Physicists Detect Rare Particle Decay, Filling Gap in Standard Model
Researchers at China's BESIII detector have observed previously unseen decay patterns of a charm particle, providing new experimental evidence about fundamental forces. The finding refines physicists' understanding of weak interactions and helps validate theoretical predictions about particle behavior—knowledge critical for developing future detection technologies and advancing nuclear physics applications.
Originaltitel: Evidence for the semileptonic decays <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msubsup> <mml:mi mathvariant="normal">Λ</mml:mi> <mml:mi>c</mml:mi> <mml:mo>+</mml:mo> </mml:msubsup> <mml:mo stretchy="false">→</mml:mo> <mml:msup> <mml:mi mathvariant="normal">Σ</mml:mi> <mml:mo>±</mml:mo> </mml:msup> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>∓</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msub> <mml:mi>ν</mml:mi> <mml:mi>e</mml:mi> </mml:msub> </mml:math>
Using <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>4.5</a:mn> <a:mrow> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:msup> <a:mi>fb</a:mi> <a:mrow> <a:mo>−</a:mo> <a:mn>1</a:mn> </a:mrow> </a:msup> </a:mrow> </a:math> of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msup> <c:mi>e</c:mi> <c:mo>+</c:mo> </c:msup> <c:msup> <c:mi>e</c:mi> <c:mo>−</c:mo> </c:msup> </c:math> collision data collected by the BESIII detector at center-of-mass energies between 4.600 and 4.699 GeV, we search for the semileptonic decays <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msubsup> <e:mi mathvariant="normal">Λ</e:mi> <e:mi>c</e:mi> <e:mo>+</e:mo> </e:msubsup> <e:mo stretchy="false">→</e:mo> <e:msup> <e:mi mathvariant="normal">Σ</e:mi> <e:mo>+</e:mo> </e:msup> <e:msup> <e:mi>π</e:mi> <e:mo>−</e:mo> </e:msup> <e:msup> <e:mi>e</e:mi> <e:mo>+</e:mo> </e:msup> <e:msub> <e:mi>ν</e:mi> <e:mi>e</e:mi> </e:msub> </e:math> and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msubsup> <j:mi mathvariant="normal">Λ</j:mi> <j:mi>c</j:mi> <j:mo>+</j:mo> </j:msubsup> <j:mo stretchy="false">→</j:mo> <j:msup> <j:mi mathvariant="normal">Σ</j:mi> <j:mo>−</j:mo> </j:msup> <j:msup> <j:mi>π</j:mi> <j:mo>+</j:mo> </j:msup> <j:msup> <j:mi>e</j:mi> <j:mo>+</j:mo> </j:msup> <j:msub> <j:mi>ν</j:mi> <j:mi>e</j:mi> </j:msub> </j:math> for the first time. Assuming their branching fractions are equal under isospin symmetry, evidence for <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:msubsup> <o:mi mathvariant="normal">Λ</o:mi> <o:mi>c</o:mi> <o:mo>+</o:mo> </o:msubsup> <o:mo stretchy="false">→</o:mo> <o:msup> <o:mi mathvariant="normal">Σ</o:mi> <o:mo>±</o:mo> </o:msup> <o:msup> <o:mi>π</o:mi> <o:mo>∓</o:mo> </o:msup> <o:msup> <o:mi>e</o:mi> <o:mo>+</o:mo> </o:msup> <o:msub> <o:mi>ν</o:mi> <o:mi>e</o:mi> </o:msub> </o:math> is reported with a significance of <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"> <t:mn>3.6</t:mn> <t:mi>σ</t:mi> </t:math> . The corresponding branching fraction is measured to be <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"> <v:mi mathvariant="script">B</v:mi> <v:mo stretchy="false">(</v:mo> <v:msubsup> <v:mi mathvariant="normal">Λ</v:mi> <v:mi>c</v:mi> <v:mo>+</v:mo> </v:msubsup> <v:mo stretchy="false">→</v:mo> <v:msup> <v:mi mathvariant="normal">Σ</v:mi> <v:mo>±</v:mo> </v:msup> <v:msup> <v:mi>π</v:mi> <v:mo>∓</v:mo> </v:msup> <v:msup> <v:mi>e</v:mi> <v:mo>+</v:mo> </v:msup> <v:msub> <v:mi>ν</v:mi> <v:mi>e</v:mi> </v:msub> <v:mo stretchy="false">)</v:mo> <v:mo>=</v:mo> <v:mo stretchy="false">(</v:mo> <v:msubsup> <v:mn>7.7</v:mn> <v:mrow> <v:mo>−</v:mo> <v:msub> <v:mn>2.3</v:mn> <v:mrow> <v:mi>stat</v:mi> </v:mrow> </v:msub> </v:mrow> <v:mrow> <v:mo>+</v:mo> <v:mn>2.5</v:mn> </v:mrow> </v:msubsup> <v:mo>±</v:mo> <v:msub> <v:mn>1.3</v:mn> <v:mrow> <v:mi>syst</v:mi> </v:mrow> </v:msub> <v:mo stretchy="false">)</v:mo> <v:mo>×</v:mo> <v:msup> <v:mn>10</v:mn> <v:mrow> <v:mo>−</v:mo> <v:mn>4</v:mn> </v:mrow> </v:msup> </v:math> , which is consistent with quark model predictions within two standard deviations.