The Glue that binds us all Probing the nature of gluonic matter with EIC: the world’s first eA collider Raju Venugopalan Brookhaven National Laboratory Phases of Matter Town Hall meeting, Jan. 12th, 2007
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
QCD explains ~ 99% of the mass of the visible universe hep-lat/0304004 Hadron mass spectrum vs quenched lattice results Quenched QCD full QCD Quenched QCD (no dynamical quark-antiquark pairs) explains hadron mass spectrum to 10% The dynamics of glue is central to our understanding of the structure of matter
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
Where is the glue ? # of partons per unit rapidity momentum fraction of hadron The proton is dominated for x < 0.01 by glue- which grows rapidly… What happens when the density of gluons becomes large ?
Large x - bremsstrahlung linear evolution (DGLAP/BFKL) Small x -gluon recombination non-linear evolution (BK/JIMWLK) Mechanism of gluon saturation in QCD p, A Saturation scale QS(x) - dynamical scale below which non-linear QCD dynamics is dominant
The Color Glass Condensate In the saturation regime: Strongest fields in nature! • CGC: Classical effective theory of QCD describing • dynamics of gluon fields in non-linear regime • Novel renormalization group equations (JIMWLK/BK) • describe how the QCD dynamics changes with energy • A universal saturation scale QS arises naturally in the theory
Saturation scale grows with energy Typical gluon momenta are large Typical gluon kT in hadron/nuclear wave function Bulk of high energy cross-sections: obey dynamics of novel non-linear QCD regime Can be computed systematically in weak coupling
Saturation scale grows with A High energy compact (1/Q < Rp) probes interact coherently across nuclear size 2 RA - experience large field strengths Pocket formula: Enhancement of QS with A => non-linear QCD regime reached at significantly lower energy in A than in proton
A New window on universal properties of the matter in nuclear wavefunctions Can we quantify the various regimes ?
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 Q2 # partons per unit rapidity
Saturation Models-excellent fits to HERA data Kowalski et al., hep-ph/0606272 Also see Forshaw et al. hep-ph/0608161
Caveat: Saturation scale extracted from HERA data inconsistent with model assumptions Model assumes Typical sat. scale is rather low... QS2 << 1 GeV2
Evidence of non-linear saturation regime at RHIC ? Global multiplicity observables in AA described in CGC models: Au-Au mult. at eta=0 Kharzeev,Levin,Nardi Krasnitz, RV
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
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.
The Electron Ion Collider Quantitative QCD studies in largely “terra incognita” small x-large Q2 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
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 ~ QSA ) diffractive final states predicted to be > 30 % of cross-section Gluon dists. measured for x < 0.01 in nuclei for first time
Strong color fields are vastly more accessible in eA at EIC relative to ep at HERA Nuclear profile more uniform- study centrality dependence
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%
EIC can cleanly access cross-over region from weak field to novel strong field QCD dynamics Weak field regime Q2 >> QS2 Strong field regime Q2 << QS2 Qualitative change in final states: eg., 1/Q6 1/Q2 change in elastic vector meson production! McDermott,Guzey,Frankfurt,Strikman
p/D-A and AA are complementary probes to eA Universality: Soft color exchange between proton and nucleus breaks factorization at order 1 / Q4 Qiu,Sterman RHIC DA and LHC AA/pA -significant discovery potential Universality => genuine discovery will require complementary probes
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.
Inclusive measurements Measure of resolution power Measure of inelasticity Measure of momentum fraction of struck quark quark+anti-quark mom. dists. gluon mom. dists
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?
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,…
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.
At the Tevatron: Predictions obtained with HERA diffractive pdfs overestimate CDF data by a factor of about 10 Dramatic breakdown of factorization between ep and pp for diffractive final states Alvero,Collins,Terron,Whitmore
A dependence of saturation scale - estimates from fits to HERA and NMC data A dependence 0.33
Space-time dist. of strong Color Fields! Data from Dipole Survival Probability In pQCD, survival probability ~ 1 A 0.3 fm qq dipole survives only 20% of the time scattering off center of the proton!
Dominant impact parameters in DIS scattering off a proton b (GeV-1) Strong color fields are localized here
Hubble is taking beautiful pictures of dark matter binding Galaxies… Hubble Can EIC obtain similar pictures of glue binding visible matter ?