Associated quarkonium production at ATLAS as a new probe of QCD

We present new measurements of the associated production of quarkonium with a vector boson or an additional quarkonium state using the ATLAS Run-1 dataset. These rare processes provide new insight into QCD models of quarkonium production, but also provide new opportunities to study double parton scattering, including cross-section measurements in single and double parton scattering dominated regimes and a precise assessment of the sigma_eff parameter governing the effective spatial area of parton-parton interactions at a variety of energy scales.