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Photon-Pomeron Interactions at RHIC

This paper presents an analysis and first results of ultra-peripheral collisions (UPC) at RHIC, focusing on photon-pomeron interactions. The study includes nuclear excitation, interference with direct pp production, and e+e- pair production. The analysis also explores the coherent coupling to extended charge and the production of coherent vector mesons. The results provide insights into high-energy QCD beyond the Pomeron.

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Photon-Pomeron Interactions at RHIC

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  1. Photon-Pomeron Interactions at RHIC Falk Meissner Lawrence Berkeley National Laboratory For the STAR Collaboration High Energy QCD, Beyond The Pomeron Brookhaven National Laboratory 22nd May 2001 Introduction to Ultra-Peripheral Collisions Analysis and First Results Nuclear Excitation in UPC Interference with Direct ppProduction Analysis of e+e- Pair Production Summary

  2. Falk Meissner, LBNL

  3. Ultra-Peripheral Collisions Two nuclei ‘miss’ each other and interact via their photon fields at impact parameters b > 2RA • Interaction via long range fields • Large cross section • Coherent coupling to both nuclei • Small transverse momentum • Nuclei may be mutually excited Equivalent Photon Approximation (Weizsaecker-Williams, Fermi) Falk Meissner, LBNL

  4. Coherent Coupling to Extended Charge • Photon(Pomeron) = plane wave • Can’t distinguish between different points of origin • Coherence condition from uncertainty principle: • Small transverse momentum:pT< 2h/RA~ 60 MeV • Longitudinal component • PL < gh/RA ~ 3 GeV/c <<P nuclei • Coupling Strength • Photon Z2(onlyZ for incoherent coupling to single nucleon) • PomeronA4/3 to A 2(A4/3  surface, in the limitsrN; A2  volume, in the weak limit ) Falk Meissner, LBNL

  5. Au g r0 P Au Coherent Vector Meson Production: gA ’VA Exclusive production ofr0mesons Au+Au --> Au+Au +r0 Large cross section: 380 mb for Au at 130 GeV/nucleon (5% of hadronic cross section) • Non-local interference of unstable particles • Vector meson spectroscopy(r*) • Production cross sections(srN) First Goal - Proof of Principle Observe exclusiver0production Au Au Collisions Falk Meissner, LBNL

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

  7. Silicon Vertex            Tracker Magnet Coils E-M Calorimeter Time Projection Chamber Trigger Barrel Electronics Platforms Forward Time Projection Chamber STAR Detector Falk Meissner, LBNL

  8. Heavy Ion Collision (Au Au, 200GeV/nucleon, University of Frankfurt) RHIC 2000 run: 130 GeV/nucleon Up to ~2000 tracks per event • Trigger: • Multiplicity in Central Trigger Barrel • Neutron Deposit in Zero Degree Calorimeter Falk Meissner, LBNL

  9. Experimental Signature of UPC Typical Event : • 0nly two oppositely charged tracks • Low total pT • Back-to-back in transverse plane • Trigger Backgrounds: • Cosmic rays • Beam-gas Events • Debris from upstream events Falk Meissner, LBNL

  10. 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 trigger Falk Meissner, LBNL

  11. Event Selection Criteria • Vertex within interaction region • |zvertex| < 200 cm and |x,yvertex| < 2 cm • 2 tracks with net charge zero • Opening angle < 3.0 rad • (reject cosmic background) • Pion identification for both tracks via dE/dx • ADC counts of ZDC <8 • (reject signals above pedestal, e.g. hadronic peripheral collisions and beam-gas events) Cosmics Falk Meissner, LBNL

  12. First Results: Invariant Mass & Transverse Momentum Spectra Peripheral Trigger: Au+Au -> Au +Au +r0 Peak at low pt => signature for coherent interaction Signal region: pT<0.1 GeV Preliminary Select Signal: Low pT region pT<0.1MeV Falk Meissner, LBNL Observe about 100 r0 events in the data for the peripheral trigger

  13. Au*+n Au g g r0 Au P Au*+n Nuclear Excitation In addition tor0production, nuclei can exchange one or more separate photons and become mutually excited. Process Au+Au -> Au*+Au* +r0 • Believed to factorize as function of impact parameter • Decay yields neutrons in Zero Degree Calorimeter (ZDC) -> minimum bias trigger • Data set: ~400,000 events with minimum bias trigger Falk Meissner, LBNL

  14. Invariant Mass & Transverse Momentum Spectra Minimum Bias Trigger: Au+Au -> Au*+Au* +r0 Signal region: pT<0.1 GeV Preliminary Select Signal: Low pT region pT<0.1 MeV Select Signal: Low pT region pT<0.1MeV Falk Meissner, LBNL Observe about 200 r0 events in ~500,000 events for the minbias trigger, a rate of 0.5*10^-3 is consistent with expectations

  15. Compare ZDC Signals (for two track events) Ultra Peripheral Trigger Minimum Bias Trigger • Single neutron peak around ADC =9 • coincident in east and west • Higher ADC values from • hadronic peripheral events • Pedestal peak at ADC sum = 4 • Higher ADC values usually in east or west only (beam gas events) Pedestal Single neutron peak Reject events ADC>30 Reject events ADC>7 Observe two different processes ! Au+Au -> Au+Au +r0and Au+Au -> Au*+Au* +r0 Falk Meissner, LBNL

  16. Monte Carlo Simulation vs. Data • STARlight - Monte Carlo Generator for UPC • Simulates photon-photon, photon-Pomeron, • photon-meson interactions • Simulates rapidity, transverse momentum • and angular distributions S.R. Klein and J. Nystrand, STAR Note 347 r0Transverse Momentum Spectrum Data: Minimum bias trigger 0.62<Mpp<0.92 GeV Preliminary • Not yet addressed by the Monte Carlo: • Background • Full spectrometer simulation • Interference Reasonable agreement between data and Monte Carlo Falk Meissner, LBNL

  17. p- p- r0 g g p+ p+ gA =>p+p-A gA =>r0A =>p+p-A Direct p+p-production • Direct p+p- is independent of energy • The two processes interfere => 1800 phase shift at M(r0) changes p+p- lineshape • good data with gp • p+p- fraction expected to decrease as A rises Falk Meissner, LBNL

  18. Fit Data r0 p+p- Fit of r0Lineshape ZEUS gp --> (r0 + p+p- )p STAR gAu --> (r0 + p+p- )Au Preliminary Fit all data to r0 + p+p- interference is significant p+p- fraction is high (background?) Set =0 for STAR Falk Meissner, LBNL

  19. e- Au g e+ Z-1 g Au Z g e+ g Z Electromagneticparticle production: gg’leptons, mesons Pair Production e+e- • Purely electromagnetic exclusive Process • Strong field QED Za~ 0.6 • Large cross section Z4a4 (RHIC 33 kb) but acceptance/efficiency small • Pair production with e- capture Limits on beam life time at LHC (Z-1 particles leave optics of machine, local heating) Falk Meissner, LBNL

  20. Analysis of e+e- Pairs • Identified e+e- Pairs • data: minimum • bias trigger • two track events • net charge zero • low momentum • only p<0.13 GeV • Use dEdx • -0.2<log(z)<0.3 Preliminary • e+e- Pairs are all at low p_T • Signature for coherent process Au Au-> Au*Au* e+e- Falk Meissner, LBNL

  21. Summary The observation of exclusiver0production in both peripherally triggered and minimum bias data sets demonstrates existence of both interactions Au + Au -> Au + Au +r Au + Au -> Au* + Au* +r0. Clear signals at ther0mass and at low transverse momentum for both processes. First observation of Ultra-Peripheral Collisions in heavy ion interactions Falk Meissner, LBNL

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