Low energy atmospheric muon neutrinos in MACRO

We present the measurement of two event samples induced by atmospheric nu /sub mu / of average energy E/sub nu /~4 GeV. In the first sample, a neutrino interacts inside the MACRO detector producing an upward-going muon leaving the apparatus. The ratio of the number of observed to expected events is 0.57+or-0.05/sub stat /+or-0.06/sub syst/+or-0.14/sub theor/ with an angular distribution similar to that expected from the Bartol atmospheric neutrino flux. The second is a mixed sample of internally produced downward-going muons and externally produced upward-going muons stopping inside the detector. These two subsamples are selected by topological criteria; the lack of timing information makes it impossible to distinguish stopping from downgoing muons. The ratio of the number of observed to expected events is 0.71+or-0.05/sub stat/+or-0.07/sub syst/+or-0.18 /sub theor/. The observed deficits in each subsample is in agreement with neutrino oscillations, although the significance is reduced by the large theoretical errors. However, the ratio of the two samples causes a large cancellation of theoretical and of some systematic errors. With the ratio, we rule out the no-oscillation hypothesis at 95% c.l. Furthermore, the ratio tests the pathlength dependence of possible oscillations. The data of both samples and their ratio favor maximal mixing and Delta m/sup 2/~10/sup -3/-10/sup -2/ eV/sup 2/. These parameters are in agreement with our results from upward throughgoing muons, induced by nu /sub mu / of much higher energies. (16 refs)