1. NUCLEAR MODIFICATION FACTORS: HIGH P T π 0 @ SPS, ISR, RHIC Measure dielectrons @ mid-rapidity in UPC AuAu – 200 GeV from: (1) Coherent J/Ψ production:  +A  A*+J/Ψ  e + e - ) (2) Dielectron continuum at high m inv :  A*)+ e + e – to study: (i) gluon distribution function, G A (x,Q 2 ), in nuclei, (ii) Pomeron-exchange factorization, (iii) vector-meson dynamics in nuclear matter. LVL1 UPC TRIGGER AuAu RUN-4 PHYSICS GOAL Global cuts: |zvtx| < 30 cm, track multiplicity <15 Single-track cuts: N 0 ≥ 2 [# of RICH phototubes fired by e + e - ]. E 1 > 0.8 GeV || E 2 > 0.8 GeV [ERT threshold]. No dead-warn tower around assoc. EMCal cluster [CNT-EMC matching. e+e- candidates]. Pair cuts: arm 1 ≠ arm 2 [back-to-back di-electrons] Background subtraction: [unlike-sign] - [like-sign] Full GEANT MC for J/Ψ & high-mass e + e – continuum based on physics input from Starlight model [1-3] Ratio of A+A to p+p high p T data: [1] David d’Enterria Nevis Labs, Columbia University, NY References: [0] G.Baur et al. Phys. Rep. 364 (2002) 359; C.A. Bertulani, S.R. Klein, J. Nystrand, nucl-ex/05052005. [1] J. Nystrand, NPA 752 (2005) 470c. [2] A.J. Baltz, S.R. Klein, J. Nystrand, PRL 89 (2002) 012301. [3] S.R. Klein, J. Nystrand, PRC 60 (1999) 014903. [4] M. Strikman, M. Tverskoy and M. Zhalov, hep-ex/0505023 Coherent photoproduction of J/Ψ & high mass e + e - pairs in Ultra-Peripheral AuAu Collisions at √s NN = 200 GeV David d’Enterria for the Collaboration ULTRAPERIPHERAL COLLISIONS (UPC) High-energy ultra-peripheral (b > 2R A ~ 10 – 100 fm) AA collisions produce strong electromagnetic fields due to the coherent action of all nucleon charges. Such strong EM fields constitute a flux of "quasi-real” photons which can fluctuate into fermion pairs or vector-mesons: and then scatter elastically with the other coherent  or nucleus =- = + Photon-photon interactions Photon-pomeron interactions UPC A+A interaction ffff V=  J/ Ψ High rates expected @ RHIC due to: (i) Large Au ion charge:  AA = Z 1 2 Z 2 2 ·  NN ~ 4·10 5 ·  NN (ii) High √s : m inv ~2  CM /R ~ 6 GeV for  CM = 100 DATA ANALYSIS Sensitive to  +A  A*+J/Ψ  e + e - ): Veto on BBC (|y| ~3-4) [exclude periph. nuclear & beam-gas] Neutron(s) in at least one ZDC [from Au* Coulomb de-excitation] Large energy (>0.8 GeV) cluster in EMCal [e + e - decay from J/ψ] UPC: (ZDCN || ZDCS) && (!BBCLL1noVtx) && (ERT2x2) (ZDCLL1N || ZDCLL1S) && (BBCLL1S ^ BBCLL1N) && (ERT2x2) PRELIMINARY RESULTS Total data set: 1352 PRDFFs * 0.8 GB/file ~ 1.04 TB Total equivalent sampled luminosity: L int ≈ 120  b -1 Detectors: DC+PCs: Full charged tracking RICH & EMCal: e + e - identification Invariant mass distribution of unlike-sign and like-sign e + e - pairs: dN/dm ee (background subtracted) w/ fit to (MC) expected dielectron continuum and J/Ψ signals: J/Ψ invariant mass distribution (e + e - pairs minus dielec. continuum) Excellent agreement with J/Ψ peak position & width from full MC. p T distribution of e + e - pairs: peaked at very low p T  coherent production (p T < √2 ℏ c/R ~ 50 MeV) dσ J/ Ψ /dy| y=0 = 44 ± 16 (stat) ± 18 (syst) μb PHENIX preliminary