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Original citation:

LHCb Collaboration (Including: Back, John J., Craik, Daniel, Dossett, D., Gershon, Timothy J., Kreps, Michal, Latham, Thomas, Pilar, T., Poluektov, Anton, Reid, Matthew M., Silva Coutinho, R., Whitehead, M. (Mark) and Williams, Matthew P.). (2013)

Observation of B+c→J/ψD+s and B+c→J/ψD∗+s decays. Physical Review D (Particles, Fields, Gravitation and Cosmology), Volume 87 (Number 11). Article number 112012 . ISSN 1550-7998

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Observation of

B

þc

!

J=

c

D

þs

and

B

þc

!

J=

c

D

decays

R. Aaijet al.* (LHCb Collaboration)

(Received 18 April 2013; published 28 June 2013)

The decays Bþc !J=cDþs and Bþc !J=cDþs are observed for the first time using a dataset, corresponding to an integrated luminosity of3 fb1, collected by the LHCb experiment in proton-proton collisions at center-of-mass energies ofpffiffiffis¼7and 8 TeV. The statistical significance for both signals is in excess of 9 standard deviations. The following ratios of branching fractions are measured to be BðBþc!J=cDþsÞ

BðBþ

c!J=cþÞ¼2:900:570:24,

BðBþc!J=cDþs Þ BðBþ

c!J=cDþsÞ ¼2:370:560:10, where the first uncertainties are statistical and the second systematic. The mass of theBþc meson is measured to bemBþ

c ¼6276:28 1:44ðstatÞ 0:36ðsystÞMeV=c2, using theBþ

c !J=cDþs decay mode.

DOI:10.1103/PhysRevD.87.112012 PACS numbers: 13.25.Hw, 13.25.k

I. INTRODUCTION

The Bþc meson, the ground state of the bc system, is unique, being the only weakly decaying heavy quarkonium system. Its lifetime [1,2] is almost 3 times smaller than that of other beauty mesons, pointing to the important role of the charm quark in weak Bþc decays. The Bþc meson was first observed through its semileptonic decay Bþc !

J=c‘þ

‘X [3]. Only three hadronic modes have been

observed so far:Bþc !J=cþ [4],Bþc !J=cþþ [5] andBþc !cð2SÞþ[6].

The first observations of the decaysBþc !J=cDþs and

c !J=cDþs are reported in this paper. The leading

Feynman diagrams of these decays are shown in Fig. 1. The decayBþc !J=cDþs is expected to proceed mainly through spectator and color-suppressed spectator dia-grams. In contrast to decays of other beauty hadrons, the weak annihilation topology is not suppressed and can contribute significantly to the decay amplitude.

Assuming that the spectator diagram dominates and that factorization holds, the following approximations can be established:

RDþ

s=þ

ðBþc !J=cDþsÞ ðBþc !J=cþÞ

ðB!DDþsÞ

ðB!DþÞ; (1a)

RDþ

s =Dþs

ðBþc !J=cDþs Þ ðBþc !J=cDþsÞ

ðB!DDþs Þ ðB!DDþsÞ ; (1b)

where B stands forBþorB0 andD denotesD0 orD.

Phase space corrections amount to Oð0:5%Þ for Eq. (1a) and can be as large as 28% for Eq. (1b), depending on the relative orbital momentum. The relative branching ratios estimated in this way, together with more detailed theoretical calculations, are listed in Table I, where the

branching fractions for the B!DDsþ and B!Dþ decays are taken from Ref. [1].

The analysis presented here is based on a data sample, corresponding to an integrated luminosity of 3 fb1,

collected with the LHCb detector during 2011 and 2012 in pp collisions at center-of-mass energies of 7 and 8 TeV, respectively. The decay Bþc !J=cþ is used as a nor-malization channel for the measurement of the branching fraction BðBþc !J=cDþsÞ. In addition, the low energy release (Q value) in the Bþc !J=cDþs mode allows a determination of the Bþc mass with small systematic uncertainty.

II. LHCB DETECTOR

The LHCb detector [12] is a single-arm forward spec-trometer covering the pseudorapidity range 2< <5, designed for the study of particles containing b or c quarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding the ppinteraction region, 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 and straw drift tubes placed downstream. The combined tracking system has momentum resolution p=p that varies from 0.4% at5 GeV=cto 0.6% at100 GeV=c, and impact parameter resolution of20mfor tracks with high transverse momentum. Charged hadrons are identified using two ring-imaging Cherenkov detectors. Photon, elec-tron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional cham-bers. The trigger [13] consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage which applies a full event reconstruction.

This analysis uses events collected by triggers that select the decay products of the dimuon decay of theJ=c meson

*Full author list given at the end of the article.

Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri-bution of this work must maintain attridistri-bution to the author(s) and the published article’s title, journal citation, and DOI.

PHYSICAL REVIEW D87,112012 (2013)

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with high efficiency. At the hardware stage either one or two identified muon candidates are required. In the case of single muon triggers the transverse momentumpT of the candidate is required to be larger than 1:5 GeV=c. For dimuon candidates a requirement on the product of the pT of the muon candidates is applied, pffiffiffiffiffiffiffiffiffiffiffiffiffiffipT1pT2>

1:3 GeV=c. At the subsequent software trigger stage, two muons with invariant mass in the interval2:97< mþ<

3:21 GeV=c2and consistent with originating from a

com-mon vertex are required.

The detector acceptance and response are estimated with simulated data. Proton-proton collisions are generated usingPYTHIA6.4 [14] with the configuration described in Ref. [15]. Particle decays are then simulated by EVTGEN

[16] in which final state radiation is generated using

PHOTOS [17]. The interaction of the generated particles with the detector and its response are implemented using theGEANT4toolkit [18] as described in Ref. [19].

III. EVENT SELECTION

Track quality of charged particles is ensured by requir-ing that the 2 per degree of freedom, 2

tr=ndf, is less

than 4. Further suppression of fake tracks created by the reconstruction is achieved by a neural network trained to discriminate between these and real particles based on information from track fit and hit pattern in the tracking detectors. A requirement on the output of this neural net-work, Pfake<0:5, allows us to reject half of the fake

tracks.

Duplicate particles created by the reconstruction are suppressed by requiring the symmetrized Kullback-Leibler divergence [20] min

KL, calculated with respect to

all particles in the event, to be in excess of 5000. In addition, the transverse momentum is required to be greater than 550ð250ÞMeV=c for each muon (hadron) candidate.

Well-identified muons are selected by requiring that the difference in logarithms of the likelihood of the muon hypothesis, as provided by the muon system, with respect to the pion hypothesis,=lnL[21], is greater than zero. Good quality particle identification by the ring-imaging Cherenkov detectors is ensured by requiring the momen-tum of the hadron candidates, p, to be between 3.2 and100 GeV=c, and the pseudorapidity to be in the range

2< <5. To select well-identified kaons (pions) the

corresponding difference in logarithms of the likelihood of the kaon and pion hypotheses [22] is required to be

K=lnL>2ð<0Þ. These criteria are chosen to be tight enough to reduce significantly the background due to mis-identification while ensuring good agreement between data and simulation.

To ensure that the hadrons used in the analysis are inconsistent with being directly produced in a pp inter-action vertex, the impact parameter 2, defined as the difference between the2of the reconstructed pp collision vertex formed with and without the considered track, is required to be 2

IP>9. When more than one vertex is

reconstructed, that with the smallest value of2

IPis chosen.

As in Refs. [23–25] the selection of J=c !þ candidates proceeds from pairs of oppositely charged muons forming a common vertex. The quality of the vertex is ensured by requiring the 2 of the vertex fit,2

vx, to be

less than 30. The vertex is forced to be well separated from the reconstructed pp interaction vertex by requiring the decay length significance Sflight, defined as the ratio of

the projected distance from pp interaction vertex to þ vertex on direction ofþ pair momentum and its uncertainty, to be greater than 3. Finally, the mass of the dimuon combination is required to be within45 MeV=c2

of the knownJ=cmass [1], which corresponds to a3:5

window, whereis the measuredJ=c mass resolution. Candidate Dþs mesons are reconstructed in the Dþs !

ðKþKÞ

þ mode using criteria similar to those in

Ref. [26]. A good vertex quality is ensured by requiring

2

vx<25. The mass of the kaon pair is required to be

consistent with the decay !KþK,jmKþKmj<

[image:3.612.147.466.48.120.2]

20 MeV=c2. Finally, the mass of the candidate is required to be within 20 MeV=c2 of the known Dþs mass [1],

TABLE I. Predictions for the ratios of Bþc meson branching fractions. In the case of RDþs =Dþs the second uncertainty is

related to the unknown relative orbital momentum.

RDþ

s=þ RDþs =Dþs

2:900:42 2:200:350:62 Eqs. (1) withB0 1:580:34 2:070:520:52 Eqs. (1) withBþ

1.3 3.9 Ref. [7]

2.6 1.7 Ref. [8]

2.0 2.9 Ref. [9]

2.2 Ref. [10]

[image:3.612.51.298.610.717.2]

1.2 Ref. [11]

FIG. 1. Feynman diagrams forBþc !J=cDþs decays: (a) spectator, (b) color-suppressed spectator and (c) annihilation topology.

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which corresponds to a 3:5 window, where is the measuredDþs mass resolution, and its transverse momen-tum to be>1 GeV=c.

Candidate Bþc mesons are formed from J=cDþs pairs with transverse momentum in excess of 1 GeV=c. The candidates should be consistent with being produced in a pp interaction vertex by requiring 2

IP<9 with respect

to reconstructed pp collision vertices. A kinematic fit is applied to theBþc candidates [27]. To improve the mass and lifetime resolution, in this fit, a constraint on the pointing of the candidate to the primary vertex is applied together with mass constraints on the intermediate J=c and Dþs states. The value of theJ=c mass is taken from Ref. [1]. For the Dþs meson the value of mDþ

s ¼1968:31 0:20 MeV=c2 is used, that is, the average of the values

given in Refs. [1,28]. The2per degree of freedom of this

fit,2

fit=ndf, is required to be less than 5. The decay time

of the Dþs candidate, cðDþsÞ, determined by this fit, is required to satisfyc >75m. The corresponding signed significanceSc, defined as the ratio of the measured decay time and its uncertainty, is required to be in excess of 3. Finally, the decay time of the Bþc candidate, cðBþcÞ, is required to be between75mand 1 mm. The upper edge, in excess of seven lifetimes ofBþc meson, is introduced to remove badly reconstructed candidates.

IV. OBSERVATION OFBþc !J=cDþs

The mass distribution of the selected Bþc !J=cDþs candidates is shown in Fig.2. The peak close to the known mass of theBþc meson [1,29] with a width compatible with the expected mass resolution is interpreted as being due to theBþc !J=cDþs decay. The wide structure between 5.9

and6:2 GeV=c2is attributed to the decayBþ

c !J=cDþs ,

followed by Dþs !Dþs or Dþs !Dþs0 decays,

where the neutral particles are not detected. The process

c !J=cDþs being the decay of a pseudoscalar particle

into two vector particles is described by three helicity amplitudes: Aþþ,A00 andA, where indices

corre-spond to the helicities of the J=c and Dþs mesons. Simulation studies show that the J=cDþs mass distribu-tions are the same for the Aþþ and A amplitudes. Thus, theJ=cDþs mass spectrum is described by a model consisting of the following components: an exponential shape to describe the combinatorial background, a Gaussian shape to describe the Bþc !J=cDþs signal and two helicity components to describe the Bþc !J=cDþs contributions corresponding to theA andA00 ampli-tudes. The shape of these components is determined using the simulation where the branching fractions for Dþs !

s andDþs !Dþs0 decays are taken from Ref. [1].

To estimate the signal yields, an extended unbinned maximum likelihood fit to the mass distribution is per-formed. The correctness of the fit procedure together with the reliability of the estimated uncertainties has been extensively checked using simulation. The fit has seven free parameters: the mass of the Bþc meson, mBþ

c,

the signal resolutionBþ

c, the relative amount of theA

helicity amplitudes of total Bþc !J=cDþs decay rate, f, the slope parameter of the exponential background and the yields of the two signal components, NBþ

c!J=cDþs

andNBþ

c!J=cDþs , and of the background. The values of the

signal parameters obtained from the fit are summarized in TableII. The fit result is also shown in Fig.2.

To check the result, the fit has been performed with different models for the signal: a double-sided Crystal Ball function [30,31], and a modified Novosibirsk function [32]. For these tests the tail and asymmetry parameters are fixed using the simulation values, while the parameters representing the peak position and resolution are left free to vary. As alternative models for the background, the product of an exponential function and a fourth-order polynomial function are used. The fit parameters obtained are stable with respect to the choice of the fit model and the fit range interval.

The statistical significance for theBþc !J=cDþs signal is estimated from the change in the likelihood function

5.6 5.8 6 6.2 6.4 6.6 0

10 20 30 40 50 60

6.2 6.25 6.3 6.35 0

5 10 15 20 25

[image:4.612.55.294.457.640.2]

FIG. 2 (color online). Mass distributions for selectedJ=cDþs pairs. The solid curve represents the result of a fit to the model described in the text. The contribution from theBþc !J=cDþs decay is shown with thin green dotted and thin yellow dash-dotted lines for theAandA00amplitudes, respectively. The inset shows a zoom of theBþc mass region.

TABLE II. Signal parameters of the unbinned extended maxi-mum likelihood fit to theJ=cDþs mass distribution.

Parameter Value

mBþc [MeV=c2] 6276:281:44

c [MeV=c2] 7:01:1

NBþ

c!J=cDþs 28:95:6

NBþc!J=cDþs

NBþc!J=cDþs 2:370:56

f[%] 5220

OBSERVATION OFBþc !J=cDþs . . . PHYSICAL REVIEW D87,112012 (2013)

[image:4.612.316.562.626.717.2]
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ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 lnLBþS

LB

q

, where LB is the likelihood of a background-only hypothesis and LBþS is the likelihood of a background-plus-signal hypothesis. The significance has been estimated separately for the Bþc !J=cDþs and

c !J=cDþs signals. To exclude the look-elsewhere

effect [33], the mass and resolution of the peak are fixed to the values obtained with the simulation. The minimal significance found varying the fit model as described above is taken as the signal significance. The statistical signifi-cance for both the Bþc !J=cDþs and Bþc !J=cDþs signals estimated in this way is in excess of 9 standard deviations.

The low Q value for the Bþc !J=cDþs decay mode allows theBþc mass to be precisely measured. This makes use of the Dþs mass value, evaluated in Sec. III, taking correctly into account the correlations between the mea-surements. The calibration of the momentum scale for the dataset used here is detailed in Refs. [28,34]. It is based upon large calibration samples of Bþ!J=cKþ and J=c !þ decays and leads to an accuracy in the momentum scale of 3104. This translates into an

uncertainty of 0:30 MeV=c2 on the Bþ

c meson mass.

A further uncertainty of 0:11 MeV=c2 arises from

the knowledge of the detector material distribution [28,29,34,35] and the signal modeling. The uncertainty on theDþs mass results in a 0:16 MeV=c2uncertainty on theBþc meson mass. Adding these in quadrature gives

mBþ

c ¼6276:281:44ðstatÞ 0:36ðsystÞ MeV=c

2:

The uncertainty on the Dþs meson mass and on the momentum scale largely cancels in the mass difference

mBþ

c mDþs ¼4307:971:44ðstatÞ

0:20ðsystÞMeV=c2:

V. NORMALIZATION TO THEBþc !J=cþ DECAY MODE

A large sample of Bþc !J=cþ decays serves as a normalization channel to measure the ratio of branching fractions for theBþc !J=cDþs andBþc !J=cþmodes. Selection of Bþc !J=cþ events is performed in a manner similar to that described in Sec. III for the signal channel. To further reduce the combinatorial back-ground, the transverse momentum of the pion for the

c !J=cþ mode is required to be in excess of

1 GeV=c. The mass distribution of the selected Bþc ! J=cþ candidates is shown in Fig.3.

To determine the yield, an extended unbinned maximum likelihood fit to the mass distribution is performed. The signal is modeled by a double-sided Crystal Ball function and the background with an exponential function. The fit gives a yield of 300979 events. As cross-checks, a modified Novosibirsk function and a Gaussian function for the signal component and a product of exponential

and polynomial functions for the background are used. The difference is treated as systematic uncertainty.

The ratio of the total efficiencies (including acceptance, reconstruction, selection and trigger) for the Bþc !

J=cDþ

s and Bþc !J=cþ modes is determined with

simulated data to be0:1480:001, where the uncertainty is statistical only. As only events explicitly selected by the J=c triggers are used, the ratio of the trigger efficiencies for the Bþc !J=cDþs andBþc !J=cþ modes is close to unity.

VI. SYSTEMATIC UNCERTAINTIES Uncertainties on the ratioRDþ

s=þrelated to differences

between the data and simulation efficiency for the selection requirements are studied using the abundant Bþc ! J=cþ channel. As an example, Fig. 4 compares the distributions of 2fitðBþcÞ and 2IPðBþcÞ for data and simu-lated Bþc !J=cþ events. For background subtraction the sPlot technique [36] has been used. It can be seen that the agreement between data and simulation is good. In addition, a large sample of selected Bþ!

J=cðKþKÞ

Kþevents has been used to quantify

differ-ences between data and simulation. Based on the deviation, a systematic uncertainty of 1% is assigned.

The agreement of the absolute trigger efficiency be-tween data and simulation has been validated to a precision of 4% using the technique described in Refs. [13,31,37] with a large sample of Bþ !J=cðKþKÞKþ events. A further cancellation of uncertainties occurs in the ratio of branching fractions resulting in a systematic uncertainty of 1.1%.

The systematic uncertainties related to the fit model, in particular to the signal shape, mass and resolution for the

6.2 6.3 6.4 6.5 0

200 400 600 800 1000 1200 1400

FIG. 3 (color online). Mass distribution for selected Bþc !

J=cþ candidates. The results of a fit to the model described

in the text are superimposed (solid line) together with the background component (dotted line).

[image:5.612.318.554.45.231.2]
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c !J=cDþs mode and the fit interval have been

dis-cussed in Secs.IVandV. The main part comes from the normalization channelBþc !J=cþ.

Other systematic uncertainties arise from differences in the efficiency of charged particle reconstruction between data and simulation. The largest of these arises from the knowledge of the hadronic interaction probability in the detector, which has an uncertainty of 2% per track [37]. A further uncertainty related to the reconstruction of two additional kaons in theBþc !J=cDþs mode with respect to the Bþc !J=cþ mode is estimated to be 20:6%

[38]. Further uncertainties are related to the track quality selection requirements 2

tr<4 and Pfake<0:5. These

are estimated from a comparison of data and simulation in the Bþc !J=cþ decay mode to be 0.4% per final state track.

The uncertainty associated with the kaon identification criteria is studied using the combinedBþc !J=cDþs and

c !J=cDþs signals. The efficiency to identify a kaon

pair with a selection onK=lnLhas been compared for data and simulation for various selection requirements. The comparison shows a ð1:82:9Þ% difference between data and simulation in the efficiency to identify a kaon pair with 2min K=logL. This estimate has been confirmed using a kinematically similar sample of recon-structed Bþ!J=cðKþKÞKþ events. An uncertainty of 3% is assigned.

The limited knowledge of theBþc lifetime leads to an additional systematic uncertainty due to the different decay time acceptance between the Bþc !J=cDþs and Bþc ! J=cþ decay modes. To estimate this effect, the decay time distributions for simulated events are reweighted to change theBþc lifetime by one standard deviation from the known value [1], as well as the value recently measured by the CDF Collaboration [2], and the efficiencies are recomputed. An uncertainty of 1% is assigned.

Possible uncertainties related to the stability of the data taking conditions are tested by studying the ratio of the

yields ofBþ!J=cKþþandBþ!J=cKþdecays for different data taking periods and dipole magnet polari-ties. This results in a further 2.5% uncertainty.

The largest systematic uncertainty is due to the knowl-edge of the branching fraction of theDþs ! ðKKþÞþ decay, with a kaon pair mass within 20 MeV=c2 of the

knownmeson mass. The value ofð2:240:110:06Þ%

from Ref. [39] is used in the analysis. The systematic uncertainties onRDþ

s=þ are summarized in TableIII.

The ratioRDþ

s =Dþs is estimated as

RDþ

s =Dþs ¼

NBþ

c!J=cDþs

NBþ

c!J=cDþs

; (2)

where the ratio of yields is given in Table II. The uncer-tainty associated with the assumption that the efficiencies for the Bþc !J=cDþs and Bþc !J=cDþs modes are equal is evaluated by studying the dependence of the relative yields for these modes for loose (or no) require-ments on the2IPðBþcÞ,2fitðBþcÞandcðBþcÞvariables. For this selection the measured ratio of Bþc !J=cDþs to

0 1 2 3 4 5

-0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0 5 10 15 20 25

0 0.05 0.1 0.15 0.2

[image:6.612.103.510.45.201.2]

FIG. 4 (color online). Distributions of (a)2fitðBþcÞand (b)2IPðBþcÞforBþc !J=cþevents: background subtracted data (red points with error bars) and simulation (blue histogram).

TABLE III. Relative systematic uncertainties for the ratio of branching fractions ofBþc !J=cDþs andBþc !J=cþ.

Source Uncertainty [%]

Simulated efficiencies 1.0

Trigger 1.1

Fit model 1.8

Track reconstruction 20:6

Hadron interactions 22:0

Track quality selection 20:4

Kaon identification 3.0

c lifetime 1.0

Stability for various data taking conditions 2.5

BðDþs ! ðKKþÞþÞ 5.6

Total 8.4

OBSERVATION OFBþc !J=cDþs . . . PHYSICAL REVIEW D87,112012 (2013)

[image:6.612.315.561.565.718.2]
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c !J=cDþs events changes to 2:270:59. An

uncer-tainty of 4% is assigned to theRDþ

s =Dþs ratio.

The uncertainty on the fraction of theA amplitude, f, has been studied with different fit models for the parameterization of the combinatorial background, as well as different mass resolution models. This is negligible in comparison to the statistical uncertainty.

VII. RESULTS AND SUMMARY

The decays Bþc !J=cDþs and Bþc !J=cDþs have been observed for the first time with statistical significan-ces in exsignifican-cess of 9 standard deviations. The ratio of branch-ing fractions for Bþc !J=cDþs and Bþc !J=cþ is calculated as

BðBþ

c !J=cDþsÞ

BðBþ

c !J=cþÞ

¼ 1

BDþ

s

"

tot

c!J=cþ

"tot

c!J=cDþs

NðBþc !J=cDþs Þ

NðBþ

c !J=cþÞ; (3)

where the value of BDþ

s ¼BðDþs ! ðKKþÞþÞ [39]

with the mass of the kaon pair within 20 MeV=c2

of the known value of the mass is used, together with the ratio of efficiencies, and the signal yields given in Secs.IVandV. This results in

BðBþ

c !J=cDþsÞ

BðBþ

c !J=cþÞ ¼

2:900:57ðstatÞ 0:24ðsystÞ:

The value obtained is in agreement with the naı¨ve expec-tations given in Eq. (1a) from B0 decays, and the values

from Refs. [8–10] but larger than predictions from Refs. [7,11] and factorization expectations from Bþ decays.

The ratio of branching fractions for theBþc !J=cDþs andBþc !J=cDþs decays is measured to be

BðBþ

c !J=cDþsÞ

BðBþ

c !J=cþÞ ¼

2:370:56ðstatÞ 0:10ðsystÞ:

This result is in agreement with the naı¨ve factorization hypothesis [Eq. (1b)] and with the predictions of Refs. [8,9].

The fraction of the A amplitude in the Bþc !

J=cDþ

s decay is measured to be

ðBþc !J=cDþs Þ totðBþc !J=cDþs Þ

¼ ð5220Þ%;

in agreement with a simple estimate of 23, the measure-ments [40,41] and factorization predictions [42] forB0!

DDþ

s decays, and expectations for Bþc !J=c‘þ‘

decays from Refs. [43,44].

The mass of the Bþc meson and the mass difference between theBþc andDþs mesons are measured to be

mBþ

c ¼6276:281:44ðstatÞ 0:36ðsystÞMeV=c

2;

mBþ

c mDþs ¼4307:971:44ðstatÞ 0:20ðsystÞMeV=c

2:

TheBþc mass measurement is in good agreement with the previous result obtained by LHCb in the Bþc !J=cþ mode [29] and has smaller systematic uncertainty.

ACKNOWLEDGMENTS

We thank A. Luchinsky and A. K. Likhoded for advice on aspects of Bþc physics. We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 and Region Auvergne (France); BMBF, DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO (Netherlands); SCSR (Poland); ANCS/IFA (Romania); MinES, Rosatom, RFBR and NRC ‘‘Kurchatov Institute’’ (Russia); MinECo, XuntaGal and GENCAT (Spain); SNSF and SER (Switzerland); NAS Ukraine (Ukraine); STFC (United Kingdom); NSF (USA). We also acknowledge the support received from the ERC under FP7. The Tier1 computing centres are supported by IN2P3 (France), KIT and BMBF (Germany), INFN (Italy), NWO and SURF (Netherlands), PIC (Spain), GridPP (United Kingdom). We are thankful for the computing resources put at our disposal by Yandex LLC (Russia), as well as to the communities behind the multiple open source software packages that we depend on.

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J. Blouw,11S. Blusk,57V. Bocci,24A. Bondar,33N. Bondar,29W. Bonivento,15S. Borghi,53A. Borgia,57 T. J. V. Bowcock,51E. Bowen,39C. Bozzi,16T. Brambach,9J. van den Brand,41J. Bressieux,38D. Brett,53 M. Britsch,10T. Britton,57N. H. Brook,45H. Brown,51I. Burducea,28A. Bursche,39G. Busetto,21,qJ. Buytaert,37 S. Cadeddu,15O. Callot,7M. Calvi,20,jM. Calvo Gomez,35,nA. Camboni,35P. Campana,18,37D. Campora Perez,37 A. Carbone,14,cG. Carboni,23,kR. Cardinale,19,iA. Cardini,15H. Carranza-Mejia,49L. Carson,52K. Carvalho Akiba,2

G. Casse,51M. Cattaneo,37Ch. Cauet,9M. Charles,54Ph. Charpentier,37P. Chen,3,38N. Chiapolini,39 M. Chrzaszcz,25K. Ciba,37X. Cid Vidal,37G. Ciezarek,52P. E. L. Clarke,49M. Clemencic,37H. V. Cliff,46 J. Closier,37C. Coca,28V. Coco,40J. Cogan,6E. Cogneras,5P. Collins,37A. Comerma-Montells,35A. Contu,15,37 A. Cook,45M. Coombes,45S. Coquereau,8G. Corti,37B. Couturier,37G. A. Cowan,49D. C. Craik,47S. Cunliffe,52

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R. Currie,49C. D’Ambrosio,37P. David,8P. N. Y. David,40A. Davis,56I. De Bonis,4K. De Bruyn,40S. De Capua,53 M. De Cian,39J. M. De Miranda,1L. De Paula,2W. De Silva,56P. De Simone,18D. Decamp,4M. Deckenhoff,9

L. Del Buono,8D. Derkach,14O. Deschamps,5F. Dettori,41A. Di Canto,11H. Dijkstra,37M. Dogaru,28 S. Donleavy,51F. Dordei,11A. Dosil Sua´rez,36D. Dossett,47A. Dovbnya,42F. Dupertuis,38R. Dzhelyadin,34 A. Dziurda,25A. Dzyuba,29S. Easo,48,37U. Egede,52V. Egorychev,30S. Eidelman,33D. van Eijk,40S. Eisenhardt,49 U. Eitschberger,9R. Ekelhof,9L. Eklund,50,37I. El Rifai,5Ch. Elsasser,39D. Elsby,44A. Falabella,14,eC. Fa¨rber,11 G. Fardell,49C. Farinelli,40S. Farry,12V. Fave,38D. Ferguson,49V. Fernandez Albor,36F. Ferreira Rodrigues,1

M. Ferro-Luzzi,37S. Filippov,32M. Fiore,16C. Fitzpatrick,37M. Fontana,10F. Fontanelli,19,iR. Forty,37 O. Francisco,2M. Frank,37C. Frei,37M. Frosini,17,fS. Furcas,20E. Furfaro,23,kA. Gallas Torreira,36D. Galli,14,c

M. Gandelman,2P. Gandini,57Y. Gao,3J. Garofoli,57P. Garosi,53J. Garra Tico,46L. Garrido,35C. Gaspar,37 R. Gauld,54E. Gersabeck,11M. Gersabeck,53T. Gershon,47,37Ph. Ghez,4V. Gibson,46V. V. Gligorov,37C. Go¨bel,58

D. Golubkov,30A. Golutvin,52,30,37A. Gomes,2H. Gordon,54M. Grabalosa Ga´ndara,5R. Graciani Diaz,35 L. A. Granado Cardoso,37E. Grauge´s,35G. Graziani,17A. Grecu,28E. Greening,54S. Gregson,46O. Gru¨nberg,59 B. Gui,57E. Gushchin,32Yu. Guz,34,37T. Gys,37C. Hadjivasiliou,57G. Haefeli,38C. Haen,37S. C. Haines,46S. Hall,52

T. Hampson,45S. Hansmann-Menzemer,11N. Harnew,54S. T. Harnew,45J. Harrison,53T. Hartmann,59J. He,37 V. Heijne,40K. Hennessy,51P. Henrard,5J. A. Hernando Morata,36E. van Herwijnen,37E. Hicks,51D. Hill,54 M. Hoballah,5C. Hombach,53P. Hopchev,4W. Hulsbergen,40P. Hunt,54T. Huse,51N. Hussain,54D. Hutchcroft,51

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O. Okhrimenko,43R. Oldeman,15,dM. Orlandea,28J. M. Otalora Goicochea,2P. Owen,52A. Oyanguren,35,o B. K. Pal,57A. Palano,13,bM. Palutan,18J. Panman,37A. Papanestis,48M. Pappagallo,50C. Parkes,53 C. J. Parkinson,52G. Passaleva,17G. D. Patel,51M. Patel,52G. N. Patrick,48C. Patrignani,19,iC. Pavel-Nicorescu,28

A. Pazos Alvarez,36A. Pellegrino,40G. Penso,24,lM. Pepe Altarelli,37S. Perazzini,14,cD. L. Perego,20,j E. Perez Trigo,36A. Pe´rez-Calero Yzquierdo,35P. Perret,5M. Perrin-Terrin,6G. Pessina,20K. Petridis,52 A. Petrolini,19,iA. Phan,57E. Picatoste Olloqui,35B. Pietrzyk,4T. Pilarˇ,47D. Pinci,24S. Playfer,49M. Plo Casasus,36

F. Polci,8G. Polok,25A. Poluektov,47,33E. Polycarpo,2D. Popov,10B. Popovici,28C. Potterat,35A. Powell,54 J. Prisciandaro,38V. Pugatch,43A. Puig Navarro,38G. Punzi,22,rW. Qian,4J. H. Rademacker,45

B. Rakotomiaramanana,38M. S. Rangel,2I. Raniuk,42N. Rauschmayr,37G. Raven,41S. Redford,54M. M. Reid,47 A. C. dos Reis,1S. Ricciardi,48A. Richards,52K. Rinnert,51V. Rives Molina,35D. A. Roa Romero,5P. Robbe,7 E. Rodrigues,53P. Rodriguez Perez,36S. Roiser,37V. Romanovsky,34A. Romero Vidal,36J. Rouvinet,38T. Ruf,37 F. Ruffini,22H. Ruiz,35P. Ruiz Valls,35,oG. Sabatino,24,kJ. J. Saborido Silva,36N. Sagidova,29P. Sail,50B. Saitta,15,d

C. Salzmann,39B. Sanmartin Sedes,36M. Sannino,19,iR. Santacesaria,24C. Santamarina Rios,36E. Santovetti,23,k M. Sapunov,6A. Sarti,18,jC. Satriano,24,mA. Satta,23M. Savrie,16,eD. Savrina,30,31P. Schaack,52M. Schiller,41

H. Schindler,37M. Schlupp,9M. Schmelling,10B. Schmidt,37O. Schneider,38A. Schopper,37M.-H. Schune,7 R. Schwemmer,37B. Sciascia,18A. Sciubba,24M. Seco,36A. Semennikov,30K. Senderowska,26I. Sepp,52N. Serra,39

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J. Serrano,6P. Seyfert,11M. Shapkin,34I. Shapoval,16,42P. Shatalov,30Y. Shcheglov,29T. Shears,51,37 L. Shekhtman,33O. Shevchenko,42V. Shevchenko,30A. Shires,52R. Silva Coutinho,47T. Skwarnicki,57

N. A. Smith,51E. Smith,54,48M. Smith,53M. D. Sokoloff,56F. J. P. Soler,50F. Soomro,18D. Souza,45 B. Souza De Paula,2B. Spaan,9A. Sparkes,49P. Spradlin,50F. Stagni,37S. Stahl,11O. Steinkamp,39S. Stoica,28

S. Stone,57B. Storaci,39M. Straticiuc,28U. Straumann,39V. K. Subbiah,37S. Swientek,9V. Syropoulos,41 M. Szczekowski,27P. Szczypka,38,37T. Szumlak,26S. T’Jampens,4M. Teklishyn,7E. Teodorescu,28F. Teubert,37 C. Thomas,54E. Thomas,37J. van Tilburg,11V. Tisserand,4M. Tobin,38S. Tolk,41D. Tonelli,37S. Topp-Joergensen,54 N. Torr,54E. Tournefier,4,52S. Tourneur,38M. T. Tran,38M. Tresch,39A. Tsaregorodtsev,6P. Tsopelas,40N. Tuning,40

M. Ubeda Garcia,37A. Ukleja,27D. Urner,53U. Uwer,11V. Vagnoni,14G. Valenti,14R. Vazquez Gomez,35 P. Vazquez Regueiro,36S. Vecchi,16J. J. Velthuis,45M. Veltri,17,gG. Veneziano,38M. Vesterinen,37B. Viaud,7 D. Vieira,2X. Vilasis-Cardona,35,nA. Vollhardt,39D. Volyanskyy,10D. Voong,45A. Vorobyev,29V. Vorobyev,33

C. Voß,59H. Voss,10R. Waldi,59R. Wallace,12S. Wandernoth,11J. Wang,57D. R. Ward,46N. K. Watson,44 A. D. Webber,53D. Websdale,52M. Whitehead,47J. Wicht,37J. Wiechczynski,25D. Wiedner,11L. Wiggers,40

G. Wilkinson,54M. P. Williams,47,48M. Williams,55F. F. Wilson,48J. Wishahi,9M. Witek,25S. A. Wotton,46 S. Wright,46S. Wu,3K. Wyllie,37Y. Xie,49,37F. Xing,54Z. Xing,57Z. Yang,3R. Young,49X. Yuan,3 O. Yushchenko,34M. Zangoli,14M. Zavertyaev,10,aF. Zhang,3L. Zhang,57W. C. Zhang,12Y. Zhang,3

A. Zhelezov,11A. Zhokhov,30L. Zhong,3and A. Zvyagin37

(LHCb Collaboration)

1

Centro Brasileiro de Pesquisas Fı´sicas (CBPF), Rio de Janeiro, Brazil

2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China

4LAPP, Universite´ de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France

5Clermont Universite´, Universite´ Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France

7LAL, Universite´ Paris-Sud, CNRS/IN2P3, Orsay, France 8

LPNHE, Universite´ Pierre et Marie Curie, Universite´ Paris Diderot, CNRS/IN2P3, Paris, France

9Fakulta¨t Physik, Technische Universita¨t Dortmund, Dortmund, Germany 10Max-Planck-Institut fu¨r Kernphysik (MPIK), Heidelberg, Germany

11Physikalisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany 12School of Physics, University College Dublin, Dublin, Ireland

13Sezione INFN di Bari, Bari, Italy 14Sezione INFN di Bologna, Bologna, Italy 15Sezione INFN di Cagliari, Cagliari, Italy 16Sezione INFN di Ferrara, Ferrara, Italy

17Sezione INFN di Firenze, Firenze, Italy

18Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 19Sezione INFN di Genova, Genova, Italy

20Sezione INFN di Milano Bicocca, Milano, Italy 21Sezione INFN di Padova, Padova, Italy

22Sezione INFN di Pisa, Pisa, Italy 23Sezione INFN di Roma Tor Vergata, Roma, Italy 24

Sezione INFN di Roma La Sapienza, Roma, Italy

25Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krako´w, Poland 26AGH-University of Science and Technology, Faculty of Physics and Applied Computer Science, Krako´w, Poland

27National Center for Nuclear Research (NCBJ), Warsaw, Poland

28Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 29Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia

30Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia 31Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia 32Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 33

Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia

34Institute for High Energy Physics (IHEP), Protvino, Russia 35Universitat de Barcelona, Barcelona, Spain

36Universidad de Santiago de Compostela, Santiago de Compostela, Spain 37European Organization for Nuclear Research (CERN), Geneva, Switzerland

OBSERVATION OFBþc !J=cDþs . . . PHYSICAL REVIEW D87,112012 (2013)

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38Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Lausanne, Switzerland 39Physik-Institut, Universita¨t Zu¨rich, Zu¨rich, Switzerland

40Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands

41Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, Netherlands 42NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine

43Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 44University of Birmingham, Birmingham, United Kingdom

45H. H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 46

Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom

47Department of Physics, University of Warwick, Coventry, United Kingdom 48STFC Rutherford Appleton Laboratory, Didcot, United Kingdom

49School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 50School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom

51Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 52Imperial College London, London, United Kingdom

53School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 54Department of Physics, University of Oxford, Oxford, United Kingdom

55Massachusetts Institute of Technology, Cambridge, Massachusetts, USA 56University of Cincinnati, Cincinnati, Ohio, USA

57Syracuse University, Syracuse, New York, USA

58Pontifı´cia Universidade Cato´lica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil

[associated with Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil]

59Institut fu¨r Physik, Universita¨t Rostock, Rostock, Germany [associated with Physikalisches Institut,

Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany]

aAlso at P. N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia.

bAlso at Universita` di Bari, Bari, Italy.

cAlso at Universita` di Bologna, Bologna, Italy.

dAlso at Universita` di Cagliari, Cagliari, Italy.

eAlso at Universita` di Ferrara, Ferrara, Italy.

fAlso at Universita` di Firenze, Firenze, Italy.

g

Also at Universita` di Urbino, Urbino, Italy.

hAlso at Universita` di Modena e Reggio Emilia, Modena, Italy.

iAlso at Universita` di Genova, Genova, Italy.

jAlso at Universita` di Milano Bicocca, Milano, Italy.

kAlso at Universita` di Roma Tor Vergata, Roma, Italy.

lAlso at Universita` di Roma La Sapienza, Roma, Italy.

mAlso at Universita` della Basilicata, Potenza, Italy.

nAlso at LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain.

oAlso at IFIC, Universitat de Valencia-CSIC, Valencia, Spain.

pAlso at Hanoi University of Science, Hanoi, Vietnam.

qAlso at Universita` di Padova, Padova, Italy.

rAlso at Universita` di Pisa, Pisa, Italy.

sAlso at Scuola Normale Superiore, Pisa, Italy.

Figure

FIG. 1.Feynman diagrams for Bþc ! J=c Dþs decays: (a) spectator, (b) color-suppressed spectator and (c) annihilation topology.
TABLE II.Signal parameters of the unbinned extended maxi-mum likelihood fit to the J=c Dþs mass distribution.
FIG. 3 (color online).Mass distribution for selectedin the text are superimposed (solid line) together with the Bþc !J=c �þ candidates
FIG. 4 (color online).Distributions of (a) �2fitðBþc Þ and (b) �2IPðBþc Þ for Bþc ! J=c �þ events: background subtracted data (red pointswith error bars) and simulation (blue histogram).

References

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