The <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>C</a:mi><a:mi>P</a:mi></a:math> asymmetry and branching fraction of the Cabibbo-Kobayashi-Maskawa-suppressed decay <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msup><c:mi>B</c:mi><c:mo>+</c:mo></c:msup><c:mo stretchy="false">→</c:mo><c:mrow><c:mi>J</c:mi><c:mo>/</c:mo><c:mi>ψ</c:mi></c:mrow><c:msup><c:mi>π</c:mi><c:mo>+</c:mo></c:msup></c:math> are precisely measured relative to the favored decay <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:msup><f:mi>B</f:mi><f:mo>+</f:mo></f:msup><f:mo stretchy="false">→</f:mo><f:mrow><f:mi>J</f:mi><f:mo>/</f:mo><f:mi>ψ</f:mi></f:mrow><f:msup><f:mi>K</f:mi><f:mo>+</f:mo></f:msup></f:math> using a sample of proton-proton collision data corresponding to an integrated luminosity of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mn>5.4</i:mn><i:mtext> </i:mtext><i:mtext> </i:mtext><i:msup><i:mi>fb</i:mi><i:mrow><i:mo>−</i:mo><i:mn>1</i:mn></i:mrow></i:msup></i:math> recorded at a center-of-mass energy of 13 TeV during 2016–2018. The results of the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>C</k:mi><k:mi>P</k:mi></k:math> asymmetry difference and branching fraction ratio are <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mrow><m:mi mathvariant="normal">Δ</m:mi><m:msup><m:mrow><m:mi mathvariant="script">A</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:mi>P</m:mi></m:mrow></m:msup><m:mo>≡</m:mo><m:msup><m:mrow><m:mi mathvariant="script">A</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:mi>P</m:mi></m:mrow></m:msup><m:mo stretchy="false">(</m:mo><m:msup><m:mrow><m:mi>B</m:mi></m:mrow><m:mrow><m:mo>+</m:mo></m:mrow></m:msup><m:mo stretchy="false">→</m:mo><m:mi>J</m:mi><m:mo>/</m:mo><m:mi>ψ</m:mi><m:msup><m:mrow><m:mi>π</m:mi></m:mrow><m:mrow><m:mo>+</m:mo></m:mrow></m:msup><m:mo stretchy="false">)</m:mo><m:mo>−</m:mo><m:msup><m:mrow><m:mi mathvariant="script">A</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:mi>P</m:mi></m:mrow></m:msup><m:mo stretchy="false">(</m:mo><m:msup><m:mrow><m:mi>B</m:mi></m:mrow><m:mrow><m:mo>+</m:mo></m:mrow></m:msup><m:mo stretchy="false">→</m:mo><m:mi>J</m:mi><m:mo>/</m:mo><m:mi>ψ</m:mi><m:msup><m:mrow><m:mi>K</m:mi></m:mrow><m:mrow><m:mo>+</m:mo></m:mrow></m:msup><m:mo stretchy="false">)</m:mo><m:mo>=</m:mo><m:mo stretchy="false">(</m:mo><m:mn>1.29</m:mn><m:mo>±</m:mo><m:mn>0.49</m:mn><m:mo>±</m:mo><m:mn>0.08</m:mn><m:mo stretchy="false">)</m:mo><m:mo>×</m:mo><m:msup><m:mrow><m:mn>10</m:mn></m:mrow><m:mrow><m:mo>−</m:mo><m:mn>2</m:mn></m:mrow></m:msup></m:mrow></m:math>, <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mrow><ab:msub><ab:mrow><ab:mi mathvariant="script">R</ab:mi></ab:mrow><ab:mrow><ab:mi>π</ab:mi><ab:mo>/</ab:mo><ab:mi>K</ab:mi></ab:mrow></ab:msub><ab:mo>≡</ab:mo><ab:mo stretchy="false">[</ab:mo><ab:mi mathvariant="script">B</ab:mi><ab:mo stretchy="false">(</ab:mo><ab:msup><ab:mrow><ab:mi>B</ab:mi></ab:mrow><ab:mrow><ab:mo stretchy="false">+</ab:mo></ab:mrow></ab:msup><ab:mo stretchy="false">→</ab:mo><ab:mi>J</ab:mi><ab:mo stretchy="false">/</ab:mo><ab:mi>ψ</ab:mi><ab:msup><ab:mrow><ab:mi>π</ab:mi></ab:mrow><ab:mrow><ab:mo stretchy="false">+</ab:mo></ab:mrow></ab:msup><ab:mo stretchy="false">)</ab:mo><ab:mo>/</ab:mo><ab:mspace linebreak="goodbreak"/><ab:mi mathvariant="script">B</ab:mi><ab:mo stretchy="false">(</ab:mo><ab:msup><ab:mrow><ab:mi>B</ab:mi></ab:mrow><ab:mrow><ab:mo stretchy="false">+</ab:mo></ab:mrow></ab:msup><ab:mo stretchy="false">→</ab:mo><ab:mi>J</ab:mi><ab:mo stretchy="false">/</ab:mo><ab:mi>ψ</ab:mi><ab:msup><ab:mrow><ab:mi>K</ab:mi></ab:mrow><ab:mrow><ab:mo stretchy="false">+</ab:mo></ab:mrow></ab:msup><ab:mo stretchy="false">)</ab:mo><ab:mo stretchy="false">]</ab:mo><ab:mo>=</ab:mo><ab:mo stretchy="false">(</ab:mo><ab:mn>3.852</ab:mn><ab:mo>±</ab:mo><ab:mn>0.022</ab:mn><ab:mo>±</ab:mo><ab:mn>0.018</ab:mn><ab:mo stretchy="false">)</ab:mo><ab:mo>×</ab:mo><ab:msup><ab:mrow><ab:mn>10</ab:mn></ab:mrow><ab:mrow><ab:mo>−</ab:mo><ab:mn>2</ab:mn></ab:mrow></ab:msup></ab:mrow></ab:math>, where the first uncertainties are statistical and the second are systematic. A combination with previous LHCb results based on data collected at 7 and 8 TeV in 2011 and 2012 yields <wb:math xmlns:wb="http://www.w3.org/1998/Math/MathML" display="inline"><wb:mi mathvariant="normal">Δ</wb:mi><wb:msup><wb:mi mathvariant="script">A</wb:mi><wb:mrow><wb:mi>C</wb:mi><wb:mi>P</wb:mi></wb:mrow></wb:msup><wb:mo>=</wb:mo><wb:mo stretchy="false">(</wb:mo><wb:mn>1.42</wb:mn><wb:mo>±</wb:mo><wb:mn>0.43</wb:mn><wb:mo>±</wb:mo><wb:mn>0.08</wb:mn><wb:mo stretchy="false">)</wb:mo><wb:mo>×</wb:mo><wb:msup><wb:mn>10</wb:mn><wb:mrow><wb:mo>−</wb:mo><wb:mn>2</wb:mn></wb:mrow></wb:msup></wb:math> and <cc:math xmlns:cc="http://www.w3.org/1998/Math/MathML" display="inline"><cc:msub><cc:mi mathvariant="script">R</cc:mi><cc:mrow><cc:mi>π</cc:mi><cc:mo>/</cc:mo><cc:mi>K</cc:mi></cc:mrow></cc:msub><cc:mo>=</cc:mo><cc:mo stretchy="false">(</cc:mo><cc:mn>3.846</cc:mn><cc:mo>±</cc:mo><cc:mn>0.018</cc:mn><cc:mo>±</cc:mo><cc:mn>0.018</cc:mn><cc:mo stretchy="false">)</cc:mo><cc:mo>×</cc:mo><cc:msup><cc:mn>10</cc:mn><cc:mrow><cc:mo>−</cc:mo><cc:mn>2</cc:mn></cc:mrow></cc:msup></cc:math>. The combined <hc:math xmlns:hc="http://www.w3.org/1998/Math/MathML" display="inline"><hc:mi mathvariant="normal">Δ</hc:mi><hc:msup><hc:mi mathvariant="script">A</hc:mi><hc:mrow><hc:mi>C</hc:mi><hc:mi>P</hc:mi></hc:mrow></hc:msup></hc:math> value deviates from zero by 3.2 standard deviations, providing the first evidence for direct <lc:math xmlns:lc="http://www.w3.org/1998/Math/MathML" display="inline"><lc:mi>C</lc:mi><lc:mi>P</lc:mi></lc:math> violation in the amplitudes of beauty decays to charmonium final states. © 2025 CERN, for the LHCb Collaboration 2025 CERN