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

Abstract

We search for a light Higgs boson ( A 0 ) decaying into a τ + τ − or μ + μ − pair in the radiative decays of Υ ( 1 S ) . The production of Υ ( 1 S ) mesons is tagged by Υ ( 2 S ) → π + π − Υ ( 1 S ) transitions, using 158 × 10 6

Υ ( 2 S ) 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 ( 1 S ) → γ A 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 ( 1 S ) → γ A 0 ( → τ + τ − ) production. For A 0 → μ + μ − , the upper limits on the product branching fractions for Υ ( 1 S ) → γ A 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 f Υ ( 1 S ) and mixing angle sin θ A 0 are also given.