| Abstract |
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\def\babar{\mbox{\slshape B\kern-0.1em{\smaller A}\kern-0.1em
B\kern-0.1em{\smaller A\kern-0.2em R}}}
We measure the dependence of $\bar B^0 \rightarrow
D^{*+}e^-\overline{\nu}_e$ on the decay angles and momentum transfer.
The data sample consists of $\sim 86 \times 10^6$ $B\bar B$-pairs
accumulated on the $\Upsilon(4{\rm S})$ resonance by the \babar\
detector at the asymmetric $e^+ e^-$ collider PEP-II. We specify the
three form factors by two ratios $R_1$ and $R_2$, and by a single
parameter $\rho^2$ characterizing the polynomial representing
$h_{A_1}$, the function which describes the momentum-transfer
dependence of the form factor $A_1$. We determine $R_1$, $R_2$, and
$\rho^2$ using an unbinned maximum likelihood fit to the full decay
distribution. The results are $R_1=1.396\pm 0.060\pm 0.035\pm 0.027$,
$R_2=0.885\pm 0.040\pm 0.022\pm 0.013$, and $\rho^2=1.145\pm 0.059\pm
0.030\pm 0.035$. The stated uncertainties are the statistical from
the data, statistical from the size of the Monte Carlo sample and the
systematic uncertainty, respectively. Though we measure the form
factor parameters in the $\bar B^0 \rightarrow
D^{*+}e^-\overline{\nu}_e$ decay, they are expected to be the same
with electron replaced by a muon, and thus apply to all ${\bar
B}^{0}\rightarrow D^{*+}\ell^{-} \nubar$ decays where $\ell$ is a
light charged lepton. In addition, based on this measurement, we give
an updated value for the CKM matrix element $|V_{cb}|$.
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