Measurement of $B^0\to\pi^+\pi^-\pi^+\pi^-$ Decays and Search for $B^0\to\rho^0\rho^0$

TL;DR

This paper reports on a search for specific charmless B meson decays involving four pions, setting upper limits on their branching fractions and extracting the CKM phase angle from the data collected by the Belle experiment.

Contribution

First measurement setting upper limits on $B^0$ decays to four-pion final states and determining the CKM phase $ heta_2$ from the $B^0 o ho^0 ho^0$ decay analysis.

Findings

Upper limit on $B^0 o ho^0 ho^0$ branching fraction: $1.0\times 10^{-6}$

Evidence for $B^0\to \rho^0\pi^+\pi^-$ with less than $12.0\times 10^{-6}$ branching fraction

Evidence for non-resonant $B^0\to 4\pi^{\pm}$ with less than $19.3\times 10^{-6}$ branching fraction

Abstract

We report on a search for the decay $B^0\to\rho^0\rho^0$ and other charmless modes with a $\pi^+\pi^-\pi^+\pi^-$ final state, including $B^0\to\rho^0\pi^+\pi^-$, non-resonant $B^0\to 4\pi^{\pm}$, $B^0\to\rho^0f_0(980)$, $B^0\to f_0(980)f_0(980)$ and $B^0\to f_0(980)\pi^+\pi^-$. These results are obtained from a data sample containing 657 million $B \overline B$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We set an upper limit on $\mathcal{B}(B^0\to\rho^0\rho^0)$ of $1.0\times 10^{-6}$ at the 90% confidence level (C.L.). From our $B^0\to\rho^0\rho^0$ measurement and an isospin analysis, we determine the Cabibbo-Kobayashi-Maskawa phase $\phi_2$ to be $91.7 \pm 14.9$ degrees. We find excesses in $B^0\to \rho^0\pi^+\pi^-$ and non-resonant $B^0\to 4\pi^{\pm}$ with 1.3$\sigma$ and 2.5$\sigma$ significance, respectively. The corresponding branching fractions are less than $12.0 \times 10^{-6}$ and $19.3 \times 10^{-6}$ at the 90% C.L. In addition, we set 90% C.L. upper limits as follows: $\mathcal{B}(B^0\to\rho^0f_0(980))< 0.3 \times 10^{-6}$, $\mathcal{B}(B^0\to f_0(980)f_0(980))< 0.1 \times 10^{-6}$, and $\mathcal{B}(B^0\to f_0(980)\pi^+\pi^-)< 3.8 \times 10^{-6}$.