Ultra-Peripheral Collisions in STAR: Current Results and Future Prospects

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Ultra-Peripheral Collisions in STAR Current
Results and Future Prospects
Spencer Klein, LBNL (for the STAR Collaboration)

• What are ultra-peripheral collisions?
• Impact Parameter tagging and multiple
  interactions
• STAR r0 Results at 130 GeV/nucleon
• Preliminary r0 and ee- Results from 200 GeV
• UPC Future Prospects
• Conclusions

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Coherent Interactions

• b gt 2RA
• no hadronic interactions
• ltbgt 20-60 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

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Unique Features of Ultra-peripheral collisions

• Very strong electromagnetic fields
• g --gt ee- and g --gt qq
• Multiple production
• Coherent Decays
• Unique Geometry
• 2-source interferometer
• Nuclear Environment
• Particle Production with capture
• Large s for e-

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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
• Multiple Vector Meson Production Decay
• 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

• One nucleus emits a photon
• Weizsacker-Williams flux
• 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(had.) at 200
  GeV/nucleon
• 120 Hz production rate at RHIC design luminosity
• RHIC r, w, f, r rates all gt 5 Hz
• J/y , Y, f, w, copiously produced, U a
  challenge
• The LHC is a vector meson factory
• s(r) 5.2 b 65 of sPbPb (had.)
• 230 kHz r0 production rate with calcium

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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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Shadowing Hard Pomerons

• If pomerons are 2-gluon ladders
• P shadowing (gluon
  shadowing)2
• Valid for cc or bb
• ds/dy s depend on gluon distributions
• reduces mid-rapidity ds/dy
• Suppression grows with energy
• s reduced 50 at the LHC
• colored glass condensates may have even bigger
  effect
• high density phase of gluonic matter

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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Multiple Interactions

• Za 0.6
• Ng gt1 quite likely
• Multiple Interactions
• Photon emission is independent
• S. N. Gupta (1950)
• (Mostly) different interactions factorize

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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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Photonuclear 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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gg with nuclear breakup
dN/dy for f2(1270) gold at RHIC

• Impact parameter tagging similar for gg
• f2(1270) cross sections
• Breakup non-negligible
• Tagging is less dramatic than for photoproduction
• s(XnXn), s(1n1n)/s(tot) smaller
• rapdity distributions less different

XnXn breakup
Rapidity
no breakup
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gg --gt mm- w / nuclear breakup

• Smaller ltbgt
• harder photon spectrum
• harder Mmm spectrum
• s(XnXn), s(1n1n)/s(tot) ratio smaller than
  f2(1270)
• smaller avg. mass

gg --gt mm- gold at RHIC
XnXn nobreak
XnXn breakup
Mmm(GeV)
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Polarized Photons

• Electric fields parallel to b
• photon linear polarization follows b
• not spin
• Use 1 reaction to determine the polarization, to
  study a 2nd reaction
• Correlated Decay angles
• Use r0 -- gt pp- as analyzer
• pp- plane tends to parallel photon pol.
• Study VM production angles, single spin
  asymmetries in gA, etc.

b,E
(transverse view)
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Polarized Photons

• Electric fields parallel to b
• photon polarization follows b
• Use 1 reaction to determine the polarization, to
  study a 2nd reaction
• Correlated Decay angles
• Use r0 -- gt pp- as analyzer
• pp- decay plane often parallels photon pol.
• reasonable analyzing power
• Study VM production angles, single spin gA
  asymmetries, etc.

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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

• 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

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Interference and Nuclear Excitation

• Smaller ltbgt --gt interference at higher ltpTgt

r0 prod at RHIC
J/Y prod at the LHC
Solid - no breakup criteria Dashed lines 1n1n and
XnXn breakup
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Multiple meson production

• P(r0) 1 at b2RA
• w/ Poisson distribution
• P(r0r0) (1)2/2 at b2RA
• 106 r0r0 /year
• Enhancement (ala HBT) for production from same
  ion (away from y0)
• Vector meson superradiance
• toward a vector meson laser
• Dp lt h/RA
• Like production coherence
• Large fraction of pairs
• Stimulated decays?

Production with gold at RHIC
P(b)
b (fermi)
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Angular Correlations in r0r0

• r0 polarization follows the photon
• r0r0 should have parallel polarization
• pp- planes should be highly correlated

b,E
(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
• upstream interactions
• out-of-time events

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Triggering Strategies

• Triggering is the hardest part of studying UPCs!!
• Ultra-peripheral final states
• 1-6 charged particles
• low pT
• rapidity gaps
• small b events
• tagged by nuclear breakup

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Peripheral Trigger Data Collection

• Level 0
• hits in opposing CTB quadrants
• Rate 20-40 /sec
• Level 3 (online reconstruction)
• vertex position, multiplicity
• Rate 1-2 /sec
• 2000 prototype
• 9 hours of data -gt 30,000 events
• 2001 early production
• few weeks -gt 1.5M events

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Exclusive r0

• 2 tracks in interaction region
• vertex in diamond
• reject cosmic rays
• non-coplanar qlt 3 rad
• 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
r0 PT
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
• single (1n) and multiple (Xn) neutron production
• Coulomb excitation
• Giant Dipole Resonance
• Xn may include hadronic interactions?
• Measure s(1n1n) s(XnXn)

Signal region pTlt0.15 GeV
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- Modified Soding
approach Interference is significant pp-
fraction is comparable to ZEUS
ee- and hadronic backgrounds
Many other fits are possible
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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 (1999/2002)

• 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
• limited by statistics for XnXn in topology
  trigger
• Frankfurt, Strikman Zhalov predict 50 higher
  cross sections
• Good agreement
• factorization works

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The 2001 data

• 200 GeV/nucleon
• higher cross sections
• much higher luminosity
• Production triggers
• Minimum Bias data
• 10X statistics
• Topology Data
• 20X statistics
• Higher STAR B field
• 0.5 T (2001) vs. 0.25 T (2000)
• lower acceptance for low pT particles
• shorter interaction region
• sz 25 cm

• Physics
• precision r0 lineshape, pT spectra and helicity
  distribution
• s(ee-) and theory comparison
• 4-prong events (r(1450/1700)?)

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Minimum Bias r0 at 200 GeV

• 70 of minimum bias data processed so far
• 1.7 million triggers
• higher purity than 2000
• Analysis same as for 2000 data

r0 PT
r0 PT
ZDC Energy (arbitrary units)
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Minimum Bias r0 at 200 GeV
Rapidity
Mpp (GeV)
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200 GeVExclusive r0
Signal region pTlt0.15 GeV

• Almost same analysis as 130 GeV
• smaller interaction diamond
• 1.5 million triggers
• improved trigger --gt higher efficiency
• B0.5T increases trigger threshold
• fewer low Mpp pairs
• still under investigation
• Same modified Mpp Soding fit

r0 PT
Preliminary
M(pp-)
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gg --gt ee-
red - e e- pairs
p

• Minimum bias trigger
• 200 GeV
• B0.25T
• small fraction of data
• Select electrons by dE/dx
• in region plt 140 MeV/c
• Select identified pairs
• pT peaked at 1/ltbgt
• Different from photoproduced pp

e
Preliminary
P (GeV/c)
Events
ee
pp
Pair Pt (GeVc)
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STAR Near Future possibilities

• VM interference measurements
• 2-prong meson decays
• r0 at higher pT
• J/Y (needs more data)
• wider r0 rapidity distribution (using FTPCs)
• 4-prong meson spectroscopy
• r0
• r0r0

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Future Possibilities RHIC LHC

• Criteria
• Interesting Physics
• At least vaguely experimentally realizable
• More multiple vector meson production
• correlated decays?
• Photoproduction of open charm/bottom/top
• Pair production with capture
• pp diffraction meson spectroscopy and hard
  diffraction

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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
• max 10 at RHIC, 20 at LHC
• 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 c,b
g
g
Production occurs in one ion
Klein, Nystrand Vogt, 2002
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Electron Production w/ Capture

• gg -- gt ee-
• Electron is bound to nucleus
• Probe of atomic physics
• non-perturbative
• s uncertain, 100-200 barns
• Focused 78Au beam
• RHIC Rate 10,000 particles/sec
• beam 40-80 mW
• easy to measure
• LHC rate 1M particles/sec
• beam 10-40 W
• can quench superconducting magnets
• limits LHC luminosity w/ Pb
• Could extract as external beam

Za 0.6 is Ng gt 1?
Klein,2001
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pp Diffraction

• Mostly double-Pomeron interactions
• at SPS significant Reggeon component
• use Roman pots to measure momentum transfer from
  diffracted protons

p
Pomerons
p
Coupling nuclear form factor
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Meson spectroscopy with pp

• CERN WA91/102 discovered pt filter
• meson type depends on momentum transfers
• low dpt selects exotics (non qq)
• at RHIC
• much smaller Reggeon component
• polarized beams
• study spin structure of Pomeron meson
  production
• very little theoretical guidance

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Hard diffraction at RHIC

• Study jets, W,Z production ala CDF
• CDF, D0 find s(D)/s(ND) 0.01
• good rates
• polarized beams
• spin structure of Pomeron
• probe quark content (for W,Z)
• compare pp vs. pp

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Physics Recap

• RHIC is a high luminosity gg and gA collider
• The strong fields and 2-source geometry allow
  many unique studies
• Coherent events have distinctive kinematics
• Ultra-peripheral collisions allow for many
  studies
• Measurement of s(VA)
• Vector meson spectroscopy
• A measurement of the r0 pT spectrum can test if
  particle decay triggers wave function collapse
• multiple vector meson production
• Tests of strong field QED
• Studies of charge content of glueball candidates

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Conclusions

• STAR has observed three peripheral collisions
  processes
• Au Au -- gt Au Au r0
• Au Au -- gt Au Au r0
• Au Au -- gt Au Au ee-
• The cross sections for r0 production are in
  agreement with theoretical models
• Interference between r0 and direct pp- is seen
• Ultra-peripheral collisions is in its infancy
• It works!!!
• Were learning a lot about techniques
• come back next year for many more results!