Photon-Pomeron Interactions at RHIC

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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 pp Production
• Analysis of ee- Pair Production
• Summary

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Ultra-Peripheral Collisions
Two nuclei miss each other and interact via
their photon fields at impact parameters b gt 2RA

Equivalent Photon Approximation (Weizsaecker-Willi
ams, Fermi)

• Interaction via long range fields
• Large cross section
• Coherent coupling to both nuclei
• Small transverse momentum
• Nuclei may be mutually excited

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Coherent Coupling to Extended Charge

• Photon(Pomeron) plane wave
• Cant distinguish between different points of
  origin
• Coherence condition from uncertainty principle
• Small transverse momentum pT lt 2h/RA 60
  MeV
• Longitudinal component
• PL lt gh/RA 3 GeV/c ltltP nuclei

• Coupling Strength
• Photon ? Z2 (only ? Z for incoherent coupling
  to single nucleon)
• Pomeron ?A4/3 to A 2 (A4/3 ? surface, in the
  limit srN?? A2 ? volume, in the
  weak limit )

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Coherent Vector Meson Production gA VA
Exclusive production of r0 mesons AuAu --gt AuAu
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 exclusive r0 production Au Au Collisions
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An Example Interference
Fundamental Quantum Mechanics at Work
Cant differentiate between projectile and target
Expected Signal
No Interference
Interference
2-slit interferometer ! r, w, f, J/y have
negative parity destructive interference at pT0

S. Klein and J. Nystrand, Phys. Rev. Lett.
84(2000)2330
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STAR Detector
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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

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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

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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

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Event Selection Criteria

• Vertex within interaction region
• zvertex lt 200 cm and x,yvertex
  lt 2 cm
• 2 tracks with net charge zero
• Opening angle lt 3.0 rad
• (reject cosmic background)
• Pion identification for both tracks via dE/dx
• ADC counts of ZDC lt8
• (reject signals above pedestal, e.g.
  hadronic peripheral collisions and beam-gas
  events)

Cosmics
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First Results Invariant Mass Transverse
Momentum Spectra
Peripheral Trigger AuAu -gt Au Au r0 Peak at
low pt gt signature for coherent interaction
Signal region pTlt0.1 GeV
Preliminary
Select Signal Low pT region pTlt0.1MeV
Observe about 100 r0 events in the data for the
peripheral trigger
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Nuclear Excitation
In addition tor0production, nuclei can exchange
one or more separate photons and become mutually
excited.
Process AuAu -gt AuAu
r0

• Believed to factorize as function of impact
  parameter
• Decay yields neutrons in Zero Degree Calorimeter
  (ZDC) -gt minimum bias trigger
• Data set 400,000 events with minimum bias
  trigger

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Invariant Mass Transverse Momentum Spectra
Minimum Bias Trigger AuAu -gt AuAu r0
Signal region pTlt0.1 GeV
Preliminary
Select Signal Low pT region pTlt0.1 MeV
Select Signal Low pT region pTlt0.1MeV
Observe about 200 r0 events in 500,000 events
for the minbias trigger, a rate of 0.510-3 is
consistent with expectations
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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 ADCgt30
Reject events ADCgt7
Observe two different processes !
AuAu -gt AuAu r0 and AuAu -gt AuAu
r0
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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
r0 Transverse Momentum Spectrum
Data Minimum bias trigger 0.62ltMpplt0.92 GeV
Preliminary

• Not yet addressed by the Monte Carlo
• Background
• Full spectrometer simulation
• Interference

Reasonable agreement between data and Monte Carlo
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Direct pp- production

• Direct p p- is independent of energy
• The two processes interfere
• gt 1800 phase shift at M(r0) changes p p-
  lineshape
• good data with gp
• p p- fraction expected to decrease as A rises

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Fit of r0 Lineshape
ZEUS gp --gt (r0 pp- )p
STAR gAu --gt (r0 pp- )Au
Preliminary
Fit all data to r0 pp- interference is
significant pp- fraction is high
(background?)
Set 0 for STAR
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Electromagnetic particle production ggleptons,
mesons
Pair Production ee-

• 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)
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Analysis of ee- Pairs

• Identified ee- Pairs
• data minimum
• bias trigger
• two track events
• net charge zero
• low momentum
• only plt0.13 GeV
• Use dEdx
• -0.2ltlog(z)lt0.3

Preliminary

• ee- Pairs are all at low p_T
• Signature for
• coherent process
• Au Au-gt AuAu ee-

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Summary

• The observation of exclusive r0 production in
  both peripherally triggered and minimum bias data
  sets demonstrates existence of both interactions
• Au Au -gt Au Au r
• Au Au -gt Au Au r0 .
• Clear signals at the r0 mass and at low
  transverse momentum for both processes.

First observation of
Ultra-Peripheral Collisions in heavy ion
interactions