Observation of $X(3872)$ in $B\to X(3872)K\pi$ decays

We report the first observation of $B^0\to X(3872)(K^{+}{\pi }^{-})$ and evidence for $B^{+}\to X(3872)(K^0{\pi }^{+})$. We measure the product of branching fractions for the former to be $\mathcal{B}(B^0\to X(3872)(K^{+}{\pi }^{-}))\times \mathcal{B}(X(3872)\to J/\psi {\pi }^{+}{\pi }^{-})=(7.9\pm 1.3(\text{stat})\pm 0.4(\text{syst}))\times 10^{-6}$ and find that $B^0\to X(3872)K^{*}(892{)}^0$ does not dominate the $B^0\to X(3872)K^{+}{\pi }^{-}$ decay mode. We also measure $\mathcal{B}(B^{+}\to X(3872)(K^0{\pi }^{+}))\times \mathcal{B}(X(3872)\to J/\psi {\pi }^{+}{\pi }^{-})=(10.6\pm 3.0(\text{stat})\pm 0.9(\text{syst}))\times 10^{-6}$. This study is based on the full data sample of $711{\mathrm{fb}}^{-1}$ ($772\times 10^{6}B\overline{B}$ pairs) collected at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB collider.

Figure 1

Projections of the ($\mathrm{\Delta }E$, $M_{J/\psi \pi \pi }$) fit for the $B^0\to X(3872)(\to J/\psi {\pi }^{+}{\pi }^{-})K^{+}{\pi }^{-}$ decay mode (top) and the $B^0\to {\psi }^{\prime }(\to J/\psi {\pi }^{+}{\pi }^{-})K^{+}{\pi }^{-}$ decay mode (bottom): (a) $\mathrm{\Delta }E$ distribution for $3.860\mathrm{GeV}/c^2<M_{J/\psi \pi \pi }<3.881\mathrm{GeV}/c^2$, (b) $M_{J/\psi \pi \pi }$ distribution for $-11\mathrm{MeV}<\mathrm{\Delta }E<8\mathrm{MeV}$, (c) $\mathrm{\Delta }E$ distribution for $3.675\mathrm{GeV}/c^2<M_{J/\psi \pi \pi }<3.695\mathrm{GeV}/c^2$, and (d) $M_{J/\psi \pi \pi }$ distribution for $-11\mathrm{MeV}<\mathrm{\Delta }E<8\mathrm{MeV}$. The curves show the signal (red long-dashed curve) and the background components (black dash-dotted line for the component peaking in $M_{J/\psi \pi \pi }$ but nonpeaking in $\mathrm{\Delta }E$, green dashed line for the one peaking in $\mathrm{\Delta }E$ but nonpeaking in $M_{J/\psi \pi \pi }$, and magenta long dash-dotted line for combinatorial background) as well as the overall fit (blue solid curve).

Figure 2

Fit to the background-subtracted $M_{K\pi }$ distribution: (a) for the $B^0\to X(3872)(K^{+}{\pi }^{-})$ decay mode, the curves show the $B^0\to X(3872)K^{*}(892{)}^0$ (red long-dashed lines), $B^0\to X(3872)(K^{+}{\pi }^{-}{)}_{\mathrm{NR}}$ (green dot-dashed lines), as well as the overall fit (blue solid lines). (b) for the $B^0\to {\psi }^{\prime }(K^{+}{\pi }^{-})$ decay mode, the curves show the $B^0\to {\psi }^{\prime }{K}^{*}(892{)}^0$ (red long-dashed lines), $B^0\to {\psi }^{\prime }(K^{+}{\pi }^{-}{)}_{\mathrm{NR}}$ (green dot-dashed lines), $B^0\to {\psi }^{\prime }{K}_2^{*}(1430{)}^0$ (magenta dashed lines) as well as the overall fit (blue solid lines).

Figure 3

Projections of the ($\mathrm{\Delta }E$, $M_{J/\psi \pi \pi }$) fit for the $B^{\pm }\to X(3872)(\to J/\psi {\pi }^{+}{\pi }^{-})K_S^0{\pi }^{\pm }$ decay mode (top) and for the $B^{\pm }\to {\psi }^{\prime }(\to J/\psi {\pi }^{+}{\pi }^{-})K_S^0{\pi }^{\pm }$ decay mode (bottom): (a) $\mathrm{\Delta }E$ distribution for $3.859\mathrm{GeV}/c^2<M_{J/\psi \pi \pi }<3.882\mathrm{GeV}/c^2$, (b) $M_{J/\psi \pi \pi }$ distribution for $-11\mathrm{MeV}<\mathrm{\Delta }E<9\mathrm{MeV}$, (c) $\mathrm{\Delta }E$ distribution for $3.675\mathrm{GeV}/c^2<M_{J/\psi \pi \pi }<3.695\mathrm{GeV}/c^2$, and (d) $M_{J/\psi \pi \pi }$ distribution for $-11\mathrm{MeV}<\mathrm{\Delta }E<9\mathrm{MeV}$. The curves show the signal (red long-dashed curves: and the background components (black dash-dotted curves for the component peaking in $M_{J/\psi \pi \pi }$ but nonpeaking in $\mathrm{\Delta }E$, green dashed lines for the one peaking in $\mathrm{\Delta }E$ but nonpeaking in $M_{J/\psi \pi \pi }$, and magenta long dash-dotted lines for combinatorial background) as well as the overall fit (blue solid curves).