Revealing baryon number fluctuations from proton number fluctuations in relativistic heavy ion collisions

Baryon number cumulants are invaluable tools to diagnose the primordial stage of heavy ion collisions if they can be measured. In experiments, however, proton number cumulants have been measured as substitutes. In fact, proton number fluctuations are further modified in the hadron phase and are different from those of the baryon number. We show that the isospin distribution of nucleons at kinetic freeze-out is binomial and factorized. This leads to formulas that express the baryon number cumulants solely in terms of proton number fluctuations, which are experimentally observable.