Measuring Parton Densities with Ultra-Peripheral Collisions in ALICE Photonuclear Interactions Measuring Structure Functions Vector Mesons Open Charm/Bottom Other Channels Experimental Feasibility in ALICE Spencer Klein, LBNL

S. Klein, LBNL A word from your sponsor: Why use UPCs to study parton distributions? n Several reactions are directly sensitive to gluon distributions u Directly probe ‘new phases of matter’ like colored glass condensates. u Understand initial state for central collisions n ~ 10X photon energies than even HERA u Much lower x than fixed target experiments n Smaller (or at least different) systematics than pA collisions

S. Klein, LBNL Photonuclear Interations n One nucleus emits a photon u Weizsacker-Williams flux n The photon fluctuates to a qq pair n The pair interacts with the other nucleus n If b>2R --> No accompanying hadronic interactions n Possible Reactions u Elastic qq Scattering --> Vector Meson Production  , J/ ,  ’,  (1s),  (2S),… F For heavy Quark mesons, production is sensitive to parton densities u If qq = cc or bb --> Heavy quark photoproduction u Diject production Au X  qq

S. Klein, LBNL Vector Meson Production n Quark-nucleus elastic scattering u Coherent enhancement to cross section Light vector mesons ( , ,  *…) u Soft (optical model) Pomeron Heavy vector mesons (J/ ,  ’,  …) u Probe short distance scales u Scattering may be described via 2-gluon exchange F Sensitive to gluon distribution at x = M V /2  m p exp(± y) Q 2 ~ M V 2 Au V  qq

S. Klein, LBNL The effect of shadowing  (VM) ~ |g(x,Q 2 )| 2 n Shadowing reduces cross section u Factor of 5 for one standard shadowing model n Rapidity maps into x  y =  ln(2x  m p /m V )  x= m V /2  m p exp(  y) n A colored glass condensate could have a larger effect No shadowing FGS shadowing Frankfurt, Strikman & Zhalov, 2001 y d  /dy, mb

S. Klein, LBNL The 2-fold ambiguity n Which nucleus emitted the photon?  X or k ~ m V /2  m p exp(  y) F Photon fluxes, cross sections for two different photon energies (directions) are different n 2 Solutions u Stick to mid-rapidity  For J/  at y=0, x= 6*10 -4 u Compare two reactions F Pb + Pb --> Pb + Pb + J/Y F Pb + Pb --> Pb* + Pb* + J/Y F Different reactions have different photon flux ratios System of 2 linear equations Au V  qq Au V  qq Or…

S. Klein, LBNL J/  --> 3.2 Hz --> 3M/ 10 6 s  ~ 300,000  ’ and 17,000  (1S) 36,000 J/  --> e + e - with |y|<1 (ALICE central)  ~360  ’ and ~100  (1s) 27,000 J/  --> e + e - with 2.5 <y< 4 (ALICE  spect.)  ~ 270  ’ and ~75  (1s) n Rates are higher with lighter ions (higher L AA ) Rates (w/o shadowing)

S. Klein, LBNL AA vs. pA vs. pp Exclusive J/  production can be studied in AA, p/dA and pp collisions u Rates are high with all species u With it’s distinctive signature, signal to noise ratio should be high for all species u In p/dA collisions the photon usually comes from the nucleus, and strikes the proton/deuteron. F Avoids two-fold ambiguity for proton targets n Measure parton distributions in protons and nuclei n Compare reactions --> low-systematics measurement of shadowing

S. Klein, LBNL Reconstruction n Exactly 2 tracks in event n p T < 150 MeV/c u leptons are nearly back- to-back n PID as leptons u 1 e in PHOS + 1 e in EMCAL? u 2 muons in spectrometer PHENIX, QM2004

S. Klein, LBNL Triggering n For UPCs, Level 0 cannot use multiplicity trigger u Need L0 from muon spectrometer (present?) or calorimetry or TRD n L1 from and/or muon spectrometer & calorimetery/TRD n L2 due to dileptons (+ low multiplicity?) n N.b. Many ‘central collision’ analyses will face the same problem with pp collisions

S. Klein, LBNL Open Charm/Bottom n Occurs via gluon exchange u Well described in QCD (& HERA) u Sensitive to gluon distribution F x and Q 2 depend on final state configuration (M(QQ)…) Wider range of Q 2 than vector meson production n High rates   (charm) ~ 1.8 barns ~ 25% of  hadronic  S(bottom) ~ 700  b ~ 1/10,000 of  hadronic u (without shadowing) n One nucleus breaks up Au V  qq Au Nuclear debris

S. Klein, LBNL Charm kinematic distributions Single Quark p T Single Quark y 1 Pair Mass (GeV) Solid – Total Dashed – Direct/2 Dot-Dashed – Resolved Photon Contribution 2 sets of curves are with EKS98 (small) and w/o shadowing SK, Joakim Nystrand & Ramona Vogt, 2002

S. Klein, LBNL Charm Reconstruction n cc reconstruction n Well-understood techniques u Direct Reconstruction of charm F Should be feasible in ALICE F Probably can’t reconstruct both c and cbar u Semi-leptonic decay F Should be able to detect leptons from dual semi-leptonic decay

S. Klein, LBNL Separating Photoproduction and Hadroproduction n Photoproduction of charm is very favorable  P(  --> cc) ~ 4/10 n Cross Sections are large   (charm photoproduction) ~ 1.8 barns F For lead at the LHC F No shadowing   (charm hadroproduction) ~ 240 barns SK, Joakim Nystrand & Ramona Vogt, 2002

S. Klein, LBNL Rejecting Hadroproduction n Single nucleon-single hadronic interactions u Eliminate more central reactions with multiplicity cut   1n1n ~ 700 mb   1n1n, with charm ~ 5.5 mb   b>2R with charm ~ 1100 mb n One nucleus remains intact u Assume <10% chance n Particle-free rapidity gap around photon emitter  P~ exp(-  y dN/dy) F dN ch /dy ~ 4.4 at midrapidity for pp the LHC 40% lower at large |y|  For  y= 2 units, P ~   remaining, with charm ~ 3  b SK, Joakim Nystrand & Ramona Vogt, 2002

S. Klein, LBNL Triggering n Direct Charm reconstruction u Trigger is tough n Semileptonic Decays u Use leptons, as with vector mesons u May get some usage out of multiplicity detectors

S. Klein, LBNL Other channels n Dijets u Large rates   (E T > 20 GeV) ~ 1/minute u Separating photoproduction and hadroproduction seems difficult n Photon + jet (Compton scattering)   (1% of dijet rate) Ramona Vogt, 2004

S. Klein, LBNL Other UPC studies in ALICE n Photoproduction of W + W -  Study  WW vertex  --> Higgs? u Rate is low, but this is important in establishing the nature of the Higgs n Vector Meson Spectroscopy u If trigger problem can be solved n e + e - pair production u A test of strong field QED u Enormous rates (13,000 barns in SPD1) F A trigger is probably not needed

S. Klein, LBNL Conclusions n UPC photoproduction can be used to measure the parton distributions in ions and protons, and test colored glass condensate models of nature. n Both vector meson and open charm can be studied with ALICE. u The analyses seem rather straightforward. n Attention to the trigger will be required to collect the data required for these analyses. n Many other UPC studies are possible