Search for a Light $CP$-odd Higgs Boson and Low-Mass Dark Matter at the Belle Experiment

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 $\mathrm{\Upsilon }(2S)\to \mathrm{\Upsilon }(1S){\pi }^{+}{\pi }^{-}$, followed by the on-shell process $\mathrm{\Upsilon }(1S)\to \gamma A^0$ with $A^0\to \chi \chi$, or by the off-shell process $\mathrm{\Upsilon }(1S)\to \gamma \chi \chi$. Utilizing a data sample of $157.3\times {10}^6$ $\mathrm{\Upsilon }(2\mathrm{S})$ decays, we find no evidence for a signal. We set limits on the branching fractions of such processes in the mass ranges $M_{A^0}<8.97\mathrm{GeV}/c^2$ and $M_{\chi }<4.44\mathrm{GeV}/c^2$. We then use the limits on the off-shell process to set competitive limits on WIMP-nucleon scattering in the WIMP mass range below $5\mathrm{GeV}/c^2$.

Figure 1

Fit result for the on-shell process mass scan point with $M_{A^0}=2.946\mathrm{GeV}/c^2$, which has the highest local signal significance; $2.1\sigma$. Top: $M_{\text{recoil}}$ distribution. Bottom: $E_{\gamma }^{*}$ distribution. The fitted components are continuum background (cyan dashed curve), $\mathrm{\Upsilon }(1S)$ decay background (magenta dashed curve), total background (black dashed curve), and the on-shell signal (red dashed curve). The blue solid curve shows the sum of all fitted components.

Figure 2

90% C.L. upper limits on the BFs of the on-shell process $\mathrm{\Upsilon }(1S)\to \gamma A^0$ with $A^0\to \chi \chi$ (top) and the off-shell process $\mathrm{\Upsilon }(1S)\to \gamma \chi \chi$ (bottom). The orange solid curves are the Belle limits and the blue dashed curves are the BABAR limits.

Figure 3

WIMP-nucleon spin-independent scattering cross-section limits at 90% C.L. The black solid and dashed curves are the upper limits obtained by assuming the WIMP couples to all quarks and only b quarks, respectively. The 90% C.L. exclusion limits of LUX [18], CRESST II [19], SuperCDMS [20], and ATLAS [21, 22] and CMS [23, 24] are shown for reference; and the 90% C.L. signal regions of CRESST II [25], CoGeNT [26], DAMA/LIBRA [27], and CDMS II (Silicon) [28] are also shown.