A search for $B^{0}_{s}\to\tau^{\pm}\mu^{\mp}$ and $B^{0}\to\tau^{\pm}\mu^{\mp}$ decays is performed using data corresponding to an integrated luminosity of 3 fb$^{1}$ of protonproton collisions, recorded with the LHCb detector in 2011 and 2012. For this search, the $\tau$ lepton is reconstructed in the $\tau^{}\to\pi^{}\pi^{+}\pi^{}\nu_{\tau}$ channel. No significant signal is observed. Assuming no contribution from $B^{0}\to\tau^{\pm}\mu^{\mp}$ decays, an upper limit is set on the $B^{0}_{s}\to\tau^{\pm}\mu^{\mp}$ branching fraction of $\mathcal{B}\left( B^{0}_{s}\to\tau^{\pm}\mu^{\mp}\right) < 4.2\times 10^{5}$ at $95\%$ confidence level. If instead no contribution from $B^{0}_{s}\to\tau^{\pm}\mu^{\mp}$ decays is assumed, a limit of $\mathcal{B}\left( B^{0}\to\tau^{\pm}\mu^{\mp}\right) < 1.4\times 10^{5}$ is obtained at $95\%$ confidence level. These are the first limit on $\mathcal{B}\left( B^{0}_{s}\to\tau^{\pm}\mu^{\mp}\right)$ and the world's best limit on $\mathcal{B}\left( B^{0}\to\tau^{\pm}\mu^{\mp}\right)$.
Normalized distributions of the reconstructed invariant mass for $ B ^0_ s $ and $ B ^0$ in simulated event samples and for samesign candidates in data, after applying the initial event selection (see the text). 
Fig1.pdf [16 KiB] HiDef png [344 KiB] Thumbnail [240 KiB] *.C file 

Final BDT output binned distributions for data and simulated signal samples. The markers are displaced horizontally to improve visibility. 
Fig2.pdf [15 KiB] HiDef png [238 KiB] Thumbnail [218 KiB] *.C file 

Distributions of the reconstructed $ B $ invariantmass in data in the four final BDT bins with the projections of the fit for the $ B ^0_ s $ signalonly hypothesis overlaid. The lowerpart of each figure shows the normalised residuals. 
Fig3a.pdf [19 KiB] HiDef png [193 KiB] Thumbnail [183 KiB] *.C file 

Fig3b.pdf [19 KiB] HiDef png [180 KiB] Thumbnail [169 KiB] *.C file 

Fig3c.pdf [19 KiB] HiDef png [181 KiB] Thumbnail [173 KiB] *.C file 

Fig3d.pdf [18 KiB] HiDef png [181 KiB] Thumbnail [173 KiB] *.C file 

The expected and observed pvalues derived with the CLs method as a function of the $ B ^0_ s \rightarrow \tau ^\pm \mu ^\mp $ branching fraction. 
Fig4.pdf [16 KiB] HiDef png [195 KiB] Thumbnail [154 KiB] *.C file 

The expected and observed pvalues derived with the CLs method as a function of the $ B ^0 \rightarrow \tau ^\pm \mu ^\mp $ branching fraction. 
Fig5.pdf [16 KiB] HiDef png [199 KiB] Thumbnail [157 KiB] *.C file 

Animated gif made out of all figures. 
PAPER2019016.gif Thumbnail 
Expected and observed $90\%$ and $95\%$ CL limits on the $ B ^0_{( s )} \rightarrow \tau ^\pm \mu ^\mp $ branching fraction. 
Table_1.pdf [68 KiB] HiDef png [58 KiB] Thumbnail [29 KiB] tex code 
Created on 20 February 2021.