Beauty enhances precision of CKM angle γ

The CP-violating angle γ of the Cabibbo–Kobayashi–Maskawa (CKM) quark-mixing matrix is a benchmark of the Standard Model, since it can be determined from tree-level beauty decays in an entirely data-driven way with negligible theoretical uncertainty. Comparisons between direct and indirect measurements of γ therefore provide a potent test for new physics. Before LHCb began taking data, γ was one of the poorest known constraints of the CKM unitarity triangle, but that is no longer the case. 

A new result from LHCb marks an important change in strategy, by including not only results from beauty decays sensitive to γ but additionally exploiting the sensitivity to CP violation and mixing of charm meson (D⁰) decays. Mixing in the D⁰–D⁰ system proceeds via flavour-changing neutral currents, which may also be affected by contributions from new heavy particles. The process is described by two parameters: the mass difference, x, and width difference, y, between the two charm flavour states (see figure 1).

The latest combination takes the results of more than 20 LHCb beauty and charm measurements to determine γ = (65.4 _–4.2^+3.8)°, which is the most precise measurement from a single experiment (see figure 2). Furthermore, various charm-mixing parameters were determined by combining, for the first time, both the beauty and charm datasets, which results in x = (0.400)% and y = (0.630)%. The latter is a factor-of-two more precise than the current world average, which is entirely due to the new methodology that harnesses additional sensitivity to the charm sector from beauty decays.

This demonstrates that LHCb has already achieved better precision than its original design goals. When the redesigned LHCb detector restarts operations in 2022, the target of sub-degree precision on γ, and the chance to observe CP violation in charm mixing, comes ever closer.