Search for a Light Higgs Boson in Single-Photon Decays of Upsilon(1S) Using (SIC)(2S) -> pi(+) pi(-) Upsilon(1S) Tagging Method

We search for a light Higgs boson (A0) 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 158×106 ϒ(2S) events accumulated with the Belle detector at the KEKB asymmetric energy electron-positron collider. No significant A0 signals in the mass range from the τ+τ- or μ+μ- threshold to 9.2 GeV/c2 are observed. We set the upper limits at 90% credibility level (C.L.) on the product branching fractions for ϒ(1S)→γA0 and A0→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)→γA0(→τ+τ-) production. For A0→μ+μ-, the upper limits on the product branching fractions for ϒ(1S)→γA0 and A0→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 sinθA0 are also given.