Combined analysis of Belle and Belle II data to determine the CKM angle ϕ ₃ using B ⁺ → D(KS0 h ⁺ h ⁻)h ⁺ decays

We present a measurement of the Cabibbo-Kobayashi-Maskawa unitarity triangle angle ϕ₃ (also known as γ) using a model-independent Dalitz plot analysis of B⁺ → D (KS0h⁺h⁻)h⁺, where D is either a D⁰ or D¯ ⁰ meson and h is either a π or K. This is the first measurement that simultaneously uses Belle and Belle II data, combining samples corresponding to integrated luminosities of 711 fb⁻¹ and 128 fb⁻¹, respectively. All data were accumulated from energy-asymmetric e⁺e⁻ collisions at a centre-of-mass energy corresponding to the mass of the Υ(4S) resonance. We measure ϕ₃ = (78.4 ± 11.4 ± 0.5 ± 1.0)°, where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is from the uncertainties on external measurements of the D-decay strong-phase parameters. [Figure not available: see fulltext.]