The Glue that binds us all Phases of Matter Town Hall meeting, Jan. 12th, 2007 Probing the nature of gluonic matter with EIC: the world’s first eA collider Raju Venugopalan Brookhaven National Laboratory

2 Talk Outline:  Outstanding questions in QCD at high energies  Lessons and open questions from HERA and RHIC  How these are addressed by measurements with EIC  The discovery potential of eA at EIC  Summary

3 QCD explains ~ 99% of the mass of the visible universe Quenched QCD (no dynamical quark-antiquark pairs) explains hadron mass spectrum to 10% hep-lat/ Hadron mass spectrum vs quenched lattice results Quenched QCDfull QCD The dynamics of glue is central to our understanding of the structure of matter

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

5 Where is the glue ? The proton is dominated for x < 0.01 by glue- which grows rapidly… What happens when the density of gluons becomes large ? # of partons per unit rapidity momentum fraction of hadron

6 Mechanism of gluon saturation in QCD p, A Large x - bremsstrahlung linear evolution (DGLAP/BFKL) Small x -gluon recombination non-linear evolution (BK/JIMWLK) Saturation scale Q S (x) - dynamical scale below which non-linear QCD dynamics is dominant

7 CGC: Classical effective theory of QCD describing dynamics of gluon fields in non-linear regime o Novel renormalization group equations (JIMWLK/BK) describe how the QCD dynamics changes with energy o A universal saturation scale Q S arises naturally in the theory The Color Glass Condensate In the saturation regime: Strongest fields in nature!

8 Saturation scale grows with energy Typical gluon momenta are large Bulk of high energy cross-sections: a)obey dynamics of novel non-linear QCD regime b)Can be computed systematically in weak coupling Typical gluon k T in hadron/nuclear wave function

9 Saturation scale grows with A High energy compact (1/Q < R p ) probes interact coherently across nuclear size 2 R A - experience large field strengths Enhancement of Q S with A => non-linear QCD regime reached at significantly lower energy in A than in proton Pocket formula:

10 New window on universal properties of the matter in nuclear wavefunctions A Can we quantify the various regimes ?

11 Evidence of non-linear saturation regime at HERA ? “Linear” pQCD describes inclusive observables well-however hints of non-linear (“higher twist”) at small x and Q 2 # partons per unit rapidity

12 Kowalski et al., hep-ph/ Also see Forshaw et al. hep-ph/ Saturation Models-excellent fits to HERA data

13 Typical sat. scale is rather low... Q S 2 << 1 GeV 2 Caveat: Saturation scale extracted from HERA data inconsistent with model assumptions Model assumes

14 Evidence of non-linear saturation regime at RHIC ? Global multiplicity observables in AA described in CGC models: Kharzeev,Levin,Nardi Krasnitz, RV Au-Au mult. at eta=0

15 DA: Kharzeev,Kovchegov,Tuchin Albacete,Armesto,Salgado,Kovner,Wiedemann D-Au pt spectra compared to CGC prediction Hayashigaki, Dumitru, Jalilian-Marian Talk by M. Leitch

16 A Estimates of the saturation scale from RHIC

17 Outstanding questions in high energy QCD (QCD Theory Workshop, DC, Dec. 15th-16th, 2006)  What is the nature of glue at high density ?  How do strong fields appear in hadronic or nuclear wavefunctions at high energies ?  How do they respond to external probes or scattering ?  What are the appropriate degrees of freedom ?  Is this response universal ? (ep,pp,eA, pA, AA) An Electron Ion Collider (EIC) can provide definitive answers to these questions.

18 The Electron Ion Collider Quantitative QCD studies in largely “terra incognita” small x-large Q 2 regime  Variable ep c.m energy up to 100 GeV and high luminosity (~100 times HERA) unpolarized e-p scattering  pol. e-pol. p - highest energies and collider mode for the first time ( parallel Town Hall discussion & tomorrow )  First eA collider with wide range of nuclear beams and c.m. energy up to 63 (90) GeV/ nucleon Precision studies of QCD in nuclear media & very high parton densities

19 What are the measurements with EIC ? See Thomas Ullrich’s talk  Momentum distributions of gluons and quarks in nuclei  Space-time distributions of quarks and gluons in nuclei Extract space-time dist. of nuclear glue from exclusive final states  Interaction of fast probes with nuclear media First semi-inclusive measurements: charm and bottom dists. & energy loss in nuclei  Role of color neutral (Pomeron) excitations in scattering off nuclei Semi-hard (M ~ Q S A ) diffractive final states predicted to be > 30 % of cross-section Gluon dists. measured for x < 0.01 in nuclei for first time

20 Strong color fields are vastly more accessible in eA at EIC relative to ep at HERA Nuclear profile more uniform- study centrality dependence

21 The nuclear oomph factor… Saturation scale significantly enhanced in nuclei ~ 6 enhancement in central Au relative to min-bias proton Matches pocket formula to 10%

22 EIC can cleanly access cross-over region from weak field to novel strong field QCD dynamics Weak field regime Q 2 >> Q S 2 Strong field regime Q 2 << Q S 2 Qualitative change in final states: eg., 1/Q 6 1/Q 2 change in elastic vector meson production! McDermott,Guzey,Frankfurt,Strikman

23 p/D-A and AA are complementary probes to eA Universality: Soft color exchange between proton and nucleus breaks factorization at order 1 / Q 4 RHIC DA and LHC AA/pA -significant discovery potential Universality => genuine discovery will require complementary probes Qiu,Sterman

24 Summary  EIC with variable energies, nuclear beams and high luminosity is a powerful tool to access and study universal properties of QCD at high parton densities  These studies have profound ramifications for our understanding of QCD dynamics at the LHC-especially in heavy ion collisions  The ability of EIC to distinguish between model predictions for measurements is discussed in the following talk by Thomas Ullrich.

25 EXTRA SLIDES

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

27 Diffractive measurements Color singlet multi-glue (Pomeron ) exchange Very sensitive to glue mom. dists. Extract spatial (impact parameter) dists. of gluon fields Deg. of freedom: classsical fields, Pomeron interactions?

28 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: the “spin crisis”.  QCD in nuclei: EMC effect, shadowing, color transparency,…

29 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.

30 Dramatic breakdown of factorization between ep and pp for diffractive final states At the Tevatron: Predictions obtained with HERA diffractive pdfs overestimate CDF data by a factor of about 10 Alvero,Collins,Terron,Whitmore

31 A dependence of saturation scale - estimates from fits to HERA and NMC data 0.33 A dependence

32 In pQCD, survival probability ~ 1 Dipole Survival Probability Data from Space-time dist. of strong Color Fields! A 0.3 fm qq dipole survives only 20% of the time scattering off center of the proton!

33 Dominant impact parameters in DIS scattering off a proton b (GeV -1 ) Strong color fields are localized here

34 Hubble Hubble is taking beautiful pictures of dark matter binding Galaxies… Can EIC obtain similar pictures of glue binding visible matter ?