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Measuring Parton Densities with Ultra-Peripheral Collisions in ALICE

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

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Measuring Parton Densities with Ultra-Peripheral Collisions in ALICE

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

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

  3. Au g qq Au X Photonuclear Interations • One nucleus emits a photon • Weizsacker-Williams flux • The photon fluctuates to a qq pair • The pair interacts with the other nucleus • If b>2R --> No accompanying hadronic interactions • Possible Reactions • Elastic qq Scattering --> Vector Meson Production • r, f, w, J/y, y’, U(1s), U(2S),… • For heavy Quark mesons, production is sensitive to parton densities • If qq = cc or bb --> Heavy quark photoproduction • Diject production S. Klein,LBNL

  4. Au qq g V Au Vector Meson Production • Quark-nucleus elastic scattering • Coherent enhancement to cross section • Light vector mesons (r,w,f,r*…) • Soft (optical model) Pomeron • Heavy vector mesons (J/y, y’, U…) • Probe short distance scales • Scattering may be described via 2-gluon exchange • Sensitive to gluon distribution at • x = MV/2gmp exp(± y) • Q2 ~ MV2 S. Klein,LBNL

  5. The effect of shadowing • s(VM) ~ |g(x,Q2)|2 • Shadowing reduces cross section • Factor of 5 for one standard shadowing model • Rapidity maps into x • y = ln(2xgmp/mV) • x= mV/2gmp exp(y) • A colored glass condensate could have a larger effect No shadowing ds/dy, mb FGS shadowing y Frankfurt, Strikman & Zhalov, 2001 S. Klein,LBNL

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

  7. Rates (w/o shadowing) • J/y --> 3.2 Hz --> 3M/ 106 s • ~ 300,000 Y’ and 17,000 U(1S) • 36,000 J/y --> e+e- with |y|<1 (ALICE central) • ~360 Y’ and ~100 Y(1s) • 27,000 J/y --> e+e- with 2.5 <y< 4 (ALICE m spect.) • ~ 270 y’ and ~75 U(1s) • Rates are higher with lighter ions (higher LAA) S. Klein,LBNL

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

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

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

  11. Open Charm/Bottom Au qq g V Au Nuclear debris • Occurs via gluon exchange • Well described in QCD (& tested @ HERA) • Sensitive to gluon distribution • x and Q2 depend on final state configuration (M(QQ)…) • Wider range of Q2 than vector meson production • High rates • s(charm) ~ 1.8 barns ~ 25% of shadronic • S(bottom) ~ 700 mb ~ 1/10,000 of shadronic • (without shadowing) • One nucleus breaks up S. Klein,LBNL

  12. Single Quark pT Single Quark y1 Pair Mass (GeV) Charm kinematic distributions 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

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

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

  15. Rejecting Hadroproduction • Single nucleon-single hadronic interactions • Eliminate more central reactions with multiplicity cut • s1n1n ~ 700 mb • s1n1n, with charm ~ 5.5 mb • sb>2R with charm ~ 1100 mb • One nucleus remains intact • Assume <10% chance • Particle-free rapidity gap around photon emitter • P~ exp(-Dy dN/dy) • dNch/dy ~ 4.4 at midrapidity for pp the LHC • 40% lower at large |y| • For Dy= 2 units, P ~ 0.005 • sremaining, with charm ~ 3 mb SK, Joakim Nystrand & Ramona Vogt, 2002 S. Klein,LBNL

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

  17. Other channels • Dijets • Large rates • s(ET> 20 GeV) ~ 1/minute • Separating photoproduction and hadroproduction seems difficult • Photon + jet (Compton scattering) • s (1% of dijet rate) Ramona Vogt, 2004 S. Klein,LBNL

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

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

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