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

Explore ultra-peripheral collisions with impact parameter tagging, multiple interactions, and unique features like strong electromagnetic fields, vector meson production, and exclusive r0 production in Au nuclei.

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

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  1. 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 r0Results at 130 GeV/nucleon: Preliminary r0 and e+e- Results from 200 GeV UPC Future Prospects Conclusions

  2. Au g, P, or meson Au Coupling ~ nuclear form factor Coherent Interactions • b > 2RA; • no hadronic interactions • <b> ~ 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/p> RA • amplitudes add with same phase • P < h/RA, ~30 MeV/c for heavy ions • P|| < gh/RA ~ 3 GeV/c at RHIC • Strong couplings --> large cross sections

  3. Unique Features of Ultra-peripheral collisions • Very strong electromagnetic fields • g --> e+e- and g --> qq • Multiple production • Coherent Decays • Unique Geometry • 2-source interferometer • Nuclear Environment • Particle Production with capture • Large s for e-

  4. VM g Production occurs in/near one ion Specific Channels • Vector meson production gA -- > ’ r0, w, f, J/y,… A • Production cross sections --> s(VN) • Vector meson spectroscopy (r*, w*, f*,…) • Wave function collapse • Multiple Vector Meson Production & Decay • Electromagnetic particle production gg -- > leptons,mesons • Strong Field (nonperturbative?) QED • Za ~ 0.6 • meson spectroscopy Ggg • Ggg ~ charge content of scalar/tensor mesons • Ggg is small for glueballs e+e-, qq,... gs Za ~ 0.6; is Ng > 1?

  5. Au g qq Au r0 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 > 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

  6. RHIC - Au HERA data + Glauber Klein & Nystrand, 1999 HERA param. 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)

  7. Shadowing & Hard Pomerons RHIC - Au • 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 No shadowing HERA param. ds/dy Leading Twist Calculation Frankfurt, Strikman & Zhalov, 2001 Shadowed Y = 1/2 ln(2k/MV)

  8. Au* Au g g(1+) r0 P Au Au* Multiple Interactions • Za ~ 0.6 • Ng >1 quite likely • Multiple Interactions • Photon emission is independent • S. N. Gupta (1950) • (Mostly) different interactions factorize

  9. Au* Au g g(1+) r0 P Au Au* Nuclear Excitation • Nuclear excitation ‘tag’s small b • Multiple photon exchange • Mutual excitation • Au* decay via neutron emission • simple, unbiased trigger • Multiple Interactions probable • P(r0, b=2R) ~ 1% at RHIC • P(2EXC, b=2R) ~ 30% • Non-factorizable diagrams are small for AA

  10. r0 with Gold @ RHIC P(b) b [fm] Photonuclear Interaction Probabilities & ds/dy r0 with gold @ RHIC • Excitation + r0 changes b distribution • alters photon spectrum • low <b> --> high <k> ds/dy y Exclusive - solid X10 for XnXn - dashed X100 for 1n1n - dotted Baltz, Klein & Nystrand (2002)

  11. gg with nuclear breakup dN/dy for f2(1270) gold at RHIC XnXn breakup • 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 Rapidity no breakup

  12. gg --> m+m- w / nuclear breakup gg --> m+m- gold at RHIC • Smaller <b> • harder photon spectrum • harder Mmm spectrum • s(XnXn), s(1n1n)/s(tot) ratio smaller than f2(1270) • smaller avg. mass XnXn nobreak XnXn breakup Mmm(GeV)

  13. + r - 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 -- > p+p- as ‘analyzer • p+p- plane tends to parallel photon pol. • Study VM production angles, single spin asymmetries in gA, etc. b,E (transverse view)

  14. + b,E r - (transverse view) 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 -- > p+p- as ‘analyzer • p+p- decay plane often parallels photon pol. • reasonable analyzing power • Study VM production angles, single spin gA asymmetries, etc.

  15. 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 - A2eip·b|2 • b is unknown • For pT << 1/<b> • destructive interference No Interference Interference y=0 r0 -->p+p- pT (GeV/c)

  16. e+ J/Y e- + b J/Y + - (transverse view) 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

  17. Interference and Nuclear Excitation • Smaller <b> --> interference at higher <pT> r0 prod at RHIC J/Y prod at the LHC Solid - no breakup criteria Dashed lines 1n1n and XnXn breakup

  18. Multiple meson production Production with gold at RHIC • P(r0) ~ 1% at b=2RA • w/ Poisson distribution • P(r0r0) ~ (1%)2/2 at b=2RA • ~ 106r0r0 /year • Enhancement (ala HBT) for production from same ion (away from y=0) • Vector meson superradiance • toward a vector meson laser • Dp < h/RA • Like production coherence • Large fraction of pairs • Stimulated decays? P(b) b (fermi)

  19. + + r r - - Angular Correlations in r0r0 • r0 polarization follows the photon • r0r0 should have parallel polarization • p+p- planes should be highly correlated b,E (transverse view)

  20. r0 Analysis • Exclusive Channels • r0 and nothing else • 2 charged particles • net charge 0 • Coherent Coupling • SpT < 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

  21. 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

  22. 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 -> 30,000 events • 2001 early production • few weeks -> 1.5M events

  23. Exclusive r0 Signal region: pT<0.15 GeV • 2 tracks in interaction region • vertex in diamond • reject cosmic rays • non-coplanar; q< 3 rad • peak for pT < 150 MeV/c • p+p+ andp-p- give background shape • p+p- pairs from higher multiplicity events have similar shape • scaled up by 2.1 • high pTr0 ? • asymmetric Mpp peak r0 PT M(p+p-)

  24. ‘Minimum Bias’ Dataset Signal region: pT<0.15 GeV • Trigger on neutron signals in both ZDCs • ~800,000 triggers • Event selection same as peripheral • p+p+ andp-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) r0 PT ZDC Energy (arbitrary units)

  25. p- p- r0 p+ p+ g g gA -- > p+p- A gA -- > r0A -- > p+p- A 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) • p+p- : r0 ratio should depend on s(pA):s(rA) • decrease as A rises?

  26. r0 lineshape ZEUS gp --> (r0 + p+p- )p STAR gAu --> (r0 + p+p- )Au* ds/dMpp (mb/GeV) ds/dMpp (mb/GeV) Preliminary Mpp Mpp Fit to r0 Breit-Wigner + p+p- Modified Soding approach Interference is significant p+p- fraction is comparable to ZEUS e+e- and hadronic backgrounds Many other fits are possible

  27. Nucl.Breakup 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

  28. 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

  29. 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(e+e-) and theory comparison 4-prong events (r*(1450/1700)?) The 2001 data

  30. 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)

  31. Minimum Bias r0 at 200 GeV Rapidity Mpp (GeV)

  32. 200 GeVExclusive r0 Signal region: pT<0.15 GeV • Almost same analysis as 130 GeV • smaller interaction diamond • 1.5 million triggers • improved trigger --> higher efficiency • B=0.5T increases trigger threshold • fewer low Mpp pairs • still under investigation • Same modified Mpp Soding fit r0 PT Preliminary M(p+p-)

  33. gg --> e+e- red - e+ e- pairs p e • ‘Minimum bias trigger • 200 GeV • B=0.25T • small fraction of data • Select electrons by dE/dx • in region p< 140 MeV/c • Select identified pairs • pT peaked at 1/<b> • Different from photoproduced pp Preliminary P (GeV/c) Events ee pp Pair Pt (GeVc)

  34. 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 • r*0 • r0r0

  35. 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

  36. Photoproduction of Open Quarks QQ--> open c,b g • gA --> ccX, bbX • sensitive to gluon structure function. • Higher order corrections problematic • Ratio s(gA)/s(gp) --> shadowing • removes most QCD uncertainties • max ~10% at RHIC, ~ 20% at LHC • Experimentally feasible (?) • high rates • known isolation techniques • Physics backgrounds are gg--> cc, gg --> cc • gg cross section is small • gg background appears controllableby requiring a rapidity gap g Production occurs in one ion Klein, Nystrand & Vogt, 2002

  37. e- gs e+ Electron Production w/ Capture • gg -- > e+e- • 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 > 1? Klein,2001

  38. pp Diffraction p • Mostly double-Pomeron interactions • at SPS significant Reggeon component • use Roman pots to measure momentum transfer from diffracted protons Pomerons p Coupling ~ nuclear form factor

  39. 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

  40. 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

  41. 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

  42. Conclusions • STAR has observed three peripheral collisions processes • Au + Au -- > Au + Au + r0 • Au + Au -- > Au* + Au* + r0 • Au + Au -- > Au* + Au* + e+e- • The cross sections for r0 production are in agreement with theoretical models • Interference between r0 and direct p+p- is seen • Ultra-peripheral collisions is in it’s infancy • It works!!! • We’re learning a lot about techniques • come back next year for many more results!

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