East-West Effect

Primary cosmic rays arrive at the near vicinity of the earth isotropically, having been randomized by interstellar magnetic fields. Super-K relies on the up-down symmetry of high energy neutrinos (with energy greater than 1 GeV) from high energy cosmic rays as one element ofour evidence for neutrino oscillation. However, low energy cosmic rays are deflected by the earths magnetic field and are therefore more complicated to study. In particular, low energy cosmic rays from the east are suppressed compared to those from the west, because the presence of the earth effectively shadows certain trajectories, which are therefore forbidden. In the 1930's this east-west effect was detected in charged cosmic rays and was used to infer that the sign of the primary cosmic rays charge must be positive. Super-K has for the first time detected this asymmetry in the flux of atmospheric neutrinos. The generally good agreement between the Super-K data and the prediction indicates that the theoretical calculation of the neutrino flux, which takes this effect into account, is reasonably well modeled.

To be published in Physical Review Letters, preprint available at the astro-ph LANL server.