  Non-monotonic variation with collision energy ($\sqrt{s_{\rm NN}}$) of the
moments of the net-baryon number distribution in heavy-ion collisions, related
to the correlation length and the susceptibilities of the system, is suggested
as a signature for the Quantum Chromodynamics (QCD) critical point. We report
the first evidence of a non-monotonic variation in kurtosis times variance of
the net-proton number (proxy for net-baryon number) distribution as a function
of \rootsnn with 3.1$\sigma$ significance, for head-on (central) gold-on-gold
(Au+Au) collisions measured using the STAR detector at RHIC. Data in
non-central Au+Au collisions and models of heavy-ion collisions without a
critical point show a monotonic variation as a function of $\sqrt{s_{\rm NN}}$.
