Saturation physics: NLO precision and quasi-collectivity

The perturbative saturation regime or Color Gass Condensate (CGC), is an exciting and qualitatively new regime of strong interactions. It is characterized by high partonic density in hadrons and has specific manifestations in the behavior of final states at high energies and large nuclei. This regime may have been achieved experimentally at RHIC and LHC in heavy ion collisions. Many aspects of p—Pb collisions at LHC however display apparent collectivity in the final state. Whether this is due to strong final state interactions, or to nontrivial structure of the CGC initial state is still a subject for debate. Observation of the saturation regime is one of the main motivations for building the Electron Ion Collider in the US. Confirmation (or negation) of observation of this phenomenon hinges crucially 011 the ability to provide precise quantitative description of data.

Our research aimed at developing a program for systematic improvement of current calculational paradigm of saturation effects. This involves systematic resummation program in the high energy evolution equations, this work is still in progress. We also proposed to study in depth to what extent the structure of initial CGC state can lead to quasi collectivity in the state produced in p—A collisions.

One of the central goals was to understand mechanism of particle production at RHIC and LHC and explain nature of various correlations observed experimentally. We produced several publications addressing this problem. We studied particle production (gluon and quark jets, photons) in central and forward kinematics, in the CGC framework. A special focus was placed on two—particle correlations, such as long range angular correlations, known as ridge. We have identified several mechanisms for these correlations, all of them related to initial state correlations, such as Bose—correlations in the wave—function of incoming proton. Pauli blocking effects for quark production were quantified too. Additional mechanism for correlations was identified as to be induced by charge density fluctuations. One of the main puzzles existing in this field is the origin of odd harmonics in the Fourier decomposition of the correlations. We have come with various original ideas about possible sources of these harmonics. We further considered three—particle correlations such dijet plus photon and quar—antiquar—gluon correlations. The latter provides a background for chiral magnetic effect, which is being searched for at RHIC.

Solid progress has been achieved in understanding collective phenomena in terms of quasi-thermodynamics. We have advanced 011 several research directions expending our original computation of entanglement entropy of hadronic wave function and the entropy production. Through thermodynamic relations, the produced entropy can be related to the temperature of the produced state. ‘We have passed a milestone understanding time—dependent effects in entropy production and understood questions related to energy dependence of partonic entanglement entropy. We published two papers 011 this topic, but further work is needed to relate the thermalisation puzzle to direct quantum mechanical computations.

Our research revealed new aspects of QCD beyond naive perturbation theory, particularly focusing on phenomena related to extreme conditions, such high energy and high density. We addressed questions fundamental to understanding the experimental results from RHIC and LHC.