Measurement of the Kaon Detection Asymmetry using Double-tagged Partially Reconstructed $D^{0}$ Decays

This note describes the measurement of the charged kaon detection asymmetry $A_K$ at the LHCb experiment. The detection efficiencies for $K^+$ and $K^-$ are measured using the process $D^{*+} \rightarrow \pi + D^0 \rightarrow \pi + [K^- π^+ π^− π^+ ]$ and performing fits to the distributions in the partially and fully reconstructed delta-mass, $\Delta M = m(D^{*+} ) − m(D^0 )$. In order to obtain a clean selection we use a $D^{*+}$ from $B^0 \rightarrow D^{*+} \mu^- \nu$ instead of prompt $D^{*+}$ . A kinematic fit is used to infer the kaon’s momentum allowing a momentum dependent measurement.