Coherent Electromagnetic Interactions in UltraPeripheral Heavy Ion Collisions

1
Coherent Electromagnetic Interactions in
Ultra-Peripheral Heavy Ion Collisions
Falk Meissner Lawrence Berkeley National
Laboratory For the STAR Collaboration Photon
2003 April 2003

• Ultra Peripheral Collisions
• Exclusive Meson Production
• Data Sets and Analysis
• Cross Sections
• Photon-Photon Interactions
• Summary

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Heavy Ion Collision
(Au Au, 200GeV/nucleon, University of Frankfurt)
RHIC
3
Ultra-Peripheral Collisions

• Nuclei miss geometrically and interact
  via long range fields
• Coupling strengthalarge cross sections
• Photon-g ? Z2
• Equivalent Photon Approximation
  (Weizsaecker-Williams, Fermi)
• Pomeron-P ?A4/3 (surface) to A 2 (volume)
  
• Coherent coupling to extended charge of both
  nuclei
• Lg, LPgt R_A
• Small transverse momentum pT lt h/2RA 90
  MeV
• Longitudinal component PL lt gh/2RA 6 GeV
• Quasi Real Photons

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Exclusive Vector Meson Production gAVA

• Factorize as function of impact parameter
• Nuclear breakup by single (1n1n) and multiple
  (xnxn )neutron emission
• single/multiple neutron emission selects
  different impact parameters

• Extrapolate gpVp to gAVA with Glauber
  calculation
• e Large cross sections
• 350 mb at sNN1/2130 GeV
• 590 mb at sNN1/2200 GeV
• S.Klein, J.Nystrand, Phys. Rev C60 014903
  (1999),
• A. Baltz, S. Klein,J. Nystrand PRL 89, 012301
  (2002)

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Experimental Signature of UPC

• Two oppositely charged tracks with vertex
• Low total pT
• Back-to-back in transverse plane

Challenge Trigger !
Topology requirements in central trigger
barrel (CTB)
Coincident signals from nucl. Breakup in zero
degree calorimeters (ZDC)
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Transverse Momentum Invariant Mass Spectra
200GeV
Topology Trigger AuAu a AuAur0

• 2000 130 GeV 9hrs dedicated,
• 30k triggers
• 2001 200 GeV trigger mix
• 1.5M topology triggers
• Peak at low pT a coherent interaction
• Background model from like-sign pairs normalized
  to data
• No neutron signal in ZDC a gold nuclei remain
  in ground state

Preliminary
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Meson Production with Nuclear Break-up
200GeV
Minimum Bias (ZDC) Trigger AuAu aAuAur0

• 2000 400k triggers L60/mb
• 2001 1.7M triggers L250/mb
• Amplitude ratio pp/r similar to lepton nucleon
  scattering

Preliminary

• ZDC Response

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Cross Section s (AuAuaAu()Au()r)
130 GeV PRL89, 272303 (2002), 200 GeV
Preliminary Luminosity normalization from 7.2b
hadronic AuAu cross section Systematic
uncertainties 20

• Theory A. Baltz, S. Klein J. Nystrand

Preliminary
Total
No Breakup
xn,xn
1n,1n
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t-Spectrum

• Determination of
• Gold Radius

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Coherent Interactions in Deuterium-Gold

• 2003 dAu-gt rdAu
• Golden photon torch on d
• Trigger on the breakup of the deuterium (ZDC)
• Level0 trigger purity 10 !!
• 0.9M events
• First results clear signal (5 minute testrun)
  data is in production

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Two-Photon Interaction AuAuaAuAuee-
Identified ee - via dE/dx only at low
momentum plt0.13 GeV Small acceptance (10-6) for
ee- pairs cross section is peaked in the
forward region and at low (non-reconstruct able)
invariant mass Manetic field cuts of very low
momentum tracks (lt60MeV) where cross section is
large
200GeV, 0.25T Magnetic Field
Acceptance corrected pair pT
Coherent ee- Pairs pT lt h/pb 20MeV
(ltbgt40fm)

• Physics Topics
• Validity of WWA and factorization
• Strong field QED Za 0.6
• Large cross section ? Z4a4

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Cross-section

• Within acceptance
• 0.14ltMeelt0.26 GeV, hlt1.15
• Total 4p cross section 33kB!

Predictions LO QED S.Klein V.Morozov
Phd. Thesis V. Morozov
STAR PRELIMINARY
Normalized to total hadronic cross-section,
10 systematic error
W
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Summary

• First measurements for coherent meson production
  in ultra-peripheral heavy ion collisions with
  and without nuclear excitation
• Au Au -gt Au Au r0 and Au Au -gt Au Au
  r0 .
• low pT coherent coupling
• Cross Sections agree with predictions
• gtApproximate factorization of
  rho-production and nuclear excitation
• gtWeizsaecker-Williams photon flux from
  large relativistic charges ok.
• gt Glauber extrapolation of gN to gAu ok
• Cross section for electron-pair production
  reproduced by LO calculations
• This is just the beginning Future analysis
  topics
• Vector meson spectroscopy ,excited states (r,)
• Multiple VM production
• Hard diffraction - higher mass states J/y
• Interference of decaying particles
• RHIC is a good place to study diffractive and
  electromagnetic processes in heavy ion
  collisions. Lots of data and physics topics.