Study of the $B\to X(3872)(\to D^{*0}{\overline{D}}^0)K$ decay

We present a study of $B\to X(3872)K$ with $X(3872)$ decaying to $D^{*0}{\overline{D}}^0$ using a sample of $657\times {10}^6$ $B\overline{B}$ pairs recorded at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. Both $D^{*0}\to D^0\gamma$ and $D^{*0}\to D^0{\pi }^0$ decay modes are used. We find a peak of ${50.1}_{-11.1}^{+14.8}$ events with a mass of $({3872.9}_{-0.4-0.5}^{+0.6+0.4})\mathrm{MeV}/c^2$, a width of $({3.9}_{-1.4-1.1}^{+2.8+0.2})\mathrm{MeV}/c^2$ and a product branching fraction $\mathcal{B}(B\to X(3872)K)\times \mathcal{B}(X(3872)\to D^{*0}{\overline{D}}^0)=(0.80\pm 0.20\pm 0.10)\times {10}^{-4}$, where the first errors are statistical and the second ones are systematic. The significance of the signal is $6.4\sigma$. The difference between the fitted mass and the $D^{*0}{\overline{D}}^0$ threshold is calculated to be $({1.1}_{-0.4-0.3}^{+0.6+0.1})\mathrm{MeV}/c^2$. We also obtain an upper limit on the product of branching fractions $\mathcal{B}(B\to Y(3940)K)\times \mathcal{B}(Y(3940)\to D^{*0}{\overline{D}}^0)$ of $0.67\times {10}^{-4}$ at 90% CL.

Figure 1

$X(3872)$ mass resolution as a function of the $X(3872)$ mass in the $D^0\gamma$ channel, obtained from MC with a $X(3872)$ mass spectrum generated for a continuous range of masses from threshold to $4.0\mathrm{GeV}/c^2$. Crosses are Gaussian resolutions for various generated $D^{*0}{\overline{D}}^0$ masses; the curve is the result of a fit with a square root function. Very similar results are obtained for the $D^0{\pi }^0$ channel.

Figure 2

Distributions of $M_{D^{*}D}$ mass for $M_{\mathrm{bc}}>5.27\mathrm{GeV}/c^2$ for $D^{*0}\to D^0\gamma$ (left) and for $D^{*0}\to D^0{\pi }^0$ (right). The result of the simultaneous fit is shown by the superimposed lines. The points with error bars are data, the dotted curve is the signal, the dashed curve is the background, the dash-dotted curve (barely visible) is the $Y(3940)$ component, and the solid curve is the total fitting function.

Figure 3

Distributions of $M_{\mathrm{bc}}$ for $M_{D^{*}D}<3.88\mathrm{GeV}/c^2$ (left) and of $M_{D^{*}D}$ for $M_{\mathrm{bc}}>5.27\mathrm{GeV}/c^2$ (right); the top row is for $D^{*0}\to D^0\gamma$ and the bottom row for $D^{*0}\to D^0{\pi }^0$. The result of the simultaneous fit is shown by the superimposed lines. The points with error bars are data, the dotted curve is the signal, the dashed curve is the background, the dash-dotted curve is the sum of the background and the $B\to D^{*}DK$ component, the dot-dot-dashed curve is the contribution from $D^0-{\overline{D}}^0$ reflections, and the solid curve is the total fitting function.