Research

I perform research on the phenomenology of the strongly interacting matter, the fireball produced in relativistic heavy ion collisions. Quantum Chromodynamics (QCD), the theory of strong interactions that govern the dynamics of the quark gluon plasma (QGP) as well as the hadronic medium that is produced in heavy ion collision (HIC) experiments is non-perturbative for the thermodynamic conditions relevant to the phenomenology of HICs. Currently, there is no systematic first principle approach to study the QCD phase diagram. This highlights the importance of the role of HIC experiments and phenomenological approach to unravel the hot and dense QCD medium properties. The physics of the QCD phase diagram is interesting due to several reasons. So far only perturbative aspects of QCD have been tested experimentally. HIC phenomenology gives us the unique opportunity to test non-perturbative aspects of QCD. Further, QGP is the state of matter expected to be in the core of neutron stars as well as the early universe. Thus, knowledge of QGP properties will have astrophysical and cosmological implications.

The timeline of a typical HIC event can be classified into three stages: (i) the intial stage just after the collision takes place and before thermalization has happened. This stage is estimated to last~1 fm/c, (ii) the expansion phase of the fireball produced maintaining local thermal equilibrium. Hydrodynamics has been quite successful in modelling this stage which lasts~10 fm/c, and (iii) the final stage is known as freezeout when the microscopic mechanism that has been maintaining local thermodynamic equilibrium can no longer cope with the expansion and the description of the fireball in terms of hydrodynamic field variables beaks down.

Some specific research topics on which I am currently interested in are outlined below:

1. Freezeout:

One of the main source of background in QGP phenomenology is that of the hadron dynamics at freezeout. A good understanding of hadronization, hadron-hadron interactions and their eventual freezeout is important to construct this background and subtract from data to uncover potential new physics (QCD critical point, chiral magnetic effect) of the QGP phase and the initial state. Flavor dependence, system size dependence, hadron-hadron correlations at freezeout built in due to conservation laws, freezeout of loosely bound composites like nuclei etc are some of the avenues related to the freezeout stage that we are pursuing.

Left: Raio of strange to non-strange phase space volumes at freezeout HIC [1, 2] Centre & Right: System size dependence in freezeout scheme [3]

2. Initial state:

This is one of the least understood stages of the Standard Model of HIC. The nuclei being extended objects, geometry plays a dominant role here. A clear understanding of the various geometrical contributions to the initial entropy deposition is necessary to understand the geometry driven background and single out the contribution from QCD dynamics. Glauber models have been quite successful in capturing various geometrical aspects of the initial state with a few parameters. The effect of nucleon shadowing, breaking of the longitudinal (along the beam axis) boost invariance etc are some current topics of interest that we are investigating. Apart from entropy deposition, the other topic of interest that has lately gained a lot of momentum is the generation of one of the strongest (in comparison to any terrestrial as well as known astrophysical sources) electromagnetic field. The evolution of the plasma in its presence and its experimental signatures are currently under investigation.

3. Flavor dynamics:

i. The plausibility of hierarchy in flavor dependence of cross section can lead to hierarchy in freezeout- most relevant for non-strange vs strange flavor. We have been trying to cook novel observables to confirm such phenomena. ii. Hierarchy in flavor dependence of quark mass has been already confirmed in high energy physics experiments. This has important consequence in HIC phenomenology. Charm and other heavier flavors have masses that are several times larger than the highest temperatures reached in these collisions, ruling out the possibility of thermal production. Thus they are unique probes that are witness to the evolution of the entire HIC event.

Left: Predicted 5 - 20 times larger D meson directed flow compared to light flavor [7] Right: Presence of electromagnetic field splits the directed flow of charmed mesons with opposite charges [8, 9]