  The single-particle inclusive differential cross-section for a reaction
$a+b\to c+X$ is written as the imaginary part of a correlation function in a
for ward scattering amplitude for $a+b\to a+b$ in a modified effective theory.
In this modified theory the interaction Hamiltonian $\tilde H_I$ equals $H_I$
in the original theory up to a certain time. Then there is a sign change and
$\tilde H_I$ becomes nonlocal. This is worked out in detail for scalar field
models and for QED plus the abelian gluon model. A suitable path integral for
direct calculations of inclusive cross sections is presented.
