Charmless Hadronic Beauty Decays at LHCb - EPJ Web of Conferences

EPJ Web of Conferences 158, 01005 (2017) QFTHEP 2017

DOI: 10.1051/epjconf/201715801005

Charmless Hadronic Beauty Decays at LHCb Timothy Williams,1 on behalf of the LHCb collaboration 1

School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, United Kingdom

Abstract. A summary of six LHCb results on the topic of charmless hadronic b-hadron decays is presented. These are comprised of: a search for the decay B0s → KS0 K + K − and updated branching fraction measurements of B0(s) → KS0 h+ h− decays (h=K,π) [1]; the first observation of the decays B0 → ppπ+ π− , B0s → ppK + K − ,B0s → ppK + π− and strong evidence for the decay B0 → ppK + K − [2]; the first observation of the decay B0s → pΛK − [3]; a search for the decay B0s → φη [4]; the first observation of the decay Ξb− → pK − K − [5] and evidence for CP-violation in Λ0b → pπ− π+ π− decays [6].

1 Introduction Charmless b-hadron decays provide fertile ground for studying CP violation and searching for the effects of new physics. These decays can typically proceed via either b → u tree level diagrams or b → s, d gluonic loop level diagrams. The former of these transisitons is heavily suppressed by the CKM matrix element Vub , which means the tree and loop level diagrams often have similar amplitudes. The consequences of this are two fold; firstly the rates of these processes are sensitive to the presence of new physics entering into the loop diagrams and secondly CP-violation can arise from the interference of the tree and loop level diagrams. Therefore, by measuring branching fractions and CP-violation observables indirect searches for physics beyond the standard model can be performed. Furthermore, these measurements provide vital input for QCD calculations. An overview of recent results from the LHCb experiment on the topic of charmless b- hadron decays is presented. These all make use of the Run I dataset collected by the LHCb detector during the years 2011 and 2012 which correspond to integrated luminosities of approximately 1 fb−1 and 2 fb−1 respectively. The LHCb detector is a single-arm forward spectrometer with a pseudorapidity coverage of 2 < η < 5, which has an acceptance for bb pairs of ∼ 25%. The detector is specialised for studying particles containing beauty and charm quarks. The unique sub-detectors relevant for the studies presented here are: the silicon strip vertex locator (VELO) surrounding the beam pipe which gives an impact parameter resolution of (15 ± p29T ) µm [7] and the two ring imaging cherenkov sub-detectors (RICH) which provide excellent particle identification ability [8].

2 Updated Branching Fraction Measurements of B0(s) → KS0 h+ h− Decays

Several extensions to the standard model, which involve the presence of new particles, introduce additional weak phases to the amplitude of b → qqs ¯ (q=s,d,u) decays [9]. As these new physics effects e-mail: [email protected]

© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).


DOI: 10.1051/epjconf/201715801005

Upstream track

VELO VELO track

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Figure 1. The different track types at LHCb. KS0 particles that decay inside the VELO produce Long tracks and KS0 particles that decay outside the VELO produce downstream tracks [12].

only enter via the loop diagrams which dominate the amplitudes of these charmless decays, the overall weak phase of b → ccs decays would remain unchanged. Therefore, deviations in the values of weak phase measurements between those determined from b → qqs ¯ (q=s,d,u) decays and from b → ccs decays could be an indication of new physics. Mixing induced CP violation has been measured in charmed decays through channels such as B0 → J/ψ KS0 [10], and a time-dependent flavour tagged analysis of the three body Dalitz plot for B0 → KS0 π+ π− and B0 → KS0 K + K − decays would facilitate a comparison with charmless decays. Unfortunately, the size of the data samples currently available at LHCb mean a flavour tagged time-dependent analysis of these decay modes is not feasible. In fact, the B0s → KS0 K + K − is still unobserved. Therefore, the first aim is to observe all decay modes and develop selections that can later be used for Dalitz analyses. Previous LHCb results on these channels, making use of 1 fb−1 of 7 TeV data collected in 2011, made the first observation of the decay modes B0s → KS0 π+ π− , B0s → KS0 K ± π∓ and measured the branching fraction of all observed B0(s) → KS0 h+ h− decays relative to the well measured channel B0 → KS0 π+ π− [11]. Updated branching fraction results, which now include the 2 fb−1 of 8 TeV data collected in 2012, are presented below along with an updated search for the decay B0s → KS0 K + K − . Due to the long lived nature of the neutral KS0 particle, two separate reconstruction categories are used in this measurement. If the KS0 decays inside the LHCb VELO the tracks from the daughter particles can be fully reconstructed. However, if the KS0 decays outside of the VELO there is less precise tracking information available and the mass resolution of the reconstructed KS0 is broader. Consequently, separate mass fits are required for each reconstruction category. As depicted in Figure 1, when the KS0 decays outside of the VELO it is labelled a downstream event and when the KS0 decays inside the VELO it is labelled as a long event. A BDT classifier is used to separate signal and background events. The variables that enter into this classifier are deliberately chosen to avoid significant variations in efficiency over the phase space of the decay as such variations would be undesirable for future three body Dalitz plot analyses. There are two separate optimisations of the requirement on the BDT output for each final state; one for the favoured mode and one for the suppressed mode. It is possible for decays such as Λ0b → KS0 pπ− to remain present as mis-ID backgrounds, therefore particle identification requirements are imposed to

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Decay B → KS0 π+ π− B0 → KS0 K ± π∓ B0 → KS0 K + K − B0s → KS0 π+ π− B0s → KS0 K ± π∓ B0s → KS0 K + K − 0

Downstream Yield 2766 ± 66 261 ± 24 1133 ± 39 146 ± 19 1100 ± 41 12 ± 6

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Long Yield 1411 ± 45 160 ± 17 685 ± 29 74 ± 11 568 ± 28 7±4

Branching Fraction (6.1 ± 0.5 ± 0.7 ± 0.3) × 10−6 (27.2 ± 0.9 ± 01.6 ± 1.1) × 10−6 (9.5 ± 1.3 ± 1.5 ± 0.4) × 10−6 (84.3 ± 3.5 ± 7.4 ± 3.4) × 10−6 ∈ [0.4 − 2.5] × 10−6 at 90% C.L.

Table 1. Signal yield and branching fraction results for each of the B0(s) → KS0 h+ h− decay modes

remove such backgrounds. There can be further backgrounds from B and Λ0b decays where the decay has proceeded through an intermediate charm (charmonium) resonance, for example B0 → DKS0 . These backgrounds are removed with explicit mass vetos of width 30(48) MeV around the world average mass of the charm (charmonium) resonance. In order to extract the signal yields, the selected data samples are separated into 12 mutually exclusive categories; three separate final states, two different opitmisations and two different reconstruction categories. An unbinned simultaneous extended maximum likelihood fit to all categories is then performed. The signal decays are modelled with the sum of two Crystal Ball functions (CB) [13] and combinatorial background is modelled with a linear function. Cross-feed backgrounds, where events from one signal final state are mis-identified as another, are modelled with emperical shapes taken from simulated samples and Gaussian constraints are applied to the relative yield of these backgrounds. Partially reconstructed backgrounds from decays such as B0s → (K ∗0 → KS0 π0 )(K ∗0 → K − π+ ) are modelled with ARGUS functions [14]. The results of this fit for the favoured and suppressed optimisations are shown in Figure 2 and Figure 3 respectively. The resulting signal yields are shown in Table 1. After correcting efficiencies for the variation over the phase space of the decay, the ratio of branching fractions (relative to the decay B0 → KS0 π+ π− ) is determined separately for each reconstruction category. These are then combined for each decay mode by performing a minimum χ2 fit where the convariance matrix includes the statistical and systematic uncertainties. The world average branching fraction measurement, with the previous LHCb result omitted, for the B0 → KS0 π+ π− mode is then used to obtain the branching fraction results shown in Table 1. The significance of the observed signal yield for the B0s → KS0 K + K − mode is equivalent to 2.5 Gaussian standard deviations; this decay mode still remains unobserved. Therefore, a 90% confidence level interval is instead derived using the Feldman-Cousins method [15]. Despite not observing the B0s → KS0 K + K − mode, these branching fraction measurements offer improved precision over, whilst still being consistent with, the previous LHCb results [11]. Work is now underway to perform Daltiz plot analyses of the dominant decay modes - B0 → KS0 π+ π− , B0s → KS0 K ± π∓ and B0 → KS0 K + K − .

3 First Observation of the Decay B0s → pΛK − The inclusive branching fraction of B decays to baryonic final states is ∼ 7% of the total decay width; many experimental studies of baryonic B decays have been carried out. At previous e+ e− collider experiments the observation of many baryonic decays of B0 and B+ mesons have been reported [16, 17]. More recently, the first observation of a baryonic B+c decay was reported by the LHCb collaboration [18]. However, the baryonic decay of a B0s meson has never previously been observed. The only

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+ Kfavoured − , K 0 K ± ⇡mode ⌥ and K 0 + − Figuremass 1: Invariant mass distributions of, from top to bottom, KS0 Kthe Figure 2. Invariant distributions for each possible final state with selection. The DownS S⇡ ⇡ 0 summing the three periodson of the dataright(left). taking, withThe the B selection optimisation chosenare for shown by the stream(Long) candidates, reconstruction category is shown (B0s ) signal candidates the favoured decay modes for (left) downstream and (right) long KS0 reconstruction categories. magenta (orange) short dashed line, partially reconstructed backgrounds are shown by the red double dot dash In each plot, data are the black points with error bars and the total fit model is overlaid (solid line, the combinatorial shown by the grey dotpink dash(orange) line andshort-dashed the blue and greenlines, long dashed lines blue line). background The B 0 (Bs0 ) is signal components are the (dotted) are mis-ID backgrounds. The full fit ismisidentified shown by the solid line. [1] while fully reconstructed decays areblue the green and dark blue dashed lines close

to the B 0 and Bs0 peaks. The sum of the partially reconstructed contributions from B to open charm decays, charmless hadronic decays, B 0 ! ⌘ 0 KS0 and charmless radiative decays are the red dash triple-dotted lines. The combinatorial background contribution is the gray long-dash dotted line.

evidence for such a decay is reported by the Belle collaboration, where 4.4σ evidence for the decay − B0s → Λc Λπ+ was seen [19]. events in the fitted data sample. The average efficiency for each decay mode: One interesting aspect of baryonic B decays is the hierachy of branching fractions; multi-body 0 !K 0 h± h0⌥ NB(s) S baryonic B decays generally have higher "branching than two body baryonic , (7)decays. It is = corr fractions N 0 !K 0 h± h0⌥ 0 − B S (s) fraction of the order of 10−6 which, based on predicted that the decay Bs → pΛK has a branching similar charmless b decays, suggests it is likely to be accessible with the LHCb Run I dataset [20, 21]. 8 It is also predicted that CP violating asymmetries in the decay B0 → pΛπ− could be as high as ∼ 12% [22], which provides extra motivation for studying the B0(s) → pΛh family of decays. For these reasons, a search for the decay B0s → pΛK − has been performed using the LHCb Run I dataset. The branching fraction of the B0s → pΛK − is measured relative to the topologically very similar decay B0 → pΛπ− in order to reduce systematic uncertainties. The branching fraction of this decay, B(B0 → pΛπ− ) = (3.14 ± 0.29) × 10−6 , has been previously measured by the BaBar and Belle collaborations [23]. For the same reasons as described in Section 2, the data is separated into two reconstruction categories, depending on whether the Λ is reoncstructed inside or outside of the LHCb

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−, K 0 ± ⌥ and K 0 + − Figure 3. Invariant distributions for each stateKS0with suppressed mode Figure 2:mass Invariant mass distributions of, possible from top tofinal bottom, K + Kthe SK ⇡ S ⇡ ⇡ selection. The candidates, summing thecategory three periods of data with the selection optimisation chosen for Downstream(Long) reconstruction is shown ontaking, the right(left). The individual fit components follow the the suppressed decay modes for (left) downstream and (right) long KS0 reconstruction categories. same convention as Figure 2. [1]

Colours and line styles follow the same conventions as in Fig. 1

0 !K 0 h± h0⌥ is the fitted signal yield, is given for each signal decay in Table 1. They where NB(s) S

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0 0 VELO. Afterare applying vetoesperiod to remove events from the decay presentedexplicit for eachmass data-taking and KSbackground reconstruction category in Tables 3–5 B → Λc p, in Appendix A. to Their relative uncertainties due to the finiteThere size of are the simulated eventPID requirean MLP classifier is used reduce combinatorial background. then further samples vary from 1% ments applied to the p and K − / to π−20%. from the B meson, but no PID requirements are placed on the p The imperfections of the simulation are corrected 0for in several + − respects. Inaccuracies no evidence forincontamination KS → πby πweighting decays the hashbeen from the Λ because + of the tracking efficiency the simulation from are mitigated andfound. h0− tracks a correction factor obtained a data by calibration samplean [49]. An analogous The signal andbycontrol channel yields are from extracted performing extended maximum likelicorrection is applied for the KS0 tracking and vertex reconstruction A control hood fit to the signal and control channel simultaneously. The results efficiency. of these fits for both years of data taking and both reconstruction categories are shown in Figure 4. These mass fit models include components for: signal, combinatorial background9 and partially reconstructed background from the decays B0 → pΣπ− and B0s → pΣK − . It is clear from Figure 4 that signal events are unambiguously present in both channels; the resulting total yields are N(B0s → pΛK − ) = 234 ± 29 and N(B0 → pΛπ− ) = 519 ± 28. This is the first observation of a baryonic B0s decay, with a statistical significance of greater than 15 gaussian standard deviations. The branching fraction is subsequently measured to be,

fs B(B0s → pΛK − ) + B( B¯0s → pΛK − ) = 5.46 ± 0.61 ± 0.57 ± 0.50(B) ± 0.32( ) × 10−6 fd

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− for the anddistribution downstream categories. The −black represent the data, Figure 4. Leftsample (Right): Thecombined invariantlong mass of B0 → pΛπ (B0s points → pΛK ) channel candidate events. 0 the area solid is blue curve the result of the fit, the red dashed curve the Bbackgrounds s ! p⇤K contribution, The grey shaded combinatorial background, partially reconstructed are represented by the the black (magenta) dotted curve the B 0 ! p⇤⇡ (Bs0 ! p⌃ 0 K ) and the green dash-dotted dot and dot dash green and pink lines and signal is represented by black and red dashed lines. The full fit is 0 0 curve the contribution from B ! p⌃ ⇡ decays. The combinatorial background distribution is shown by the solid bluebyline. [3] indicated the shaded histogram.

Bs0 ! p⇤K decay. The combinatorial background yield and shape parameters are treated

independently each subsamplethe andsecond are allowed to vary in is thesystematic, fit. where the first uncertaintyin is statistical, uncertainty the third is due to the 1 presents the fitand to the p⇤h invariant mass distributions for allof subsamples control channel Figure branching fraction the fourth is due to the knowledge the ratio of B0s /B0 b 0 combined. Both B 0 ! f s p⇤⇡ and Bs ! p⇤K signals are prominent. In particular, quark fragmentation ( fd ).isThe sum of thea branching fractions from B0s and B0s is quoted the Bs0 !fractions p⇤K decay observed with statistical significance aboveboth 15 standard deviations,final estimated the it change in possible log-likelihood between fitsthe withflavour and without because the identical statesfrom mean is not to determine of the B0s meson the Bs0 ! p⇤K signal component [33]. It constitutes the first observation of a baryonic without a flavour tagged analysis. Bs0 decay. The yields summed over all subsamples are N (B 0 ! p⇤⇡ ) = 519 ± 28 and Another area the±study of baryonic B decays is threshold !interest p⇤K ) =in234 N (Bs0of 29, where the uncertainties are statistical only.enhancement in the baryon anti-baryon system. This technique effect hasispreviously beenthe observed by the Bellespace collaborations in The sPlot used to subtract background andBaBar obtain and the phase 0 − distributions 0 distribution of signal candidates. Figure 2 shows the m(p⇤) invariant mass several baryonic B decays [17]. This effect is investigated in the B → pΛπ and B → pΛK − decays s for the B 0 ! p⇤⇡ and Bs0 ! p⇤K candidates after correcting for the distribution by using the sPlot technique to perform a background subtraction and correct for efficiency variations selection efficiencies. Both distributions show a pronounced enhancement at threshold in across the phase space of the decay [24]. The corrected background invariant the baryon-antibaryon invariant mass,efficiency first suggested in Ref. and [5] and observed insubtracted several mass distributions forBthe p Λmodes. system are shown in Figure 5; clear threshold enhancement is observed. baryonic decay Normalized Weights

Normalized Weights

The sources of systematic uncertainty arise from the fit model, the knowledge of the selection efficiencies, and the uncertainties on the0.5 B 0 ! p⇤⇡ branching fraction and on 0.5 the ratio of hadronization probabilities f /f . Uncertainties on the selection efficiencies s d LHCb LHCb 0.4 arise0.4from residual di↵erences between data and simulation in the trigger, reconstruction, selection and particle identification. Additional uncertainties arise due to the limited size 0.3 0.3 of the simulation samples and the corresponding uncertainty on the distribution of the 0.2 0.2 efficiencies across the decay phase space. As the efficiencies depend on the signal decay-time distribution, the e↵ect coming from the di↵erent lifetimes of the Bs0 mass eigenstates has 0.1 0.1 been evaluated [34]. Pseudoexperiments are used to estimate the e↵ect of using alternative shapes0 for the fit components, of including additional backgrounds in the fit such as 0 0 0 0 0 partially reconstructed decays, excluding the B2500 ! p⌃ and B4000 2500 3000 3500 4000 and 4500 of5000 3000 ⇡ 3500 4500 K s ! p⌃ 2 decays that show no significant Intrinsic biases in the fitted signal[ MeV/c yields2are m(p⇤)contribution. [ MeV/c ] m(p⇤) ] Figure 2: Efficiency-corrected and background-subtracted m(p⇤) invariant mass distributions

Figure 5. Left Normalised, background subtracted andThe efficiency corrected m(p Λ) toinvariant mass for(Right): (left) B 0 ! p⇤⇡ and (right) Bs0 ! p⇤K distributions are normalized 5candidates. 0 − 0 − unity. distributions for the B → pΛπ (Bs → pΛK ) candidates. [3] 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. + − 4 Search The fortotal B0(s)systematic → pphuncertainty h decays on the Bs0 ! p⇤K branching fraction is given by the sum of all uncertainties added in quadrature and amounts to 10.5%; it is dominated by systematic on decay the fit model. As describedthe in Section 3 uncertainty the baryonic of a B0s meson has now been observed. However, a four 0 The uncertainty on the branching fractionand of previous the normalization decay, results for the B0 meson only body baryonic decay of the Bs meson is still to6 be observed B(B 0 ! p⇤⇡ ) = (3.14 ± 0.29) ⇥ 10 [25], is taken as a systematic uncertainty from external inputs. The 5.8% uncertainty on the latest fs /fd combination from LHCb, fs /fd = 0.259 ± 0.015 [35], is taken as a second source of systematic uncertainty from external inputs. 6 The Bs0 ! p⇤K branching fraction, determined relative to that of the B 0 ! p⇤⇡ normalization channel according to Eq. 1, is measured to be


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set an upper limit B(B0 → ppπ+ π− ) < 2.5 × 10−4 at 90% confidence level[25]. There is additional motivation in studying four body baryonic B meson decays because they are an ideal place to search for CP violation using triple-product correlations (TPCs); in contrast to the case of three body decays, they do not involve the spins of the final state particles. This motivation is strengthened by the first evidence of CP violation in baryonic B decays being seen in B+ → ppK + decays [26] and predictions of CP violation as high as 20% in other baryonic B decays [27]. A search for the family of decays B0(s) → pph+ h− is reported, where h is a kaon or pion. The branching fractions of these decays are measured relative to the decay B0 → J/ψ K ∗0 , where the resonances are reconstructed through the J/ψ → pp and K ∗0 → K + π− decay channels. In order to remove charm and charmonium resonances in the signal channels the requirement m(pp) < 2.85 GeV is imposed on the signal channels and specfic mass vetoes are applied to remove Λ+c and D0 decays. The selection further consists of a BDT, which makes use of 15 variables, to reduce combinatorial background and PID requirements remove mis-ID backgrounds. The signal yields are extracted with an extended maximum likelihood fit which is simultaneous across the invariant masses of all 3 possible final states: m(ppK + π− ), m(ppK + K − ) and m(ppπ+ π− ). A separate fit is performed for the B0 → J/ψ K ∗0 control channel which is multi-dimensional for the m(ppK + π− ), m(pp) and m(K + π− ) invariant masses. These fits are shown in Figure 6 and the resulting yields and branching fractions are shown in Table 2. This is the first observation of the decays B0 → ppπ+ π− and B0s → ppK + K − with significances of > 25σ. The decay B0s → ppK + π− is also observed with a significance of 6.5σ, there is strong evidence for the decay B0 → ppK + K − at the level of 4.1σ and evidence at the level of 2.6σ is seen for the B0s → ppπ+ π− decay. A limit is set on the B0s → ppπ+ π− channel by integrating the likelihood with a uniform prior in the region of positive branching fraction.

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Figure 6. Topright) Left,ppK⇡, Top Right and left) Bottom channel simultaneous mass fit to all three final states. B0 (bottom pp⇡⇡Left: finalSignal state and (bottom right) invariant mass distribution 0 0! 0 in the signal is shown the green line,ppK⇡ Bs signal by theThe dotresults dash red line of Bby J/ dotted K ⇤ (892) final state. of the fitsand are cross shownfeed withbackground blue solid is shown lines. In the firstBottom three figures signals for B 0 of andthe Bs03D decays are channel shown, respectively, with green by the dot dash purple line. Right: Projection control mass fit. [2] dotted and red dot-dashed lines, combinatorial backgrounds are shown with black dashed lines and cross-feed backgrounds are shown with violet dot-dashed lines. In the bottom right figure the normalization signal is shown with a green dotted line, the K⇡ S-wave component is displayed with a red dot-dashed line and the combinatorial background with a black dashed line.

where the B 0 ! J/ K ⇤ (892)0 branching fraction is taken from Ref. [35] and the others 7 from Ref. [11].


Channel B → ppK + K − B0 → ppK + π− B0 → ppπ+ π− B0s → ppK + K − B0s → ppK + π− B0s → ppπ+ π− 0

Yield 68 ± 17 4155 ± 83 902 ± 35 635 ± 32 246 ± 39 39 ± 16

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Branching Fraction/10−6 0.113 ± 0.028 ± 0.011 ± 0.008 5.9 ± 0.3 ± 0.3 ± 0.4 2.7 ± 0.1 ± 0.1 ± 0.2 4.2 ± 0.3 ± 0.2 ± 0.3 ± 0.3 1.3 ± 0.21 ± 0.11 ± 0.09 ± 0.08 < 0.66 at 90% CL

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0 + − Table 2. Signal yields and branching 0.08 fraction results for the six B(s) → pph h decays. The first uncertainty is statistical, the second systematic, the third is due to the uncertainty on the normalisation channel branching LHCb LHCb 0.07 fraction and the fourth (in the case of B0s mesons) is due to the ratio of B0s /B0 fragmentation fractions.

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0 distributions from (top left) ncy-corrected background-subtracted m(hh Figureand 7. Normalised, background subtracted and) efficiency corrected m(pp) distribution for the B0 → ppK + π− 0 0 op right) decay B !channel. pp⇡⇡, [2] and (bottom left) B ! ppK⇡ candidates and (bottom tributions from B 0 ! ppK⇡ candidates. All distributions are normalized to

5 Search for the decay B0s → η φ the efficiency the hardware stage the trigger. the efficiencies Currentof experimental knowledge of of B0s decays to a lightAs pseudoscalar (P) meson and a vector meson ignal decay-time the e↵ectpredictions coming from di↵erent lifetimes (V) is verydistribution, limited and theoretical for thethe branching fractions of such decays have large uncertainties and cover a large range. For example, a QCD factorisation approach eigenstates has been evaluated. The systematic uncertainties due to the can yield the result 0 −6 η φ) = (0.051.18 [28], whereas a perturbative QCD approach yields the prediction hadronsB(B are0s → also included. −0.19 ) × 10 +16.3 −6 ) × 10 (see Table 1 in Ref. [4] pphh for a summary of theoretical preB(B → η φ) = (20.0 0 s the four-body −9.1 , a search for charmless[29] baryonic decays B 0) ! has dictions). These large uncertainties are a consequence of the(scurrently limited knowledge of penguin t by the LHCb collaboration with a sample of proton-proton collision data contributions, the ω- φ mixing angle, the s-quark mass and form factors. Therefore, a measurement of −1 o an integrated luminosity of 3 fb . First forand would subsequently 0 B(Bs → η φ) would be a useful input to the observations knowledge of B0sare to φobtained form factors 0 0 0 0 ! pp⇡⇡, improve nonresonant B !ofppK⇡, Bsfraction ! ppKK and B !s →ppK⇡, while Pφ decays. the accuracy branching predictions fors B s reported for the B 0 ! ppKK mode and an upper limit is set on the nching fraction. In particular, four-body baryonic Bs0 decays are observed e and a threshold enhancement in the baryon-antibaryon mass spectra is 8 0 aryonic Bs decays [2].


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A search for the decay B0s → η φ is reported, where the η is reconstructed through the channel η → π+ π− γ and the φ through the channel φ → K + K − . The well studied decay B+ → K + η is used as a control channel due to its relatively large branching fraction B(B+ → K + η ) = (70.6 ± 2.5) × 10−6 and the presence of only combinatorial background [23, 30]. The selection makes use of a BDT with 9 variables to reject the majority of combinatorial background and particle identification requirements are imposed on the hadrons and the photon from the η . The signal yields are extracted with an extended maximum likelihood fit to the B0s → η φ and + B → K + η channels simultaneously. The PDF used in the B0s → η φ (B+ → K + η ) channel is two dimensional for the m(η φ) and m(π+ π− γ) (m(η K + ) and m(π+ π− γ)) variables. In the case of the B0s → η φ channel, there is an irreducible background from B0s → φφ decays where one of the φ mesons decays through φ → K + K − and the other through φ → π+ π− π0 but one of the photons from the decay of the π0 is not reconstructed. This background is therefore modelled in the fit with a two-dimensional Gaussian kernel function [31]. The fit results for the B0s → η φ channel are shown in Figure 8. Candidates/(8 MeV/c2)

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+5.0 + + 2 The yields from this fit are N(B0s → mηππφ) = −3.2 −3.8 and N(B → K η ) = 11081 ± 127, therefore γ [MeV/c ] 0 a uniform prior. The resulting limit is, bayesian upper limits are set on B(Bs → η φ) using + − 0 + − 900

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Figure 2: Distributions of the (top left) ⇡ ⇡ γ and (top right) ⌘ K K masses of the selected Bs0 ! ⌘ 0 φ candidates, as of the (bottom left) ⇡ +−6 ⇡ − γ and (bottom right) ⌘ 0 K + masses of 0 as well + candidates. η φ) < 0.82(1.01) × curves 10 represent at 90% the (95%) the selected B +B(B ! ⌘ 0sK→ The solid blue resultCL, of the simultaneous two-dimensional fit described in the text, with the following components: Bs0 ! ⌘ 0 φ and B + ! ⌘ 0 K + upper signals limit (red dashed), combinatorial backgrounds (blue dot-dot-dashed), combinatorial first to be set on B(B0s → η φ). This result is consistent with the lower backgrounds with real ⌘ 0 (green dotted), and Bs0 ! φφ background (black dot-dashed).

which is the end of the range of predictions available for this branching fraction; the central values of several predictions are inconsistent with this result, see for example Refs. [29, 32–34]. This result therefore provides very Sets pseudoexperiments are used to to evaluate possible fit biases. Fits onand samples useful constraints onofthe theoretical models used predict branching fractions CP asymmetries from probability density (PDF) with the of charmless generated B0s decays to the a pseudoscalar and function vector meson finalparameters state. obtained from 0 data are found to be unbiased. The procedure is then repeated using simulated Bs ! events instead of generating the corresponding background component from the PDF. Biases of −1.3 ± 0.3 on the−signal− yield and−of − (−1.16 ± 0.33) ⇥ 10−4 on the ratio of yields 6 Search are forobserved. the decays The resultsΞobtained withph datahare corrected for these biases and systematic b , Ωb → uncertainties computed as the quadratic sum of the statistical uncertainty on the bias Prior to the collection of the dataset by LHCb, opportunities to study charmless decays of band half of the biasRun valueIare assigned. 0 0 − Additional systematic uncertainties signal yield and thewere yield Λ ratio. The −mass baryons were very limited. Consequently, thea↵ect onlythe observed decays b → pπ and Λb → pK 0 0 0 − fit is repeated with di↵erent background PDFs: linear are replaced by the CDF collaboration [35]. Morecombinatorial recently, LHCb has observed thefunctions Λb decays Λb → pKS π [36], with0 exponential and is replaced with a third-order + − functions, + the − parabolic function 0 Λb → ΛK The π ,quadratic Λ0b → ΛK K [37] and Λ → Λφ [38]. There has also been evidence at Λ0b → Λπ+ π− ,polynomial. sum of the di↵erencesb between the values obtained in these 0 − − → Λη [21]. However a charmless decay of a Ξ the level of 3σ seen for the decay Λ alternative fits and the nominal result is assigned as a systematic uncertainty. The limited b or Ωb baryon is b 0 size of the simulated Bs ! sample leads to an uncertainty on the determination of the nonparametric PDF for the physics background, which is propagated as a systematic uncertainty. The e↵ect of fixing some of the model parameters in the fit is studied by performing a large number of fits on the 9 data, with the fixed parameters sampled


DOI: 10.1051/epjconf/201715801005

yet to be observed, therefore a search for the decays Ξb− , Ωb− → ph− h− where h=K − , π− is presented. It is not currently possible to measure the absolute branching fractions of Ξb− and Ωb− decays because the fragmentation fractions, fΞb− and fΩb− , are not known. This search uses the topologically similar and well measured decay B− → K + K − K − as a control channel, therefore the reported results are the product of branching fraction and ratio of fragmentation fractions, denoted R ph− h− . The event selection firstly consists of a neural network, which makes use of 8 variables, to reduce combinatorial background. A tight particle identification requirement is then imposed on the proton to reject background from B− → K + h− h− decays. Further particle identification requirements on the other hadrons are optimised in a manner that ensures mutually exclusive samples for each possible final state. There are then vetoes applied to remove charm backgrounds from decays such as Ξb− → Ξc0 h− → pK − h− and B− → D0 K − → K + K − K − . The signal yields are extracted using an extended unbinned maximum likelihood fit which is simultaneous across the invariant mass distributions of the three possible final states (m(pK − K − ), m(pK − π− ) and m(pπ− π− )). The use of a simultaneous fit allows cross-feed background, where candidates from one final state are mis-identified as another, to be constrained to expected values. The control channel yield is extracted with a separate fit to the m(K + K − K − ) distribution. There are also partially reconstructed backgrounds present that have to be modelled in the fit. In the m(ph− h− ) distributions these arise from decays such as Ξb− → N + h− h− → pπ0 h− h− where the π0 is not reconstructed. These backgrounds are modelled with an ARGUS function [14] convolved with a Gaussian function. The projections of the simultaneous fit and the separate control channel fit are shown in Figure 9; the fitted yields and subsequent branching fraction times fragmentation fraction ratios are shown in Table 3. The significance of the Ξb− → pK − K − signal yield is 8.7σ; this is the first observation of a charmless Ξb− decay. There is also 3.4σ evidence for the decay Ξb− → pK − π− , but all other signal yields have a significance of < 2.0σ meaning the charmless decay of an Ωb− baryon still remains unobserved. For these channels with signifiance < 2.0σ an upper limit is set on R ph− h− by integrating the likelihood after multiplying by a prior probability distribution which is uniform in the positive region. Although it is not possible to measure absolute branching fractions, the observation of the Ξb− → − − pK K decay means it is possible to measure the relative branching fractions of the Ξb− decays. This cancels the dependance on fragmentation fractions, which is useful for comparisons with theoretical predictions. The measured relative branching fractions are, B(Ξb− → pK − π− ) = 0.98 ± 0.27(stat) ± 0.09(syst), B(Ξb− → pK − K − ) B(Ξb− → pπ− π− ) = 0.28 ± 0.16(stat) ± 0.13(syst) < 0.56(0.63), B(Ξb− → pK − K − ) where the upper limit quoted is at the 90%(95%) CL. It is of interest to study the intermediate resonances that occur in the Ξb− → pK − K − channel; it is hoped that when larger data samples are available an amplitude analysis to dis-entangle the intermediate resonances and to search for CP violation can be performed. Whilst this is not currently possible, it is still of interest to inspect the intermediate resonances. Figure 10 shows the background subtracted and efficiency corrected m(pK − )min distribution for the Ξb− → pK − K − channel; m(pK − )min is the smaller of the two possible m(pK − ) values for each candidate. This distribution shows clear peaks consistent with the Λ(1520) and a combination of the Λ(1670) and Λ(1690) states. The less prominent peaks at higher mass suggest the possibility of further states being present.

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Figure 2: Mass distributions for b-hadron candidates in the (top left) pK K , (top right)

− − Figure 9. ToppK Left, Rightleft) andp⇡Bottom Projections fit to the m(ph h ) final states. ⇡ Top , (bottom ⇡ andLeft: (bottom right) K +of Kthe K simultaneous final states. Results of the fits are + − − and B distribution. (⌦b ) decays are pinkshown (light by the blue dark channel blue solid fit lines. SignalsKforK⌅b) mass Bottom Right:shown Thewith control to m(K Theshown full with fits are − lines, combinatorial greyline, long-dashed lines,components cross-feed are shown solid line, Ξb−green) and Bdashed signal components arebackgrounds shown by are theshown pink with dashed Ωb− signal backgrounds are shown with red dot-dashed lines, and partially reconstructed backgrounds are by the green shown dashedwith line, cross-feed backgrounds are shown by the red dot-dash line, partially reconstructed dark blue double-dot-dashed lines. backgrounds are shown by the blue double dot-dash line and combinatorial background is shown by the grey long dashed line. [5]

of reconstruction and selection efficiencies in the simulation has been validated with large control samples; the impact on the results of possible residual di↵erences between data and simulation is negligible. Channel Yield R ph− h− (10−5 ) For the ⌅b ! pK ! pK ⇡ and B ! K + K K channels, efficiency cor− K ,⌅ −b − Ξb → pK K 82.9 ± 10.4 265 ± 35 ± 47 rections for each candidate are−applied using the method of Ref. [46] to take the vari− − → pK π 59.6Using ± 16.0 259 ± 64the ± 49 Ξ b space into account. ation over the phase this procedure, efficiency-corrected π− )min 33.2 ± 17.9 shown < 147(166) Ξb− → pπ−m(pK and background-subtracted distribution in Fig. 3 is obtained from ⌅b ! pK K candidates. )min±indicates the< smaller K − m(pK −2.8 2.5 18(22)of the two m(pK ) Ωb− → pK −Here − candidate, − −evaluated with the ⌅b and the final-state particle masses values for each signal −7.6 ± 9.2 < 51(62) Ωb → pK π fixed to their known values [40, − 44]. The distribution contains a clear peak from the Ω− → pπ− πthat 20.1 ± 13.8 < 109(124) ⇤(1520) resonance,− ab structure is consistent with being a combination of the ⇤(1670) + − − → K K K 50490 ± 250 B and ⇤(1690) states, and possible additional contributions at higher mass. Compared to the pK structures seen in the amplitude analysis of ⇤0b ! J/ pK [47], the contributions − is the product of Table 3. Yield andbroad R ph− h−⇤(1600) resultsand for each of the decay channels R ph− hamplitude from the ⇤(1810) states appear to be studied, smaller. where A detailed fΞ − /Ω− analysis will be of interest when are available. − −larger samples − b b branching fraction and the ratio of Ξb /Ωb and B fragmentation fractions f − . The limits set are at the B over phase space For channels without significant signal yields the efficiency averaged (90%)95% confidence level. is assigned from the variation is used in Eq. (1). A corresponding systematic uncertainty of the efficiency over the phase space; this is the dominant source of systematic uncertainty

7 Search for CP violation in Λ0b → pπ− 5π+ π− decays

The levels of CP violation predicted by the standard model, through the CKM mechanism, are not enough to explain the matter-antimatter imbalance in the universe. It is therefore of great interest to search for additional sources of CP violation to explain the existence of a matter dominated universe. Thus far, CP violation has been observed in B0s [39], B+ [40] and B0 [41] meson decays but has not been seen in the decay of a Λ0b baryon. The LHC produces a copious amount of Λ0b baryons; approximately 20% of all b hadrons prodcued are Λ0b baryons. Therefore, LHCb exploits these unprecedented Λ0b samples by searching for CP violation in Λ0b → pπ− π+ π− decays.

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Figure 10. Background subtracted and efficiency corrected m(pK − )min distributionmin for Ξb− → pK − K − candidates. ⌅b ! pK K candidates. [5] Table 1: Fitted yields, efficiencies and relative branching fractions multiplied by fragmentation fractions (Rph− h0− ). The two uncertainties quoted on Rph− h0− are statistical and systematic. ThisUpper search is performed studying asymmetries Tˆ operator, which is the unitary limits are quoted by at 90 (95) % confidence levelin forthe modes with signal significance less thanoperator 1 triple products are defined, that reverses both momentum spin three-vectors 3 σ. Uncertainties on theand efficiencies are not given .asScalar only the relative uncertainties a↵ect the branching fraction measurements.

C¯ Tˆ = p p · (pπ+ × pπ− )

CTˆ = p p · (pπ− × pπ+ ),

(1)

Mode Yield N Efficiency ✏ (%) Rph− h0− (10 5 ) 0 ! momentum pK K ± 10.4 0.398h, in the Λ265centre-of-mass ± 35 ± 47 isb the of 82.9 the final state hadron, frame. The π− with where ph ⌅ b 0 − 59.6frame ± 16.0is used (π 0.293). Asymmetries 259 ± 64 ± then 49 be defined as, can the largest⌅momenta in the Λb rest b ! pK ⇡ f ast ⌅b ! p⇡ ⇡ 33.2 ± 17.9 0.573 74 ± 40 ± 36 < 147 (166) ⌦b ! pK N(C K ˆ > 0)−2.8 ± ˆ2.5 0.375 −N(− < ¯9 ±C¯ Tˆ9 >± 0)6− N(− ¯ 18 − N(C C¯ Tˆ 0) + N(C < 0) CTˆ (124) < 0) N(− Tˆ Tˆ > ⌦b ! p⇡ ⇡ Tˆ 20.1 ± 13.8 0.536 48 ±C33 ± 0) 28+ N(− < 109 B ! K +K K 50 490 0± 250 0.643 —

¯ is the number of Λ0 (Λb ) decays satisfying the given condition. Using these asymmetries, where N(N) b CP violation and P violation observables are defined:

for those channels. The quantities entering Eq. (1), and the results for Rph− h0− , are reported in Table 1. When1the signal significance ˆ is less 1than 3 σ, upper limits are set Tˆ −odd aCP = (A A¯ Tˆ ), aT −odd = (ATˆ + A¯ Tˆ ). (3) Tˆ −multiplying by integrating the likelihood by pa prior probability distribution that is 2 after 2 uniform in the region of positive branching fraction. Any deviation zero these operators would indicate thefitpresence of CP/P violation.of The Thefrom sources of in systematic uncertainty arise from the model and the knowledge the advantage of using triple product asymmetries is that they are largely insensitive to prodcution asymmetries efficiency. The fit model is changed by varying the fixed parameters of the model, using and charge asymmetries the LHCb detector. alternative shapescaused for thebycomponents, and by including components that are omitted the selection baseline fit. Intrinsic in to thereduce fitted combinatorial yields are investigated withand simulated The in event makes use ofbiases a BDT background optimised parpseudoexperiments, and areAn found to be negligible. in the efficiency arisem(pπ due − π+ π− ) ticle identification requirements. extended unbinnedUncertainties maximum likelihood fit to the 0 to the limited size of the simulation samples and possible residual di↵erences between data 0 invariant mass distribution is used to extract the total signal yield of both Λb and Λb decays. This fit is and simulation in the trigger and particle identification efficiencies [48]. Possible biases shown in Figure 11 and has several components: signal decays are modelled by a Gaussian core with in the results due to the vetoes of charm hadrons are also accounted for. The efficiency power law tails on [13]; background is modelled with the an exponential function; depends thecombinatorial signal decay-time distribution, and therefore precision of the ⌅b andpartially 0 decays where one particle is not reconstructed are modreconstructed backgrounds from five body Λ ⌦b lifetime measurements [40–42] is a bsource of uncertainty. Similarly, the pT distribution elled with an ARGUS [14] function convolved with a Gaussian and mis-ID backgrounds are modelled assumed for signal decays in the simulation a↵ects the efficiency. Since the pT spectra for

with shapes fixed from simulation. The signal yield from this fit is N(Λ0b → pπ− π+ π− ) = 6646 ± 105 and is the first observation of the decay Λ0b → pπ− π+ π− . 6 In order to extract the CP and P observables in Eqn. (3), the data sample is split into four categories 0 by Λ0b or Λb flavour and sign of CTˆ or C¯ Tˆ . An extended unbinned maximum likelihood fit is then 1 The

Tˆ operator is different to the time-reversal operator

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Figure 2: Reconstructed mass usedoverlayed. to extract Figure 11. The invariant mass distribution of Λ0b → pπ− π+ π−invariant candidates with thefits fit results The fitthe signa 0 − + −distributions for (a) Λ0 → pπ − π + π − and (b) Λ0 → pπ − K + K − dec invariant mass has several components: the Λb → pπ π π signal component is shown by the solid red line, combinatorial b b background is shown by theAblue line, partially 5 body shown fit dashed is overlaid on top reconstructed of the data backgrounds points, withfrom solid and decays dottedarelines describing − + − by the light blue double dotofdashed line,results mis-identified Λ0b →ofpK π components π events are shown by the in purple dashand listed the fit for each the described thedot text line and mis-identified B0 → K + π− π− π+ candidates are shownonly. by the brown dashed line. The full fit is shown by Uncertainties are statistical the solid blue line. [6]

reconstructed Λ0b decays are described by an empirical function [41] conv Gaussian function to account for resolution effects. The shapes of back performed simultaneously to all four categories extract the asymmetries ATˆ and A¯ Tˆ (defined other b-hadron decays todue to incorrectly identified particles, e.g. in kaons iden Tˆ −odd Tˆ −odd Eqn (2)), from which theorCPprotons and P observables and a are calculated (defined in Eqn (3)). identified aas kaons, are modelled using simulated events. These c p CP of Λ0b → pK − π + π − and B 00 → K +−π −+ π−− π + decays for the Λ0b → pπ − π + π − s Many resonances are expected in the four body Λb → pπ π π +decay and it is expected difsimilar final states for the Λ0b → pπIt− K K − sample, as shown in Fig. 2. ferent resonances will exhibit different levels of CP violation. is entirely possible that the CP contributions areif obtained from fits to data reconstructed violation from different these resonances would cancel a phase space integrated asymmetry was calcu- under th 0 mass hypotheses for the final-state particles. The signal lated, which would considerably reduce the sensitivity of this search. Therefore, differentyields regionsof Λb → p − + − 0 ˆ −odd and 1030 ˆ T105 Λb → pπ Kby K are 6646 56, respectively. of the phase space are investigated calculating the±aCP and aTp −odd±observables for two This sepa- is the firs rate binning schemes of of thethese phase decay space. modes. The first binning scheme, denoted “Scheme A”, makes use of two body invariant masses in order to exploit the from according decays suchto as Λ0b or Λ Signal candidates are strong split resonant into fourstructure categories ++ + B” consists in the angle Φ which is the angle between the p ∆(1232) → pπ . “Scheme the sign of CTofor10Cbins T in order to calculate the asymmetries defined in planes, shown in Figure 12. The objective of this scheme is to with probe C > 0 π−f ast and π−slow π+ decay and (2). The reconstruction efficiency for binning signal candidates T the interference betweenthat different resonant contributions. with CT < 0 within the statistical uncertainties of the control sample ˆ Tˆ −oddindicates 2 , which the binning detector and the program do C Tfor and aTp −oddthat in each scheme. A χreconstruction test is used to asFigure 13 shows the for results aCP − + Thiswith check is performed both on the Λ0bthe →p-values Λ+ (pK π )π − sess the compatibility ofmeasurement. each binning scheme the null hypothesis of CP symmetry; c −4 are 4.9 × 10−2 (2.0σ) andsample 7.1 × 10and (3.4σ) for the binningofschemes A and B respectively. The about com- 30 time on large samples simulated events, using yields 0 − + − → pπ π π decays is determined bined significance of thein measured level of CP violation in Λ data, which are generated with no CP asymmetry. The CP usasymmetry b + − T-odd 0 0 the 40,000 controlpseudoexperiments sample is aCP are (Λgenerated π ) = (0.15 ± 0.31)%, compatible with C /Λ ing a permutation test where with randomly assigned Λ c b b flavour. The product of The p-values from both binning compared asymmetries AT andschemes AT inisthe signal between samplesthese are pseudoexmeasured with a periments and the valuesunbinned obtained inmaximum data, this leads to a combined for CP violation in likelihood fit to significance the invariant mass distributions of 3.3σ. This is the first evidence for CP violation in a baryonic decay and Λ0b → pπ− π+ π− decays of signal categories, and are found to be uncorrelated. The values itof aTCP-odd a will be of great interest to study this asymmetry when data samples are available. . ATlarger then calculated fromfurther AT and In four-body particle decays, the CP asymmetries may vary over the ph to resonant contributions or their interference effects, possibly cancelling wh 13


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⇡+ p 0

⇡slow Figure 3: Definition of the

⇡fast angle. The decay planes formed by the p⇡fast and the ⇡slow ⇡ +

Figure 12. Asystems diagram theframe. defnition of the angle Φ which is used for binning scheme B. [6] in showing the ⇤0b rest The momenta of the particles, represented by vectors, determine the two decay planes and the angle

2 [−⇡, ⇡] [19] measures their relative orientation.

Asymmetries [%]

Asymmetries [%]

LHCb A Scheme B with background components are Scheme also measured in the fit; theyLHCb are found to be consistent 20 do not lead to significant systematic uncertainties 20 zero, and in the signal asymmetries. b b The values 0 of aTCP-odd and aTP -odd are then calculated from 0 ATb and ATb . In −four-body particle decays, the CP asymmetries 20 −20 may vary over the phase space due to resonant contributions or their interference e↵ects, possibly when integrated aTP -odd χ2/ndf=27.9/12 aTP -odd cancelling χ2/ndf=20.7/10 5 10 1 2 3 over the whole phase space. Therefore, the asymmetries are measured in di↵erent regions of phase space for the ⇤0b ! p⇡ ⇡ + ⇡ decay using two binning schemes, defined before 20 20 examing the data. Scheme A, defined in Table 1, is designed to isolate regions of phase 0 0 space according to their dominant resonant contributions. Scheme B exploits in more 20 interference of contributions which could −20be visible as a function of the angle detail−the -odd χ2/ndf=21.1/12 aTCPdecay χ+2/ndf=30.5/10 Φ between the planes formed by the p⇡fast and theaTCP ⇡-odd slow ⇡ systems, as illustrated in Fig. 3. Scheme B has bins of width ⇡/10 5 10 non-overlapping 10 1 in |Φ|. For 2 every bin 3 in each of the schemes, thePhase ⇤0b efficiencies for CTb > 0 and CTb < 0 are compared found space bin |Φand | [rad] to be equal within uncertainties, and likewise the ⇤0b efficiencies for C Tb > 0 and C Tb < 0. + The analysis technique isofvalidated on the ⇤0b !The ⇤+ (pK ⇡of )⇡ control sample, which Figure 4: Distributions the asymmetries. the fit in each region for of binning c results Figure 13. Left The CP(bottom) and P planes (top) violation asymmetries binning scheme A(B). [6] 0! -oddpK Tb-odd the (Right): angle Φ is by the ofaTPbthe ⇡ +for ⇡ ⇤for pairs, and aand p⇡and ⇡ +on ⇡ simulated decays are schemes A and Bdefined are shown. Thedecay asymmetries CP b signal events. represented by open boxes and filled circles, respectively. The error bars represent one standard + the statistical uncertainty resulting from the deviations, calculated asmeasured the sum in The asymmetries in quadrature ⇤0b ! p⇡ ⇡of ⇡ decays with these two binning schemes fitare to shown the invariant distribution and the systematic estimatedmeasurements. as described in in Fig.mass 4 and reported in Table 2, togetheruncertainties with the integrated 2 /ndf are quoted for the P - and CP -conserving hypotheses 8 Summary the The individually, values of the χthe Formain eachtext. scheme compatibility with the CP -symmetry hypothesis is b-odd for each binning scheme.of a χ2 test, with χ2 = RT V 1 R, where R is the array of aTCP evaluated by means The study of charmless band hadron decays continues towhich yield ismany interesting results, and and LHCb is measurements V is the covariance matrix, the sum of the statistical 0 0 + − covariance An average systematic uncertainty, → K his h family leading the systematic way in studying thismatrices. sector. The branching fractions results forwhose the Bevaluation S (s) discussed is assigned all bins. The systematicfraction uncertainties + be of decays have been below, updated and the for limits on the branching of theare B0sassumed → KS0 Kto K − have been fully correlated; their contribution is small compared to the statistical uncertainties. The narrowed. p-values These results set the foundations for further Dalitz plot studies ofschemes these decay channels. 4 + ⇡ . Binning of the CP -symmetry ⇥decay 10 2 and 7.1 ⇥ for ⇡ A and0 Table 1: Definition of binninghypothesis scheme Aare for4.9 the mode ⇤0b10 ! p⇡ → pΛK − Several baryonic decays of b mesons have been observed for the first time; the decay B s B, respectively, corresponding to statistical significances of 2.0the andresonant 3.4 Gaussian standard scheme A is defined to exploit0interference patterns arising from structure of0 the b 2 meson to be observed. T -odd The ++ ! was the first baryonic decay ofthe a χBregion four decays → ppπ+ π− , deviations (σ). A similar is performed on a∆(1232) measurements with p-values for decay. Bins 1-4 focus on by the p⇡ +body resonance. TheBother s test dominated P 3 2 0 +the− P -symmetry 0 + − hypothesis ofregions 5.8been ⇥ 10 (2.8σ) and ⇥ 10 (2.3σ), for scheme A up and B, Bs → ppKeight π and Bs → ppK have observed for2.4the first time, (5–8) which opportunities bins are defined to K study where p⇡ resonances are present onopen either side of 0 0 ! ⇡+ respectively. The overall significance forb CPV in ⇤decays. p⇡ | ⇡A ⇡stringent decays from thelimit results the ⇢(770) ⇡ four resonances (5–12). Further splitting |+lower or greater than ⇡/2has is been set b !for for CP violation studies in body baryonic meson upper

done to reduce potential dilution of0asymmetries, suggested Ref. [19]. Masses −6 in units φη ) =in0.82(1.01) × 10are at the 90%(95%) on the branching fraction of the decay Bs → φη , B(Bas0s → of GeV/c2 . CL, which is a very useful input to the theoretical 5models used to predict the properties of B0s → PV − − − of decays. In the baryonic the + ) observation + ⇡ →),pK + ⇡ is) the first observation m(p⇡slow ) of the decay m(⇡Ξ | | Phase space binsector, m(p⇡ b slow m(⇡K fast − a charmless Ξb decay and opens up opportunities to search for CP violation in charmless Ξ⇡b− decays. 1 (1.07, 1.23) (0, 2 ) Lastly, 3.3σ evidence for CP violation in Λ0b → pπ− π+ π− decays has been found; this ⇡) the first 2 (1.07, 1.23) ( ⇡2 , is evidence for CP violation in(1.23, a baryonic beauty decay. With further studies and larger data it 3 1.35) (0, ⇡2samples ) ⇡ 4 (1.23, contribute 1.35) , ⇡) is hoped these differences could to understanding the absence of anti-matter in(the universe. 2 5 6 7 8 9 10 11

(1.35, 5.34) (1.35, 5.34) (1.35, 5.34) (1.35, 5.34) (1.35, 5.34) (1.35, 5.34) (1.35, 5.34)

(1.07, 2.00) (1.07, 2.00) (1.07, 2.00) (1.07, 2.00) (2.00, 4.00) (2.00, 4.00) (2.00, 4.00)

m(⇡ + ⇡slow ) < 0.78 or m(⇡ + ⇡fast ) < 0.78 m(⇡ + ⇡slow ) < 0.78 or m(⇡ + ⇡fast ) < 0.78 m(⇡ + ⇡slow ) > 0.78 and m(⇡ + ⇡fast ) > 0.78 m(⇡ + ⇡slow ) > 0.78 and m(⇡ + ⇡fast ) > 0.78 m(⇡ + ⇡slow ) < 0.78 or m(⇡ + ⇡fast ) < 0.78 m(⇡ 14 + ⇡slow ) < 0.78 or m(⇡+ ⇡fast ) < 0.78 m(⇡ + ⇡slow ) > 0.78 and m(⇡ + ⇡fast ) > 0.78 +

+

(0, ⇡2 ) ( ⇡2 , ⇡) (0, ⇡2 ) ( ⇡2 , ⇡) (0, ⇡2 ) ( ⇡2 , ⇡) (0, ⇡2 ) ⇡


DOI: 10.1051/epjconf/201715801005

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DOI: 10.1051/epjconf/201715801005

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