Ultra-Peripheral Collisions at STAR

1. STAR Ultra-Peripheral Collisions at STAR • STAR Detector • Ultra-Peripheral Collisions • Triggers • First results • Coherent rho production • Electron pair production • Direct pion production Janet Seger, Creighton University for the Collaboration

2. Silicon Vertex            Tracker Magnet Coils E-M Calorimeter Time Projection Chamber Trigger Barrel Electronics Platforms Forward Time Projection Chamber STAR Detector

4. Central Gold-Gold Collisions at 130 GeV/nucleon • Minimum-bias Trigger: • Coincident neutrons in both zero-degree calorimeters • ~800,000 triggers (Au Au, 200GeV/nucleon, University of Frankfurt) Up to 2000 tracks/event

5. Ultra-Peripheral Collisions Two nuclei ‘miss’ each other and interact via their photon fields at impact parameters b > 2RA • Mostly 2-track events • Coherent coupling to both nuclei • Nuclei may be mutually excited • Large background • Incoherent photo-nuclear interactions, Beam-gas events, Cosmic rays,Hadronic interactions (peripheral AA),Upstream interactions… •  Triggering is Crucial!

6. Coherent Coupling Coupling Strength: • Photon  Z2 • ( Z for incoherent coupling to single nucleon) • Pomeron A4/3 to A 2  Cross sections are large Coherence condition: • Longitudinal momentum • pL < gh/RA ~ 3 GeV/c << p nuclei • Small Transverse momentum • pT < 2h/RA~ 50 MeV  Clear signature of coherent events; acceptance may be small

7. e- Au g e+ g Au Photon-Photon Interactions: ggleptons, mesons • Purely electromagnetic • Exclusive process • Strong field QED Za~ 0.6 • Large cross section Z4a4 (33 kb at RHIC) • Acceptance/efficiency small Example: electron pair production

8. Photonuclear interactions • Photon fluctuates to pair • pair can emerge as vector meson • r, w, F, J/y • Large cross section – 380 mb for r0 in Au collisions at 130 GeV • Two sub-channels– with and without nuclear excitation

9. Au*+n Au g g r0 Au P Au*+n Nuclear Excitation Nuclei may exchange one or more separate photons and become mutually excited. Au+Au  Au*+Au* +r0 • Cross section factorizes • Decay of excited nuclei yields neutrons in Zero Degree Calorimeter

10. Direct p+p - production p- p- r0 p+ • Two diagrams interfere • Constructive below M(r0); destructive above it • Well-studied with gp • greater for p+p- than for r0 • p+p- fraction should decrease as A increases p+ g g gA p+p- A gA r0A p+p- A

11. No Interference Interference S. Klein and J. Nystrand, Phys. Rev. Lett. 84(2000)2330 Interference Fundamental Quantum Mechanics Can’t differentiate between projectile and target Expected Signal 2-slit interferometer! r, w, f, J/yhave negative parity: destructive interference at pT=0

12. Ultra-Peripheral Collisions Trigger • Level 0 • Back-to-back hits in Central • Trigger Barrel • Coincidence • 1 North + 1 South hit • Veto on top + bottom • (reject cosmic rays) • Rate: 20-40 Hz • Level 3-online reconstruction • Vertex position • Charged multiplicity • Accepted 1-2 Hz • Data Set • ~ 7 hours of dedicated data collection • 30,000 triggers

13. Pedestal Single neutron peak Reject events ADC>30 Reject events ADC>7 ZDC Signals for 2-track events • Ultra-Peripheral Trigger • Pedestal peak at ADC sum = 4 • Higher ADC values usually in east or west only (beam gas events) • Minimum Bias Trigger • Single neutron peak around ADC =9 • coincident in east and west • Higher ADC values from hadronic peripheral events Observe two different processes !

14. Au*+n Au g r0 Au g r0 g P Au Au P Au*+n Observed with Peripheral Trigger: Au+Au  Au+Au +r0 Observed with Minimum-Bias Trigger: Au+Au  Au*+Au* +r0

15. Cosmics Event Selection Criteria • Vertex within interaction region • |zvertex| < 200 cm and |x,yvertex| < 2 cm • Exactly 2 tracks with net charge zero • ZDC cuts • Opening angle < 3.0 rad • (reject cosmic background)

16. Preliminary Signal region: pT<0.1 GeV Rho events with Peripheral Trigger • Au+AuAu+Au+r • Uncorrected • mpp spectrum peaks near r0 mass • peak at pT < 100 MeV/c • Clear evidence that production was coherent

17. Signal region: pT<0.1 GeV Preliminary Rho events with Minimum-Bias Trigger • Au+AuAu*+Au*+r • Uncorrected • mpp spectrum peaks near r0 mass • peak at pT < 100 MeV/c • Clear evidence that production was coherent

18. Preliminary STARlight Monte Carlo Simulation vs. data: r0 pt spectrum • STARlight Monte Carlo S.R. Klein and J. Nystrand, STAR Note 347 • Simulates rapidity, transverse momentum and angular distributions for photon-photon, photon-Pomeron and photon-meson interactions • Does not (yet) include interference, backgrounds, and full detector simulation • Min. Bias dataset • 0.62 GeV < mpp < 0.92 GeV • uncorrected Reasonable match between Monte Carlo and data

19. • Min-bias data with two oppositely-charged tracks • Identified electrons Preliminary Analysis of e+e- Pairs • Select electrons from min-bias data: • p < 0.13 GeV • -0.2 < log(z) <0.3 Selected pairs show a peak at very low pT Coherent process Au+AuAu*+Au*+e++e-

20. Preliminary r0 p+p- Set =0 for STAR Fit r0 mass lineshape ZEUS gp --> (r0 + p+p- )p STAR gAu --> (r0 + p+p- )Au Fit Data interference is significant p+ p- fraction is high (electron pairs have not been removed)

21. Summary • We observe exclusive r0 production in both peripherally-triggered and minimum bias data sets  Observation of both interactions: Au + Au -> Au + Au + r0 Au + Au -> Au* + Au* + r0 • Peaks at low transverse momentum indicate both processes are coherent. • We observe interference between rho production and direct pion production. • We observe coherent electron-pair production. First observation of Ultra-Peripheral Collisions in heavy ion interactions

22. Year 2001 Run • Full field (0.5 T) • Peripheral Trigger in parallel with Central • Better calibrations • May have some functionality from MWC • Acceptance at higher h • Better statistics  more channels