J/ µ + µ – measurement in Cu+Cu collision at √s = 200 GeV

1. 173 Tb initially produced diagonal cc pair 50 Tb formation using allcc pairs which can recombine in the medium New ! Level2 filtering: MuTr + MuID Reconstruction 2.5 Tb 500 Mb Physics motivations in the J/ yield measurement Volume of data –1.95 < y < –1.70 Due to cold nuclear effects Predicted by recombination model • The J/ is a promising hard probe to study the hot and dense matter created in relativistic heavy ion collisions: • large charm quark mass  J/ produced in the early stages of the collision only • hard enough to resolve sub-hadronic scales • Most theoretical models indicate that producing a quark-gluon plasma will result in changes to the J/ yield: • its dissociation requires harder gluons than those present in hadrons • possible only in a deconfined medium • J/ will be suppressed by color screening in a deconfined medium at ~1.2 Tc(Matsui and Satz, Phys. Lett. B178, 416) • coalescence/recombination models rather predict an enhancement of the J/ yield at RHIC energy (Grandchamp, Rapp and Brown, Phys.Rev.Lett. 92:212301 (2004), Thews, Schroedter, Rafelski, Phys. Rev. C63, 054905 (2001)) • NA50 (CERN) experiment: anomalous J/ suppression in Pb+Pb collisions, in excess of the normal suppression expected from the nuclear absorption (NA50 Collaboration, Phys. Lett. B477 (2000) 28) • To have a proper baseline for charmonium suppression in AA collisions, one need to carefully understand/predict cold nuclear matter effects in charmonium production : • absorption (final-state effect) • where the formed cc pre-resonance pair is broken by interactions with primary target/projectile nucleons • shadowing (initial-state effect) • where the free nucleon structure function is modified by the nuclear environnement, and hence affects heavy quark production • theoretical predictions on RAA vs rapidity by R. Vogt (nucl-th/0507027): If large number of c and c are produced in nucleus collision, then the probability of c and c incoherent recombination will no longer be negligible (off-diagonal recombination in a deconfined medium). This could be the case at RHIC energy. J/ formed by recombination will exhibit a narrower rapidity spectra. Since recombination process is larger for central collisions, the width of the rapidity distribution of all J/ should decrease with increasing centrality: Thews (nucl-th/0505055): PHENIX Muon Arms Predicted rapidity spectra of J/ in Au+Au at 200 GeV (pQCD calculation). Diagonal pair = cc pair coming from the same hard collision. Muon Arms: 0 <  < 2  |p| > 2 GeV The AA/pp ratio with the EKS98 parametrization as a function of y for octet absorption for Cu+Cu at 200 GeV. The curves are σabs = 0 (solid), 1 (dashed), 3 (dot-dashed) and 5~mb (dotted). 35° MuID:muon identification with penetration depth/momentum matching. MuTr: tracking and momentum measurement with cathode strip chambers 35° 12° MuTr 10° BBC BBC : particle production measurement in 3.0 < |η| < 3.9. Luminosity monitor + vertex measurement. Minimum Bias trigger: BBC MuID • fast online reconstruction of 1D road in MuID • di-road accepted if last MuID gap reached (with a minimum number of hit gaps) • J/ measurement is done via the decay channel: J/  µ+µ– • data presented here = forward (backward) rapidities only Level1 trigger: MuID Deep-Deep Cu+Cu 200 GeV data taking: triggers and level2 filtering North Muon arm : 1.2 < y <2.4 South Muon arm : -2.2 < y <-1.2 • reconstruction of 2D road in MuID used as seed for MuTr track finding • rough track momentum estimate  invariant mass cut for tracks pairs at 2 GeV/c² J/ yield per centrality slice vs rapidity Signal extraction Di-muon invariant mass distribution • Combinatoric background from uncorrelated di-muon: • Nbgd = 2√(N++. N– –) • Signal = number of counts within J/ invariant mass region 2.6 – 3.6 GeV/c² after substracting Nbgd to the distribution of opposite sign dimuons • Systematic errors: ~10% from varying fits of background subtracted signal. Also account for physical background that can be included into previous counting. • Cuts: • Event cuts • |Z-vertex| < 30 cm • Pair cuts • 2.6 < mass < 3.6 GeV/c² • 1.2 < |rapidity| < 2.2 • Level2-like cut • Track cuts • pz < 0 (>0) to match the studied arm • determined using Monte Carlo J/ embedded in real data: • cut on pair approach to BBC vertex • cut on track quality • cuts on the matching between MuID road and MuTr track for rapidity bin i, centrality bin j Recombination model predicts a narrowing of the distribution with centrality (Thews, nucl-th/0505055). Data indicate no significant change in the shape with increasing centrality. But, this model takes into account recombination only, and no other effect like cold nuclear matter effect. Nuclear modification factor RAA vs rapidity per centrality slice RAA at forward |y| are computed using reference dimuon p+p data fitted with a Gaussian. This fit serves as an interpolation to the available p+p points in the dimuon channel. Substantial uncertainty due to chosen fit 20% added in quadrature to systematics. Close-up: RAA at |y| =2 vs centrality Getting acceptance*efficiency correction factors RAA • Using Monte Carlo J/ generated by PYTHIA over 4π • embed the J/ within muon arm acceptance into real minimum bias Cu+Cu data • use a realistic detector response • apply to them the same triggers and signal extraction method as the ones applied to the data • Acc*eff(i) is the probability for a J/ thrown by PYTHIA in a given bin i to survive the whole process followed by the data The study of the nuclear modification factor as a function of the rapidity exhibits a similar pattern between the different centrality bins. A comparison of the data with thecurves from cold nuclear matter predictions (Vogt, nucl-th/0507027) shows a possible agreement with the general trend of these curves within our large PHENIX Preliminary errors (shadowing + σabs = 1mb dot curve, shadowing + σabs = 3 mb dashed curve). Going from the most peripheral to the most central bin, a suppression (increasing with centrality) of the J/ production is observed. This suppression is indeed clearly seen on the centrality dependence of the J/ yield per binary collision (see the poster by D. Silvermyr for more information): Data is compared to predictions at y = 2 from cold nuclear matter effects (Vogt, nucl-th/0507027, σabs = 3 mb and EKS98 parametrization at y=2, showing shadowing dependance vs centrality). The most central point seems to be underpredicted: is there any additionnal suppression mechanism? RCP vs |y| per centrality slice Systematic of Glauber calculation for peripheral is shown as a band. Acc*eff vs rapidity for the range 1.2 < |y| < 2.2 and integrated over all centralities (statistical error only). • Systematic errors: • 5% from track/pair cuts and uncertainities in pT, y and z-vertex input distribution Acc*eff vs rapidity (4 bins) and for 4 slices in centrality (0-20%, 20-40%, 40-60%60-94%) for North (top) and South (bottom) muon arms (statistical error only). • Rcp useful because: • removes uncertainities due to p+p reference • We can see: • most central (0-20%) Rcp distribution significantly below the peripheral (40-60%) Rcp distribution • 8% from run to run variation (mainly due to the varying number of dead FEM in MuTr). J/µ+µ– measurement in Cu+Cu collision at √s = 200 GeV Rapidity dependence Andry Rakotozafindrabe – LLR For the PHENIX collaboration Quark Matter 05 Budapest, August 2005 Theoretical expectations: rapidity dependence of the nuclear modification RAA • Summary: • Baseline cold nuclear matter effects seems to describe RAA dependence with rapidity for the centrality slices. But most central bin looks underpredicted: hint of additionnal suppression mechanism? • J/ yield distribution with rapidity does not seem to get narrower with increasing centrality: this is expected in model where only recombination is taken into account.