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

Area of Science:

• Particle Physics
• High Energy Physics
• Standard Model Extensions

Background:

• Searches for new physics beyond the Standard Model often involve looking for exotic particles like the Higgs boson.
• Radiative decays of heavy quarkonium states, such as the ϒ(1S) meson, provide a unique environment to search for such particles.
• Previous experiments have constrained the properties of light Higgs bosons through various decay channels.

Purpose of the Study:

• To search for a light Higgs boson (A^{0}) decaying into tau ($\tau^{+}\tau^{-}$) or muon ($\mu^{+}\mu^{-}$) pairs.
• To investigate the radiative decays of the ϒ(1S) meson, specifically ϒ(1S) → γA^{0}.
• To set new upper limits on the product branching fractions and probe the Yukawa coupling and mixing angle of the hypothetical A^{0} boson.

Main Methods:

• Utilized 158 million ϒ(2S) events collected by the Belle detector at the KEKB collider.
• Tagged ϒ(1S) meson production via the ϒ(2S) → π^{+}π^{-}ϒ(1S) transition.
• Analyzed ϒ(1S) radiative decays searching for A^{0} signals in the $\tau^{+}\tau^{-}$ and $\mu^{+}\mu^{-}$ final states within a specific mass range.

Main Results:

• No significant signal for the light Higgs boson (A^{0}) was observed in the mass range from the lepton pair threshold up to 9.2 GeV/c^{2}.
• Established new upper limits at 90% credibility level (C.L.) for the product branching fractions of ϒ(1S) → γA^{0} and A^{0} → $\tau^{+}\tau^{-}$, showing a twofold improvement over world best limits.
• Set upper limits for A^{0} → $\mu^{+}\mu^{-}$ that are comparable to existing world average limits.

Conclusions:

• The search for a light Higgs boson in ϒ(1S) radiative decays yielded no evidence for its existence within the studied mass range.
• The improved upper limits on branching fractions for A^{0} → $\tau^{+}\tau^{-}$ provide stronger constraints on new physics models.
• The results contribute to the ongoing efforts to explore physics beyond the Standard Model by constraining parameters related to hypothetical Higgs bosons.