Higgs portal interpretation of the Belle II <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>B</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:msup><mml:mi>K</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mi>ν</mml:mi><mml:mi>ν</mml:mi></mml:math> measurement
David McKeen, John N. Ng, Douglas Tuckler
Abstract
The Belle II experiment recently observed the decay <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>B</a:mi><a:mo>+</a:mo></a:msup><a:mo stretchy="false">→</a:mo><a:msup><a:mi>K</a:mi><a:mo>+</a:mo></a:msup><a:mi>ν</a:mi><a:mi>ν</a:mi></a:math> for the first time, with a measured value for the branching ratio of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mo stretchy="false">(</d:mo><d:mn>2.3</d:mn><d:mo>±</d:mo><d:mn>0.7</d:mn><d:mo stretchy="false">)</d:mo><d:mo>×</d:mo><d:msup><d:mn>10</d:mn><d:mrow><d:mo>−</d:mo><d:mn>5</d:mn></d:mrow></d:msup></d:math>. This result exhibits a <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mo>∼</h:mo><h:mn>3</h:mn><h:mi>σ</h:mi></h:math> deviation from the Standard Model (SM) prediction. The observed enhancement with respect to the Standard Model could indicate the presence of invisible light new physics. In this paper, we investigate whether this result can be accommodated in a minimal Higgs portal model, where the SM is extended by a singlet Higgs scalar that decays invisibly to dark sector states. We find that current and future bounds on invisible decays of the 125 GeV Higgs boson completely exclude a new scalar with a mass <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mo>≳</j:mo><j:mn>10</j:mn><j:mtext> </j:mtext><j:mtext> </j:mtext><j:mi>GeV</j:mi></j:math>. On the other hand, the Belle II results can be successfully accommodated if the new scalar is lighter than <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi>B</l:mi></l:math> mesons but heavier than kaons. We also investigate the cosmological implications of the new states and explore the possibility that they are part of an Abelian Higgs extension of the SM. Future Higgs factories are expected to place stringent bounds on the invisible branching ratio of the 125 GeV Higgs boson, and will be able to definitively test the region of parameter space favored by the Belle II results. Published by the American Physical Society 2024