e+ 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 ?  Open questions and what we need to get there

High Energy QCD & Hadron Structure QCD is the “nearly perfect” fundamental theory of the strong interactions (F.Wilczek, hep-ph/ ) 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 -

Why Physics at an e+A collider is interesting Nuclei open a new precision window into fundamental questions about hadron structure in QCD At large x and large Q 2 : 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) 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 Terra Incognita

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

What are the measurements? Precision inclusive measurements of structure functions : : (Inclusive diffraction) Semi-inclusive measurements of final state distributions Exclusive final states Q: What do these measurements tell us about the partonic structure of nuclei? Multiple handles: x, Q 2, t, M X 2 for light and heavy nuclei

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

HERA data on inclusive DIS x= fraction of momentum of hadron carried by parton Parton Density Gluon distribution from scaling violations of F 2 pQCD works…?

HERA data on inclusive DIS # partons per unit rapidity For Q 2 ≤ 5 GeV 2, leading twist (“parton gas”) description problematic. Sign of higher twist (multi-parton correlation) effects? Recent HERA data on F L (H1: Q 2 = GeV 2 ; ZEUS Q 2 = GeV 2 ) eRHIC can add significantly to world F L data set -- even for protons. Important test of QCD evolution Golec-Biernat, Stasto, arXiv:

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

Inclusive DIS in saturation models q q P ** z 1-z rr = 0.3; x 0 = 3* Kowalski, Teaney Machado, hep-ph/

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 Fit of S+H in restricted range Extrapolation to full range of data CGC model S+H model Difficult to fit HERA data with soft + hard Pomeron model Forshaw and Shaw

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

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 Interesting pattern of enhancement and suppression-can be tested Large  = small M X Small  = large M X

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

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

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

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 GeV 2 Caldwell-Kowalski

Exclusive final states: saturation model Kowalski,Motyka,Watt

Geometrical Scaling: Evidence from HERA Golec-Biernat, Stasto,Kwiecinski  F2F2 F2DF2D VM, DVCS  = Q 2 / Q S 2 DD VV Marquet, Schoeffel hep-ph/  Scaling confirmed by “Quality factor” analysis Gelis et al., hep-ph/  Scaling seen forF2DF2D and VM,DVCS for same Q S as F 2  Recent NLO BK analysis: Albacete, Kovchegov, hep-ph x< < Q 2 < 450 GeV 2

Evidence of geometrical scaling in nuclear DIS Freund et al., hep-ph/ Nuclear shadowing: Geometrical scaling  Data scale as a function of  = Q 2 / Q S 2 Large error bars obscured in log plot - can explore for less inclusive final states in nuclear DIS

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

Universal gluodynamics & energy dependence of Q S A 1/3 Q S 2 (b) A p Small x QCD RG eqns. predict (fixed b) Q S approaches universal behavior with increasing energy (Y) for all hadrons and nuclei -can the approach to this behavior be tested ? A.H. Mueller, hep-ph/

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?

EIC Pie Chart

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