Decoding the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>B</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:mi>K</mml:mi><mml:mi>ν</mml:mi><mml:mi>ν</mml:mi></mml:mrow></mml:math> excess at Belle II: Kinematics, operators, and masses

Abstract

An excess in the branching fraction for <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msup><a:mrow><a:mi>B</a:mi></a:mrow><a:mrow><a:mo>+</a:mo></a:mrow></a:msup><a:mo stretchy="false">→</a:mo><a:msup><a:mrow><a:mi>K</a:mi></a:mrow><a:mrow><a:mo>+</a:mo></a:mrow></a:msup><a:mi>ν</a:mi><a:mi>ν</a:mi></a:mrow></a:math> recently measured at Belle II may be a hint of new physics. We perform thorough likelihood analyses for different new physics scenarios such as <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mi>B</d:mi><d:mo stretchy="false">→</d:mo><d:mi>K</d:mi><d:mi>X</d:mi></d:math> with a new invisible particle <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>X</g:mi></g:math>, or <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>B</i:mi><i:mo stretchy="false">→</i:mo><i:mi>K</i:mi><i:mi>χ</i:mi><i:mi>χ</i:mi></i:math> through a scalar, vector, or tensor current with <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi>χ</l:mi></l:math> being a new invisible particle or a neutrino. We find that vector-current three-body decay with <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mrow><n:msub><n:mrow><n:mi>m</n:mi></n:mrow><n:mrow><n:mi>X</n:mi></n:mrow></n:msub><n:mo>≃</n:mo><n:mn>0.6</n:mn></n:mrow></n:math> GeV—which may be dark matter—is most favored, while two-body decay with <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mrow><p:msub><p:mrow><p:mi>m</p:mi></p:mrow><p:mrow><p:mi>X</p:mi></p:mrow></p:msub><p:mo>≃</p:mo><p:mn>2</p:mn><p:mtext> </p:mtext><p:mtext> </p:mtext><p:mi>GeV</p:mi></p:mrow></p:math> is also competitive. The best-fit branching fractions for the scalar and tensor cases are a few times larger than for the two-body and vector cases. Past measurements provide further discrimination, although the best-fit parameters stay similar. Published by the American Physical Society 2024

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