MuGirl -- Muon identification in the ATLAS detector from the inside out

HEP experiments have traditionally identified muons by associating muon hits or segments with particle tracks measured in the inner tracking detector. Due to the toroid magnet system of the muon spectrometer in the ATLAS experiment, muon tracks can be reconstructed independently in the muon spectrometer. The ATLAS experiment has two programs, MuonBoy and MOORE, which perform this "standalone" muon reconstruction. Following this step, the muon tracks or segments are combined with inner detector tracks to obtain the muon momentum at the interaction point. We present a new program that identifies muons in the traditional way, associating muon hits and segments to an inner detector track in order to flag the track as a muon. The muon momentum and position are taken from the inner detector track parameters, giving a good measure of the muon momentum for muons with pTup to about 50 GeV. This approach has advantages for muons with low pT, which lose a significant part of their energy in the calorimeter. Some of them cannot be reconstructed in the muon spectrometer because there are not sufficient hits for the track fit, or because the track was modified too much by energy loss and multiple scattering. This approach is also advantageous where the muon spectrometer coverage is incomplete because of layout issues or malfunctioning chambers. This orthogonal approach allows cross-checking the results of the other two programs. The structure and physics performance of this program are presented here.