DIS06, Tsukuba Japan 1 Ultraperipheral J/  and di- electron Production at RHIC (PHENIX) Mickey Chiu University of Illinois at Urbana-Champaign

DIS06, Tsukuba Japan 2 Hadronic Interaction: Au-Au --> X 6.8 barns  -  : AuAu --> AuAu + e + e - 33 kbarns AuAu --> AuAu + 2(e + e - ) 680 barns AuAu --> AuAu + 3(e + e - ) 50 barns  -N: L(  -N )=10 29 cm -2 s -1 2<E  <300GeV AuAu --> Au+Au* 92 barns X+neutrons AuAu --> Au*+Au* 3.67  0.26 barns X+neutrons Y+neutrons “Hadronic” Collider Processes You’re probably familiar with the “Hadronic Interactions” But there are a lot more processes going on at a hadron collider Hadronic Interaction Peripheral Interaction

DIS06, Tsukuba Japan 3  +  and  +A Luminosities UPC A+A interaction The electromagnetic field is equivalent to a large flux of quasi-real photons, and can be calculated per (Fermi)-Weizsacker-Williams: Can get large flux due to the coherent effect of protons in nucleus (~Z 2 ) However, coherence condition limits maximum energy and p T RHIC, LHC L  A /L AA

DIS06, Tsukuba Japan 4 Ultraperipheral Collisions Photon-photon interactions Photon-nucleus interactions UPC A+A interaction V=  J/ Ψ = + This large flux of quasi-real photons means a hadron collider is also a photon collider Pro: Much cleaner environment - QCD backgrounds are far lower Con: Effectively a Lower energy Collider

DIS06, Tsukuba Japan 5 Physics Potential of UPC J/Psi Direct Measurement of gluon distributions at low-x x~10 -2 at RHIC, but in nucleus – earlier onset of saturation due to gluon overlap – Color Glass Condensate? Ryskin, Roberts, Martin, Levin, Z. Phys C 76 (1997) 231: Quadratic dependence on xG A Dynamics of heavy vector meson propagation through nuclear matter J/Psi+N cross-section Nucleus dynamics Black body limit Color transparency Search for new physics At LHC, possible to go up to W  = 180 GeV

DIS06, Tsukuba Japan 6 Ultraperipheral Trigger UPC: (ZDCN || ZDCS) && (!BBCLL1noVtx) && (ERT2x2) BBC 1.Veto on BBC coincidence to reduce hadroproduction contamination Similar to rapidity gap selection 2.Relatively large energy deposit (E>0.8 GeV) in EMCAL to select e  from J/Psi decay and high mass continuum Good J/Psi efficiency,  J/  = 1 – (1 -  e ) 2 ~ 0.9  At least 30 GeV neutral energy deposited in one or both ZDCs to select Au+Au events with forward neutron emission from single or double Au* decay Significantly reduces background rates from cosmics, beam gas, beam scrape,

DIS06, Tsukuba Japan 7 Coulomb Dissociation Tag RHIC Zero Degree Calorimeter The ZDC requirement reduces the trigger rate from O(10kHz) to 0.4% of Au+Au hadronic rate Reduces photoproduction rates by only about 50% due to impact parameter selection. Large flux of photons means there is high probability to have additional photon exchanges Effect is factorizable, and therefore calculable

DIS06, Tsukuba Japan 8 Data Analysis and Cuts 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/  & high-mass e + e – continuum based on physics input from Starlight model Detectors: DC+PCs: Full charged tracking RICH & EMCal: e + e - identification

DIS06, Tsukuba Japan 9 Di-electron UltraPeripheral Production dN/dm ee (background subtracted) w/ fit to (MC) expected dielectron continuum and J/Ψ signals:

DIS06, Tsukuba Japan 10 J/Psi UltraPeripheral Production J/Ψ invariant mass distribution (e + e - pairs minus dielec. continuum) N J/  = 10  3(stat)  3(syst)

DIS06, Tsukuba Japan 11 Di-electron Pair P T p T distribution of e + e - pairs: peaked at very low p T  coherent production (p T < √2 ℏ c/R ~ 50 MeV) Excess at high pt due to incoherent production? Or is it contamination from grazing hadronic collisions?

DIS06, Tsukuba Japan 12 Coherent vs Incoherent t-Dist Strikman, Tverskoy, Zhalov, PLB 626 p Strikman et al calculate that quasi- elastic (incoherent) J/  cross-section comparable to coherent production Incoherent J/  produced from photo-nucleon interaction Much larger t distribution expected for incoherent J/ 

DIS06, Tsukuba Japan 13 UPC J/Psi Theory Comparison dσ J/ Ψ /dy| y=0 = N J/  /(BR J/  ee *Acc*  reco *  trig *Lint*  y) = 44 ± 16 (stat) ± 18 (syst) μb PHENIX preliminary Impulse Approx  J/  = 3 mb Glauber Approach

DIS06, Tsukuba Japan 14 Future Outlook in PHENIX Reduce systematic errors (well under progress) Improved study of Trigger efficiency Increased Monte Carlo statistics Better (tighter) electron id cuts Look for UPC production of open charm Trigger was on single leptons Expect order of magnitude increase in Au+Au statistics from future runs as part of heavy ion program Start to discriminate between different approaches (Impulse Approximation, Glauber) Start to measure gluon distributions at low x Possibility to trigger on muons at 1.2 < | c | < 2.4 Above  >2, quasi-elastic (incoherent) dominated Improve discrimination between coherent and quasi-elastic signal

DIS06, Tsukuba Japan 15 Summary and Conclusions Hadron colliders naturally are also  +  and  +N(or A) colliders, and this can be exploited for physics Direct Measurement of Gluons at Low-x Color dipole interactions with a nucleus Searches for new physics Among others…. In PHENIX at RHIC we have initiated a program that we hope lays the foundation for future work at the LHC, where for instance L  is higher than at any existing lepton collider While energies are low, RHIC is a first proof of principle that these measurements can be done Can learn how to effectively trigger on UPC Tests of existing theoretical framework