# Observation of the $B^0_s\to\eta'\eta'$ decay

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## Abstract

The first observation of the $B^0_s\to\eta'\eta'$ decay is reported. The study is based on a sample of proton-proton collisions corresponding to $3.0$ ${\rm fb^{-1}}$ of integrated luminosity collected with the LHCb detector. The significance of the signal is $6.4$ standard deviations. The branching fraction is measured to be $[3.31 \pm 0.64\,{\rm (stat)} \pm 0.28\,{\rm (syst)} \pm 0.12\,{\rm (norm)}]\times10^{-5}$, where the third uncertainty comes from the $B^{\pm}\to\eta' K^{\pm}$ branching fraction that is used as a normalisation. In addition, the charge asymmetries of $B^{\pm}\to\eta' K^{\pm}$ and $B^{\pm}\to\phi K^{\pm}$, which are control channels, are measured to be $(-0.2 \pm1.3)\%$ and $(+1.7\pm1.3)\%$, respectively. All results are consistent with theoretical expectations.

## Figures and captions

 Distributions of the (top) $\eta ^{\prime} \eta ^{\prime}$, (bottom) $(\pi\pi\gamma )_1$ and $(\pi\pi\gamma )_2$ invariant masses for the $B ^0_ s \!\rightarrow \eta ^{\prime} \eta ^{\prime}$ candidates with fit results overlaid. The $(\pi\pi\gamma )_1$ and $(\pi\pi\gamma )_2$ mass distributions are shown for the candidates with an $\eta ^{\prime} \eta ^{\prime}$ invariant mass within three standard deviations of the $B ^0_ s$ mass. The components are the following: (dashed red curves) $B ^0_ s \!\rightarrow \eta ^{\prime} \eta ^{\prime}$ signal, (long-dashed blue curves) combinatorial background without an $\eta ^{\prime}$ resonance in the final state, (dot-long-dashed black curves) partially reconstructed background without an $\eta ^{\prime}$ resonance, (short-dashed red, short-dashed blue curves) combinatorial background with one $\eta ^{\prime}$ resonance, and (dot-dashed green, dot-dashed black curves) partially reconstructed background with one $\eta ^{\prime}$ resonance. The total fit function is shown as the solid blue curves. fig1.pdf [37 KiB] HiDef png [473 KiB] Thumbnail [142 KiB] *.C file Distributions of the (top) $\eta ^{\prime} K ^\pm$ and (bottom) $\pi ^+ \pi ^- \gamma$ invariant masses for the $B ^\pm \!\rightarrow \eta ^{\prime} K ^\pm$ candidates with fit results overlaid. The components are the following: (dashed red curves) $B ^\pm \!\rightarrow \eta ^{\prime} K ^\pm$ signal, (long-dashed blue curves) combinatorial background without an $\eta ^{\prime}$ resonance in the final state, and (dotted blue curves) combinatorial background with an $\eta ^{\prime}$ resonance. The total fit function is shown as the solid blue curves. fig2.pdf [30 KiB] HiDef png [366 KiB] Thumbnail [106 KiB] *.C file Distributions of the (top) $\phi K ^\pm$ and (bottom) $K ^+ K ^-$ invariant masses for the $B ^\pm \!\rightarrow \phi K ^\pm$ candidates with fit results overlaid The components are the following: (dashed red curves) $B ^\pm \!\rightarrow \phi K ^\pm$ signal, (long-dashed red curves) non-resonant $B ^\pm \!\rightarrow K ^+ K ^- K ^\pm$ background, partially reconstructed $b$-hadron background (long-dashed blue curves) with or (dot-dashed blue curves) without a $\phi$ resonance in the decay chain. The combinatorial background with or without a $\phi$ resonance is too small to be visible. The total fit function is shown as the solid blue curves. fig3.pdf [34 KiB] HiDef png [382 KiB] Thumbnail [109 KiB] *.C file Animated gif made out of all figures. PAPER-2014-065.gif Thumbnail

Created on 23 February 2019.Citation count from INSPIRE on 23 February 2019.