Physics Opportunities at an Electron-Ion Collider (EIC) Thomas Ullrich Phases of QCD Matter Town Meeting Rutgers University January 12, 2006 Lots of hard work from and violent discussion with: A. Bruell (JLAB), J. Dunlop (BNL), R. Ent (JLAB), D. Morrison (BNL), P. Steinberg (BNL), B. Surrow (MIT), R. Venugopalan (BNL), W. Vogelsang (BNL), Z. Xu (BNL)

2 Crouching Quarks, Hidden Glue Gluons: mediator of the strong interactions  Responsible for > 98% of the visible mass in universe  Determine all the essential features of strong interactions l QCD w/o quarks l QCD w/o gluons  QCD vacuum has non-perturbative structure driving:  Color confinement  Chiral symmetry breaking  In large due to fluctuations in the gluon fields in the vacuum Hard to “see” the glue in the low-energy world  Does not couple to electromagnetism  Gluon degrees of freedom “missing” in hadronic spectrum l but dominate the structure of baryonic matter at low-x l are the dominant player at RHIC and LHC

3 What Do We Know About Glue in Matter? Established Model: linear DGLAP evolution scheme  works well for quarks   cannot simultaneously describe gluons  l negative at low Q 2 ? l explosion of G(x,Q 2 ) at low-x  violation of unitarity l problems in describing diffractive events (HERA) New picture: BK based models introduce non-linear effects   saturation  characterized by a scale Q s (x,A)  grows with decreasing x and increasing A  arises naturally in the CGC framework Deep Inelastic Scattering: Measure of resolution power Measure of momentum fraction of struck quark Measure of inelasticity “Perfect” Tomography

4 Understanding Glue in Matter Understanding the role of the glue in matter involves understanding its key properties which in turn define the required measurements: WWhat is the momentum distribution of the gluons in matter? WWhat is the space-time distributions of gluons in matter? HHow do fast probes interact with the gluonic medium? DDo strong gluon fields effect the role of color neutral excitations (Pomerons)? What system to use? 1.e+p works, but more accessible by using e+A 2.have analogs in e+p, but have never been measured in e+A 3.have no analog in e+p

5 eA: Ideal to Study Non-Linear Effects Scattering of electrons off nuclei: u Small x partons cannot be localized longitudinally to better than size of nucleus u Virtual photon interacts coherently with all nucleons at a given impact parameter  Amplification of non-linear effects at small x. k k’ p W Nuclear “Oomph” Factor: e+A Collisions are Ideal for Studying “Glue” u Gain deeper understanding of QCD u Terra incognita: Physics of Strong Color Fields

6 eA Landscape and a new Electron Ion Collider The x, Q 2 plane looks well mapped out – doesn’t it? Except for ℓ+A ( A) many of those with small A and very low statistics Electron Ion Collider (EIC): E e = 10 GeV (20 GeV) E A = 100 GeV  s eN = 63 GeV (90 GeV) High L eAu ~ 6·10 30 cm -2 s -1 Terra incognita: small-x, Q  Q s high-x, large Q 2

7 How EIC will Address the Important Questions  What is the momentum distribution of the gluons in matter? l Gluon distribution G(x,Q 2 )  F L ~  s G(x,Q 2 ) (BTW: requires  s scan)  Extract from scaling violation in F 2 :  F 2 /  lnQ 2 §2+1 jet rates (needs modeling of hadronization) §inelastic vector meson production (e.g. J/  )  What is the space-time distributions of gluons in matter?  How do fast probes interact with the gluonic medium?  Do strong gluon fields effect the role of color neutral excitations (Pomerons)?

8 F 2 at EIC: Sea (Anti)Quarks Generated by Glue at Low x F 2 will be one of the first measurements at EIC nDS, EKS, FGS: pQCD models with different amounts of shadowing EIC will allow to distinguish between pQCD and saturation models predictions

9 F L at EIC: Measuring the Glue Directly Q 2 /xs = y Needs  s scan EIC will allow to measure G(x,Q 2 ) with great precision EIC: (10+100) GeV  Ldt = 2/A fb -1

10 l Measurement of structure functions for various mass numbers A (shadowing, EMC effect) and its impact parameter dependence l Deep virtual compton scattering (DVCS) l color transparency  color opacity exclusive final states (e.g. vector meson production , J/ , …) How EIC will Address the Important Questions  What is the momentum distribution of the gluons in matter?  What is the space-time distributions of gluons in matter?  How do fast probes interact with the gluonic medium?  Do strong gluon fields effect the role of color neutral excitations (Pomerons)?

11 How EIC will Address the Important Questions  What is the momentum distribution of the gluons in matter?  What is the space-time distributions of gluons in matter?  How do fast probes interact with the gluonic medium?  Do strong gluon fields effect the role of color neutral excitations (Pomerons)? l Hadronization, Fragmentation l Energy loss (charm!)

12 Charm at EIC 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

13 How EIC will Address the Important Questions  What is the momentum distribution of the gluons in matter?  What is the space-time distributions of gluons in matter?  How do fast probes interact with the gluonic medium?  Do strong gluon fields effect the role of color neutral excitations (Pomerons)? diffractive cross-section  diff /  tot HERA/ep: 10% of all events are hard diffractive EIC/eA: 30%? diffractive structure functions shadowing == multiple diffractive scattering ? diffractive vector meson production - very sensitive to G(x,Q 2 )

14 Diffractive Structure Function F 2 D at EIC EIC allows to distinguish between linear evolution and saturation models x IP = momentum fraction of the Pomeron with respect to the hadron  = momentum fraction of the struck parton with respect to the Pomeron x IP = x/ 

15 Thermalization:  At RHIC system thermalizes (locally) fast (   ~ 0.6 fm/c)  We don’t know why and how? Initial conditions? Jet Quenching:  Refererence: E-loss in cold matter  d+A alone won’t do l  need more precise handles  no data on charm from HERMES Forward Region:  Suppression at forward rapidities l Color Glass Condensate ? l Gluon Distributions ? Connection to RHIC & LHC Physics FF modification (parton energy loss) Accardi et al., hep-ph/ , CERN A Even more crucial at LHC: Ratios of gluon distribution functions for Pb versus x from different models at Q 2 = 5 GeV 2 : ?

16 Many New Questions w/o Answers … Latest News:  Observe “E-loss” of direct photons l Are we seeing the EMC effect? Many (all?) of these questions cannot be answered by studying A+A or p+A alone. EIC provides new level of precision: Handle on x, Q 2 Means to study effects exclusively RHIC is dominated by glue  Need to know G(x,Q 2 ) In short we need ep but especially eA  EIC

17 EIC Collider Aspects Requirements for EIC:  ep/eA program l polarized e, and p l maximal ion mass A l  s ~ 100 GeV l high luminosity (L > L Hera ) There are two complementary concepts to realize EIC:  eRHIC l construct electron beam to collide with the existing RHIC ion complex l high luminosity (6·10 30 cm -2 s -1 ), ions up to U,  s ~ 100 GeV  ELIC l construct ion complex to collide with the upgraded CEBAF accelerator l very high luminosity (4·10 34 cm -2 s -1 /A), only light ions,  s ~ 50 GeV

18 Experimental Aspects Concepts: 1.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 2.Focus on a wide acceptance detector system similar to HERA experiments  allow for the maximum possible Q 2 range. I. Abt, A. Caldwell, X. Liu, J. Sutiak, hep-ex J. Pasukonis, B.Surrow, physics/

19 Summary eA collisions at an EIC allow us to:  Study the Physics of Strong Color Fields l Establish (or not) the existence of the saturation regime l Explore non-linear QCD l Measure momentum & space-time of glue  Study the nature of color singlet excitations (Pomerons)  Study and understand nuclear effects l shadowing, EMC effect, Energy Loss in cold matter  Test and study the limits of universality (eA vs. pA)  Cross-fertilization: DIS (Hera), RHIC/LHC, JLAB In Short: EIC allows us to expand and deepen our understanding of QCD Now is a good time to get started! EIC White Paper: Soon: EIC/eA Specific Position Paper:

20 BACKUP

21 Structure Functions in DIS Quantitative description of electron-proton scattering Measure of resolution power Measure of momentum fraction of struck quark Measure of inelasticity

22 eA From a “Dipole” Point of View Coherence length of virtual photon’s fluctuation into  qq: L ∼ 1/2m N x valid in the small-x limit k k’ p r : dipole size L << 2R  Energy Loss  color transparency  EMC effect L >> 2R  Physics of strong color fields  Shadowing  Diffraction In the rest frame of the nucleus: Propagation of a small pair, or “color dipole”

23 Vector Meson Production HERA: Survival prob. of  qq pair of d=0.32 fm scattering off a proton from elastic vector meson production. Strong gluon fields in center of p at HERA (Q s ~ 0.5 GeV 2 )? b profile of nuclei more uniform and Q s ~ 2 GeV 2 Survival Probability color opacity color transparency “color dipole” picture

24 What Do We Know About Glue in Matter? Deep Inelastic Scattering: Distribution functions G(x,Q 2 ) evaluated through models  rise steeply at low Bjorken x Is nature well-described by model evolution? Gluons and QuarksGluons

25 Diffractive DIS is … momentum transfer t = (P-P’) 2 < 0 diffractive mass of the final state M X 2 = (P-P’+l-l’) 2 … when the hadron/nuclei remains intact x pom = x/  rapidity gap :  = ln(1/x pom ) hadron P P  ~ momentum fraction of the struck parton with respect to the Pomeron x pom ~ momentum fraction of the Pomeron with respect to the hadron HERA/ep: 10% of all events are hard diffractive EIC/eA: 30%? Black Disk Limit: 50% Pomeron