Longitudinal polarization at high xF in muon pair production

We present results from experiment E615 at Fermilab, a high-statistics study of high-mass muon pair production by a 252-GeV π- beam on tungsten. The detector has large acceptance at high xF and at large /cosθ/, making it ideal to study polarization of virtual photons and J/ψ's at high xF. We show that at high xF both the continuum and J/ψ angular distributions show longitudinal polarization, in qualitative agreement with higher-twist models. The mechanism for J/ψ and ψ' production is also shown to be consistent with quark-antiquark annihilation at high xF