Search for Hadronic Decays of a Light Higgs Boson in the Radiative Decay ${\rm Y}\to \gamma A^0$

We search for hadronic decays of a light Higgs boson ($A^0$) produced in radiative decays of an $\mathit{{\rm Y}}(2S)$ or $\mathit{{\rm Y}}(3S)$ meson, $\mathit{{\rm Y}}\to \gamma A^0$. The data have been recorded by the BABAR experiment at the $\mathit{{\rm Y}}(3S)$ and $\mathit{{\rm Y}}(2S)$ center-of-mass energies and include $(121.3\pm 1.2)\times {10}^6$ $\mathit{{\rm Y}}(3S)$ and $(98.3\pm 0.9)\times {10}^6$ $\mathit{{\rm Y}}(2S)$ mesons. No significant signal is observed. We set $90\%$ confidence level upper limits on the product branching fractions $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(nS)\to \gamma A^0\mathbf{)}\mathcal{B}(A^0\to \mathrm{\text{hadrons}})$ ($n=2$ or 3) that range from $1\times {10}^{-6}$ for an $A^0$ mass of $0.3\mathrm{GeV}/c^2$ to $8\times {10}^{-5}$ at $7\mathrm{GeV}/c^2$.

Figure 1

Candidate mass spectrum in the (a) $CP$-all and (b) $CP$-odd analyses. The top curve in each plot is the on-peak data overlaid (in red) with the background fit described in the text, while the bottom curve (blue) is the scaled continuum data. The prominent initial-state radiation resonances are labeled.

Figure 2

$A^0$ candidate mass spectrum after continuum subtraction, overlaid with fit. (a) $CP$-all analysis; (b) $CP$-odd analysis.

Figure 3

Statistical significance (events divided by statistical error) of the $A^0$ signal as a function of mass, for (a) $CP$-all analysis and (b) $CP$-odd analysis.

Figure 4

Histogram of the statistical significance of the $A^0$ signal for (a) the 6710 masses considered in the $CP$-all analysis and for (b) the 6701 masses in the $CP$-odd analysis. The overlaid curve shows the distribution expected in the absence of signal.

Figure 5

$90\%$ C.L. upper limits on product branching fractions (BF) (left axis) $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(3S)\to \gamma A^0\mathbf{)}\mathcal{B}(A^0\to \mathrm{\text{hadrons}})$ and (right axis) $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(2S)\to \gamma A^0\mathbf{)}\mathcal{B}(A^0\to \mathrm{\text{hadrons}})$, for (a) $CP$-all analysis and (b) $CP$-odd analysis. The overlaid curves (red online) are the limits expected from simulated experiments, while the light gray curves (blue online) are the limits from statistical errors only. The $\mathit{{\rm Y}}(2S)$ limits do not include the phase space factor, which is at most a $3.5\%$ correction.