Physics Opportunities with an Electron Ion Collider

1. Physics Opportunities with an Electron Ion Collider Thomas Ullrich, BNL ISMD 2007, LBL Berkeley August 6, 2007

2. Theory of Strong Interactions: QCD • “Emergent” Phenomena not evident from Lagrangian • Asymptotic Freedom & Color Confinement • In large part due to non-perturbative structure of QCD vacuum • Gluons: mediator of the strong interactions • Determine essential features of strong interactions • Dominate structure of QCD vacuum (fluctuations in gluon fields) • Responsible for > 98% of the visible mass in universe • Hard to “see” the glue in the low-energy world • Gluon degrees of freedom “missing” in hadronic spectrum • but drive the structure of baryonic matter at low-x • are crucial players at RHIC and LHC QCD requires fundamental investigation via experiment

3. Gluons dominate low-x wave function What Do We Know About Glue in Matter? • Scaling violation: dF2/dlnQ2 and linear DGLAP Evolution  G(x,Q2)

4. The Issue With Our Current Understanding Established Model: LinearDGLAP evolution scheme • Weird behavior of xG and FLfrom HERA at small x and Q2 • Could signal saturation, higher twist effects, need for more/better data? • Unexpectedly large diffractive cross-section more severe: Linear Evolution has a built in high energy “catastrophe” • xG rapid rise for decreasing x and violation of (Froissart) unitary bound •  must saturate • What’s the underlying dynamics?  Need new approach

5. proton N partons any 2 partons can recombine into one proton N partons new partons emitted as energy increases could be emitted off any of the N partons Non-Linear QCD - Saturation • BFKLEvolution in x • linear • explosion of color field? • New: BK/JIMWLK based models • introduce non-lineareffects saturation • characterized by a scale Qs(x,A) • arises naturally in the Color Glass Condensate (CGC)framework Regimes of QCD Wave Function

6. e+A: Studying Non-Linear Effects Scattering of electrons off nuclei: • Probes interact over distances L ~ (2mN x)-1 • For L > 2 RA ~ A1/3 probe cannot distinguish between nucleons in front or back of nucleon • Probe interacts coherently with all nucleons Nuclear “Oomph” Factor Pocket Formula: Enhancement of QS with A non-linear QCD regime reached at significantly lower energy in A than in proton

7. Nuclear “Oomph” Factor More sophisticated analyses  more detailed picture even exceeding the Oomph from the pocket formula (e.g. Armesto et al., PRL 94:022002, Kowalski, Teaney, PRD 68:114005)

8. Universality & Geometric Scaling Crucial consequence of non-linear evolution towards saturation: • Physics invariant along trajectories parallel to saturation regime (lines of constant gluon occupancy) • Scale with Q2/Q2s(x) instead of x and Q2 separately  Geometric Scaling • Consequence of saturation which manifests itself up to kT > Qs x < 0.01

9. Qs a Scale that Binds them All Geometrical scaling Nuclear shadowing: proton  5 nuclei Freund et al., hep-ph/0210139 Are hadrons and nuclei wave function universal at low-x ?

10. Understanding Glue in Matter Understanding the role of the glue in matter involves understanding its key properties which in turn define the required measurements: • What is the momentum distribution of the gluons in matter? • e+p and e+A • Exploration of saturation regime better in e+A (Oomph Factor) • What is the space-time distributions of gluons in matter? • e+p and e+A • Unknown in e+A • How do fast probes interact with the gluonic medium? • Strength of e+A • Do strong gluon fields effect the role of color neutral excitations (Pomerons)? • e+p and e+A • Unknown in e+A

11. Well mapped in e+p Not so for ℓ+A (nA) Electron Ion Collider (EIC): L(EIC) > 100  L(HERA) Different EIC Concepts: eRHIC ELIC eA Landscape and a New Electron Ion Collider Terra incognita:small-x, Q  Qs high-x, large Q2

12. ElectronCooling e-cooling (RHIC II) Snake eRHIC (Linac-Ring) IR IR PHENIX Snake Main ERL (2 GeV per pass) STAR ELIC Four e-beam passes Electron Ion Collider Concepts • eRHIC (BNL): Add Energy Recovery Linac to RHIC Ee = 10 (20) GeV EA = 100 GeV (up to U) seN = 63 (90) GeV LeAu (peak)/n ~ 2.9·1033 cm-2 s-1 • ELIC (JLAB): Add hadron beam facility to existing electron facility CEBAF Ee = 9 GeV EA = 90 GeV (up to Au) seN = 57 GeV LeAu (peak)/n ~ 1.6·1035 cm-2 s-1 Both allow for polarized e+p collisions !

13. What is the Momentum Distribution of Gluons? Gluon distribution G(x,Q2) • Shown here: • Scaling violation in F2: dF2/dlnQ2 • FL ~ as G(x,Q2) • Other Methods: • 2+1 jet rates (needs jet algorithm and modeling of hadronization for inelastic hadron final states) • inelastic vector meson production (e.g. J/) • diffractive vector meson production ~ [G(x,Q2)]2

14. F2 : Sea (Anti)Quarks Generated by Glue at Low x F2 will be one of the first measurements at EIC nDS, EKS, FGS: pQCD based models with different amounts of shadowing Syst. studies of F2(A,x,Q2):  G(x,Q2) with precision  distinguish between models

15. FL at EIC: Measuring the Glue Directly FL requires s scan Q2/xs = y Here: Ldt = 5/A fb-1 (10+100) GeV =5/A fb-1 (10+50) GeV =2/A fb-1 (5+50) GeV statistical error only  G(x,Q2) with great precision

16. The Gluon Space-Time Distribution • What we know is mostly the momentum distribution of glue? • How is the glue distributed spatially in nuclei? • Gluon density profile: small clumps or uniform ? • Various techniques & methods: • Exclusive final states (e.g. vector meson production r, J/y, DVCS) • color transparency  color opacity • Deep Virtual Compton Scattering (DVCS) • Integrated DVCS cross-section: sDVCS ~ A4/3 • Measurement of structure functions for various mass numbers A (shadowing, EMC effect) and its impact parameter dependence

17. Hadronization and Energy Loss nDIS: • Suppression of high-pT hadrons analogous but weaker than at RHIC • Clean measurement in ‘cold’ nuclear matter Fundamental question: When do colored partons get neutralized? Parton energy loss vs. (pre)hadron absorption Energy transfer in lab rest frame EIC: 10 < n < 1600 GeV HERMES: 2-25 GeV EIC: can measure heavy flavor energy loss zh = Eh/n

18. Connection to p+A Physics F. Schilling, hex-ex/0209001 • e+A and p+A provide excellent information on properties of gluons in the nuclear wave functions • Both are complementary and offer the opportunity to perform stringent checks of factorization/universality  • Issues: • p+A lacks the direct access to x, Q2 Breakdown of factorization (e+p HERA versus p+p Tevatron) seen for diffractive final states.

19. RHIC LHC Connection to RHIC & LHC Physics Matter at RHIC: • thermalizes fast (t0 ~ 0.6 fm/c) • We don’t know why and how? • Initial conditions?  G(x, Q2) Role of saturation ? • RHIC → forward region • LHC → midrapidity • bulk (low-pT matter) & semi-hard jets Jet Quenching: • Need Refererence: E-loss in cold matter • No HERMES data for • charm energy loss • in LHC energy range EIC provides new essential input: • Precise handle on x, Q2 • Means to study exclusive effects

20. J. Pasukonis, B.Surrow, physics/0608290 I. Abt, A. Caldwell, X. Liu, J. Sutiak, hep-ex 0407053 Experimental Aspects Concepts: • Focus on the rear/forward acceptance and thus on low-x / high-x physics • compact system of tracking and central electromagnetic calorimetry inside a magnetic dipole field and calorimetric end-walls outside • Focus on a wide acceptance detector system similar to HERA experiments • allow for the maximum possible Q2 range.

21. EIC Timeline & Status • NSAC Long Range Plan 2007 • Recommendation: $6M/year for 5 years for machine and detector R&D • Goal for Next Long Range Plan 2012 • High-level (top) recommendation for construction • EIC Roadmap (Technology Driven) • Finalize Detector Requirements from Physics 2008 • Revised/Initial Cost Estimates for eRHIC/ELIC 2008 • Investigate Potential Cost Reductions 2009 • Establish process for EIC design decision 2010 • Conceptual detector designs 2010 • R&D to guide EIC design decision 2011 • EIC design decision 2011 • “MOU’s” with foreign countries? 2012

22. Summary The EIC presents a unique opportunity in high energy nuclear physics and precision QCD physics • e+A • Study the Physics ofStrong Color Fields • Establish (or not) the existence of the saturation regime • Explore non-linear QCD • Measure momentum & space-time of glue • Study the nature ofcolor singlet excitations(Pomerons) • Test and study the limits ofuniversality(eA vs. pA) • e+p (polarized) • Precisely image the sea-quarks and gluons to determine the spin, flavor and spatial structure of the nucleon For more see: http://web.mit.edu/eicc/

23. The EIC Collaboration 17C. Aidala, 28E. Aschenauer, 10J. Annand, 1J. Arrington, 26R. Averbeck, 3M. Baker, 26K. Boyle, 28W. Brooks, 28A. Bruell, 19A. Caldwell, 28J.P. Chen, 2R. Choudhury, 10E. Christy, 8B. Cole, 4D. De Florian, 3R. Debbe, 26,24-1A. Deshpande, 18K. Dow, 26A. Drees, 3J. Dunlop, 2D. Dutta, 7F. Ellinghaus, 28R. Ent, 18R. Fatemi, 18W. Franklin, 28D. Gaskell, 16G. Garvey, 12,24-1M. Grosse-Perdekamp, 1K. Hafidi, 18D. Hasell, 26T. Hemmick, 1R. Holt, 8E. Hughes, 22C. Hyde-Wright, 5G. Igo, 14K. Imai, 10D. Ireland, 26B. Jacak, 15P. Jacobs, 28M. Jones, 10R. Kaiser, 17D. Kawall, 11C. Keppel, 7E. Kinney, 18M. Kohl, 9H. Kowalski, 17K. Kumar, 2V. Kumar, 21G. Kyle, 13J. Lajoie, 3M. Lamont, 16M. Leitch, 27A. Levy, 27J. Lichtenstadt, 10K. Livingstone, 20W. Lorenzon, 145. Matis, 12N. Makins, 6G. Mallot, 18M. Miller, 18R. Milner, 2A. Mohanty, 3D. Morrison, 26Y. Ning, 15G. Odyniec, 13C. Ogilvie, 2L. Pant, 26V. Pantuyev, 21S. Pate, 26P. Paul, 12J.-C. Peng, 18R. Redwine, 1P. Reimer, 15H.-G. Ritter, 10G. Rosner, 25A. Sandacz, 7J. Seele, 12R. Seidl, 10B. Seitz, 2P. Shukla, 15E. Sichtermann, 18F. Simon, 3P. Sorensen, 3P. Steinberg, 24M. Stratmann, 22M. Strikman, 18B. Surrow, 18E. Tsentalovich, 11V. Tvaskis, 3T. Ullrich, 3R. Venugopalan, 3W. Vogelsang, 28C. Weiss, 15H. Wieman,15N. Xu,3Z. Xu, 8W. Zajc. 1Argonne National Laboratory, Argonne, IL; 2Bhabha Atomic Research Centre, Mumbai, India; 3Brookhaven National Laboratory, Upton, NY; 4University of Buenos Aires, Argentina; 5University of California, Los Angeles, CA; 6CERN, Geneva, Switzerland; 7University of Colorado, Boulder,CO; 8Columbia University, New York, NY; 9DESY, Hamburg, Germany; 10University of Glasgow, Scotland, United Kingdom; 11Hampton University, Hampton, VA; 12University of Illinois, Urbana-Champaign, IL; 13Iowa State University, Ames, IA; 14University of Kyoto, Japan; 15Lawrence Berkeley National Laboratory, Berkeley, CA; 16Los Alamos National Laboratory, Los Alamos, NM; 17University of Massachusetts, Amherst, MA; 18MIT, Cambridge, MA; 19Max Planck Institüt für Physik, Munich, Germany; 20University of Michigan Ann Arbor, MI; 21New Mexico State University, Las Cruces, NM; 22Old Dominion University, Norfolk, VA; 23Penn State University, PA; 24RIKEN, Wako, Japan; 24-1RIKEN-BNL Research Center, BNL, Upton, NY; 25Soltan Institute for Nuclear Studies, Warsaw, Poland; 26SUNY, Stony Brook, NY; 27Tel Aviv University, Israel; 28Thomas Jefferson National Accelerator Facility, Newport News, VA 96 Scientists, 28 Institutions, 9 countries

24. Additional Slides

25. Regimes of QCD Wave Function in 3D

26. Charm at EIC in e+A EIC: allows multi-differential measurements of heavy flavor covers and extend energy range of SLAC, EMC, HERA, and JLAB allowing study of wide range of formation lengths Based on HVQDIS model, J. Smith

27. Diffractive Physics in e+A ‘Standard DIS event’ Diffractive event ? Detector activity in proton direction • HERA/ep: 15% of all events are hard diffractive • Diffractive cross-section sdiff/stot in e+A ? • Predictions: ~25-40%? • Look inside the “Pomeron” • Diffractive structure functions • Diffractive vector meson production ~ [G(x,Q2)]2

28. = x/xIP Diffractive Structure Function F2D at EIC xIP= momentum fraction of the pomeron w.r.t the hadron • Distinguish between linear evolution and saturation models • Insight into the nature of pomeron • Search for exotic objects (Odderon) Curves: Kugeratski, Goncalves, Navarra, EPJ C46, 413

29. A Truly Universal Regime ? • Radical View: • Nuclei and all hadrons have a component of their wave function with the same behavior • This is a conjecture! Needs to be tested Small x QCD evolution predicts: • QSapproaches universal behavior for all hadrons and nuclei  Not only functional form f(Qs) universal but even Qsbecomes the same ? “Research is what I'm doing when I don't know what I'm doing. “ Wernher von Braun A.H. Mueller, hep-ph/0301109

30. Spin Physics at the EIC - The Quest for G Spin Structure of the Proton ½ = ½  + G + Lq + Lg • quark contribution ΔΣ ≈ 0.3 • gluon contribution ΔG ≈ 1 ± 1 ? G: a “quotable” property of the proton (like mass, charge) Measure through scaling violation: Superb sensitivity to g at small x!

31. Deep Inelastic Scattering: Measure of resolution power: ~1/wavelength2 Measure of momentum fraction of struck quark Gluons dominate low-x wave function Measure of inelasticity “Perfect” Tomography What Do We Know About Glue in Matter? • Scaling violation: dF2/dlnQ2 and linear DGLAP Evolution  G(x,Q2)