A test of tau neutrino interactions with atmospheric neutrinos and K2K
  The presence of a tau component in the flux of atmospheric neutrinos inside the Earth, due to flavor oscillations, makes these neutrinos a valuable probe of interactions of the tau neutrino with matter. We study -- analytically and numerically -- the effects of nonstandard interactions in the nu_e-nu_tau sector on atmospheric neutrino oscillations, and calculate the bounds on the exotic couplings that follow from combining the atmospheric neutrino and K2K data. We find very good agreement between numerical results and analytical predictions derived from the underlying oscillation physics. While improving on existing accelerator bounds, our bounds still allow couplings of the size comparable to the standard weak interaction. The inclusion of new interactions expands the allowed region of the vacuum oscillation parameters towards smaller mixing angles, 0.2 ~< sin^2 theta_{23} ~< 0.7, and slightly larger mass squared splitting, 1.5 * 10^{-3} eV^2 ~< |\Delta m^2_{23}| ~< 4.0 * 10^{-3} eV^2, compared to the standard case. The impact of the K2K data on all these results is significant; further important tests of the nu_e-nu_tau exotic couplings will come from neutrino beams experiments such as MINOS and long baseline projects. 
Alexander Friedland, Cecilia Lunardini