Study of $B^{\pm }\to J/\psi {\pi }^{\pm }$ and $B^{\pm }\to J/\psi K^{\pm }$ Decays: Measurement of the Ratio of Branching Fractions and Search for Direct $CP$ Violation

We study $B^{\pm }\to J/\psi {\pi }^{\pm }$ and $B^{\pm }\to J/\psi K^{\pm }$ decays in a sample of about $89\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric $B$ factory at SLAC. We observe a signal of $244\pm 20$ $B^{\pm }\to J/\psi {\pi }^{\pm }$ events and determine the ratio $\mathcal{B}(B^{\pm }\to J/\psi {\pi }^{\pm })/\mathcal{B}(B^{\pm }\to J/\psi K^{\pm })$ to be $[5.37\pm 0.45(\mathrm{stat})\pm 0.11(\mathrm{syst})]\%$. The charge asymmetries for the $B^{\pm }\to J/\psi {\pi }^{\pm }$ and $B^{\pm }\to J/\psi K^{\pm }$ decays are determined to be ${\mathcal{A}}_{\pi }=0.123\pm 0.085(\mathrm{stat})\pm 0.004(\mathrm{syst})$ and ${\mathcal{A}}_K=0.030\pm 0.015(\mathrm{stat})\pm 0.006(\mathrm{syst})$, respectively.

Figure 1

(a) The $m_{\mathrm{ES}}$ distribution for the $B^{\pm }$ candidates in data. A fit to the sum of a Gaussian and an empirical threshold function (dashed curve) is superimposed. The fitted resolution is approximately $2.5\mathrm{MeV}/c^2$. (b) The $\Delta E_K$ distribution for the $B^{\pm }$ candidates in data with $m_{\mathrm{ES}}>5.27\mathrm{GeV}\mathrm{/}{c}^2$. A fit to the sum of a double Gaussian and a 3rd order polynomial function (dashed curve) is superimposed. The fitted resolution is approximately 10.5 MeV.

Figure 2

The distribution of (a) $\Sigma$ and (b) $\Pi$ for events in the $m_{\mathrm{ES}}$ sideband in data. The curve corresponds to the projection of the best fit.

Figure 3

The $\Delta E_{\pi }$ distribution in data (points) compared with the distribution obtained from a simulated experiment (histogram). The distributions for each simulated component in the sample, normalized to the fitted event yields, are also displayed.