Atomic-mass dependence of the transverse-momentum distribution of massive muon pairs from 225-GeV/c. pi. /sup -/-nucleus collisions

We have measured the atomic-mass dependence of the P/sub T/ distribution of massive muon pairs produced in 225-GeV/c ..pi../sup -/-nucleus collisions at Fermilab. We find that is consistent with being independent of the atomic mass A, with the value of 1.69 +- 0.10 (GeV/c)/sup 2/ for muon pairs of mass between 4.0 and 8.5 GeV/c/sup 2/. When the dependence of on A is parametrized as = a+bA/sup 1/3/, we find that b = -0.079 +- 0.073 (GeV/c)/sup 2/ which corresponds to an upper limit of 0.015 (GeV/c)/sup 2/ at the 90% confidence level.