A recent study has used the first indication of the diffuse supernova neutrino background (DSNB) by the Super-Kamiokande collaboration to establish a new upper limit on the axion-proton coupling constant, $|g_{ap}|$. This result, with a statistical significance of $2.6σ$ for the DSNB detection, opens a new avenue for the search for weakly interacting particles, such as axions, which could influence stellar evolution.

Axions are hypothetical particles proposed to solve the strong CP problem in quantum chromodynamics. If they exist, they could be produced inside neutron stars, altering their cooling. The DSNB is the cumulative flux of neutrinos emitted by all core-collapse supernovae that have occurred throughout the history of the universe. By comparing the observed DSNB flux with models that include axion cooling, researchers have been able to constrain the strength with which these particles interact with matter.

The analysis has established a $1σ$ upper limit for the axion-proton coupling of $|g_{ap}| < 1.3 \times 10^{-9}$. This value is comparable to conventional limits obtained from the neutrino burst of supernova SN 1987A. However, unlike the SN 1987A bound, which depends on the properties of a single event, the DSNB-based constraint is derived from a cosmic population of supernovae, giving it a different and complementary robustness. This method can be extended to other weakly interacting particles that modify the cooling of protoneutron stars, offering a powerful tool for physics beyond the Standard Model.