Measurement of branching fraction and first evidence ofCPviolation inB0→a1±(1260)π∓decays

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Abstract

We present a measurement of the branching fraction and time-dependent $CP$ violation parameters in ${B}^{0}\ensuremath{\rightarrow}{a}_{1}^{\ifmmode\pm\else\textpm\fi{}}(1260){\ensuremath{\pi}}^{\ensuremath{\mp}}$ decays. The results are obtained from the final data sample containing $772\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $B\overline{B}$ pairs collected at the $\ensuremath{\Upsilon}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ collider. We obtain the product branching fraction $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{a}_{1}^{\ifmmode\pm\else\textpm\fi{}}(1260){\ensuremath{\pi}}^{\ensuremath{\mp}})\ifmmode\times\else\texttimes\fi{}\mathcal{B}({a}_{1}^{\ifmmode\pm\else\textpm\fi{}}(1260)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})=\phantom{\rule{0ex}{0ex}}(11.1\ifmmode\pm\else\textpm\fi{}1.0(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}1.4(\mathrm{syst}))\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and an upper limit on the product branching fraction for a possible decay with the same final state $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{a}_{2}^{\ifmmode\pm\else\textpm\fi{}}(1320){\ensuremath{\pi}}^{\ensuremath{\mp}})\ifmmode\times\else\texttimes\fi{}\mathcal{B}({a}_{2}^{\ifmmode\pm\else\textpm\fi{}}(1320)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})<2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{1em}{0ex}}\mathrm{\text{at}}\text{ }\text{ }90%\text{ }\mathrm{CL}$. In a time-dependent measurement to extract $CP$ asymmetries, we obtain the $CP$ violation parameters ${\mathcal{A}}_{CP}=\ensuremath{-}0.06\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.07(\mathrm{syst})$, ${\mathcal{C}}_{CP}=\ensuremath{-}0.01\ifmmode\pm\else\textpm\fi{}0.11(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.09(\mathrm{syst})$, ${\mathcal{S}}_{CP}=\ensuremath{-}0.51\ifmmode\pm\else\textpm\fi{}0.14(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.08(\mathrm{syst})$, representing time- and flavor-integrated direct, flavor-dependent direct and mixing-induced $CP$ violation, respectively. Simultaneously, we also extract the $CP$-conserving parameters $\ensuremath{\Delta}\mathcal{C}=+0.54\ifmmode\pm\else\textpm\fi{}0.11(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.07(\mathrm{syst})$, $\ensuremath{\Delta}\mathcal{S}=\ensuremath{-}0.09\ifmmode\pm\else\textpm\fi{}0.14(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{syst})$, which, respectively, describe a rate difference and strong phase difference between the decay channels where the ${a}_{1}^{\ifmmode\pm\else\textpm\fi{}}$ does not contain the spectator quark and those where it does. We find first evidence of mixing-induced $CP$ violation in ${B}^{0}\ensuremath{\rightarrow}{a}_{1}^{\ifmmode\pm\else\textpm\fi{}}(1260){\ensuremath{\pi}}^{\ensuremath{\mp}}$ decays with $3.1\ensuremath{\sigma}$ significance. The rate where the ${a}_{1}^{\ifmmode\pm\else\textpm\fi{}}$ does not contain the spectator quark from the $B$ meson is found to dominate the rate where it does at the $4.1\ensuremath{\sigma}$ level. However, there is no evidence for either time- and flavor-integrated direct $CP$ violation or flavor-dependent direct $CP$ violation.

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