Fresh look at axions and SN 1987A

We reexamine the very stringent limit on the axion mass based on the strength and duration of the neutrino signal from SN 1987A, in the light of new measurements of the axial-vector coupling strength of nucleons, possible suppression of axion emission due to many-body effects, and additional emission processes involving pions. The suppression of axion emission due to nucleon spin fluctuations induced by many-body effects degrades previous limits by a factor of about 2. Emission processes involving thermal pions can strengthen the limits by a factor of 3–4 within a perturbative treatment that neglects saturation of nucleon spin fluctuations. Inclusion of saturation effects, however, tends to make the limits less dependent on pion abundances. The resulting upper limit to the axion mass also depends on the precise couplings of the axion and ranges from $0.5\times {10}^{-3}$ eV to $6\times {10}^{-3}$ eV, consistent with previous results.