Universal features of QCD dynamics in hadrons & nuclei at high energies Raju Venugopalan DNP (APS/JPS) meeting, Hawaii, October 13, 2009

2  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 What can this teach us about confinement, universal features of the theory (infrared fixed point?) -- hard vs soft Pomerons, Strong Fields, in-medium hadronization - High Energy QCD

3 High Energy QCD-the role of Glue Self-interacting carriers of the strong force Localized energy fluctuation of Gluon Field (D. Leinweber) Dominate structure of QCD vacuum MILC Coll.: hep-lat/ Hadron mass spectrum vs quenched lattice results Quenched QCD full QCD Nearly all visible matter in the universe is made of Glue

4 Measuring Glue: what are our options? Gluon jet event in e + e - collisions at LEP  p+A and e+A provide complementary information on role of glue  Need both to test what’s universal and what’s not in QCD processes  Some final states differ dramatically (diffractive/exclusive states) Nothing matches DIS for precision-vast majority of world data for pdfs (especially for glue) from DIS

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

6 HERA data on inclusive DIS x= fraction of momentum of hadron carried by parton Parton Density Gluon distribution from scaling violations of F 2 Proton is almost entirely glue by x=0.01 for Q 2 = 10 GeV 2

7 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 ) EIC can add significantly to world F L data set -- even for protons. Important test of QCD evolution Golec-Biernat, Stasto, arXiv:

8 increasing Q 2 But… the phase space density decreases-the proton becomes more dilute Resolving the hadron in the Bjorken limit of QCD -DGLAP evolution Important for precision/ beyond standard model physics

9 - Large x - Small x IMF picture: Gluon phase space grows - saturates at occupation # f = Resolving the hadron in the Regge-Gribov limit of QCD -BFKL evolution Rest frame picture: Scattering amplitude is unity…

10 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 (“higher twist”) QCD dynamics is dominant Gribov,Levin,Ryskin Mueller,Qiu

11 CGC: Classical effective theory of QCD describing dynamical gluon fields + static color sources 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! McLerran, RV Jalilian-Marian,Kovner,Weigert Iancu, Leonidov,McLerran

12 Saturation scale grows with energy Bulk of high energy cross-sections: a)obey dynamics of novel non-linear QCD regime b)Can be computed systematically in weak coupling unintegrated gluon dist. from NLL RG evolution Q S (x ) Y=ln(x 0 /x)  0,1,3,9 Exact analogy to physics of “pulled” travelling wave fronts in stat. mech. Munier,Peschanski

13 Evidence from HERA for geometrical scaling 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 Recent caveats: Avsar, Gustafson, hep-ph/

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

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

16 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  Diffractive structure functions in DIS measure quark and gluon content of Pomeron Collins

17 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

18 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 Kowalski, Lappi, RV c ~ 1 in saturation model fit to HERA extrapolated to nuclei

19 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

20 Evidence of non-linear saturation RHIC ? Global multiplicity observables in AA described in CGC models: Input is Q S (x,A) Kharzeev,Levin,Nardi Krasnitz, RV PHOBOS central Au+Au mult. vs models

21 DA: Kharzeev,Kovchegov,Tuchin Albacete,Armesto,Salgado,Kovner,Wiedemann Blaizot, Gelis, RV D-Au pt spectra compared to CGC prediction Hayashigaki, Dumitru, Jalilian-Marian Review: Jalilian-Marian, Kovchegov, hep-ph/ Forward RHIC as well Boer, Dumitru, PRD 74, (2006)

22 Extrapolation of BK-fit to RHIC LF data to LHC dn/dy|_{y=0} = in A+A at LHC Gelis,Stasto, RV, hep-ph/ Natural explanation for limiting fragmentation + deviations in CGC Jalilian-Marian

23 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 Terra Incognita  Large x and Q 2 : Precision study of propagation of colored probes in extended QCD medium. QCD showering and fragmentation in nuclei  Small x: Explore physics of strong, non-linear color fields

24 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, Q 2, t, M X 2 for light and heavy nuclei Would like answers to:  What is the momentum distribution of gluons in matter  What is the space-time distribution of gluons in matter  How do fast probes interact with the gluonic medium?  What is the nature of color neutral exchanges (Pomerons) Tools/Measurements:

25 EIC: 10 GeV GeV/n - estimate for 10 fb -1 Gluon distribution from F eRHIC Eskola,Paukkunen,Salgado Quark and Glue contribution to Hadron-hadron inclusive Cross-section

26 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 squared. Diffractive cross-section ~ 25% of total cross-section! Kowalski,Lappi,Marquet,RV Interesting pattern of enhancement and suppression-can be tested Large  = small M X Small  = large M X

27 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 t = GeV 2 Caldwell-Kowalski

28 Universal gluodynamics & energy dependence of Q S 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/

29 In eA DIS, 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; Review: Frankfurt, Strikman, Weiss

30 Semi-inclusive DIS Virtual photon with short coherence length scatters off quark or gluon (photon-gluon fusion) in medium - jet propagates through medium Vastly extended reach at EIC relative to HERMES, Jlab, EMC Precision studies of heavy quark energy loss Accardi,Dupre,Hafidi,

31 What does a heavy ion collision look like ? Color Glass Condensates Initial Singularity Glasma sQGP - perfect fluid Hadron Gas t

32 What does a heavy ion collision look like ? Color Glass Condensates Initial Singularity Glasma sQGP - perfect fluid Hadron Gas t

33 Glasma flux tubes from small x dynamics After: boost invariant Glasma flux tubes of size 1/Q S Krasnitz,Nara, RV; Lappi Before: transverse E & B “Weizsacker-Williams fields Lappi,McLerran,NPA 772 (2006) parallel color E & B fields Kharzeev, Krasnitz, RV, Phys. Lett. B545 (2002) generate Chern-Simons topological charge

34 Imagining the Glasma through long range rapidity correlations Causality dictates: Au+Au 200 GeV, % PHOBOS preliminary  4 At LHC can probe color field dynamics for  << 1 fm… Very sensitive to gluon correlations in wave fn.

EIC High Energy Physics (LHC,LHeC Cosmic Rays) Lattice Gauge Theory Condensed Matter Physics (B-E Condensates, Spin Glasses Graphene)

36 Inclusive Diffraction-II Impact parameter dipole (CGC) models give  -sq. fits ~ 1 to HERA e+p inclusive and diffractive cross-section: H. Kowalski, C. Marquet, T. Lappi and R. Venugopalan, Phys. Rev. C 78, (2008).  = parton mom./Pomeron mom. x P = Pomeron mom./ Hadron mom.

37 A Estimates of the saturation scale from RHIC

38 The unstable Glasma Small rapidity dependent quantum fluctuations of the LO Yang-Mills fields grow rapidly as E  and B  fields as large as E L and B L at time Romatschke, RV:PRL 96 (2006) Possible mechanism for rapid isotropization Problem: collisions can’t ‘catch up’  Turbulent “thermalization” may lead to “anomalously” low viscosities Asakawa, Bass, Muller; Dumitru, Nara, Schenke, Strickland  Significant energy loss in Glasma because of synchroton like radiation? Shuryak, Zahed; Zakharov; Kharzeev

39 P and CP violation: Chiral Magnetic Effect Kharzeev,McLerran,Warringa L or B + External (QED) magnetic field Chiral magnetic effect = STAR Preliminary Possible experimental signal of charge separation ( Voloshin, Quark Matter 2009 ) Topological fluctuations -sphaleron transitions in Glasma N CS = Effect most significant, for transitions at early times

40 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 EIC kinematics

41

42 Forming a Glasma in the little Bang Glasma (\Glahs-maa\): Noun: non-equilibrium matter between Color Glass Condensate (CGC)& Quark Gluon Plasma (QGP)  Problem: Compute particle production in QCD with strong time dependent sources Gelis, Lappi, RV; arXiv : , ,  Solution: for early times (t  1/Q S ) -- n-gluon production computed in A+A to all orders in pert. theory to leading log accuracy

43 New window on universal properties of the matter in nuclear wavefunctions A Can we quantify the various regimes ? Iancu, RV, hep-ph/

44 Strong color fields may be more accessible in eA collisions relative to ep Nuclear profile more uniform-can study centrality dependence of distributions

45 Outline of talk  Whither the “perfect” theory ? - QCD at high energies  QCD coherence at small x => Universality - Saturation in hadrons & nuclei; the Color Glass Condensate picture  Exploring the structure of high energy nuclei with EIC - entering terra incognita  From Glue to Glasma & QGP - how multi-parton correlations in nuclei generate extreme states of quark-gluon matter