Resolution of outstanding puzzles in $B^+ \!\to K^+ \pi^+ \pi^-$ decays

Abstract

An amplitude analysis of $B^+ \!\to K^+ \pi^+ \pi^-$ decays is presented, based on $pp$ collision data recorded with the LHCb detector in 2011--2012, corresponding to an integrated luminosity of $3\,\text{fb}^{-1}$. Previous studies of the $B \!\to K \pi \pi$ sector have left key unresolved questions concerning the model of the S-wave contributions. A pivotal finding is that relaxing unitarity-based assumptions about the relation between the $K^*_0(1430)^0$ resonance and the slowly varying scalar part in $K^+\pi^-$ leads to considerably better agreement between the model and data. The $B^+ \!\to K^*_0(1430)^0 \pi^+$ branching fraction now challenges the experimental consensus that $B \!\to K^*_0(1430) \pi$ decays dominate the $B \!\to K \pi \pi$ phase space, aligning with the predictions of QCD factorisation rather than perturbative QCD, thus reversing the agreement found in previous measurements. With this increased flexibility, it also becomes possible to model the scalar $\pi^+ \pi^-$ amplitude using established states, eliminating the need for the ad-hoc ``$f_X(1300)$'' component included in previous analyses of the $B \!\to K\pi\pi$ sector. These advances facilitate the discovery of ten intermediate decays.