Variations of the crossover and first-order phase transition curve in modeling the QCD equation of state

Lattice QCD calculations have shown that the transition from hadrons to quarks and gluons is a rapid crossover at $T = 155-160$ MeV at vanishing chemical potential. Many model calculations show that the transition is first-order at sufficiently high baryon chemical potential. It is then natural to expect the existence of a critical point where the crossover and first-order phase transition lines meet. We show how to embed a phase boundary that terminates at the critical point in a smooth background equation of state, using several different but closely related criteria, so as to yield the critical exponents and critical amplitude ratios expected of a transition in the 3D Ising and liquid-gas universality class. The crossover curves can be tuned to pass through experimental freeze-out data from heavy ion collisions at RHIC and the LHC. The resulting equations of state can be used in hydrodynamic simulations of these collisions to probe the existence of a critical point and corresponding first-order phase transition.