Physicists observe rare particle decay, filling gap in standard model data
Researchers using China's BESIII detector have directly observed electromagnetic decays of two exotic particles for the first time, completing measurements needed to validate theoretical physics models. The finding adds precision to our understanding of particle interactions and could guide future experiments in high-energy physics.
Originaltitel: Observation of the Electromagnetic Radiative Decays of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi mathvariant="normal">Λ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1520</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi mathvariant="normal">Λ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1690</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>γ</mml:mi> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">Σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math>
Using <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mo stretchy="false">(</a:mo> <a:mn>10087</a:mn> <a:mo>±</a:mo> <a:mn>44</a:mn> <a:mo stretchy="false">)</a:mo> <a:mo>×</a:mo> <a:msup> <a:mrow> <a:mn>10</a:mn> </a:mrow> <a:mrow> <a:mn>6</a:mn> </a:mrow> </a:msup> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>J</a:mi> <a:mo>/</a:mo> <a:mi>ψ</a:mi> </a:mrow> </a:math> events collected with the BESIII detector, we present a study of resonant structures in the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>J</e:mi> <e:mo>/</e:mo> <e:mi>ψ</e:mi> <e:mo stretchy="false">→</e:mo> <e:mi mathvariant="normal">Λ</e:mi> <e:mover accent="true"> <e:mi mathvariant="normal">Λ</e:mi> <e:mo stretchy="false">¯</e:mo> </e:mover> <e:mi>γ</e:mi> </e:math> and <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mi mathvariant="normal">Λ</l:mi> <l:msup> <l:mover accent="true"> <l:mi mathvariant="normal">Σ</l:mi> <l:mo stretchy="false">¯</l:mo> </l:mover> <l:mn>0</l:mn> </l:msup> <l:mi>γ</l:mi> </l:math> processes, reporting the first observation of the electromagnetic radiative decays of the <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"> <r:mi mathvariant="normal">Λ</r:mi> <r:mo stretchy="false">(</r:mo> <r:mn>1520</r:mn> <r:mo stretchy="false">)</r:mo> </r:math> and <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mi mathvariant="normal">Λ</w:mi> <w:mo stretchy="false">(</w:mo> <w:mn>1690</w:mn> <w:mo stretchy="false">)</w:mo> </w:math> to <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"> <bb:mi>γ</bb:mi> <bb:msup> <bb:mi mathvariant="normal">Σ</bb:mi> <bb:mn>0</bb:mn> </bb:msup> </bb:math> , with a statistical significance of <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"> <eb:mn>16.6</eb:mn> <eb:mi>σ</eb:mi> </eb:math> and <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"> <gb:mn>23.2</gb:mn> <gb:mi>σ</gb:mi> </gb:math> , respectively. The ratio of the branching fractions <ib:math xmlns:ib="http://www.w3.org/1998/Math/MathML" display="inline"> <ib:mrow> <ib:mo stretchy="false">{</ib:mo> <ib:mi mathvariant="script">B</ib:mi> <ib:mo stretchy="false">[</ib:mo> <ib:mi mathvariant="normal">Λ</ib:mi> <ib:mo stretchy="false">(</ib:mo> <ib:mn>1520</ib:mn> <ib:mo stretchy="false">)</ib:mo> <ib:mo stretchy="false">→</ib:mo> <ib:mi>γ</ib:mi> <ib:mi mathvariant="normal">Λ</ib:mi> <ib:mo stretchy="false">]</ib:mo> <ib:mo>/</ib:mo> <ib:mi mathvariant="script">B</ib:mi> <ib:mo stretchy="false">[</ib:mo> <ib:mi mathvariant="normal">Λ</ib:mi> <ib:mo stretchy="false">(</ib:mo> <ib:mn>1520</ib:mn> <ib:mo stretchy="false">)</ib:mo> <ib:mo stretchy="false">→</ib:mo> <ib:mi>γ</ib:mi> <ib:msup> <ib:mrow> <ib:mi mathvariant="normal">Σ</ib:mi> </ib:mrow> <ib:mrow> <ib:mn>0</ib:mn> </ib:mrow> </ib:msup> <ib:mo stretchy="false">]</ib:mo> <ib:mo stretchy="false">}</ib:mo> </ib:mrow> </ib:math> is determined to be <cc:math xmlns:cc="http://www.w3.org/1998/Math/MathML" display="inline"> <cc:mrow> <cc:mn>3.19</cc:mn> <cc:mo>±</cc:mo> <cc:mn>0.34</cc:mn> <cc:mo stretchy="false">(</cc:mo> <cc:mrow> <cc:mi>stat</cc:mi> </cc:mrow> <cc:mo stretchy="false">)</cc:mo> <cc:mo>±</cc:mo> <cc:mn>0.19</cc:mn> <cc:mo stretchy="false">(</cc:mo> <cc:mrow> <cc:mi>syst</cc:mi> </cc:mrow> <cc:mo stretchy="false">)</cc:mo> </cc:mrow> </cc:math> , which is in agreement with flavor SU(3) symmetry within two standard deviations. The branching fraction of <ic:math xmlns:ic="http://www.w3.org/1998/Math/MathML" display="inline"> <ic:mi mathvariant="normal">Λ</ic:mi> <ic:mo stretchy="false">(</ic:mo> <ic:mn>1520</ic:mn> <ic:mo stretchy="false">)</ic:mo> <ic:mo stretchy="false">→</ic:mo> <ic:mi>γ</ic:mi> <ic:msup> <ic:mi mathvariant="normal">Σ</ic:mi> <ic:mn>0</ic:mn> </ic:msup> </ic:math> is measured to be <pc:math xmlns:pc="http://www.w3.org/1998/Math/MathML" display="inline"> <pc:mrow> <pc:mi mathvariant="script">B</pc:mi> <pc:mo stretchy="false">[</pc:mo> <pc:mi mathvariant="normal">Λ</pc:mi> <pc:mo stretchy="false">(</pc:mo> <pc:mn>1520</pc:mn> <pc:mo stretchy="false">)</pc:mo> <pc:mo stretchy="false">→</pc:mo> <pc:mi>γ</pc:mi> <pc:msup> <pc:mrow> <pc:mi mathvariant="normal">Σ</pc:mi> </pc:mrow> <pc:mrow> <pc:mn>0</pc:mn> </pc:mrow> </pc:msup> <pc:mo stretchy="false">]</pc:mo> <pc:mo>=</pc:mo> <pc:mo stretchy="false">[</pc:mo> <pc:mn>2.66</pc:mn> <pc:mo>±</pc:mo> <pc:mn>0.28</pc:mn> <pc:mo stretchy="false">(</pc:mo> <pc:mrow> <pc:mi>stat</pc:mi> </pc:mrow> <pc:mo stretchy="false">)</pc:mo> <pc:mo>±</pc:mo> <pc:mn>0.50</pc:mn> <pc:mo stretchy="false">(</pc:mo> <pc:mrow> <pc:mi>syst</pc:mi> </pc:mrow> <pc:mo stretchy="false">)</pc:mo> <pc:mo stretchy="false">]</pc:mo> <pc:mo>×</pc:mo> <pc:msup> <pc:mrow> <pc:mn>10</pc:mn> </pc:mrow> <pc:mrow> <pc:mo>−</pc:mo> <pc:mn>3</pc:mn> </pc:mrow> </pc:msup> </pc:mrow> </pc:math> , corresponding to a partial width of <fd:math xmlns:fd="http://www.w3.org/1998/Math/MathML" display="inline"> <fd:mrow> <fd:mi mathvariant="normal">Γ</fd:mi> <fd:mo stretchy="false">[</fd:mo> <fd:mi mathvariant="normal">Λ</fd:mi> <fd:mo stretchy="false">(</fd:mo>