Abstract
The Beam Energy Scan program at the Relativistic Heavy Ion Collider (RHIC) is searching for the QCD critical point. The main signal for the critical point is the kurtosis of the distribution of proton yields obtained on an event by event basis where one expects a peak at the critical point. However, its exact behavior is still an open question due to out-of-equilibrium effects and uncertainty in the equation of state. Here we use a simplistic hydrodynamic model that enforces strangeness-neutrality, selecting trajectories that pass close to the critical point. We vary the initial conditions to estimate the effect of out-of-equilibrium hydrodynamics on the kurtosis signal.
Highlights
As the search for the QCD critical point [1,2,3,4,5] continues, it becomes increasingly important to have a thorough understanding of its signals, including what factors influence them and how strong those influences are
One such implementation is done in Ref. [8] in which a critical region with the expected 3D Ising universality class is smoothly matched to the Lattice QCD equation of state (EoS) at zero net baryon density and uses a Taylor expansion for the non critical contribution to the pressure at finite net baryon densities
The size, shape, and strength of the critical region can be selected through the use of multiple numerical parameters, each of which produce a realization of a QCD EoS with a critical point that can be constrained against data and theoretical consistency checks (e.g. 0 ≤ c2s ≤ 1)
Summary
As the search for the QCD critical point [1,2,3,4,5] continues, it becomes increasingly important to have a thorough understanding of its signals, including what factors influence them and how strong those influences are One such signal is non-monotonic behavior of the kurtosis of the equilibrium net baryon distribution [6, 7], a large peak in the vicinity of the critical region signifying large fluctuations of net baryon number. Since there is no first principles way to calculate the QCD equation of state (EoS) at finite baryon densities due to the sign problem, one must rely on phenomenological methods and models. It does give a qualitative idea of the kinds of effects one might expect to encounter in realistic model to data comparison
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