Measurement of the W Boson Mass in Proton-Proton Collisions at s = 7 TeV with the ATLAS Detector

A measurement of the mass of the $W$ boson is presented based on proton--proton collision data recorded in 2011 at a centre-of-mass energy of 7 TeV with the ATLAS detector at the LHC, and corresponding to 4.6 fb$^{-1}$ of integrated luminosity. The selected data sample consists of $7.8\times 10^6$ candidates in the $W\rightarrow \mu \nu$ channel and $5.9\times 10^6$ candidates in the $W\rightarrow e \nu$ channel. The $W$-boson mass is determined using template fits to the Jacobian edges of the reconstructed distributions of the transverse momentum of the charged leptons and the transverse mass of the $W$ boson in the electron and muon decay channels, yielding \begin{eqnarray} \nonumber m_W &=& 80370 \pm 7 (\textrm{stat.}) \pm 11 (\textrm{exp. syst.}) \pm 14 (\textrm{mod. syst.}) \textrm{ MeV}\\ \nonumber &=& 80370 \pm 19 \textrm{ MeV}, \end{eqnarray} where the first uncertainty is statistical, the second corresponds to the experimental systematic uncertainty, and the third to the physics-modelling systematic uncertainty. A measurement of the mass difference between the $W^+$ and $W^-$ bosons yields $m_{W^+}-m_{W^-} = -29 \pm 28$ MeV.