# Measurement of the $B_s^0 \to \phi \phi$ branching fraction and search for the decay $B^0 \to \phi \phi$

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## Abstract

Using a dataset corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV, the $B_s^0 \to \phi \phi$ branching fraction is measured to be $\mathcal{B}(B_s^0 \to \phi \phi) = ( 1.84 \pm 0.05 (\text{stat}) \pm 0.07 (\text{syst}) \pm 0.11 (f_s/f_d) \pm 0.12 (\text{norm}) ) \times 10^{-5},$ where $f_s/f_d$ represents the ratio of the $B_s^0$ to $B^0$ production cross-sections, and the $B^0 \to \phi K^*(892)^0$ decay mode is used for normalization. This is the most precise measurement of this branching fraction to date, representing a factor five reduction in the statistical uncertainty compared with the previous best measurement. A search for the decay $B^0 \to \phi \phi$ is also made. No signal is observed, and an upper limit on the branching fraction is set as $\mathcal{B}(B^0 \to \phi \phi) < 2.8 \times 10^{-8}$ at 90% confidence level. This is a factor of seven improvement compared to the previous best limit.

## Figures and captions

 The $K^+K^-K^+K^-$ invariant mass distribution. The total fitted function as described in the text is shown by the (red) solid line, the $B ^0_ s \rightarrow \phi\phi$ component by the (blue) long-dashed line, and the combinatorial background as the (purple) dotted line. Fig1.pdf [9 KiB] HiDef png [235 KiB] Thumbnail [167 KiB] *.C file The $K^+K^-K^+\pi^-$ invariant mass distribution. The total fitted function is shown by the (red) solid line, the $B ^0 \rightarrow \phi K ^*$ component by the (blue) short-dashed line, the $B ^0_ s \rightarrow \phi\overline{ K }{} {}^* (892)^0$ component by the (blue) long-dashed line, the $\Lambda ^0_ b \rightarrow \phi p \pi^-$ and $\Lambda ^0_ b \rightarrow \phi p K^-$ contribution by the (green) dashed-dotted line, and the combinatoral background by the (purple) dotted line. Fig2.pdf [10 KiB] HiDef png [258 KiB] Thumbnail [184 KiB] *.C file The $K^+K^-K^+K^-$ invariant mass with the tight BDT selection applied. A fit to the total PDF as described in the text is shown as a (red) solid line, $B ^0_ s \rightarrow \phi\phi$ as a (blue) long-dashed line, $B ^0 \rightarrow \phi\phi$ as a (blue) short-dashed line, and the combinatorial background as a (purple) dotted line. Fig3.pdf [9 KiB] HiDef png [234 KiB] Thumbnail [179 KiB] *.C file Results of the $\text{CL}_\text{s}$ scan as a function of the $B ^0 \rightarrow \phi\phi$ branching fraction ($BF$). The observed $\text{CL}_\text{s}$ distribution is given by the (black) points and solid line, while the expected distribution is given by the (black) dashed line. The dark (green) and light (yellow) bands mark the $1\sigma$ and $2\sigma$ confidence regions on the expected $\text{CL}_\text{s}$. The upper limits at 90$\%$ and 95$\%$ confidence level are where the observed $\text{CL}_\text{s}$ line intercepts the (red) solid and dashed horizontal lines, respectively. Fig4.pdf [5 KiB] HiDef png [114 KiB] Thumbnail [82 KiB] *.C file Animated gif made out of all figures. PAPER-2015-028.gif Thumbnail

## Tables and captions

 Predictions for the $B ^0_ s \rightarrow \phi \phi$ branching fraction. The first and second uncertainties of Refs. [8,9] reflect the knowledge of CKM parameters and power corrections, respectively. Table_1.pdf [53 KiB] HiDef png [68 KiB] Thumbnail [33 KiB] tex code Summary of the systematic uncertainties on the measurement of the ratio of branching fractions $\mathcal{B}( B ^0_ s \rightarrow \phi \phi)/\mathcal{B}( B ^0 \rightarrow \phi K^*)$. Table_2.pdf [56 KiB] HiDef png [104 KiB] Thumbnail [49 KiB] tex code

Created on 21 October 2019.