PhotonPomeron Interactions at RHIC Falk Meissner Lawrence Berkeley

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

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Ultra-Peripheral Collisions Two nuclei ‘miss’ each other and interact via their photon fields at impact parameters b > 2 RA Equivalent Photon Approximation (Weizsaecker-Williams, Fermi) • • Interaction via long range fields Large cross section Coherent coupling to both nuclei Small transverse momentum • Nuclei may be mutually excited 3 Brookhaven, May 2001 Falk Meissner, LBNL

Coherent Coupling to Charge Extended Photon(Pomeron) = plane wave Can’t distinguish between different points of origin Coherence condition from uncertainty principle: • Small transverse momentum: p. T < 2 h/RA~ 60 Me. V • Longitudinal component PL < gh/RA ~ 3 Ge. V/c <<P Coupling Strength • Photon Z 2 (only Z for incoherent coupling to single nucleon) • Pomeron A 4/3 to A 2 Brookhaven, May 2001 nuclei (A 4/3 surface, in the limit sr. N ; A 2 volume, in the weak limit ) Falk Meissner, LBNL 4

Coherent Vector Meson Production: g. A ’VA Exclusive production of r 0 mesons Au+Au --> Au+Au + r 0 Large cross section: 380 mb for Au at 130 Ge. V/nucleon section) Au (5% of hadronic cross g P r 0 Au • Non-local interference of unstable particles • Vector meson spectroscopy (r*) • Production cross sections (sr. N) First Goal - Proof of Principle 0 Observe exclusive r production Au Au Collisions 5 Brookhaven, May 2001 Falk Meissner, LBNL

An Example : Interference Fundamental Quantum Mechanics at Work 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 6 Brookhaven, May 2001 Falk Meissner, LBNL

STAR Detector Coils Magnet Silicon Vertex Tracker E-M Calorimeter Time Projection Chamber Trigger Barrel Electronics Platforms Forward Time Projection Chamber 7 Brookhaven, May 2001 Falk Meissner, LBNL

Heavy Ion Collision (Au Au, 200 Ge. V/nucleon, University of Frankfurt) RHIC 2000 run: 130 Ge. V/nucleon Up to ~2000 tracks per event Trigger: • Multiplicity in Central Trigger Barrel • Neutron Deposit in Zero Degree Calorimeter Brookhaven, May 2001 Falk Meissner, LBNL 8

Experimental Signature of UPC Typical Event : 0 nly two oppositely charged tracks • Low total p. T • Back-to-back in transverse plane Trigger Backgrounds: • Cosmic rays • Beam-gas Events • Debris from upstream events 9 Brookhaven, May 2001 Falk Meissner, LBNL

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 10 Brookhaven, May 2001 Falk Meissner, LBNL

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 Cosmics (reject cosmic background) • Pion identification for both tracks via d. E/dx • ADC counts of ZDC <8 (reject signals above pedestal, e. g. hadronic peripheral collisions and beam-gas events) 11 Brookhaven, May 2001 Falk Meissner, LBNL

First Results: Invariant Mass & Transverse Momentum Spectra Peripheral Trigger: Au+Au -> Au +r 0 Peak at low pt => signature for coherent interaction Signal region: p. T<0. 1 Ge. V Pre lim ina ry Select Signal: Low p. T region p. T<0. 1 Me. V 12 Brookhaven, May 2001 Falk Meissner, LBNL

Nuclear Excitation In addition tor 0 production, nuclei can exchange one or more separate photons and become mutually excited. Au Au*+n Process g P r 0 Au+Au -> Au*+Au* + g r 0 Au Au*+n • 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 13 Brookhaven, May 2001 Falk Meissner, LBNL

Invariant Mass & Transverse Momentum Spectra Minimum Bias Trigger: Au+Au -> Au*+Au* + r 0 Signal region: p. T<0. 1 Ge. V Pre Select Signal: Low p. T region p. T<0. 1 Me. V lim ina ry Select Signal: Low p. T region p. T<0. 1 Me. V 14 Brookhaven, May 2001 Falk Meissner, LBNL

Compare ZDC Signals (for two track events) Ultra Peripheral Trigger Minimum Bias Trigger • Pedestal peak at ADC sum = 4 • Higher ADC values usually in east or west only (beam gas events) • Single neutron peak around ADC =9 coincident in east and west • Higher ADC values from hadronic peripheral events Pedestal Single neutron peak Reject events ADC>30 Reject events ADC>7 Observe two different processes ! Au+Au -> Au+Au + r 0 and Au+Au -> Au*+Au* + r 0 15 Brookhaven, May 2001 Falk Meissner, LBNL

Monte Carlo Simulation vs. Data STARlight - Monte Carlo Generator for UPC S. R. Klein and J. Nystrand, STAR Note 347 • Simulates photon-photon, photon-Pomeron, photon-meson interactions • Simulates rapidity, transverse momentum and angular distributions r 0 Transverse Momentum Spectrum Data: Minimum bias trigger 0. 62<Mpp<0. 92 Ge. V Prel imin Not yet addressed by the Monte Carlo: ary • Background • Full spectrometer simulation • Interference Reasonable agreement between data and Monte Carlo Brookhaven, May 2001 Falk Meissner, LBNL 16

Direct p p + - production p- g p+ g. A => p+ p- A g r 0 pp+ g. A => r 0 A => p+ p- A • Direct p+ p- is independent of energy • The two processes interfere => 1800 phase shift at M(r 0) changes p+ p- lineshape • good data with gp • p+ p- fraction expected to decrease as A rises 17 Brookhaven, May 2001 Falk Meissner, LBNL

0 Fit of r Lineshape ZEUS gp --> (r 0 + p+p- )p STAR g. Au --> (r 0 + p+p- )Au Fit Data Prel r 0 imin p+p- ary Fit all data to r 0 + p+pinterference is significant p+p- fraction is high (background? ) Set =0 for STAR Brookhaven, May 2001 Falk Meissner, LBNL 18

Electromagnetic particle production: gg’leptons, mesons Pair Production e+e. Au g g • Purely electromagnetic exclusive Process • Strong field QED Za ~ 0. 6 ee+ • Large cross section Au Z 4 a 4 (RHIC 33 kb) but acceptance/efficiency small Pair production with e- capture Limits on beam life time at LHC Z-1 Z g g (Z-1 particles leave optics of machine, local heating) e+ Z 19 Brookhaven, May 2001 Falk Meissner, LBNL

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 Ge. V • Use d. Edx -0. 2<log(z)<0. 3 Preliminary e+e- Pairs are all at low p_T Þ Signature for coherent process Au Au-> Au*Au* e+e. Brookhaven, May 2001 Falk Meissner, LBNL 20

Summary The observation of exclusive r 0 production in both peripherally triggered and minimum bias data sets demonstrates existence of both interactions Au + Au -> Au + r Au + Au -> Au* + r 0. r Clear signals at the 0 mass and at low transverse momentum for both processes. First observation of Ultra. Peripheral Collisions in heavy ion interactions 21 Brookhaven, May 2001 Falk Meissner, LBNL