STAR Collaboration Meeting, UCD group meeting, February 2008 1 Ultra Peripheral Collisions • What is a UPC? • Photonuclear interaction • Two nuclei “miss” each other (b > 2R A ), electromagnetic interaction dominates over strong interaction • Photon flux ~ Z 2 • Weizsacker-Williams Equivalent Photon Approximation • No hadronic interactions . . Au Au d 3 N( k , r) dkd 2 r = Z 2 αx 2 π 2 kr 2 K 1 2 ( x) x = kr γ
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STAR Collaboration Meeting, UCD group meeting, February 2008 1 Ultra Peripheral Collisions What is a UPC? Photonuclear interaction Two nuclei “miss” each.
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STAR Collaboration Meeting, UCD group meeting, February 2008
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Ultra Peripheral Collisions
• What is a UPC?• Photonuclear interaction• Two nuclei “miss” each other (b > 2RA), electromagnetic
interaction dominates over strong interaction
• Photon flux ~ Z2
• Weizsacker-Williams
Equivalent Photon Approximation
• No hadronic interactions
..
Au
Au
€
d3N(k,r)
dkd 2r=
Z 2α x 2
π 2kr2K1
2(x)
€
x =kr
γ
STAR Collaboration Meeting, UCD group meeting, February 2008
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Exclusive o Production
• Photon emitted by a nucleus fluctuates to virtual qq pair
• Virtual qq pair elastically scatters from other nucleus
• Real vector meson (i.e. J/, o) emerges
• Photon and pomeron are emitted coherently
• Coherence condition limits transverse momentum of produced
Courtesy of F. Meissner
Au+Au Au+Au+o
€
pT <h
2RA
STAR Collaboration Meeting, UCD group meeting, February 2008
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o Production With Coulomb Excitation
Au+Au Au*+Au*+o
Au
Au
P
Au*
Au*
(2+)0
Courtesy of S. Klein
• Coulomb Excitation• Photons exchanged between
ions give rise to excitation and subsequent neutron emission
• Process is independent of o production
€
σ(AuAu → Au*Au* + ρ o) = d2bP∫ρ(b)PXnXn (b)
STAR Collaboration Meeting, UCD group meeting, February 2008
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Courtesy of S. Klein
Nucleus 1 emits photon which scatters from Nucleus 2
Nucleus 2 emits photon which scatters from Nucleus 1-Or-
Interference
• Amplitude for observing vector meson at a distant point is the convolution of two plane waves:
STAR Collaboration Meeting, UCD group meeting, February 2008
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Central Trigger Barrel
Time Projection Chamber
STAR Analysis Detectors
Zero Degree
Calorimeter
Zero Degree
Calorimeter
STAR Collaboration Meeting, UCD group meeting, February 2008
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UPC Topology• Central Trigger Barrel divided into four quadrants• Verification of decay candidate with hits in North/South quadrants• Cosmic Ray Background vetoed in Top/Bottom quadrants
Au+Au Au+Au+o
Triggers
UPC Minbias• Minimum one neutron in each Zero
Degree Calorimeter required • Low Multiplicity• Not Hadronic Minbias!