First Measurement of R(Xτ/ℓ) as an Inclusive Test of the b<mml:mo stretchy="…

We measure the tau-to-light-lepton ratio of inclusive <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>B</a:mi></a:math>-meson branching fractions <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mrow><c:mi>R</c:mi><c:mo stretchy="false">(</c:mo><c:msub><c:mrow><c:mi>X</c:mi></c:mrow><c:mrow><c:mi>τ</c:mi><c:mo>/</c:mo><c:mo>ℓ</c:mo></c:mrow></c:msub><c:mo stretchy="false">)</c:mo></c:mrow><c:mo>≡</c:mo><c:mspace linebreak="goodbreak"/><c:mrow><c:mi mathvariant="script">B</c:mi><c:mo stretchy="false">(</c:mo><c:mi>B</c:mi><c:mo stretchy="false">→</c:mo><c:mrow><c:mi>X</c:mi><c:mi>τ</c:mi><c:mi>ν</c:mi></c:mrow><c:mo stretchy="false">)</c:mo></c:mrow><c:mo>/</c:mo><c:mrow><c:mi mathvariant="script">B</c:mi><c:mo stretchy="false">(</c:mo><c:mi>B</c:mi><c:mo stretchy="false">→</c:mo><c:mrow><c:mi>X</c:mi><c:mo>ℓ</c:mo><c:mi>ν</c:mi></c:mrow><c:mo stretchy="false">)</c:mo></c:mrow></c:mrow></c:math>, where <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mo>ℓ</p:mo></p:math> indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:mi>B</r:mi></r:math> meson and a charged lepton candidate from <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:mrow><t:mn>189</t:mn><t:mtext> </t:mtext><t:mtext> </t:mtext><t:msup><t:mrow><t:mi>fb</t:mi></t:mrow><t:mrow><t:mo>−</t:mo><t:mn>1</t:mn></t:mrow></t:msup></t:mrow></t:math> of electron-positron collision data collected with the Belle II detector. We find <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mrow><v:mrow><v:mi>R</v:mi><v:mo stretchy="false">(</v:mo><v:msub><v:mrow><v:mi>X</v:mi></v:mrow><v:mrow><v:mi>τ</v:mi><v:mo>/</v:mo><v:mo>ℓ</v:mo></v:mrow></v:msub><v:mo stretchy="false">)</v:mo></v:mrow><v:mo>=</v:mo><v:mn>0.228</v:mn><v:mo>±</v:mo><v:mn>0.016</v:mn><v:mo stretchy="false">(</v:mo><v:mrow><v:mi>stat</v:mi></v:mrow><v:mo stretchy="false">)</v:mo><v:mo>±</v:mo><v:mn>0.036</v:mn><v:mo stretchy="false">(</v:mo><v:mrow><v:mi>syst</v:mi></v:mrow><v:mo stretchy="false">)</v:mo></v:mrow></v:math>, in agreement with standard-model expectations. This is the first direct measurement of <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mi>R</db:mi><db:mo stretchy="false">(</db:mo><db:msub><db:mi>X</db:mi><db:mrow><db:mi>τ</db:mi><db:mo>/</db:mo><db:mo>ℓ</db:mo></db:mrow></db:msub><db:mo stretchy="false">)</db:mo></db:math>. Published by the American Physical Society 2024