Search for a light Higgs boson in single-photon decays of ${\rm Y}(1S)$ using ${\rm Y}(2S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ tagging method

Abstract

We search for a light Higgs boson ($A^0$) decaying into a ${\tau }^{+}{\tau }^{-}$ or ${\mu }^{+}{\mu }^{-}$ pair in the radiative decays of ${\rm Y}(1S)$. The production of ${\rm Y}(1S)$ mesons is tagged by ${\rm Y}(2S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ transitions, using 158 million ${\rm Y}(2S)$ 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 ${\tau }^{+}{\tau }^{-}$ or ${\mu }^{+}{\mu }^{-}$ 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 ${\rm Y}(1S)\to \gamma A^0$ and $A^0\to {\tau }^{+}{\tau }^{-}$ varying from $3.8\times 10^{-6}$ to $1.5\times 10^{-4}$. Our results represent an approximately twofold improvement on the current world best upper limits for the ${\rm Y}(1S)\to \gamma A^0(\to {\tau }^{+}{\tau }^{-})$ production. For $A^0\to {\mu }^{+}{\mu }^{-}$, the upper limits on the product branching fractions for ${\rm Y}(1S)\to \gamma A^0$ and $A^0\to {\mu }^{+}{\mu }^{-}$ are at the same level as the world average limits, and vary from $3.1\times 10^{-7}$ to $1.6\times 10^{-5}$. The upper limits at 90\% C.L. on the Yukawa coupling $f_{{\rm Y}(1S)}$ and mixing angle $\sin {\theta }_{A^0}$ are also given.

Comments: 9 pages, 5 figures; accepted for publication as a Letter in Physical Review Letters