Axino as a Sterile Neutrino

Current data from the atmospheric and solar neutrino experiments are beautifully explained by oscillations among three active neutrino species . Another data in favor of neutrino oscillation has been obtained in the LSND experiment . Reconciliation of these experimental results requires three distinct mass-squared differences, implying the existence of a sterile neutrino νs. In the four-neutrino oscillation framework, there are two possible scenarios : the 2 + 2 scheme in which two pairs of close mass eigenstates are separated by the LSND mass gap ∼ 1 eV and the 3 + 1 scheme in which one mass is isolated from the other three by the LSND mass gap. It has been claimed that the LSND results can be compatible with various short-baseline experiments only in the context of the 2+2 scheme. However, according to the new LSND results , the allowed parameter regions are shifted to smaller mixing angle, thereby allowing the 3 + 1 scheme to be phenomenologically viable . Although it can be realized in a rather limited parameter space, the 3+1 scheme is attractive since the fourth (sterile) neutrino can be added without changing the most favorable picture that the atmospheric and solar neutrino data are explained by the predominant νμ → ντ and νe → νμ, ντ oscillations, respectively. In particular, the 3 + 1 scheme with the heaviest νs would be an interesting explanation of all existing neutrino data. In this talk, we present a supersymmetric axion model with gauge-mediated supersymmetry (SUSY) breaking in which the fermionic superpartner of axion, i.e. the axino, corresponds to a sterile neutrino realizing the 3+1 scheme of four neutrino oscillation . In this model, axino can be as light as 1 eV, and a proper axino-neutrino mixing is induced by R-parity violating couplings which appear as a consequence of spontaneous U(1)PQ breaking. It turns out that only the large angle MSW solution to the solar neutrino problem is allowed in this model.