Search for a Light Higgs Boson in Single-Photon Decays of <math><mrow><mi>ϒ</mi><mo>(</mo><mn>1</mn><mi>S</mi><mo>)</mo></mrow></math> Using <math><mrow><mi>ϒ</mi><mo>(</mo><mn>2</mn><mi>S</mi><mo>)</mo><mo>→</mo><msup><mrow><mi>π</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>π</mi></mrow><mrow><mo>−</mo></mrow></msup><mi>ϒ</mi><mo>(</mo><mn>1</mn><mi>S</mi><mo>)</mo></mrow></math> 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&#215;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&#215;10−6 to 1.5&#215;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&#215;10−7 to 1.6&#215;10−5. The upper limits at 90% credibility level on the Yukawa coupling fϒ(1S) and mixing angle sinθA0 are also given.