Interpretation of excess in H→Zγ using a light axionlike particle

We interpret the recent excess in a rare decay of the Higgs boson, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>H</a:mi><a:mo stretchy="false">→</a:mo><a:mi>Z</a:mi><a:mi>γ</a:mi></a:math>, using a light axionlike particle (ALP) in the mass range 0.05–0.1 GeV. The dominant decay of such a light ALP is into a pair of collimated photons, whose decay is required to happen before reaching the ECAL detector, such that it mimics a single photon in the detector. It can explain the excess with a coupling <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msubsup><d:mi>C</d:mi><d:mrow><d:mi>a</d:mi><d:mi>Z</d:mi><d:mi>H</d:mi></d:mrow><d:mi>eff</d:mi></d:msubsup><d:mo>/</d:mo><d:mi mathvariant="normal">Λ</d:mi><d:mo>∼</d:mo><d:mn>4</d:mn><d:mo>×</d:mo><d:msup><d:mn>10</d:mn><d:mrow><d:mo>−</d:mo><d:mn>5</d:mn></d:mrow></d:msup><d:mtext> </d:mtext><d:mtext> </d:mtext><d:msup><d:mrow><d:mi>GeV</d:mi></d:mrow><d:mrow><d:mo>−</d:mo><d:mn>1</d:mn></d:mrow></d:msup></d:math>, while the decay of the ALP before reaching the ECAL requires the diphoton coupling <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msubsup><g:mi>C</g:mi><g:mrow><g:mi>γ</g:mi><g:mi>γ</g:mi></g:mrow><g:mi>eff</g:mi></g:msubsup><g:mo>/</g:mo><g:mi mathvariant="normal">Λ</g:mi><g:mo>≥</g:mo><g:mn>0.35</g:mn><g:mtext> </g:mtext><g:mtext> </g:mtext><g:msup><g:mi>TeV</g:mi><g:mrow><g:mo>−</g:mo><g:mn>1</g:mn></g:mrow></g:msup><g:mo stretchy="false">(</g:mo><g:mn>0.1</g:mn><g:mtext> </g:mtext><g:mtext> </g:mtext><g:mi>GeV</g:mi><g:mo>/</g:mo><g:msub><g:mi>m</g:mi><g:mi>a</g:mi></g:msub><g:msup><g:mo stretchy="false">)</g:mo><g:mn>2</g:mn></g:msup></g:math>. A potential test would be the rare decay of the <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi>Z</l:mi></l:math> boson <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mi>Z</n:mi><n:mo stretchy="false">→</n:mo><n:mi>a</n:mi><n:msup><n:mi>H</n:mi><n:mo>*</n:mo></n:msup><n:mo stretchy="false">→</n:mo><n:mi>a</n:mi><n:mo stretchy="false">(</n:mo><n:mi>b</n:mi><n:mover accent="true"><n:mi>b</n:mi><n:mo stretchy="false">¯</n:mo></n:mover><n:mo stretchy="false">)</n:mo></n:math> at the Tera-<v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mi>Z</v:mi></v:math> option of the future FCC and CEPC. However, it has a branching ratio of only <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"><x:mi>O</x:mi><x:mo stretchy="false">(</x:mo><x:msup><x:mn>10</x:mn><x:mrow><x:mo>−</x:mo><x:mn>12</x:mn></x:mrow></x:msup><x:mo stretchy="false">)</x:mo></x:math>, and thus barely testable. The production cross section for <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:mi>p</bb:mi><bb:mi>p</bb:mi><bb:mo stretchy="false">→</bb:mo><bb:msup><bb:mi>Z</bb:mi><bb:mo>*</bb:mo></bb:msup><bb:mo stretchy="false">→</bb:mo><bb:mi>a</bb:mi><bb:mi>H</bb:mi></bb:math> via the same coupling <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:msubsup><fb:mi>C</fb:mi><fb:mrow><fb:mi>a</fb:mi><fb:mi>Z</fb:mi><fb:mi>H</fb:mi></fb:mrow><fb:mi>eff</fb:mi></fb:msubsup><fb:mo>/</fb:mo><fb:mi mathvariant="normal">Λ</fb:mi></fb:math> at the LHC is too small for detection.