 production in proton-nucleus and indium-indium collisions Michele Floris University and INFN, Cagliari, Italy. on behalf of the NA60 Collaboration ISMD 2005, Kromeriz, Czech Republic, August, 14 th Outline Motivation Apparatus Collected data Results in p-A Results in In-In Ongoing work for   KK

14/08/2005ISMD Motivation The study of  meson production in heavy ion collisions carries information about strangeness production Two channels have been studied:      Muons not influenced by the medium Previous SPS measurements: NA50 Acceptance limited to high p T     KK Better mass resolution No physical BG Previous SPS measurements: NA49 Broad p T coverage, but dominated by low p T The discrepancy between the values of the inverse slope T measured by these experiments led to the so-called  puzzle New measurements from NA60 in In-In collisions  NA60 measures the  channel with very good p T coverage  Has access to the   KK channel

17m The vertex region: The NA60 experiment Fixed target dimuon experiment at the CERN SPS Apparatus composed of 4 main detectors Zero degree calorimeter (centrality measurements) Muon Spectrometer Analysis strategy Create sample of matched muons Subtract combinatorial BG via event mixing Estimate fakes via overlay MC Fit experimental spectra with expected sources Hadronic cocktail (Genesis) IMR continuum (open charm and Drell-Yan or empirical)

14/08/2005ISMD Data Taking Two samples discussed in this talk  p-A Collisions (400 GeV protons) Six targets, three nuclei (Be, In, Pb) Microstrips silicon vertex detector 4 days in 2002, dimuons collected (at “low” beam intensity: 1–3 x 10 8 protons/burst) New high statistics sample collected in 2004, still to be analyzed  In-In collisions (158 GeV ions) Seven indium targets Silicon pixels vertex detector 5-week-long run in 2003, very good statistics (~ 230 million dimuon triggers on tape) ~ 50% statistics discussed in this talk

14/08/2005ISMD Detector performance Dimuon mass resolution at M ~ 1 GeV: In-In: ~ 23 MeV (independent of centrality) p-A: ~ 30 MeV Previous dimuon experiments: ~ 80 MeV p beam 400 GeV InPb 3 x Be Be Clear separation of all targets (Z vertex resolution ~ 600–900  m in p-A, better than 200  m in In-In) target box windows 7 In targets z-vertex (cm) Transverse vertexing with 20 µm accuracy Beam tracker station Indium beam 158 A GeV The acceptance of NA60 extends, all the way down to small M and p T A (%)

14/08/2005ISMD    BG+charm  production in proton-nucleus collisions Mass spectra described as a superposition of the low mass resonances decays into muons + charm + DY Fit to the mass spectra to extract the  ratio  p-Be  0.04 p-In  0.07 p-Pb  0.06

14/08/2005ISMD  production in indium-indium collisions 4 Centrality bins: N part estimated from a Glauber fit to the E ZDC spectrum Fit input: Hadron cocktail (genesis) Low-level (empirical) continuum source with exponential fall-off (to mimic continuum under the vector mesons) Parameters allowed to vary: , ,  and the continuum Arbitrarily normalize to the  Total sample: events after BG subtraction 50% of the full statistics S/B = 1/4 peripheral all p T Signal Cocktail

14/08/2005ISMD Peripheral Bin Peripheral bin studied in three p T bins  The normalizations of the hadron decay cocktail and of the continuum are independently fit in each p T bin   and  ratios are nearly p T independent  The peripheral bin is well described in terms of expected sources but: “Too many” low p T  mesons Peripheral In-In is not quite pp, it’s more like CC or OO Effect of pion annhilation cannot be neglected

14/08/2005ISMD  /  cross section ratio – Vacuum  Vacuum  contribution (  annihilation) important at low p T even in peripheral collisions Effect becomes dramatic in more central collisions  complicated continuum below the  However, the excellent mass resolution of NA60 allows us to extract a robust   yield Vacuum  Cocktail 

14/08/2005ISMD  /  cross section ratio – Centrality Dependence As a function of centrality: We restrict analysis to p T > 1 GeV Increase of a factor ~2 from peripheral to central collisions

14/08/2005ISMD  /  comparison to NA50  A direct comparison is impossible, due to the contribution from pion annihilation, which must be even higher in Pb-Pb collisions, and which NA50 cannot isolate NA50 points converted to the window p T >1.1 GeV/c assuming T=228 MeV      used (lower limit for NA50  ratio)

14/08/2005ISMD  /  comparison to NA49  /  Same trend as a function of N part  constant If we set the ratio  to 0.07–0.08, as suggested by statistical models, then the NA60   yield is a factor 1.5–2 higher than the NA49 value

14/08/2005ISMD  transverse momentum spectrum We select the events on the  peak and use two side mass windows to estimate the p T distribution of the continuum under the peak Then we correct for the acceptance, calculated (by Monte Carlo) as a 2-dim matrix: p T and y background total 

14/08/2005ISMD There is no significant variation of the extracted inverse slope parameter, T, with rapidity There is a clear increase from peripheral to central collisions With full statistics, extension up to p T > 3 GeV/c should be feasible  p T spectrum versus y and centrality

Comparison to NA49/NA50 Pb-Pb In-In Si-Si C-C pp The In-In measurement of NA60 follows the NA49 systematics, The disagreement between NA49 and NA50 is not due to the different decay channel Average NA60 value 1. All p T : 253 ± 2 MeV 2. NA50 range (m T > 1.65 GeV/c): 244 ± 5 MeV 3. NA49 range (p T < 1.5 GeV/c): 260 ± 5 MeV  Only small variations…

14/08/2005ISMD p T (GeV/c) MC   KK, Analysis strategy   KK can be studied using charged tracks reconstructed in vertex telescope Brute force method (no PID):  Assume all tracks are kaons  Make invariant mass from all track pairs Huge combinatorial BG  Subtracted by event mixing technique Kinematical cuts  Single tracks    avoid phase space boundaries p T and p  improve Signal/BG  Pair Opening angle  improve Signal/BG

14/08/2005ISMD Signal Very clean signal! MC   KK, Status Measured spectrum Combinatorial BG DATA Something yet to be tuned (BG?) Subtraction Detailed Monte Carlo (Venus) studies (full detector description) First attempts to get a signal out of the data MC Semi-peripheral In-In collisions

14/08/2005ISMD Summary NA60 is well suited to help understanding the “  puzzle”  New In-In measurements  Better p T coverage than previous  experiments  Capability to measure   KK  ratio:  Rise with N part consistent with NA49 and NA50  Absolute values between NA49 and NA50 Inverse slope T of the  p T -distribution:  Agreement between NA49 and NA60  The difference between NA49 and NA50 is not due to the different channels probed   KK  Full MC simulation shows the feasibility of the study  Final tuning still needed for background subtraction in real data

14/08/2005ISMD Lisbon CERN Bern Torino Yerevan Cagliari Lyon Clermont BNL Riken Stony Brook Palaiseau Heidelberg BNL 56 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. Damjanovic, 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 The NA60 Collaboration

14/08/2005ISMD BACKUP

14/08/2005ISMD Measuring Dimuons MWPC & trigger hodos MWPC & trigger hodos Toroidal magnet Iron Wall Hadron absorber Target Last trigger station Limiting Factor: Enegy loss Multiple Scattering Tracking before the hadron absorber Vertex Detector High multiplicity + High luminosity Rad-hard silicon pixels

14/08/2005ISMD hadron absorber and trackingmuon trigger magnetic field iron wall muon other targets Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them to muon measured in the spectrometer  Improved kinematics (~20 MeV/c 2 at  instead of 80 MeV/c 2 in NA50) Origin of muons can be accurately determined 2.5 T dipole magnet beam tracker vertex tracker Concept of NA60

14/08/2005ISMD Peripheral bin 0.5 < p T < 1.0 GeV/cp T > 1.0 GeV/cp T < 0.5 GeV/c Signal Cocktail

14/08/2005ISMD Clear increase of the extracted slope parameter T with multiplicity Purely statistical errors  p T spectra vs multiplicity

14/08/2005ISMD Fit of the  p T distribution in different p T ranges Differential” fits  Fix fit interval at  p T =0.8 GeV and move the extremes Dynamic range of 40 MeV in T eff (all centralities) Flat trend for peripheral collisions Indication for flow in In-In collisions

14/08/2005ISMD Agreement of data and mixed CB over several orders of magnitude Accuracy of agreement ~1% Combinatorial Background from ,K →  decays

14/08/2005ISMD A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with  2 < 3 are selected. Fake matches are subtracted by a mixed-events technique (CB) and an overlay MC method (only for signal pairs, see below) Matching between the muons in the Muon Spectrometer (MS) and the tracks in the Vertex Telescope (VT) is done using the weighted distance (  2 ) in slopes and inverse momenta. For each candidate a global fit through the MS and VT is performed, to improve kinematics. Muon track matching

14/08/2005ISMD Fake-match contribution localized in mass (and p T ) space   = 23 MeV  fake = 110 MeV Example of overlay MC: the 

14/08/2005ISMD Comparison of data to RW, BR and Vacuum  Vacuum  Cocktail 

14/08/2005ISMD Mass spectrum in semi-central In-In collisions Complicated continuum under the  in more central collisions However, the excellent mass resolution of NA60 allows us to extract a robust  yield

14/08/2005ISMD Predictions for In-In by Rapp et al (2003) for = 140, covering all scenarios Theoretical yields, folded with acceptance of NA60 and normalized to data in mass interval < 0.9 GeV Only broadening of  ( RW) observed, no mass shift (BR) Comparison of data to RW, BR and Vacuum 