Mixing and $CP$ violation in $D^0 \to K^- \pi^+$ decays

TL;DR

This paper reviews experimental methods for measuring mixing and $CP$ violation in $D^0 o K^- \pi^+$ decays, highlighting biases and sensitivities, and proposing approaches to improve phase measurements in charm meson decays.

Contribution

It provides a detailed analysis of biases in current measurements and estimates the sensitivity of untagged $D^0$ decays to weak mixing phases, offering new insights into $CP$ violation studies.

Findings

Bias in $y_{CP}$ measurement can be around 40% of current precision.

Untagged $D^0$ decays can measure weak phases without decay amplitude nuisance.

Sensitivity to weak phases is lower by a factor of six compared to other channels.

Abstract

We review the experimental methods to measure mixing and time-dependent $C\!P$ violation in $D^0$ decays into two hadrons. While these phenomena are usually neglected for $D^0 \to K^- \pi^+$ decays, this approximation is not always justified. In particular, it produces a bias on the measurement of the parameter $y_{CP}$, when this is performed by relying on $D^0 \to K^- \pi^+$ decays as a normalisation channel, whose size is around 40% of the precision of the current world average. Finally, we estimate the sensitivity to the weak mixing phases achievable by studying $D^0 \to K^- \pi^+$ and untagged $D \to K^- \pi^+$ decays, where $D$ stands for either of the $D^0$ and $\bar{D}^0$ mesons. Contrary to Cabibbo-suppressed $D^0 \to h^+ h^-$ decays, these decay channels allow to measure these phases without final-state dependent nuisance contributions from the decay amplitudes, but their sensitivity is lower by a factor of six.