Lam-Tung relation breaking in $Z^0$ hadroproduction as a probe of parton transverse momentum

Leszek Motyka; Mariusz Sadzikowski; Tomasz Stebel

Paper

Lam-Tung relation breaking in $Z^0$ hadroproduction as a probe of parton transverse momentum

Abstract

The Lam-Tung relation breaking coefficient

in the Drell-Yan dilepton angular distributions in the

boson mass region at the LHC is analyzed in the

-factorization approach. This observable was recently measured with high precision by ATLAS collaboration. Within the

-factorization approach we perform an approximate

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

channel and the

channel. The resulting

exhibits high sensitivity to the gluon transverse momentum distribution (TMD). Several gluon TMDs are probed derived from the CCFM and BFKL 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

. It is verified that the proposed approach with the WW gluon TMD describes well also the

and

angular coefficients at the

peak, as well as the Drell-Yan dilepton mass distribution at lower masses. We conclude that inclusion of gluon transverse momentum effects improves description of the angular distributions of Drell-Yan dileptons and that the Drell-Yan scattering provides an excellent probe of the parton TMDs.