# First observation of a baryonic $B_s^0$ decay

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## Abstract

The first observation of a baryonic $B ^0_ s$ decay, $B ^0_ s \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ , is reported using proton-proton collision data corresponding to an integrated luminosity of 3.0 $\,fb^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 $\mathrm{\,Te\kern -0.1em V}$ . The branching fraction is measured to be $$\mathcal{B} ( B ^0_ s \rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^- + \kern 0.18em\overline{\kern -0.18em B }{} {}^0_ s \rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^- ) = \left[\vphantom{\sum} {5.48}^{+ 0.82}_{- 0.80} \pm 0.60 \pm 0.51(\mathcal{B} ) \pm 0.32(f_s/f_d) \right]\times 10^{-6}\,,$$ where the first uncertainty is statistical and the second systematic, the third uncertainty accounts for the experimental uncertainty on the branching fraction of the $B ^0 \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi ^-$ decay used for normalisation, and the fourth uncertainty relates to the knowledge of the ratio of $b$-quark hadronisation probabilities $f_s/f_d$. The notation $B ^0_ s \rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^- + \kern 0.18em\overline{\kern -0.18em B }{} {}^0_ s \rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ emphasises the fact that both $B ^0_ s$ and $\kern 0.18em\overline{\kern -0.18em B }{} {}^0_ s$ can decay to the $p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ final state. This branching fraction measurement is not corrected for a possible bias due to the finite width difference of the $B ^0_ s$ system.

## Figures and captions

 Results of the fit for the 2011 data sample reconstructed under (left) the $p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi ^-$ invariant mass hypothesis and (right) the $p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ invariant mass hypothesis for (top) the long sample and (bottom) the downstream sample. The black points represent the data, the solid blue curve the complete fit model, the black (red) dotted curve the $B ^0 \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi ^-$ ( $B ^0_ s \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ ) contribution, and the green (magenta) dashed curve the contribution from $B ^0 \!\rightarrow p \overline \Sigma ^0 \pi ^-$ ( $B ^0_ s \!\rightarrow p \overline \Sigma ^0 K ^-$ ) decays. cLLpi_11.pdf [46 KiB] HiDef png [238 KiB] Thumbnail [82 KiB] cLLK_11.pdf [46 KiB] HiDef png [248 KiB] Thumbnail [78 KiB] cDDpi_11.pdf [47 KiB] HiDef png [234 KiB] Thumbnail [81 KiB] cDDK_11.pdf [47 KiB] HiDef png [248 KiB] Thumbnail [83 KiB] Results of the fit for the 2012 data sample reconstructed under (left) the $p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi ^-$ invariant mass hypothesis and (right) the $p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ invariant mass hypothesis for (top) the long sample and (bottom) the downstream sample. The black points represent the data, the solid blue curve the complete fit model, the black (red) dotted curve the $B ^0 \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi ^-$ ( $B ^0_ s \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ ) contribution, and the green (magenta) dashed curve the contribution from $B ^0 \!\rightarrow p \overline \Sigma ^0 \pi ^-$ ( $B ^0_ s \!\rightarrow p \overline \Sigma ^0 K ^-$ ) decays. cLLpi_12.pdf [47 KiB] HiDef png [234 KiB] Thumbnail [83 KiB] cLLK_12.pdf [48 KiB] HiDef png [253 KiB] Thumbnail [86 KiB] cDDpi_12.pdf [48 KiB] HiDef png [236 KiB] Thumbnail [83 KiB] cDDK_12.pdf [48 KiB] HiDef png [250 KiB] Thumbnail [86 KiB] Dalitz plot distribution for (left) the $B ^0_ s \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ and (right) the $B ^0 \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi ^-$ decay mode. The bin contents are given by the sum of the sPlot weights determined from the fits to the $p \kern 0.1em\overline{\kern -0.1em\Lambda} h^-$ invariant mass for each data sample. The only structure visible is a pronounced enhancement at the $m( p \kern 0.1em\overline{\kern -0.1em\Lambda} )$ threshold. The distributions are not corrected for efficiency. Dalitz[..].pdf [77 KiB] HiDef png [759 KiB] Thumbnail [156 KiB] Dalitz[..].pdf [70 KiB] HiDef png [679 KiB] Thumbnail [148 KiB] Invariant mass distributions of (top) $m( p \kern 0.1em\overline{\kern -0.1em\Lambda} )$, (middle) $m( p h^-)$, and (bottom) $m(\kern 0.1em\overline{\kern -0.1em\Lambda} h^-)$ for the (left) $B ^0 \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi ^-$ and the (right) $B ^0_ s \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ mode. The binning is chosen to be sensitive to typical widths of baryon resonances in $m( p h^-)$ and $m(\kern 0.1em\overline{\kern -0.1em\Lambda} h^-)$. The background subtraction is performed using the sPlot technique. The distributions are not corrected for efficiency. m_pLba[..].pdf [52 KiB] HiDef png [152 KiB] Thumbnail [33 KiB] m_pLba[..].pdf [50 KiB] HiDef png [149 KiB] Thumbnail [32 KiB] m_ph_BLppi.pdf [59 KiB] HiDef png [160 KiB] Thumbnail [36 KiB] m_ph_BsLpK.pdf [57 KiB] HiDef png [156 KiB] Thumbnail [36 KiB] m_Lbar[..].pdf [60 KiB] HiDef png [168 KiB] Thumbnail [39 KiB] m_Lbar[..].pdf [58 KiB] HiDef png [164 KiB] Thumbnail [38 KiB]

## Tables and captions

 Summary of systematic uncertainties relative to the measured $B ^0_ s \!\rightarrow p \kern 0.1em\overline{\kern -0.1em\Lambda} K ^-$ branching fraction. The total corresponds to the sum of all contributions added in quadrature. Table_1.pdf [62 KiB] HiDef png [128 KiB] thumbnail [18 KiB] tex code

Created on 16 December 2017.Citation count from INSPIRE on 16 December 2017.