CMS DT Chambers: Optimized Measurement of Cosmic Rays Crossing Time in absence of Magnetic Field

Two spare drift chambers produced in the I.N.F.N. Legnaro Laboratory (Padova, Italy) for the barrel muon spectrometer of the LHC CMS experiment have been extensively tested using cosmic-ray muons. A fitting algorithm was developed to optimize the determination of the time of passage of the particle. A timing resolution of $\approx$2~ns has been obtained. The algorithm permits the measurement of the track reconstruction precision of the chambers by using cosmic-ray data with the same accuracy obtained using high-energy test-beam data.