Measurement of the branching fraction, polarization, and time-dependent CPCP asymmetry in B0ρ+ρB^0 \to \rho^+\rho^- decays and constraint on the CKM angle ϕ2\phi_2

Collaboration
Dec 27, 2024
15 pages
e-Print:
Report number:
  • Belle II preprint 2024-024,
  • KEK preprint 2024-23
Experiments:

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Abstract: (arXiv)
We present a measurement of the branching fraction and fraction of longitudinal polarization of B0ρ+ρB^0 \to \rho^+ \rho^- decays, which have two π0\pi^0's in the final state. We also measure time-dependent CPCP violation parameters for decays into longitudinally polarized ρ+ρ\rho^+ \rho^- pairs. This analysis is based on a data sample containing (387±6)×106(387\pm6) \times 10^6\BBbar pairs collected with the Belle~II detector at the SuperKEKB asymmetric-energy e+ee^+e^- collider in 2019-2022. We obtain B(B0ρ+ρ)=(2.880.22+0.230.27+0.29)×105,fL=0.9210.025+0.0240.015+0.017{B}(B^0\to\rho^+\rho^-) = (2.88 ^{+0.23}_{-0.22} {}^{+0.29}_{-0.27}) \times 10^{-5}, f_{L} = 0.921 ^{+0.024}_{-0.025} {}^{+0.017}_{-0.015}, S=0.26±0.19±0.08S = -0.26\pm0.19\pm0.08, and C=0.02±0.120.05+0.06C = -0.02\pm0.12^{+0.06}_{-0.05}, where the first uncertainties are statistical and the second are systematic. We use these results to perform an isospin analysis to constrain the CKM angle ϕ2\phi_2 and obtain two solutions; the result consistent with other Standard Model constraints is ϕ2=(92.64.8+4.5)\phi_2 = (92.6^{+4.5}_{-4.8})^\circ.