Search for a Light <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi><mml:mi>P</mml:mi></mml:math>-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, $\ensuremath{\chi}$, in final states with a single photon and missing energy. We search for events produced via the dipion transition $\mathrm{\ensuremath{\Upsilon}}(2S)\ensuremath{\rightarrow}\mathrm{\ensuremath{\Upsilon}}(1S){\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, followed by the on-shell process $\mathrm{\ensuremath{\Upsilon}}(1S)\ensuremath{\rightarrow}\ensuremath{\gamma}{A}^{0}$ with ${A}^{0}\ensuremath{\rightarrow}\ensuremath{\chi}\ensuremath{\chi}$, or by the off-shell process $\mathrm{\ensuremath{\Upsilon}}(1S)\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\chi}\ensuremath{\chi}$. Utilizing a data sample of $157.3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $\mathrm{\ensuremath{\Upsilon}}(2\mathrm{S})$ decays, we find no evidence for a signal. We set limits on the branching fractions of such processes in the mass ranges ${M}_{{A}^{0}}&lt;8.97\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ and ${M}_{\ensuremath{\chi}}&lt;4.44\text{ }\text{ }\mathrm{GeV}/{c}^{2}$. We then use the limits on the off-shell process to set competitive limits on WIMP-nucleon scattering in the WIMP mass range below $5\text{ }\text{ }\mathrm{GeV}/{c}^{2}$.