First observation of a baryonic $B_s^0$ decay

TL;DR

This paper reports the first observation of a baryonic decay of the $B_s^0$ meson, measuring its branching fraction using LHCb data at 7 and 8 TeV, with detailed uncertainty analysis.

Contribution

It presents the first experimental observation and measurement of the branching fraction for the baryonic decay $B_s^0 o p ar{ extLambda} K^-$.

Findings

First observation of the decay mode.

Measured branching fraction of approximately 5.46 x 10^{-6}.

Uncertainty analysis includes statistical, systematic, normalization, and hadronization ratio uncertainties.

Abstract

We report the first observation of a baryonic $B_s^0$ decay, $B_s^0 \to p \kern 0.1em\overline{\kern -0.1em\Lambda} K^-$, using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of $3.0\,\mathrm{fb}^{-1}$. The branching fraction is measured to be $ \mathcal{B}(B_s^0 \to p \kern 0.1em\overline{\kern -0.1em\Lambda} K^-) + \mathcal{B}(B_s^0 \to \bar{p} \Lambda K^+) = \left[5.46 \pm 0.61 \pm 0.57 \pm 0.50 (\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 $B^0 \to p \kern 0.1em\overline{\kern -0.1em\Lambda} \pi^-$ decay used for normalization, and the fourth uncertainty relates to the knowledge of the ratio of $b$-quark hadronization probabilities $f_s/f_d$.

Equations

$$\mathcal{B}(B_s^0\to p\overline{\text{mathchar}28931}{K}^{-})+\mathcal{B}(B_s^0\to \overline{p}\text{mathchar}28931K^{+})=\frac{f_d}{f_s}\frac{N(B_s^0\to p\overline{\text{mathchar}28931}{K}^{-})}{N(B^0\to p\overline{\text{mathchar}28931}{\pi }^{-})}\frac{{ϵ}_{B^0\to p\overline{\text{mathchar}28931}{\pi }^{-}}}{{ϵ}_{B_s^0\to p\overline{\text{mathchar}28931}{K}^{-}}}\mathcal{B}(B^0\to p\overline{\text{mathchar}28931}{\pi }^{-}),$$

$$\mathcal{B}(B_s^0\to p\overline{\text{mathchar}28931}{K}^{-})+\mathcal{B}(B_s^0\to \overline{p}\text{mathchar}28931K^{+})=\left[5.46\pm 0.61\pm 0.57\pm 0.50(\mathcal{B})\pm 0.32(f_s/f_d)\right]\times 10^{-6},$$

Full text

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

​​​

CERN-EP-2017-067

LHCb-PAPER-2017-012

25 July 2017

First observation

of a baryonic ${B}^{0}_{s}$ decay

The LHCb collaboration†††Authors are listed at the end of this paper.

We report the first observation of a baryonic ${B}^{0}_{s}$ decay, ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$, using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7 and 8$\mathrm{\,Te\kern-1.00006ptV}$, corresponding to an integrated luminosity of 3.0$\mbox{\,fb}^{-1}$. The branching fraction is measured to be ${\mathcal{B}}({{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}})+{\mathcal{B}}({{B}^{0}_{s}}\!\rightarrow{\overline{p}}{\mathchar 28931\relax}{{K}^{+}})=\left[\vphantom{\sum}{5.46}\pm{0.61}\pm 0.57\pm 0.50({\mathcal{B}})\pm 0.32(f_{s}/f_{d})\right]\times 10^{-6},$ where the first uncertainty is statistical and the second systematic, the third uncertainty accounts for the experimental uncertainty on the branching fraction of the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ decay used for normalization, and the fourth uncertainty relates to the knowledge of the ratio of $b$-quark hadronization probabilities $f_{s}/f_{d}$.

Published in Phys. Rev. Lett. 119 (2017) 041802

© CERN on behalf of the LHCb collaboration, licence CC-BY-4.0.

The experimental study of $B$-meson decays to baryonic final states has a long history, starting with the first observation of baryonic $B$ decays by the CLEO collaboration in 1997 [1]. The asymmetric $e^{+}e^{-}$ collider experiments BaBar and Belle reported numerous searches and observations of decays of ${B}^{0}$ and ${B}^{+}$ mesons to baryonic final states [2]. The LHCb collaboration published the first observation of a baryonic ${B}_{c}^{+}$ decay in 2014 [3]. Until now, no baryonic ${B}^{0}_{s}$ decay has ever been observed with a significance in excess of five standard deviations; the Belle collaboration provided the only evidence for such a process in the study of ${{B}^{0}_{s}}\!\rightarrow{{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{}^{-}_{c}}{\mathchar 28931\relax}{{\pi}^{+}}$ decays, with a significance of 4.4 standard deviations [4].

Areas of particular interest in baryonic $B$ decays are the study of the hierarchy of branching fractions and the threshold enhancement in the baryon-antibaryon mass spectrum [5, 2]. Multi-body baryonic $B$ decays are expected to have higher branching fractions than two-body decays [6, 7]. The ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ branching fractions are predicted to be of the order of $10^{-6}$ [8]. The notation ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ is used hereafter for the sum of both accessible final states ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{\overline{p}}{\mathchar 28931\relax}{{K}^{+}}$. As emphasized in Ref. [8], which studied the decays ${{B}^{0}_{s}}\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-}$, the decay ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ is a unique baryonic $B$ decay in that it is the only presently known decay where all four processes, namely the decays of a ${B}^{0}_{s}$ or a ${\kern 1.79993pt\overline{\kern-1.79993ptB}{}}{}^{0}_{s}$ meson to either the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ or the ${\overline{p}}{\mathchar 28931\relax}{{K}^{+}}$ final state, can occur. A $B$-flavor-tagged decay-time-dependent study is required in order to separate the two possible final states and measure their individual branching fractions as well as $C\!P$ violation observables.

The current experimental knowledge on the family of \mbox{{B}^{0}{({s})}}\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-} decays ($h={\pi},{K}$) and related modes such as \mbox{{B}^{0}{({s})}}\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}h^{-}, with ${{\overline{\mathchar 28934\relax}}{}^{0}}\rightarrow{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}\gamma$, is rather scarce. The ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ decay has been studied by the BaBar [9] and Belle [10, 11] collaborations and the Belle collaboration has reported the 90% confidence level upper limits ${\mathcal{B}}({{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}})<8.2\times 10^{-7}$ and ${\mathcal{B}}({{B}^{0}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{\pi}^{-}})<3.8\times 10^{-6}$ [10].

Manifestations of $C\!P$ and $T$ violation in baryonic $B$ decays have been studied from a theoretical viewpoint, see for example Ref. [12] and references therein. A large $C\!P$-violation asymmetry of order 10% is expected for the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ decay mode [12], which further motivates the experimental study of \mbox{{B}^{0}_{({s})}}\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-} decays.

This Letter presents the first observation of a charmless baryonic ${B}^{0}_{s}$ decay. The branching fraction of the ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ decay is measured relative to that of the topologically identical ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ decay to suppress common systematic uncertainties:

[TABLE]

where $N$ represents yields determined from mass fits, $f_{q}$ stands for the ${b}$ hadronization probability to the meson $B_{q}$, and $\epsilon$ represents the selection efficiencies. The inclusion of charge-conjugate processes is implied, unless otherwise stated.

The data sample analyzed corresponds to an integrated luminosity of 1.0$\mbox{\,fb}^{-1}$ of proton-proton collision data collected by the LHCb experiment at center-of-mass energies of 7$\mathrm{\,Te\kern-1.00006ptV}$ in 2011 and 2.0$\mbox{\,fb}^{-1}$ at 8$\mathrm{\,Te\kern-1.00006ptV}$ in 2012. The LHCb detector is a single-arm forward spectrometer covering the pseudorapidity range $2<\eta<5$, designed for the study of particles containing $b$ or $c$ quarks [13, 14]. The pseudorapidity is defined as $\eta=-{\rm ln}\left[\tan(\theta/2)\right]$, where $\theta$ is the polar angle with respect to the proton in the positive $z$ direction. The detector elements that are particularly relevant to this analysis are a silicon-strip vertex detector surrounding the proton-proton interaction region that allows heavy hadrons to be identified from their characteristically long flight distance; a tracking system that provides a measurement of momentum, $p$, of charged particles; two ring-imaging Cherenkov detectors that are able to discriminate between different species of charged hadrons; a calorimeter system for the measurement of photons and neutral hadrons; and multiwire proportional chambers for the detection of muons. Simulated data samples, produced as described in Refs. [15, *Sjostrand:2006za, 17, 18, 19, 20, *Agostinelli:2002hh, 22], are used to evaluate the response of the detector and to investigate and characterize possible sources of background.

Events are selected in a similar way for both the signal decay ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ and the normalization channel ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$, where ${\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}\rightarrow{\overline{p}}{{\pi}^{+}}$. Real-time event selection is performed by a trigger [23] consisting of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which performs a full event reconstruction. The hardware trigger stage requires events to have a muon with high transverse momentum, $p_{\mathrm{T}}$, or a hadron, photon or electron with high transverse energy deposited in the calorimeters. For this analysis, the hardware trigger decision can either be made on the signal candidates or on other particles in the event. The software trigger requires a two- or three-track secondary vertex with a significant displacement from all the primary $pp$ interaction vertices (PVs). At least one charged particle must have high $p_{\mathrm{T}}$ and be inconsistent with originating from a PV. A multivariate algorithm [24] is used for the identification of secondary vertices consistent with the decay of a $b$ or $c$ hadron.

The $\mathchar 28931\relax$ decays are reconstructed in two different categories: the first consists of $\mathchar 28931\relax$ baryons that decay early enough for the proton and pion to be reconstructed in the vertex detector, while the second contains those that decay later such that track segments cannot be reconstructed in the vertex detector. These reconstruction categories are referred to as long and downstream, respectively.

The selection of ${B}^{0}_{({s})}$ candidates, formed by combining a $\mathchar 28931\relax$ candidate with a proton and a pion or kaon, is carried out with a filtering stage, a requirement on the response of a multilayer perceptron [25] (MLP) classifier, and particle identification (PID) criteria discussed below. The proton and pion or kaon, of opposite charge, both decay products of the $B$ meson, are hereafter referred to as the charged hadrons. Unless stated otherwise, the terms proton and pion refer to the charged hadrons from the $B$-meson decay, not to the $\mathchar 28931\relax$ decay products. Both the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and the ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ decay chains are refitted [26] employing a mass constraint on the $\mathchar 28931\relax$ candidates.

In the filtering stage the $\mathchar 28931\relax$ decay products are required to have a minimum momentum, $p$, form a good quality vertex and satisfy $|m({p}{{\pi}^{-}})-m_{\mathchar 28931\relax}|<20(15){\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$ for downstream (long) candidates, where $m_{\mathchar 28931\relax}$ is the $\mathchar 28931\relax$ mass [27]. They must have a large impact parameter (IP) with respect to all PVs, where the IP is defined as the minimum distance of a track to a PV. A minimum $\chi^{2}_{\text{IP}}$ with respect to any PV is imposed on each $\mathchar 28931\relax$ decay product, where $\chi^{2}_{\text{IP}}$ is defined as the difference between the vertex-fit $\chi^{2}$ of a PV reconstructed with and without the particle in question. A loose PID requirement, based primarily on information from the ring-imaging Cherenkov detectors, is imposed to select the proton candidate from the $\mathchar 28931\relax$ baryon to remove background from ${K}^{0}_{\mathrm{\scriptscriptstyle S}}$ decays. For downstream $\mathchar 28931\relax$ candidates a minimum momentum is also required.

A minimum requirement is imposed on the scalar sum of the $p_{\mathrm{T}}$ of the $\mathchar 28931\relax$ candidate and the two charged hadrons. The distance of closest approach among any pair from (p, $\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}$, $h^{-}$) divided by its uncertainty must be small. The $B$ candidate must have a good quality vertex, have a minimum $p_{\mathrm{T}}$ and a small $\chi^{2}_{\text{IP}}$ with respect to the associated PV as its reconstructed momentum vector should point to its production vertex; the associated PV is the one with which it forms the smallest $\chi^{2}_{\text{IP}}$. The pointing condition of the $B$ candidate is further reinforced by requiring that the angle between the $B$-candidate momentum vector and the line connecting the associated PV and the $B$-decay vertex ($B$ direction angle, $\theta_{B}$) is close to zero.

Backgrounds from the ${{B}^{0}}\!\rightarrow{{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{}^{-}_{c}}{p}$ decay with ${{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{}^{-}_{c}}\!\rightarrow{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ (${{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{}^{-}_{c}}\!\rightarrow{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$) are removed from the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ (${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$) samples with a veto around the ${\mathchar 28931\relax}^{+}_{c}$ mass [27] of three times the ${\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ (${\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$) invariant mass resolution of approximately 6${\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$. No veto is found to be necessary to suppress backgrounds from $B$ decays to charmonia and a ${{\pi}^{+}}{{\pi}^{-}}$ pair final states.

Further separation between signal and combinatorial background candidates relies on MLPs implemented with the TMVA toolkit [28]. The MLPs are trained using simulated ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ samples, generated according to a constant matrix element without intermediate resonances, to represent the signal, and with data from the high-mass sideband region $5400<m({p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}})<5600{\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$ for the background, to avoid partially reconstructed backgrounds. Separate MLPs are trained and optimized for each year of data taking and for the two $\mathchar 28931\relax$ reconstruction categories. Each MLP is used to select both ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ candidates.

The seventeen variables used in the MLP classifiers are properties of the $B$ candidate, the charged hadrons and the $\mathchar 28931\relax$ decay products. The input variables are the following: the $\chi^{2}$ per degree of freedom of the kinematic fit of the decay chain [26]; the IP for all particles calculated with respect to the associated PV; the distance of closest approach between the two charged hadrons and the sum of their corresponding $\chi^{2}_{\text{IP}}$; the $\mathchar 28931\relax$ candidate decay-length significance with respect to the $B$ vertex, i.e. the decay length divided by its uncertainty; the angle between the $\mathchar 28931\relax$ momentum and the spacial vector connecting the $B$ and $\mathchar 28931\relax$ decay vertices in the $B$ rest frame; the $\mathchar 28931\relax$ decay time; the $B$-meson $p_{\mathrm{T}}$, pseudorapidity, direction angle $\theta_{B}$, decay-length significance and decay time; the $\mathchar 28931\relax$ helicity angle defined by the $\mathchar 28931\relax$ momentum in the $B$ rest frame and the boost axis of the $B$ meson, which is given by the $B$-meson momentum in the laboratory frame; the pointing variable defined as P=[\sum_{p,{\kern 0.70004pt\overline{\kern-0.70004pt\mathchar 28931\relax}},h^{-}}{p}\times\sin{\theta_{B}}]/[\sum_{p,{\kern 0.70004pt\overline{\kern-0.70004pt\mathchar 28931\relax}},h^{-}}{p}\times\sin{\theta_{B}}+\sum_{p,{\kern 0.70004pt\overline{\kern-0.70004pt\mathchar 28931\relax}},h^{-}}{\mbox{p_{\mathrm{T}}}}]. The optimal MLP requirement for each of the four subsamples is determined by maximizing the signal significance of the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ normalization decay, with the variation of the signal efficiency with MLP cut value determined from simulation.

A PID selection is applied to the charged hadrons after the MLP selection. No additional PID requirement is applied to the proton from the $\mathchar 28931\relax$ candidate since no contamination from misidentified ${{K}^{0}_{\mathrm{\scriptscriptstyle S}}}\!\rightarrow{{\pi}^{+}}{{\pi}^{-}}$ decays is observed. The optimization of the PID requirements follows the same procedure as the optimization of the MLP selection. If more than one candidate is selected in any event of any subsample, which occurs in about 5% of selected events, one is chosen at random.

Large data control samples of ${{D}^{0}}\rightarrow{{K}^{-}}{{\pi}^{+}}$, $\mathchar 28931\relax\!\rightarrow{p}{{\pi}^{-}}$ and ${{\mathchar 28931\relax}^{+}_{c}}\!\rightarrow{p}{{K}^{-}}{{\pi}^{+}}$ decays are employed [29] to determine the efficiency of the PID requirements. All other selection efficiencies are determined from simulation. It is necessary to account for the distribution of signal candidates and the variation of the efficiency over the phase space of the decay. The variation is well described by the factorized efficiencies in the two-dimensional space of the variables $m^{2}(p{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}})$ and $m^{2}(ph^{-})$ defining the Dalitz plot. Simulated events are binned in $m^{2}(p{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}})$ in order to determine the selection efficiencies, the variation in $m^{2}(ph^{-})$ being mild and therefore integrated out. The distribution of signal decays in the phase space is obtained separately for each spectrum with the sPlot technique [30] with the $B$-meson candidate invariant mass used as the discriminating variable. The overall efficiencies of this analysis are of order $10^{-4}$.

The efficiency of the software trigger selection on both decay modes varied during the data-taking period. During the 2011 data taking, downstream tracks were not reconstructed in the software trigger. Such tracks were included in the trigger during the 2012 data taking and a further significant improvement in the algorithms was implemented mid-year. The corresponding changes to the trigger efficiency are taken into account.

Potential sources of background to the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-}$ spectra are investigated using simulation samples. Cross-feed between the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ decay modes is the dominant source of peaking background. The loop-mediated decays ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ are suppressed and estimated to be insignificant [8]. Pion-kaon misidentification from $b$-baryon decays such as the recently observed decays ${{\mathchar 28931\relax}^{0}_{b}}\!\rightarrow{\mathchar 28931\relax}h^{+}h^{\prime-}$ [31] is found to be negligible. The influence of proton-pion misidentification in the reconstruction and selection of the $\mathchar 28931\relax$ baryon arising from ${K}^{0}_{\mathrm{\scriptscriptstyle S}}$ cross-feed is checked since the PID requirement on the proton from the $\mathchar 28931\relax$ is rather loose. It is verified with Armenteros-Podolanski plots [32] that the ${K}^{0}_{\mathrm{\scriptscriptstyle S}}$ contamination can be ignored. Cross-feed from the presently unobserved decay ${{\mathchar 28931\relax}^{0}_{b}}\!\rightarrow{\mathchar 28931\relax}{p}{\overline{p}}$ due to proton-pion and proton-kaon misidentification is assumed to be negligible considering that the proton misidentification rate is small. Partially reconstructed decays such as the unobserved ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\rho}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{*-}}$ modes are treated as a source of systematic uncertainty. Decay modes containing a ${\overline{\mathchar 28934\relax}}{}^{0}$ baryon decaying into ${{\overline{\mathchar 28934\relax}}{}^{0}}\rightarrow{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}\gamma$, where the $\gamma$ is not detected, can pollute the signal regions due to the small mass difference $m({{\mathchar 28934\relax}^{0}})-m({\mathchar 28931\relax})\approx 77{\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$ [27]. The decay ${{B}^{0}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{\pi}^{-}}$ is expected to have a branching fraction at the level of $10^{-6}$ [33], though searches for the \mbox{{B}^{0}_{({s})}}\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}h^{-} family of decays have found no signal [10]. The decays ${{B}^{0}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{K}^{-}}$ are expected to be the dominant members of the family and are included in the fits to the data.

The yields of the signal and background candidates in eight subsamples are determined from a simultaneous unbinned extended maximum likelihood fit to the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-}$ invariant mass distributions. The eight subsamples correspond to the 2011 and 2012 data-taking periods, the two $\mathchar 28931\relax$ reconstruction categories, and the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ final state hypotheses. This approach allows the use of common shape parameters, and the level of cross-feed background can be better constrained by fitting all subsamples simultaneously. The probability density function in each subsample is defined as the sum of components accounting for the signal decay, the cross-feed contribution, the ${{B}^{0}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{K}^{-}}$ decays, and combinatorial background.

The signal and normalization modes are modeled with the sum of two Novosibirsk functions [34]. All shape parameters are fixed to the values obtained separately for each subsample from simulation samples. The ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ peak positions are free parameters determined simultaneously in all subsamples. The cross-feed ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ (${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$) in the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ (${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$) invariant mass distribution is modeled with the sum of a Gaussian and a modified Fermi function defined as the product of an exponential and a Fermi-Dirac function. The ${{B}^{0}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{K}^{-}}$ decays are modeled differently according to the $\mathchar 28931\relax$ reconstruction category and the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-}$ invariant mass hypothesis under which they are reconstructed. Depending on the category a modified Fermi function, a sum of two Novosibirsk functions, the sum of a Novosibirsk and a Gaussian function, or the sum of a Novosibirsk and a modified Fermi function are used. A combinatorial background component described by an exponential function is present for both ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-}$ final states.

The yields of the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ candidates are determined in the fit together with the ratio of the ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ to ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ branching fractions, which is determined simultaneously across all subsamples accounting for differences in selection efficiencies. These depend on the data-taking period, $\mathchar 28931\relax$ reconstruction category and mass hypothesis of the meson from the $B$ decay. The uncertainties arising from the ratios of efficiencies are included in the fit as Gaussian constraints. The yields of the ${{B}^{0}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{K}^{-}}$ decays are defined relative to those of the corresponding ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ decays, respectively. These two ${{\mathchar 28934\relax}^{0}}$-to-${\mathchar 28931\relax}$ decay yield ratios are determined simultaneously in the fit across all subsamples following the same procedure as for the ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ decay. The combinatorial background yield and shape parameters are treated independently in each subsample and are allowed to vary in the fit.

Figure 1 presents the fit to the ${p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}h^{-}$ invariant mass distributions for all subsamples combined. Both ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ signals are prominent. In particular, the ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ decay is observed with a statistical significance above 15 standard deviations, estimated from the change in log-likelihood between fits with and without the ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ signal component [35]. It constitutes the first observation of a baryonic ${B}^{0}_{s}$ decay. The yields summed over all subsamples are $N({{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}})=519\pm 28$ and $N({{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}})=234\pm 29$, where the uncertainties are statistical only.

The sPlot technique is used to subtract the background and obtain the phase space distribution of signal candidates. Figure 2 shows the $m({p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}})$ invariant mass distributions for the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ candidates after correcting for the distribution selection efficiencies. Both distributions show a pronounced enhancement at threshold in the baryon-antibaryon invariant mass, first suggested in Ref. [5] and observed in several baryonic $B$ decay modes.

The sources of systematic uncertainty arise from the fit model, the knowledge of the selection efficiencies, and the uncertainties on the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ branching fraction and on the ratio of hadronization probabilities $f_{s}/f_{d}$. Uncertainties on the selection efficiencies arise from residual differences between data and simulation in the trigger, reconstruction, selection and particle identification. Additional uncertainties arise due to the limited size of the simulation samples and the corresponding uncertainty on the distribution of the efficiencies across the decay phase space. As the efficiencies depend on the signal decay-time distribution, the effect coming from the different lifetimes of the ${B}^{0}_{s}$ mass eigenstates has been evaluated [36]. Pseudoexperiments are used to estimate the effect of using alternative shapes for the fit components, of including additional backgrounds in the fit such as partially reconstructed decays, and of excluding the ${{B}^{0}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{\pi}^{-}}$ and ${{B}^{0}_{s}}\!\rightarrow{p}{{\overline{\mathchar 28934\relax}}{}^{0}}{{K}^{-}}$ decays that show no significant contribution. Intrinsic biases in the fitted signal yields are investigated with ensembles of simulated pseudoexperiments. A small bias is found and added to the systematic uncertainty on the fit model. The systematic uncertainty due to the knowledge of the efficiencies involved in the definition of fit constraints is negligible. The total systematic uncertainty on the ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ branching fraction is given by the sum of all uncertainties added in quadrature and amounts to 10.5%; it is dominated by the systematic uncertainty on the fit model.

The uncertainty on the branching fraction of the normalization decay, ${\mathcal{B}}({{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}})=(3.14\pm 0.29)\times 10^{-6}$ [27], is taken as a systematic uncertainty from external inputs. The 5.8% uncertainty on the latest $f_{s}/f_{d}$ combination from LHCb, $f_{s}/f_{d}=0.259\pm 0.015$ [37], is taken as a second source of systematic uncertainty from external inputs.

The ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ branching fraction, determined relative to that of the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ normalization channel according to Eq. 1, is measured to be

[TABLE]

where the first uncertainty is statistical and the second systematic, the third uncertainty accounts for the experimental uncertainty on the branching fraction of the ${{B}^{0}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{\pi}^{-}}$ decay, and the fourth uncertainty relates to the knowledge of $f_{s}/f_{d}$.

In summary, the first observation of the three-body charmless baryonic decay ${{B}^{0}_{s}}\!\rightarrow{p}{\kern 1.00006pt\overline{\kern-1.00006pt\mathchar 28931\relax}}{{K}^{-}}$ is reported using a proton-proton collision data sample collected by the LHCb experiment, corresponding to an integrated luminosity of 3.0$\mbox{\,fb}^{-1}$. The decay is observed with a statistical significance above 15 standard deviations, which constitutes the first observation of a baryonic ${B}^{0}_{s}$ decay.

Decays of $B$ mesons to final states containing baryons are now observed for all $B$-meson species. Their study provides valuable information on the dynamics of hadronic decays of $B$ mesons. The present analysis motivates further theoretical studies of baryonic ${B}^{0}_{s}$ decays in addition to those currently published [8, 38, 39, 6].

Acknowledgements

We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); MOST and NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); NWO (The Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MinES and FASO (Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (United Kingdom); NSF (USA). We acknowledge the computing resources that are provided by CERN, IN2P3 (France), KIT and DESY (Germany), INFN (Italy), SURF (The Netherlands), PIC (Spain), GridPP (United Kingdom), RRCKI and Yandex LLC (Russia), CSCS (Switzerland), IFIN-HH (Romania), CBPF (Brazil), PL-GRID (Poland) and OSC (USA). We are indebted to the communities behind the multiple open source software packages on which we depend. Individual groups or members have received support from AvH Foundation (Germany), EPLANET, Marie Skłodowska-Curie Actions and ERC (European Union), Conseil Général de Haute-Savoie, Labex ENIGMASS and OCEVU, Région Auvergne (France), RFBR and Yandex LLC (Russia), GVA, XuntaGal and GENCAT (Spain), Herchel Smith Fund, The Royal Society, Royal Commission for the Exhibition of 1851 and the Leverhulme Trust (United Kingdom).