Evidence for the decay $B^{0}\to \eta \eta$

TL;DR

This paper presents the first evidence for the decay of B0 mesons into two eta mesons, based on a large data sample collected at the KEKB collider, with a measured branching fraction indicating a rare decay process.

Contribution

It provides the first experimental evidence for the decay mode B0 → ηη, measuring its branching fraction with statistical significance.

Findings

Branching fraction measured as (7.6^{+2.7 +1.4}_{-2.3 -1.6}) × 10^{-7}

Evidence at 3.3 standard deviations above zero

First observation of this decay mode

Abstract

We report a search for $B^{0}\to \eta \eta $ with a data sample corresponding to an integrated luminosity of $698 \,{\rm fb}^{-1}$ containing $753 \times 10^{6}$ $B\bar{B}$ pairs collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The branching fraction is measured to be $\mathcal{B}(B^{0} \to \eta \eta ) = (7.6^{+2.7 +1.4}_{-2.3 -1.6}) \times 10^{-7}$ at the level of 3.3 standard deviations above zero, which provides the first evidence for the decay $B^{0} \to \eta \eta$.