  We propose an intensity-dependent pion-nucleon coupling Hamiltonian within a
unitary multiparticle-production model of the Auerbach-
Avin-Blankenbecler-Sugar (AABS) type in which the pion field is represented by
the thermal-density matrix.Using this Hamiltonian, we explain the appearance of
the negative-binomial (NB) distribution for pions and the well-known empirical
relation, the so-called Wr\' oblewski relation, in which the dispersion $D$ of
the pion- multiplicity distribution is linearly related to the average
multiplicity $<n>$ : $D = A <n> + B$, with the coefficient $A < 1$. The
Hamiltonian of our model is expressed linearly in terms of the generators of
the $SU(1,1)$ group.We also find the generating function for the pion field,
which reduces to the generating function of the NB distribution limit $T \to
0$.
