Discovery potential of future electron-positron colliders for a 95 GeV scalar

Abstract

The Large Electron Positron collider observed an indication for a new Higgs boson with a mass around 95 GeV-100 GeV in the process $e^{+}{e}^{-}\to Z^{*}\to ZS$ with $S\to b\overline{b}$. The interest in this excess re-emerged with the di-photon signature at $\approx$ 95 GeV at the Large Hadron Collider. In fact, a combined global significance of $3.4\sigma$ is obtained once $WW$ and $\tau \tau$ signals are included in addition. In this article, we perform a feasibility study for discovering such a new scalar $S$ at future electron-positron colliders using the recoil-mass method applied to $e^{+}{e}^{-}\to ZS$ with $Z\to {\mu }^{+}{\mu }^{-}$ and $S\to b\overline{b}$. For this, we employ a Deep Neural Network to enhance the separation between the Standard Model background and the signal, reducing the required integrated luminosity necessary for discovery by a factor of two to three. As a result, an $SU{\left(2\right)}_L$ singlet Higgs with a mass of $\approx$ 95 GeV can be observed with more than 5$\sigma$ significance at a 250 GeV centre-of-mass energy collider with $5{ab}^{-1}$ integrated luminosity if it has a mixing angle of at least 0.1 with the Standard Model Higgs, which means that a discovery can be achieved within the whole 95 % confidence-level region preferred by Large Electron Positron excess. Furthermore, including more decay channels such as $S\to \tau \tau$ and $Z\to e^{+}{e}^{-}$ further enhances the discovery potential of future $e^{+}{e}^{-}$ accelerators, like CEPC, CLIC, FCC-ee and ILC.