Abstract : Based on a sample of 500 million e+e−→cc¯ events recorded by the BaBar detector at c.m. energies of close to 10.6 GeV, we report on a study of the decay D0→π−e+νe. We measure the ratio of branching fractions RD=B(D0→π−e+νe)/B(D0→K−π+)=0.0713±0.0017stat.±0.0024syst., and use the present world average for B(D0→K−π+) to obtain B(D0→π−e+νe)=(2.770±0.068stat.±0.092syst.±0.037ext.)×10−3 where the third error accounts for the uncertainty on the branching fraction for the reference channel. The measured dependence of the differential branching fraction on q2, the four-momentum transfer squared between the D and the π meson, is compared to various theoretical predictions for the hadronic form factor, fπ+,D(q2), and the normalization |Vcd|×fπ+,D(q2=0)=0.1374±0.0038stat.±0.0022syst.±0.0009ext. is extracted from a fit to data. Using the most recent LQCD prediction of fπ+,D(q2=0)=0.666±0.029, we obtain |Vcd|=0.206±0.007exp.±0.009LQCD. Assuming instead, |Vcd|=|Vus|=0.2252±0.0009, we obtain fπ+,D(q2=0)=0.610±0.020exp.±0.005ext.. The q2 dependence of fπ+,D(q2) is compared to a variety of multi-pole parameterizations. This information is applied to B0→π−e+νe decays and, combined with an earlier B0→π−e+νe measurement by BaBar, is used to derive estimates of |Vub|.
