Multiplicity distributions associated to subthreshold events in heavy-ion collisions

Abstract

Subthreshold events (pion production, for instance, at energies $E<{m}_{\ensuremath{\pi}})$ in heavy-ion collisions are treated as rare cluster-cluster collision events. On kinematical grounds such events are forbidden in free nucleon-nucleon and even nucleon-nucleus collisions. We show, by using effective mass clustering arguments, that the associated distribution ${P}_{c}(n),$ when the pion trigger is present, and the unconstrained distribution $P(n),$ when there is no pion trigger, are related by the universal relation ${P}_{c}{(n)=(n}^{2}/〈{n}^{2}〉)P(n).$ This relation, and in particular an improved version taking into account the number of clustering nucleons, is in fair agreement with data. Moreover, it allows information to be extracted from the multiplicity data on the number of nucleons involved in subthreshold meson production processes.