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, A0, that decays into low mass dark matter, χ, in final states with a single photon and missing energy. We search for events produced via the dipion transition Υ(2S)→Υ(1S)π+π−, followed by the on-shell process Υ(1S)→γA0 with A0→χχ, or by the off-shell process Υ(1S)→γχχ. Utilizing a data sample of 157.3×106 Υ(2S) decays, we find no evidence for a signal. We set limits on the branching fractions of such processes in the mass ranges MA0<8.97 GeV/c2 and Mχ<4.44 GeV/c2. 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 GeV/c2.