawbacks: High mass dimuons are rare: we are forced to study the J/ with statistical errors imposed by the 100 times smaller reference process We cannot have more than 3 centrality bins

  10. Comparison with previous results An “anomalous suppression” is present already in Indium-Indium The normal absorption curve is based on the NA50 results. Its uncertainty (~ 8%) at 158 GeV is dominated by the (model dependent) extrapolation from the 400 and 450 GeV data need p-A measurements at 158 GeV:data collected in 2004 (analysis under way)

  11. εvertex dimuon > 99.5 % εvertex Direct J/ sample The idea is to directly compare the measured J/ sample (as a function of centrality) with the theoretical distribution expected in case of pure nuclear absorption dNJ/y/dEZDC J/ sample: matched sample of J/ (cleaner spectrum). J/ events correspond to the signal after the combinatorial background subtraction Inefficiencies introduced by the cuts, used in the events selection, affect in a negligible way the J/y sample EZDC (GeV) Advantages: small statistical errors  possibility to obtain a detailed pattern as a function of centrality only one trigger involved (i.e. dimuon trigger) inefficiencies are negligible Drawbacks: there is no intrinsic absolute normalization. does not show a centrality dependence

  12. Direct J/ sample: the result Data are compared with a theoretical J/ distribution, obtained within the Glauber model, taking into account the survival probability to the nuclear absorption. Nuclear absorption The ratio Measured / Expected is normalized to thestandard analysis The following pattern is observed: Onset of anomalous suppression in the range 80 < NPart < 100 Saturation at large NPart EZDC(TeV)

  13. J/ analysis (2 TeV EZDC bins) J/ analysis (1 TeV EZDC bins) Direct J/ sample: stability of the result J/ analysis shifted bins J/ analysis The observed pattern is confirmed by a similar analysis with a reduced number of bins Shifting the 16 EZDC bins by half the bin width does not change the observed suppression pattern

  14. Direct J/ sample: comparison with previous results The J/y suppression patterns are in fair agreement in the Npart variable The S-U, In-In and Pb-Pb data pointsdo not overlap in the L variable

  15. NA60 In-In NA60 In-In NA50 Pb-Pb NA50 Pb-Pb Direct J/ sample: comparison with previous results (2) very preliminary Bjorken energy density, estimated using VENUS A more significant comparison requires Pb-Pb points with reduced error bars

  16. Direct J/ sample: comparison with theoretical models Good accuracy of NA60 data allows a quantitive comparison with the available theoretical predictions It is important to emphasize that these models were previously tuned on the p-A, S-U and Pb-Pb suppression patterns obtained by NA38 and NA50 We consider models for which we have predictions specifically made for In-In collisions: J/y absorption by produced hadrons (comovers) Capella and Ferreiro, hep-ph/0505032; J/y suppression in the QGP and hadronic phasesincluding thermal regeneration and in-medium properties of open charm and charmonium statesGrandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301; hep-ph/0403204 cc suppression by deconfined partons when geometrical percolation sets inDigal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.

  17. Suppression by produced hadrons (“comovers”) The model takes into account nuclear absorption and comovers interaction with sco = 0.65 mb (Capella-Ferreiro) In-In @ 158 GeV J/y / NColl nuclear absorption comover + nuclear absorption (E. Ferreiro, private communication) NA60 In-In 158 GeVpreliminary The smeared form (dashed line) is obtained taking into account the resolution on NPart due to our experimental resolution Pb-Pb @ 158 GeV

  18. QGP + hadrons + regeneration + in-medium effects The model simultaneously takes into account dissociation and regeneration processes in both QGP and hadron gas (Grandchamp, Rapp, Brown) In-In @ 158 GeV fixed thermalization time fixed thermalization time centrality dependent thermalization time BmmsJ/y/sDY centrality dependent thermalization time Nuclear Absorption Suppression + Regeneration QGP+hadronic suppression Regeneration Number of participants NA60 In-In 158 GeVpreliminary The smeared form (dashed line) is obtained taking into account the resolution on NPart due to our experimental resolution Pb-Pb @ 158 GeV

  19. NA60 In-In 158 GeVpreliminary NA60 In-In 158 GeVpreliminary The measured data show a similar pattern but the anomalous suppression sets in atNpart ~ 90 Suppression due to a percolation phase transition Model based on percolation (Digal-Fortunato-Satz) Sharp onset (due to the disappearance of the cc meson) at Npart ~ 125 for Pb-Pb and ~ 140 for In-In The dashed line includes thesmearing due to the ZDC resolution Pb-Pb @ 158 GeV

  20. Summary of models comparison Satz, Digal, Fortunato Rapp, Grandchamp, Brown Capella, Ferreiro

  21. Summary • The J/y shows an anomalous suppression already in Indium-Indium • The suppression is centrality dependent and sets in at a number of participants ~ 90 • None of the available models properly describes the observed suppression pattern

  22. CERN Heidelberg Bern Palaiseau BNL Riken Yerevan Stony Brook Torino Lisbon Cagliari Clermont Lyon The NA60 experiment http://cern.ch/na60 ~ 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, P. Pillot,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri

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