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

TL;DR

This paper reports the first search by Belle for a light CP-odd Higgs boson decaying into low-mass dark matter, setting limits on its production and constraining low-mass WIMP-nucleon interactions.

Contribution

First experimental search for a light CP-odd Higgs decaying into dark matter at Belle, providing new limits on such processes and WIMP interactions.

Findings

No evidence for the signal was observed.

Limits were set on the branching fractions for the processes.

Constraints on WIMP-nucleon scattering for masses below 5 GeV/c².

Abstract

We report on the first Belle search for a light CP-odd Higgs boson, $A^{0}$, that decays into low mass dark matter, $\chi$, in final states with a single photon and missing energy. We search for events produced via the dipion transition $\Upsilon(\textrm{2S})\rightarrow\Upsilon(\textrm{1S})\pi^{+}\pi^{-}$, followed by the on-shell process $\Upsilon(\textrm{1S})\rightarrow\gamma A^{0}$ with $A^{0} \rightarrow\chi\chi$, or by the off-shell process $\Upsilon(\textrm{1S})\rightarrow\gamma\chi\chi$. Utilizing a data sample of 157.3 $\times$ 10$^{6}$ $\Upsilon(\textrm{2S})$ 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 $\textrm{GeV/}\textit{c}^{2}$ and $M_{\chi} <$ 4.44 $\textrm{GeV/}\textit{c}^{2}$. 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 $\textrm{GeV/}\textit{c}^{2}$.