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The Color glass COndensate A classical effective theory of high energy QCD Raju Venugopalan Brookhaven National Laboratory ICPAQGP, Feb. 8th-12th, 2005
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Outline of talk: Introduction A classical effective theory (and its quantum evolution) for high energy QCD Hadronic scattering and k_t factorization in the Color Glass Condensate What the CGC tells us about the matter produced in dA and AA collisions at RHIC. Open issues
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Much of the discussion in pQCD has focused on the Bjorken limit: Asymptotic freedom, the Operator Product Expansion (OPE) & Factorization Theorems: machinery of precision physics in QCD…
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Coefficient functions - C - computed to NNLO for many processes, e.g., gg -> H Harlander, Kilgore; Ravindran,Van Neerven,Smith; … Splitting functions -P - computed to 3-loops recently! Moch, Vermaseren, Vogt + higher twist (power suppressed) contributions… STRUCTURE OF HIGHER ORDER CONTRIBUTIONS IN DIS
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DGLAP evolution: Linear RG in Q^2 Dokshitzer-Gribov-Lipatov-Altarelli-Parisi # of gluons grows rapidly at small x…
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increasing But… the phase space density decreases -the proton becomes more dilute Resolving the hadron -DGLAP evolution
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The other interesting limit-is the Regge limit of QCD: Physics of strong fields in QCD, multi-particle production- possibly discover novel universal properties of theory in this limit
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- Large x - Small x Gluon density saturates at f= BFKL evolution: Linear RG in x Balitsky-Fadin-Kuraev-Lipatov
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Proton Proton is a dense many body system at high energies QCD Bremsstrahlung Non-linear evolution: Gluon recombination
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Mechanism for parton saturation: Competition between “attractive” bremsstrahlung and “repulsive” recombination effects. Maximal phase space density => Saturated for Gribov,Levin,Ryskin Mueller, Qiu Blaizot, Mueller
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Need a new organizing principle- beyond the OPE- at small x. Higher twists (power suppressed-in ) are important when : Leading twist “shadowing’’ of these contributions can extend up to at small x.
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Born-Oppenheimer: separation of large x and small x modes Dynamical Wee modes Valence modes-are static sources for wee modes McLerran, RV; Kovchegov; Jalilian-Marian,Kovner,McLerran, Weigert In large nuclei, sources are Gaussian random sources MV, Kovchegov, Jeon, RV
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Hadron at high energies is a Color Glass Condensate Random sources evolving on time scales much larger than natural time scales-very similar to spin glasses Typical momentum of gluons is Bosons with large occupation # ~ - form a condensate Gluons are colored
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Quantum evolution of classical theory: Wilsonian RG Fields Sources Integrate out Small fluctuations => Increase color charge of sources JIMWLK (Jalilian-Marian, Iancu, McLerran, Weigert, Leonidov, Kovner)
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JIMWLK RG Eqns. Are master equations-a la BBGKY hierarchy in Stat. Mech. -difficult to solve Preliminary numerical studies. Mean field approximation of hierarchy in large N_c and large A limit- the BK equation. Rummukainen, Weigert Balitsky; Kovchegov
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The hadron at high energies Mean field solution of JIMWLK = B-K equation Balitsky-Kovchegov DIS: Dipole amplitude N satisfies BFKL kernel
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BK: Evolution eqn. for the dipole cross-section From saturation condition, 1 1/2 Rapidit y:
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How does Q_s behave as function of Y? Fixed coupling LO BFKL: LO BFKL+ running coupling: Re-summed NLO BFKL + CGC: Triantafyllopolous Very close to HERA result!
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Remarkable observation: Munier-Peschanski B-K same universality class as FKPP equation FKPP = Fisher-Kolmogorov-Petrovsky-Piscunov FKPP-describes travelling wave fronts - B-K “anomalous dimensions” correspond to spin glass phase of FKPP
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Stochastic properties of wave fronts => sFKPP equation W. Saarlos D. Panja Exciting recent development: Can be imported from Stat. Mech to describe fluctuations (beyond B-K) in high energy QCD. Tremendous ramifications for event-by-event studies At LHC and eRHIC colliders!
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Novel regime of QCD evolution at high energies “Higher twists” Leading twist shadowing
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Universality: collinear versus k_t factorization Collinear factorization: Di-jet production at colliders
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k_t factorization: Are these “un-integrated gluon distributions” universal? “Dipoles”-with evolution a la JIMWLK / BK
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Solve Yang-Mills equations for two light cone sources: For observables average over HADRONIC COLLISIONS IN THE CGC FRAMEWORK
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K_t factorization seen “trivially” in p-p Also holds for inclusive gluon production lowest order in but all orders in Adjoint dipole -includes all twists Breaks down at next order in Krasnitz,RV; Balitsky Systematic power counting-inclusive gluon production
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Quark production to all orders in pA Blaizot, Gelis, RV Two point-dipole operator in nucleus 3- & 4- point operators More non-trivial evolution with rapidity…
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The demise of the Structure function Dipoles (and multipole) operators may be more relevant observables at high energies Are universal-process independent. RG running of these operators - detailed tests of high energy QCD. Jalilian-Marian, Gelis; Kovner, Wiedemann Blaizot, Gelis, RV
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Strong hints of the CGC from Deuteron-Gold data at RHIC
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Colliding Sheets of Colored Glass at High Energies Classical Fields with occupation # f= Initial energy and multiplicity of produced gluons depends on Q_s Krasnitz,Nara,RV; Lappi Straight forward extrapolation from HERA: Q_s = 1.4 GeV
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McLerran, Ludlam In bottom up scenario, ~ 2-3 fm at RHIC Baier,Mueller,Schiff,Son Exciting possibility - non-Abelian “Weibel” instabilities speed up thermalization - estimates: Isotropization time ~ ~ 0.3 fm Mrowczynski; Arnold,Lenaghan,Moore,Yaffe Romatschke, Strickland; Jeon, RV, Weinstock
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Are there contributions in high energy QCD beyond JIMWLK? Are “dipoles” the correct degrees of freedom at high energies? Do we have a consistent phenomenological picture? Can we understand thermalization from first principles?
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