Atmospheric Neutrinos and the Oscillations Bonanza

We are lucky, you and I, to be born here on planet Earth and to have as our birthright the unrestricted use of a splendid neutrino beamline. Truly remarkable is that the originating hadronic primary beam, namely the cosmic ray flux of protons and assorted stable nuclei, is isotropic to high degree. Moreover, the beamline target region, which is the terrestrial atmosphere, is very nearly spherically symmetric. Together, these two attributes ensure that neutrino fluxes of this beamline, in the absence of neutrino oscillations, must be up/down symmetric with respect to the horizon [1, 2]. Consequently, observation of a sizable neutrino flux up/down asymmetry by the user community is clear evidence that new physics is happening with neutrinos. The null oscillation up/down symmetry for neutrino fluxes is however, not complete. There are geomagnetic effects which produce mild distortions in the fluxes of low energy neutrinos and of horizontal neutrinos. These distortions, which are latitude-dependent, provide useful tests for data verity. The beamline delivers neutrino fluxes which are wide-band in Eν and which contain the flavors (νμ + νμ) and (νe + νe). In the absence of oscillations, these flavors must occur in the ratio 2:1 for sub-GeV neutrinos; for multi-GeV neutrinos, the ratio should increase gradually with Eν . At or below the Earth’s surface, the atmospheric flux is about 103 ν ’s incident per human body per second [2], an amount which is adequate for experimentation, but does not pose a radiation safety hazard. The neutrino path lengths L which are possible in this beamline range from 20 km for ν ’s incident from the local zenith, to 13,000 km for ν ’s arriving from the opposite side of the globe. Within the beamline there are regions of different, roughly uniform, matter densities. These include the Earth’s mantle (density 4.5 g/cm3) and the Earth’s core (density 11.5 g/cm3). This arrangement may eventually permit experimental strategies to be tried which are akin to utilization of regeneration plates in K0 beams. The neutrinos from this ever-running beamline give rise to useful reaction final states both in and below any detector deployed underground. By investigating the full