Resolving the ϕ 2 (α) ambiguity in B → ρρ

I propose an alternative method for measuring the CP violating phase ϕ2 (α) without ambiguity in an extended SU(2) isospin triangle analysis, which can ultimately be achieved by exploiting interference effects between B0 → ρ0ρ0 and B0 → a1±π∓ in a time-dependent flavour-tagged amplitude analysis. Under certain assumptions on the effective ϕ2 in each channel, I demonstrate with an idealised amplitude model that potential deviations in the measured ϕ2 due to penguin contamination in B0 → a1±π∓ are sufficiently large within current experimental uncertainties that this programme could be executed with Run 3 data at LHCb and easily at Belle II.