Multiplicity distributions associated to subthreshold events in heavy-ion collisions

Subthreshold events (pion production, for instance) at energies $E< m_{\pi}$ are treated as rare events. The associated multiplicity distribution $P_c (n)$ and the unconstrained distribution P(n), when there is no rare event-trigger, are related by the model independent relation $P_c (n) = {n^2 \over \average{n^2}} P(n)$. This relation, and in particular its improved version inspired in clustering of nucleons, is in fair agreement with data. Moreover, it allows to extract from the multiplicity data information on the number of nucleons involved in meson production processes, occuring at energies below the production threshold in free nucleon collisions.