  We propose a method to experimentally study the equation of state of strongly
interacting matter created at the early stage of nucleus--nucleus collisions.
The method exploits the relation between relative entropy and energy
fluctuations and equation of state. As a measurable quantity, the ratio of
properly filtered multiplicity to energy fluctuations is proposed. Within a
statistical approach to the early stage of nucleus-nucleus collisions, the
fluctuation ratio manifests a non--monotonic collision energy dependence with a
maximum in the domain where the onset of deconfinement occurs.

High Energy Physics - Phenomenology