J/ ψ production in Indium collisions

1. J/ψ productionin Indium collisions Projectile J/ψ L Target all events after rejecting beam pile-up& non-interacting beam ions after muon quality cuts & in dimuon phase space window Background J/y y’ DY Charm Physics motivation What was previously observed at the SPS? According to lattice QCD calculations, above a critical temperature or energy density, strongly interacting matter undergoes a phase transition to a new state where thequarks and gluons are no longer confinedin hadrons. Since 1986, several SPS experiments searched for such a phase transition in high-energy nuclear collisions. Among the several “signatures” suggested by theory, the suppression of strongly bound heavy quarkonia statesis particularly important because it is directly related to the critical behavior associated with the deconfinement phase transition. NA38 and NA50 extensively studied the J/ψsuppression pattern in p-A, S-Uand Pb-Pb collisions, identifying the J/ψ through the muon pair decay We need to measure the J/ψproduction pattern in a nuclear collision system other than Pb-Pb in order to understand which is the variable driving the charmonium suppression: L, Npart, local energy density, etc. The J/ψ suppression patternseen in p-A and S-U datacan be understood in terms of“normal nuclear absorption” Results with Indium-Indium collisions are being studied from the data that NA60 collected in 2003 NA60 To understand the heavy-ion results we need a solid reference baseline from p-A data, with respect to which we can identify behaviors specific to the nuclear data. To improve our basic understanding of J/ψ production, NA60 will study the impact of the c feed-down on the observed J/ψ production pattern, in p-A collisions In central Pb-Pb collisions the J/ψ productionis suppressed with respect to the yield expected from the proton-nucleus and S-U data. Such p-A results will be obtained from the data that NA60 will collect in 2004 NA60 Event selection and centrality determination Minimum bias ZDC trigger Indium peak • Severe quality cuts have been used in this preliminary analysis (statistics will increase in the future, mainly for peripheral collisions) • Beam pile-up is rejected using Beam Tracker timing information • Non-interacting beam ions are rejected using the Interaction Counter • Dimuon data analysis performed for events with EZDC < 15 TeV • and in the phase space window: 0 < ycms < 1 ; |cos CS| < 0.5 The J/ψ suppression must be studied as a function of the centrality of the collision, which can be estimated by means of: In order to compare the Indium-Indium results with the previous data we can use L or number of participants, for instance • the energy released in the ZDC (EZDC) or • the multiplicity of charged particles detected with the pixel telescope • L is the average length of nuclear matter traversed bythe charmonium state after its production • Number of nucleons participating in the collision all events after rejecting beam pile-up & non-interacting Indium ions central collision peripheral collision Dimuon trigger central collision Using, for example, a Glauber fit to the EZDC minimum bias spectrum it is possible to extract the correlation between EZDC and centrality estimators, as the number of nucleons participating in the collision Correlation between L and number of participant nucleons for different collision systems peripheral collision ψ / DY results in Indium-Indium collisions at 158 GeV The J/ψ and DY yields are extracted from a fit to the dimuon mass spectrum Dividing the J/Y and DY yields, after acceptance corrections, we obtain B s(J/y) / s(DY) = 19.5 ± 1.6 (integrating all the events with EZDC in the range 0-15 TeV) In-In collisions of EZDC < 15 TeVL = 7.0 fm and NPart=133 (from Glauber fit to the minimum bias EZDC distribution) The dimuon mass region above 2 GeV is composed of the J/ψ and ψ’ resonances on the top of a continuum made of Drell-Yan dimuons and semi-leptonic decays of D mesons, besides the combinatorial background, • Dimuon data from the 6500 A event sample • No muon track matching used in this preliminary analysis • Mass resolution at the J/ψ : ~107 MeV • Combinatorial background from π and K decays estimated from the measured like-sign muon pairs • Signal mass shapes from Monte Carlo:PYTHIA and MRS A (Low Q2) parton densities GEANT 3.21 for detector simulation same reconstruction algorithm as for the raw data • Acceptances from Monte Carlo simulation: •  for J/ψ : 12.4 % •  for DY : 13.4 % (in mass window 2.9–4.5 GeV) Ratio between measurement and normal nuclear absorption: 0.87 ± 0.07 A multi-step fit (max likelihood) is performed: a)M > 4.2 GeV : normalise the DY Preliminary DY yield = 162± 131302 ± 104 in range 2.9–4.5 GeV b) 2.2<M<2.5 GeV: normalise the charm (with DY fixed) c) 2.9<M<4.2 GeV: get the J/ψ yield (with DY & charm fixed) J/ψ yield = 23532 ± 298 These are the ranges of L and number of participants that NA60 will be able to study, once the data analysis is more advanced