They have shown that very large flavor conversion can occur between neutrinos and antineutrinos in SN ejecta.

Continuous efforts of numerical studies on SN explosions revealed detailed conditions in the deep region of SN ejecta (e.g., [@jl07]). One of the findings is the time evolution of electron mole fraction $Y_e$. The electron fraction of the central core of a presupernova is between 0.4 and 0.5 [@hw01]. After the core bounce in a scenario of delayed SN explosion, neutrino heating helps the explosion of a SN. The neutrino interaction also changes the electron fraction of the inner region of the exploding materials [@aj07]. The electron fraction in the neutrino-heated bubbles and the neutrino-driven winds exceeds 0.5 in several seconds. This excess in $Y_e$ opens a new nucleosynthesis process, which is called the $\nu p$ process [@pw05; @ph06; @fr06; @fm06; @wa06]. The $\nu p$ process is one of the most promising processes to produce proton-rich nuclei. The electron fraction is expected to decrease eventually below 0.5 in several seconds. After electron fraction becomes small, $r$ process is expected to occur in neutrino-driven winds (e.g., [@ww94; @ot00; @wi02]).

We expect that the change of the electron fraction also affects the neutrino oscillations, especially 