Measurement of the lifetime of the B+c meson using the B+c→J/ψπ+ decay mode

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Measurement of the lifetime of the B+c meson using the B+c → J/ψπ+ decay mode

J. Aaij, L. Beaucourt, M. Chefdeville, D. Decamp, N. Déléage, Ph. Ghez, J.-P. Lees, J.F. Marchand, M.-N. Minard, B. Pietrzyk, et al.

To cite this version:

J. Aaij, L. Beaucourt, M. Chefdeville, D. Decamp, N. Déléage, et al.. Measurement of the lifetime of the B+c meson using the B+c → J/ψπ+ decay mode. Physics Letters B, Elsevier, 2015, 742, pp.29-37.

�10.1016/j.physletb.2015.01.010�. �in2p3-01088069�

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurement of the lifetime of the B ⁺ _c meson using the B ⁺ _c → ^J / ψπ ⁺ decay mode

.LHCb Collaboration

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

Articlehistory:

Received26November2014

Receivedinrevisedform19December2014 Accepted9January2015

Availableonline13January2015 Editor: M.Doser

The difference in total widths between the B⁺_c and B⁺ mesons is measured using a data sample correspondingtoanintegratedluminosityof3.0 fb⁻¹collectedbytheLHCbexperimentin7 and8 TeV centre-of-massenergyproton–proton collisionsattheLHC. Throughthestudyofthetimeevolutionof B⁺_c →^J/ψπ⁺^and^B⁺→^J/ψ^K⁺^decays,^the^width^difference^is^measured^to^be

≡_B+

c −_B+=4.46±0.14±0.07 mm⁻¹c,

wheretheﬁrstuncertaintyisstatisticalandthesecondsystematic.TheknownlifetimeoftheB⁺meson isusedtoconvertthistoaprecisemeasurementoftheB⁺_c lifetime,

τ_B+

c =513.4±11.0±5.7 fs,

wheretheﬁrstuncertaintyisstatisticalandthesecondissystematic.

©²⁰¹⁵^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Forweakly decaying beauty hadrons the heavy quark expan- sion [1–4] predicts lifetime differences of the order (QCD/mb), whereQCD isthe scale parameterof thestrong interactionand m_b isthe b-quark mass.Inagreementwiththeexpectations,differences between B⁺, B⁰, B⁰_s, ⁰_b, ⁰_b and ⁻_b lifetimes not ex- ceedinga few per centare found experimentally [5–11].The B⁺_c meson is a bound state of an anti-b quark anda charm quark, and Cabibbo-favoured decays of the charm quark are expected to account for 70% of its total width, resulting in a signiﬁcantly shorterlifetimethanfor otherB mesons.In addition,non-spectator topologies,inparticularannihilationamplitudes,arenot Cabibbo- suppressed. These could give sizeable contributions to the total width[12–18].Understandingtherelative contributionsofbeauty andcharm quarksto the total widthof theB⁺_c meson isimpor- tantforpredictingthepropertiesofunobservedbaryonswithtwo heavyquarks[19,20].

The lifetime of the B⁺_c meson was ﬁrst measured by the CDF[21–23] andD0[24] Collaborations usingsemileptonicB⁺_c → J/ψμ⁺νμX and hadronicB⁺_c →^J/ψπ⁺ ^decays.¹ ^The ^average ^value of these measurements is τB⁺_c =⁴⁵²±^{32 fs} ^[25]. ^Recently, ^the LHCbCollaboration made the mostprecise measurement to date

1 Theinclusionofcharge-conjugateprocessisimpliedthroughthisLetter.

oftheB⁺_c mesonlifetimeusingsemileptonicB⁺_c →^J/ψμ⁺νμX decays,τB⁺_c =⁵⁰⁹±^{14 fs}^[26].

InthisLetter wereport ameasurement oftheB⁺_c mesonlife- time obtainedvia the difference between the total width ofthe B⁺_c and B⁺ mesons in the hadronic modes B⁺_c →^J/ψπ⁺ ^and B⁺→^J/ψ^K⁺^, ^using ^the ^technique ^developed ⁱⁿ ^Refs. ^[7–11,27].

The measurement uses a data sample corresponding to an inte- gratedluminosityof3.0 fb⁻¹collectedbytheLHCbexperimentin proton–proton(pp) collisionsatcentre-of-mass energies of7 and 8 TeV.This studyiscomplementarytothemeasurementoftheB⁺_c lifetimeusingthesemileptonicB⁺_c →^J/ψμ⁺νμX decaysdescribed inRef.[26].

The B⁺_c lifetimeisdetermined asfollows.The decaytimedis- tribution for signal, NB(t), can be described as the product of an acceptance function εB(t) and an exponential decay EB(t)= exp(−^t/τB) convolved with the decaytime resolution ofthe detector.TheeffectofthedecaytimeresolutionontheratioR(t)≡ N_B+

c(t)/NB⁺(t)isfoundtobesmallandisabsorbedintotheratio ofacceptancefunctions.Thisleadstothesimpliﬁedexpression

R(t)∝ε_B+ c(t) ε_B+(t)

E_τ

B+ c(t)

E_τ_B₊(t)≡Rε(t)e⁻^t with≡_B+

c −_B+= ¹

τ_B⁺_c − ¹ τB

, (1)

http://dx.doi.org/10.1016/j.physletb.2015.01.010

0370-2693/©²⁰¹⁵^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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30 LHCb Collaboration / Physics Letters B 742 (2015) 29–37

wherethefactor Rε(t) denotesthe ratiooftheacceptancefunc- tions.Thisallowsa precisemeasurementof andhenceofthe lifetimeoftheB⁺_c meson.

2. Detectorandeventsimulation

The LHCb detector [28] is a single-arm forward spectrometer covering the pseudorapidity range 2<η<5, designed for the studyofparticlescontaining b or c quarks.Thedetectorincludes ahigh-precisiontrackingsystemconsistingofasilicon-stripvertex detectorsurrounding the pp interactionregion [29], a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon- strip detectors andstraw drift tubes [30] placed downstream of themagnet. Thetrackingsystemprovides a measurementofmo- mentum, p, witha relative uncertainty that varies from0.4% at low momentumto 0.6% at100 GeV/c.The minimumdistance of a track to a primary vertex, the impact parameter, is measured witha resolutionof(15+²⁹/pT)μm,wherepT isthecomponent of momentum transverse to the beam, in GeV/c. Different types ofcharged hadronsaredistinguished usinginformationfromtwo ring-imagingCherenkovdetectors(RICH)[31].Photon,electronand hadroncandidatesareidentiﬁedbya calorimetersystemconsisting of scintillating-pad and preshower detectors, an electromagnetic calorimeterandahadroniccalorimeter.Muonsareidentiﬁedbya systemcomposedofalternatinglayers ofironandmultiwirepro- portionalchambers[32].

Thetrigger[33] comprisestwostages.Eventsareﬁrstrequired topassthehardwaretrigger,whichselectsmuoncandidateswith pT>1.5 GeV/c or pairs of opposite-sign muon candidates with a requirementthattheproductofthemuontransversemomentais largerthan1.7(2.6)GeV²/c² fordatacollectedat√

s=⁷(8)TeV.

The subsequent software trigger is composed of two stages, the first of which performs a partial event reconstruction, while full eventreconstructionisdoneatthesecondstage.Atthefirststage of the software trigger the invariant mass of well-reconstructed pairsofoppositelychargedmuonsforminga goodtwo-prongver- texisrequiredtoexceed2.7 GeV/c²,andthetwo-prongvertexis required to be significantly displaced with respect to the recon- structedpp collisionvertex.

Inthesimulation,pp collisionsaregeneratedusingPythia[34]

with a speciﬁc LHCb conﬁguration [35]. A dedicated generator, Bcvegpy[36],whichimplements explicitleading-ordermatrix el- ementcalculations[37–39],isused forproductionofB⁺_c mesons.

The kinematicdistributions ofB⁺_c mesons are reproduced bythe Bcvegpy generator withpercent-level precision [26,40–47],while thesimulatedB⁺ samples,producedwithPythia,arecorrectedto reproducetheobservedkinematicdistributions.Decaysofhadronic particlesaredescribed byEvtGen [48],inwhichﬁnal-stateradia- tionisgeneratedusingPhotos[49].Theinteractionofthegener- atedparticleswiththedetectorandthedetectorresponseareim- plementedusingtheGeant4toolkit[50]asdescribedinRef.[51].

3. Eventselection

The offline selection of B⁺_c →^J/ψπ⁺ ând^B⁺→^J/ψ^K⁺ ^candidates is divided into two parts. An initial selection is applied to reducethecombinatorialbackground.Subsequently,a multivariate estimatorbasedonan artificialneural networkalgorithm [52,53], configuredwitha cross-entropycostestimator[54],inthefollow- ingreferredasMLPclassifier,isapplied.Thesamecriteriaareused forboththeB⁺_c andB⁺candidates.

Theselection startsfromwell-identiﬁedmuon candidatesthat have a transverse momentum in excess of 550 MeV/c. Pairs of muon candidates are required to form a common vertexand to

have an invariant mass within ±^{60 MeV}/c² of the known J/ψ mass[5].ToensurethattheJ/ψ^candidateôriginatesⁱⁿ^{a b-hadron} decay, a significant decaylength withrespect to the pp collision vertex is required. The charged pions and kaons must be posi- tively identified using the combined information fromthe RICH, calorimeter and muon detectors. The B⁺_c and B⁺ candidates are formed fromJ/ψπ⁺ând^J/ψ^K⁺ combinations,respectively. Toim- prove the invariant massanddecay time resolutionsfor selected candidates,a kinematicfit[55]isappliedinwhichaprimaryver- texpointingconstraintanda massconstraintontheintermediate J/ψ ^statesâre âpplied.^To^reducecombinatorial background,a requirement on the χ² of this fit, χ_fit², is imposed and the decay time of thereconstructed B⁺_c (B⁺) candidateisrequired tobe in therange50<t<1000 μm/c.

The final selection of candidates using the MLP classifier is based onthetransverse momentaandrapiditiesof reconstructed B⁺_c (B⁺) and J/ψ^, ^the ^transverse ^momentum ând pseudorapidity of the π⁺ ^(K⁺) candidate, the cosine of the decay angle θ between the momentum of the π⁺ ^(K⁺⁾ ⁱⁿ ^the ^rest ^frame ôf ^the B⁺_c (B⁺) candidate andthe boostdirectionfromthe B⁺_c (B⁺) rest frametothelaboratoryframe,the χ²oftheB⁺_c (B⁺) vertexfit,and

χ_fit². These variables provide good discrimination between signal and background whilst keepingthe selection efficiency indepen- dent ofthe B⁺_c (B⁺) decaytime. The MLPclassifier is trainedon asimulatedsampleofB⁺_c →^J/ψπ⁺ êventsândâ^background^data samplefromthemasssidebandsoftheB⁺_c signalpeak.Itistested onindependentsamplesfromthesamesources.Theworkingpoint oftheclassifierischosentominimize σ(S)/S^,^whereS îs^the^B⁺c signal yield andσ(S) is the yield uncertainty, asdetermined by themassfitdescribedinthenextsection.ThesameMLPclassifier isusedfortheB⁺→^J/ψ^K⁺^mode.

4. Measurementof

The invariant massdistributions forselected B⁺_c and B⁺ candidates are presented in Fig. 1. The signal yields are determined using an extended unbinned maximum likelihood fit in which thesignal distributionsare modelledbya Gaussianfunctionwith power-lawtailsonbothsidesofthepeak[56],andthebackground is modelled by the product of an exponential and a first-order polynomialfunction.Simulationstudiessuggestthatthesametail parameters apply for theB⁺_c andB⁺ signals. The tailparameters determined from the data for the large B⁺ signal are in good agreementwiththesimulation.Thefitgives2886±^{71 signal} ^B⁺c decays and586065±^{798 signal}^B⁺ ^decays.^The ^fitted^values^for the B⁺_c and B⁺ invariant masses are consistent with the known values[5]andthefittedmass resolutionsagreewiththeexpecta- tionfromsimulation.

ThesignalyieldsofB⁺_c andB⁺mesonsinbinsofdecaytimeare showninFig. 2(a).A non-uniformbinningschemeischosenwith a minimal binwidthof25 μm/c atlow t increasingto200 μm/c at thelargest decaytimes, tokeep the B⁺_c signal yield above 20 for all t bins. In the mass fits of the individual decay time bins the peak positions and mass resolutions are fixed to the values obtainedfromthefitintheentireregion,50<t<1000 μm/c.

The decay time resolution function is estimated using simu- lated samplesandfound to be well described by tripleGaussian functions with overall rms widths of 10.9 μm/c and 11.5 μm/c for B⁺_c andB⁺ decays,respectively. Theratio ofacceptancefunctions, Rε(t), is determined using the simulation and shown in Fig. 2(b). The variation in the acceptance ratio is caused by the requirement onthe J/ψ ^decay^length ^imposed ⁱⁿ^the ^trigger^and the subsequent selection. The acceptanceis calculated asthe ratio of decaytime distributions ofthe reconstructed and selected simulatedeventstothetheoretical(exponential)distributionscon-

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Fig. 1.Invariantmassdistributionsfor(a) selectedB⁺c →J/ψπ⁺ and(b) B⁺→J/ψK⁺candidates.Theﬁtresultwiththefunctiondescribedinthetextisshownbythered solidline;thesignal(background)componentsareshownwithgreen(blue)dottedlines.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderis referredtothewebversionofthisarticle.)

Fig. 2.(a) DecaytimedistributionsforselectedB⁺_c →J/ψπ⁺(redsolidcircles)andB⁺→J/ψK⁺(blueopensquares)decays,withthedatapointspositionedwithinthe tbinsaccordingtoEq. (6)inRef.[57];(b) Ratio ofacceptancefunctionsRε(ct).Theuncertaintiesareduetosamplesizeonly.Forvisualizationpurposestheeﬃciencyratio isnormalizedasRε(0.5 mm/c)=1.

Fig. 3.Ratiooftheeﬃciency-correcteddecaytimedistributions(pointswitherror bars).Thecurveshowstheresultoftheﬁtwithanexponentialfunction.Thedata pointsarepositionedwithinthet binsaccordingtoEq. (6)inRef.[57]andthe horizontalerrorbarsdenotethebin widths.

volved withthe resolution function. This effectively includes the correctionsduetoresolutioneffects,neglectedinEq.(1).It ises- timated that any residual bias is smaller than 0.1% in the range 50<t<1000 μm/c.

The eﬃciency-corrected ratio R(t)/Rε(t) is shown in Fig. 3.

A minimum χ² ﬁt with an exponential function, according to Eq.(1),gives

=⁴.46±⁰.14 mm⁻¹c, (2)

Table 1

Summaryofsystematicuncertaintiesfor.

Source σ[^mm⁻¹^c]

Fit model (signal and background) 0.012

ctﬁt range 0.040

ctbinning 0.016

Acceptance

Simulation sample size 0.011

MLPﬁltering 0.025

J/ψdisplacement 0.050

Total 0.072

wheretheuncertaintyis statistical.The qualityoftheﬁtis good, witha p-valueof 42%.

5. Systematicuncertaintiesandcross-checks

Several sources of systematic uncertainty are considered, as summarizedinTable 1anddiscussedbelow.

Theuncertaintyrelatedtothedeterminationofthesignalyields in t bins is estimated by comparing the nominal results with thoseobtainedusingdifferentﬁtmodels.Asanalternative model for the B⁺_c and B⁺ signals, a modiﬁed Novosibirsk function [58]

and a Gaussian function are used. Although the latter provides poordescription for thelarge B⁺ sample for all decaytime bins and the low-background B⁺_c signal for bins with t>150 μm/c, there is no effect on the determination of . For the combi-