Matter Effects in Upward-Going Muons and Sterile Neutrino Oscillations

The angular distribution of upward-going muons produced by atmospheric neutrinos in the rock below the MACRO detector show anomalies in good agreement with two flavor neutrino-mu ==> neutrino-tau oscillations with maximum mixing and Delta m**2 around 0.0024 eV**2. Exploiting the dependence of magnitude of the matter effect on oscillation channel, and using a set of 809 upward-going muons observed in MACRO, we show that the two flavor neutrino-mu ==> neutrino-sterile oscillation is disfavored with 99% C.L. with respect to neutrino-mu ==> neutrino-tau.