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e+ A @ eRHIC

e+ A @ eRHIC. Raju Venugopalan. RHIC-AGS Users Meeting, BNL, June 2nd, 2009. Talk Outline. Why is e+A interesting ? Why is it important ? What are the measurements ? What do they tell us about 1 & 2 ? How will what we learn impact other fields ?

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e+ A @ eRHIC

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  1. e+ A @ eRHIC Raju Venugopalan RHIC-AGS Users Meeting, BNL, June 2nd, 2009

  2. Talk Outline • Why is e+A interesting ? • Why is it important ? • What are the measurements ? • What do they tell us about 1 & 2 ? • How will what we learn impact other fields ? • Open questions and what we need to get there

  3. High Energy QCD & Hadron Structure • QCD is the “nearly perfect” fundamental theory of the strong interactions (F.Wilczek, hep-ph/9907340) • We are only beginning to explore the high energy, many body dynamics of this theory What are the right effective degrees of freedom at high energies? -- gluons & sea quarks, dipoles, pomerons, strong fields? How do these degrees of freedom interact with each other and with hard probes? -- Multi-Pomeron interactions, Higher Twist effects, Saturation/CGC -- Rapidity Gaps, Energy loss, Multiple Scattering, Color Transparency -- Glasma, Quark Gluon Plasma (Brian Cole’s talk) What can this teach us about confinement, universal features of the theory (infrared fixed point?) -- hard vs soft Pomerons, Strong Fields, in-medium hadronization -

  4. Terra Incognita Why Physics at an e+A collider is interesting Not your grand aunt’s e+A: world’s first such collider, first measurements of a range of final states… eg. rapidity gaps, jets, impact parameter dependent distributions Nuclei open a new precision window into fundamental questions about hadron structure in QCD At large x and large Q2: precision study of the propagation of colored probes in extended QCD medium, QCD evolution in nuclei At small x: coherent multiple scattering, strong color fields and the stability of QCD matter (saturation)

  5. Why Physics at an e+A collider is important Intrinsic interest: Deeper understanding of QCD in media and at high energies. Unambiguosly establish/ independently confirm discovery of gluon saturation …quantify theoretical understanding of fundamental prediction of theory Great uncertainity: In gluon distributions in nuclei, impact parameter distribution of glue, Energy loss of heavy quarks, nuclear fragmentation. Relevance for HI experiments at RHIC & LHC Eskola,Paukkunen, Salgado Plot by Vogelsang

  6. What are the measurements? Precision inclusive measurements of structure functions : : (Inclusive diffraction) Semi-inclusive measurements of final state distributions Exclusive final states Multiple handles: x, Q2, t, MX2 for light and heavy nuclei Q: What do these measurements tell us about the partonic structure of nuclei?

  7. Inclusive DIS Measure of resolution power Measure of inelasticity Measure of momentum fraction of struck quark quark+anti-quark mom. dists. gluon mom. dists

  8. HERA data on inclusive DIS Gluon distribution from scaling violations of F2 pQCD works…? Parton Density x= fraction of momentum of hadron carried by parton

  9. HERA data on inclusive DIS # partons per unit rapidity For Q2 ≤ 5 GeV2, leading twist (“parton gas”) description problematic. Sign of higher twist (multi-parton correlation) effects? Recent HERA data on FL (H1: Q2 = 12-90 GeV2 ; ZEUS Q2 = 24-110 GeV2) eRHIC can add significantly to world FL data set -- even for protons. Important test of QCD evolution Golec-Biernat, Stasto, arXiv: 0905.1321

  10. Gluon distribution from FL @ eRHIC e+A whitepaper (2007) eRHIC: 10 GeV + 100 GeV/n - estimate for 10 fb-1

  11. q z * r 1-z q P Inclusive DIS in saturation models • = 0.3; x0 = 3* 10-4 Machado, hep-ph/0512264 Kowalski, Teaney

  12. Inclusive DIS: Soft + Hard Pomeron model Soft non-pert. Pomeron exchange gives good fits to total hadronic cross-sections. A hard Pomeron is predicted by LO BFKL Forshaw and Shaw Fit of S+H in restricted range CGC model Extrapolation to full range of data S+H model Difficult to fit HERA data with soft + hard Pomeron model

  13. Inclusive diffraction MX “Pomeron” Rapidity Gap Big surprise at HERA:~ 15% of all events are hard diffractive (MX > 3 GeV) events Coherent + incoherent In rest frame: 50 TeV electron hits proton - in 1/7 events proton remains intact Coherent In e+A, saturation models predict rapidity gaps in ~ 25% of all events!

  14. Inclusive diffraction In pQCD, expect exponential suppression of large gaps -- parametrize data with diffractive structure functions which obey QCD evolution…not universal In CGC, diffractive structure functions depend on universal dipole cross-section Kowalski,Lappi,Marquet,RV Large  = small MX Small  = large MX Interesting pattern of enhancement and suppression-can be tested

  15. Semi-inclusive DIS Virtual photon with short coherence length scatters off quark or gluon (photon-gluon fusion) in medium - jet propagates through medium

  16. Semi-inclusive DIS Models: Hadron Attenuation Energy loss Nuclear Fragmentation Fns. Kopeliovich, Accardi,… Wang, Guo, Majumder, Arleo,… Sassot,Stratmann,… Vastly extended reach at eRHIC relative to HERMES, Jlab, EMC Precision studies of heavy quark energy loss Accardi,Dupre,Hafidi, EIC note-in preparation

  17. Semi-inclusive DIS At small x: Hadron distributions and multiplicities sensitive to QS(x,A) Kang,Qiu Ratio to x = 10-2 proportional to ratio of QS2(x,A) Need more quantitative studies for eRHIC kinematics Marquet,Xiao,Yuan

  18. Exclusive final states in DIS Brodsky et al. Frankfurt,Koepf,Strikman In the dipole model: Kowalski,Motyka,Watt Extract b dist. of glue In nuclei? Claim: can extract t dependence in photo-production of J/ down to 10-4 GeV2 Caldwell-Kowalski

  19. Exclusive final states: saturation model Kowalski,Motyka,Watt

  20. Geometrical Scaling: Evidence from HERA Golec-Biernat, Stasto,Kwiecinski x< 0.01 0 < Q2 < 450 GeV2   = Q2 / QS2 F2 D VM, DVCS V F2D Marquet, Schoeffel hep-ph/0606079 • Scaling seen for F2D and VM,DVCS for same QS as F2 Gelis et al., hep-ph/0610435 • Scaling confirmed by “Quality factor” analysis • Recent NLO BK analysis: Albacete, Kovchegov,hep-ph-0704.0612

  21. Evidence of geometrical scaling in nuclear DIS Freund et al., hep-ph/0210139 Geometrical scaling Nuclear shadowing: • Data scale as a function of  = Q2 / QS2 Large error bars obscured in log plot - can explore for less inclusive final states in nuclear DIS

  22. Quantifying the various regimes T.Ullrich-based on Kowalski, Lappi, RV ; PRL 100, 022303 (2008)

  23. Universal gluodynamics & energy dependence of QS A.H. Mueller, hep-ph/0301109 QS2(b) A p A1/3 Small x QCD RG eqns. predict (fixed b) QS approaches universal behavior with increasing energy (Y) for all hadrons and nuclei -can the approach to this behavior be tested ?

  24. Impact of e+A program • Initial conditions for heavy ion and p/D+A Program at RHIC/ LHC • Improve understanding of QCD evolution through precision data • Fundamental understanding of strong color fields universal behavior of QCD at high energies • Analogy of CGC RG evolution to B-E Condensates and Spin Glasses • Improve understanding of QCD final states at LHC • Partonic origin of nuclear forces • Fundamental insight into confinement?

  25. EIC Pie Chart

  26. Open questions Firstly, did not address many topics in this work … Jets, DVCS, Intrinsic Charm, fluctuations & correlations … Few detailed calculations for nuclei -- beyond fully Inclusive and diffractive studies … beginning to change An e+A event generator is important for further progress Physics notes in preparation on Diffraction, Jets and Hadron Attenuation -- need to extend scope

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