Glauber gluons in pion-induced Drell-Yan processes

We point out that the existence of Glauber gluons in the kT factorization theorem can account for the violation of the Lam-Tung relation, namely, the anomalous lepton angular distribution observed in pion-induced Drell-Yan processes. The emission of a final-state parton, that balances the lepton-pair transverse momentum, causes the responsible spin-transverse-momentum correlation in the Glauber-gluon background. It is argued that the Glauber effect is significant in the pion due to its unique role of being a Nambu-Goldstone boson and a qq¯ bound state simultaneously. This mechanism is compared to other resolutions in the literature by means of vacuum effects and Boer-Mulders functions. We propose to discriminate the above resolutions by measuring the pp¯ Drell-Yan process at GSI and J-PARC.