Searching for axions with kaon decay at rest

We describe a novel search strategy for axions (or hadronically coupled axionlike particles) in the mass range of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>m</a:mi><a:mi>a</a:mi></a:msub><a:mo>≲</a:mo><a:mn>350</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>MeV</a:mi></a:math>. The search relies on kaon decay at rest, which produces a monoenergetic signal in a large volume detector (e.g., a tank of liquid scintillator) from axion decays <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>a</c:mi><c:mo stretchy="false">→</c:mo><c:mi>γ</c:mi><c:mi>γ</c:mi></c:math> or <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi>a</f:mi><f:mo stretchy="false">→</f:mo><f:msup><f:mi>e</f:mi><f:mo>+</f:mo></f:msup><f:msup><f:mi>e</f:mi><f:mo>−</f:mo></f:msup></f:math>. The decay modes <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msup><i:mi>K</i:mi><i:mo>+</i:mo></i:msup><i:mo stretchy="false">→</i:mo><i:msup><i:mi>π</i:mi><i:mo>+</i:mo></i:msup><i:mi>a</i:mi></i:math> and <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi>a</l:mi><l:mo stretchy="false">→</l:mo><l:mi>γ</l:mi><l:mi>γ</l:mi></l:math> are induced by the axion’s coupling to gluons, which is generic to any model which addresses the strong <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mrow><o:mi>C</o:mi><o:mi>P</o:mi></o:mrow></o:math> problem. We recast a recent search from MicroBooNE for <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msup><q:mi>e</q:mi><q:mo>+</q:mo></q:msup><q:msup><q:mi>e</q:mi><q:mo>−</q:mo></q:msup></q:math> pairs and study prospects at <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mrow><s:msup><s:mrow><s:mi>JSNS</s:mi></s:mrow><s:mrow><s:mn>2</s:mn></s:mrow></s:msup></s:mrow></s:math> and other near-term facilities. We find that <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mrow><u:msup><u:mrow><u:mi>JSNS</u:mi></u:mrow><u:mrow><u:mn>2</u:mn></u:mrow></u:msup></u:mrow></u:math> will have world-leading sensitivity to hadronically coupled axions in the mass range of <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mn>40</w:mn><w:mtext> </w:mtext><w:mtext> </w:mtext><w:mi>MeV</w:mi><w:mo>≲</w:mo><w:msub><w:mi>m</w:mi><w:mi>a</w:mi></w:msub><w:mo>≲</w:mo><w:mn>350</w:mn><w:mtext> </w:mtext><w:mtext> </w:mtext><w:mi>MeV</w:mi></w:math>.