Lam-Tung relation breaking in Z0 hadroproduction as a probe of parton transverse momentum
AbstractThe Lam-Tung relation breaking coefficient ALT=A0-A2 in the Drell-Yan dilepton angular distributions in the Z0 boson mass region at the LHC is analyzed in the kT-factorization approach. This observable was recently measured with high precision by the ATLAS Collaboration. Within the kT-factorization approach we perform an approximate O(αemαs2) calculation of the off-shell parton hard matrix elements in which we include the leading tree-level contributions of valence quarks and off-shell gluons: the qvalg*→qZ0 channel and the g*g*→qq¯Z0 channel. The resulting ALT exhibits high sensitivity to the gluon transverse momentum distribution (TMD). Several gluon TMDs are probed derived from the Catani-Ciafaloni-Fiorani-Marchesini and Balisky-Fadin-Kuraev-Lipatov evolution equations, and given by QCD-inspired phenomenological parameterizations. The ATLAS data favor a simple “Weizsäcker-Williams” (WW) hard gluon TMD with the asymptotic behavior of one-gluon exchange at large gluon transverse momenta and moderate x. It is verified that the proposed approach with the WW gluon TMD describes well also the A0 and A2 angular coefficients at the Z0 peak, as well as the Drell-Yan dilepton mass distribution at lower masses. We conclude that the inclusion of gluon transverse momentum effects improves the description of the angular distributions of Drell-Yan dileptons and that the Drell-Yan scattering provides an excellent probe of the parton TMDs.