Physicists measure rare particle decay for the first time
Researchers at China's BEPCII collider have made the first direct measurement of how a charm meson decays into a kaon and muon. The finding fills a gap in the Standard Model's predictions and provides a precision test of fundamental physics that could reveal new particles or forces beyond current theory.
Originaltitel: First Measurement of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mi>s</mml:mi> <mml:mo>+</mml:mo> </mml:msubsup> <mml:mo stretchy="false">→</mml:mo> <mml:msup> <mml:mi>K</mml:mi> <mml:mn>0</mml:mn> </mml:msup> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msub> <mml:mi>ν</mml:mi> <mml:mi>μ</mml:mi> </mml:msub> </mml:math> Decay
We report the first measurement of the semileptonic decay <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msubsup> <a:mrow> <a:mi>D</a:mi> </a:mrow> <a:mrow> <a:mi>s</a:mi> </a:mrow> <a:mrow> <a:mo>+</a:mo> </a:mrow> </a:msubsup> <a:mo stretchy="false">→</a:mo> <a:msup> <a:mrow> <a:mi>K</a:mi> </a:mrow> <a:mrow> <a:mn>0</a:mn> </a:mrow> </a:msup> <a:msup> <a:mrow> <a:mi>μ</a:mi> </a:mrow> <a:mrow> <a:mo>+</a:mo> </a:mrow> </a:msup> <a:msub> <a:mrow> <a:mi>ν</a:mi> </a:mrow> <a:mrow> <a:mi>μ</a:mi> </a:mrow> </a:msub> </a:mrow> </a:math> , using a sample of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:msup> <d:mi>e</d:mi> <d:mo>+</d:mo> </d:msup> <d:msup> <d:mi>e</d:mi> <d:mo>−</d:mo> </d:msup> </d:math> annihilation data corresponding to an integrated luminosity of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mn>7.33</f:mn> <f:mtext> </f:mtext> <f:mtext> </f:mtext> <f:msup> <f:mi>fb</f:mi> <f:mrow> <f:mo>−</f:mo> <f:mn>1</f:mn> </f:mrow> </f:msup> </f:math> collected at center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector at the BEPCII collider. The branching fraction of the decay is measured to be <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mrow> <h:mi mathvariant="script">B</h:mi> <h:mo stretchy="false">(</h:mo> <h:msubsup> <h:mrow> <h:mi>D</h:mi> </h:mrow> <h:mrow> <h:mi>s</h:mi> </h:mrow> <h:mrow> <h:mo>+</h:mo> </h:mrow> </h:msubsup> <h:mo stretchy="false">→</h:mo> <h:msup> <h:mrow> <h:mi>K</h:mi> </h:mrow> <h:mrow> <h:mn>0</h:mn> </h:mrow> </h:msup> <h:msup> <h:mrow> <h:mi>μ</h:mi> </h:mrow> <h:mrow> <h:mo>+</h:mo> </h:mrow> </h:msup> <h:msub> <h:mrow> <h:mi>ν</h:mi> </h:mrow> <h:mrow> <h:mi>μ</h:mi> </h:mrow> </h:msub> <h:mo stretchy="false">)</h:mo> <h:mo>=</h:mo> <h:mo stretchy="false">(</h:mo> <h:mn>2.89</h:mn> <h:mo>±</h:mo> <h:mn>0.2</h:mn> <h:msub> <h:mrow> <h:mn>7</h:mn> </h:mrow> <h:mrow> <h:mi>stat</h:mi> </h:mrow> </h:msub> <h:mo>±</h:mo> <h:mn>0.1</h:mn> <h:msub> <h:mrow> <h:mn>2</h:mn> </h:mrow> <h:mrow> <h:mi>syst</h:mi> </h:mrow> </h:msub> <h:mo stretchy="false">)</h:mo> <h:mo>×</h:mo> <h:msup> <h:mrow> <h:mn>10</h:mn> </h:mrow> <h:mrow> <h:mo>−</h:mo> <h:mn>3</h:mn> </h:mrow> </h:msup> </h:mrow> </h:math> , where the first uncertainty is statistical and the second is systematic. Based on a simultaneous fit to the partial decay rates in <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:msup> <p:mi>q</p:mi> <p:mn>2</p:mn> </p:msup> </p:math> intervals measured in <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"> <r:mrow> <r:msubsup> <r:mrow> <r:mi>D</r:mi> </r:mrow> <r:mrow> <r:mi>s</r:mi> </r:mrow> <r:mrow> <r:mo>+</r:mo> </r:mrow> </r:msubsup> <r:mo stretchy="false">→</r:mo> <r:msup> <r:mrow> <r:mi>K</r:mi> </r:mrow> <r:mrow> <r:mn>0</r:mn> </r:mrow> </r:msup> <r:msup> <r:mrow> <r:mi>μ</r:mi> </r:mrow> <r:mrow> <r:mo>+</r:mo> </r:mrow> </r:msup> <r:msub> <r:mrow> <r:mi>ν</r:mi> </r:mrow> <r:mrow> <r:mi>μ</r:mi> </r:mrow> </r:msub> </r:mrow> </r:math> and <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:msubsup> <u:mi>D</u:mi> <u:mi>s</u:mi> <u:mo>+</u:mo> </u:msubsup> <u:mo stretchy="false">→</u:mo> <u:msup> <u:mi>K</u:mi> <u:mn>0</u:mn> </u:msup> <u:msup> <u:mi>e</u:mi> <u:mo>+</u:mo> </u:msup> <u:msub> <u:mi>ν</u:mi> <u:mi>e</u:mi> </u:msub> </u:math> decays, the product value of the form factor <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"> <x:msubsup> <x:mi>f</x:mi> <x:mo>+</x:mo> <x:msup> <x:mi>K</x:mi> <x:mn>0</x:mn> </x:msup> </x:msubsup> <x:mo stretchy="false">(</x:mo> <x:mn>0</x:mn> <x:mo stretchy="false">)</x:mo> </x:math> and the Cabibbo-Kobayashi-Maskawa matrix element <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"> <bb:mo stretchy="false">|</bb:mo> <bb:msub> <bb:mi>V</bb:mi> <bb:mrow> <bb:mi>c</bb:mi> <bb:mi>d</bb:mi> </bb:mrow> </bb:msub> <bb:mo stretchy="false">|</bb:mo> </bb:math> is measured to be <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="i