Disentangling axionlike particle couplings to nucleons via a delayed signal in Super-Kamiokande from a future supernova

In this work, we show that, if axionlike particles (ALPs) from core-collapse supernovae (SNe) couple to protons, they would produce very characteristic signatures in neutrino water Cherenkov detectors through their scattering off free protons via <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>a</a:mi><a:mi>p</a:mi><a:mo stretchy="false">→</a:mo><a:mi>p</a:mi><a:mi>γ</a:mi></a:math> interactions. Specifically, sub-MeV ALPs would generate photons with energies <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mo>∼</d:mo><d:mn>30</d:mn><d:mtext> </d:mtext><d:mtext> </d:mtext><d:mi>MeV</d:mi></d:math>, which could be observed by Super-Kamiokande and Hyper-Kamiokande as a delayed signal after a future detection of SN neutrinos. We apply this to a hypothetical neighboring SN (at a maximum distance of 100 kpc) and demonstrate that the region in the parameter space with ALP masses between <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:msup><f:mrow><f:mn>10</f:mn></f:mrow><f:mrow><f:mo>−</f:mo><f:mn>4</f:mn></f:mrow></f:msup><f:mtext> </f:mtext></f:mrow></f:math> and 1 MeV and ALP-proton couplings in the range <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mn>3</h:mn><h:mo>×</h:mo><h:msup><h:mrow><h:mn>10</h:mn></h:mrow><h:mrow><h:mo>−</h:mo><h:mn>6</h:mn></h:mrow></h:msup><h:mi>–</h:mi><h:mn>4</h:mn><h:mo>×</h:mo><h:msup><h:mrow><h:mn>10</h:mn></h:mrow><h:mrow><h:mo>−</h:mo><h:mn>5</h:mn></h:mrow></h:msup></h:mrow></h:math> could be probed. We argue that this new signature, combined with the one expected at <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mo>∼</j:mo><j:mn>7</j:mn><j:mtext> </j:mtext><j:mtext> </j:mtext><j:mi>MeV</j:mi></j:math> from oxygen deexcitation, would allow us to disentangle ALP-neutron and ALP-proton couplings.