Atmospheric neutrino induced muons in the MACRO detector

A measurement of the flux of neutrino-induced muons using the MACRO detector is presented. Different event topologies, corresponding to different neutrino parent energies can be detected. The upward throughgoing muon sample is larger event sample. For this sample, produced by neutrinos having an average energy around 100 GeV, the ratio of the number of observed to expected events integrated over all zenith angles is 0.74 +/- 0.036(stat) +/- 0.046(syst) +/- 0.13(theor). We have investigated whether the observed number of events and the shape of the zenith distribution can be explained by an hypothesis of v(mu) --> v(tau) oscillation. The best probability (17%) is obtained for sin(2) 2 theta similar or equal to 1.0 and Delta m(2) of a few times 10(-3) eV(2), while the probability for the no oscillation hypothesis is 0.1 %. The other samples are due to the internally produced events and to upward-going stopping muons; the average parent neutrino energy is of the order of 4 GeV. The low energy data sets show a deficit of observed events similar to the one predicted by the oscillation model with maximum mixing suggested from the upward throughgoing muon sample.