A Measurement of the Low Mass Drell-Yan Differential Cross Section in the Di-Muon Channel with $\sqrt{s}=7$ TeV Proton-Proton Collisions at the ATLAS Experiment

A measurement of the Drell-Yan differential cross section at low invariant mass is presented in the di-muon channel. A 1.64 pb$^{-1}$ dataset of $\sqrt{s}=7$ TeV proton-proton collision data collected by the ATLAS experiment at the LHC is used. The measurement is made in an invariant mass range of $26 < M_{\mu\mu} < 66$ GeV where $M_{\mu\mu}$ is the invariant mass of the muon pair. A review of the relevant theoretical physics and the ATLAS detector is made. The analysis is described with particular attention paid to the determination of the isolation efficiency corrections for the Monte Carlo and the estimate of the multijet background. The fiducial differential cross section is calculated with a statistical uncertainty that varies between 0.8% and 1.2%. The systematic uncertainty is seen to vary between 2.4% and 4.1%. A cross section extrapolated to the full phase space is also presented. This is dominated by theoretical uncertainties from the variation of the factorisation and renormalisation scales. The obtained fiducial differential mass cross section is compared to theoretical predictions at NLO and NNLO in perturbative QCD. It is shown that a move beyond NLO is needed to describe the distribution well due to the restrictions of using a fixed order theoretical prediction. A combination with the electron channel measurement is also briefly discussed as well as comparisons to a di-muon measurement in an extended invariant mass range. This allows similar, but stronger conclusions to be drawn. A discussion is made of a PDF fit that uses the measurement presented here. The fit demonstrates the impact of the measurement on the PDFs and further supports the conclusion that a move to NNLO in pQCD is needed to describe the data