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High-Pt Physics in p-A Collisions: Collisions at RHIC and the LHC and Connections to e-A Physics

This article discusses the physics goals of p-A collisions, including nuclear effects, shadowing/saturation, jet structure, pT broadening/energy loss, modifications of baryon production, tests of pQCD, nucleus as a filter, diffraction, proton break-up, baryon junction excitation, and soft phenomenology.

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High-Pt Physics in p-A Collisions: Collisions at RHIC and the LHC and Connections to e-A Physics

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  1. p-A Collisions at RHIC and the LHC and Connections to e-A Physics Prof. B.A. Cole Columbia University

  2. p-A Physics Goals • Nuclear effects • Shadowing / saturation @ low xA. • Jet structure / mono-jets @ low xA. • pT broadening / energy loss. • Modifications of baryon production. • Tests of pQCD: factorization / universality. • Nucleus as a filter • Diffraction. • Proton break-up, color transparency. • Baryon junction excitation. • Soft phenomenology.

  3. View in nucleus rest frame For mid-rapidity jet with MT Relative to nucleus, y=5.4 E  pL = MT cosh(y)  100 MT Also, Jet formation time:  ~1/ MT Lorentz boost:  = cosh(y)  100 Giving jet formation length (LF) LF = 20 GeV fm / MT From this simple analysis we can conclude: All for the “action” for mid-rapidity particle production (and forward) occurs along the straight path of the incoming nucleon. Even high-pT and heavy quark production processes may be affected by coherence in the multiple scattering process. New at RHIC: Ability to select on “centrality” (poor man’s impact parameter) Coherence in p/d-A @ RHIC

  4. We don’t have to look very hard to see the effects of coherence. Effects near mid- disappear by pT ~ 6 (?) @  = 3.2 kinematic limit: pT  8 GeV/c. Limited phase space for truly high-pT physics (Semi) Hard Scattering in d-A @ RHIC Brahms

  5. d-A J/ Production (from M. Leitch) E866: PRL 84, 3256 (2000)NA3: ZP C20, 101 (1983)  compared to lower s RdA Low x2 ~ 0.003 (shadowing region) • Not universal versus X2 : not shadowing !?? • BUT does scale with xF ! - why? • Initial-state gluon energy loss depends on x1~xF - weak at RHIC energy? • But Kopeliovich: • Effect can be due to “energy loss” (in gold) xF = xd - xAu Klein,Vogt, PRL 91:142301,2003 Kopeliovich, NP A696:669,2001 • Data favors (weak) shadowing + (weak) absorption ( > 0.92) • With current statistics hard to separate different nuclear effects • Will need more d-Au data!

  6. Studying Jet Properties @ RHIC • Use hadron pairs to study jet properties • pout dist. has both non-pert. (Gaussian) + hard (power) contributions. Pout Pout Jet PHENIX, From J. Jia, DNP’04 Talk Radiative tails pp PHENIX Preliminary

  7. Jet Properties in d-Au • Compare pout dist’s in p-p and d-Au. • Evidence for effects of re-scattering, modified radiation, … ? • Not so far! • But this is just the beginning! • Such measurements w/ one jet @  > 2 would be very interesting!! • But not possible yet

  8. Radiative Effects on (di)Jets • Conclude: large radiative component to di-jet kT • Also see Vitev, Qiu : Phys.Lett.B570:161-170,2003. • Without accounting for radiation initial parton intrinsic kT ~ 2 GeV/c (RMS). • After accounting for radiation ~ 1 GeV/c Analysis of STAR di-hadron  distribution by Boer & Vogelsang, Phys. Rev. D69 094025, 2004

  9. Radiative contributions from initial & final state Initial state radiation due to parton shower prior to the hard scattering The development of the initial-state shower must be different in nucleus. In fact, an important part of saturation is the change in the initial-state shower “Quantum evolution” Hard Scattering – IS/FS Radiation • “Model-independent approach” • Study di-jet acoplanarity • Better: -jet and -  (hard) processes

  10. kT broadening and evolution of parton distributions will modify  production. If there are mono-jets, are there mono-photons?? -jet angular correlations more sensitive because less broadening from jet. Di-  production even more interesting – kinematics completely determined. Need good photon/0 separation. Direct Photon Production J. Jalilian-Marian, hep-ph/0501222

  11. The ability to select on centrality in d(p)-A collisions is NEW and very important. Potentially the first opportunity to measure the impact parameter dependence of: Initial-state broadening, Shadowing, … Observations of centrality dependence have already been important. But, there are some limitations: Rely on Glauber model to indirectly relate “centrality” observables to impact parameter. Kopeliovich: Flaw in Glauber models due to neglect of diffraction – which I think is a real issue. May be important for understanding RCP. Centrality in d(p)-A

  12. p-A Collisions @ LHC • Summary of LHC “Yellow Report” on p-A

  13. p-A in ATLAS (CMS) Electromagnetic Calorimeter Muon chambers Hadronic Calorimeter Superconducting Solenoid Inner Detectors Silicon Pixels Silicon Strips Transition Radiation Tracker Superconducting Coils for Toroidal Field for Muon System • p-p detectors @ LHC ideal for studying high-pT physics in p-A collisions. For CMS: EMCal covers ||<5 Had. Cal: ||<5 TOTEM: ||<7

  14. p-A @ LHC • p-A @ LHC can reach low x at high Q2 • Rates for high-pT processes are enormous • Concerns • No p-p measurements at same s (?). • Centrality selection will require care. • Little particle (baryon) identification away from mid-rapidity Parameters from LHC Yellow Report Rates for pT > 100 GeV/c

  15. Measurable shadowing even at 100 GeV. Modest effects at mid-rapidity (but going away slowly) Low-x Effects @ LHC Q=100 GeV Q=10 GeV Q=2 GeV Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005) Frankfurt, Strikman: Shadowing

  16. p-A @ LHC: Plans • p-A is considered an “upgrade” @ LHC • Straight-forward but needs second timing system. • Cabling will be in place but the $ are not yet committed (but small change: few 100k$) • Meeting on p-A @ LHC May 25-28. • We will know much more then. • “Guaranteed” that there will be p-A @ the LHC but when? • Presumably all three experiments will run. • LHC p-A complementary to RHIC p-A and e-RHIC.

  17. q q-bar pair + … (evolution) interacts with target In many ways similar to p-A collision. But: Transverse size controlled by Q2 Kinematics much better determined For moderate Q2 get multiple scattering Shadowing “Centrality” !!? Connection between structure function & unintegrated PDFs But only at leading twist! Can we directly measure violation using p-A/e-A ? e-A in Target Rest Frame

  18. p-A: What is Unique to RHIC • At LHC we will not be able to measure into the fragmentation region. • At RHIC, we could in principle cover a large part of the fragmentation region • Simultaneously measure • Proton break-up • Hard/semi-hard processes with good efficiency for capturing di-jet, -jet, - • Nuclear break-up • In a detector that would look much like an e-A detector w/ similar requirements (?)

  19. Semi-inclusive DIS • HERMES has very interesting results on modifications of quark fragmentation in nuclei. • In target rest-frame: • Nucleus as filter of different dipole+… configurations

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