We solve the problem of coherent Mikheyev-Smirnov-Wolfenstein (MSW) resonant active-to-sterile neutrino flavor conversion driven by an initial lepton number in the early universe. We find incomplete destruction of lepton number in this process and a sterile neutrino energy distribution with a distinctive cusp and high energy tail. These features imply alteration of the non-zero lepton number primordial nucleosynthesis paradigm when there exist sterile neutrinos with rest masses ~ 1 eV. This could result in better light element probes of (constraints on) these particles.
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