Signatures of light new particles in <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:msup> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mo>*</mml:mo> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>miss</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math>

Abstract

The recent Belle II observation of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>B</a:mi> <a:mo stretchy="false">→</a:mo> <a:mi>K</a:mi> <a:msub> <a:mi>E</a:mi> <a:mrow> <a:mi>miss</a:mi> </a:mrow> </a:msub> </a:math> challenges theoretical interpretations in terms of Standard Model neutrino final states. Instead, we consider new physics scenarios where up to two new light-invisible particles of spin 0 up to <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mn>3</d:mn> <d:mo>/</d:mo> <d:mn>2</d:mn> </d:mrow> </d:math> are present in the final state. We identify viable scenarios by reconstructing the (binned) likelihoods of the relevant <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mi>B</f:mi> <f:mo stretchy="false">→</f:mo> <f:msup> <f:mi>K</f:mi> <f:mrow> <f:mo stretchy="false">(</f:mo> <f:mo>*</f:mo> <f:mo stretchy="false">)</f:mo> </f:mrow> </f:msup> <f:msub> <f:mi>E</f:mi> <f:mrow> <f:mi>miss</f:mi> </f:mrow> </f:msub> </f:math> and also <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:msub> <k:mi>B</k:mi> <k:mi>s</k:mi> </k:msub> <k:mo stretchy="false">→</k:mo> <k:msub> <k:mi>E</k:mi> <k:mrow> <k:mi>miss</k:mi> </k:mrow> </k:msub> </k:math> experimental analyses and present preferred regions of couplings and masses. In particular, we find that the current data prefer two-body decay kinematics involving the emission of a single massive scalar or a vector particle or, alternatively, three-body decays involving pairs of massive scalars or spin <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mrow> <n:mn>1</n:mn> <n:mo>/</n:mo> <n:mn>2</n:mn> </n:mrow> </n:math> fermions. When applicable, we compare our findings with existing literature and briefly discuss some model-building implications. Published by the American Physical Society 2024

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