  We have constructed an equation of state which smoothly interpolates between
an excluded volume hadron resonance gas at low energy density to a plasma of
quarks and gluons at high energy density. This crossover equation of state
agrees very well with lattice calculations at both zero and nonzero baryon
chemical potential. We use it to compute the variance, skewness, and kurtosis
of fluctuations of baryon number, and compare to measurements of proton number
fluctuations in central Au-Au collisions as measured by the STAR collaboration
in a beam energy scan at the Relativistic Heavy Ion Collider. The crossover
equation of state can reproduce the data if the fluctuations are frozen out at
temperatures well below than the average chemical freeze-out.
