Time-integrated CP asymmetries in meson and baryon decays

TL;DR

This article surveys time-integrated CP asymmetries in meson and baryon decays, focusing on direct CP violation in decay amplitudes and the CKM framework. It details measurements of the CKM angle $\gamma$ from $B \to Dh$ decays, constrained by quantum-correlated $D\bar D$ decays, with current global results around $\gamma \approx 65^\circ$ and uncertainties down to about $2.5^\circ$. The first observation of CP violation in baryon decays is highlighted, with large asymmetries in multibody resonant decays of $\Lambda_b^0$ indicating significant strong-interaction dynamics. In the charm sector, direct CP asymmetries in several channels are measured with sub-percent sensitivity, with results broadly consistent with Standard Model expectations but featuring tensions that motivate further study as larger data samples become available.

Abstract

Measurements of CP asymmetries in hadron decays integrated over time provide access to direct CP violation, which arises from differences between the amplitudes of CP-conjugate decay processes. These proceedings present an overview of recent measurements from the LHCb and Belle II experiments, including progress on the determination of the CKM angle $γ$, the first observation of CP violation in baryon decays, and new studies of direct CP violation in $D$ mesons. The outlook for measurements with datasets whose collection is underway is also discussed.

Figures (10)

• Figure 1: Cross-sections of the (left) LHCb and (right) Belle II experimental setups.
• Figure 2: Invariant mass distributions for $B^- \to D[\pi^-K^+]K^{*-}$ (left) and $B^+ \to D[\pi^+K^-]K^{*+}$ (center) candidates with an eye-guide to highlight the observed asymmetry. (Right) Confidence level contours in the $\gamma$--$\delta_B^{DK^*}$ plane (right) from a combined analysis of all examined $D$ decay modes of the $B^\pm\to DK^{*\pm}$ process in Ref. LHCb:2024ett .
• Figure 3: Confidence level distributions for $\gamma$ from (left) the LHCb 2024 combination LHCb:2024yxi, (center) Belle/Belle II 2024 combination Belle:2024knt, and (right) 2025 global analysis Betti:2024ldy.
• Figure 4: Invariant mass distributions for (left) $\Lambda_b^0 \to pK^-$ and (right) $\bar{\Lambda}_b^0 \to \bar{p}K^+$ candidates from Ref. LHCb:2024iis. An eye-guide is included for ease of comparison.
• Figure 5: (Left) Region of $\Lambda_b^0 \to \Lambda K^+K^-$ phase space where largest asymmetries are observed, and (right): invariant mas distributions of selected (center) $\Lambda_b^0 \to \Lambda K^+K^-$ and (right) $\overline{\Lambda}_b^0 \to \overline{\Lambda} K^+K^-$ events in this region from Ref. LHCb:2024yzj, with an eye-guide for reference.