Search for a low-mass scalar Higgs boson decaying to a tau pair in single-photon decays of ${\rm Y}(1S)$

We search for a low-mass scalar $CP$-odd Higgs boson, $A^0$, produced in the radiative decay of the upsilon resonance and decaying into a ${\tau }^{+}{\tau }^{-}$ pair: ${\rm Y}(1S)\to \gamma A^0$. The production of ${\rm Y}(1S)$ mesons is tagged by ${\rm Y}(2S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ transitions, using a sample of $(98.3\pm 0.9)\times {10}^6$ ${\rm Y}(2S)$ mesons collected by the BABAR detector. We find no evidence for a Higgs boson in the mass range $3.5\le m_{A^0}\le 9.2\mathrm{GeV}$, and combine these results with our previous search for the tau decays of the light Higgs in radiative ${\rm Y}(3S)$ decays, setting limits on the coupling of $A^0$ to the $b\overline{b}$ quarks in the range 0.09–1.9. Our measurements improve the constraints on the parameters of the next-to-minimal-supersymmetric Standard Model and similar theories with low-mass scalar degrees of freedom.

Figure 1

Signal efficiency as a function of $m_{A^0}$ in the $\mathtt{L}$ and $\mathtt{H}$ mass ranges (separated by the vertical blue line).

Figure 2

Fits to the $m_X^2$ distributions in (a) $\mathtt{L}$ and (b) $\mathtt{H}$ ranges for the two particular $m_{A^0}$ points that return the largest upward fluctuations: (a) $m_{A^0}=6.36\mathrm{GeV}$ and (b) $m_{A^0}=8.93\mathrm{GeV}$. The red solid line shows the signal PDF, the green dot-dashed line is the background contribution, and the blue solid line shows the total PDF. The top plots show the fit residuals normalized by the error (pulls). The signal peaks corresponds to a statistical significance of (a) $2.7\sigma$ and (b) $3.0\sigma$.

Figure 3

90% C.L. upper limits for $\mathcal{B}({\rm Y}(1S)\to \gamma A^0)\times \mathcal{B}(A^0\to {\tau }^{+}{\tau }^{-})$.

Figure 4

90% C.L. upper limits for Yukawa coupling $g_b^2\times \mathcal{B}(A^0\to {\tau }^{+}{\tau }^{-})$. Shown are combined BABAR results (red solid line), results from this analysis only (dashed green line), the previous BABAR measurement [9] (dotted blue line), and results from the CLEO experiment [12] (dot-dashed black line). The shaded vertical bar shows the region around ${\chi }_b(2P)$ mass excluded from Ref. [9].