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.
