Measurement of Semileptonic $B$ Decays into Orbitally Excited Charmed Mesons

We present a study of $B$ decays into semileptonic final states containing charged and neutral $D_1(2420)$ and $D_2^{*}(2460)$. The analysis is based on a data sample of $208{\mathrm{fb}}^{-1}$ collected at the $\mathit{{\rm Y}}(4S)$ resonance with the BABAR detector at the PEP-II asymmetric-energy $B$ factory at SLAC. With a simultaneous fit to four different decay chains, the semileptonic branching fractions are extracted from measurements of the mass difference $\Delta m=m(D^{**})-m(D)$ distributions. Product branching fractions are determined to be $\mathcal{B}(B^{+}\to D_1^0{\ell }^{+}{\nu }_{\ell })\times \mathcal{B}(D_1^0\to D^{*+}{\pi }^{-})=(2.97\pm 0.17\pm 0.17)\times {10}^{-3}$, $\mathcal{B}(B^{+}\to D_2^{*0}{\ell }^{+}{\nu }_{\ell })\times \mathcal{B}(D_2^{*0}\to D^{(*)+}{\pi }^{-})=(2.29\pm 0.23\pm 0.21)\times {10}^{-3}$, $\mathcal{B}(B^0\to D_1^{-}{\ell }^{+}{\nu }_{\ell })\times \mathcal{B}(D_1^{-}\to D^{*0}{\pi }^{-})=(2.78\pm 0.24\pm 0.25)\times {10}^{-3}$ and $\mathcal{B}(B^0\to D_2^{*-}{\ell }^{+}{\nu }_{\ell })\times \mathcal{B}(D_2^{*-}\to D^{(*)0}{\pi }^{-})=(1.77\pm 0.26\pm 0.11)\times {10}^{-3}$. In addition we measure the branching ratio $\Gamma (D_2^{*}\to D{\pi }^{-})/\Gamma (D_2^{*}\to D^{(*)}{\pi }^{-})=0.62\pm 0.03\pm 0.02$.

Figure 1

$\Delta m$ spectra for the selected data and the results of the fitted functions. The solid line represents the complete fit function, dotted ($D_1$) and dashed ($D_2^{*}$) lines for the signal and dash-dotted the for background. (a) to (d) show the mode $D^{**0}\to D^{*+}{\pi }^{-}$ with increasing values for $|\cos {\vartheta }_h|$, (e) the mode $D^{**0}\to D^{+}{\pi }^{-}$. (f) to (i) show the corresponding bins in $|\cos {\vartheta }_h|$ for the mode $D^{**+}\to D^{*0}{\pi }^{+}$ and (k) the mode $D^{**+}\to D^0{\pi }^{+}$.