Atomic-Mass Dependence of the Transverse-Momentum Distribution of Massive Muon Pairs from 225-GeV/cπ−-Nucleus Collisions

We have measured the atomic-mass dependence of the $P_T$ distribution of massive muon pairs produced in 225-GeV/c ${\pi }^{-}$-nucleus collisions at Fermilab. We find that $〈P_T^2〉$ is consistent with being independent of the atomic mass $A$, with the value of 1.69±0.10 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ for muon pairs of mass between 4.0 and 8.5 GeV/${\mathit{c}}^2$. When the dependence of $〈P_T^2〉$ on $A$ is parametrized as $〈P_T^2〉=a+b{A}^{\frac{1}{3}}$, we find that $b=-0.079\mathrm{}\pm 0.073$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ which corresponds to an upper limit of 0.015 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ at the 90% confidence level.