Measurement of $B \to \D^*$ Form Factors in the Semileptonic Decay $B^{0}\to D^{*-}\ell^+ \nu$

Abstract

We present a preliminary measurement of \rone, \rtwo and \rhosq which are the three parameters used to characterize the $B\rar D^*\ell\bar\nu_e$ form factors ($A_1$, $V$, and $A_2$). We use $85 \times 10^6$ $B\bar B$-pairs accumulated on the \upsfs resonance at PEP-II. In this analysis we use the decay mode $\bar B^0\rar D^{+*}e^-\bar\nu$ and its charge conjugate. The $D^{*+}$ is reconstructed in the channel $D^{*+}\to D^0\pi^+$ and the $D^0$ in the channel $\dtokpi$. We parameterize the \ff s in terms of their ratios (determined by the parameters $R_1$ and $R_2$) and the common slope $\rhosq$ in the variable $w$ (a quantity related to the momentum transfer in the decay process). These parameters are determined via an unbinned maximum likelihood fit to the distributions in four kinematic variables (three decay angles and $w$). The results are $R_1= 1.328 \pm 0.055\pm 0.025\pm 0.025$ and $R_2=0.920 \pm 0.044\pm 0.020\pm 0.013 $ for the ratios and $\rhosq= 0.769\pm 0.039 \pm 0.019\pm 0.032 $ for the slope. The stated errors are the statistical uncertainty from the data, statistical uncertainty from the size of the Monte Carlo sample and the systematic uncertainty, respectively.