Physics of Ultraperipheral Nuclear Collisions Janet Seger

May 23, Introduction to UPC physics Introduction to UPC physics Experimental results from RHIC Experimental results from RHIC Looking toward the LHC Looking toward the LHC

May 23, Ultraperipheral Nuclear Collisions Nuclei miss each other geometrically Nuclei miss each other geometrically b > R 1 + R 2 b > R 1 + R 2 Long-range electromagnetic interaction Long-range electromagnetic interaction Exchange of nearly-real photon(s) Exchange of nearly-real photon(s) Weizsacker-Williams formalism Weizsacker-Williams formalism Photon flux ~ Z 2 Photon flux ~ Z 2 Exclusive interaction Exclusive interaction Coherent emission limits p T and energy of photon Coherent emission limits p T and energy of photon Z Z b > 2R

May 23, Photon interactions   Non-pert. QED Non-pert. QED Produces lepton or quark pairs Produces lepton or quark pairs Photonuclear Photonuclear Vector Meson Dominance Vector Meson Dominance Photon fluctuates to a vector meson (  ) Photon fluctuates to a vector meson (  ) Vector meson photoproduction -- dominant coherent process Vector meson photoproduction -- dominant coherent process Incoherent processes  g,  q Incoherent processes  g,  q Shadowing, exotics Shadowing, exotics

May 23, High Photon Fluxes Photon fluxes high at ion colliders Photon fluxes high at ion colliders High probability of multiple photon exchange High probability of multiple photon exchange Vector meson can be accompanied by nuclear Coulomb excitation Vector meson can be accompanied by nuclear Coulomb excitation 3-  exchange at lowest order 3-  exchange at lowest order Coulomb excitation  neutrons Coulomb excitation  neutrons Useful for tagging UPCs Useful for tagging UPCs

May 23, Modeling Photonuclear Interactions Klein/Nystrand: Phenomenological model based on scaling data of  p to  A Starlight Monte Carlo agrees well with data Photon spectrum: Weizsäcker-Williams Input photon-nucleon data: parameterized from results at HERA and fixed target Scaling  p   A: Neglecting cross terms -  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/ J. Nystrand, S. Klein PRC 60(1999)014903

May 23, Starlight predictions No Breakup With Breakup (Xn,Xn) With Breakup (1n,1n) A.Baltz, S.Klein, J.Nystrand Phys. Rev. Lett. 89(2002)012301

May 23, Heavy Vector Mesons J/ ,   (  p  Vp) calculable from pQCD 2-gluon exchange Sensitive probe of g(x), g 2 (x) Low-mass states at high rapidity probe low 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)

May 23, Kinematic range of UPCs  at LHC J/  at LHC J/  at RHIC y=0 J/  RHICW  p = 25 GeV x ≈ 2 x LHC PbPb W  p = 130 GeV x ≈ 6 x W  p = 230 GeV x ≈ 2 x W  p : photon-proton CM energy x : Bjorken-x of gluon Q 2 = M V 2 /4

May 23, Gluon shadowing suppresses VM photoproduction Gluon shadowing suppresses VM photoproduction FSZ, Acta Physics Polonica B34 Blue = impulse approx. Red = leading twist shadowing

May 23, Gluon shadowing alters rapidity dist. FSZ, Phys Lett B540 Black  Impulse Approx. Red  Alvero et al. gluon density Blue  H1 Gluon density

May 23, Experimental Characteristics of UPCs 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 Require: good tracking, particle ID, selective triggering

May 23, Triggering on UPCs Typically require Typically require Low multiplicity Low multiplicity Dissociation of excited nucleus (neutrons in ZDC) Dissociation of excited nucleus (neutrons in ZDC) Reduces statistics but increases triggering efficiency Reduces statistics but increases triggering efficiency Sometimes include Sometimes include EM Calorimeter towers for J/psi EM Calorimeter towers for J/psi Back-to-back event topology Back-to-back event topology

May 23, UPCs at RHIC 200 GeV Au-Au collisions 200 GeV Au-Au collisions k max ~ 3 GeV, W  N ~ 35 GeV k max ~ 3 GeV, W  N ~ 35 GeV Electron pairs, vector meson photoproduction studied so far Electron pairs, vector meson photoproduction studied so far Proof of principle for UPC studies Proof of principle for UPC studies Develop trigger algorithms Develop trigger algorithms Test UPC models Test UPC models Consistent with HERA measurements Consistent with HERA measurements

May 23, Electron pairs 2-photon interaction 2-photon interaction Z  ~ 0.6 Z  ~ 0.6 Expect non-perturbative QED effects Expect non-perturbative QED effects Pair p T M inv Lowest order Higher order A. J. Baltz, Phys. Rev. Lett. 100, (2008). S T A RS T A R

May 23, Coherent  photoproduction at RHIC Select coherent events with p T < 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 ρ 0 B: amplitude for direct  +  - S T A RS T A R

May 23, Many properties consistent with ZEUS Ratio of non-resonant to resonant pion production Ratio of non-resonant to resonant pion production 200 GeV: |B/A| = 0.84 ± 0.11 GeV -1/2 200 GeV: |B/A| = 0.84 ± 0.11 GeV -1/2 130 GeV: |B/A| = 0.81 ± 0.28 GeV -1/2 130 GeV: |B/A| = 0.81 ± 0.28 GeV -1/2 No angular dependence or rapidity dependence No angular dependence or rapidity dependence s-channel helicity conservationParameterSTARZEUS ± 0.03 ± ± ± ± 0.03 ± ± 0.02 S T A RS T A R

May 23, Extend p T range for measurement of ρ 0 production Extend p T range for measurement of ρ 0 production Fit function: Fit function: Incoherent production Incoherent production d = 8.8 ±1.0 GeV -2 – access to the nucleon form factor d = 8.8 ±1.0 GeV -2 – access to the nucleon form factor Coherent production Coherent production b = ±24.8 GeV -2 – access to nuclear form factor b = ±24.8 GeV -2 – access to nuclear form factor  (incoh)/  (coh) ~ 0.29 ±0.03 Incoherent Production To the p T 2 range: (0.002,0.3) GeV 2 Coherent Incoherent S T A RS T A R

May 23, Model predictions for  cross section Klein, Nystrand: vector dominance model (VDM) & classical mechanical approach for scattering, based on γp→ρp experiments results Klein, Nystrand: vector dominance model (VDM) & classical mechanical approach for scattering, based on γp→ρp experiments results PRC 60 (1999) PRC 60 (1999) Frankfurt, Strikman, Zhalov: generalized vector dominance model + Gribov-Glauber approach Frankfurt, Strikman, Zhalov: generalized vector dominance model + Gribov-Glauber approach PRC 67 (2003) PRC 67 (2003) Goncalves, Machado: QCD dipole approach (nuclear effects and parton saturation phenomenon) Goncalves, Machado: QCD dipole approach (nuclear effects and parton saturation phenomenon) Eur.Phys.J. C29 (2003) Eur.Phys.J. C29 (2003)

May 23, Energy and A-dependence of  cross section STAR Preliminary 62 GeV Au-Au 62 GeV STAR Preliminary 200 GeV d-Au STAR Preliminary

May 23, Excited  state(s) γ Au  ρ  π + π – π + π – γ Au  ρ  π + π – π + π – STAR observes broad peak around 1510 MeV/c 2 STAR observes broad peak around 1510 MeV/c 2 May be production of excited states  (1450) and/or  (1700) May be production of excited states  (1450) and/or  (1700) STAR preliminary

May 23, J/Psi at RHIC (PHENIX) D’Enterria, nucl-ex/ dN/dm ee (background subtracted) w/ fit to (MC) expected dielectron continuum and J/Ψ signals

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

May 23, UPCs at the LHC 2.75 TeV Pb beams 2.75 TeV Pb beams k max = 81 GeV, W  p ~ 950 GeV k max = 81 GeV, W  p ~ 950 GeV Compared to RHIC: Compared to RHIC: Greater energy Greater energy Greater photon flux Greater photon flux Increased cross sections Increased cross sections Lower x Lower x

May 23, New UPC physics at the LHC Elastic Vector Meson production  +A  J  +A expected prod rate ~ 1x10 7 / year  +A   +A expected prod rate ~ 1x10 5 / year sensitive probe of g(x,Q 2 ) Photonuclear production of heavy quarks  +g  cc Photonuclear jet production; photon+parton  jet+jet; e.g.  +g  q+q R. Vogt hep-ph/ , M. Strikman, R. Vogt, S. White PRL 96(2006)

May 23, LHC detectors CMS ALICE ATLAS Very good tracking, PID Extends to p T =0.05 GeV/c, but |  | < 1 No ZDC trigger Tracking to |  | 0.2 GeV/c Good rapidity coverage– can measure rapidity gaps Tracking to |  | 0.5 GeV/c Good rapidity coverage– can measure rapidity gaps

May 23, Conclusions UPCs allow study of photon-induced interactions UPCs allow study of photon-induced interactions Low-multiplicity environment Low-multiplicity environment Can be separated from hadronic background Can be separated from hadronic background RHIC and LHC are high-luminosity  A colliders RHIC and LHC are high-luminosity  A colliders RHIC energies comparable to HERA RHIC energies comparable to HERA LHC energies will extend beyond LHC energies will extend beyond Experience at RHIC Experience at RHIC demonstrated feasibility of UPC studies demonstrated feasibility of UPC studies Developed trigger algorithms Developed trigger algorithms  and J/  cross sections  and J/  cross sections Agreement with HERA results Agreement with HERA results LHC will probe interesting new physics LHC will probe interesting new physics Higher energy, lower x Higher energy, lower x Shadowing effects, jets Shadowing effects, jets