Search for a light CP-odd Higgs boson and low-mass dark matter at the Belle experiment

Abstract

We report on the first Belle search for a light CP-odd Higgs boson, $A^0$, that decays into low mass dark matter, $\ddot{I}‡$, in final states with a single photon and missing energy. We search for events produced via the dipion transition $\hat{I}\yen (\text{2S})\to \hat{I}\yen (\text{1S})\ddot{I}{€}^{+}\ddot{I}{€}^{-}$, followed by the on-shell process $\hat{I}\yen (\text{1S})\to {\hat{I}}^3{A}^0$ with $A^0\to \ddot{I}‡\ddot{I}‡$, or by the off-shell process $\hat{I}\yen (\text{1S})\to {\hat{I}}^3\ddot{I}‡\ddot{I}‡$. Utilizing a data sample of 157.3 $\times$ 10${}^6$ $\hat{I}\yen (\text{2S})$ decays, we find no evidence for a signal. We set limits on the branching fractions of such processes in the mass ranges $M_{A^0}<$ 8.97 $\text{GeV/}{\text{c}}^2$ and $M_{\ddot{I}}‡<$ 4.44 $\text{GeV/}{\text{c}}^2$. We then use the limits on the off-shell process to set competitive limits on WIMP-nucleon scattering in the WIMP mass range below 5 $\text{GeV/}{\text{c}}^2$.