aa r X i v : . [ nu c l - e x ] S e p Ultra-peripheral Au+Au collisions at PHENIX
M´at´e Csan´ad for the PHENIX Collaboration1 - E¨otv¨os University - Department of Atomic PhysicsP´azm´any P´eter s. 1/a, Budapest, H-1117 HungaryUltra-peripheral collisions (UPC) of heavy-ions involve long range electromagnetic in-teractions at impact parameters twice larger than the nuclear radius, where no nucleon-nucleon collisions occur. The first measurement of photoproduction of J/ψ and oftwo-photon production of high-mass e + e − pairs in ultra-peripheral nucleus-nucleus in-teractions will be presented, using Au+Au data at √ s NN = 200 GeV. The measuredcross sections at midrapidity are consistent with various theoretical predictions. [1] The study of photoproduction at hadron colliders has attracted an increased interest inrecent years [2, 3, 4]. Electromagnetic interactions can be studied without background fromhadronic processes in ultra-peripheral collisions without nuclear overlap. This study focuseson the measurement of exclusively produced high-mass e + e − –pairs in Au+Au collisions at √ s NN = 200 GeV, Au + Au → Au + Au + e + e − at midrapidity. The results [5] have beenobtained with the PHENIX detector [6] at the BNL Relativistic Heavy Ion Collider (RHIC).The electromagnetic field of a relativistic particle can be represented by a spectrum ofequivalent photons. This is the Weizsacker-Williams approximation. The virtualities of theequivalent photons when the field couples coherently to the entire nucleus are restricted bythe nuclear form factor to Q = (cid:0) ω /γ + q ⊥ (cid:1) . ~ /R A , where γ is the Lorentz factor of thebeam and R A is the nuclear radius. At RHIC energies, γ = 108 and the maximum photonenergy in the center-of-mass (lab) system is ω max ≈ W maxγN ≈
34 GeV.The exclusive production of an e + e − –pair can proceed either through a purely electro-magnetic process or through coherent photonuclear production of a vector meson, whichdecays into a dilepton pair. The Feynman diagrams for the two leading order processes areshown in Fig. 1.The production cross section of vector mesons is a good probe of the nuclear gluondistribution, G A ( x, Q ), as well as of vector-meson dynamics in nuclear matter [7, 8, 9]. For J/ψ -production, PHENIX acceptance at y = 0 corresponds to a mean photon energy of h E γ i = 300 GeV and nuclear Bjorken- x values of x = m J/ψ /W γA ≈ . · − .Measurements of coherent photonuclear production of the ρ meson [10], as well as γ γ production of low-mass e ± pairs [11] have been performed by the STAR collaboration. ThePHENIX analysis presented in Refs. [12, 5] and summarized in this paper is the first onheavy final states in ultra-peripheral nucleus-nucleus collisions. The cross section for J/ψ and e + e − photoproduction are compared with model calculations [9, 13, 14, 15, 16]. The data presented here were collected with the PHENIX detector at RHIC during the2004 Au+Au run at √ s NN = 200 GeV (Run-4). The PHENIX detector is equipped with DIS 2009 γ γγ ...... + e _ e γ J/ γ Au AuAu* Au*Au AuAu Aua) b)
Figure 1: Lowest order Feynman diagrams for exclusive photoproduction of
J/ψ (left) anddielectrons (right) in ultra-peripheral Au+Au collisions. The photons to the right of thedashed line are soft photons that may excite the nuclei but do not lead to particle production.multi-layer drift chambers (DC) followed by multi-wire proportional chambers (PC) withpixel-pad readout. The tracking arms also contain Ring-Imaging- ˇCerenkov (RICH) detectorsand electromagnetic calorimeters (EMCal) for electron and positron identification.The events used in this analysis were collected by a level-1 Ultra-Peripheral Collision(UPC) trigger set up for the first time in PHENIX in Run-4 as follows. A veto on coincidentsignals in both Beam-Beam Counters (BBC) selected exclusive-type events characterised bya large rapidity gap on either side of the central arm. The EMCal and the RICH decetorwere used to form a trigger (ERT) to select events with at least one of the two high-energy e ± coming from the e + e − pair. Finally at least 30 GeV energy deposited in one or both ofthe ZDCs was required to select events with forward neutron emission.The total number of events collected by the UPC trigger was 8.5 M, of which 6.7 Msatisfied standard quality assurance criteria. The useable event sample corresponds to anintegrated luminosity L int = 141 ± µ b − .The following cuts were applied to enhance the sample of genuine γ -induced events:1. A standard offline vertex cut | vtx z | <
30 cm was required2. Only events with exactly two charged particles were analyzed. This cut allows tosuppress most of non photoproduction contamination in the UPC trigger.3. A RICH cut selects e ± which fire 2 or more tubes separated by the nominal ring radius.4. Good Track–EMCal matching is also required.5. An EMCal energy cut ( E > || E > e ± above the ERT trigger threshold.6. Events with back-to-back e + e − candidates (detected in opposite arms) were selected. DIS 2009 (GeV/c ee m ( c oun t s ) ee d N / d m (unlike-sign pairs) - e + e Ψ J/ coherent continuum - e + e continuum - e + max/min e a) ) (GeV/c ee m ( c oun t s ) ee d N / d m -2-1012345678 subtracted) γ γ ( - e + e b) Figure 2: Left: (a) Invariant mass distribution of e + e − pairs fitted to the combinationof (shaded) a dielectron continuum [exponential distribution] and (cross hatched) a J/ψ [Gaussian] signal. The two additional dashed curves indicate the maximum and minimumcontinuum contributions considered in this analysis (see text). (b)
J/ψ invariant massdistribution after subtracting the fitted dielectron continuum signal in (a).
After the above cuts we find 28 events with e + e − pairs and none with like-sign pairs for m e ± e ± > . The measured e + e − invariant mass distribution for the sample is shownin Fig. 2 a). This distribution is fitted with a continuum (exponential) curve combined witha Gaussian function at the J/ψ peak, as shown by the solid curve in Fig. 2 a). Simulationsbased on events generated by the starlight
Monte Carlo [13, 17, 18] and processed through geant3 [19] have shown that the shape of the measured continuum contribution is welldescribed by an exponential function dN/dm e + e − = A · e c m e + e − . Those simulations allowus to fix the exponential slope parameter to c = − . ± . − c . The combined datafit is done with three free parameters: the exponential normalisation ( A ), the J/ψ yieldand the
J/ψ peak width (the Gaussian peak position has been fixed at the known
J/ψ mass of m J/ψ = 3.097 GeV/c [20]). The J/ψ and continuum yields and the correspondingstatistical errors are calculated from the fit and summarized in Table 1. Fig. 2 b) shows theresulting invariant mass distribution obtained by subtracting the fitted exponential curve ofthe dielectron continuum from the total experimental e + e − pairs distribution.Physical cross-sections were obtained after correcting the raw number of signal counts forthe geometrical acceptance of our detector system, and the efficiency losses introduced bythe previously described analysis cuts. Acceptance and efficiency corrections were obtainedwith a full Monte Carlo of the experimental apparatus with realistic starlight MonteCarlo. Such a model reproduces well the existing d N/dydφdp T distribution of coherent ρ production in UPC Au+Au events measured at RHIC by STAR [10]. The coherentevents were simulated in the PHENIX detector using geant3 and passed through the samereconstruction program as the real data. DIS 2009 e + e − [GeV/c ] Yield Cross-section [ µ b/(GeV/c )] J/ψ peak 9 . ± . ± . ±
31 (st) ±
15 (sy) [ µ b] e + e − cont. [2.0,2.8] 13 . ± . ± . ±
23 (st) ±
16 (sy) [ starl. : 90] e + e − cont. [2.0,2.3] 7 . ± . ± . ±
47 (st) ±
28 (sy) [ starl. : 138] e + e − cont. [2.3,2.8] 6 . ± . ± . ±
24 (st) ±
14 (sy) [ starl. : 61]Table 1: Measured
J/ψ and e + e − continuum photoproduction yields and cross-sectionsat midrapidity in ultra-peripheral Au+Au collisions (accompanied with forward neutronemission) at √ s NN = 200 GeV (obtained from the fit of the data to an exponential plusGaussian function) per invariant mass range are shown in the second column. For thecontinuum cross-sections starlight predictions are taken from Ref. [18].For J/ψ at midrapidity the differential cross section is calculated as: dσ J/ψ + Xn dy (cid:12)(cid:12)(cid:12)(cid:12) | y | < . = 1 BR · N J/ψ
Acc · ε · ε trigg · L int · y . Here
Acc is the detector acceptance, ε is the track reconstruction efficiency, ε trigg is thetrigger efficiency. The integrated luminosity L int is given in Section 2, ∆ y is the rapidityinterval of the measurement. These correction factors and corresponding uncertainties arequoted in Ref. [5],and BR = 5.94% is the known J/ψ dielectron branching ratio [20].For dielectrons at midrapidity ( y is the rapidity of the pair) the double differential crosssection is: d σ e + e − + Xn dy dm e + e − (cid:12)(cid:12)(cid:12)(cid:12) | y | < . , ∆ m e + e − = N e + e − Acc · ε · ε trigg · L int · y · m e + e − , where the factors are defined as for the previous equation.The measured dielectron cross sections at midrapidity are in very good agreement withthe starlight predictions for coherent dielectron photoproduction (rightmost column ofTable 1) [18].The final J/ψ cross section is in good agreement, within the (still large) statistical errors,with the theoretical values computed in [17, 14, 13, 16, 18, 21] as shown in Fig. 3. For detailsof the comparision see Ref. [5]
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DIS 2009 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 b ) µ / d y ( σ d PHENIXStarlight, coh.Strikman et al, coh.Strikman et al, incoh.Goncalves-Machado, coh.Kopeliovich et al, coh.Kopeliovich et al, incoh. a) y-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 b ) µ / d y ( σ d PHENIXStarlight, coh.Strikman et al, coh. + incoh.Goncalves-Machado, coh.Kopeliovich et al, coh. + incoh. b) Figure 3: Measured cross section of
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