Highlights from NA60 experiment Study of dimuon production in p-A and A-A collisions at CERN SPS Alessandro Ferretti (University of Torino and INFN) for the NA60 collaboration NA60 concept In-medium modification of vector mesons Intermediate mass range excess: prompt or charm? J/ψ production in p-A and A-A

Layout of the NA60 experiment Muon trigger and tracking: 2.5 T dipole magnet NA10/38/50 spectrometer beam tracker vertex tracker magnetic field Iron wall hadron absorber targets ZDC Matching in coordinate and momentum space! Muon Other NA60 concept: place a radiation-hard silicon tracking telescope in the vertex region to measure the muon tracks before they suffer multiple scattering in the absorber and match them to the tracks measured in the muon spectrometer. Origin of muons can be accurately determined Mass resolution @ :~20 MeV/c2 (vs. 80 MeV/c2) Mass resolution @ J/:~70 MeV/c2 (vs. 105 MeV/c2) first of all, let’s give a look to the NA60 experimental setup. NA60 INHERITED THE na50 muon spectrometer, composed of tracking chambers, trigger hodoscopes, a solenoidal magnet and a zdc to measure the spectator nucleons energy which is used to determine the centrality of the collision. in addition to this, we or ! prompt displaced

Search for in-medium modifications of vector mesons Low dimuon masses – In-In data Search for in-medium modifications of vector mesons Peripheral data: well reproduced by the hadronic cocktail Central data: isolate the excess by subtracting the cocktail Phys. Rev. Lett. 96 (2006) 162302   f  and : fix yields such as to get, after subtraction, a smooth underlying continuum : = set upper limit, defined by saturating the measured yield in the mass region close to 0.2 GeV (lower limit for excess). = use yield measured for pT > 1.4 GeV/c (where the excess is small) the first physics topic I will show you is the study of the dimuon low mass spectrum. In this range, CERES reported an excess w.r.t. pA data and peripheral collisions: this excess could be due to rho broadening or mass shift from chiral symmetry restoration. Because of insufficient mass resolution CERES was not able to individuate the source of the excess, but thanks to its vertex tracker, NA60 can measure the dimuon yields with unprecedented mass resolution: the hadronic states are clearly resolved NA 60 peripheral data are well … : to isolate the excess present in central bins we subtracted the hadronic cocktail obtained in peripheral data.

Excess spectra from difference Fine analysis in 12 centrality bins data – cocktail (all pT) No cocktail r and no DD subtracted Clear excess above the cocktail , centered at the nominal r pole and rising with centrality Excess even more pronounced at low pT these are the results in 12 centrality bins. Cocktail rho was not subtracted because we expect some modification of its yield, so the rho is inside the excess: cocktail rho is shown for comparison cocktail / =1.2

Evolution of the excess shape as a function of centrality Quantify the peak and the broad symmetric continuum with a mass interval C around the peak (0.64 <M<0.84 GeV) and two equal bins L, U on either side “continuum” = 3/2(L+U) “peak” = C-1/2(L+U) Fine analysis in 12 centrality bins continuum/r Peak/cocktail r drops by a factor 2 from peripheral to central: the peak is not simply the cocktail r residing over a broad continuum. peak/r nontrivial changes of all three variables at dNch/dy>100? peak/continuum

Comparison to theory Predictions for In-In by Rapp et al. (2003) for <dNch/d> = 133, covering all scenarios Data and predictions as shown, after acceptance filtering, roughly mirror the respective spectral functions, averaged over space-time and momenta Theoretical yields normalized to data in mass interval < 0.9 GeV/c2 Only broadening of  (RW) No mass shift (BR) observed

Hadron-parton duality? Description of the mass region above 1 GeV Rapp / Hees, hep-ph/0604269 (2006) Ruppert / Renk, Phys.Rev.C (2005) Mass region above 1 GeV described in terms of hadronic processes, 4 p …, sensitive to vector-axialvector mixing and therefore to chiral symmetry restoration Mass region above 1 GeV described in terms of partonic processes, dominated by q-qbar annihilation Hadron-parton duality?

First look at transverse momentum distributions Differential fits with gliding windows of pT=0.8 GeV  local slope Teff At high pT, -like region hardest, high mass region softest! Not yet explained by theory Weak centrality dependence Trend at small mT different to what expected from radial flow (not for f!) High mass interval shows steepest slope  smaller T slope the transverse mass spectra show a weak centality dependence. Concerning the slope, the trend...

Conclusions I (low dimuon masses) Mass spectra Pion annihilation seems to be a major contributor to the lepton pair excess at SPS energies Strong broadening, but no significant mass shift of the r pT spectra Strong mass dependence of pT spectra Spectra behave opposite to expected from radial flow pT spectra could serve as a handle to disentangle partonic from hadronic sources (breaking parton-hadron duality)

Intermediate dimuon masses (IMR – 1.1<mmm<2.5 GeV) Observed IMR excess in In-In over expected Charm and Drell-Yan yields. Where it comes from? NA60 measures the muon offsets Dm: distance between interaction vertex and track impact point Dimuon offset Fix prompt contribution to the expected DY– leave open charm free Leave DY free and fix open charm according to expectations An excess in the imr was measured by NA50: it could be explained either by thermal radiation from the medium produced in the collision or by an enhancement of charm production. To discriminate between these two sources it is necessary to measure the offset of the muon tracks w.r.t. interaction vertex: NA60 can do it! Good Fit Bad Fit The excess is a prompt source ~2 times higher than the expected DY yield

Centrality dependence of the excess Slight increase as a function of number of participants with respect to Drell-Yan All data preliminary Corrected for acceptance

Mass spectrum (1.16<M<2.56 GeV/c2) Contributions to IMR corrected for the acceptance in -0.5 < cos  < 0.5 2.92 < ylab < 3.92 (both 4000 and 6500 A data sample used) preliminary Excess is not drell-yan

pT dependence of the excess (1.16<M<2.56 GeV/c2) No acceptance correction 6500 A High pT tail strongly depends on the correctness of Drell-Yan description by Pythia  Teff fits are performed in 0< pT <2.5 GeV/c preliminary Corrected for acceptance Corrected for acceptance

Towards a “unification” of low and intermediate dimuon mass regions: evolution of excess Teff vs. Mmm preliminary

Conclusions II (intermediate mass range) Prompt dimuons production is ~2 times higher than the expected Drell-Yan in Indium-Indium collisions at 1.16 < m < 2.56 GeV/c2. Charm production is compatible with expectations. Prompts/Drell-Yan slightly increases with number of participants. Excess contribution is dominated by low pT’s, reaching a factor 3.50.4 for pT<0.5 GeV/c. The effective temperature of the excess (~190 MeV) is considerably lower than the temperatures observed at lower masses (both for the resonances and the low-mass excess) Results are preliminary, since they depend on the correctness of used Drell-Yan and Charm contribution pT distributions. Need to be verified with pA data

Direct J/ sample – In-In data The measured J/ vs. EZDC distribution is compared to the distribution expected in case of pure nuclear absorption. An anomalous suppression of the j/psi yield w.r.t the normal nuclear absorption has been measured by NA50 in lead-lead collisions: to understand which physics mechanism drive this suppression, NA60 took data with a different colliding system, namely In-In. Here you can see the results of na60 Normalization of the nuclear absorption curve: the ratio measured/expected, integrated over centrality, is fixed to the one resulting from J/ψ/DY analysis (0.87 ± 0.05).

(J/)/DY in p-A collisions at 158 GeV L = 3.4 fm Preliminary! To claim that an anomalous suppression of the j psi as been observed, j/psi production in p-A collision must be accurately known. Up to now,this knowledge came from the pA collisions with a proton energy of 400 GeV, and to allow a comparison we had to rescale the cross section for elementary collisions to the rather different energy an kinematical domain of the 158 GeV collisions. Preliminary NA60 result shows that the rescaling of the J/ production cross section from 450(400) GeV to 158 GeV is correct! Next step: obtain absJ/ at 158 GeV

J/ polarization Quarkonium polarization  test of production models Color Singlet Model: transverse polarization Color Evaporation Model: no polarization Non-Relativistic QCD: transverse polarization at high pT Deconfinement should lead to a higher degree of polarization (Ioffe,Kharzeev PRC 68(2003) 094013) CS = -0.03  0.17 2/ndf =1.42 0 < pT < 5 GeV 0.4 < yCM < 0.75 H = 0.03  0.06 2/ndf =1.01 0.5 < pT < 5 GeV 0.1 < yCM < 0.6

pT2 vs centrality If pT broadening is due to gluon scattering in the initial state  pT2 = pT2pp + gN · L NA60 In-In points are in fair agreement with Pb-Pb results We get gNInIn = 0.067  0.011 (GeV/c)2/fm pT2ppInIn = 1.15  0.07 (GeV/c)2 2/ndf = 0.62 to be compared with gNPbPb = 0.073  0.005 (GeV/c)2/fm pT2ppPbPb = 1.19  0.04 (GeV/c)2 2/ndf = 1.22 gNPbPb96 = 0.081  0.003 (GeV/c)2/fm pT2ppPbPb96 = 1.10  0.03 (GeV/c)2 2/ndf = 1.38 (NA50 2000 event sample) pT broadening consistent with initial state gluon scattering

’ suppression in In-In and pA collisions Study limited by statistics in In-In (N’ ~ 300) Normalized to Drell-Yan yields In-In: Preliminary! Most peripheral point (Npart ~ 60) does not show an anomalous suppression Good agreement with Pb-Pb results pA: Also the ’ value measured by NA60 at 158 GeV is in good agreement with the normal absorption pattern, calculated from 450 (400) GeV data

Conclusions III (J/ψ suppression) NA60 has performed a high-quality study of J/ production in In-In collisions at the SPS which confirms, for a much lighter system, the anomalous suppression seen in Pb-Pb collisions by NA50 Preliminary results from p-A collisions at 158 GeV show that the normalization of the absorption curve is correct Peripheral In-In and Pb-Pb results are compatible with p-A Absence of J/ polarization in the kinematical window probed by NA60 pT distributions sensitive to initial state effects Study of J/ suppression for other collision systems, with the accuracy allowed by a vertex spectrometer, would be very interesting

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. Grigoryan, J.Y. Grossiord, N. Guettet, A. Guichard, H. Gulkanyan, 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, T. Poghosyan, 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

spare slides

Set A (lower ACM current) Set B (higher ACM current) J/ / DY analysis Set A (lower ACM current) Set B (higher 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) Tuning of quality cut for muon spectrometer tracks  < 3%

Smooth effect or sharp drop ? Npart Meas/Exp 1 Step position A1 A2 Step position: Npart = 86 ± 8 ( Bj ~ 1.6 GeV/fm3 ) A1= 0.98 ± 0.02 A2= 0.84 ± 0.01 2/dof = 0.7 Taking into account the EZDC resolution data are compatible with a sharp drop An onset smoother than our resolution on Npart (~20) is disfavored Work in progress to extend our Npart range towards more peripheral events

T vs centrality Fitting functions Used by NA50 Gives slightly higher T values (~ 7 MeV) 1) dN/dpT = pT mT K1(mT/T) 2) dN/dpT = pT e -mT/T

Comparison with recent results (HERA-B, E866) Helicity Helicity Collins Soper HERA-B, in p-A collisions at 920 GeV, sees (mostly in the Collins-Soper reference system) a significant longitudinal polarization at low pT (P. Faccioli et al., Hard Probes 2006) No polarization in NA60, which covers a higher xF region E866, at still larger xF, sees a (slight) transverse polarization (T.H. Chang et al., PRL 91(2003), 211801)

Azimuthal distribution of the J/ central peripheral More peripheral data  hint for a non isotropic emission pattern? Only 50% of the statistics analyzed

Event selection 2 event selections have been used for J/ analysis 1) No matching required Extrapolation of muon tracks must lie in the target region Higher statistics Poor vertex resolution (~1 cm) 2) Matching between muon tracks and vertex spectrometer tracks Dimuon vertex in the most upstream interaction vertex (MC correction to account for centrality bias due to fragment reinteraction) Better control of systematics Good vertex resolution (~200 m) Lose 40% of the statistics After quality cuts  NJ/ ~ 45000 (1), 29000 (2) 2 analyses a) Use selection 1 and normalize to Drell-Yan b) Use selection 2 and normalize to calculated J/ nuclear absorption

J/ kinematical distributions Study of differential distributions important in order to assess Role of initial state effects  pT distributions Production mechanisms and/or deconfinement  polarization Technique 3-D acceptance correction (pT, y, cos) Fine binning (0.1 GeV/c pT, 0.05 y-units, 0.1 cos-units) Define fiducial region (zone with local acceptance >1%) -0.1<cosH<0 pproj ptarg z CS + y x Viewed from J/ rest frame H J/ Collins Soper Frames for polarization studies Helicity

Low mass dimuons Net data sample: 360 000 events Fakes / CB < 10 % ω and  peaks clearly visible in dilepton channel; even μμ seen Mass resolution: 23 MeV at the  position Progress over CERES: statistics: factor >1000 resolution: factor 2-3 ω  

Role of baryons Baryons important in the low mass tail Calculations for In-In by Rapp et al. (11/2005) for <dNch/d> = 140 Improved model: Fireball dynamics 4  processes spectrum described in absolute terms Baryons important in the low mass tail

pT spectra - acceptance correction Reduce 3-dimensional acceptance correction in M-pT-y to a 2-dimensional correction in M-pT, using measured y distribution as an input. Use  for control Use slices of m = 0.1 GeV pT = 0.2 GeV Check behaviour on 3 extended mass windows 0.4<M<0.6 GeV 0.6<M<0.9 GeV 1.0<M<1.4 GeV Subtract charm from the data (based on NA60 IMR results) before acceptance correction

Systematics of low-pT data: combinatorial background Enhanced yield at low-pT seen at all centralities, including the peripheral bin Errors at low pT, due to subtraction of combinatorial background: peripheral 1% semiperipheral 10% semicentral 20% central 25% Enhanced yield at low pT not due to incorrect subtraction of combinatorial background

Excess pT spectra: 3 centrality bins Hardly any centrality dependence BUT Significant mass dependence