Cumulants of the chiral order parameter in a nonequilibrium Bjorken expansion with a QCD critical point

To understand experimentally obtained net-proton number cumulants in the search for the QCD critical point, we study a dynamical model based on an effective quark-meson Lagrangian with chiral symmetry. We investigate the evolution of the expanding medium created in a heavy-ion collision using a spatially homogeneous fluid and a time-dependent order parameter, the sigma field evolved by a Langevin equation. We extract cumulants of the sigma field along a parametrized freeze-out curve and match the obtained freeze-out points to corresponding beam energies. These cumulants can be related to cumulants of the net-proton number through the sigma-proton coupling to provide a qualitative comparison to experimental data from STAR’s beam energy scan program. We demonstrate that the presence of the spinodal or mixed phase region around the first-order chiral phase transition allows for a wide interval of cumulants at the lowest beam energies.

Critical exponents and transport properties near the QCD critical endpoint from the statistical bootstrap model

We present an estimate of the behavior of the shear and bulk viscosity coefficients when the QCD critical point is approached from the hadronic side, describing hadronic matter within the statistical bootstrap model of strong interactions. The bootstrap model shows critical behavior near the quark-hadron transition temperature if the parameter characterizing the degeneracy of Hagedorn states is properly chosen. We calculate the critical exponents and amplitudes of relevant thermodynamic quantities near the QCD critical point and combine them with an Ansatz for the shear and bulk viscosity coefficients to derive the behavior of these coefficients near the critical point. The shear viscosity to entropy density ratio is found to decrease when the temperature is increased, and to approach the Kovtun–Son–Starinets bound $$1/(4\pi )$$ 1 / ( 4 π ) faster near the critical point, while the bulk viscosity coefficient is found to rise very rapidly.

The Little-Bang and the femto-nova in nucleus-nucleus collisions

We make a theoretical and experimental summary of the state-of-the-art status of hot and dense QCD matter studies on selected topics. We review the Beam Energy Scan program for the QCD phase diagram and present the current status of the search for the QCD critical point, particle production in high baryon density region, hypernuclei production, and global polarization effects in nucleus-nucleus collisions. The available experimental data in the strangeness sector suggests that a grand canonical approach in the thermal model at high collision energy makes a transition to the canonical ensemble behavior at low energy. We further discuss future prospects of nuclear collisions to probe properties of baryon-rich matter. Creation of a quark-gluon plasma at high temperature and low baryon density has been called the “Little-Bang” and, analogously, a femtometer-scale explosion of baryon-rich matter at lower collision energy could be called the “femto-nova”, which could possibly sustain substantial vorticity and a magnetic field for non-head-on collisions.