The Angular Matching Method for the Muon Charge Sign Measurement in the OPERA Experiment

The long baseline neutrino oscillation experiment OPERA has observed the direct appearance of $\nu_{\tau}$ in the CNGS $\nu_{\mu}$ beam. The task of the two muon magnetic spectrometers of the detector is to identify muons produced in the tau lepton decay and in $\nu_{\mu}$ CC interactions, and measure their charge and momentum. Apart from the kinematic analysis of muonic tau lepton decays, this helps in reducing the background resulting from the decay of charmed particles produced in $\nu_{\mu}$ CC interactions. In the new method for the charge sign determination described below, a weight based on the angular matching between straight track segments reconstructed on both sides of a dipole magnet arm is attributed to each of up to four charge sign measurements. Results based on the analysis of Monte Carlo simulated and real data are presented. They are compared to those produced by the method used so far where weights are based on the precision on the measurement of the track segment angles. A significant reduction of up to 40% of the fraction of wrongly determined charges is obtained.