LSND effect as a Chooz-restricted "sterile" perturbation of three-neutrino texture

Abstract

Considering the hypothesis of mixing of three active neutrinos with, at least, one sterile neutrino, we report on a simple $4\times 4$ texture whose $ 3 \times 3 $ part arises from the popular bimaximal texture for three active neutrinos $ \nu_e, \nu_\mu, \nu_\tau $, where $ c_{12} = 1/\sqrt{2} = s_{12}$, $ c_{23} = 1/\sqrt{2} = s_{23}$ and $ s_{13} = 0$. Such a $3\times 3 $ bimaximal texture is perturbed through a rotation in the 14 plane, where $\nu_4 $ is the extra neutrino mass state induced by the sterile neutrino $\nu_s $ which becomes responsible for the LSND effect. Then, with $m^2_1\simeq m^2_2$ we predict that $\sin^2 2\theta_{\rm atm} = {1/2}(1+ c^2_{14}) \sim 0.99$ and $\sin^2 2\theta_{\rm LSND} = {1/2}s^4_{14} \sim 4.5\times10^{-4}$, and in addition $ \Delta m^2_{\rm atm} = \Delta m^2_{32}$ and $ \Delta m^2_{\rm LSND} = |\Delta m^2_{41}|$, where $c^2_{14} = \sin^2 2\theta_{\rm sol} \sim 0.97$ and $\Delta m^2_{21} = \Delta m^2_{\rm sol} \sim 10^{-7} {\rm eV}^2$ if {\it e.g.} the LOW solar solution is applied. In this four-neutrino texture with $m^2_1 \simeq m^2_2 $ the sum rule $\sin^2 2 \theta_{\rm sol} + {1/2}\sin^2 2 \theta_{\rm Chooz} + \sin^2 2 \theta_{\rm LSND} = 1$ holds in the two-flavor approximation (for each of three cases), leaving room for the LSND effect, depending on the magnitude of Chooz effect that, not observed so far, leads (at present) to the upper bound $\sin^2 2\theta_{\rm LSND}\stackrel{<}{\sim} 1.3\times10^{-3}$ and the lower bound $\sin^2 2\theta_{\rm sol} \stackrel{>}{\sim} 0.95$. At the end a four-neutrino seesaw mechanism is sketched.