Based on a sample of 500 million $e^+e^- to cbar c$ events recorded by the BaBar detector at c.m. energies of close to 10.6 GeV, we report on a study of the decay $D^0 to pi^- e^+ u_e$. We measure the ratio of branching fractions $R_D = {cal B}(D^0
to pi^- e^+ u_e)/{cal B}(D^0 to K^- pi^+) = 0.0713 pm 0.0017_{stat.} pm 0.0024_{syst.}$, and use the present world average for ${cal B}(D^0 to K^- pi^+)$ to obtain ${cal B}(D^0 to pi^- e^+ u_e) = (2.770 pm 0.068_{{rm stat.}} pm 0.092_{{rm syst.}} pm 0.037_{{rm ext.}})times 10^{-3}$ 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 $q^2$, the four-momentum transfer squared between the $D$ and the $pi$ meson, is compared to various theoretical predictions for the hadronic form factor, $f_{+,D}^{pi}(q^2)$, and the normalization $|V_{cd}| times f_{+,D}^{pi}(q^2=0)=0.1374pm0.0038_{{rm stat.}} pm0.0022_{{rm syst.}} pm0.0009_{{rm ext.}}$ is extracted from a fit to data. Using the most recent LQCD prediction of $f_{+,D}^{pi}(q^2=0)=0.666 pm 0.029$, we obtain $|V_{cd}|=0.206 pm 0.007_{rm exp.} pm 0.009_{rm LQCD}$. Assuming instead, $|V_{cd}|=|V_{us}|=0.2252 pm 0.0009$, we obtain $f_{+,D}^{pi}(q^2=0)=0.610 pm 0.020_{rm exp.} pm 0.005_{rm ext.}$. The $q^2$ dependence of $f_{+,D}^{pi}(q^2)$ is compared to a variety of multi-pole parameterizations. This information is applied to $B^0 to pi^- e^+ u_e$ decays and, combined with an earlier $B^0 to pi^- e^+ u_e$ measurement by BaBar, is used to derive estimates of $|V_{ub}|$.
The vertex detectors are crucial detectors for future linear e+e- colliders since they must give the most accurate location of any outgoing charged particles originating from the interaction point. The DEPFET collaboration is developing a new type of
pixel sensors which provide very low noise and high spatial resolution. In order to precisely determine the track and vertex positions, multiple scattering in the detector has to be reduced by minimizing the material in the sensors, cooling, and support structures. A new method of cooling by blowing air over the sensors has been developed and tested. It is applied in the design and construction of the Belle-II detector and may be used in the new generation of vertex detectors for linear colliders.
