Color Glass Condensate : Theory and Phenomenology Azfar Adil PHENIX Journal Club

Overview Evolution Saturation and CGC Phenomenology DGLAP ( log(Q2) evolution ) BFKL ( log(1/x) evolution) Saturation and CGC Non-Linearities at low x Dipole versus Classical Field Phenomenology Observables at RHIC Implications 11/21/2018

A look at DIS Factored x-sections x and Q2 set via kinematics Function f(x,Q2) can be predicted given f(x0,Q02) This is evolution 11/21/2018

The “Phase Diagram” BFKL DGLAP Weigert hep-ph/0501087 11/21/2018

Leading Order Evolution “Wee” radiation is leading order process Parton starts at distribution f(x0,Q02) and multiply radiates to get to f(x,Q2) Get “large logarithms” we need to resum 11/21/2018

DGLAP Evolution Experimentally access various momentum transfers (X1,Q12) (X2,Q22) (X4,Q42) (X3,Q32) Experimentally access various momentum transfers Need evolution in Q2 Collinear Factorization 11/21/2018

Gluons and Quarks at Low-x Distribution functions xq(x,Q2) and xG(x,Q2) rise steeply at low Bjorken x. Gluons and Quarks Gluons only Is all this well-described by the standard DGLAP evolution?

Negative gluon distribution! NLO global fitting based on leading twist DGLAP evolution leads to negative gluon distribution MRST PDF’s have the same features Does it mean that we have no gluons at x < 10-3 and Q=1 GeV? No! 11/21/2018

BFKL Evolution Experimentally access various COM energies (X1,Q12) (X2,Q22) (X4,Q42) (X3,Q32) Experimentally access various COM energies Need evolution in 1/x kT Factorization 11/21/2018

BFKL from HERA HERA DIS data shown to be explainable using BFKL type dynamics Still have the x- singular behavior (violates unitarity) 11/21/2018

A Different Point of View Boost calculation to “dipole” frame Calculations factorizes into wave function and cross section Evolution encompassed in dipole cross section Linear Non Linear 11/21/2018

Hints of a Solution A scaling property seen in DIS e-A dat  = Q2/Q0(x)2 Suggests generation of an x dependent scale Q0(x) Characteristic of ‘saturation’ model of Golec-Biernat-Wusthoff 11/21/2018

Non Linearity to Saturation Resum pomeron loops to get non linear effects Pomerons are effective at large energies and large densities Get the BK equations and the Balitsky Heirarchy 11/21/2018

Classical Field Picture In the saturation regime we get N ~ 1/ Get a background classical field description Quantum evolution comes from separation between field and source dof JIMWLK Equation Note : Can also be formulated as evolution of Qs or  11/21/2018

McLerran-Venugopalan Model Assume a Gaussian weight (MV model) JIMWLK with MV Initial Conditions gives evolution for Qs and   can now be used with kT factorized formula to calculate production 11/21/2018

RHIC Phenomenology 11/21/2018

CGC Forward Suppression Suppression in RdAu at forward rapidities is said to be indicative of saturation physics This is unclear because other physics also “works” 11/21/2018

RHIC Bulk Production Models “inspired by” CGC explain well the total particle production, e.g. KLN Are a viable alternative to phenomenological models as in HIJING 11/21/2018

Other Implications !! Need high viscosity with CGC !!!! Hirano et al. Nucl-th/0511046 Need high viscosity with CGC !!!! Hirano et al. Nucl-th/0511046 !! 11/21/2018

The CGC/KLN Bulk Model Use kT factorized GLR formula Gluon Distributions depend on Qs Qs determined locally (Not factorized!!!) 11/21/2018

Eccentric CGC Initial spatial eccentricity causes v2 For Participant For CGC 11/21/2018

Problems with KLN model Not factorized as Has trouble getting multiplicities for smaller systems (d-Au, p-p) and larger systems (Au-Au) consistently 11/21/2018

The Correct Limits To get a universal CGC theory we need We also need for Solution is … 11/21/2018

Factorized KLN (fKLN) Make the replacements Explicitly factorized Correct nuclear edge limit Can now consistently investigate small systems 11/21/2018

Start with p-p Use GLR formula to calculate production Normalize to p-p data Set average Qs to 2 GeV2 at RHIC 11/21/2018

Move on to A-A 11/21/2018

Asymmetric Collisions… 11/21/2018

The Bottom Line fKLN is an improvement Has different eccentricity Theoretically Consistent Phenomenologically Successful Has different eccentricity Need to run hydro with it 11/21/2018

Conclusions - I Parton Evolution is Key Prediction of QCD Get log scaling violations in Q2 and 1/x DGLAP, BFKL and DLLA not unitary CGC - a QCD effective field theory Takes into account fully non perturbative non linear correction Includes generation of a large scale Qs Need more work in proving kT factorization as well as complete solutions for JIMWLK 11/21/2018

Conclusions - II KLN one implementation of CGC fKLN improves KLN Gets centrality dependence Not factorized Not good with small systems and nuclear edge Predicts large spatial eccentricity giving large v2 fKLN improves KLN Gets improved and consistent results with smaller systems Explicitly factorized Gets smaller eccentricity, needs to be input into hydro 11/21/2018