Study of Time-Dependent $CP$ Asymmetry in Neutral $B$ Decays to $J/\psi {\pi }^0$

We present the first study of the time-dependent $CP$-violating asymmetry in $B^0\to J/\psi {\pi }^0$ decays using $e^{+}{e}^{-}$ annihilation data collected with the BABAR detector at the ${\rm Y}\mathrm{(}\mathrm{4}\mathrm{S}\mathrm{)}$ resonance during the years 1999–2002 at the PEP-II asymmetric-energy $B$ Factory at SLAC. Using approximately $88\times {10}^6$ $B\overline{B}$ pairs, our results for the coefficients of the cosine and sine terms of the $CP$ asymmetry are $C_{J/\psi {\pi }^0}=0.38\pm 0.41\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}\pm 0.09\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}$ and $S_{J/\psi {\pi }^0}=0.05\pm 0.49\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}\pm 0.16\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}$.

Figure 1

Projection in $m_{\mathrm{E}\mathrm{S}}$ for the results of the $CP$ fit, displayed with the added requirement $-0.11<\mathrm{\Delta }E<0.11\mathrm{G}\mathrm{e}\mathrm{V}$. In contrast, the $CP$ fit uses the full $\mathrm{\Delta }E$ region. In the further restricted region $m_{\mathrm{E}\mathrm{S}}>5.27\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$, there are 49 data events (points), of which about 12 events are fit as background. The background under the signal peak is composed of less than one event from $B\overline{B}$ generic background and is roughly evenly divided among the other three background categories. Here, $B^0\to J/\psi K_S^0({\pi }^0{\pi }^0)$ and inclusive $J/\psi$ decays contribute to the enhancement in the background distribution at large $m_{\mathrm{E}\mathrm{S}}$.

Figure 2

Distributions of events (a) with a $B^0$ tag ($N_{B^0}$), (b) with a ${\overline{B}}^0$ tag ($N_{{\overline{B}}^0}$), and (c) the raw asymmetry $(N_{B^0}-N_{{\overline{B}}^0})/(N_{B^0}+N_{{\overline{B}}^0})$, as functions of $\Delta t$. Candidates in these plots are required to satisfy $-0.11<\mathrm{\Delta }E<0.11\mathrm{G}\mathrm{e}\mathrm{V}$ and $m_{\mathrm{E}\mathrm{S}}>5.27\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$. Of the 49 signal and background events in this region, 25 have a $B^0$ tag and 24 have a ${\overline{B}}^0$ tag, with fit background contributions of approximately five and seven events, respectively. The curves are projections that use the values of the other variables in the likelihood to determine the contributions to the signal and backgrounds.