  A mixed quantal-semiquantal theory is presented in which the semiquantal
squeezed-state wave packet describes the heavy degrees of freedom. We first
derive mean-field equations of motion from the time-dependent variational
principle. Then, in order to take into account the interparticle correlation,
in particular the 'quantum backreaction' beyond the mean-field approximation,
we introduce the stochastic particle description for both the quantal and
semiquantal parts. A numerical application on a model of O2 scattering from a
Pt surface demonstrates that the proposed scheme gives correct asymptotic
behavior of the scattering probability, with improvement over the mixed
quantum-classical scheme with Bohmian particles, which is comprehended by
comparing the Bohmian and the stochastic trajectories.
