Observation of Radiative B Meson Decays into Higher Kaonic Resonances

TL;DR

The paper investigates exclusive radiative B decays via $b \to s \gamma$ into higher kaonic resonances using $21.3\ \mathrm{fb}^{-1}$ of Belle data. It employs two- and three-body final-state analyses, including a helicity-angle approach to separate resonant contributions, to measure branching fractions for $B \to K_2^*(1430)\gamma$, $B \to K^*\pi\gamma$, and $B \to K\rho\gamma$, and to set limits on $B \to K_1(1270)\gamma$. The results yield ${\cal B}(B^0 \to K_2^*(1430)^0 \gamma) = (1.26 \pm 0.66 \pm 0.10) \times 10^{-5}$, ${\cal B}(B \to K^*\pi\gamma; M_{K^*\pi}<2.0) = (5.6 \pm 1.1 \pm 0.9) \times 10^{-5}$, and ${\cal B}(B \to K\rho\gamma; M_{K\rho}<2.0) = (6.5 \pm 1.7^{+1.1}_{-1.2}) \times 10^{-5}$, with no significant signal for $B \to K_1(1270)\gamma$ in the $K\rho\gamma$ channel. Collectively, these exclusive channels account for a substantial fraction of the inclusive $b \to s \gamma$ rate when combined with $B \to K^*(892)\gamma$, providing crucial tests of hadronization models and relativistic form-factor predictions in radiative B decays.

Abstract

We have studied radiative B meson decays into higher kaonic resonances decaying into a two-body or three-body final state, using a data sample of 21.3 fb$^{-1}$ recorded at the $Υ(4S)$ resonance with the Belle detector at KEKB. For the two-body final state, we extract the $B \to K_2^{*}(1430)^0 γ$ component from an analysis of the helicity angle distribution, and obtain ${\cal B}(B^0 \to K_2^{*}(1430)^0 γ) = (1.26\pm 0.66\pm 0.10)\times 10^{-5}$. For the three-body final state, we observe a $B \to Kππγ$ signal that is consistent with a mixture of $B \to K^* πγ$ and $B \to K ργ$. This is the first time that $B \to K^* πγ$ and $B \to K ργ$ have been observed separately. We find their branching fractions to be ${\cal B}(B \to K^* πγ; M_{K^*π}<2.0 GeV/c^2) = (5.6\pm 1.1\pm 0.9)\times 10^{-5}$ and ${\cal B}(B \to K ργ; M_{Kρ} < 2.0 GeV/c^2) = (6.5\pm 1.7^{+1.1}_{1.2})\times 10^{-5}$, respectively.

Figures (11)

• Figure 1: The $M_{\mathrm{bc}}$ distributions for $B^0 \to K_2^*(1430)^0 \gamma$ (left) and $B^+ \to K_2^*(1430)^+ \gamma$ (right) candidates. The solid line is the fitting result. The background component is shown as the dashed line.
• Figure 2: The $\Delta E$ distributions for $B^0 \to K_2^*(1430)^0 \gamma$ (left) and $B^+ \to K_2^*(1430)^+ \gamma$ (right) candidates.
• Figure 3: The background subtracted $K_2^*(1430)$ helicity angle distribution. The solid curve is the fitting result. The theoretical curve for $B \to K_2^*(1430) \gamma$ ($B \to K^*(1410) \gamma$) is shown as the dashed (dotted) line.
• Figure 4: The background subtracted $K\pi$ invariant mass distribution.
• Figure 5: The $M_{\mathrm{bc}}$ distribution for $B \to K^* \pi \gamma$ candidates.