Charge asymmetry of same-sign dileptons in $B^0-{\overline{B}}^0$ mixing

We report a measurement of the charge asymmetry for same-sign dileptons in $B^0-{\overline{B}}^0$ mixing, $A_{\mathrm{sl}}$. The data were collected with the Belle detector at KEKB. Using a data sample of $78{\mathrm{fb}}^{-1}$ recorded at the ${\rm Y}(4S)$ resonance and $9{\mathrm{fb}}^{-1}$ recorded at an energy 60 MeV below the resonance, we measure $A_{\mathrm{sl}}=(-1.1\pm 7.9(\mathrm{\text{stat}})\pm 8.5(\mathrm{\text{sys}}))\times {10}^{-3}$.

Figure 1

${\theta }_{\mathrm{lab}}$-averaged track finding efficiencies as a function of c.m. momentum for positron tracks ${\epsilon }_{+}$ (a), electron tracks ${\epsilon }_{-}$ (b), and charge dependence defined as $({\epsilon }_{+}-{\epsilon }_{-})/({\epsilon }_{+}+{\epsilon }_{-})$ (c). Corresponding plots for muon tracks are shown in (d), (e), and (f).

Figure 2

${\theta }_{\mathrm{lab}}$-averaged identification efficiencies as a function of c.m. momentum for positrons ${\epsilon }_{+}$ (a), electrons ${\epsilon }_{-}$ (b), and charge dependence defined as $({\epsilon }_{+}-{\epsilon }_{-})/({\epsilon }_{+}+{\epsilon }_{-})$ (c). Corresponding plots for muons are shown in (d), (e), and (f).

Figure 3

Rates of hadrons faking leptons vs laboratory momentum: (a) $\pi \to e$, (b) $K\to e$, (c) $p\to e$, (d) $\pi \to \mu$, (e) $K\to \mu$, (f) $p\to \mu$. Filled circles are for positive tracks and open circles are for negative tracks. The increase in the rate of kaons faking electrons at low momentum clearly visible in (b) is due to the overlap of the electron and kaon energy-loss bands; the other distributions are discussed in the text. Note that the minimum values of vertical axes are set to negative values. The dotted lines shows fake rate equal to zero.

Figure 4

$\cos {\theta }_{\ell \ell }^{*}$ distributions for dilepton samples in the on-resonance (open histogram) and scaled off-resonance (filled histogram) data. (a),(b) show $ee$ events, (c),(d) show $\mu \mu$ events and (e),(f) are from $e\mu$ combinations. (a),(c) and (e) are the $++$ charge case and (b), (d) and (f) are the $--$ charge case. The arrows indicate the selection requirements.

Figure 5

$\cos {\theta }_{\ell \ell }^{*}$ dependence of the weighting factor for the fraction of the dilepton yield originating from $B\overline{B}$ decays in the on-resonance data for $e^{+}{e}^{+}$ (a) and $e^{-}{e}^{-}$ (b) and corresponding quantities for $\mu \mu$ (c) and (d) and $e\mu$ (e) and (f).

Figure 6

Relative multiplicities of hadrons as a function of $p_{\mathrm{lab}}$ with respect to leptons. (a), (b) and (c) are for electrons and (d), (e) and (f) are for muons. (a) and (d) are pion to lepton, (b) and (e) are kaon to lepton, (c) and (f) are proton to lepton ratios. Filled circles are for positive tracks and open circles are for negative tracks. The effects of decay-in-flight and interaction with detector are included. Though in (a), (b), (d) and (e), the difference between positive and negative cases is less than 1%, in both (c) and (f), the proton rate is larger than the antiproton rate by about 8%. The increases in multiplicities for pions and kaons around $3.5\mathrm{GeV}/c$ are due to two-body hadronic $B$ decays.

Figure 7

Mass distributions for $J/\psi \to e^{+}{e}^{-}$ (a) and $J/\psi \to {\mu }^{+}{\mu }^{-}$ (b). The arrows indicate the selection criteria for each decay mode. The dashed lines indicate the fitted background component and the solid lines show the total fit results. The cross hatched area shows the estimated combinatorial background in the signal region and the single hatched area is the sideband region used to estimate the background $\Delta z$ distribution. The $\Delta z$ distributions for $J/\psi \to e^{+}{e}^{-}$ (c) and $J/\psi \to {\mu }^{+}{\mu }^{-}$ (d). Open histograms are for all $J/\psi$ candidates in the signal region and hatched histograms are for the background.

Figure 8

(a) $\Delta z$ distribution for the true dilepton events ($++$ and $--$ are combined). Points with error bars are data. The dot-dashed line shows the contribution from CT backgrounds, the dotted line shows the WT background contributions, the dashed line indicates the signal component and the solid line indicates the fitted total. (b) Difference between data and fit result as a function of $|\Delta z|$.

Figure 9

$A_{\mathrm{sl}}$ as a function of $|\Delta z|$.