The adiabatic conversion of solar electron neutrinos to muon neutrinos via the Mikheyev-Smirnov-Wolfenstein mechanism is discussed as a level-crossing problem. Solutions to the ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$ puzzle obtained numerically by Mikheyev and Smirnov, Rosen and Gelb, and others are shown to form the boundary of the region in the $\delta m^2-{sin}^{2}2\theta$ plane where adiabatic conversion occurs. Most of the region within this boundary is ruled out by the nonzero signal in Davis's experiment. The Landau-Zener description of the adiabatic boundary is employed.

• Received 31 March 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.1271