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We study the semileptonic decay $\bar{B}_s \to K^{*+} \ell^-\bar\nu_\ell$,which is induced by $b\to u \ell^- \bar\nu_\ell$ transitions at the quarklevel. We take into account the standard model (SM) operator from $W$-bosonexchange as well as possible extensions from physics beyond the SM. Thesecondary decay $K^{*+}\to K\pi$ can be used to study a number of angularobservables, which are worked out in terms of short-distance Wilsoncoefficients and hadronic form factors. Our analysis allows for an independentextraction of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ub}|$ and forthe determination of certain ratios of $\bar{B}_s\to K^*$ form factors.Moreover, a future precision measurement of the forward-backward asymmetry inthe $\bar{B}_s \to K^{*+} \ell^-\bar\nu_\ell$ decay can be used tounambiguously verify the left-handed nature of the transition operator aspredicted by the SM. We provide numerical estimates for the relevant angularobservables and the resulting decay distributions on the basis of availableform-factor information from lattice and sum-rule estimates. In addition, wepay particular attention to suitable combinations of angular observables in thedecays $\bar{B}_s \to K^{*+}(\to K\pi)\ell^-\bar\nu_\ell$ and $\bar{B} \toK^{*0}(\to K\pi)\ell^+\ell^-$, and find that they provide complementaryconstraints on the relevant $b\to s$ short-distance coefficients. As aby-product, we perform a SM fit on the basis of selected experimental decayrates and hadronic input functions, which results in $|V_{ub}| = (4.07 \pm0.20) \cdot 10^{-3}$.

Analyzing $b\to u$ transitions in semileptonic $\bar{B}_s \to K^{*+}(\to K \pi)\ell^-\bar\nu_\ell$ decays