Observation of the Lambda(0)(b) -> Lambda+cK+K-pi(-) decay

University of Groningen

Observation of the Lambda(0)(b) -> Lambda+cK+K-pi(-) decay

Aaij, R.; Adeva, B.; Adinolfi, M.; Ajaltouni, Z.; Akar, S.; Albrecht, J.; Alessio, F.; De Bruyn, K.;

Dufour, L.; Mulder, M

Published in:

Physics Letters B

DOI:

10.1016/j.physletb.2021.136172

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Publication date:

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Aaij, R., Adeva, B., Adinolfi, M., Ajaltouni, Z., Akar, S., Albrecht, J., Alessio, F., De Bruyn, K., Dufour, L.,

Mulder, M., Onderwater, C. J. G., Pellegrino, A., Tolk, S., van Veghel, M., & LHCb Collaboration (2021).

Observation of the Lambda(0)(b) -> Lambda+cK+K-pi(-) decay. Physics Letters B, 815, [136172].

https://doi.org/10.1016/j.physletb.2021.136172

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Physics Letters B 815 (2021) 136172

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Observation

of

the

Λ

_b

0

→ Λ

+

_c

K

+

K

−

π

−

decay

.

LHCb

Collaboration

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received4December2020

Receivedinrevisedform18February2021 Accepted18February2021

Availableonline23February2021 Editor:L.Rolandi

The Λ0_b→ Λ+cK+K−

π

− decay is observed for the first time using a data sample of proton-proton collisionsatcentre-of-massenergiesof√s=7 and8TeV collectedbytheLHCb detector,corresponding toanintegratedluminosityof3fb−1.TheratioofbranchingfractionsbetweentheΛ0_b→ Λ+cK+K−

π

− andtheΛ0_b→ Λ+cD−s decaysismeasuredtobe

B(Λ0_b→ Λ+_cK+K−

π

−) B(Λ0_b→ Λ+cD−s)

= (9.26±0.29±0.46±0.26)×10−2,

wherethefirstuncertaintyisstatistical,thesecondsystematicandthethirdisduetotheknowledgeof the D−s →K+K−

π

− branchingfraction.Nostructureonthe invariantmassdistributionoftheΛ+cK+ systemisfound,consistentwithnoopen-charmpentaquarksignature.

©2021TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Over the last two decades, a wealth of information has been accumulated on the decays of hadrons containing b quarks [1]. Measurementsoftheirdecayratesandpropertieshavebeenused to testthe Cabibbo-Kobayashi-Maskawamechanism [2,3] describ-ing weakinteractionswithintheStandardModel,andtoexamine various theoretical approaches, such asthe heavy quark effective theory [4] and thefactorizationhypothesis [5–8]. Althoughmany

b-hadron decays have been observed with their branching

frac-tions measured, a large number of them remains either unob-served or poorly measured, most notably decays of

Λ

0_b,

Ξ

b and

Ω

_b− baryons.Inthelast years,theLHCbexperimenthasobserved many new

Λ

0_b decays to final states such as

Λ

+c

π

−

π

+

π

− [9],

Λ

+_c

π

−p p [10],

Λ

+_c D−_s [11],

χ

c1p K−,

χ

c2p K−[12],

ψ(

2S

)

p K−and

J

/

ψ

π

+

π

−p K−[13].1

In thisLetter, the first observationof the

Λ

_b0

→ Λ

+c K+K−

π

− decay (referred to hereafter assignal channel)is reported, along with a measurement of its branching fraction relative to that of the

Λ

0_b

→ Λ

+c D−s decay(normalisationchannel).Theanalysisuses a data sample of proton-proton(pp) collisions atcentre-of-mass energies of

√

s

=

7 and 8 TeV collectedby the LHCb experiment, corresponding to an integrated luminosity of 3 fb−1. The obser-vation ofthe

Λ

0_b

→ Λ

+_cK+K−

π

− decay provides a laboratory to search for open-charm pentaquarks with valence quark content

1 _{Thecharge-conjugateprocessisimpliedthroughoutthisLetter.}

Fig. 1. FeynmandiagramoftheleadingcontributiontotheΛb0→ Λ+cK+K−π− sig-naldecay.

c

¯

suud that could decay strongly to the

Λ

+cK+ final state. These

statesareanaturalextensionofthethreenarrowpentaquark can-didateswithquark contentccuud observed

¯

in

Λ

0_b

→

J

/

ψ

p K− de-cays [14], with the c quark

¯

replaced by an

¯

s quark. The recent discoveryofa D+K− structureinB−

→

D−D+K−decays [15,16], consistentwithopen-charmtetraquarks,alsomotivatesthesearch foropen-charmpentaquarks.

Fig.1showstheleadingdiagramcontributingtothesignal de-cay. Contributions to the companion K+K−

π

− system could be through intermediate a−₁ mesons, such as the a1

(

1260

)

− state,

whichis found to dominatein B

→

D(∗)K∗0K− decays [17]. De-caysof

Σ

c0

→ Λ

+c

π

−oreven

Ξ

c0

→ Λ

+cK− could alsocontribute tothesignal.

https://doi.org/10.1016/j.physletb.2021.136172

0370-2693/©2021TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

2. Detectorandsimulation

The LHCb detector [18,19] is a single-armforward spectrom-eter covering the pseudorapidity range 2

<

η

<

5, designed for the study ofparticles containing b or c quarks.The detector in-cludesahigh-precisiontrackingsystemconsistingofasilicon-strip vertex (VELO) detector surrounding the pp interaction region, a large-areasilicon-stripdetectorlocatedupstreamofadipole mag-net with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream of the magnet. The tracking system provides a measurement of themomentum,p,ofchargedparticleswitharelativeuncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The minimum distance of a trackto a primary pp collision ver-tex (PV), the impact parameter (IP), is measured with a reso-lution of

(

15

+

29

/

pT

)

μ

m, where pT is the component of the

momentum transverse to the beam, in GeV/c. Different types of charged hadrons are distinguished using information from two ring-imagingCherenkovdetectors.Photons,electrons andhadrons are identified by a calorimeter systemconsisting of scintillating-pad and preshowerdetectors,an electromagneticanda hadronic calorimeter(HCAL).Muonsareidentifiedbyasystemcomposedof alternatinglayersofironandmultiwireproportionalchambers.

The onlineeventselectionis performedbya triggerbased on signal information only.The trigger consistsofa hardware stage, basedon informationfromthecalorimetersystem, followedbya software stage, whichapplies a full event reconstruction [20]. At the hardware trigger stage, referred to as L0 trigger in the fol-lowing, the

Λ

0_b

→ Λ

c+K+K−

π

− and

Λ

0b

→ Λ

+c D−s candidates are required to include a hadron having high transverse energy de-posited in the calorimeters. The transverse energy threshold is 3.5 GeV. Thesoftwaretrigger,alsonamedhigh-level trigger(HLT), requiresa two-,three- orfour-track vertexwithasignificant dis-placement fromany PV. At leastone charged particlemust have a large transverse momentum andbe inconsistent with originat-ing from any PV. A multivariate algorithm [21] is used for the identificationofdisplaced verticesconsistent withthedecayofa

b-hadron.

Simulation is used to model the effects of the detector ac-ceptanceandthe selection requirements,tovalidate thefit mod-els and to evaluate efficiencies. In the simulation, pp collisions

are generated using Pythia 8 [22] with a specific LHCb con-figuration [23]. Decays of unstable particles are described by EvtGen [24], in which final-state radiation is generated using Photos _{[25]. The interaction of the generated particles with the} detector, and its response, are implemented using the Geant4 toolkit [26] asdescribedinRef. [27].

3. Eventselection

Candidate

Λ

+_c andD−_s hadronsarereconstructedthroughtheir decaystothe p K−

π

+andK+K−

π

− finalstates,respectively.The offline candidateselection isperformedbyapplyingaloose pres-election,followedbyamultivariateanalysis(MVA)tofurther sup-presscombinatorialbackgroundoriginatingfromrandom combina-tions.Toreducesystematicuncertaintiesontheratioofefficiencies betweenthe signal andthenormalisation channels, theselection criteriaof

Λ

+c candidatesareidenticalbetweenthetwochannels.

A good-quality track with pT

>

100 MeV/c and p

>

1 GeV/c is

required foreach final-stateparticle. Protonsandantiprotonsare required to have a momentum greater than 10 GeV/c to improve their identification.Allfinal-stateparticlesare alsorequiredtobe inconsistent with originating from any PV by requiring a large

χ

2

IP,where

χ

IP2 isdefined asthe difference in the

χ

2 of a given

PV fit withandwithout the trackunder consideration. Each

Λ

+c baryon candidateis required to have at leastone decayproduct

with pT

>

500 MeV/c and p

>

5 GeV/c, a good-quality vertex (i.e.

small

χ

2

vtx), andinvariant mass within

±

15 MeV/c2 of theknown

Λ

+_c mass [1]. Forthe

Λ

+_c candidates, thesumof transverse mo-mentaoftheirdecayproductsmustexceed1.8 GeV/c.Theselection criteriafor D−_s candidates aresimilar to thoseof

Λ

+c candidates. The K+K−

π

−invariantmassisrequiredtobewithin

±

35 MeV/c2

fromtheknown D−_s mesonmass.

Thesignalchannelisreconstructedbycombining

Λ

+_c,K+, K−

and

π

− candidates, while the normalisation channel is recon-structed by combining a

Λ

+_c with a D−_s candidate. The combi-nationsabove form

Λ

0_b candidates, whichare required to havea small

χ

2

vtx and

χ

IP2, and a decay time with respect to its

asso-ciated PV greater than 0

.

2 ps. The associated PV is the one that givesthe smallest

χ

2

IP, wherethe

χ

IP2 denotes the IP significance

ofcandidate’s trajectory returned by the kinematicalfit. The an-gle between the

Λ

_b0 momentum and the vector pointing from the associated PV to the

Λ

0_b decay vertex,

θ

p, is required to be smallerthan 11mrad.The

Λ

0_b candidateis alsorequiredtohave atleastonefinal-state particlewith pT

>

1

.

7 GeV/c, andits decay

vertexsignificantly displaced fromanyPV. The latteris achieved byrequiringthesignificanceoftheflightdistancebetweenthe

Λ

0_b decayvertexandanyPVtobe largerthan4.Final-statetracksof signal andnormalisationcandidates mustpass stringent particle-identification requirements based on the information from RICH detectors,calorimetersystemand muonstations. Toreject tracks thatsharethesamesegmentintheVELOdetector,anytwotracks withthesamechargeusedtoformthe

Λ

0_b candidatearerequired tohaveanopeninganglelargerthan0.5mrad.Akinematicfit [28] ofthedecaychain constrains the

Λ

0_b candidateto originate from the associated PV and the

Λ

+_c candidate invariant mass to its knownvalue [1].

The

Λ

0_b candidatecould originate from B0

→

D+K+K−

π

− or

B0_s

→

D+_sK+K−

π

− decays, where a pion or kaon in

D+

→

K+

π

−

π

+ or D+_s

(

D+

)

→

K+K−

π

+ decays is misidenti-fiedasaproton.Thesebackgroundcontributionsarevetoedifthe invariant massesof the

Λ

+c and

Λ

b0 candidates,evaluated by re-placingtheprotonbyeitherthepionorkaonmasshypothesis,are within

±

15 MeV/c2_oftheknownD+

₍

_D+

s

)

massand

±

25 MeV/c2 of theknown B0

₍

_B0

s

)

mass [1].Thesevetoesare appliedtoboththe signal and the normalisation channels. For the signal decay, ad-ditionalvetoesare appliedifthe invariant massofthe K+

π

− or

K+K−

π

− companiontracksfallswithin

±

30 MeV/c2 ofthe D0 or

D−_s knownmass,respectively [1].

Reconstructedcandidates arefurther requiredtopassan MVA output threshold based upon a multilayer perceptron (MLP) fil-ter [29], designed to reject the combinatorial background. The MLP classifier is trained using a signal sample of simulated

Λ

0_b

→ Λ

+cK+K−

π

− decays tuned on datato reproduce correctly the

Λ

0_b production kinematics based on the pT and y

distribu-tionsandabackgroundsample takenfromtheuppersidebandof the

Λ

_b0 invariant mass spectrum in the range of 5.75 – 7 GeV/c2_.

Afour-bodyphase-spacesimulationisusedforthesignal sample tokeeptheMLPefficiencyasuniformaspossible,asincluding in-termediate resonancesinthe simulationcould potentially leadto small MLP efficiencies for less represented phase-space regions. The lower sideband is not used to avoid potential background contributions frompartially reconstructed decays.The MLP input includes thefollowing variables: pT sumof the

Λ

+c decay prod-ucts,minimal

χ

2

IPamongthe

Λ

c+decayproducts,minimal pT and

minimal

χ

2

IPamongthekaonsoriginatingdirectlyfromthe

Λ

b0 de-cay,pTand

χ

_IP2 ofthe

π

−fromthe

Λ

0_bdecay,pT sumofall

π

and K originating directlyfrom the

Λ

0_b decay,

χ

2

vtx of the

Λ

+c candi-date,

χ

2 _{ofthe flightdistancebetweenthe}

_Λ

0

b decayvertexand theassociatedPV,cos

θ

p,

χ

2 _{probabilityofthe}

_Λ

0

ver-LHCb Collaboration Physics Letters B 815 (2021) 136172

Fig. 2. Invariantmassdistributionof(a)Λ0

b→ Λ+cK+K−π− and(b)Λ0b→ Λ+cD−s candidates.Fitprojectionsareoverlaidasabluesolidline.For(a),theredsolidline representsthesignalcomponent,thebluedashedlineisthebackgroundduetorandomcombinations,andthevioletdottedlineisthecontributionfromΛ0

b→ Σc+K+K−π− decays.For(b),theredsolidlineisthenormalizationchannelcomponent,thevioletdottedlineisthe Λ0b→ Λ+cD∗−s background,thegreendashed-dottedlineisthe contributionfromΛ0b→ Λ+cK+K−π−decays,andthebluedashedlinerepresentscombinatorialbackground.

texfit,andthedifferenceoflongitudinalpositionbetweenthe

Λ

+_c andthe

Λ

0_b decayvertices.

The MLP response obtained from the training is also applied to the normalisation channel sample. The optimal thresholds on the MLP response are obtained for the signal and normalisation channels separately by maximising a figure-of-merit, defined as

S

/

√

S

+

B,whereS andB aretheexpectedsignalandbackground yieldsfor

Λ

0_bcandidateswithina

±

2

.

5

σ

masswindowaroundthe known

Λ

0_bmass [1],where

σ

isthemassresolutioncorresponding to about12 MeV/c2_{.Both S and B aredetermined bymultiplying}

theinitialyieldsofsignal andbackgroundwiththecorresponding MLPselectionefficienciesestimatedfromsimulationandsideband data,respectively.Theinitialsignalandbackgroundyieldsare ob-tainedfromapreliminaryfittothepreselecteddatasamplebefore theMLPrequirementapplied,wherethesignal

Λ

_b0peakisalready seeninthe

Λ

+cK+K−

π

− invariantmassdistribution.Theoptimal point corresponds toa signalefficiency of90%anda background rejectionof 85%.About 0.6%eventsinthe signal channelcontain multiple candidates, onlyone candidate is retainedby a random selection.

4. Signalyieldsandsearchforintermediatestates

The yields in both the signal andnormalisation channels are determined from an unbinned extended maximum-likelihood fit to the corresponding invariant mass spectraof the

Λ

+c K+K−

π

− system. The signal component is modelled by a sum of two Crystal Ball functions [30] with a common mean of the Gaus-sian cores, with tail parameters fixed to the values obtained from simulation. For both the signal and normalisation chan-nels, the combinatorial background is described by an expo-nential function, whose parameters are varied freely and al-lowed to be different between the signal and normalisation channels. For the signal channel, a significant contribution from

Λ

0_b

→ Σ

c+

[→ Λ

+c

π

0

]

K+K−

π

− decays ispresentin thelower in-variant mass region, which has the same final state as the

π

0

is not reconstructed. The shape of this background is obtained from a simulation of

Λ

_b0

→ Σ

c

(

2455

)

+K+K−

π

− decays. For the normalisation channel, the

Λ

0_b

→ Λ

+_c D∗−_s decay may be recon-structed as

Λ

0_b

→ Λ

+_cD_s− due to photon emission in the D∗−_s

decay. The shape of thisbackground is obtained from simulated

Λ

0_b

→ Λ

+cDs∗− decays. The signal decay can also contribute to the normalisation channel forming a background under the D−_s

mass peak. This background contribution is estimated from the

D−_s sidebandsofthenormalisationdatasample,wherethewidth of the sideband is chosen to be the same as that of the D−_s

mass window used in the normalisation channel selection. The

Fig. 3. InvariantmassdistributionsofΛ+cK+candidatesintheΛ0b→ Λ+cK+K−π− signalchannelforthe simulation(red line)andthe background-subtracteddata (bluepointswitherrorbars).

invariantmassdistributionsforthesignalandnormalisation chan-nelsare showninFig.2 withthefitprojects overlaid.The signal yieldsareobtainedtobeN

(Λ

0_b

→ Λ

+c K+K−

π

−

)

=

3400

±

80 and

N

(Λ

0_b

→ Λ

+_cD−_s

[

K+K−

π

−

])

=

2550

±

60, respectively,wherethe uncertaintiesarestatisticalonly.

Anopen-charmpentaquarkstate couldbe revealedasa struc-ture in the invariant mass distribution of the

Λ

+c K+ system, shownin Fig.3 fordata andsimulation. The datadistribution is backgroundsubtractedthroughthesPlot weightingtechnique [31], usingthe

Λ

+c K+K−

π

− invariantmassasdiscriminating variable. Nostructureisobserved.Afullamplitudeanalysisisneededto es-timatethe limitof thepentaquark contribution,whichis beyond thescopeofthisLetter.

Instead,arich structureofknownhadroncontributions is vis-ible in thebackground-subtracted invariant mass distributions of the

Λ

+c

π

−, K+

π

− and K+K−

π

− systems, shown inFig. 4. The

Σ

c

(

2455

)

0and

Σ

c

(

2520

)

0resonancesarevisibleinthe

Λ

+c

π

− dis-tribution. A large K∗

(

892

)

0 resonance is observed in the K+

π

−

projection.In the K+K−

π

− system,a broadpeakingstructure at about1

.

5 GeV/c2 isalsoobserved.Asimilar structureis alsoseen inB

→

D(∗)K∗0K−decaysbytheBelleexperiment [17],andis ex-plainedasthetailcontributionofthea1

(

1260

)

−resonance.

5. Branchingfractionratioandefficiencies

Theratioofthebranchingfractionsofthe

Λ

_b0

→ Λ

+cK+K−

π

− decayincludingresonance contributionswith respectto the nor-malisationchannelisdeterminedby

Fig. 4. Invariantmassdistributionsof(a)theΛ+cπ−,(b)K+π−,and(c)K+K−π− systemsintheΛ0b→ Λ+cK+K−π−signalchannel,forthebackground-subtracteddata. ThereddashedverticallinesindicatethevetomassintervalsforD0_mesonsintheK+_π−_{distribution,andD}−

s intheK+K−π−distribution.

B

(Λ

0_b

→ Λ

+_c K+K−

π

−

)

B

(Λ

_b0

→ Λ

+c D−s

)

=

N

(Λ

0b

→ Λ

+c K+K−

π

−

)

N

(Λ

_b0

→ Λ

+c Ds−

[

K+K−

π

−

])

×

(1)



tot

(Λ

0_b

→ Λ

+c D−s

[

K+K−

π

−

])



tot

(Λ

0_b

→ Λ

+c K+K−

π

−

)

×

B

(

D−_s

→

K+K−

π

−

),

where

B

standsforthebranchingfractionofthecorresponding de-cay.ThesignalandnormalisationyieldsarereportedinSec.4.The totalefficiencies



tot ofthesignalandthenormalisationchannels

aredeterminedbytheproduct



tot

=



acc

×



sel

×



L0

×



HLT

×



PID

,

(2)

where



acc accountsfor the LHCb geometrical acceptance,



sel is

the efficiency of reconstructing and selecting a candidate within the acceptance,



L0 is the L0 trigger efficiency for the selected

candidates,



HLT is the HLT efficiency for the selected

candi-dates passingtheL0triggerrequirement,and



PID isthe

particle-identification(PID)efficiencyfortheselectedcandidatesthat sur-vive all trigger requirements. All efficiencies except for



L0 and



PIDaredeterminedfromsimulation,andthe

(

pT

,

y

)

distributions

ofthe simulated

Λ

0_b baryonsareweighted tomatchthat ofdata, where y istherapidityofthecandidate.Theweightsareobtained usingthenormalisationchannelandappliedtothesignaldecay.

To take into account the resonance contributions to the sig-nal decay channel, the simulation uses a mixture of three de-cay modes:

Λ

_b0

→ Λ

+_ca1

(

1260

)

−

(

→

K∗0K−

)

,

Λ

+c K∗0K− and non-resonant four-body phase space. The fractions are determined by fitting the two-dimensional data distribution of K+

π

− and

K+K−

π

− invariantmasses.

The L0 efficiency of each hadron is computed using samples of well identified pions and kaons from D0

_→

_K−

_π

+ _decays and protons from

Λ

→

pπ− decays [32]. The efficiency is cal-culated in bins of transverse energy for the particles incident on the HCAL surface, separately for its inner and outer regions. The PID efficiency is determined by the calibration samples of

D∗+

→

D0

₍

_→

_K−

_π

+

₎

_π

+ _and

_Λ

_→

_pπ− _{decays andis evaluated} asafunctionoftrackmomentum,trackpseudorapidityandevent multiplicity,wherethelatterisrepresentedbythenumberofthe reconstructedtracksintheevent.

Theratiobetweenthetotalefficiencies forthesignal and nor-malisation channels in Eq. (1), is determined to be 0

.

78

±

0

.

02, where the uncertainty accounts only for the size of the simula-tionsample.Thevaluediffersfromunityprimarilyduetodifferent selectionefficienciesontheMVAresponsesforthesignaland nor-malisationchannels.

External inputs are used for the branching fractions

B(

D−_s

→

K+K−

π

−

)

= (

5

.

39

±

0

.

15

)

×

10−2 _{[1] and}

B(Λ

0

b

→ Λ

+c D−s

)

= (

1

.

10

±

0

.

10

)

×

10−2 [11]. In the latter case, whilethevalueismeasuredbytheLHCbcollaboration [11],its un-certainty isdominated by thebranching fractionof B0

_→

_D+_D−

s decays,andisessentially uncorrelatedwiththepresent measure-ment.

6. Systematicuncertainties

Allsystematicuncertaintiesonthemeasurementoftheratioof branching fractions are listed inTable 1. The total uncertaintyis determined fromthe sumofall contributions in quadrature.The dominantuncertaintyisrelatedtotheresonancestructurethatis notperfectlymodelled bythesimulation.

Uncertaintiesduetothefitmodelareconsidered.Forthe back-groundduetorandomcombinationsoffinal-stateparticlesinboth

LHCb Collaboration Physics Letters B 815 (2021) 136172

Table 1

Summaryofsystematicuncertaintiesontheratioofbranchingfractions.

Source Uncertainty (%)

Combinatorial background 0.9

Shape ofΛ0

b→ Σc+K+K−π−contribution 0.3

Λ0

b→ Λ+cK+K−π−background in normalisation channel 0.8

Signal fit model 0.5

Simulation sample size 2.5

PID efficiency 0.4

Trigger efficiency 0.1

(pT, y)weight 0.8

Track multiplicity weight 0.8

Λ+c Dalitz structure 1.4

Mixture fraction in simulation 0.2

Resonance structure 3.6

Multiple candidates 0.3

MVA selection 0.5

Total 4.9

thesignalandnormalisationchannels, theexponentialfunctionis replaced by a second-order polynomial function. From the com-parison tothedefaultresult, therelativeuncertaintyon theratio of branching fractions is 0.9%. In the signal channel, the uncer-tainty duetothe

Λ

_b0

→ Σ

_c+K+K−

π

− background contributionis assessedbyperformingthefitwithawidenedmassregion, result-inginarelativeuncertaintyof0.3%.Forthenormalisationchannel, changingtheyieldofthe

Λ

0_b

→ Λ

+_cK+K−

π

− contributionwithin itsuncertaintyresultsinarelative0

.

8% variation.

Thesystematicuncertaintyduetothemodelforbothsignaland normalisationchannels,isstudiedbychangingtoasingleHypatia function [33],wherethemeanandwidthparametersareleftfree whileallother parametersaretakenfromsimulation.Thisresults inarelativeuncertaintyof0

.

5%.

Theuncertaintiesontheratioofefficienciesareevaluated.The uncertainty dueto the finite simulationsample size isevaluated fromtheexpectedefficiencyvariation inbins of pT and y of the

Λ

0_bcandidateas

σ



=



i



i

(

1

−



i

)

Niwi





i Niwi

,

(3)

for each bin i, where Ni isthe number of generated events, wi isacorrectionweight,and



i isthecandidateefficiency.The nor-malisationof theweightsis chosensuch that thedenominator is equaltototalnumberofgeneratedeventswithouttheweighting. Therelativeuncertaintyisfoundtobe2

.

5%.

Pseudoexperiments areused toevaluate thesystematiceffects due to uncertainties on the weights or efficiencies in different bins. Foragiven source,manypseudoexperimentsare generated, in which each produces a new set ofweights or efficiencies ac-cordingtothecentralvaluesanduncertaintiesfollowingGaussian distributions.Theefficiencyratiobetweenthesignaland normali-sationchannelsisrecomputed.Theresultingefficiencyratiosfrom manypseudoexperimentsofthissourceproduceaGaussian distri-bution centering atthe baseline value. The standard deviationof the Gaussian distributionis takenasabsolute uncertaintyon the efficiency ratiofor the givensource. The procedure is applied to obtainthesystematicuncertaintyrelatedtothePIDandtrigger ef-ficienciesandto

(

pT

,

y

)

andtrackmultiplicityweighting.

Thetrackingefficiencyreturnedby thesimulationiscalibrated using adata-driven method [34].The uncertainty onthe calibra-tionsamplesizeispropagatedtotheefficiencyratiousing pseudo-experiments,resultinginasystematicuncertaintyof0.8%.Because the final states forsignal and normalisation modesare identical,

possible data-simulation differences in hadron interactions with thedetectormaterialareassumedtobenegligible.

The agreement between data and simulation for the

Λ

+_c

→

p K−

π

+ channel is tested by comparing the Dalitz structure. The signal simulation sample is weighted in the m

(

p K−

)

versus

m

(

K−

π

+

)

plane to match the distribution of the background-subtracteddata.Theuncertaintyrelatedtothelimitedsamplesize used for obtaining these weights is 1.1%, obtained from pseudo-experiments. The uncertainty related to the choice of binning is 0.8%,determinedbyusinganalternativebinning.Atotalof1.4%is assignedassystematicuncertainty.

Thecontributionsofthe

Λ

0_b decaysthroughthemixtureofthe threedecaymodesareconsideredwhengeneratingthesimulated eventsofthesignalchannel,andtheirfractionsareobtainedby fit-tingthetwo-dimensionaldistributionoftheK+K−

π

−andK+

π

−

systemsinthebackground-subtractedsignaldata.Thefractionsare changed according to the statistical uncertainty of the fit result, yielding0.2%ofrelativeuncertainty.

The simulation does not fully model the resonance structure,

e.g. the contribution of

Σ

c0 resonances, which is clearly seen in

the

Λ

+_c

π

− invariant mass distribution, as illustrated in Fig. 4. Byweighting thesimulation tomatchthem

(Λ

+_c

π

−

)

distribution in the data, a 1.3% variation of the ratio of branching fractions is found and assigned as systematic uncertainty. Besides, differ-ences between background-subtracted data andsimulated signal eventsarealsoobservedintheinvariantmassdistributionsofthe

Λ

+_c K+K− and K+K− systems. To account for this discrepancy, the simulatedsample is weighted according to the

Λ

+_c K+K− or

K+K− mass distribution of background-subtracted data, andthe ratioofbranchingfractionsisreevaluated.Thetwoprocedures re-turnchanges of 2.6%and 2.0%,respectively. The three valuesare added inquadrature to account for the uncertainty dueto reso-nancestructure.

Simulation does not account well for multiple candidates, whichisfoundto beabout0.6%ofthe datasampleinthe signal channel.Halfofthisfractionisassignedassystematicuncertainty duetotherandomchoicetoretainonlyonecandidate.

TheMVAselectioncriteriaareoptimizedseparatelyforthe sig-nalandnormalisationchannels.Asanalternativechoice,theMVA selectionofthe normalisationchannel isfixed tobe thesameas thatofthesignalchanneltotesttherobustnessoftheMVA selec-tion.Therelative variationofthe branchingfractionratiois0.5%, whichisassignedassystematicuncertainty.

7. Resultsandsummary

The first observation of the

Λ

0_b

→ Λ

_c+K+K−

π

− decay is presented, and the branching fraction is determined using the

Λ

0_b

→ Λ

+cD−s decay as a normalisation channel. The relative branchingfractionismeasuredtobe

B

(Λ

0_b

→ Λ

+c K+K−

π

−

)

B

(Λ

0_b

→ Λ

+c D−s

)

= (

9

.

26

±

0

.

29

±

0

.

46

±

0

.

26

)

×

10−2

,

wherethefirstuncertaintyisstatistical,thesecondsystematic,and thethirdisduetotheknowledgeofthe D−_s

→

K+K−

π

− branch-ingfraction [1].Usingthisratio,the

Λ

_b0

→ Λ

+c K+K−

π

−branching fractionisdeterminedtobe

B

(Λ

_b0

→ Λ

+c K+K−

π

−

)

= (

1

.

02

±

0

.

03

±

0

.

05

±

0

.

10

)

×

10−3

,

where the third term includes the uncertainty on the branching fractionofthe

Λ

0_b

→ Λ

+c D−s decay [1]. Theinvariant mass distri-butionofthe

Λ

+cK+systemisinspectedforpossiblestructuredue toopen-charmpentaquarks,andnocontributionisobserved.

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

Acknowledgements

We express our gratitude to our colleagues in the CERN ac-celerator departments for the excellent performance of the LHC. We thankthe technicaland administrative staff at the LHCb in-stitutes. We acknowledge support from CERN and from the na-tional agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); MOST and NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); NWO (Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MSHE (Russia); MICINN (Spain); SNSF andSER (Switzerland); NASU(Ukraine); STFC (United King-dom); DOE NP and NSF (USA). We acknowledge the computing resourcesthatareprovidedbyCERN,IN2P3(France),KITandDESY (Germany), INFN (Italy), SURF (Netherlands), PIC (Spain), GridPP (United Kingdom),RRCKIandYandexLLC(Russia), CSCS (Switzer-land),IFIN-HH(Romania),CBPF(Brazil),PL-GRID(Poland)andOSC (USA). We are indebtedto the communities behind the multiple open-source software packages on which we depend. Individual groups or members havereceived support fromAvH Foundation (Germany); EPLANET, Marie Skłodowska-Curie Actions and ERC (European Union); A*MIDEX, ANR, Labex P2IO and OCEVU, and Région Auvergne-Rhône-Alpes (France); Key Research Program of FrontierSciencesofCAS,CASPIFI,ThousandTalentsProgram,and Sci. & Tech. Program of Guangzhou (China); RFBR, RSF and Yan-dex LLC (Russia); GVA,XuntaGal and GENCAT (Spain); The Royal Society andtheLeverhulmeTrust(UnitedKingdom).

Appendix A. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttps://doi.org/10.1016/j.physletb.2021.136172.

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47

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V. Franco Lima

59

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M. Franco Sevilla

65

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M. Frank

47

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E. Franzoso

20

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G. Frau

16

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C. Frei

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D.A. Friday

58

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J. Fu

25

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Q. Fuehring

14

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W. Funk

47

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E. Gabriel

31

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T. Gaintseva

41

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45

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19

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57

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Y. Gan

3

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M. Gandelman

2

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P. Gandini

25

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Y. Gao

4

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M. Garau

26

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L.M. Garcia Martin

55

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P. Garcia Moreno

44

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J. García Pardiñas

49

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B. Garcia Plana

45

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11

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L. Garrido

44

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D. Gascon

44

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C. Gaspar

47

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31

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16

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L.L. Gerken

14

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E. Gersabeck

61

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M. Gersabeck

61

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T. Gershon

55

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D. Gerstel

10

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Ph. Ghez

8

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V. Gibson

54

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22

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A. Gioventù

45

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P. Gironella Gironell

44

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36

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20

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K. Gizdov

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E.L. Gkougkousis

47

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12

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69

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83

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D. Golubkov

38

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A. Golutvin

60

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1

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S. Gomez Fernandez

44

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F. Goncalves Abrantes

69

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M. Goncerz

33

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G. Gong

3

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38

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39

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24

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16

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44

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12

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E. Graugés

44

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E. Graverini

48

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21

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A. Grecu

36

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L.M. Greeven

31

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P. Griffith

20

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L. Grillo

61

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S. Gromov

80

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L. Gruber

47

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B.R. Gruberg Cazon

62

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C. Gu

3

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M. Guarise

20

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P.A. Günther

16

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E. Gushchin

40

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A. Guth

13

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Y. Guz

43

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47

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T. Gys

47

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T. Hadavizadeh

68

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G. Haefeli

48

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47

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J. Haimberger

47

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54

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T. Halewood-leagas

59

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P.M. Hamilton

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Q. Han

7

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X. Han

16

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T.H. Hancock

62

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16

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N. Harnew

62

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T. Harrison

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C. Hasse

47

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M. Hatch

47

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J. He

5

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M. Hecker

60

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K. Heijhoff

31

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K. Heinicke

14

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A.M. Hennequin

47

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K. Hennessy

59

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L. Henry

25

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46

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J. Heuel

13

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A. Hicheur

2

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D. Hill

62

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M. Hilton

61

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S.E. Hollitt

14

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P.H. Hopchev

48

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J. Hu

16

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J. Hu

71

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W. Hu

7

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W. Huang

5

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X. Huang

72

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W. Hulsbergen

31

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R.J. Hunter

55

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M. Hushchyn

81

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D. Hutchcroft

59

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D. Hynds

31

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P. Ibis

14

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M. Idzik

34

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D. Ilin

37

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P. Ilten

64

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A. Inglessi

37

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A. Ishteev

80

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K. Ivshin

37

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R. Jacobsson

47

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S. Jakobsen

47

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E. Jans

31

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B.K. Jashal

46

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A. Jawahery

65

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V. Jevtic

14

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M. Jezabek

33

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F. Jiang

3

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M. John

62

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D. Johnson

47

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C.R. Jones

54

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T.P. Jones

55

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B. Jost

47

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N. Jurik

47

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S. Kandybei

50

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Y. Kang

3

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M. Karacson

47

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M. Karpov

81

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N. Kazeev

81

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F. Keizer

54

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47

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M. Kenzie

55

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T. Ketel

32

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B. Khanji

47

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A. Kharisova

82

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S. Kholodenko

43

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K.E. Kim

67

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T. Kirn

13

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V.S. Kirsebom

48

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O. Kitouni

63

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S. Klaver

31

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K. Klimaszewski

35

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S. Koliiev

51

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A. Kondybayeva

80

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A. Konoplyannikov

38

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P. Kopciewicz

34

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R. Kopecna

16

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P. Koppenburg

31

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M. Korolev

39

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I. Kostiuk

31

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51

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O. Kot

51

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S. Kotriakhova

37

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30

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P. Kravchenko

37

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L. Kravchuk

40

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47

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M. Kreps

55

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F. Kress

60

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S. Kretzschmar

13

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P. Krokovny

42

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v

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W. Krupa

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W. Krzemien

35

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33

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M. Kucharczyk

33

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G.J. Kunde

66

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T. Kvaratskheliya

38

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D. Lacarrere

47

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G. Lafferty

61

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A. Lai

26

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A. Lampis

26

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D. Lancierini

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J.J. Lane

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R. Lane

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22

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C. Langenbruch

13

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J. Langer

14

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O. Lantwin

49

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80

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T. Latham

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28

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R. Le Gac

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S.H. Lee

84

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R. Lefèvre

9

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A. Leflat

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S. Legotin

80

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O. Leroy

10

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T. Lesiak

33

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B. Leverington

16

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H. Li

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62

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P. Li

16

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X. Li

66

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Y. Li

6

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Y. Li

6

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14

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26

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3

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J. Lomba Castro

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3

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20

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28

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8

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P. Marino

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J. Moron

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R. Mountain

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F. Muheim

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M. Mukherjee

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P. Muzzetto

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M. Needham

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28

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M. Palutan

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9

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23

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L. Pica

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M. Piccini

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B. Pietrzyk

8

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G. Pietrzyk

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M. Pili

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D. Pinci

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J. Pinzino

47

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F. Pisani

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10

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V. Placinta

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J. Plews

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M. Plo Casasus

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M. Poli Lener

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M. Poliakova

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A. Poluektov

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N. Polukhina

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E. Polycarpo

2

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G.J. Pomery

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D. Popov

5

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S. Popov

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K. Prasanth

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28

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5

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J. Qin

5

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R. Quagliani

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8

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2

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81

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G. Raven

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M. Reboud

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F. Reiss

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V. Renaudin

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J.A. Rodriguez Lopez

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N. Sagidova

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N. Sahoo

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26

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