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 kbarnsAuAu --> AuAu + 2(e+e-) 680 barnsAuAu --> AuAu + 3(e+e-) 50 barns

-N: L(-N )=1029 cm-2s-1 2<E<300GeVAuAu --> Au+Au* 92 barns X+neutronsAuAu --> Au*+Au* 3.670.26 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 (~Z2)•However, coherence condition limits maximum energy and pT

RHIC, LHC LA/LAA

DIS06, Tsukuba Japan 4

Ultraperipheral Collisions

Photon-photon interactions

Photon-nucleusinteractionsUPC 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 xGA

•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 TriggerUPC: (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 0.1

3. 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

RHICZero 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 CutsGlobal cuts: |zvtx| < 30 cm, track multiplicity <15

Single-track cuts:

• N0 ｡ 2 [# of RICH phototubes fired by e+e- ].

• E1 > 0.8 GeV || E2 > 0.8 GeV [ERT threshold].

• No dead-warn tower around assoc. EMCal cluster

•[CNT-EMC matching. e+e- candidates].

Pair cuts: arm1 ≠ arm2 [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)

NJ/ = 10 3(stat) 3(syst)

DIS06, Tsukuba Japan 11

Di-electron Pair PT• p

T distribution of e+e- pairs: peaked at very low p

T

coherent production (pT < √2 ℏc/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-DistStrikman, Tverskoy, Zhalov, PLB 626 p. 72-79

•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 = NJ//(BRJ/ee*Acc*reco*trig*Lint*y)

= 44 ± 16 (stat) ± 18 (syst) μbPHENIX 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