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eA Physics @ eRHIC

eA Physics @ eRHIC. Raju Venugopalan Brookhaven National Laboratory. eRHIC discussion group, Oct. 18th 2006. eRHIC at BNL. THE PROPOSAL

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eA Physics @ eRHIC

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  1. eA Physics @ eRHIC Raju Venugopalan Brookhaven National Laboratory eRHIC discussion group, Oct. 18th 2006

  2. eRHIC at BNL THE PROPOSAL A high energy, high intensity polarized electron/positron beam facility at BNL to colliding with the existing heavy ion and polarized proton beam would significantly enhance RHIC’s ability to probe fundamental, universal aspects of QCD Ring Ring Option Linac Ring Option Main Design

  3. eRHIC vs. Other DIS Facilities • New kinematic region • Ee = 10 GeV (~5-12 GeV variable) • Ep = 250 GeV (~50-250 GeV variable) • EA= 100 GeV • Sqrt[Sep] = 30-100 GeV • Kinematic reach of eRHIC: • X = 10-4 --> 0.7 (Q2 > 1 GeV2) • Q2 = 0 --> 104 GeV2 • Polarization of e,p beams (~70%) and He3 beams ~30-40% polarized • Heavy ions of ALL species at RHIC • Luminosity Goal: • L(ep) ~1033-34 cm-2 sec-1 eRHIC DIS

  4. CM vs. Luminosity • eRHIC • Variable beam energy • P-U ion beams • Light ion polarization • Huge luminosity ELIC-Jlab eRHIC

  5. The DIS Paradigm Kinematics: Cross-section: g_1,…g_5 - polarized structure functions in pol. e - pol. p scattering

  6. DIS highlights: • Bjorken scaling: the parton model. • Scaling violations: QCD- asymptotic freedom, renormalization group; precision tests of pQCD. • Rapid growth of gluon density at small x, significant hard diffraction. • Measurement of polarized structure functions: scaling violations, the “spin crisis”. • QCD in media: the EMC effect, shadowing, color transparency,..

  7. How can we probe glue with a high luminosity lepton-ion collider ? Key Measurements Precision inclusive measurements of structure functions -wide sweep of nuclei from Protons to Uranium Direct (photon-gluon fusion) semi-inclusive and exclusive probes of final states Deeply virtual Compton scattering • observables in blue will be measured for the first time in a wide • kinematic domain - CGC/saturation models predict significant differences from DGLAP type pQCD models for Q_s > (p_t, M)

  8. Lego plots a la Bjorken and Khoze et al. Such multi-gap events can also be studied in DIS Bj, hep-ph/9601363

  9. Nobel to Friedman, Kendall, Taylor Bj-scaling - apparent scale invariance of structure functions…

  10. Fraction of hadron momentum carried by a parton… • Feynman and Bjorken explanation of scaling puzzle… Parton model Parton constituents of proton are “quasi-free” on interaction time scale 1/q

  11. Puzzle resolved in QCD QCD Parton model Logarithmic scaling violations

  12. The Hadron at high energies: I Parton model QCD-logarithmic corrections

  13. # of valence quarks # quarks… “sea” quarks Valence quarks “X”-QCD--RG evolution

  14. DGLAP evolution: Linear RG in Q^2 Dokshitzer-Gribov-Lipatov-Altarelli-Parisi # of gluons grows rapidly at small x…

  15. STRUCTURE OF HIGHER ORDER CONTRIBUTIONS IN DIS + higher twist (power suppressed) contributions… • Coefficient functions - C - computed to NNLO for many processes, e.g., gg -> H Harlander, Kilgore; Ravindran,Van Neerven,Smith; … • Splitting functions -P - computed to 3-loops recently! • Moch, Vermaseren, Vogt

  16. Resolving the hadron -DGLAP evolution increasing But… the phase space density decreases -the proton becomes more dilute

  17. RG evolution…

  18. Resolving the hadron -BFKL evolution - Large x - Small x Gluon density saturates at f=

  19. Non-linear evolution: Gluon recombination QCD Bremsstrahlung Proton Proton is a dense many body system at high energies

  20. Saturated for Mechanism for parton saturation: Competition between “attractive” bremsstrahlung and “repulsive” recombination effects. Maximal phase space density =>

  21. Higher twists (power suppressed-in ) are important when: • Leading twist “shadowing’’ of these contributions can extend up to at small x. Need a new organizing principle- beyond the OPE- at small x.

  22. F_L is a positive definite quantity- more sensitive to higher twists than F_2 ? - clarify comparision with leading twist NLO pQCD at low x and moderate

  23. Golec-Biernat & Wusthoff’s model where & Parameters:

  24. Geometrical scaling at HERA (Golec-Biernat,Kwiecinski,Stasto) Scaling seen for all x < 0.01and

  25. Comparison with Data FS modelwith/without saturation and IIMCGC model hp-ph/0411337. Fit F2 and predict xIPF2D(3) FS(nosat) F2 CGC FS(sat) F2 x

  26. Comparison with Data Exclusive J/Psi production: Kowalski-Teaney

  27. R_{A1,A2} = 1 => Pomeron flux is A -independent = f(A1,A2) - universal form Diffractive Vector Meson Production: Brodsky,Gunion,Mueller, Frankfurt,Strikman Very sensitive to small x glue!

  28. III: Hard diffractive processes “Pomeron” exchange 30 % of eRHIC eA events may be hard diffractive events- Study sizes and distributions of Rapidity Gaps

  29. Viewing the hadron in the transverse plane at high energies… Y_0 ---> Y+Y_0 Gluon radiation & recombination Gluon radiation

  30. Overlapping gluon clouds… at large impact parameters, interplay between perturbative (hard Pomeron) and non-perturbative (soft Pomeron) physics… • By studying the “t” dependence of diffractive final states , can we learn more about the transition regime ? S-matrix:

  31. Shadowing and diffraction: • Is shadowing a non-perturbative leading twist phenomenon, or is generated by weak coupling, high parton density effects? • What is the relation of shadowing to diffraction? AGK rules relating the two are valid at low parton densities-how do these generalize to large parton densities? Armesto,Capella,Kaidalov, Salgado

  32. Cartoon of ratio of nuclear structure functions Uncertainity in ratio of Ca to nucleon gluon distributions -Hirai,Kumano,Nagai, hep-ph/0404093 Armesto

  33. Ratio of Gluon densities in Lead to Proton at in x range x Factor 3 uncertainity in glue => Factor 9 uncertainity in Semi-hard HI-parton cross-sections at LHC!

  34. The nuclear “oomph” factor! ? eA at eRHICsame parton density as ep at LHC energies!

  35. Hayashigaki et al. Reasonable agreement with forward spectra

  36. Virtual photon coherence length: • x_Bj << 0.01 : Photon coherence length exceeds nuclear size • 0.01 < x_Bj < 0.1: Intermediate length scale between R_p & R_A • x_Bj >> 0.1: Photon localized to longitudinal size smaller than nucleon size

  37. Strong hints from AA multiplicity dists. and D Au forward data of novel physics of strong color fields A dependence of saturation scale ? Numbers on “phase” diagram ? Breakdown of linear QCD evolution eqns. ?

  38. II: Extracting gluon distributions in pA relative to eA Direct photons Open charm Drell-Yan As many channels…but more convolutions, kinematic constraints-limit precision and range.

  39. Drell-Yan Impressive reach… But very difficult to see scaling violations

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