Bounds on ν μ Oscillations from Atmospheric Neutrinos

A reanalysis of identified muon neutrino interactions from IMB 3 yields bounds on sin 2 (2θ) and ∆m 2. The limit sin 2 (2θ) < 0.72 is in conflict with the recent announcement of a neutrino mass. 1 Atmospheric neutrinos have always held the possibility of providing a large range of propagation coupled with a near source to provide a convenient calibration. With a reasonable choice of angular cuts a sample of neutrinos traveling about 10,000 km can easily be compared with a sample traveling on the order of 20 km collected in the same experiment at the same time. Early work [1] using these concepts was hampered by low statistics and poor particle identification. In [1] a mixed sample of 25 upward neutrino interactions was compared with a sample of 25 downward neutrino interactions. The neutrino propagation distance is a function of the neutrino direction, so scattering can, in principle, make the distance estimate ambiguous. In reality the propagation distance varies very slowly with angle near the vertical so as long as regions near the vertical are compared scattering will have a negligible effect on the sensitivity. In most analyses regions containing 20% of the solid angle over the upward and downward direction have been used. Depending on the location of the detector the upward and downward fluxes may be influenced differently by geomagnetic effects [2]. IMB was located at 52 • north geomagnetic latitude. This meant that the local flux, coming from above, had a lower geomagnetic cut off than the Earth at large. As such one would expect a greater rate of downward going events since the lower cut off permitted more of the extraterrestrial cosmic ray flux to descend and hit the atmosphere. But such effects are small and have not been observed in a statistically significant way even in a sample of 401 neutrino interactions [2]. In the 401 IMB 1 sample 55 events were measured in the upward going 20% of the solid angle and 65 were found in a comparable portion of downward solid angle. comparison of the 15 upward events and 21 downward events which were classified as muon neutrino induced because of the presence of a muon decay in the event yielded limits on ∆m 2 in the range of 5 × 10 −5 eV 2. IMB 3 provided a comparable sample of events but with better light collection and with …