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Physics of Ultraperipheral Nuclear Collisions

Physics of Ultraperipheral Nuclear Collisions. Janet Seger. Introduction to UPC physics Experimental results from RHIC Looking toward the LHC. Ultraperipheral Nuclear Collisions. Z. b > 2R. Z. Nuclei miss each other geometrically b > R 1 + R 2

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Physics of Ultraperipheral Nuclear Collisions

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  1. Physics of Ultraperipheral Nuclear Collisions Janet Seger

  2. Introduction to UPC physics • Experimental results from RHIC • Looking toward the LHC

  3. Ultraperipheral Nuclear Collisions Z b > 2R Z • Nuclei miss each other geometrically • b > R1 + R2 • Long-range electromagnetic interaction • Exchange of nearly-real photon(s) • Weizsacker-Williams formalism • Photon flux ~ Z2 • Exclusive interaction • Coherent emission limits pT and energy of photon

  4. Photon interactions • gg • Non-pert. QED • Produces lepton or quark pairs • Photonuclear • Vector Meson Dominance • Photon fluctuates to a vector meson (r, w, f) • Vector meson photoproduction -- dominant coherent process • Incoherent processes gg, gq • Shadowing, exotics

  5. High Photon Fluxes • Photon fluxes high at ion colliders • High probability of multiple photon exchange • Vector meson can be accompanied by nuclear Coulomb excitation • 3-g exchange at lowest order • Coulomb excitation  neutrons • Useful for tagging UPCs

  6. Modeling Photonuclear Interactions Klein/Nystrand: Phenomenological model based on scaling data of gp to gA Starlight Monte Carlo agrees well with data • Photon spectrum: • Weizsäcker-Williams • Input photon-nucleon data: • parameterized from results at HERA and fixed target • Scaling gp  gA: • Neglecting cross terms - g fluctuates into V which scatters elastically • Shadowing through a Glauber model • nuclear momentum transfer from form factor (excellent analytical parameterization) J. Nystrand, S. Klein nucl-ex/9811007 J. Nystrand, S. Klein PRC 60(1999)014903

  7. Starlight predictions No Breakup With Breakup (Xn,Xn) With Breakup (1n,1n) A.Baltz, S.Klein, J.Nystrand Phys. Rev. Lett. 89(2002)012301

  8. Heavy Vector Mesons • J/Y,U • s(gpVp) calculable from pQCD • 2-gluon exchange • Sensitive probe of g(x), g2(x) • Low-mass states at high rapidity probe low x Ryskin, Roberts, Martin, Levin, Z. Phys C 76 (1997) 231, Frankfurt LL, McDermott MF, Strikman M, J. High Energy Physics 02:002 (1999) and Martin AD, Ryskin MG, Teubner T Phys.Lett. B454:339 (1999)

  9. Kinematic range of UPCs U at LHC J/Y at LHC J/Y at RHIC • Wgp: photon-proton CM energy • x : Bjorken-x of gluon • Q2= MV2/4

  10. Gluon shadowing suppresses VM photoproduction Blue = impulse approx. Red = leading twist shadowing FSZ, Acta Physics Polonica B34

  11. Gluon shadowing alters rapidity dist. FSZ, Phys Lett B540 Black  Impulse Approx. Red  Alvero et al. gluon density Blue  H1 Gluon density

  12. Low central multiplicities “cleaner” than hadronic collisions Zero net charge Low total transverse momentum Low virtualities Narrow dN/dy peaked at mid-rapidity Large probability of multiple electromagnetic interactions Coulomb excitations Emission of neutrons Experimental Characteristics of UPCs Require: good tracking, particle ID, selective triggering

  13. Triggering on UPCs • Typically require • Low multiplicity • Dissociation of excited nucleus (neutrons in ZDC) • Reduces statistics but increases triggering efficiency • Sometimes include • EM Calorimeter towers for J/psi • Back-to-back event topology

  14. UPCs at RHIC • 200 GeV Au-Au collisions • kmax ~ 3 GeV, WgN ~ 35 GeV • Electron pairs, vector meson photoproduction studied so far • Proof of principle for UPC studies • Develop trigger algorithms • Test UPC models • Consistent with HERA measurements

  15. Electron pairs STAR Pair pT Minv • 2-photon interaction • Za ~ 0.6 • Expect non-perturbative QED effects Lowest order Higher order A. J. Baltz, Phys. Rev. Lett. 100, 062302 466 (2008).

  16. Coherent r photoproduction at RHIC STAR • Select coherent events with pT < 0.15 GeV/c • Mass distribution fit with • Breit-Wigner signal • Söding interference term for direct +- production • Second order polynomial to describe background A: amplitude for ρ0 B: amplitude fordirect +-

  17. Many properties consistent with ZEUS STAR • Ratio of non-resonant to resonant pion production • 200 GeV: |B/A| = 0.84 ± 0.11 GeV -1/2 • 130 GeV: |B/A| = 0.81 ± 0.28 GeV -1/2 • No angular dependence or rapidity dependence • s-channel helicity conservation

  18. Incoherent Production STAR • Extend pT range for measurement of ρ0production • Fit function: • Incoherent production • d = 8.8 ±1.0 GeV-2– access to the nucleon form factor • Coherent production • b = 388.4 ±24.8 GeV-2 – access to nuclear form factor • s(incoh)/s(coh) ~ 0.29 ±0.03 Incoherent Coherent To the pT2 range: (0.002,0.3) GeV2

  19. Model predictions for r cross section • Klein, Nystrand: vector dominance model (VDM) & classical mechanical approach for scattering, based on γp→ρp experiments results • PRC 60 (1999) 014903 • Frankfurt, Strikman, Zhalov: generalized vector dominance model + Gribov-Glauber approach • PRC 67 (2003) 034901 • Goncalves, Machado: QCD dipole approach (nuclear effects and parton saturation phenomenon) • Eur.Phys.J. C29 (2003) 271-275

  20. Energy and A-dependence of r cross section 62 GeV Au-Au 200 GeV d-Au STAR Preliminary STAR Preliminary STAR Preliminary 62 GeV

  21. Excited r state(s) STAR preliminary • γAu ρπ+ π– π+ π– • STAR observes broad peak around 1510 MeV/c2 • May be production of excited states r(1450) and/or r(1700)

  22. dN/dmee (background subtracted) w/ fit to (MC) expected dielectron continuum and J/Ψ signals J/Psi at RHIC (PHENIX) D’Enterria, nucl-ex/0601001

  23. Large error bars! Need more/better data Comparison with Theory D’Enterria, nucl-ex/0601001 Strikman, et al., Phys. Lett B626

  24. UPCs at the LHC • 2.75 TeV Pb beams • kmax = 81 GeV, Wgp ~ 950 GeV • Compared to RHIC: • Greater energy • Greater photon flux • Increased cross sections • Lower x

  25. New UPC physics at the LHC • Elastic Vector Meson production • g+A J/Y +A • expected prod rate ~ 1x107/ year • g+A U +A • expected prod rate ~ 1x105/ year • sensitive probe of g(x,Q2) • Photonuclear production of heavy quarks • g+gcc • Photonuclear jet production; photon+partonjet+jet; e.g. g+g  q+q • R. Vogt hep-ph/0407298, M. Strikman, R. Vogt, S. White PRL 96(2006)082001.

  26. LHC detectors ALICE CMS ATLAS Very good tracking, PID Extends to pT =0.05 GeV/c, but |h| < 1 No ZDC trigger Tracking to |h| < 2.4, but pT > 0.2 GeV/c Good rapidity coverage– can measure rapidity gaps Tracking to |h| < 2.4, but pT > 0.5 GeV/c Good rapidity coverage– can measure rapidity gaps

  27. Conclusions • UPCs allow study of photon-induced interactions • Low-multiplicity environment • Can be separated from hadronic background • RHIC and LHC are high-luminosity gA colliders • RHIC energies comparable to HERA • LHC energies will extend beyond • Experience at RHIC • demonstrated feasibility of UPC studies • Developed trigger algorithms • r and J/Y cross sections • Agreement with HERA results • LHC will probe interesting new physics • Higher energy, lower x • Shadowing effects, jets

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