Research Projects

The recent discovery of spin polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons at the Relativistic Heavy Ion Collider (RHIC), Large Hadron Collider (LHC), and GSI Helmholtz Centre for Heavy Ion Research has opened an intriguing possibility to explore the vortical structure of the QCD plasma produced in non-central heavy-ion collision experiments. Spin polarization of hadrons due to the fluid vorticity is a plausible scenario because of the interesting speculation that peripheral heavy-ion collisions can be associated with large angular momentum. After the collision, most of this angular momentum is carried away by the ``spectators", but there could be a sizable fraction of the initial angular momentum that can remain in the created thermalized partonic medium due to the inhomogeneous density profile of the colliding nuclei. Such a nonvanishing angular momentum can give rise to nonzero vorticity or rotational structure in the QCD plasma. If such vorticity exists, then due to the spin vorticity coupling, it can give rise to the spin polarization of hadrons. Geometrically, in heavy-ion collisions, the reaction plane (typically considered as the XZ-plane) is constituted by the beam direction (typically considered along the Z-axis) and the impact parameter vector (typically considered along the X-axis). The transverse plane (typically considered as the XY-plane) is normal to the reaction plane. Experimentally there are two distinct spin observable, these are global spin polarization, i.e., the spin polarization of hadrons along the direction of global angular momentum (direction normal to the reaction plane of heavy ions, i.e., Y axis), and local spin polarization, i.e., the longitudinal (z-component of the spin polarization vector) spin polarization as a function of the azimuthal angle in the transverse plane. This azimuthal angle dependence of spin polarization originates from the momentum dependence on the transverse plane. Interestingly, theoretical models that take into account only spin-thermal vorticity coupling in equilibrium do not provide a satisfactory explanation for experimental data on the sign of the longitudinal spin polarization measurements of hyperons. Different theoretical models fail to explain the azimuthal angle dependence of the longitudinal spin polarization on the plane perpendicular to the reaction plane. The theoretical estimation of the azimuthal angle dependence is completely opposite to the observation. This is usually called the “spin sign problem”. Our goal is to develop a hydrodynamic framework incorporating the dynamical evolution of the spin of microscopic constituents and explain the observation data on the spin polarization in QCD medium. Fluid dynamics allows us to account for out-of-equilibrium physics in the evolution of spin polarization. In this project, we develop the ``Muller-Israel-Stewart" type second-order dissipative spin hydrodynamic framework considering the spin chemical potential in the leading order in the hydrodynamic gradient expansion. Spin chemical potential captures the dynamical evolution of the spin degree of freedom within the hydrodynamic framework. Spin hydrodynamics represents a cutting-edge research direction in the field of heavy-ion collision physics. We aim to look into the theoretical complexities of spin hydrodynamic frameworks, exploring aspects such as analytical solutions for spin hydrodynamic equations, causality properties, etc. Second-order hydrodynamic theories that are expected to be both causal and stable, and are commonly applied in the realm of heavy-ion physics phenomenology. We aim to generalize a well-explored numerical dissipative hydrodynamic framework to the spin hydrodynamic framework for the realistic simulation of QCD medium. Subsequently, this computational model will be utilized to investigate polarization data and address the ``spin sign problem" along with other phenomenological aspects, e.g., anisotropy in dilepton spectra, etc.