Geomagnetic effects on atmospheric neutrinos

Abstract

Geomagnetic effects distort the zenith angle distribution of sub-GeV and few-GeV atmospheric neutrinos, breaking the up-down symmetry that would be present in the absence of neutrino oscillations and without a geomagnetic field. The geomagnetic effects also produce a characteristic azimuthal dependence of the fluxes, related to the well-known east-west effect, that should be detectable in neutrino experiments of sufficiently large mass. We discuss these effects quantitatively. Because the azimuthal dependence is in first-order independent of any oscillation effect, it is a useful diagnostic tool for studying possible systematic effects in the search for neutrino oscillations.

Observation of the East-West Anisotropy of the Atmospheric Neutrino Flux

“…The smaller anisotropy observed in µ-like data is consistent with a statistical fluctuation. The expected anisotropy is similar for ν e and ν µ [3,4]; the calculated anisotropy for ν e is only about 1.1 times larger than that for ν µ in the relevant energy region.…”

“…These data were compared with Monte Carlo simulations with two independent flux calculations [3,4]. These calculations represented the geomagnetic field by a multipole expansion of the spherical harmonic function.…”

The Muon Charge Ratio In Cosmic Ray Air Showers

“…The East-West effect, whose detailed calculation has been discussed in several studies, e.g. in [1,7,24,25] is transferred to the secondary particles of air showers induced in the atmosphere, in particular to muons and to neutrinos [6,26,27]. The azimuthal variation of the muon charge ratio of atmospheric (inclusive) muons has been recently measured for low-energy muons for the latitude of the Bucharest (Romania) with a geomagnetic cut off of 5.6 GV [28].…”

“…Several studies have re-examined the influence of the Earth's magnetic field on muon and neutrino fluxes [6,7]. At lower energies, the fluxes of the primaries and secondaries are influenced by the Earth's magnetic field, in particular by a magnetic rigidity cut-off of the primary cosmic rays penetrating the Earth's atmosphere from the cosmos [7], resulting in latitude dependence and in an East-West asymmetry of the incident flux [8].…”

“…At lower energies, the fluxes of the primaries and secondaries are influenced by the Earth's magnetic field, in particular by a magnetic rigidity cut-off of the primary cosmic rays penetrating the Earth's atmosphere from the cosmos [7], resulting in latitude dependence and in an East-West asymmetry of the incident flux [8]. There are numerous studies of the charge ratio of the atmospheric muon flux, on sea level (see the compilations [2,[9][10][11]) and also of the vertical dependence by balloon experiments e.g.…”