Search for Sub-GeV Invisible Particles in Inclusive Decays of J/ψ to ϕ

A search for an invisible particle, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>X</a:mi></a:math>, with a mass between 0 and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mn>0.96</c:mn><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi>GeV</c:mi><c:mo>/</c:mo><c:msup><c:mrow><c:mi>c</c:mi></c:mrow><c:mrow><c:mn>2</c:mn></c:mrow></c:msup></c:mrow></c:math>, is performed in the process <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mi>J</e:mi><e:mo>/</e:mo><e:mi>ψ</e:mi><e:mo stretchy="false">→</e:mo><e:mi>ϕ</e:mi><e:mo>+</e:mo><e:mi>X</e:mi></e:mrow></e:math> using <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mo stretchy="false">(</h:mo><h:mn>8774.0</h:mn><h:mo>±</h:mo><h:mn>39.4</h:mn><h:mo stretchy="false">)</h:mo><h:mo>×</h:mo><h:msup><h:mrow><h:mn>10</h:mn></h:mrow><h:mrow><h:mn>6</h:mn></h:mrow></h:msup><h:mtext> </h:mtext><h:mtext> </h:mtext><h:mi>J</h:mi><h:mo>/</h:mo><h:mi>ψ</h:mi></h:mrow></h:math> events collected with the BESIII detector from 2017 to 2019. The <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi>ϕ</l:mi></l:math> meson is fully reconstructed and an efficient veto of photons and neutral and charged hadrons up to twice the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:msubsup><n:mi>K</n:mi><n:mi>L</n:mi><n:mn>0</n:mn></n:msubsup></n:math> mass is applied to the rest of the event and the recoil mass against the <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mi>ϕ</p:mi></p:math> is obtained precisely from the kinematic constraint in the event. No significant signal over the expected background is observed in the investigated region and the upper limit on the inclusive branch fraction of <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:mi>J</r:mi><r:mo>/</r:mo><r:mi>ψ</r:mi><r:mo stretchy="false">→</r:mo><r:mi>ϕ</r:mi><r:mo>+</r:mo><r:mi>X</r:mi></r:math> is determined to be <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mn>7.0</u:mn><u:mo>×</u:mo><u:msup><u:mn>10</u:mn><u:mrow><u:mo>−</u:mo><u:mn>8</u:mn></u:mrow></u:msup></u:math> at 90% confidence level. Upper limits at a 90% confidence level are also given for this branch fraction as a function of the invisible particle mass, varying from <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mn>4</w:mn><w:mo>×</w:mo><w:msup><w:mn>10</w:mn><w:mrow><w:mo>−</w:mo><w:mn>9</w:mn></w:mrow></w:msup></w:math> to <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mn>4</y:mn><y:mo>×</y:mo><y:msup><y:mn>10</y:mn><y:mrow><y:mo>−</y:mo><y:mn>8</y:mn></y:mrow></y:msup></y:math> over the investigated mass range. Additionally, a 90% confidence level upper limit on the branch fraction of <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mrow><ab:mi>η</ab:mi><ab:mo stretchy="false">→</ab:mo><ab:mtext>invisible</ab:mtext></ab:mrow></ab:math> is determined to <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mn>2.4</db:mn><db:mo>×</db:mo><db:msup><db:mn>10</db:mn><db:mrow><db:mo>−</db:mo><db:mn>5</db:mn></db:mrow></db:msup></db:math>, which improves the previous best results by more than 4 times. The analysis technique in this Letter offers a clean window to search for sub-GeV invisible particles, which can be adapted for other <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:mi>J</fb:mi><fb:mo>/</fb:mo><fb:mi>ψ</fb:mi></fb:math> decays and direct <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline"><hb:msup><hb:mi>e</hb:mi><hb:mo>+</hb:mo></hb:msup><hb:msup><hb:mi>e</hb:mi><hb:mo>−</hb:mo></hb:msup></hb:math> annihilation experiments in future studies, and improve the sensitivity by orders of magnitude.