Abstract
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.
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