Determination of the muon charge sign with the dipolar spectrometers of the OPERA experiment

TL;DR

The paper presents a new method for determining muon charge sign in the OPERA experiment using dipolar spectrometers, significantly reducing charge misidentification compared to previous techniques.

Contribution

A novel weighted angular matching technique for muon charge sign determination in dipolar spectrometers is introduced and validated with Monte Carlo and real data.

Findings

Up to 40% reduction in charge misidentification.

Effective background suppression in tau decay analysis.

Validation of the method with real experimental data.

Abstract

The OPERA long-baseline neutrino-oscillation experiment has observed the direct appearance of $\nu_\tau$ in the CNGS $\nu_\mu$ beam. Two large muon magnetic spectrometers are used to identify muons produced in the $\tau$ leptonic decay and in $\nu_\mu^{CC}$ interactions by measuring their charge and momentum. Besides the kinematic analysis of the $\tau$ decays, background resulting from the decay of charmed particles produced in $\nu_\mu^{CC}$ interactions is reduced by efficiently identifying the muon track. A new method for the charge sign determination has been applied, via a weighted angular matching of the straight track-segments reconstructed in the different parts of the dipole magnets. Results obtained for Monte Carlo and real data are presented. Comparison with a method where no matching is used shows a significant reduction of up to 40\% of the fraction of wrongly determined charges.