Preliminary Results from MINOS on Muon Neutrino Disappearance Based on an Exposure of 2.5x10²0 120 GeV Protons on the NuMI Target

Updating our previous measurements with new data and analysis modifications, we report preliminary results on the energy-dependent deficit of muon neutrinos from the Fermilab NuMI beam as observed with the MINOS Far Detector located 735 km away in the Soudan Underground Laboratory. From an exposure of 2.50x10^20 protons on target, we observe 563 charged-current nu_mu interaction candidates in the Far Detector, where 738+/-30 events are expected in the absence of neutrino oscillations. We have analyzed these data assuming two-flavor nu_mu to nu_tau oscillations. From a simultaneous fit to the reconstructed nu_mu energy spectra obtained during two running periods we obtain the neutrino squared-mass difference Delta m_{32}^2 = (2.38 +0.20/-0.16) x 10^-3 eV^2/c^4 with errors at 68% confidence level (CL), and mixing angle sin^2(2 theta_23) > 0.84 at 90% CL. The uncertainties and confidence intervals include both statistical and systematic errors. All results and plots presented here are preliminary.