Using 7.3 fb−1 of p¯p collisions collected by the D0 detector at the Fermilab Tevatron, we measure the distribution of the variable ϕ∗η, which probes the same physical effects as the Z/γ∗ boson transverse momentum, but is less susceptible to the effects of experimental resolution and efficiency. A QCD prediction is found to describe the general features of the ϕ∗η distribution, but is unable to describe its detailed shape or dependence on boson rapidity. A prediction that includes a broadening of transverse momentum for small values of the parton momentum fraction is strongly disfavored.Received 1 October 2010DOI:https://doi.org/10.1103/PhysRevLett.106.122001© 2011 American Physical Society