Photoproduction of heavy QCD axions in supernovae

Compact stellar objects like supernovae and neutron stars are believed to cool by emitting axions predominantly via bremsstrahlung (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>N</a:mi><a:mi>N</a:mi><a:mo stretchy="false">→</a:mo><a:mi>N</a:mi><a:mi>N</a:mi><a:mi>a</a:mi></a:math>), pion conversion (<d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msup><d:mi>π</d:mi><d:mo>−</d:mo></d:msup><d:msup><d:mi>p</d:mi><d:mo>+</d:mo></d:msup><d:mo stretchy="false">→</d:mo><d:mi>N</d:mi><d:mi>a</d:mi></d:math>), and photoproduction (<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>γ</g:mi><g:mi>N</g:mi><g:mo stretchy="false">→</g:mo><g:mi>N</g:mi><g:mi>a</g:mi></g:math>). In this paper, we study new contributions to the photoproduction channel pertaining to axions. We first point out the impact of anomaly-induced unavoidable Wess-Zumino-Witten term <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mo>∝</j:mo><j:msup><j:mi>ε</j:mi><j:mrow><j:mi>μ</j:mi><j:mi>ν</j:mi><j:mi>α</j:mi><j:mi>β</j:mi></j:mrow></j:msup><j:msub><j:mi>F</j:mi><j:mrow><j:mi>μ</j:mi><j:mi>ν</j:mi></j:mrow></j:msub><j:msub><j:mo>∂</j:mo><j:mi>α</j:mi></j:msub><j:mi>a</j:mi><j:msub><j:mi>ω</j:mi><j:mi>β</j:mi></j:msub></j:math> in the photoproduction of such axions. We then perform a complementary data-driven study where the rate of photoproduction can be estimated from low-energy pion photoproduction data. We found that for such heavier axions, i.e., <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msub><l:mi>m</l:mi><l:mi>a</l:mi></l:msub><l:mo>∼</l:mo><l:mi mathvariant="script">O</l:mi><l:mo stretchy="false">(</l:mo><l:mn>100</l:mn><l:mo stretchy="false">)</l:mo><l:mtext> </l:mtext><l:mtext> </l:mtext><l:mi>MeV</l:mi></l:math>, photoproduction processes can be dominant compared to the usual emission processes. In addition, the spectrum of these particles emitted in the process is significantly harder than those originating from bremsstrahlung. Published by the American Physical Society 2024