Observation of $B_s^0\to J/\psi f_0(980)$ and Evidence for $B_s^0\to J/\psi f_0(1370)$

We report the observation of $B_s^0\to J/\psi f_0(980)$ and evidence for $B_s^0\to J/\psi f_0(1370)$, which are $CP$ eigenstate decay modes. These results are obtained from $121.4{\mathrm{fb}}^{-1}$ of data collected at the ${\rm Y}(5S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We measure the branching fractions $\mathcal{B}\mathbf{(}{B}_s^0\to J/\psi f_0(980);f_0(980)\to {\pi }^{+}{\pi }^{-}\mathbf{)}=\mathbf{(}{1.16}_{-0.19}^{+0.31}(\mathrm{stat}{)}_{-0.17}^{+0.15}(\mathrm{syst}{)}_{-0.18}^{+0.26}(N_{B_s^{(*)}{\overline{B}}_s^{(*)}})\mathbf{)}\times {10}^{-4}$ with a significance of $8.4\sigma$, and $\mathcal{B}\mathbf{(}{B}_s^0\to J/\psi f_0(1370);f_0(1370)\to {\pi }^{+}{\pi }^{-}\mathbf{)}=\mathbf{(}{0.34}_{-0.14}^{+0.11}(\mathrm{stat}{)}_{-0.02}^{+0.03}(\mathrm{syst}{)}_{-0.05}^{+0.08}(N_{B_s^{(*)}{\overline{B}}_s^{(*)}})\mathbf{)}\times {10}^{-4}$ with a significance of $4.2\sigma$. The last error listed is due to uncertainty in the number of produced $B_s^{(*)}{\overline{B}}_s^{(*)}$ pairs.

Figure 1

Data fit projections to $M_{\pi \pi }$ for (a) $-79.7\mathrm{MeV}<\Delta E<-19.7\mathrm{MeV}$, and to $\Delta E$ for (b) $0.80\mathrm{GeV}/c^2<M_{\pi \pi }<1.16\mathrm{GeV}/c^2$ and (c) $1.3\mathrm{GeV}/c^2<M_{\pi \pi }<1.5\mathrm{GeV}/c^2$. The total PDF is shown with a solid line. The dash-dotted curves represent the total background, and the dashed curves show other $J/\psi$ background. The dotted curves are the nonresonant components, which are barely visible due to the smallness of their yields.

Figure 2

The cosine of the angle ${\theta }_{\mathrm{hel}}$ between $l^{+}$ and the direction opposite to that of the $B_s^0$ in the $J/\psi$ rest frame. $\cos ({\theta }_{\mathrm{hel}})$ is projected in the $\Delta E$ signal region and $f_0(980)$ and $f_0(1370)$ signal regions as $0.80\mathrm{GeV}/c^2<M_{\pi \pi }<1.16\mathrm{GeV}/c^2$ (left) and $1.3\mathrm{GeV}/c^2<M_{\pi \pi }<1.5\mathrm{GeV}/c^2$ (right). The expected distributions from the fit assuming a longitudinally polarized $J/\psi$, which would result from a $B_s^0\to J/\psi +\mathrm{\text{scalar}}$ decay, are superimposed. The curves follow the convention in Fig. 1.