Observation of the ${B^0 \to \rho^0 \rho^0}$ decay from an amplitude analysis of ${B^0 \to (\pi^+\pi^-)(\pi^+\pi^-)}$ decays

TL;DR

This paper reports the first observation of the ${B^0 o ho^0 ho^0}$ decay using amplitude analysis of ${B^0 o ( ho^0)( ho^0)}$ decays, measuring its branching fraction and polarization with high significance.

Contribution

It provides the first observation and detailed measurement of the ${B^0 o ho^0 ho^0}$ decay, including polarization and branching fraction, from LHCb data.

Findings

First observation of ${B^0 o ho^0 ho^0}$ decay with 7.1 sigma significance.

Measured longitudinal polarization fraction ${f_L} = 0.745^{+0.048}_{-0.058}$.

Branching fraction ${ imes} 10^{-6} = (0.94 \, ext{±} \, 0.17 \, ext{(stat)} \, ext{±} \, 0.09 \, ext{(syst)} \, ext{±} \, 0.06 \, ext{(BF)})$.

Abstract

Proton-proton collision data recorded in 2011 and 2012 by the \lhcb experiment, co\-rres\-pon\-ding to an integrated luminosity of 3.0\invfb, are a\-na\-lysed to search for the charmless ${B^0 \to \rho^0 \rho^0}$ decay. More than 600 ${\B^0 \to (\pi^+\pi^-)(\pi^+\pi^-)}$ signal decays are selected and used to perform an amplitude analysis from which the ${B^0 \to \rho^0 \rho^0}$ decay is observed for the first time with 7.1 standard deviations significance. The fraction of ${B^0 \to \rho^0 \rho^0}$ decays yielding a longitudinally polarised final state is measured to be $\fL = 0.745^{+0.048}_{-0.058} ({\rm stat}) \pm 0.034 ({\rm syst})$. The ${B^0 \to \rho^0 \rho^0}$ branching fraction, using the ${\B^0 \to \phi K^*(892)^{0}}$ decay as reference, is also reported as ${\BF(B^0 \to \rho^0 \rho^0) = (0.94 \pm 0.17 ({\rm stat}) \pm 0.09 ({\rm syst}) \pm 0.06 ({\rm BF})) \times 10^{-6}}$.