A team of researchers from the BESIII and LHCb collaborations report a new, precise measurement of the CKM angle γ, a number that quantifies one source of matter–antimatter difference in particle physics. They combine electron–positron collision data from BESIII (8 fb⁻¹) with proton–proton collision data from LHCb (9 fb⁻¹) and use a novel, unbinned, model‑independent analysis to find γ = (71.3 ± 5.0)°. According to the paper excerpt, this is the most precise direct determination of γ to date and it agrees with previous measurements and world averages.

The measurement uses decays of charged B mesons of the form B± → D(→ K0_S h'+ h'−) h±, where h (or h') is a charged pion or kaon, recorded by LHCb. To interpret those B decays, the analysis needs information about the strong‑phase difference between D0 and D̄0 decays across their decay phase space. Those strong‑phase parameters are measured with quantum‑correlated D D̄ pairs produced at BESIII in e+e− collisions at the ψ(3770) resonance. The two datasets are fitted together, so the CP‑violating observables from the B data and the strong‑phase parameters from the D data are determined at the same time. The BESIII sample gives the dominant constraint on the strong‑phase parameters.

The key technical advance is an unbinned, model‑independent weighting method. Past model‑independent measurements divided the D decay phase space into bins and averaged information inside each bin. That binned approach avoids relying on a decay model, but it loses information inside each bin and keeps about 85% of the possible sensitivity. The new method builds many continuous weight functions across the D decay phase space. One part of the weights is a Fourier expansion of the strong‑phase difference to capture how the phase changes across the decay space. The other part, called the optimal weight, upweights regions with larger expected CP effects and accounts for how the experiment sees the decays (efficiency and background). Amplitude models are used only to build these weights; they do not directly model the observed data. Because of that, the authors emphasize the result remains model‑independent even though models guide the choice of weights.