Measurement of the ratio of branching fractions $\mathcal{B}(B^{\pm }\to J/\psi {\pi }^{\pm })/\mathcal{B}(B^{\pm }\to J/\psi K^{\pm })$

We report a measurement of the ratio of branching fractions of the decays $B^{\pm }\to J/\psi {\pi }^{\pm }$ and $B^{\pm }\to J/\psi K^{\pm }$ using the CDF II detector at the Fermilab Tevatron Collider. The signal from the Cabbibo-suppressed $B^{\pm }\to J/\psi {\pi }^{\pm }$ decay is separated from $B^{\pm }\to J/\psi K^{\pm }$ using the $B^{\pm }\to J/\psi K^{\pm }$ invariant mass distribution and the kinematical differences of the hadron track in the two decay modes. From a sample of $220{\mathrm{pb}}^{-1}$ of $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$, we observe $91\pm 15$ $B^{\pm }\to J/\psi {\pi }^{\pm }$ events together with $1883\pm 34$ $B^{\pm }\to J/\psi K^{\pm }$ events. The ratio of branching fractions is found to be $\mathcal{B}(B^{\pm }\to J/\psi {\pi }^{\pm })/\mathcal{B}(B^{\pm }\to J/\psi K^{\pm })=(4.86\pm 0.82(\mathrm{stat})\pm 0.15(\mathrm{syst}))\%$.

Figure 1

The $\alpha$ distributions of $B^{\pm }\to J/\psi K^{\pm }$, which are obtained with Monte Carlo simulation and background obtained from the nonsignal data sample. The solid curves are the corresponding parametrization functions from Eqs. (3) and (4). The $\alpha$ distributions of the two signals are very similar in shape due to the almost identical kinematics of the two decay modes. To avoid confusion from it, we plot the $\alpha$ distribution of $B^{\pm }\to J/\psi K^{\pm }$ only.

Figure 2

The invariant mass distribution in the data sample (points) projected with the results of the likelihood fit; overall (solid line), $B^{\pm }\to J/\psi K^{\pm }$ (dotted line), $B^{\pm }\to J/\psi {\pi }^{\pm }$ (dashed line), and background (dash-dotted line). The inset shows the magnified region of the $B^{\pm }\to J/\psi {\pi }^{\pm }$ signal.

Figure 3

The $\alpha$ distribution in the data sample (points) compared with the results of the likelihood fit; overall (solid line), $B^{\pm }\to J/\psi K^{\pm }$ (dotted line), $B^{\pm }\to J/\psi {\pi }^{\pm }$ (dashed line), and background (dash-dotted line).