WHY THERE IS NO NUCLEAR-DEPENDENCE IN DIMUON PRODUCTION

Comparison of dimuon production in medium mass nuclei with that in the deuteron shows no effect of the nuclear environment for values of 0.1 < x < 0.3, just the region in which the pion excess model has produce d an enhancement. In the lowest-order approximation there is no meson excess, which can be understood by the common scaling of all masses an d cutoffs m(N)/m(N) = m(sigma)*/m(sigma) = m(omega)*/m(omega) = LAMBD A/LAMBDA, etc. In the next order, correction must be made for the fac t that the pion mass m(pi) does not scale in-medium. This is small. A second correction, pion rescattering by nucleonic excitations, is also small because of the local field correction g0'(q). The g0'(q) is onl y approximately 0.525 for q = 0, but its growth with q provides an eff ectively large g0', and this cancels the attraction in the pionic resc attering channel. Thus the pionic enhancement in the small-x region is suppressed. Inclusion of isobar excitation in the rescattering provid es a small enhancement, and the coupling of the rho-meson between nucl eon and isobar can be chosen within the range of preferred values so t hat there is no net effect of the nuclear environment in the small-x r egion considered.