Search for a Light Higgs Boson in Single-Photon Decays of ϒ(1S) Using ϒ(2S)→π$^{+}$π$^{-}$ϒ(1S) Tagging Method

We search for a light Higgs boson (A$^{0}$) decaying into a τ$^{+}$τ$^{-}$ or μ$^{+}$μ$^{-}$ pair in the radiative decays of Υ(1S). The production of Υ(1S) mesons is tagged by Υ(2S)→π$^{+}$π$^{-}$Υ(1S) transitions, using 158×10$^{6}$ Υ(2S) events accumulated with the Belle detector at the KEKB asymmetric energy electron positron collider. No significant A$^{0}$ signals in the mass range from the τ$^{+}$τ$^{-}$ or μ$^{+}$μ$^{-}$ threshold to 9.2 GeV/c$^{2}$ are observed. We set the upper limits at 90% credibility level (C.L.) on the product branching fractions for Υ(1S)→γA$^{0}$ and A$^{0}$→τ$^{+}$τ$^{-}$ varying from 3.8×10$^{-6}$ to 1.5×10$^{-4}$. Our results represent an approximately twofold improvement on the current world best upper limits for the Υ(1S)→γA$^{0}$(→τ$^{+}$τ$^{-}$) production. For A$^{0}$→μ$^{+}$μ$^{-}$, the upper limits on the product branching fractions for Υ(1S)→γA$^{0}$ and A$^{0}$→μ$^{+}$μ$^{-}$ are at the same level as the world average limits, and vary from 3.1×10$^{-7}$ to 1.6×10$^{-5}$. The upper limits at 90% credibility level on the Yukawa coupling fΥ(1S) and mixing angle sin θ$_{A0}$ are also given.