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Photoproduction at proton and ion colliders

Photoproduction at proton and ion colliders. Spencer Klein, LBNL & UC Berkeley. Photoproduction at hadron colliders Photonuclear and gg interactions Physics from Photoproduction Unique Possibilities RHIC & Tevatron results Future Prospects: the LHC & RHIC

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Photoproduction at proton and ion colliders

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  1. Photoproduction at proton and ion colliders Spencer Klein, LBNL & UC Berkeley Photoproduction at hadron colliders Photonuclear and gg interactions Physics from Photoproduction Unique Possibilities RHIC & Tevatron results Future Prospects: the LHC & RHIC LHC is the highest energy gg/gp collider in the world Conclusions r0 photoproduction in STAR

  2. Au X g, P, or meson Au Coupling ~ form factor Photons from Hadrons • Relativistic hadrons carry strong electromagnetic fields • Weizsacker-Wiliams: a field of almost-real photons • Virtuality Q2 < (h/RA)2 • Q20 significant for e+e- production, and (maybe) with proton beams • Photon Emax ~ gh/RA • 3 GeV with gold at RHIC • 80 GeV with lead at the LHC • ~10%+ of proton energy at both machines • Photon flux ~ Z2 • Higher intensity with heavy ions • Important for considering multiple interactions between a single ion pair • RHIC & LHC have higher luminosities for light ions --> often better overall rates • “Optimum” ion depends on the problem

  3. Au Photonuclear Interactions Au g X • Photon coupling Z2a ~ 0.6 for heavy ions • Multi-photon reactions common • g-gluon/Pomeron/meson interactions • g-Pomeron/meson can be coherent • Exclusive final states • Coupling ~ A 2 (bulk)A 4/3 (surface) • Large cross-sections for vector meson production • Require b > 2RA • no hadronic interactions • <b> ~ 20-60 fermi at RHIC • Cross sections are huge • ~ 5 barns for coherent r0 production with lead at the LHC • Need plots P/gluons/meson Coupling ~ form factor

  4. gg interactions Au g X g • Rate ~s ~ Z4 • Rates to produce hadrons are much smaller than for photoproduction of vector mesons • Prolific production of e+e- pairs • s = 200,000 barns with lead at the LHC • QED process --> proposed as luminosity monitor. • Coulomb corrections are significant • Wmax ~ 2 kmax • 6 GeV with gold at RHIC • 160 GeV with lead at the LHC • Higher for lighter ions Au gg Luminosity

  5. Unique Possibilities at hadron colliders • Highly charged photon emitters • Multi-photon interactions • Impact parameter tagging • Symmetric initial state • Quantum interference • Ion interactions • Pair production with capture

  6. Factorization Au* • Multiple photons are emitted independently (Gupta, 1950) • To lowest order, interactions are independent • (neglecting quantum correlations) • True for gg + nuclear breakup with heavy ions • Seems to work well for e+e- and r0 at RHIC • Cf. Yuri Gorbunov’s talk Au g g r0 g P Au Au* G. Baur et al, Nucl. Phys. A729, 787 (2003)

  7. Au* Au g g(1+) r0 P Au Au* Impact Parameter Tagging • Nuclear excitation ‘tag’s small b • Multiple photon exchange • Mutual Coulomb excitation • Au* decay via neutron emission • Detected in zero degree calorimeters • 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

  8. Tagged Photon Spectra • Single photon exchange (low energy limit) • N(g) ~ 1/k • The presence of additional tagging photons biases the collision toward small b • For single tagging (2-photon exchange) • N(g) ~ independent of k N(k) k (GeV) Photon spectra at RHIC G. Baur et al, Nucl. Phys. A729, 787 (2003)

  9. Au* Au g 2+g r0 P Au Au* Strong Coupling/Multiple Interactions • Z2a ~ 0.6 with gold/lead • ‘Extra’ photons are cheap • Higher order diagrams could be important • Multi-photon reactions are important • They factorize • Mutual Coulomb excitation of colliding nuclei is a useful tag • Simple experimental trigger • Selects events with small impact parameters

  10. Zero Degree Calorimeters & Luminosity measurement • Mutual Coulomb dissociation is used as to monitor luminosity • Calculable in QED • Accuracy limited by uncertainty in hadronic interaction radius • Require 1 neutron in each ZDC (& no activity in central detector) • via Giant Dipole Resonance • PHENIX used dA(g) --> pnA to monitor s*luminosity • gg -->l+l- proposed at LHC • Uncertainty in hadronic interaction radius limits accuracy

  11. e+e- production w/ STAR • With Coulomb excitation in 200 GeV gold-on-gold collisions • 52 events • Mee > 140 MeV/c2, |Yee|< 1 • Equivalent photon method fails for pair pT spectrum • Photon virtuality is important • Cross-section is consistent with all-orders QED calculation • ~ Inconsistent with lowest order calculation • ~ 30% higher than all-orders in STAR acceptance Points data Solid line – LO calc Dashed – all orders Pair pT Lepton pT STAR data, calcs by A. Baltz PRL 100, 062302 (2007)

  12. e+e- production in CDF • 1.8 TeV pp collisions • 16 events with Mee > 10 GeV • Fits theoretical calculations • LPAIR • Proposed e+e- pairs as a luminosity monitor at the LHC • QED is well understood • Problem: what is the effective bmin, at which there are no hadronic interactions? CDF, PRL 98,112001 (2007)

  13. g qq Au r0 STAR - r0 Photoproduction Au • Exclusive r0 triggered w/ ‘Topology’ trigger • r0-with mutual Coulomb excitation triggered with ZDC coincidence • Select events with 2 primary tracks • + some ‘global’ tracks STAR, Phys. Rev. C77, 34910 (2008)

  14. r0 Photoproduction Mass Spectra • Coherent production events • pT < ~ 150 MeV/c • Mpp fit to r0 + direct p+p- spectrum • w/ interference term • Relativistic Breit-Wigner w/ phase space • r0:direct p+p- ratio same as gp @ HERA r0 r0:pp interference Gray -background STAR, Phys. Rev. C77, 34910 (2008) r0 + Mutual Coulomb Excitation Exclusive r0

  15. STAR r0 rapidity distribution • r0 w/ mutual Coulomb excitation • Photon energy • k = Mv/2 exp(y) • 2-fold directional ambiguity leads to symmetric distributions • Cross-section depends on sqq-N, which depends on the hadronic model • Saturation models of Goncalves & Machado is ruled out • sgA->rA(k) can be determined by combining exclusive r and r w/ mutual Coulomb excitation • Errors are large STAR, Phys. Rev. C77, 34910 (2008)

  16. STAR r0 pT spectra • Coherent + Incoherent form factors • Fit to dual exponential • Incoherent production • bN = 8.8 ±1.0 GeV-2 • nucleon form factor • Coherent production • bAu = 388.4 ±24.8 GeV-2 • bAu~RA2 • Data sensitive to RA • Measure hadronic radius of gold • 3% statistical uncertainty • Significant theoretical corrections/uncertainty • s(incoh)/s(coh) ~ 0.29 ±0.03 • Extrapolated form factor to t=0 pT2 ~ t STAR, Phys. Rev. C77, 34910 (2008)

  17. + 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.

  18. 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)

  19. 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

  20. X/p meas. X/p meas. Einstein-Podolsky-Rosen paradox • For each p+from r0 decay, can choose to measure x or p • Both x - localize production point (sometimes) • Both p - measure p of VM; see interference; delocalize production • The p+p- system is an example of the Einstein-Podolsky-Rosen paradox • (Gedanken) test for superluminal collapse Klein & Nystrand, 2003 r0 p+ p- b r0 p+ p-

  21. Interference and Nuclear Excitation • Smaller <b> --> interference at higher <pT> • Suppression for pT < h/<b> Noint ~ exp(-bt) Int. Noint ~ exp(-bt) Int. dN/dt dN/dt t t No breakup criteria XnXn r0 prod at RHIC: 0.1 < |y| < 0.6

  22. Interference Analysis • ‘topology’ (multiplicity+) trigger for exclusive r0, • Neutrons in both ZDCs trigger for mutual Coulomb excitation. • Tight cuts -- > clean r0, but lower efficiency • Exactly 2 tracks • 1 vertex with Q=0 • 0.55 & GeV<Mpp<0.92 GeV • Fit dN/dt spectrum • Require accurate dN/dt w/o interference • Efficiency is ~ independent of t • Momentum smearing affects two low-t bins • Included in Monte Carlo • Interference is maximal at y=0, decreasing as |y| rises • 2 rapidity bins 0/0.05 < |y| < 0.5 & 0.5<|y|<1.0

  23. r0 pT spectrum in UPCs • Scattering (Pomeron) pT • Modified nucleon form factor • Glauber calculation for absorption • position-dependent photon flux • Photon pT • Weizsacker-Williams + form factor • Add components in quadrature • w/o interference dN/dt ~ exp(-bt) • b = x * RA2 • STAR simulations • Woods-Saxon + Glauber calculation • (H. Alvensleben et al., 1970) • Modifications change x • Interference • Only cause for dN/dt -->0 at small t Woods-Saxon Form Factor

  24. Analysis Technique dN/dt for XnXn, 0.1<|y|<0.6 Data (w/ fit) Noint Int LS Background dN/dt (a.u.) • 2 Monte Carlo samples: • Interference • No interference • w/ detector simulation • Detector Effects Small • Data matches Int • Inconsistent with Noint • Interference clearly observed • 973 events • Fit to dN/dt = A exp(-kt)* [1+c(R(t)-1)] • Exponential for nuclear form factor • R = Int(t)/NoInt(t) • Separates nuclear form factor (exponential) & interference STAR Preliminary t (GeV2) = pT2 R = Int(t)/Noint(t) t

  25. Results • Efficiency corrected spectra • Two rapidity bins, 2 triggers • Cut |y|<0.05 for topology to remove cosmic rays

  26. Results • c2/DOF > 1 for 2 samples • Much study, no answers… • Scale errors up by  c2/DOF (PDG prescription) • Systematic errors due to trigger (10% for topology), other detector effects (4%), background (1%), fitting & nuclear radius(4%), theoretical issues (5%) • Final, combined result: c=0.87 ± 0.05 (stat.) ± 8 (syst.)% • Limit on decoherence, due to decay or other factors < 23% at a 90% confidence level

  27. k values Au Ng(b) ~ 1/(b-RA)2 Ng(b) ~ 1/b2 Ng(b) ~ 1/(b+RA)2 Au • photon flux ~ 1/b2 • P(b) ~ Ng (b)r(b) • For b ~ few RA, production is asymmetric • Smaller apparent size • Small change in s(tot) • <b> ~ 3 RA for minimum bias data • F(t) is Fourier transform of production density • Parameter b ~ (diameter)2 • Affects the interference pattern • Impact parameter beff < bgeom • Neglected here A linear (toy) model Woods-Saxon (b) WS weighted by 1/b2 P(b) = r(b)Ng(b) b (fm)

  28. STAR r0 photoproduction in deuteron-gold interactions • Photon is almost always emitted by gold • No two-fold ambiguity--> easy s extraction • gd --> dr0, pnr0 both seen • Deuteron dissociation tagged by neutron in Zero Degree calorimeter • Allows cleaner trigger • Coherent reaction has larger t-slope • Incoherent process reduced at small t • Not enough momentum transfer for dissociation Neutron-tagged data t, GeV2

  29. STAR STAR STAR preliminary Photoproduction of pppp • Expected to be largely through a radially excited r • r(1450) and/or r(1700) • Peak at low pT from coherent enhancement • Studies of resonant substructure are in progress Mpppp (GeV) pT(GeV/c) B. Grube, Wkshp. on HE Photon Collisions at the LHC

  30. J/y photoproduction at RHIC • e+e- pair + 1 nucleus breakup • Nuclear breakup needed for trigger • J/y + continuum gg --> e+e- • ~ 12 events in peak • Cross sections for both J/y and continuum e+e- ~ as expected D’Enterria, PHENIX, nucl-ex/0601001

  31. J/y photoproduction at the Tevatron • CDF selects exclusive J/y photoproduction is sensitive to gluon structure of nuclei • s ~ |g(x,MV2/4)|2 • 334 exclusive m+m- signal events. • “Background” from double Pomeron production • cc --> gJ/y • Some y’ • Cross section determination in progress M(mm) (GeV/c2) J. Pinfold, Wkshp. on HE Photon Collisions at the LHC

  32. UPCs at the LHC • CMS, ALICE and ATLAS plan programs • “Yellow Book” gives physics case • K. Hencken et al., Phys. Rept. 458, 1 (2008). • Gluon structure Functions at low-x • Including saturation tests • The ‘black disc’ regime of QCD • Search for exotica/new physics • gg --> Higgs, Magnetic monoples, etc. • Some techniques, and some final states overlap with pp diffraction (double-Pomeron) studies • Roman pots can be used for full kinematic reconstruction in pp • t_perp can help separate gg, gP and PP interactions

  33. Structure Functions at the LHC • Many photoproduction reactions probe structure functions • g --> qq; the quarks interact with target gluons • Q2 ~ (Mfinal state/2)2 • x ~ 10-4 at midrapidity • x ~ 10-6 in forward regions • J/y, y’,U states • Open charm/bottom/top? • Dijets • The twofold ambiguity can be avoided using pA collisions or by comparing ds/dy with and w/o mutual Coulomb excitation

  34. J/y photoproduction • s ~ g(x,Q2)2 • x ~ few 10-4 for J/y@ the LHC • x ~ few 10-2 for J/y@ RHIC • Q2 ~ Mv2/4 • Coherent production • pT < h/RA • High rates • 3.2 Hz production with Pb • Detection is easy • Shadowing is ~ factor of several • Theoretical uncertainties cancel in s(pp)/s(AA) or s(pA) y y M. Strikman, F. Strikman and M. Zhalov, PL B540, 220 (2002)

  35. Dijets • With calorimetery to |y|<3, probe down to x~10-4 in 1 month • Use standard jet triggers Rate/bin/month @ L=4*1026/cm2/s M. Strikman, R. Vogt and S. White, PRL86, 082001 (2006)

  36. “Black Disk” Regime • At high enough energies/low enough x, nuclei look like black (absorptive) disks • Studies of HERA data suggest that the Black Disk Regime (BDR) should be visible at the LHC for Q2 < 4 GeV2 • Photons fluctuate to qq dipoles, with separation d • sdipole-target ~ pd2 • Smaller and smaller dipoles interact • Photons contain many small dipoles --> increasing number of interactions --> stot (gp) rises rapidly • Increase in high pT interactions • Fraction of diffractive events increases M. Strikman

  37. e+e- pairs at the LHC • s ~ 200,00 barns with Pb beams • Significant background for small-radius detectors (also @RHIC) • 20 pairs/beam crossing (at 1027/cm2/s & 100 nsec crossing) • e+, e- have small pT (typically me) • Hard to trigger • ALICE strategy – random events • Expect ~ MHz rates in inner detectors • Measure cross-section, Mee, pT spectra, etc. • Multiple pairs likely from 1 collision

  38. LHC – Plans & Issues • J/y,y’, U --> leptons is relatively easy • CMS, ALICE, ATLAS are pursuing • Di-jets • ATLAS is pursuing • Triggering is problematic • Beam gas, neutrons, grazing hadronic collisions... even cosmic ray muons • ZDC coincidences might help with backgrounds • Not always available at Level 0 • FP420, TOTEM & other forward detectors • Use tagging to select gg--> Higgs, sleptons…

  39. Pb 81+ IP Pb 82+ Bound Free Pair Production • A + A --> A + Ae- + e+ • 1e- atom has lower Z/A • Less bending in dipoles • Momentum ~ unchanged --> beam • s ~ 280 barns w/ lead at the LHC • 280,000 ions/s at L = 1027/cm2/s • 28 watts! • Hits beampipe ~ 136 m from the IP • Enough energy to quench superconducting magnets? • Limits LHC luminosity w/ heavy ions • Limit is ~ planned luminosity • Quench calculations are touchy SK, NIM A459, 51 (2001); R. Bruce et al. PRL 99, 144801 (2007)

  40. Observation of BFPP at RHIC • Copper beams at RHIC • stheory ~ 200 mb • D(magnetic rigidity) ~ 1/29 • 1eAu defocuses before striking beampipe • 1eCu beam hits beampipe ~ 144 m from interaction point • Look for showers using pre-existing PIN diodes • Small enough that acceptance simulations are problematic • Observe correlations between luminosity (measured at IP) and PIN diode rates R. Bruce et al. PRL 99, 144801 (2007)

  41. RHIC (ZDC) luminosity monitor BFPP results Colors are PIN photodiode signals at different locations Blue is optimal location BFPP has been observed, with cross section ~ expected

  42. Future Directions at RHIC • STAR “DAQ1000” increases data rate by a factor of 10 & TOF system improves particle identification • Substructure in 4-pion events • r0r0 pairs • Roman pots for STAR • Mostly for pp diffraction, but might be able to measure scattered deuteron in dA collisions, improving UPC reconstruction • Phase I optimized for elastic scattering • Requires special low-b beam optics • Phase II has larger acceptance

  43. r0r0 production 4 diagrams interfere • Depends on pT sum and difference between two r • Away from y=0, the top and bottom diagrams dominate • Stimulated emission at low pT • Possibility to look for stimulated decays • Needs more luminosity, and background rejection from r’ decays, but looks possible

  44. Angular Correlations in r0r0 & mutual nuclear excitation to Giant Dipole Resonances • ~ any states decaying via a vector decay • In linearly polarized r0 decays, the angle between the r0 polarization and the p+/p- pT (wrt the direction of motion) follows cos(f) • p+/p- direction acts as an analyzer • The r0 polarization follows the g polarization • The angle between the p+/p- pT in r0r0 decays is the convolution of the two cos(f) distributions • C(Df) = 1 + 1/2cos(2Df) • Possible way to study linear polarization Angle between p+/p-pT in double VM decays G. Baur et al.,Nucl. Phys. A729, 787 (2003)

  45. Conclusions • Many photoproduction reactions can be studied at hadron colliders. • STAR & PHENIX at RHIC, and CDF at the Tevatron have studied a variety of topics, including vector meson production and e+e- pair production. • Many unique reactions can be studied with heavy ion collisions. • Multiple interactions among a single ion pair. • Bound-free e+e- production will limit the LHC luminosity with lead. • The LHC and RHIC have promising future programs. • Until the ILC is complete, the LHC will be the highest energy gg and gp collider in the world.

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