Probing the Nucleus with Ultra-Peripheral Collisions

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Probing the Nucleus with Ultra-Peripheral
Collisions
Spencer Klein, LBNL (for the STAR Collaboration)

• Ultra-peripheral Collisions What and Why
• Photoproduction as a nuclear probe
• STAR Results at 130 GeV/nucleon
• Au Au --gt Au Au r0
• r0 production with nuclear excitation
• Direct pp- production interference
• A peek at 200 GeV/nucleon beyond
• Conclusions

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Coherent Interactions

• b gt 2RA
• no hadronic interactions
• ltbgt 25-50 fermi at RHIC
• Ions are sources of fields
• photons
• Z2
• Pomerons or mesons (mostly f0)
• A 2 (bulk) A 4/3 (surface)
• Fields couple coherently to ions
• Photon/Pomeron wavelength l h/pgt RA
• amplitudes add with same phase
• P? lt h/RA, 30 MeV/c for heavy ions
• P lt gh/RA 3 GeV/c at RHIC
• Strong couplings --gt large cross sections

Au
g, P, or meson
Au
Coupling nuclear form factor
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Specific Channels

• Vector meson production
• gA -- gt r0, w, f, J/y, A
• Production cross sections --gt s(VN)
• Vector meson spectroscopy (r, w, f,)
• Wave function collapse
• Electromagnetic particle production
• gg -- gt leptons,mesons
• Strong Field (nonperturbative?) QED
• Za 0.6
• meson spectroscopy Ggg
• Ggg charge content of scalar/tensor mesons
• Ggg is small for glueballs

ee-, qq,...
gs
Za 0.6 is Ng gt 1?
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Exclusive r0 Production
Au
g
qq
Au

• One nucleus emits a photon
• The photon fluctuates to a qq pair
• The pair scatters elastically from the other
  nucleus
• qq pair emerges as a vector meson
• s(r) 590 mb 8 of sAuAu at 200 GeV/nucleon
• 120 Hz production rate at RHIC design luminosity
• r, w, f, r rates at RHIC all gt 5 Hz
• J/y , Y, f, w, copiously produced, U a
  challenge

r0
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Elastic Scattering with Soft Pomerons

• Glauber Calculation
• parameterized HERA data
• Pomeron meson exchange
• all nucleons are the same
• s A2 (weak scatter limit)
• All nucleons participate
• J/y
• s A 4/3 (strong scatter limit)
• Surface nucleons participate
• Interior cancels (interferes) out
• s A 5/3 (r0)
• depends on s(Vp)
• sensitive to shadowing?

Y 1/2 ln(2k/MV)
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Elastic Scattering with Hard Pomerons

• Valid for cc or bb
• ds/dy s depend on gluon distributions
• shadowing reduces mid-rapidity ds/dy
• Effect grows with energy
• s reduced 50 at the LHC
• colored glass condensates may have even bigger
  effect

RHIC - Au
No shadowing
HERA param.
ds/dy
Leading Twist Calculation Frankfurt, Strikman
Zhalov, 2001
Shadowed
Y 1/2 ln(2k/MV)
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Nuclear Excitation

• Nuclear excitation tags small b
• Multiple photon exchange
• Mutual excitation
• Au decay via neutron emission
• simple, unbiased trigger
• Multiple Interactions probable
• P(r0, b2R) 1 at RHIC
• P(2EXC, b2R) 30
• Non-factorizable diagrams are small for AA

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Interaction Probabilities ds/dy

• Excitation r0 changes b distribution
• alters photon spectrum
• low ltbgt --gt high ltkgt

r0 with gold _at_ RHIC
ds/dy
y
Exclusive - solid X10 for XnXn - dashed X100 for
1n1n - dotted
Baltz, Klein Nystrand (2002)
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Photoproduction of Open Quarks

• gA --gt ccX, bbX
• sensitive to gluon structure function.
• Higher order corrections problematic
• Ratio s(gA)/s(gp) --gt shadowing
• removes most QCD uncertainties
• Experimentally feasible (?)
• high rates
• known isolation techniques
• Physics backgrounds are gg--gt cc, gg --gt cc
• gg cross section is small
• gg background appears controllable by requiring a
  rapidity gap

QQ--gt open charm
g
g
Production occurs in one ion
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Interference

• 2 indistinguishable possibilities
• Interference!!
• Similar to pp bremsstrahlung
• no dipole moment, so
• no dipole radiation
• 2-source interferometer
• separation b
• r,w, f, J/y are JPC 1- -
• Amplitudes have opposite signs
• s A1 - A2eipb2
• b is unknown
• For pT ltlt 1/ltbgt
• destructive interference

No Interference
Interference
y0
r0 --gt pp- pT (GeV/c)
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Entangled Waveforms
e

• VM are short lived
• decay before traveling distance b
• Decay points are separated in space-time
• no interference
• OR
• the wave functions retain amplitudes for all
  possible decays, long after the decay occurs
• Non-local wave function
• non-factorizable Yp p- ? Yp Yp-
• Example of the Einstein-Podolsky-Rosen paradox

J/Y
e-
?
b
J/Y
?
?-
(transverse view)
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r0 Analysis

• Exclusive Channels
• r0 and nothing else
• 2 charged particles
• net charge 0
• Coherent Coupling
• SpT lt 2h/RA 100 MeV/c
• back to back in transverse plane
• Backgrounds
• incoherent photonuclear interactions
• grazing nuclear collisions
• beam gas interactions

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Exclusive r0

• (prototype) trigger on 2 roughly back-to-back
  tracks
• 30,000 events in 9 hours
• 2 tracks in interaction region
• reject cosmic rays
• peak for pT lt 150 MeV/c
• pp and p-p- give background shape
• pp- pairs from higher multiplicity events have
  similar shape
• scaled up by 2.1
• high pT r0 ?
• asymmetric Mpp peak

Signal region pTlt0.15 GeV
Preliminary
r0 PT
pTlt0.15 GeV
M(pp-)
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Minimum Bias Dataset

• Trigger on neutron signals in both ZDCs
• 800,000 triggers
• Event selection same as peripheral
• pp and p-p- model background
• neutron spectrum has single (1n) and multiple
  (Xn) neutron components
• Coulomb excitation
• Xn may include hadronic interactions?
• Measure s(1n1n) s(XnXn)

Preliminary
r0 PT
ZDC Energy (arbitrary units)
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Direct p p- production

• The two processes interfere
• 1800 phase shift at M(r0)
• changes p p- lineshape
• good data with gp (HERA fixed target)
• pp- r 0 ratio should depend on s(pA)s(rA)
• decrease as A rises?

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r0 lineshape
ZEUS gp --gt (r0 pp- )p
STAR gAu --gt (r0 pp- )Au
ds/dMpp (mb/GeV)
ds/dMpp (mb/GeV)
Preliminary
Mpp
Mpp
Fit to r0 Breit-Wigner pp- Interference is
significant pp- fraction is comparable to
ZEUS
ee- and hadronic backgrounds
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dN/dy for r0(XnXn)
Soft Pomeron, no-shadowing, XnXn

• r ds/dy are different with and without breakup
• XnXn data matches simulation
• Extrapolate to insensitive region

After detector simulation
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Cross Section Comparison
Baltz, Klein Nystrand (2002)
Preliminary

• Normalized to 7.2 b hadronic cross section
• Systematic uncertainties luminosity, overlapping
  events, vertex tracking simulations, single
  neutron selection, etc.
• Exclusive r0 bootstrapped from XnXn
• Good agreement
• factorization works

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A peek at the 2001 data

• 200 GeV/nucleon
• higher ss
• higher luminosity
• Production triggers
• Minimum Bias data
• 10X statistics
• Topology Data
• 50X statistics
• Physics
• precision r0 s and pT spectra
• s(ee-) and theory comparison
• 4-prong events (r(1450/1700)???)

r0 spectra - 25 of the min-bias data
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Conclusions

• RHIC is a high luminosity gg and gA collider
• Coherent events have distinctive kinematics
• Photonuclear Interactions probe the nucleus
• s(AA --gt AAV) is sensitive to s(VA)
• probes gluon density (shadowing)
• STAR has observed three peripheral collisions
  processes
• Au Au -- gt Au Au r0
• Au Au -- gt Au Au r0
• The r0direct pp- is similar to gA nteractions
• The r0 cross sections agree with theoretical
  expectations