UltraPeripheral Collisions at STAR Janet Seger Creighton University

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

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

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

Ultra-Peripheral Collisions Two nuclei ‘miss’ each other and interact via their photon fields at impact parameters b > 2 RA • 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!

Coherent Coupling Strength: • Photon Z 2 ( Z for incoherent coupling to single nucleon) • Pomeron A 4/3 to A 2 Cross sections are large Coherence condition: • Longitudinal momentum p. L < gh/RA ~ 3 Ge. V/c << p nuclei • Small Transverse momentum • p. T < 2 h/RA~ 50 Me. V Clear signature of coherent events; acceptance may be small

Photon-Photon Interactions: gg leptons, mesons Example: electron pair production Au g g ee+ Au • Purely electromagnetic • Exclusive process • Strong field QED Za ~ 0. 6 • Large cross section Z 4 a 4 (33 kb at RHIC) • Acceptance/efficiency small

Photonuclear interactions • Photon fluctuates to pair • pair can emerge as vector meson – r, w, F, J/y • Large cross section – 380 mb for r 0 in Au collisions at 130 Ge. V • Two sub-channels– with and without nuclear excitation

Nuclear Excitation Nuclei may exchange one or more separate photons and become mutually excited. Au*+n Au g r 0 g P Au+Au Au*+Au* + r 0 Au Au*+n • Cross section factorizes • Decay of excited nuclei yields neutrons in Degree Calorimeter Zero

Direct p+p - production pp+ g g. A p+ p- A r 0 g pp+ g. A r 0 A p+ p- A • Two diagrams interfere – Constructive below M(r 0); destructive above it • Well-studied with gp • greater for p+p- than for r 0 – p+ p- fraction should decrease as A increases

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

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

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) Pedestal Minimum Bias Trigger • Single neutron peak around ADC =9 coincident in east and west • Higher ADC values from hadronic peripheral events Single neutron peak Reject events ADC>7 Observe two different processes ! Reject events ADC>30

Observed with Peripheral Trigger: Au+Au + r 0 Au g r 0 P Au Observed with Minimum-Bias Trigger: Au+Au Au*+Au* + r 0 Au Au*+n g P r 0 g Au

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) Cosmics

Rho events with Peripheral Trigger Signal region: p. T<0. 1 Ge. V P i l re i m y r a n • Au+Au+r • Uncorrected • mpp spectrum peaks near r 0 mass • peak at p. T < 100 Me. V/c – Clear evidence that production was coherent

Rho events with Minimum-Bias Trigger Signal region: p. T<0. 1 Ge. V y r na Pr i m i l e • Au+Au Au*+r • Uncorrected • mpp spectrum peaks nea r 0 mass • peak at p. T < 100 Me. V/c – Clear evidence that production was coherent

STARlight Monte Carlo Simulation vs. data: r 0 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 photonmeson interactions – Does not (yet) include interference, backgrounds, and full detector simulation Pr eli mi na ry • Min. Bias dataset – 0. 62 Ge. V < mpp < 0. 92 Ge. V – uncorrected Reasonable match between Monte Carlo and data

Analysis of e+e- Pairs Selectrons from -bias data: • p < 0. 13 Ge. V • -0. 2 < log(z) <0. 3 min y r a n i m • Min-bias data with two oppositely-charged tracks • Identified electrons i l re P Selected pairs show a peak at very low p. T Coherent process Au+Au Au*+e++e-

Fit r 0 mass lineshape ZEUS gp --> (r 0 + p+p- )p STAR g. Au --> (r 0 + p+p- )Au Fit Data Prel imin ary r 0 p+p- Set =0 for STAR interference is significant p+ p- fraction is high (electron pairs have not been removed)

Summary • We observe exclusive r 0 production in both peripherally-triggered and minimum bias data sets Observation of both interactions: Au + Au -> Au + r 0 Au + Au -> Au* + r 0 • 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

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