Testing axion couplings to leptons in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Z</mml:mi></mml:math> decays at future <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>e</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>e</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math> colliders
Lorenzo Calibbi, Zijie Huang, Shaoyang Qin, Yiming Yang, Xiaoyue Yin
Abstract
We study the possibility of probing the existence of a light, invisible, axionlike particle (ALP) $a$ in leptonic decays of the $Z$ boson at the proposed high-energy ${e}^{+}{e}^{\ensuremath{-}}$ colliders, Circular Electron-Positron Collider (CEPC) and Future Circular Collider (FCC-ee). Both projects plan to run at the $Z$ pole, collecting ${10}^{12}--{10}^{13}$ visible $Z$ decays. We show that, searching for the emission of an invisible ALP from leptons in leptonic $Z$ decays, this enormous statistics could allow us to constrain the ALP couplings to leptons at an unprecedented level for laboratory experiments. In particular, within a Monte Carlo simulation framework, we estimate that CEPC/FCC-ee can be sensitive to the coupling of an invisible ALP to muons up to ${f}_{a}/{C}_{\ensuremath{\mu}\ensuremath{\mu}}^{A}\ensuremath{\approx}1\text{ }\text{ }\mathrm{TeV}$---where ${f}_{a}$ is the ALP decay constant---corresponding to $\mathrm{BR}(Z\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}a)\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$.