A 3-dimensional calculation of the atmospheric neutrino fluxes

An extensive 3-dimensional Monte Carlo calculation of the atmospheric neutr ino flux is in progress with the FLUKA Monte Carlo code. The results are co mpared to those obtained under the 1-dimensional approximation, where secon dary particles and decay products are assumed to be collinear to the primar y cosmic ray, as usually done in most of the already existing flux calculat ions. It is shown that the collinear approximation gives rise to a wrong an gular distribution of neutrinos, essentially in the Sub-GeV region. However , the angular smearing introduced by the experimental inability of detectin g recoils in neutrino interactions with nuclei is large enough to wash out, in practice. most of the differences between 3-dimensional and 1-dimension al flux calculations. Therefore, the use of the collinear approximation sho uld not have introduced a significant bias in the determination of the flav or oscillation parameters in current experiments. (C) 2000 Elsevier Science B.V. All rights reserved.