Abstract
We discuss the applicability of the k_T factorization theorem to low-p_T hadron production in hadron-hadron collision in a simple toy model, which involves only scalar particles and gluons. It has been shown that the k_T factorization for high-pT hadron hadroproduction is broken by soft gluons in the Glauber region, so we restored the k_T factorization in the sense that a TMD parton density maintains its universality. These procedure guaranteed our next study, in which we point out that the existence of Glauber gluons in the k_T 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. Radiations, that balance the lepton-pair transverse momentum, cause 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.