Measurement of the branching ratio $\Gamma (\Lambda_{b}^{0}\rightarrow \psi (2S)\Lambda ^{0})/\Gamma (\Lambda _{b}^{0}\rightarrow J/\psi \Lambda ^{0})$ with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Measurement of the branching ratio

(

→ ψ( ^2S )

)/ (

→ ^J /ψ

) with the ATLAS detector

.ATLASCollaboration^

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

Articlehistory:

Received29July2015

Receivedinrevisedform1October2015 Accepted3October2015

Availableonline9October2015 Editor:W.-D.Schlatter

An observation of the ⁰_b→ ψ(^2S)⁰ decay and a comparison of its branching fraction with that of the ⁰_b→ ^J/ψ⁰ decay has been made with the ATLAS detector in proton–proton collisions at

√s=^{8 TeV at theLHCusing anintegratedluminosity of20.6 fb−1.The J}/ψ and ψ(2S) mesonsare reconstructed in their decays to a muon pair, while the ⁰→^pπ⁻ decay is exploited for the ⁰ baryonreconstruction.The⁰_b baryonsarereconstructedwithtransversemomentumpT>10 GeV and pseudorapidity|η_|<2.1.Themeasuredbranchingratioofthe_b⁰→ ψ(^2S)⁰and_b⁰→^J/ψ⁰decays is(⁰_b→ ψ(^2S)⁰)/ (⁰_b→ ^J/ψ⁰)=⁰.501±⁰.033(stat)±⁰.019(syst),lowerthantheexpectation fromthecovariantquarkmodel.

©^{2015CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

The ⁰_b baryon properties have been extensively studied at theLarge HadronCollider (LHC) [1–7]. The decaychannel ⁰_b→ J/ψ(μ⁺μ⁻)⁰(pπ⁻)¹ has been primarily used by the LHC ex- perimentsinthesestudies,althoughanumberofother ⁰_b decay channels have been exploited by the LHCb experiment. In par- ticular,a measurement ofthe differential branching fractionand angularanalysisoftheraredecay⁰_b→μ⁺μ⁻⁰was performed byLHCb [8,9]followingobservationofthisdecayby theCDFex- periment[10]attheTevatroncollider.However, noresultsforthe decaymode ⁰_b→ ψ(^2S)⁰ haveyet been reported, although a measurementofthedecaypropertieswouldbeusefulforverifica- tionoftheoreticalpredictions[11].

The⁰_b→ ψ(^2S)⁰ branchingfractionshould be ofthesame order as that of the decay ⁰_b→ ^J/ψ⁰ as suggested by the branching fraction values of the B⁰, B⁺ and B⁰_s meson decays to ψ(2S)/J/ψ and either a pseudoscalar (K⁰, K⁺, η) or vector (K^∗0, K^∗+,φ) meson. The branching ratiosof such B meson de- cays to ψ(2S)X and J/ψX are within the 0.5–0.8 range [12], and are generally reproduced by factorisation calculations [13].

Theonlyavailabletheoreticalcalculationofthebranching ratioof the ⁰_b→ ψ(^2S)⁰ and _b⁰→ ^J/ψ⁰ decays, performedin the frameworkofthecovariantquarkmodel[14],predicts0.8 withan uncertaintyofapproximately0.1[11].

 E-mailaddress:atlas.publications@cern.ch.

1 Hereafter,chargeconjugationisimplied,unlessexplicitlystatedotherwise.

An observation of the ⁰_b→ ψ(^2S)⁰ decay anda measure- ment of the branching ratio of the _b⁰→ ψ(^2S)⁰ and ⁰_b→ J/ψ⁰ decays is reported in this Letter. The J/ψ and ψ(2S) mesons are reconstructed in their decays to a muon pair, while the ⁰→^pπ⁻ decayisexploited forthe⁰ baryonreconstruc- tion. The ⁰_b baryons are reconstructed withtransverse momen- tumpT>10 GeV andpseudorapidity|η_|<2.1.

2. TheATLASdetector,dataandMonteCarlosimulationsamples

AdetaileddescriptionoftheATLASdetectorcanbefoundelse- where[15].Abriefoutlineofthecomponentsmostrelevanttothis analysisisgivenbelow.

The ATLAS inner detector(ID) has full coverage² in φ, covers the pseudorapidity range |η_|<2.5 and operates inside an axial magneticfieldof2 T.Itconsistsofasiliconpixeldetector(Pixel), a silicon microstrip detector (semiconductor tracker, SCT) and a transitionradiation tracker(TRT). The inner-detector barrel (end- cap)partsconsistof3(2×^{3)Pixellayers,4(2}×⁹⁾double-layersof single-sidedSCTstripsand73(2×^{160)layers ofTRTstraws.The} ATLAS muon spectrometer (MS) covers the pseudorapidity range

|η|<2.7. It consists of precision tracking chambers, fast trigger detectorsanda largetoroidal magnetsystemgeneratingan aver-

2 TheATLAScoordinatesystemisaCartesianright-handedsystem,withtheco- ordinateoriginatthenominalinteractionpoint.Theanti-clockwisebeamdirection definesthepositivez-axis,withthex-axispointingtothecentreoftheLHCring.

Polar(θ)andazimuthal(φ)anglesaremeasuredwithrespecttothisreferencesys- tem.Thepseudorapidityisdefinedasη= −ln tan(θ/2).

http://dx.doi.org/10.1016/j.physletb.2015.10.009

0370-2693/©^{2015CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

agefield of0.5 Tinthebarrelregion(|η|<1.05)and1 Tinthe end-capregions(1.05<|η_|<2.7).

The ATLAS detector hasa three-leveltrigger system [16]: the hardware-basedLevel-1systemandthetwo-stageHighLevelTrig- ger (HLT). For this measurement, dimuon triggers are used. At Level-1,thedimuontriggers search forpatterns ofMShitscorre- spondingtodimuonspassingvariouspTthresholds.Sincetherate from the low-pT dimuon triggers was too high, prescale factors were applied toreduce their outputrates. The datasample used inthisanalysiswascollectedusingthreedimuontriggerswithpT thresholdsof4 GeVforbothmuons,4 GeVand6 GeVforthetwo muons, and6 GeV forbothmuons. At theHLT, the dimuontrig- gersusedrequiremuonswithoppositechargesanddimuonmass intherange2.5<m(μ⁺μ⁻)<4.3 GeV.

This analysis uses 20.6 fb⁻¹ of proton–proton collision data with a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC in 2012. The uncertainty on the integrated luminosity is ±^{2.8%. It is derived following the same methodol-} ogyasthatdetailedin[17].The eventsample isprocessed using the standard offline ATLAS detector calibration and event recon- structioncode.Therearetypicallyafewprimaryvertexcandidates ineach eventdueto multiplecollisions per bunchcrossing.Only eventswith atleastfour reconstructed trackswith pT>0.4 GeV andatleastone reconstructedprimary vertexcandidate arekept forfurtheranalysis.

Tomodelinelasticpp eventscontaining⁰_b→^J/ψ(μ⁺μ⁻)⁰,

⁰_b→ ψ(μ⁺μ⁻)⁰, B⁰→ ^J/ψ(μ⁺μ⁻)K⁰_S or B⁰→ ψ(μ⁺μ⁻)K⁰_S decays,³ fourlargesamplesofMonteCarlo(MC)simulatedevents arepreparedusingthePYTHIA 8.1[18] MCgenerator.The B⁰ MC samplesare neededto control reflections from B⁰ decays tothe

⁰_b signal distributions. The generationis basedon leading-order matrix elements for all 2→^{2 QCD processes.} Initial- and final- statepartonshoweringisusedtosimulatehigher-orderprocesses.

Generated events with both muons from J/ψ or ψ(2S) decays having transverse momenta above 3.5 GeV and pseudorapidities within ±².5, and, for ⁰_b MC samples, with the ⁰ transverse momentum above 1 GeVare passed through a full simulation of thedetectorusingtheATLAS simulationframework[19] basedon GEANT4[20,21]andprocessedwiththesamereconstruction pro- gramasusedforthedata.Anemulationofthethreetriggersused forthedatacollectionisapplied totheMC samples.The angular decaydistributions ofthe ⁰_b→ ^J/ψ(μ⁺μ⁻)⁰(pπ⁻) decayare modelled using the helicity amplitudes measured by ATLAS [2].

For the ⁰_b→ ψ(μ⁺μ⁻)⁰(pπ⁻) decay, the helicity amplitudes aresettothepredictedvalues[11].

3. Eventand⁰_bcandidateselection 3.1. Charmoniumcandidateselection

Events are required to contain at least two muons identified bythe MSwithtracks reconstructedintheID. Thereconstructed muonsare requiredto match the muon candidates identified by thetrigger.ThemuontrackparametersaretakenfromtheIDmea- surementalone, since the MS does not significantly improvethe precision in the momentum range relevant for the charmonium measurements presentedhere. Toensureaccurate measurements, each muon trackmust contain at least sixSCT hitsand atleast onePixelhit.Muoncandidatessatisfyingthesecriteriaarerequired tohaveoppositechargesandasuccessfulfittoacommonvertex with χ²/Ndof<10,where χ²isthefitqualitywiththenumberof degreesoffreedom N_dof=^{1.Eventswithm}(μ⁺μ⁻)valueswithin

3 InthisLetter,ψ(2S)isreferredtoasψwhenitsdecaychannelisindicated.

±^{200 MeV intervals around the J}/ψ and ψ(2S) world average masses[12]areusedtosearchfor⁰→^pπ⁻candidates.

3.2. ⁰and ¯⁰candidateselection

In all events with J/ψ or ψ(2S) candidates, pairs of tracks from particles with opposite charge are combined to form ⁰ candidates. Each track is required to have at least one Pixel or SCT hit. Only pairs successfully fitted to a common vertex with

χ²/Ndof<5 arekept.Thetrackwithlargermomentumisassigned the proton masshypothesis since the protonalways has a larger momentum than the pion for ⁰ baryons withmomenta larger than0.3 GeV.Tosuppresscombinatorialbackgroundthefollowing requirementsareused:

• ^pT(p)>1.7 GeV.

• |^z0(p)|<25 mm, where z0(p)is theproton longitudinalim- pactparameterwithrespecttothedimuonvertex.MCstudies showtherequirementproducesnolossofsignal.

• ^LBLxy(⁰)> 7 mm, where L^BL_xy(⁰) is the transverse decay length⁴ofthe⁰ candidatemeasuredfromthebeamline.

Events with m(pπ⁻) values within an interval of ±^{20 MeV} aroundthe⁰worldaveragemass[12]arekeptforfurtheranaly- sis.

3.3. ⁰_breconstruction

Tracks ofthe selected charmoniumand ⁰ candidates are si- multaneouslyrefittedwiththedimuonanddihadronmassescon- strained to the world average masses of J/ψ (mJ/ψ) or ψ(2S) (mψ (2S))and ⁰ (m_0) [12],respectively. Thecombined momen- tumoftherefitted⁰trackpairisrequiredtopointtothedimuon vertex.TocontrolB⁰reflectionstothe⁰_bsignaldistributions,aB⁰ decaytopologyfitisalsoattemptedforeachtrackquadrupletsuc- cessfully fittedto the⁰_b topology,i.e.thepion massisassigned tobothhadrontracksandthedihadronmassisconstrainedtothe worldaveragemassof K⁰_S [12].TosuppresscombinatorialandB⁰ backgroundsthefollowingrequirementsareused:

•χ²(⁰_b)/N_dof<3,where χ² isthequalityofthefittothe⁰_b topologywithN_dof=^6.

• ^Lxy(⁰)>10 mm, where Lxy(⁰) is the transverse decay length of the refitted ⁰ vertex measured from the ⁰_b (dimuon)vertex.

• ^pT(⁰)>2.5 GeV.

• ^pT(π⁻)>0.45 GeV.

•τ(⁰_b)>0.35 ps,where τ(⁰_b)=^Lxy(⁰_b)·^m_⁰

b/pT(⁰_b)isthe

⁰_b properdecaytime, Lxy(⁰_b)isthetransversedecaylength ofthe⁰_b vertexmeasured fromtheprimary vertexandm_0 b

is the ⁰_b world averagemass [12].The primary vertexcan- didatewithatleastthreetracksandthesmallestvalueofthe three-dimensionalimpactparameterofthe⁰_bcandidateisse- lectedastheactualprimaryvertex.

•P(⁰_b)>P(^B0), where P(⁰_b) and P(^B0) are the χ² prob- abilities of the quadruplet fits with ⁰_b and B⁰ topologies, respectively.

4 Thetransversedecaylengthofaparticle isthetransversedistancebetween theprimaryorproductionvertexandtheparticledecayvertexprojectedalongthe transversemomentumoftheparticle.

Fig. 1. Theinvariantmassdistributionsm(J/ψ⁰)(leftplot)andm(J/ψ ¯⁰)(rightplot)forselected_b⁰and ¯⁰_bcandidates,respectively.Thesolidhistogramsrepresentthe fitresults(seetext).The⁰_bsignals(dashedlines)andtheB⁰reflectionsarealsoshown.

Fig. 2. Theinvariantmassdistributionsforthecombinedsampleoftheselected_b⁰and ¯⁰_bcandidatesobtainedaftertheirfitstothe⁰_b→^J/ψ⁰(leftplot)and B⁰→ J/ψK⁰_S(rightplot)topologies.Thesolidhistogramsrepresentfitresults(seetext).The⁰_bandB⁰signalsandtheirmutualreflectionsarealsoshown.

Themuon transversemomenta andpseudorapiditiesarerequired tobeintherangeswithhighvaluesofthetriggerandreconstruc- tionacceptances:

p_T(μ^±) >4 GeV,|η(μ^±)| <².3.

Thekinematicrangeofthe⁰_b measurementisfixedto

pT(⁰_b) >10 GeV,|η(⁰_b)| <².1.

The invariant mass distribution m(J/ψ⁰), calculated using trackparametersfromthe⁰_btopologyfits,isshowninFig. 1sep- aratelyforthe selected_b⁰ and ¯_b⁰ candidates.Clear signalswith similarsizeareseeninthetwodistributionsaroundtheworldav- eragemassofthe ⁰_b baryon.Figs. 2 and 3 showthe m(J/ψ⁰) andm(ψ(2S)⁰)distributionsforthecombinedsampleofthe⁰_b and ¯⁰_b candidates. The invariant mass distributions m(J/ψK⁰_S) andm(ψ(2S)K⁰_S)fromthe B⁰ topology fits arealsoshown. Clear signalsareseeninthem(J/ψ⁰)andm(ψ(2S)⁰)distributions⁵ aroundthe worldaverage massofthe _b⁰ baryon. There arealso signalsinthe m(J/ψK_S⁰)andm(ψ(2S)K⁰_S) distributions nearthe world average mass of the B⁰ meson [12]. The B⁰ signals are smallerthanthe⁰_bsignalsduetotheselectionrequirements.

5 StudieswithMCsimulatedeventsshowthatthefractionofreconstructed_b⁰→ J/ψ⁰decayswhichcancontributetothereconstructed⁰_b→ ψ(2S)⁰signalis

∼^10−5.

Them(J/ψ⁰)andm(J/ψK⁰_S)distributionsaresimultaneously fitted to sums of signal and two-component background distri- butions. The signals are described by modified Gaussian func- tions[22].ThemodifiedGaussianfunctionisdefinedas

Gauss^mod∝^exp[−⁰.5·^{x1+1/(1+0.5·x)}],

wherex= |(^m−^m0)/σ_|.Thisfunctional form,introduced totake intoaccount the non-Gaussian tailsofresonant signals, describes bothdataandMCsignalswell.Thesignalposition,m0,andwidth,

σ,aswellasthenumberofthesignaleventsarefreeparameters of thefit. The non-resonant backgrounds inthe distributions are described by independent exponential functions. The mutual B⁰ and⁰_b reflectionsare described by MC templates normalised to thenumbersofB⁰ and_b⁰hadronsobtainedinthefit.Thereflec- tionnormalisationsarecorrectedforsmalllosses(2–6%)of⁰_band B⁰ hadronsthat passed the ⁰_b reconstruction butfailed the B⁰ reconstruction.The correctionsare obtainedusingMC simulation.

A similar fit is performed forthe m(ψ(2S)⁰) andm(ψ(2S)K⁰_S) distributions.Intheanalysisofthecombined⁰_b and ¯⁰_b samples, theratiooftheMC ⁰_b and ¯⁰_b eventsissettothedataratioob- tainedintheseparate⁰_b→^J/ψ⁰and ¯⁰_b→^J/ψ ¯⁰fits(Fig. 1).

The⁰_b and ¯⁰_bfittedyieldsare3523±^{89 and3414}±^92,respec- tively,providingtheratio1.03±⁰.04(stat).

The results of the fits for the combined _b⁰ and ¯⁰_b samples aresummarisedinTable 1.The⁰_b massvaluesobtainedfromthe fits of the m(J/ψ⁰) and m(ψ(2S)⁰) distributions agree with