Transverse momentum distributions in the nonperturbative region

We study the low transverse momentum (p_T) distribution of the Z-boson at hadron colliders for p_T∼Λ_QCD using a factorization and resummation formula derived in the soft-collinear effective theory (SCET). In the region p_T∼Λ_QCD, new nonperturbative effects arise that cannot be entirely captured by the standard parton distribution functions, and require an additional new nonperturbative transverse momentum function (TMF). The TMF is field-theoretically defined in SCET, fully gauge-invariant, and captures the nonperturbative dynamics that affect the p_T-distribution in the region p_T∼Λ _QCD. The TMF also reduces to the expected perturbative result in the region p_TΛ_QCD. We develop phenomenological models for these TMFs in the nonperturbative region, and discuss in detail fits to Tevatron data in which both experimental and theoretical errors are carefully treated.