Determination of the strange quark density of the proton from ATLAS measurements of the $W \to \ell \nu$ and $Z \to \ell\ell$ cross sections

A QCD analysis is reported of ATLAS data on inclusive W and Z boson production in pp collisions at the LHC, jointly with ep deep inelastic scattering data from HERA. The ATLAS data exhibit sensitivity to the light quark sea composition and magnitude at Bjorken x ~ 0.01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1.00(+0.25-0.28) at absolute four-momentum transfer squared Q^2 = 1.9 GeV^2 and x = 0.023.