Measurement of the Differential Branching Fraction and Forward-Backword Asymmetry for B->K(*)l+l-

TL;DR

The differential branching fraction, isospin asymmetry, K* polarization, and the forward-backward asymmetry (A(FB)) as functions of q2 = M(ll)(2)c2 and the fitted A(FB) spectrum exceeds the standard model expectation by 2.7 standard deviations.

Abstract

We study B->K(*)l+l- decays (l=e,mu) based on a data sample of 657 million BB pairs collected with the Belle detector at the KEKB e+e- collider. We report the differential branching fraction, isospin asymmetry, K* polarization, and the forward-backward asymmetry (AFB) as functions of q^2 = M_{ll}^2*c^2. The fitted AFB spectrum exceeds the Standard Model expectation by 2.7 standard deviations. The measured branching fractions are B(B->K*l+l-)=(10.7+1.1-1.0+-0.9)*10^-7 and B(B->Kl+l-)=(4.8+0.5-0.4+-0.3)*10^-7, where the first errors are statistical and the second are systematic, with the muon to electron ratios R_K*=0.83+-0.17+-0.05 and R_K=1.03+-0.19+-0.06, respectively.

Figures (3)

• Figure 1: Differential branching fractions for the (a) $K^* \ell^+ \ell^-$ and (b) $K \ell^+ \ell^-$ modes as a function of $q^2$. The two shaded regions are veto windows to reject $J/\psi(\psi^\prime) X$ events. The solid curves show the SM theoretical predictions with the minimum and maximum allowed form factors Ali:01. (c) and (d) show the fit results for $F_L$ and $A_{FB}$ in $K^* \ell^+ \ell^-$ as a function of $q^2$, together with the solid (dotted) curve representing the SM ($C_7=-C^{SM}_7$) prediction Ali:01. (e) is the $A_I$ asymmetry as a function of $q^2$ for the $K^* \ell^+ \ell^-$ (filled circles) and $K \ell^+ \ell^-$ (open circles) modes.
• Figure 2: Fits to $M_{K\pi}$, $M_{\rm bc}$, $\cos \theta_{K^*}$, and $\cos \theta_{B\ell}$ for the $K^{*}\ell^+\ell^-$ decays in 6 $q^2$ (GeV$^2$/$c^2$) bins: (a)$\sim$(d) 0.00--2.00, (e)$\sim$(h) 2.00--4.30, (i)$\sim$(l) 4.30--8.68, (m)$\sim$(p) 10.09--12.86, (q)$\sim$(t) 14.18--16.00, and (u)$\sim$(x) $>16.00$. The solid, long-dashed, short-dashed, and dotted curves represent the combined fit result, fitted signal, combinatorial background, and $J/\psi(\psi^\prime) X$ background, respectively.
• Figure 3: Fits to $M_{\rm bc}$ and $\cos \theta_{B\ell}$ for the $K\ell^+\ell^-$ decays in 6 $q^2$ (GeV$^2$/$c^2$) bins: (a)$\sim$(b) 0.00--2.00, (c)$\sim$(d) 2.00--4.30, (e)$\sim$(f) 4.30--8.68, (g)$\sim$(h) 10.09--12.86, (i)$\sim$(j) 14.18--16.00, and (k)$\sim$(l) $>16.00$. The solid, long-dashed, and short-dashed represent the combined fit result, fitted signal, and combinatorial background, respectively. The$J/\psi(\psi^\prime) X$ background is so small and not represented.