Search for the decays $B_s^0\to\tau^+\tau^-$ and $B^0\to\tau^+\tau^-$

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Abstract

A search for the rare decays $B_s^0\to\tau^+\tau^-$ and $B^0\to\tau^+\tau^-$ is performed using proton--proton collision data collected with the LHCb detector. The data sample corresponds to an integrated luminosity of 3fb$^{-1}$ collected in 2011 and 2012. The $\tau$ leptons are reconstructed through the decay $\tau^-\to\pi^-\pi^+\pi^-\nu_{\tau}$. Assuming no contribution from $B^0\to\tau^+\tau^-$ decays, an upper limit is set on the branching fraction $\mathcal{B}(B_s^0\to\tau^+\tau^-) < 6.8\times 10^{-3}$ at 95 confidence level. If instead no contribution from $B_s^0\to\tau^+\tau^-$ decays is assumed, the limit is $\mathcal{B}(B^0\to\tau^+\tau^-)< 2.1 \times 10^{-3}$ at 95 confidence level. These results correspond to the first direct limit on $\mathcal{B}(B_s^0\to\tau^+\tau^-)$ and the world's best limit on $\mathcal{B}(B^0\to\tau^+\tau^-)$.

Figures and captions

 Two-dimensional distribution of the invariant masses $m_{\pi ^+ \pi ^- }$ of the two oppositely charged two-pion combinations for simulated $B ^0_ s \rightarrow \tau ^+\tau ^-$ candidates. The distribution is symmetric by construction. The vertical and horizontal lines illustrate the sector boundaries. Fig1.pdf [37 KiB] HiDef png [855 KiB] Thumbnail [582 KiB] *.C file (left) Normalised NN output distribution in the signal ($\widehat{\mathcal{N}}^{\text{SR}}_{\text{sim}}$) and control ($\widehat{\mathcal{N}}^{\text{CR}}_{\text{sim}}$) region for $B ^0_ s \rightarrow \tau ^+\tau ^-$ simulated events. (right) Normalised NN output distribution in the data control region $\widehat{\mathcal{N}}^{\text{CR}}_{\text{data}}$. The uncertainties reflect the statistics of the (simulated) data. Fig2a.pdf [15 KiB] HiDef png [195 KiB] Thumbnail [175 KiB] *.C file Fig2b.pdf [14 KiB] HiDef png [100 KiB] Thumbnail [52 KiB] *.C file Distribution of the NN output in the signal region $\mathcal{N}^{\text{SR}}_{\text{data}}$ (black points), with the total fit result (blue line) and the background component (green line). The fitted $B ^0_ s \rightarrow \tau ^+\tau ^-$ signal component is negative and is therefore shown multiplied by $-1$ (red line). For each bin of the signal and background component the combined statistical and systematic uncertainty on the template is shown as a light-coloured band. The difference between data and fit divided by its uncertainty (pull) is shown underneath. Fig3.pdf [15 KiB] HiDef png [169 KiB] Thumbnail [157 KiB] *.C file Invariant mass distribution of the reconstructed $B ^0 \rightarrow D^-D_s^{+}$ candidates in data (black points), together with the total fit result (blue line) used to determine the $B ^0 \rightarrow D^-D_s^{+}$ yield. The individual components are described in the text. Fig4.pdf [32 KiB] HiDef png [301 KiB] Thumbnail [234 KiB] *.C file Distribution of the NN output in the signal region $\mathcal{N}^{\text{SR}}_{\text{data}}$ (black points), with the total fit result (blue line), and the background component (green line). Shown is the fit using the "background only" model. For each bin of the background component the combined statistical and systematic uncertainty is shown as a light-coloured band. The difference between data and fit divided by its uncertainty (pull) is shown underneath. Fig5.pdf [15 KiB] HiDef png [154 KiB] Thumbnail [142 KiB] *.C file Profile likelihood scan of the $B ^0_ s \rightarrow \tau ^+\tau ^-$ signal yield. The intersections of the likelihood curve with the horizontal lines define the 68.3%, 95.4% and 99.7% likelihood intervals. Fig6.pdf [13 KiB] HiDef png [135 KiB] Thumbnail [109 KiB] *.C file The $p$-value derived with the CL$_{\text{s}}$ method as a function of $\mathcal{B} ( B ^0_ s \rightarrow \tau ^+\tau ^- )$. Expected (observed) values are shown by a dashed (full) black line. The green (yellow) band covers the regions of 68% and 95% confidence for the expected limit. The red horizontal line corresponds to the limit at 95% CL. Fig7.pdf [15 KiB] HiDef png [179 KiB] Thumbnail [138 KiB] *.C file Normalised NN output distribution in the signal ($\widehat{\mathcal{N}}^{\text{SR}}_{\text{sim}}$) and control ($\widehat{\mathcal{N}}^{\text{CR}}_{\text{sim}}$) region for $B ^0 \rightarrow \tau ^+\tau ^-$ simulated events. Fig8.pdf [15 KiB] HiDef png [197 KiB] Thumbnail [176 KiB] *.C file Fit results for $B ^0 \rightarrow \tau ^+\tau ^-$. Distribution $\mathcal{N}^{\text{SR}}_{\text{data}}$ (black points), overlaid with the total fit result (blue line), and background component (green line), assuming all signal events originate from $B ^0 \rightarrow \tau ^+\tau ^-$ decays. The $B ^0 \rightarrow \tau ^+\tau ^-$ signal component is also shown (red line), multiplied by $-1$. For each bin of the signal and background component the combined statistical and systematic uncertainty is shown as a light-coloured band. The difference between data and fit divided by its uncertainty (pull) is shown underneath. Fig9.pdf [15 KiB] HiDef png [169 KiB] Thumbnail [157 KiB] *.C file The $p$-value derived with the CL$_{\text{s}}$ method as a function of $\mathcal{B} ( B ^0 \rightarrow \tau ^+\tau ^-$). Expected (observed) values are shown by a dashed (full) black line. The green (yellow) band covers the regions of 68% and 95% confidence for the expected limit. The red horizontal line corresponds to the limit at 95% CL. Fig10.pdf [14 KiB] HiDef png [184 KiB] Thumbnail [143 KiB] *.C file Animated gif made out of all figures. PAPER-2017-003.gif Thumbnail

Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [106 KiB] This ZIP file contains supplemetary material for the publication LHCb-PAPER-2017-003. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in various formats Fig1-S.pdf [53 KiB] HiDef png [989 KiB] Thumbnail [678 KiB] *C file

Created on 11 July 2020.