Searches for Λ0b and Ξ0b decays to K0Spπ− and K0SpK− final states with first observation of the Λ0b→K0Spπ− decay

JHEP04(2014)087 Published for SISSA by Springer Received:February 5, 2014 Accepted:March 14, 2014 Published:April 11, 2014 Searches for Λ0 band Ξ0 bdecays to K0 Spπ−and K0 SpK−final states with first observation of the Λ0 b→K0 Spπ−decay The LHCb collaboration E-mail: [email protected] Abstract: A search for previously unobserved decays of beauty baryons to the final states K0 Spπ−and K0 SpK−is reported. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0 fb−1of pp collisions. The Λ0 b→K0pπ−decay is observed with a significance of 8.6σ, with branching fraction B(Λ0 b→K0pπ−) = (1.26 ±0.19 ±0.09 ±0.34 ±0.05) ×10−5, where the uncertainties are statistical, systematic, from the ratio of fragmentation fractions fΛ0 b/fd, and from the branching fraction of the B0→K0π+π−normalisation channel, respectively. A first measurement is made of the CP asymmetry, giving ACP (Λ0 b→K0pπ−) = 0.22 ±0.13 (stat) ±0.03 (syst) . No significant signals are seen for Λ0 b→K0 SpK−decays, Ξ0 bdecays to both the K0 Spπ− and K0 SpK−final states, and the Λ0 b→D− s(→K0 SK−)pdecay, and upper limits on their branching fractions are reported. Keywords: Hadron-Hadron Scattering, Branching fraction, B physics, Flavor physics ArXiv ePrint: 1402.0770 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP04(2014)087 JHEP04(2014)087 Contents 1 Introduction 1 2 Detector and data set 2 3 Selection requirements, efficiency modelling and background studies 2 4 Fit model and results 6 5 Systematic uncertainties 7 6 Branching fraction results 13 7 Direct CP asymmetry 14 8 Conclusions 15 The LHCb collaboration 19 1 Introduction The study of beauty baryon decays is still at an early stage. Among the possible ground states with spin-parity JP=1 2 +[1], no hadronic three-body decay to a charmless final state has been observed. These channels provide interesting possibilities to study hadronic decays and to search for CP violation effects, which may vary significantly across the phasespace [2,3], as recently observed in charged Bmeson decays to charmless three-body final states [4,5]. In contrast to three-body neutral Bmeson decays to charmless final states containing K0 Smesons [6], conservation of baryon number allows CP violation searches without the need to identify the flavour of the initial state. In this paper, a search is presented for Λ0 band Ξ0 bbaryon decays to final states containing a K0 Smeson, a proton and either a kaon or a pion (denoted Λ0 b(Ξ0 b)→K0 Sph− where h=π, K).1No published theoretical prediction or experimental limit exists for their branching fractions. Intermediate states containing charmed hadrons are excluded from the signal sample and studied separately: the Λ0 b→Λ+ c(→pK0 S)π−decay is used as a control channel, while the Λ0 b→Λ+ c(→pK0 S)K−and Λ0 b→D− s(→K0 SK−)pdecays are also searched for. The Λ0 b→Λ+ c(→pK−π+)K−decay has recently been observed [7], while the Λ0 b→D− spdecay has been suggested as a source of background to the B0 s→D∓ sK± mode [8]. All branching fractions are measured relative to that of the well-known control 1The inclusion of charge-conjugate processes is implied throughout this paper, except where asymmetries are discussed. – 1 – JHEP04(2014)087 channel B0→K0π+π−[6,9,10], relying on existing measurements of the ratio of fragmentation fractions fΛ0 b/fd, including its transverse momentum (pT) dependence [11–13]. When quoting absolute branching fractions, the results are expressed in terms of final states containing either K0or K0mesons, according to the expectation for each decay, following the convention in the literature [1,14]. The paper is organised as follows. A brief description of the LHCb detector and the data set used for the analysis is given in section 2. The selection algorithms, the method to determine signal yields, and the systematic uncertainties on the results are discussed in sections 3–5. The measured branching fractions are presented in section 6. Since a significant signal is observed for the Λ0 b→K0 Spπ−channel, a measurement of its phasespace integrated CP asymmetry is reported in section 7. Conclusions are given in section 8. 2 Detector and data set The LHCb detector [15] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing bor cquarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream. The combined tracking system provides momentum measurement with relative uncertainty that varies from 0.4% at 5 GeV/c to 0.6% at 100 GeV/c, and impact parameter (IP) resolution of 20 µm for tracks with high transverse momentum. Charged hadrons are identified using two ring-imaging Cherenkov (RICH) detectors [16]. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers [17]. The trigger [18] consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which applies a full event reconstruction. The analysis is based on a sample, corresponding to an integrated luminosity of 1.0 fb−1 of pp collision data at a centre-of-mass energy of 7 TeV, collected with the LHCb detector during 2011. Samples of simulated events are also used to determine the signal selection efficiency, to model signal event distributions and to investigate possible background contributions. In the simulation, pp collisions are generated using Pythia 6.4 [19] with a specific LHCb configuration [20]. Decays of hadronic particles are described by EvtGen [21], in which final-state radiation is generated using Photos [22]. The interaction of the generated particles with the detector and its response are implemented using the Geant4 toolkit [23,24] as described in ref. [25]. 3 Selection requirements, efficiency modelling and background studies Events are triggered and subsequently selected in a similar way for both Λ0 b(Ξ0 b)→K0 Sph− signal modes and the B0→K0 Sπ+π−normalisation channel. Events are required to be – 2 – JHEP04(2014)087 triggered at hardware level either by a calorimeter signal with transverse energy ET> 3.5 GeV associated with one of the particles in the signal decay chain, or by a particle in the event that is independent of the signal decay. The software trigger requires a two-, threeor four-track secondary vertex with a large sum of the transverse momentum of the tracks and significant displacement from the primary pp interaction vertices (PVs). At least one track should have pT>1.7 GeV/c and χ2 IP with respect to any PV greater than 16, where χ2 IP is defined as the difference in χ2of a given PV reconstructed with and without the considered particle. A multivariate algorithm [26] is used for the identification of secondary vertices consistent with the decay of a bhadron. An initial set of loose requirements is applied to filter the events selected by the trigger. Each bhadron (Λ0 b,Ξ0 bor B0) decay is reconstructed by combining two charged tracks with aK0 Scandidate. The K0 Scandidates are reconstructed in the π+π−final state, and are classified into two categories. The first includes candidates that have hits in the vertex detector and the tracking stations downstream of the dipole magnet, hereafter referred to as “Long”. The second category includes those decays in which track segments for the two pions are not found in the vertex detector, and use only the tracking stations downstream of the vertex detector (“Downstream”). The pions are required to have momentum p > 2 GeV/c and to form a vertex with χ2 vtx <12. In addition, for Downstream (Long) K0 S type the pions must have minimum χ2 IP with respect to any PV greater than 4 (9), and the pair must satisfy |m(π+π−)−mK0 S|<30 (20) MeV/c2, where mK0 Sis the known K0 S mass [1]. The K0 Scandidate is associated to the PV that minimises the χ2 IP, and the square of the separation distance between the K0 Svertex and the associated PV divided by its uncertainty (χ2 VS), must be greater than 50 (90) for Downstream (Long) candidates. For Downstream K0 Scandidates p > 6 GeV/c is also required. For both signal modes and the normalisation channel, the selection exploits the topology of the three-body decay and the bhadron kinematic properties. The scalar sum of the transverse momenta of the daughters is required to be greater than 3 GeV/c and at least two of the daughters must have pT>0.8 GeV/c. The IP of the charged daughter with the largest pTis required to be greater than 0.05 mm. The minimum for each pair of two daughters of the square of the distance of closest approach divided by its uncertainty must be less than 5. Furthermore, it is required that the bhadron candidate has χ2 vtx <12, χ2 IP <4, χ2 VS >50, that its vertex separation from the PV must be greater than 1 mm, that the cosine of the “pointing” angle between its momentum vector and the line joining its production and decay vertices must be greater than 0.9999, and that it has pT>1.5 GeV/c. Additional requirements are imposed to reduce background: the separation between the K0 Sand bhadron candidate vertices must be positive in the zdirection;2and the K0 Sflight distance must be greater than 15 mm. The bhadron candidates are required to have invariant mass within the ranges 5469 < m(K0 Sph−)<5938 MeV/c2, evaluated for both h=K, π hypotheses, and 4779 < m(K0 Sπ+π−)<5866 MeV/c2. To avoid potential biases during the selection optimisation, regions of ±50 MeV/c2(cf. the typical resolution of 15 MeV/c2) around both the Λ0 band Ξ0 bknown masses were not examined until the selection criteria were established. 2The zaxis points along the beam line from the interaction region through the LHCb detector. – 3 – JHEP04(2014)087 Further separation of signal from combinatorial background candidates is achieved with a boosted decision tree (BDT) multivariate classifier [27,28]. The BDT is trained using the B0→K0 Sπ+π−control channel as a proxy for the signal decays, with simulated samples used for the signal and data from the sideband region 5420 < m(K0 Sπ+π−)<5866 MeV/c2 for the background. Potential baryonic contributions in the sidebands from Λ0 b→K0 Spπ− and Λ+ c→K0 Spdecays are reduced by vetoing the relevant invariant masses in appropriate ranges. In order to avoid bias in the training, the sample is split randomly into two, and two separate BDT trainings are used. The set of input variables is chosen to optimise the performance of the algorithm, and to minimise efficiency variation across the phase-space. The input variables for the BDTs are the pT,η,χ2 IP,χ2 VS, pointing angle and χ2 vtx of the bhadron candidate; the sum of the χ2 IP values of the h+and h−tracks (here h=π, K, p); and the χ2 IP,χ2 VS and χ2 vtx of the K0 Scandidate. The choice of the optimal BDT cut value is determined separately for each K0 Scategory, and separately for the charmless signal modes and for the channels containing intermediate Λ+ cor D− shadrons. An appropriate figure of merit for previously unobserved modes is [29], Q=sig a/2 + √B,(3.1) where a= 5 quantifies the target level of significance in units of standard deviations, sig is the efficiency of the signal selection determined from the simulation, and Bis the expected number of background events in the signal region, which is estimated by extrapolating the result of a fit to the invariant mass distribution of the data sidebands. An alternative optimisation approach, which minimises the expected upper limit [30], is also investigated and provides a similar result. Potential sources of remaining background are suppressed with particle identification (PID) criteria. This is of particular importance for reducing cross feed between the signal channels due to kaon/pion misidentification. Particle identification information is provided by the RICH detectors [16], in terms of the logarithm of the likelihood ratio between the kaon/proton and pion hypotheses (DLLKπ and DLLpπ). A tight DLLpπ criterion on the proton candidate suppresses most possible backgrounds from misidentified bhadron decays. An additional DLLKπ requirement is imposed to reduce cross feed between K0 Spπ− and K0 SpK−modes. In addition, candidates containing tracks with associated hits in the muon detectors are rejected. The DLL requirements are optimised using eq. (3.1), and their efficiencies are determined using high-purity data control samples of Λ→pπ−and D0→K−π+decays, reweighted according to the expected signal kinematic (momentum and pT) distributions from the simulation. The efficiency of the selection requirements is studied with simulation. A multibody decay can in general proceed through intermediate states and through a nonresonant amplitude. It is therefore necessary to model the variation of the efficiency, and to account for the distribution of signal events, over the phase-space of the decay. The phase-space of the decay of a spin-zero particle to three spin-zero particles can be completely described by the Dalitz plot [31] of any pair of the two-body invariant masses squared. The situation for a baryon decay is more complicated due to the spins of the initial and final state fermions, but – 4 – JHEP04(2014)087 the conventional Dalitz plot can still be used if spin effects are neglected.3For three-body b hadron decays, both signal decays and the dominant combinatorial backgrounds populate regions close to the kinematic boundaries of the conventional Dalitz plot. For more accurate modelling of those regions, it is convenient to transform to a rectangular space (hereafter referred to as the square Dalitz plot [33]) described by the variables m0and θ0where m0≡1 πarccos 2m(K0 Sp)−mmin(K0 Sp) mmax(K0 Sp)−mmin(K0 Sp)−1, θ0≡1 πθ(K0 Sp).(3.2) Here m(K0 Sp) is the invariant mass of the K0 Sand proton, mmax(K0 Sp) = mΛ0 b−mh−and mmin(K0 Sp) = mK0 S+mpare the boundaries of m(K0 Sp), θ(K0 Sp) is the angle between the pand the h−track in the K0 Sprest frame. Simulated events are binned in the square Dalitz plot variables in order to determine the selection efficiencies. If no significant bhadron signal is seen, the efficiency corresponding to a uniform distribution across the square Dalitz plot is used as the nominal value, and a systematic uncertainty is assigned due to the variation across the phase-space. When the signal yield has significance (evaluated as described in the next section) greater than 3 σ, the signal distribution in the square Dalitz plot is obtained with the sPlot technique [34] (with the bhadron candidate invariant mass used as the control variable), and the efficiency corresponding to the observed distribution is used. There is limited prior knowledge of the branching fractions of bbaryon decays that may form backgrounds to the current search. Numerous modes are investigated with simulation, and the only significant potential background contribution that is found to peak in the candidate mass distribution is from Λ0 b→Λ+ c(→pK−π+)h−decays, where the kaon is misidentified as a pion, and the πK pair can form a K0 Scandidate. To suppress this background, candidates that have pK−π+masses within 30 MeV/c2of the known Λ+ cmass are vetoed. The decays Λ0 b→Λ+ c(→pK0 S)h−and Λ0 b→D− s(→K0 SK−)pshare the same final state as the charmless signal modes and are removed by vetoing regions in m(K0 Sp) and m(K0 SK) within ±30 MeV/c2of the known Λ+ cand D− smasses. These vetoes are reversed to select and study the decay modes with intermediate charmed states. The additional requirement for the charmed modes reduces the combinatorial background. Therefore the optimal BDT requirement is obtained separately for each channel. The backgrounds to the normalisation channel are treated as in ref. [6]. The main contributions are considered to be charmless decays with an unreconstructed photon in the final state (e.g. B0→K0 Sπ+π−γor B0→η0(→ρ0γ)K0 S), charmless decays of B0or B+ mesons into two vector particles (e.g. B0→K∗0(→K0 Sπ0)ρ0and B+→K∗+(→K0 Sπ+)ρ0) where a soft pion is not reconstructed, and charmed decays (e.g. B−→D0(→K0 Sπ+π−)π−) where a pion is not reconstructed. 3Note that Λ0 bbaryons produced in pp collisions at √s= 7 TeV have been measured to have only a small degree of polarisation [32]. – 5 – JHEP04(2014)087 4 Fit model and results All signal and background yields are determined simultaneously by performing an unbinned extended maximum likelihood fit to the bhadron candidate invariant mass distribution of each final state and K0 Scategory. The probability density function (PDF) in each invariant mass distribution is defined as the sum of several components (signal, cross-feed contributions, combinatorial and other backgrounds), with shapes derived from simulation. Signal PDFs are known to have asymmetric tails that result from a combination of the effects of final state radiation and stochastic tracking imperfections. The Λ0 b(Ξ0 b)→K0 Sph− signal mass distributions are modelled by the sum of a “core” Gaussian and a bifurcated Gaussian function, that share the same mean value. The core resolution is allowed to be different for each K0 Scategory, whilst the two widths of the bifurcated Gaussian are common to Downstream and Long types. Alternative shapes are studied using simulation, and this choice is found to provide the most stable and accurate description for a given number of parameters. The significant yield of Λ0 b→Λ+ c(→pK0 S)π−decays allows a subset of fit parameters common to the unobserved bbaryon decays to be determined from data. The core width and the relative fraction between the Gaussian and bifurcated Gaussian component are therefore expressed in terms of the parameters obtained from the fit to Λ0 b→Λ+ c(→pK0 S)π− candidates, with deviations from those values allowed within ranges as seen in the simulation. Explicitly, the function used for each unobserved channel jand K0 Stype cis PDF(m;µ, σc core, σR, σL) = sc,j ffcG(m;µ, sc,j σσc core) + (1 −sc,j ffc)B(m;µ, σL, σR),(4.1) where mis the invariant mass of the bhadron candidate and Gand Brepresent the Gaussian and bifurcated Gaussian distributions respectively. The parameters σLand σR are respectively the left and right widths of the bifurcated Gaussian function, σc core and fcare the width and the fraction of the core Gaussian for Λ0 b→Λ+ c(→pK0 S)π−candidates, while sc,j σand sc,j fare the corresponding scale factors for the channel j, determined from simulation. The peak position µfor Λ0 bdecays is shared among all modes, while that for Ξ0 bdecays is fixed according to the measured Λ0 band Ξ0 bmass difference, mΞ0 b−mΛ0 b= 168.6±5.0 MeV/c2[1]. The scale factors for Λ0 band Ξ0 bsignal shapes are allowed to differ but are found to be consistent. The fit model and its stability are validated with ensembles of pseudo-experiments, and no significant bias is found. The normalisation channel is parametrised following ref. [6]. The signal distribution of the Bcandidate invariant mass is modelled by the sum of two Crystal Ball (CB) functions [35], where the power law tails are on opposite sides of the peak. The two CB functions are constrained to have the same peak position and resolution, which are floated in the fit. The tail parameters and the relative normalisation of the two CB functions are taken from the simulation and fixed in the fit to data. To account for B0 s→K0 Sπ+π−decays [6] an additional component, parametrised in the same way as the B0channel, is included. Its peak position is fixed according to the known B0 s−B0mass difference [1], its width is constrained to be the same as that seen for the B0mode to within the difference found in simulation, and its yield is allowed to vary independently. – 6 – JHEP04(2014)087 An exponential shape is used to describe the combinatorial background, which is treated as independent for each decay mode and K0 Stype. Cross-feed contributions are also considered for each K0 Sph−final state. For the normalisation channel, a contribution from B0 s→K0 SK±π∓decays is included, while yields of other possible misidentified backgrounds are found to be negligible [6]. Cross-feed and misidentified B0 s→K0 SK±π∓shapes are modelled by double CB functions, with independent peak positions and resolutions. The yields of these components are constrained to be consistent with the number of signal candidates in the corresponding correctly identified spectrum, multiplied by the relevant misidentification probability. The peaking backgrounds to the normalisation channel reported in section 3are modelled by a generalised ARGUS function [36] convolved with a Gaussian function with width determined from simulation. The yield of each contribution is constrained within uncertainty according to the corresponding efficiency and branching fraction. The results of the fit to data are shown in figure 1for Λ0 b(Ξ0 b)→K0 Sph−candidates, figure 2for Λ0 b→Λ+ c(→pK0 S)h−and Λ0 b→D− spcandidates and figure 3for the B0→ K0 Sπ+π−normalisation channel, separated by K0 Stype. The fitted yields and relevant efficiencies are gathered in table 1. The statistical significance of each signal is computed as p2 ln(Lsig/L0), where Lsig and L0are the likelihoods from the nominal fit and from the fit omitting the signal component, respectively. These statistical likelihood curves for each K0 Scategory are convolved with a Gaussian function of width given by the systematic uncertainty on the fit yield. The total significance, for Downstream and Long K0 Stypes combined, is found to be 8.6σand 2.1σfor Λ0 b→K0 Spπ−and Λ0 b→K0 SpK−decays, respectively. Moreover, the statistical significance for the Λ0 b→Λ+ c(→pK0 S)K−decay is found to be 9.4σand 8.0σfor Downstream and Long categories respectively, confirming the recent observation of this channel [7]. The significances of all other channels are below 2 σ. The Dalitz plot distribution of Λ0 b→K0 Spπ−decays, shown in figure 4, is obtained using the sPlot technique and applying event-by-event efficiency corrections based on the position of the decay in the square Dalitz plot. A structure at low pπ−invariant mass, which may originate from excited nucleon states, is apparent but there are no clear structures in the other two invariant mass combinations. 5 Systematic uncertainties The choice of normalisation channel is designed to minimise systematic uncertainties in the branching fraction determination. Since no bbaryon decay has been previously measured with sufficient precision to serve as a normalisation channel, the B0→K0 Sπ+π−channel is used. The remaining systematic uncertainties are summarised in table 2separately for each signal mode and K0 Stype. The efficiency determination procedures rely on the accuracy of the simulation. Uncertainties on the efficiencies arise due to the limited size of the simulation samples, differences between data and the simulation and, for the three-body modes, the variation of the efficiency over the phase-space. The selection algorithms exploit the difference between signal and background in several variables. For the pTand decay length variables, the distributions in data and simula- – 7 – JHEP04(2014)087 ] 2 c) [MeV/ − π p S 0 K(m 5500 5600 5700 5800 5900 ) 2 cCandidates / ( 16.75 MeV/ 0 20 40 60 80 100 LHCb S 0 KDownstream ] 2 c) [MeV/ − π p S 0 K(m 5500 5600 5700 5800 5900 ) 2 cCandidates / ( 16.75 MeV/ 0 10 20 30 40 50 60 LHCb S 0 KLong ] 2 c) [MeV/ − pK S 0 K(m 5500 5600 5700 5800 5900 ) 2 cCandidates / ( 16.75 MeV/ 0 5 10 15 20 25 30 35 40 LHCb S 0 KDownstream ] 2 c) [MeV/ − pK S 0 K(m 5500 5600 5700 5800 5900 ) 2 cCandidates / ( 16.75 MeV/ 0 2 4 6 8 10 12 14 16 18 20 22 LHCb S 0 KLong Figure 1. Invariant mass distribution of (top) K0 Spπ−and (bottom) K0 SpK−candidates for the (left) Downstream and (right) Long K0 Scategories after the final selection in the full data sample. Each significant component of the fit model is displayed: Λ0 bsignal (violet dot-dashed), Ξ0 bsignal (green dashed) and combinatorial background (red dotted). The overall fit is given by the solid blue line. Contributions with very small yields are not shown. tion are known to differ, which can lead to a bias in the estimated efficiency. The pTdistribution for Λ0 b→Λ+ cπ−decays in data is obtained with the sPlot technique, and compared to that in the simulation. The corresponding possible bias in the efficiency is assigned as systematic uncertainty to each decay. The value of the Λ0 blifetime used in the simulation differs from the most recent measurement [37]. A similar reweighting of the efficiency as done for the pTdistribution results in an estimate of the associated systematic uncertainty for the Λ0 bmodes. The Ξ0 blifetime is not yet measured, and no uncertainty is assigned to the value used in the simulation (1.42 ps) — unless the true lifetime is dramatically different from this value, the corresponding bias will in any case be negligible compared to other uncertainties. The uncertainties due to simulation, including also the small effect of limited simulation samples sizes, are combined in quadrature and listed as a single contribution in table 2. For modes without significant signals, the effect of efficiency variation across the phasespace (labelled ∆PHSP in table 2) is evaluated from the spread of the per-bin efficiency after dividing the square Dalitz plot in a coarse binning scheme. The large systematic uncertainties reflect the unknown distribution of signal events across the phase-space and the large efficiency variation. Conversely, the uncertainties on the normalisation and – 8 – JHEP04(2014)087 where N¯ f/f is the observed yield for Λ0 b/¯ Λ0 bdecays. To obtain the physical CP asymmetry, this has to be corrected for small detection (AD) and production (AP) asymmetries, ACP =ARAW CP −AP−AD. This can be conveniently achieved with Λ0 b→Λ+ c(→pK0 S)π−decays, which share the same final state as the mode of interest, and have negligible expected CP violation. The measured inclusive raw asymmetry for Λ0 b→Λ+ c(→pK0 S)π−decays is found to be ARAW CP =−0.047 ±0.027, indicating that the combined detection and production asymmetry is at the few percent level. The fitted raw asymmetry for Λ0 b→K0 Spπ−decays is ARAW CP = 0.17 ±0.13, where the uncertainty is statistical only. The raw asymmetry for each of the background components is found to be consistent with zero, as expected. Several sources of systematic uncertainties are considered. The uncertainty on AP+AD comes directly from the result of the fit to Λ0 b→Λ+ c(→pK0 S)π−decays. The effect of variations of the detection asymmetry with the decay kinematics, which can be slightly different for reconstructed Λ0 b→K0 Spπ−and Λ0 b→Λ+ c(→pK0 S)π−decays, is negligible. The possible variation of the CP asymmetry across the phase-space of the Λ0 b→K0 Spπ−decay, and the non-uniform efficiency results in a systematic uncertainty that is evaluated by weighting events using the sPlot technique and obtaining an efficiency-corrected value of ARAW CP . The 0.003 difference with respect to the nominal value is assigned as uncertainty. Effects related to the choices of signal and background models, and possible intrinsic fit biases, are evaluated in a similar way as for the branching fraction measurements, leading to an uncertainty of 0.001. These uncertainties are summed in quadrature to yield the total systematic uncertainty. The phase-space integrated CP asymmetry is found to be ACP (Λ0 b→K0 Spπ−)=0.22 ±0.13 (stat) ±0.03 (syst) , which is consistent with zero. 8 Conclusions Using a data sample collected by the LHCb experiment corresponding to an integrated luminosity of 1.0 fb−1of pp collisions at √s= 7 TeV, searches for the three-body charmless decay modes Λ0 b(Ξ0 b)→K0 Spπ−and Λ0 b(Ξ0 b)→K0 SpK−are performed. Decays with intermediate charmed hadrons giving the same final state are also investigated. The decay channel Λ0 b→K0 Spπ−is observed for the first time, with a significance of 8.6σ, allowing a measurement of its phase-space integrated CP asymmetry, which shows no significant deviation from zero. All presented results, except for those of the branching fractions of Λ0 b→Λ+ cπ−and Λ0 b→Λ+ cK−, are the first to date. The first observation of a charmless three-body decay of a bbaryon opens a new field of possible amplitude analyses and CP violation measurements that will be of great interest to study with larger data samples. Acknowledgments 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 – 15 – JHEP04(2014)087 at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 and Region Auvergne (France); BMBF, DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO (The Netherlands); SCSR (Poland); MEN/IFA (Romania); MinES, Rosatom, RFBR and NRC “Kurchatov Institute” (Russia); MinECo, XuntaGal and GENCAT (Spain); SNSF and SER (Switzerland); NAS Ukraine (Ukraine); STFC (United Kingdom); NSF (U.S.A.). We also acknowledge the support received from the ERC under FP7. 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Zvyagin37. 1Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China 4LAPP, Universit´e de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France 5Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France 7LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France 8LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, CNRS/IN2P3, Paris, France 9Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany 10 Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany 11 Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 12 School of Physics, University College Dublin, Dublin, Ireland 13 Sezione INFN di Bari, Bari, Italy 14 Sezione INFN di Bologna, Bologna, Italy 15 Sezione INFN di Cagliari, Cagliari, Italy 16 Sezione INFN di Ferrara, Ferrara, Italy 17 Sezione INFN di Firenze, Firenze, Italy 18 Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 19 Sezione INFN di Genova, Genova, Italy 20 Sezione INFN di Milano Bicocca, Milano, Italy 21 Sezione INFN di Padova, Padova, Italy 22 Sezione INFN di Pisa, Pisa, Italy 23 Sezione INFN di Roma Tor Vergata, Roma, Italy 24 Sezione INFN di Roma La Sapienza, Roma, Italy 25 Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland 26 AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science, Krak´ow, Poland 27 National Center for Nuclear Research (NCBJ), Warsaw, Poland 28 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 29 Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia 30 Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia 31 Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia – 21 – JHEP04(2014)087 32 Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 33 Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia 34 Institute for High Energy Physics (IHEP), Protvino, Russia 35 Universitat de Barcelona, Barcelona, Spain 36 Universidad de Santiago de Compostela, Santiago de Compostela, Spain 37 European Organization for Nuclear Research (CERN), Geneva, Switzerland 38 Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland 39 Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland 40 Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands 41 Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands 42 NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine 43 Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 44 University of Birmingham, Birmingham, United Kingdom 45 H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 46 Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 47 Department of Physics, University of Warwick, Coventry, United Kingdom 48 STFC Rutherford Appleton Laboratory, Didcot, United Kingdom 49 School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 50 School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 51 Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 52 Imperial College London, London, United Kingdom 53 School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 54 Department of Physics, University of Oxford, Oxford, United Kingdom 55 Massachusetts Institute of Technology, Cambridge, MA, United States 56 University of Cincinnati, Cincinnati, OH, United States 57 University of Maryland, College Park, MD, United States 58 Syracuse University, Syracuse, NY, United States 59 Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to 2 60 Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to 11 61 National Research Centre Kurchatov Institute, Moscow, Russia, associated to 30 62 KVI - University of Groningen, Groningen, The Netherlands, associated to 40 63 Celal Bayar University, Manisa, Turkey, associated to 37 aUniversidade Federal do Triˆangulo Mineiro (UFTM), Uberaba-MG, Brazil bP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia cUniversit`a di Bari, Bari, Italy dUniversit`a di Bologna, Bologna, Italy eUniversit`a di Cagliari, Cagliari, Italy fUniversit`a di Ferrara, Ferrara, Italy gUniversit`a di Firenze, Firenze, Italy hUniversit`a di Urbino, Urbino, Italy iUniversit`a di Modena e Reggio Emilia, Modena, Italy jUniversit`a di Genova, Genova, Italy kUniversit`a di Milano Bicocca, Milano, Italy lUniversit`a di Roma Tor Vergata, Roma, Italy mUniversit`a di Roma La Sapienza, Roma, Italy nUniversit`a della Basilicata, Potenza, Italy oAGH - University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Krak´ow, Poland pLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain – 22 – JHEP04(2014)087 qHanoi University of Science, Hanoi, Viet Nam rUniversit`a di Padova, Padova, Italy sUniversit`a di Pisa, Pisa, Italy tScuola Normale Superiore, Pisa, Italy – 23 –