1. 1 Roberta Arnaldi INFN Torino (Italy) on behalf of the NA60 Collaboration J/  production in Indium-Indium collisions: new results from NA60 Introduction Updated In-In event sample Results on (J/  )/Drell-Yan Results on J/  Comparison with other nuclear systems Evaluation of systematic errors Conclusions 2 nd International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions June 9 – 16 2006, Asilomar Conference Grounds

2. 2 J/  production studied in p-A, S-U and Pb-Pb collisions by NA38/NA50 (CERN SPS) J/  suppression one of the most direct signatures of QGP formation (Matsui-Satz 1986) J/  suppression from p-A to Pb-Pb collisions NA60 : study In-In collisions Light systems, peripheral Pb-Pb collisions  J/  suppression scales with L Central Pb-Pb collisions L-scaling broken  Anomalous suppression

3. 3 MUON FILTER BEAM TRACKER TARGET BOX VERTEX TELESCOPE Dipole field 2.5 T BEAM IC not to scale Origin of muons can be accurately determined Improved dimuon mass resolution  Matching in coordinate and in momentum space ZDC  allows studies vs. collision centrality NA60 has a high granularity and radiation-hard silicon tracking telescope in the vertex region  muons are measured before multiple scattering and energy loss in the absorber beam ~ 1m Muon Spectrometer MWPC’s Trigger Hodoscopes Toroidal Magnet (ACM) Iron wall Hadron absorber ZDC Target area   NA60’s detector concept

4. 4 What’s new? QM2005 J/  results obtained with a first preliminary data reconstruction Updated reconstruction now available Much better alignment (a few  m accuracy)  see A. David’s talk Tracking quality in the vertex spectrometer improved Full statistics now available (factor 2 increase for the J/  ) Preliminary J/  results from In-In collisions Quark Matter 2005 Tracks  2 QM2005 reconstruction new reconstruction

5. 5 measured yield expected yield in case of pure nuclear absorption of the J/  For both analysis we compare, for the various centrality bins, Analysis methods Two different (complementary) approaches: less sensitive to systematic effects limited by high-mass DY statistics few centrality bins more sensitive to systematic effects very small statistical errors several centrality bins b) direct study of the J/  centrality distribution a) ratio of J/  and Drell-Yan production cross sections

6. 6 Event selection In-In interaction in one of the 7 targets (z of the collision determined with ~ 200 µm accuracy) Phase space window (remove acceptance edges) -0.5 < cos  CS < 0.5 & 0 < y CM < 1 Beam pile-up removed (in ±12 ns window) Matching between muon spectrometer and vertex telescope tracks Require the dimuon production point to coincide with the most upstream interaction vertex Accuracy: a few hundred  m Available statistics (after cuts): ~ 29000 J/  Muon spectrometer target cut: based on the distance between the beam axis and the extrapolated muon tracks at the target position. Reject events where the dimuon is NOT produced in the target region (where the primary interaction took place) Accuracy: a few cm Available statistics (after cuts): ~ 45000 J/  ~ 320 Drell-Yan (M >4.2 GeV) J/  : can use stricter quality cuts J/  / DY: keep maximum number of events Most cuts are common to the two analysis methods

7. 7 J/  / DY analysis Set A ( lower ACM current) Combinatorial background ( , K decays) from event mixing method (negligible) Multi-step fit: a) DY (M>4.2 GeV), b) IMR (2.2<M<2.5 GeV), c) charmonia (2.9<M<4.2 GeV) Mass shape of signal processes from MC (PYTHIA+GRV94LO pdf) Results from set A and B statistically compatible  use their average in the following Stability of the J/  / DY ratio: change of input distributions in MC calculation  0.3% (cos ), 1% (rapidity) level of muon spectrometer target cut  < 3% Set B ( higher ACM current)

8. 8 Centrality estimate N part Centrality of the collisions: Charged track multiplicity E ZDC  used in this analysis Take 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 Indium peak) N part distribution for various E ZDC bins (bin width 1 TeV) Target Projectile Target Projectile E ZDC (GeV)

9. 9 Data points have been normalized to the expected J/  normal nuclear absorption, calculated with as measured with p-A NA50 data J/  / DY vs. centrality  J/  abs = 4.18  0.35 mb Qualitative agreement with NA50 results plotted as a function of N part bin1   N part  = 63 bin2   N part  = 123 bin3   N part  = 175 B. Alessandro et al., Eur. Phys. J. C39(2005) 335 3 centrality bins Anomalous suppression present in Indium-Indium

10. 10 J/  E ZDC distribution: event samples good compatibility between Sets A and B in the following, we will show results obtained summing up the two event samples a (small) E ZDC bias due to nuclear fragment reinteractions is corrected for  2 /dof = 0.8 ratio Set B / Set A first bin peripheralcentral

11. 11 Comparison with nuclear absorption Data are compared to the expected J/  centrality distribution, calculated assuming nuclear absorption (with  abs =4.18 mb) as the only suppression source Nuclear absorption Normalization of the nuclear absorption curve we require the ratio measured/expected, integrated over centrality, to be equal to the same quantity for the J/  /DY analysis (0.87 ± 0.05)

12. 12 Measured / Expected vs. N part Departure from the expected normal nuclear absorption already in peripheral events Saturation in more central events ?

13. 13 Comparison with other SPS results The J/  suppression patterns are in fair agreement when plotted against N part

14. 14 Comparison with the extreme case of a step-like function N part Meas/Exp 1 Step position A1 A2 Step position: N part = 82 ± 9 A1= 0.98 ± 0.03 A2= 0.85 ± 0.01  2 /dof = 2.0 Resolution on N part estimate (due to the measured E ZDC resolution) taken into account A certain amount of physics smearing can be accommodated by the data

15. 15 Contrary to the J/  / DY analysis this new approach gives negligible statistical errors (< 2%) systematic errors must be carefully evaluated Systematic errors

16. 16 Study of systematic errors: event selection An alternative (less severe) event selection has been tested: require J/  to be produced in the target region, but not necessarily in the upstream vertex apply a Monte-Carlo correction for events where the J/  is produced by re-interaction of a nuclear fragment in the target region The suppression patterns obtained with the two different event selections agree within 1-2% Event selection well under control

17. 17 Study of systematic errors: inputs to Glauber model The expected J/  nuclear absorption, as a function of N part, is obtained by means of a Glauber calculation The choice of the inputs to the Glauber calculation may influence the shape of the calculated J/  centrality distribution Only for very central events, corresponding to our most central bin (E ZDC <3 TeV) an effect is clearly visible Try different nuclear density distribution functions with respect to the default parameterization (De Vries) De Vries et al. Atomic Data and Nuclear Data Tables 36, 495-536 (1987) Landolt – Bornstein Numerical Data…(Springer-Verlag 1967)

18. 18 E ZDC = 158 x N Spect + E sec Link E ZDC - centrality given by: Study of systematic errors: centrality assignment E sec  energy released in ZDC by forward (  secondary particles estimated by means of a fit to the E ZDC distribution of minimum bias events We study the influence on the nuclear absorption curve of a 10% systematic error on the evaluation of E sec Expected to affect mainly central events (higher relative contribution of secondaries) Sizeable effect for E ZDC < 3 TeV

19. 19 Dominant source of uncertainty is due to the rescaling from 450 to 158 GeV Study of systematic errors:  abs  J/  pp (450 GeV) and  abs J/   from NA50 (G. Borges et al., EPJ C43(2005)161 ) Rescale to 158 GeV (taking into account phase space factors) using NA50 and NA3/NA38 data from  s and x F parametrization of  J/  pp Combining the two errors we have a ~ 10% uncertainty, (almost) independent of centrality

20. 20 Summary on systematic errors Various sources of systematic errors have been investigated and their effect on the measured suppression pattern is the following: The most central bin is affected by a sizeable systematic error relatively to the others. There is also a ~10% systematic error, independent on centrality We can accurately evaluate the shape of the suppression pattern, but its absolute normalization is more uncertain Event selection  1-2% Input to Glauber model (In density distributions) Link E ZDC – N part Error on  J/  pp (450 GeV)  8% centrality independent Error on  abs  3-4 % (almost) centrality independent Error due to the J/  /DY normalization  ~ 6% centrality independent >10% for E ZDC < 3 TeV negligible elsewhere 5 -10 % for E ZDC < 3 TeV negligible elsewhere

21. 21 Work in progress: Azimuthal distribution of J/  More peripheral data  hint for a non isotropic emission pattern? Only 50% of the statistics analyzed central peripheral

22. 22 Conclusions NA60 has measured J/  production as a function of centrality in In-In collisions Updated results, obtained with much improved alignment/reconstruction The full statistics has been used (~ factor 2 with respect to Quark Matter 2005) Results show that the J/  is anomalously suppressed in In-In collisions, with a drop followed by a plateau… Accurate study of the systematic errors affecting the suppression pattern has been performed Next step: results on the J/  production in p-A collisions at 158 GeV  reduce systematic error on the normalization of the absorption curve

23. 23 The NA60 collaboration http://cern.ch/na60 Lisbon CERN Bern Torino Yerevan Cagliari Lyon Clermont Riken Stony Brook Palaiseau Heidelberg BNL ~ 60 people 13 institutes 8 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