Observation of the Λb 0→J/ψΛϕ decay in proton-proton collisions at √s = 13 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Observation of the 

→ ^J /ψ  φ ^{decay in} proton-proton collisions at

√ s = ^{13 TeV}

.TheCMS Collaboration^

CERN,Switzerland

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received9November2019

Receivedinrevisedform3January2020 Accepted6January2020

Availableonline9January2020 Editor:M.Doser

Keywords:

CMS bphysics

Heavy-flavorspectroscopy Computing

The observation ofthe ⁰_b→^J/ψφ ^{decayis reportedusing} proton-protoncollisiondata collectedat

√s=^{13 TeV by the CMS experiment at the LHC in2018,} corresponding toan integrated luminosity of60fb⁻¹.The ratio ofthe branchingfractions B(⁰b→^J/ψφ)/B(⁰b→ ψ(^2S))is measuredto be (8.26±⁰.90 (stat)±⁰.68 (syst)±⁰.11(B))×^{10−2,wherethefirst}uncertaintyisstatistical,thesecondis systematic,andthelastuncertaintyreflectstheuncertaintiesintheworld-averagebranchingfractionsof φ^andψ(^2S)decaystothereconstructedfinalstates.

©^{2020TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Studiesofb baryondecaysareofgreatimportanceforprobing thedynamicsof heavy-flavor decayprocesses.Since the observa- tion of the lightest b baryon ⁰_b by the UA1 Collaboration [1]

atthe CERN SppS, followedby extensivestudies at theFermilab Tevatron by the CDF [2–11] and D0 [12–17] Collaborations, the ATLAS,CMS, andLHCbexperiments haveaccomplishednumerous

⁰_b baryon studies, made possible by the large production cross sectionofbb pairsattheCERNLHC.Amongthesestudiesarepre- cision mass measurements of the groundand excited states [18, 19], as well as lifetime and polarization measurements [20–23].

Mostofthesestudieshavebeenperformedinthe⁰_b→^J/ψ^de- caychannel.Recently,anobservationofthe⁰_bbaryondecaytoan excitedcharmoniumstate⁰_b→ ψ(^2S)hasbeenreportedbythe ATLAS Collaboration [24], while theLHCb Collaboration observed other,higher-multiplicitydecaysinvolvingcharmoniumstates [25, 26]. Decays of the ⁰_b baryon also proved to be a rich sourceof exoticspectroscopy,ashasbeendemonstratedbytheobservation byLHCb [27,28] ofnewpentaquarkstatesPc(4312)⁺,Pc(4380)⁺, andP_c(4450)⁺ intheinvariant massdistributionoftheJ/ψ^{p sys-} temproduced in the ⁰_b→^J/ψ^{pK− decay. Furtherstudies of the}

⁰_b baryon decay modes involving charmonium states may shed

 E-mailaddress:cms-publication-committee-chair@cern.ch.

light on thestrong interaction processes inhadronicdecays of b baryonsandontheproductionofexoticmultiquarkstates.

This Letter reports the observation of the ⁰_b→^J/ψφ ^de- cay mode and the measurement of the branching fraction ra- tio B(⁰_b→^J/ψφ)/B(⁰_b→ ψ(^2S)), by the CMS experiment.

Here and thereafter, φ ^{refers to the} φ(¹⁰²⁰) meson. The J/ψ^,

, φ^{, and} ψ(^2S) candidates are reconstructed in μ⁺μ^{−, p}π^−, K⁺K⁻,andJ/ψπ⁺π^{−finalstates,}respectively.The⁰_b→ ψ(^2S)→ J/ψπ⁺π^−pπ⁻→ μ⁺μ⁻π⁺π^−pπ^{−decayisusedasthe}normalization channel,owingtoitssimilardecaytopology.

ThebranchingfractionratioB(⁰_b→^J/ψφ)/B(⁰_b→ ψ(^2S)) ismeasuredas:

B(⁰_b→J/ψφ)

B(⁰_b→ ψ(^2S))= ^N(⁰_b→J/ψφ) N(⁰_b→ ψ(^2S))

×(⁰_b→ ψ(2S))B(ψ(2S)→J/ψπ⁺π⁻)

(⁰_b→^J/ψφ)B(φ →^K+K−) , (1)

where N(⁰_b→^J/ψφ) ^{and N}(⁰_b→ ψ(^2S)) are the measured

⁰_b yields forthe signalandnormalizationchannels, respectively.

The terms (⁰_b→^J/ψφ) ^and (⁰_b→ ψ(^2S))are the respec- tive overall efficiencies that include the detector acceptance and the reconstruction efficiency. The branching fractions B(ψ(^2S)→ J/ψπ⁺π⁻) and B(φ →^K+K−) are taken from the Particle Data Group(PDG) [29].

https://doi.org/10.1016/j.physletb.2020.135203

0370-2693/©^{2020TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

The⁰_b→^J/ψφ^{decayisexpectedtoproceedviatheb}→^ccs process, similarly to the ⁰_b→^J/ψ ^{decay, but requires an ad-} ditional ss pair. Consequently, the measurement of its branching fractioncould enhancetheunderstandingofthefinal-statestrong interactionsinb baryondecaysandtestheavy-quarkeffectivethe- ory [30].Inaddition,the ⁰_b→^J/ψφ^{decayisa baryonicanalog} oftheB⁺→^J/ψφ^{K+decay,wherearichresonantstructureinthe} J/ψφ ^{system has been observed by several} experiments [31–34].

Therefore,detailedstudiesoftheJ/ψφspectrumproducedinbary- onic decaysmayprovide an importanttest for theproductionof thesestates.Recently,theexistenceofahidden-charmpentaquark spectrawas predictedfortheJ/ψ ^final state [35],which canbe investigatedinthe⁰_b→^J/ψφ^{decay,onceasufficientnumberof} signaleventsisaccumulated.

2. TheCMSdetector

The central feature of the CMS apparatus [36] is a supercon- ducting solenoid of 6 m internal diameter, providing a magnetic field of3.8 T.Withinthe solenoidvolumeare a siliconpixel and strip tracker,a leadtungstatecrystalelectromagnetic calorimeter, andabrass andscintillatorhadron calorimeter,eachcomposedof abarrelandtwoendcapsections.Forwardcalorimetersextendthe pseudorapidity(η)coverageprovidedbythebarrelandendcapde- tectors.Muonsaredetectedingas-ionizationchambersembedded inthe steelflux-returnyokeoutsidethe solenoid. Themain sub- detectorsusedforthepresentanalysisarethesilicontrackerand themuonsystem.

Thesilicontrackermeasureschargedparticleswithintherange

|η|<2.5.During the LHCrunning periodwhen thedata used in this Letter were recorded, the silicon tracker consisted of 1856 silicon pixel and 15 148 silicon strip detector modules. For non- isolatedparticleswithtransversemomentum1<p_T<10GeV and

|η_|<1.4,thetrackresolutionistypically1.5%inpT.

Muons are measured within |η_|<2.4, with detection planes made using three technologies: drift tubes, cathode strip cham- bers,andresistive-platechambers.Tracksinthemuonsystemare matchedtothose measuredinthesilicontracker.The relative pT resolutionismeasuredtobeintherange0.8–3.0%formuonswith pT<10GeV usedinthisanalysis,dependingonthemuon|η_|[37].

Events of interest are selected using a two-tiered triggersys- tem [38]. Thefirstlevel (L1),composedofcustom hardware pro- cessors, usesinformationfromthe calorimetersandmuon detec- tors toselect eventsata rateup to 100 kHz withina fixed time intervaloflessthan4μs.Thesecondlevel,knownasthehigh-level trigger (HLT), consistsof a farm of processors running a version of the full event reconstruction software optimized for fast pro- cessing, andreduces the event rateto around 1 kHz before data storage.

AmoredetaileddescriptionoftheCMSdetector,togetherwith adefinitionofthecoordinatesystemused andthe relevantkine- maticvariables,canbefoundinRef. [36].

3. Datasampleandeventselection

Theanalysisdescribed inthisLetterisbasedonadatasample of proton-protoncollisions at a center-of-mass energyof 13TeV, collectedwiththeCMSdetectorin2018andcorresponding toan integratedluminosityof60fb⁻¹.

Datawererecordedwithadedicatedtrigger,optimizedforthe selectionof b hadronsdecayingto J/ψ(μ⁺μ⁻)andtwo additional tracksfromthechargedhadronsemergingfromthedecay.TheL1 triggerrequirestwooppositely chargedmuonswith pT ofatleast 4GeV,ortwomuonsinthebarrelregion(|η_|<1.479)withoutany p_T threshold. At the HLT, a J/ψ ^{candidate decaying into a} μ⁺μ⁻

pairdisplacedfromtheinteractionpointisrequired,alongwithat leasttwo tracks consistentwiththe displacedvertex. Eachmuon pT is required to be at least 4GeV, while the dimuon pT is re- quired to exceed 6.9GeV.The J/ψ ^candidates reconstructed from dimuons arerequiredtohavean invariant massbetween2.9and 3.3GeV.Thethree-dimensionaldistanceofclosestapproachofthe two muonsto each other isrequired tobe lessthan 0.5 cm. The fitteddimuonvertexisrequiredtohaveatransversedecaylength significance Lxy(J/ψ)/σLxy(J/ψ)>3,where Lxy(J/ψ) ^and σLxy(J/ψ) are, respectively, the distance from the common vertex to the beam axisinthetransverseplaneanditsuncertainty.Finally,thedimuon vertexfit probability,calculatedusingthe χ² andthenumberof degreesoffreedomofthefit,isrequiredtoexceed10%,whilethe angle αbetweenthedimuonpT vectorandthedirectionconnect- ingthebeamaxisandthedimuonvertexinthetransverseplaneis requiredtosatisfycosα>0.9.Giventhelackofadedicatedkaon identification, thetwoadditionaltracksare assigneda kaonmass hypothesis andrequired to have p_T>0.8GeV, |η|<2.5, andan invariantmassinarangeof0.95–1.30GeV.

In thesubsequentoffline analysiswe followcloselythe selec- tion of Ref. [39]. The pT threshold on the two muon candidates of 4GeV and the requirement of |η_|<2.4 are kept. Two oppo- sitely charged muon candidatesare paired andrequired to origi- nate from a commonvertex. The vertexrequirements applied at the HLT are confirmed in the offline selection. Also both muon candidates must match those that triggered the event readout.

Dimuoncandidateswithaninvariantmasswithin100MeV,which corresponds to approximately four effective widths, around the J/ψ ^{mesonmass MPDG}J/ψ are selected (hereafter, M^PDG_X denotes the world-average massofhadronX [29]),andthe p_T oftheJ/ψ ^me- sonisrequiredtoexceed7GeV.

To reconstructa ⁰_b candidate,the J/ψ ^{candidateis combined} withtwo oppositely charged, high-purity [40] tracks, assumedto be kaon candidates, and a  candidate.The pT of the tracks is required to exceed 0.8GeV, and their invariant mass must sat- isfy 0.99<M(K⁺K⁻)<1.05GeV. The  candidates are formed from displaced two-prong vertices under the assumption of the

→^pπ^{− decay, asdescribed in Ref. [41]. Daughter particles of} the  candidateare refitted to a common vertex withtheir in- variant mass constrained to M_^PDG, and the vertex fit probabil- ity isrequired to exceed 1%. The protonmass is assignedto the higher-momentum daughtertrack.Toselectthecandidatesinthe

 signal region, the following additionalrequirement isapplied:

|^M(pπ⁻)−^MPDG_ |<7.5MeV.The widthofthiswindowischosen tocorrespondtoapproximatelythreetimestheeffectivewidthof thereconstructedcandidates.Inaddition,thecandidateisre- quiredtohaveatransversemomentuminexcessof1GeV.

Asthelaststepofthereconstruction,afittothecommonver- texofthecandidate,thetwokaontracks,andthedimuonpairis performed,withthedimuonmassconstrainedtoM^PDG_J/ψ ;thisvertex isreferredtoasthe⁰_b vertex.Thekinematicvertexfitprobability ofthe⁰_b candidateisrequiredtoexceed1%.Theselectedcandi- datesarerequiredtohavep_T(⁰_b)>10GeV.

Multiple proton-proton interactions in the same or nearby beam crossing (pileup) are presentin the data, withan average multiplicity of 32,resulting in multiple reconstructed vertices in an event.Thevertexwiththelowestthree-dimensionalanglebe- tween thelineconnectingthisvertexwiththe⁰_b vertexandthe

⁰_b candidate momentum is chosen as the primary vertex (PV).

The following requirement is used to select ⁰_b candidates con- sistent withoriginatingfromthePV:cosα(⁰_b,PV)>0.99,where

α(⁰_b,PV)isthetwo-dimensionalangleinthetransverseplanebe- tweenthe⁰_b candidatemomentumandthevectorpointingfrom

Fig. 1. Theinvariantmassdistributionsof(left)J/ψK⁺K⁻ and(right)background-subtractedK⁺K⁻.Thepointsarethedata,withtheverticalbarsgivingthestatistical uncertainties,andthelinesshowtheresultsofthefitsdescribedinthetext.

the PV to the ⁰_b vertex. The following requirement on the ⁰_b vertexdisplacement is also applied: L_xy(⁰_b)/σ_Lxy(0

b)>3,where Lxy(⁰_b) is thedistance betweenthe primary and⁰_b verticesin thetransverseplane,and σ_Lxy(⁰

b)isitsuncertainty.

Candidatedecaysforthenormalizationchannel⁰_b→ ψ(^2S), withψ(^2S)→^J/ψπ⁺π^{−,areselectedusingthesame}reconstruction chain.IdenticalrequirementsareusedtoselecttheJ/ψ ^candidate, π^{+ and}π^{− tracks,and}candidate.Anadditionalrequirementis placedon the J/ψπ⁺π^{− invariant mass,} |^M(J/ψπ⁺π⁻)−^MPDG_ψ(2S)|<

15MeV, to select ψ(^2S) candidates, where this window corre- spondstoapproximatelythreeeffectivewidthsofareconstructed ψ(^2S)candidate.

Incaseofmultiple⁰_b candidates perevent, theonewiththe highestvertexfitprobabilityischosenforboththesignalandnor- malizationchannels.Thereare18.9and7.4% ofeventswithtwoor morereconstructedcandidatesforsignal andnormalizationchan- nels, respectively. When there are two ormore candidates in an event, the MC simulation predicts that the correct candidate is chosen 84±^{5 and 93}±^{13% ofthe time for the signal andnor-} malizationchannels,respectively.

Tocalculatethereconstructionefficiency,astudybasedonsim- ulated signal events for both channels is performed. The events are generatedwith pythia 8.230 [42]. The ⁰_b baryon decays are modeled with evtgen [43] v1.6.0 for both the ⁰_b→^J/ψφ ^and

⁰_b → ψ(^2S) decay channels, following the three-body phase spacemodel.The eventsarethen passed throughadetailedCMS detectorsimulationbasedon Geant4 [44].

4. Signalyieldextraction

The invariant mass distribution of the ⁰_b→^J/ψ^{K+K− can-} didatesselectedusingthestrategy described intheprevious sec- tion isshown inFig. 1 (left). An unbinned,extended maximum- likelihoodfittoasignalplusbackgroundhypothesisisperformed onthisobservableandfurthermassdistributions.

The signal is described by a double-Gaussian function with a floating common mean and total normalization, while the two widthsandtherelativefractionofthetwoGaussianfunctionsare fixedtothevaluesobtainedfromsimulation.Thedouble-Gaussian function was chosen as a model that provides the best descrip- tionofthesimulatedsample.Thebackgroundisparameterizedby athird-orderBernsteinpolynomial.Thefitresultsinasignalyield of380±^{32 events.Thesignal} significanceiscalculatedto be9.7 standard deviations in the asymptotic approximation [45], using theprofilelikelihoodratioofthesignal plusbackgroundoverthe

background-onlyhypothesisasthetest statistic.Includingmodel- ing uncertainties inthe signal andbackgroundshapes (described inSection6)resultsinareductionofthesignificancevalueto9.4 standarddeviations.

There isa binwiththe yieldsignificantly higherthan theav- eragebackground levelin theleft panel ofFig. 1,justbelow the signal⁰_bpeak.Thelocalsignificanceoftheexcessisestimatedto be lessthan three standard deviations.Severalcross-checks have been performed to investigate this enhancement. The M(⁰_b→ J/ψ^K+K−)distributionwiththerequirementontheφ^candidates tohaveamasswithin10MeV ofthenominalvalueshowsnosig- nificant excess in this bin. The statistical significance of the ⁰_b signalis10.3standarddeviationsintheasymptoticapproximation.

Astatisticallyindependentdataset,collectedin2017,hasbeenex- aminedwiththe sameselection, andnosignificant excess below the ⁰_b peak was observed.As a resultofthesecross-checks, we attributetheexcesstoastatisticalfluctuation.

Anunbinnedlikelihoodfittothe M(⁰_b→^J/ψ^K+K−)observ- able is employed to separate the signal andbackground compo- nentsstatistically,whichisthenusedwiththesPlot technique [46]

to obtain the M(K⁺K⁻) data distributioncorresponding to signal

⁰_b→^J/ψ^{K+K− decays.Toextractthe} ⁰_b→^J/ψφ ^decayyield, the background-subtracted M(K⁺K⁻) distribution is fitted with theconvolutionofadouble-GaussianandrelativisticBreit–Wigner functions forthe φ ^{signal anda first-order Bernstein polynomial} forthe nonresonant component. The natural widthof theφ ^me- son isfixed to theworld-average value [29]. It was checkedthat the naturalwidth ofthe φ ^{mesonobtained fromthe fit whenit} was allowedtofloatwas consistentwiththeworld-averagevalue within the uncertainties.Both widthsandthe relative fractionof thetwoGaussiansarefixedtothevaluesobtainedfromfittingthe simulatedsignalsample.Thefitresultsinasignalyieldof286±²⁹ events.The M(K⁺K⁻)invariant mass distribution,along withthe resultofthefit,areshowninFig.1(right).

Fig.2displaystheinvariantmassdistributionof⁰_b→ ψ(^2S)

candidates. The points represent the data and the curve is the resultofthefit.Thesignalisdescribedbyadouble-Gaussianfunc- tion with floating common mean andtotal normalization, while theindividualwidthsandtherelativefractionofthetwoGaussians arefixedfromthefittoasimulatedsignalsample.Thebackground isdescribedbyathird-orderBernsteinpolynomialfunction.Thefit resultsinasignalyieldof884±37 events.Thenon-ψ(2S)contri- butioninthe⁰_b→^J/ψπ⁺π⁻signalwasestimatedtobenegligi- bleintheselectedmasswindow|^M(J/ψπ⁺π⁻)−^MPDG_ψ(2S)|<15MeV.