Measurement of the branching fraction B(Λ_b⁰ -> Λ_c⁺π⁻π⁺π⁻) at CDF

Article

Reference

Measurement of the branching fraction B(Λ

-> Λ

π

π

π

) at CDF

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al.

Abstract

We report an analysis of the Λ0b→Λ+cπ−π+π− decay in a data sample collected by the CDF II detector at the Fermilab Tevatron corresponding to 2.4  fb−1 of integrated luminosity. We reconstruct the currently largest samples of the decay modes Λ0b→Λc(2595)+π− (with Λc(2595)+→Λ+cπ+π−), Λ0b→Λc(2625)+π− (with Λc(2625)+→Λ+cπ+π−), Λ0b→Σc(2455)++π−π− (with Σc(2455)++→Λ+cπ+), and Λ0b→Σc(2455)0π+π− (with Σc(2455)0→Λ+cπ−) and measure the branching fractions relative to the Λ0b→Λ+cπ−

branching fraction. We measure the ratio

B(Λ0b→Λ+cπ−π+π−)/B(Λ0b→Λ+cπ−)=3.04±0.33(stat)+0.70−0.55(syst) which is used to derive B(Λ0b→Λ+cπ−π+π−)=(26.8+11.9−11.2)×10−3.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Measurement of the branching fraction B(Λ

-> Λ

π

π

π

) at CDF. Physical Review. D , 2012, vol. 85, no. 03, p. 032003

DOI : 10.1103/PhysRevD.85.032003

Available at:

http://archive-ouverte.unige.ch/unige:38809

Disclaimer: layout of this document may differ from the published version.

Measurement of the branching fraction Bð

!

Þ at CDF

T. Aaltonen,²¹B. A´ lvarez Gonza´lez,^9,yS. Amerio,^40aD. Amidei,³²A. Anastassov,^15,wA. Annovi,¹⁷J. Antos,¹² G. Apollinari,¹⁵J. A. Appel,¹⁵T. Arisawa,⁵⁴A. Artikov,¹³J. Asaadi,⁴⁹W. Ashmanskas,¹⁵B. Auerbach,⁵⁷A. Aurisano,⁴⁹ F. Azfar,³⁹P. Azzurri,^42a,42bW. Badgett,¹⁵T. Bae,²⁵A. Barbaro-Galtieri,²⁶V. E. Barnes,⁴⁴B. A. Barnett,²³P. Barria,^42a,42c P. Bartos,¹²M. Bauce,^40a,40bF. Bedeschi,^42aS. Behari,²³G. Bellettini,^42a,42bJ. Bellinger,⁵⁶D. Benjamin,¹⁴A. Beretvas,¹⁵

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I. Yu,²⁵S. S. Yu,¹⁵J. C. Yun,¹⁵A. Zanetti,^50aY. Zeng,¹⁴and S. Zucchelli^6a,6b (CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3University of Athens, 157 71 Athens, Greece

4Institut de Fisica d’Altes Energies, ICREA, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

5Baylor University, Waco, Texas 76798, USA

6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy

6bUniversity of Bologna, I-40127 Bologna, Italy

7University of California, Davis, Davis, California 95616, USA

8University of California, Los Angeles, Los Angeles, California 90024, USA

9Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

10Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

11Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

12Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia

13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

14Duke University, Durham, North Carolina 27708, USA

15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

16University of Florida, Gainesville, Florida 32611, USA

17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

18University of Geneva, CH-1211 Geneva 4, Switzerland

19Glasgow University, Glasgow G12 8QQ, United Kingdom

20Harvard University, Cambridge, Massachusetts 02138, USA

21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

22University of Illinois, Urbana, Illinois 61801, USA

23The Johns Hopkins University, Baltimore, Maryland 21218, USA

24Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany

25Center for High Energy Physics, Kyungpook National University, Daegu 702-701, Korea;

Seoul National University, Seoul 151-742, Korea;

Sungkyunkwan University, Suwon 440-746, Korea;

Korea Institute of Science and Technology Information, Daejeon 305-806, Korea;

Chonnam National University, Gwangju 500-757, Korea;

Chonbuk National University, Jeonju 561-756, Korea

26Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

27University of Liverpool, Liverpool L69 7ZE, United Kingdom

28University College London, London WC1E 6BT, United Kingdom

29Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

30Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

31Institute of Particle Physics, McGill University, Montre´al, Que´bec, Canada H3A 2T8;

Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6;

University of Toronto, Toronto, Ontario, Canada M5S 1A7;

and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3

32University of Michigan, Ann Arbor, Michigan 48109, USA

33Michigan State University, East Lansing, Michigan 48824, USA

34Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

T. AALTONENet al. PHYSICAL REVIEW D 032003 (2012)

35University of New Mexico, Albuquerque, New Mexico 87131, USA

36The Ohio State University, Columbus, Ohio 43210, USA

37Okayama University, Okayama 700-8530, Japan

38Osaka City University, Osaka 588, Japan

39University of Oxford, Oxford OX1 3RH, United Kingdom

40aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

40bUniversity of Padova, I-35131 Padova, Italy

41University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

42aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy

42bUniversity of Pisa, I-56127 Pisa, Italy

42cUniversity of Siena, I-56127 Pisa, Italy

42dScuola Normale Superiore, I-56127 Pisa, Italy

43University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

44Purdue University, West Lafayette, Indiana 47907, USA

45University of Rochester, Rochester, New York 14627, USA

46The Rockefeller University, New York, New York 10065, USA

47aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy

47bSapienza Universita` di Roma, I-00185 Roma, Italy

48Rutgers University, Piscataway, New Jersey 08855, USA

49Texas A&M University, College Station, Texas 77843, USA

50aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy

50bUniversity of Udine, I-33100 Udine, Italy

51University of Tsukuba, Tsukuba, Ibaraki 305, Japan

52Tufts University, Medford, Massachusetts 02155, USA

53University of Virginia, Charlottesville, Virginia 22906, USA

54Waseda University, Tokyo 169, Japan

55Wayne State University, Detroit, Michigan 48201, USA

56University of Wisconsin, Madison, Wisconsin 53706, USA

57Yale University, New Haven, Connecticut 06520, USA (Received 14 December 2011; published 13 February 2012)

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We report an analysis of the ⁰_b !^þ_c^þ decay in a data sample collected by the CDF II detector at the Fermilab Tevatron corresponding to2:4 fb¹of integrated luminosity. We reconstruct the currently largest samples of the decay modes ⁰_b!_cð2595Þ^þ (with _cð2595Þ^þ!^þ_c^þ), ⁰_b!_cð2625Þ^þ (with _cð2625Þ^þ!^þ_c^þ), ⁰_b!_cð2455Þ^þþ (with _cð2455Þ^þþ! ^þ_c^þ), and ⁰_b!_cð2455Þ^0þ (with _cð2455Þ⁰!^þ_c) and measure the branching fractions relative to the⁰_b!^þ_c branching fraction. We measure the ratioBð⁰_b!^þ_c^þÞ=

Bð⁰_b!^þ_cÞ ¼3:040:33ðstatÞ^þ0:70_0:55ðsystÞ which is used to derive Bð⁰_b!^þ_c^þÞ¼ ð26:8^þ11:9_11:2Þ10³.

DOI:10.1103/PhysRevD.85.032003 PACS numbers: 14.20.Mr, 14.20.Lq

I. INTRODUCTION

Because of the high b-quark mass, weak decays of baryons containing abquark are a good testing ground of some approximations in quantum chromodynamics calculations, such as heavy-quark effective theory [1].

Alternatively, when one uses such calculations, the ⁰_b may provide a determination of the Cabibbo-Kobayashi- Maskawa couplings with systematic uncertainties different from the determinations from the decays ofBmesons [2].

While theBmesons are well studied, less is known about the ⁰_b baryon. Only nine decay modes of the ⁰_b have been observed so far, with the sum of their measured branching fractions of the order of only 0.1 and with large uncertainties on the measurements [3]. While theoretical predictions are available for the ⁰_b!^þ_c branching fraction [4], no prediction is currently available for the ⁰_b!^þ_c^þdecay mode. The LHCb Collaboration recently reported the measurement of the ratio of branching fractions Bð⁰_b !^þ_c^þÞ=Bð⁰_b !^þ_cÞ ¼ 1:430:16ðstatÞ 0:13ðsystÞ[5].

This paper reports a study of the ⁰_b!^þ_c^þ decay mode and is especially distinguished by the high yields and high precision measurement of the ⁰_b ! ^þ_c^þ resonant contributions, the following decay modes:

⁰_b!_cð2595Þ^þ; ⁰_b!_cð2625Þ^þ; ⁰_b!_cð2455Þ^þþ; ⁰_b!_cð2455Þ^0þ: We measure the branching fraction of each resonant decay mode relative to the ⁰_b!^þ_c decay mode, and the ratio of branching fractions Bð⁰_b!^þ_c^þÞ=

Bð⁰_b!^þ_cÞ. The measurement is performed using a sample of pp collisions corresponding to 2:4 fb¹ inte- grated luminosity collected by CDF II between February 2002 and May 2007. We reconstruct ⁰_b decays from particles whose trajectory projections in the plane trans- verse to the beam line do not intersect the beam line (displaced tracks). The signal yields of interest are ex- tracted by fitting mass differences to minimize the effect of systematic uncertainties. As a cross-check, we repeat the analysis on the reference decay modesB⁰ !D^þþ andB⁰ !D^þ.

The structure of the paper is as follows. Section II describes the detector systems relevant to this analysis.

Event selection and ⁰_b!^þ_c^þ and ⁰_b! ^þ_c candidate reconstruction are described in Sec.III.

In Sec. IV we present the signal yields. In Sec. V we describe the evaluation of the detector acceptance and the relative branching fraction measurements, while in Sec.VI the systematic uncertainties are discussed. Final results are reported in Sec.VII, and we conclude in Sec.VIII.

II. THE CDF II DETECTOR AND TRIGGER The CDF II detector is a multipurpose magnetic spec- trometer surrounded by calorimeters and muon detectors.

The components relevant to this analysis are briefly de- scribed here. A more detailed description can be found elsewhere [6]. A silicon microstrip detector (SVX and ISL) [7] and a cylindrical drift chamber (COT) [8] immersed in a 1.4 T solenoidal magnetic field allow the reconstruction of charged particle trajectories in the pseudorapidity [9]

rangejj<1:0[10]. The SVX detector consists of micro- strip sensors arranged in six cylindrical shells around the beam line with radii of between 1.5 and 10.6 cm, and with a totalzcoverage of 90 cm. The first SVX layer, also referred to as the L00 detector, is made of single-sided sensors mounted on the beryllium beam pipe. The remaining five SVX layers are made of double-sided sensors and divided into three contiguous five-layer sections along the beam direction z. The two additional silicon layers of the ISL help to link tracks in the COT to hits in the SVX. The COT has 96 measurement layers between 40 and 137 cm in radius, organized into alternating axial and 2 stereo superlayers. The charged particle transverse momentum resolution is p_T=pT ’0:07%pT ðGeV=cÞ, and the reso- lution on the transverse distance of closest approach of the particle trajectory to the beam line (impact parameter,d0) is 40m, including a30m contribution from the beam line.

Candidate events for this analysis are selected by a three-level online event selection system (trigger). At level 1, charged particles are reconstructed in the COT axial superlayers by a hardware processor, the ‘‘extremely fast tracker’’ (XFT) [11]. Two charged particles are required with transverse momenta pT 2 GeV=c. At level 2, the Silicon Vertex Trigger (SVT) [12] associates SVXr

T. AALTONENet al. PHYSICAL REVIEW D 032003 (2012)

position measurements with XFT tracks. This provides a precise measurement of the track impact parameterd0. We select b-hadron candidates by requiring two SVT tracks with 120md0 1000 m. To reduce background from light-quark jet pairs, the two trigger tracks are re- quired to have an opening angle in the transverse plane of 290. The tracks must also satisfy the require- mentLT>200m, whereLTis defined as the distance in the transverse plane from the beam line to the two-track intersection point, projected onto the two-track momentum vector. The level 1 and level 2 trigger requirements are then confirmed at trigger level 3, where the event is fully reconstructed.

III. EVENT RECONSTRUCTION

The search for ⁰_b!^þ_c^þ and ⁰_b!^þ_c candidates begins with the reconstruction of the^þ_c using the three-body decay ^þ_c !pK^þ [13]. Three tracks, assumed to be a kaon, a proton, and a pion, with a total charge of þ1, are fit to a common vertex. No particle identification is used in this analysis. All particle hypoth- eses consistent with the candidate decay chain are consid- ered. Additional selection criteria (cuts) are applied on fit probability (Pð²ð^þ_cÞÞ>10⁵), transverse momentum (p_Tð^þ_cÞ>4:0 GeV=c), and transverse decay length rela- tive to the beam line (LTð^þ_cÞ>200m). We also require pTðpÞ> pTð^þÞ, to suppress random-track combinatorial background. The reconstructed ^þ_c mass (mð^þ_cÞ) distri- bution is comparable to the one reported in Ref. [14]. The reconstructed^þ_c mass is required to be close to the known ^þ_c mass (2:240–2:330 GeV=c²) [3]. Since mass differ- ences are used to search for the resonances, no mass constraint is applied in the^þ_c reconstruction. The⁰_b ! ^þ_c^þ (⁰_b !^þ_c) candidate is reconstructed by performing a fit to a common vertex of the reconstructed ^þ_c and three (one) additional tracks, assumed to be pions, withpT>0:4 GeV=c, and a total charge of1. For all the possible track pairs out of the six (four) tracks that form the ⁰_b candidate, we require the difference between the z coordinate of the points of closest approach of the two tracks to the beam to be less than 5 cm. Additional cuts on the⁰_bcandidate fit probability (Pð²ð⁰_bÞÞ>10⁴), trans- verse momentum (p_Tð⁰_bÞ>6:0 GeV=c), transverse decay length relative to the beam line (LTð⁰_bÞ>

200m), and ^þ_c transverse decay length relative to the beam line (L_Tð^þ_cÞ>200m) and to the ⁰_b vertex (LTð^þ_c from⁰_bÞ>200m) are applied. We also require that the transverse momentum of the pion produced in the ^þ_c decay is larger than the transverse momentum of the same-charge pion produced in the ⁰_b decay, which con- siderably reduces the combinatorial background due to the larger boost of the pion produced in the ^þ_c decay. To improve the purity of the ⁰_b!^þ_c^þ signal, we optimize the analysis cuts to maximize the signal signifi- cance S= ffiffiffiffiffiffiffiffiffiffiffiffiffiffi

SþB

p . The number of ⁰_b!^þ_c^þ

candidates Sand the number of background eventsBare estimated in data by performing a fit of the mð⁰_bÞ distribution. This procedure determines the final selection criteria: pTð⁰_bÞ>9:0 GeV=c, LTð⁰_bÞ=_LTð⁰

bÞ>16, d0ð⁰_bÞ<70m, and Rð^þÞ<1:2, where d0ð⁰_bÞ is the impact parameter of the reconstructed ⁰_b candidate relative to the beam line and Rð^þÞ is the maximum ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

²þ²

p distance between the two pions in each of the three possible pairs of pions. We verified that, by splitting the data sample in two indepen- dent samples, the optimization procedure yields the same final selection criteria when applied separately to the two samples, and that the⁰_b !^þ_c^þ yield is evenly

FIG. 1 (color online). The reconstructed invariant mass spectra after applying all the selection criteria: (a) the mass difference mð⁰_bÞ mð^þ_cÞdistribution of the⁰_b!^þ_c^þ candi- dates; (b) themð⁰_bÞdistribution of the⁰_b!^þ_ccandidates.

distributed. This ensures that our optimization procedure does not introduce a bias on the branching fraction mea- surement. To reduce possible systematic effects in the estimate of the reconstruction efficiency due to Monte Carlo simulation model inaccuracy, the same selec- tion cuts optimized for ⁰_b!^þ_c^þ are also ap- plied to the selection of the⁰_b!^þ_csignal, except for theRð^þÞcut.

IV. DETERMINATION OF THE SIGNAL YIELDS Figure 1(a) shows the distribution of the difference between the reconstructed ⁰_b and ^þ_c masses, mð⁰_bÞ mð^þ_cÞ, of the selected ⁰_b !^þ_c^þ candidates with the fit projection overlaid. A significant signal of ⁰_b!^þ_c^þ is visible centered approximately at 3:330 GeV=c². Backgrounds include misreconstructed multibody b-hadron decays (physics background) and

FIG. 2 (color online). The⁰_b!_cð2595Þ^þand⁰_b!_cð2625Þ^þsignals: (a)mð^þc Þ mð^þcÞdistribution for candidates in a 3 range (57 MeV=c²) around the ⁰_b mass; (b) mð⁰_bÞ mð^þcÞ distribution restricted to candidates in the region mð^þc Þ mð^þcÞ<0:325 GeV=c²; (c) mð⁰_bÞ mð^þcÞ distribution restricted to candidates in the region 0:325< mð^þc Þ mð^þcÞ<0:360 GeV=c².

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random combinations of charged particles that accidentally meet the selection requirements (combinatorial back- ground). We use an unbinned extended maximum- likelihood fit to estimate the ⁰_b!^þ_c^þ signal yield. The signal peak is modeled with a Gaussian, with mean and width left floating in the fit. The combinatorial background is modeled with an exponential function of mð⁰_bÞ mð^þ_cÞ with floating slope and normalization.

The distribution of the main physics backgrounds, due to theB⁰_ðsÞ!D^ðÞ_ðsÞ ^þþ decay modes, are derived from simulation and included in the fit with fixed shape and floating normalization. The ⁰_b!^þ_c^þ yield es- timated by the fit of the data is1087101candidates, the world’s largest sample currently available of this decay mode. Figure 1(b)shows the ⁰_b mass distribution of the selected⁰_b!^þ_c candidates. The ⁰_b mass distribu- tion is described by several components: the⁰_b!^þ_c Gaussian signal, a combinatorial background, recon- structedB mesons that pass the^þ_c selection criteria, partially reconstructed ⁰_b decays (e.g. ⁰_b!^þ_cl_l), and fully reconstructed⁰_bdecays other than^þ_c (e.g.

⁰_b!^þ_cK). Also in this case the distributions of phys- ics backgrounds are derived from simulation and included in the fit with fixed shapes and floating normalization, as detailed in Ref. [15]. The⁰_b!^þ_cyield estimated by the fit of the data is305278candidates.

In the reconstructed ⁰_b !^þ_c^þ sample we searched for the resonant decay modes: ⁰_b ! _cð2595Þ^þ, ⁰_b!_cð2625Þ^þ, ⁰_b! _cð2455Þ^þþ, and ⁰_b!_cð2455Þ^0þ. The available energy transferred to the decay products in the decays of the charmed baryons (_cð2595Þ^þ, _cð2625Þ^þ, _cð2455Þ^þþ, and_cð2455Þ⁰) into^þ_c is small. Therefore the differences of the reconstructed masses mð^þ_c Þ mð^þ_cÞ, mð_cð2455Þ^þþÞ mð^þ_cÞ, and mð_cð2455Þ⁰Þ mð^þ_cÞ are determined with better resolution than the masses of the charmed baryons, since the mass resolution of the ^þ_c signal and most of the mass systematic uncertainties cancel in the difference. Figure 2(a) shows themð^þ_c Þ mð^þ_cÞdistribution, for⁰_b!^þ_c^þ candidates with mass in a 3 range (57 MeV=c²) around the⁰_bmass. The_cð2595Þ^þand_cð2625Þ^þsignals are clearly visible. Although there are two possible ^þ_c candidates for each ⁰_b!^þ_c^þ decay, only the candidate made with thewith lowerpT has a value of mð^þ_c Þ mð^þ_cÞin the mass region where the_cð2595Þ^þ and_cð2625Þ^þsignals are expected. To check that the^þ_c signal is entirely due to the⁰_b!^þ_c^þdecay, we verified that themð^þ_c Þ mð^þ_cÞ distribution [Fig.2(a), (yellow) filled histogram] for the⁰_b candidates from the high-mass sideband of the mð⁰_bÞ mð^þ_cÞ distribution [Fig.1(a)] has negligible statistics in the^þ_c signal mass window. The_cð2595Þ^þ and_cð2625Þ^þ signal yields are estimated with an unbinned extended maximum-likelihood

fit. The _cð2595Þ^þ and _cð2625Þ^þ signals are modeled with two nonrelativistic Breit-Wigner functions convolved with the same Gaussian resolution function, since the mass difference between the two resonances is tiny. The back- ground is modeled by a linear function. The _cð2595Þ^þ natural width is mass dependent to take into account the threshold effects, as reported in Ref. [14]. The_cð2625Þ^þ natural width and the width of the Gaussian resolution function are free parameters of the fit. Table Ireports the estimated signal yields and significances, evaluated by means of the likelihood ratio test,LR L=Lbck, whereL and Lbck are the likelihood of the signal and no-signal hypotheses, respectively [16].

Figures2(b)and2(c)show the mð⁰_bÞ mð^þ_cÞ distri- bution restricted to candidates with mð^þ_c Þ mð^þ_cÞ<

0:325 GeV=c² and 0:325< mð^þ_c Þ mð^þ_cÞ<

0:360 GeV=c², respectively, i.e. compatible with the _cð2595Þ^þ and_cð2625Þ^þ expected signals. Each signal is modeled with a Gaussian function, with floating mean and width. The combinatorial background is modeled with an exponential function with floating slope and normaliza- tion, and the physics background, which is mainly due to semileptonic⁰_b!^þ_c^þlldecays, is derived from simulation and introduced in the fit with fixed shape and floating normalization. We verified that the ⁰_b! _cð2595Þ^þ and⁰_b !_cð2625Þ^þ yields estimated by fitting themð⁰_bÞ mð^þ_cÞdistribution are compatible (with lower statistical significance) with the yields ex- tracted from the resonance mass distributions and reported in TableI.

To extract the ⁰_b!_cð2455Þ^þþ and ⁰_b! _cð2455Þ^0þsignals, the contributions due to the⁰_b! _cð2595Þ^þand⁰_b!_cð2625Þ^þ decay modes are removed by applying the veto requirement mð^þ_c Þ mð^þ_cÞ>0:380 GeV=c². In Figs.3(a)and3(b)the result- ing mð_cð2455Þ^þþÞ mð^þ_cÞ and mð_cð2455Þ⁰Þ mð^þ_cÞ distributions are shown. Prominent _cð2455Þ^þþ and_cð2455Þ⁰ signals are visible. While there is only one _cð2455Þ^þþcandidate for each⁰_b!^þ_c^þdecay, two_cð2455Þ⁰ candidates are possible. Also in this case, only the candidate made with thewith lowerpTis in the _cð2455Þ⁰ mass region. The potential background contri- bution due to ^þþ;0_c candidates not produced in ⁰_b! ^þ_c^þ decays is excluded since the mð_cð2455Þ^þþ;0Þ mð^þ_cÞ distributions ([yellow] filled TABLE I. Yields and significances of the⁰_b!^þ_c^þ decay modes. The quoted uncertainty is statistical only.

⁰_b decay mode Yield Significance () _cð2595Þ^þ!^þ_c^þ 46:08:2 6.2 _cð2625Þ^þ!^þ_c^þ 13515 >8 _cð2455Þ^þþ!^þ_c^þ 11019 6.6 _cð2455Þ^0þ!^þ_c^þ 3611 3.4 ^þ_c^þðotherÞ 790100 >8

histograms in Figs. 3(a) and 3(b)] obtained from the ⁰_b candidates in the higher mass sideband [Fig.1(a)] show no evidence of a^þþ;0_c signal. The_cð2455Þ^þþand_cð2455Þ⁰ signals are modeled with nonrelativistic Breit-Wigner func- tions convolved with a Gaussian resolution function, with the addition of an empirical background [17,18]. The _cð2455Þ^þþ and _cð2455Þ⁰ natural widths are Gaussian constrained to the world average values [3], while the width of the Gaussian resolution function is determined to be 1 MeV=c² from larger statistics samples of _cð2455Þ^þþ and_cð2455Þ⁰ in the⁰_blower mass region and is fixed in

the fit. The effect of this approximation is taken into account in the systematic uncertainties. The estimated ⁰_b! _cð2455Þ^þþ and ⁰_b!_cð2455Þ^0þ yields and significances are reported in TableI.

In Figs.3(c)and3(d)themð⁰_bÞ mð^þ_cÞdistributions are shown restricted to candidates with 0:160<

mð_cð2455Þ^þþ;0Þ mð^þ_cÞ<0:176 GeV=c², where the _cð2455Þ^þþ and _cð2455Þ⁰ signals are contained. The ⁰_b signal is modeled with a Gaussian distribution, with floating mean and width, while the combinatorial back- ground is an exponential function with floating slope and

FIG. 3 (color online). The⁰_b!_cð2455Þ^þþand⁰_b !_cð2455Þ^0þsignals: (a)mðcð2455Þ^þþÞ mð^þcÞdistribution for candidates in a3range (57 MeV=c²) around the⁰_bmass; (b)mð_cð2455Þ⁰Þ mð^þ_cÞdistribution for candidates in a3 range around the ⁰_b mass; (c) mð⁰_bÞ mð^þcÞ distribution restricted to candidates in the region 0:160< mðcð2455Þ^þþÞ mð^þcÞ<0:176 GeV=c²; (d)mð⁰_bÞ mð^þcÞdistribution restricted to candidates in the region0:160< mðcð2455Þ⁰Þ mð^þcÞ<

0:176 GeV=c².

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