Centrality in the Muon Analysis Jane M. Burward-Hoy Physics Review July 17, 2003

Physics Motivation Measure J/  production as a function of event centrality in dAu collisions. Why? Parton densities within a single proton change when bound within a nucleus (“shadowing”) Observe a centrality dependence in yield of J/  in dAu relative to yield in pp Gluon shadowing model predictions (S.R. Klein and R. Vogt). dAu sensitive to gluon distributions in nucleus J/  “probes” this gluon distribution (g fusion) Ultimately understand all nuclear effects (including shadowing) as measured in cold nuclear matter (simpler systems such as dAu) Essential before we interpret the J/  particles produced in hot dense nuclear matter.

Theoretical predictions Gluon shadowing in dAu Spatial dependence predicted (S.B. Klein and R. Vogt). Measure J/psi yield in Peripheral and Central dAu events Baseline is J/psi yield in pp measurements. Central Peripheral Most energy transverse to beam axis Most energy along beam axis μ

Centrality in dAu Central events may be measured from the heavy-ion breakup High momentum protons at forward angles relative to the beam axis (“grey” protons) Low momentum protons (“black”) Peripheral events occur when either the n or the p in the d do not interact with the nucleus Quark Matter 2002 “Measuring Centrality in pA/dA Collisions at RHIC with Grey Protons” Stephen C. Johnson, Ron A. Solz, Gerd J. Kunde d Au

Number of Participant Collisions Model-dependent calculation called Glauber for Au nucleus Discrete nucleons for d Thickness of nuclear matter direct path of each oncoming nucleon Inelastic nucleon-nucleon cross section determine if collision occurs Au b Collision when b <   inel nn /   inel nn = 40 mb n2n2 n1n1 nini Woods-Saxon density profile distribution Reference: PHENIX Internal Analysis note Woods-Saxon Density Distribution d

Beam Axis Detectors: Initial Time, Vertex, and Centrality Collision axis Central = small impact parameter Peripheral = large impact parameter d Au Spectator nucleons Nucleons “participating” in the collisions Au Npart, d Npart, and Ncoll MVD BBC SouthBBC North p n Dx ZDC North ZDC South FCAL NorthFCAL South Dx

Centrality with the BBC in ZDC Hits in BBC South ~ number of Au participant nucleons ( Au Npart) Use a Glauber model to obtain distribution of Au Npart in d and Au and number of collisions (Ncoll) Fit measured hit distribution using negative binomial distribution Glauber model weighting Deduce BBC efficiency and centrality MVD BBC SouthBBC North p n Dx ZDC North ZDC South FCAL North FCAL South Dx

Number of Collisions and Participants (using BBC and ZDC) Centrality (%) d N part ± ± ± ± ± ± ± ± 0.03 In d-Au collisions, Au Npart ~ Ncoll d Npart for most peripheral bin using BBC and ZDC is 1.4 In order to measure centralities > 88% need to measure the p in FCAL.

BBC South FCAL South ZDC North FCAL North Centrality with the FCAL n p MVD BBC SouthBBC North p n Dx ZDC North ZDC South FCAL North FCAL South Dx ZDC South FCAL South Central Peripheral Tagging events Centrality

R. Vogt Centrality Dependence Estimated Expectations Estimated relative error for J/Ψ in 0-40% central events: 13% 40-88% peripheral: 17% Includes and N J/Ψ statistical uncertainties

What’s Next? Process entire data sample d+Au (p+p) and measure J/Ψ->µ+μ- channel using PHENIX Muon Arms. In addition to the minimum bias measurement, measure J/Ψ in different event centrality bins (0-40%, 40-80%, > 80%). Use the FCAL in combination with other beam axis detectors. It is important to note that the protons at forward angles measured in the FCAL enable one to select peripheral events >88% in d-Au events. Correct for muon tracking efficiency, pair acceptance, detector response, etc. Compare to theoretical predictions. Present first results to the APS/DNP meeting in October This measurement is unique to PHENIX (other experiments at RHIC cannot measure J/Ψ centrality dependence!).