Observation of <em>B_s⁰ -&gt; J/ψK*</em>(892)⁰ and <em>B_s⁰</em> -&gt; <em>J/ψK_S⁰</em> decays

Article

Reference

Observation of B

-> J/ψK* (892)

and B

-> J/ψK

decays

CDF Collaboration

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

Abstract

We report the first observation of two Cabibbo-suppressed decay modes of the B0s meson.

Using a sample of pp collisions at s√=1.96  TeV corresponding to 5.9  fb−1 of integrated luminosity collected with the CDF II, the collider detector at the Fermilab Tevatron, we search for new B0s decay modes in a sample of events containing J/ψ→μ+μ− decays. We reconstruct a B0s→J/ψK∗(892)0 signal with K∗(892)0→K+π−, observing a yield of 151±25 events with a statistical significance of 8.0σ. We also reconstruct a B0s→J/ψK0S signal with K0S→π+π−, observing a yield of 64±14 events with a statistical significance of 7.2σ. From these yields, we extract the branching ratios B(B0s→J/ψK∗(892)0)=(8.3±3.8)×10−5 and B(B0s→J/ψK0)=(3.5±0.8)×10−5, where statistical, systematic, and fragmentation-fraction uncertainties are included in the combined uncertainty.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Observation of B

-> J/ψK* (892)

and B

-> J/ψK

decays. Physical Review. D, 2011, vol. 83, no. 05, p. 052012

DOI : 10.1103/PhysRevD.83.052012

Available at:

http://archive-ouverte.unige.ch/unige:38663

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

Observation of B

! J= c K

ð892Þ

and B

! J= c K

decays

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S. S. Yu,¹⁵J. C. Yun,¹⁵A. Zanetti,^52aY. Zeng,¹⁴and S. Zucchelli^6b,6a (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 Fı´sica 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, and 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; and 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

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

36Northwestern University, Evanston, Illinois 60208, USA

37The Ohio State University, Columbus, Ohio 43210, USA

38Okayama University, Okayama 700-8530, Japan

39Osaka City University, Osaka 588, Japan

40University of Oxford, Oxford OX1 3RH, United Kingdom

41aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

41bUniversity of Padova, I-35131 Padova, Italy

42LPNHE, Universite´ Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris F-75252 France

43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

44aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy

44bUniversity of Pisa, I-56127 Pisa, Italy

44cUniversity of Siena, I-56127 Pisa, Italy

44dScuola Normale Superiore, I-56127 Pisa, Italy

45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

46Purdue University, West Lafayette, Indiana 47907, USA

47University of Rochester, Rochester, New York 14627, USA

48The Rockefeller University, New York, New York 10065, USA

49aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, Italy

49bSapienza Universita` di Roma, I-00185 Roma, Italy

50Rutgers University, Piscataway, New Jersey 08855, USA

51Texas A&M University, College Station, Texas 77843, USA

52aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy

52bUniversity of Trieste/Udine, I-33100 Udine, Italy

53University of Tsukuba, Tsukuba, Ibaraki 305, Japan

54Tufts University, Medford, Massachusetts 02155, USA

55University of Virginia, Charlottesville, Virginia 22906, USA

56Waseda University, Tokyo 169, Japan

57Wayne State University, Detroit, Michigan 48201, USA

aDeceased

bVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA

cVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy

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eVisitor from University of California Santa Barbara, Santa Barbara, CA 93106, USA

fVisitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA

gVisitor from CERN, CH-1211 Geneva, Switzerland

hVisitor from Cornell University, Ithaca, NY 14853, USA

iVisitor from University of Cyprus, Nicosia CY-1678, Cyprus

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aaOn leave from J. Stefan Institute, Ljubljana, Slovenia

83,

58University of Wisconsin, Madison, Wisconsin 53706, USA

59Yale University, New Haven, Connecticut 06520, USA (Received 10 February 2011; published 28 March 2011)

We report the first observation of two Cabibbo-suppressed decay modes of the B⁰s meson. Using a sample ofpp collisions at ffiffiffi

ps

¼1:96 TeVcorresponding to5:9 fb¹of integrated luminosity collected with the CDF II, the collider detector at the Fermilab Tevatron, we search for newB⁰s decay modes in a sample of events containingJ=c !^þ decays. We reconstruct aB⁰s !J=cKð892Þ⁰ signal with Kð892Þ⁰!K^þ, observing a yield of15125events with a statistical significance of8:0. We also reconstruct a B⁰s !J=cK⁰_S signal with K⁰_S!^þ, observing a yield of 6414 events with a statistical significance of 7:2. From these yields, we extract the branching ratios BðB⁰s ! J=cKð892Þ⁰Þ ¼ ð8:33:8Þ 10⁵andBðB⁰_s!J=cK⁰Þ ¼ ð3:50:8Þ 10⁵, where statistical, sys- tematic, and fragmentation-fraction uncertainties are included in the combined uncertainty.

DOI:10.1103/PhysRevD.83.052012 PACS numbers: 14.40.Nd, 12.15.Ff, 12.15.Hh, 13.25.Hw

I. INTRODUCTION

This paper presents the first observation of the Cabibbo- suppressed decaysB⁰_s !J=cKð892Þ⁰ andB⁰_s!J=cK⁰_S (and the corresponding charge conjugate decays) using a sample derived from an integrated luminosity of5:9 fb¹of proton-antiproton collisions at ffiffiffi

ps

¼1:96 TeVproduced at the Fermilab Tevatron. In addition to isolating these signals, we normalize the observed yields to the corresponding Cabibbo-favoredB⁰ decay modes (B⁰ !J=cK⁰, where K⁰ refers toKð892Þ⁰, and B⁰ !J=cK_S⁰) to extract the branching ratios for these newly observedB⁰sdecay modes.

With the establishment of the decay modes presented here, future measurements can be considered that will further aid our experimental investigation into the physics of theB⁰s system. The success of the Cabibbo-Kobayashi- Maskawa (CKM) three-generation description of charge conjugation-parity inversion (CP) violation [1] in the bot- tom and kaon sectors has continued to motivate additional, more precise tests ofCPviolation in the flavor sector. In recent years, attention has turned to theB⁰s meson as new territory to explore the possibility of nonstandard-model contributions, specifically in the CKM matrix elementVts. Precise measurement of the frequency ofB⁰s flavor oscil- lations [2] has significantly limited the magnitude of new physics amplitudes. However, possible large new physics phases remain poorly constrained.

Cabibbo-suppressed B⁰s modes could provide comple- mentary information on the B⁰s mixing phase and on the width difference_B⁰

s ¼_B⁰

sL_B⁰

sH, where_B⁰

sLð_B⁰

sHÞis the width of the light, even (heavy, odd)B⁰sCPeigenstate [3]. The decay B⁰s !J=cKð892Þ⁰ is a pseudoscalar to vector-vector transition and can be used to help disentangle penguin contributions in B⁰s !J=c [4]. With a suffi- ciently large data sample, it would be possible to measure _B⁰

s and the polarization amplitudes. Furthermore, the Cabibbo-suppressed decayB⁰s !J=cK_S⁰is aCP-odd final state (ignoring CP violation in the kaon system), and therefore a measurement of the lifetime in this decay mode is a direct measure of_B0

sH ¼1=_B⁰

sH. With a larger

data sample, a taggedCPasymmetry analysis of theB⁰s! J=cK⁰_Smode, in conjunction with our precise knowledge ofCPviolation inB⁰!J=cK⁰_S, can yield information on the angleof the unitarity triangle [5].

In the naı¨ve spectator model, the ratio of branching ratios is given by the ratio of the squares of the CKM elements,

BðB⁰_s!J=cKÞ

BðB⁰!J=cKÞ ¼jV_cdj²

jV_csj² ¼0:0510:006; (1) where K represents K⁰_S or K⁰. The numerical value is derived from jV_cdj ¼0:2300:011andjV_csj ¼1:023 0:036[3].

Experimentally, we extract the relative branching ratios using the relation

BðB⁰s !J=cKÞ

BðB⁰ !J=cKÞ ¼Arelf_d fs

NðB⁰_s !J=cKÞ NðB⁰ !J=cKÞ; (2) where Arelis the relative acceptance,fs=fd is the ratio of fragmentation fractions, and NðB⁰_s!J=cKÞ=NðB⁰! J=cKÞis the measured ratio of yields.

We can use the result from Eq. (1) to estimate the relative yield in the spectator model. The value forf_s=f_d is extracted from the most recent Collider Detector at Fermilab Tevatron (CDF) measurement [6] of f_s=ðf_uþ f_dÞ BðD_s!Þ and f_u=f_d, along with the current world-average value [3] for BðDs!Þ. Combining the value fs=fd¼0:2690:033 with the assumption thatArel¼1, Eq. (2) yields

NðB⁰_s !J=cKÞ

NðB⁰ !J=cKÞ ¼BðB⁰s!J=cKÞ BðB⁰!J=cKÞ

fs

f_d 1 Arel

¼0:0140:002: (3) While the result holds only in the simple spectator case, it provides useful guidance that we might expect one to two Cabibbo-suppressed B⁰s!J=cK events for every 100 Cabibbo-favoredB⁰ !J=cK events.

After a description of the detector, data sample, and simulated samples utilized here, we describe the

B⁰_s !J=cKð892Þ⁰ analysis in Sec. III, followed by the B⁰_s !J=cK_S⁰analysis in Sec.IV. SectionVthen describes the acceptance calculation for both modes, followed by the results in Sec.VI.

II. CDF DETECTOR, DATA, AND MONTE CARLO SAMPLES

The data used in these analyses correspond to an inte- grated luminosity of 5:9 fb¹ and were collected by the CDF II detector from March 2002 to February 2010 using di-muon triggers. The CDF II detector is a general purpose, cylindrically symmetric detector. A more detailed descrip- tion can be found elsewhere [7]. The subdetectors relevant for these analyses are briefly discussed here. Charged particle trajectories (tracks) are measured by a system comprising eight layers of silicon microstrip detector (SVX) and an open-cell wire drift chamber (COT), both immersed in a 1.4 T axial magnetic field. The silicon detector [8] extends from a radius of 1.5 to 22 cm and has a single-hit resolution of approximately15m. The COT drift chamber [9] provides up to 96 measurements from radii of 40 to137 cm and covers the rangejj 1 [10]. The combinedCOTþSVXcharged particle momen- tum resolution is p_T=ðpTÞ² ¼0:07%½GeV=c¹, which leads to a mass resolution on theK⁰_S!^þ signal of 0:006 GeV=c². Outside the calorimeters reside four layers of planar drift chambers [11] (CMU) that detect muons with transverse momentump_T>1:4 GeV=cwithinjj<

0:6. Additional chambers and scintillators [12] (CMX) cover0:6<jj<1:0for muons withp_T >2 GeV=c.

The di-muon triggers collect a sample ofJ=c !^þ candidates. At the first level of a three-level trigger system, an electronic track processor (XFT) [13] uses COT

information to find tracks and extrapolate [14] those with p_T>1:5ð2:0Þ GeV=c to track segments in the CMU (CMX) muon-chambers. Events pass this first trigger level if two or more XFT tracks are matched to muon-chamber track segments. The second trigger level requires those tracks to have opposite charge and an appropriate opening angle in the plane transverse to the beam line. Finally, at level 3, full tracking information is used to reconstruct J=c !^þ candidates. Events with a candidate in the mass range2:7–4:0 GeV=c² are accepted.

To identify B⁰ and B⁰s decay candidates, we pair J=c candidates with K_S⁰!^þ and K⁰!K^þ candi- dates. The reconstruction of K⁰_S!^þ and K⁰! K^þ candidates starts from pairs of oppositely charged tracks fit to a common interaction point (vertex). In the B⁰s!J=cK⁰_S analysis, we reconstruct two tracks as pions and combine them to define a K_S⁰ candidate, where the invariant mass of the two pions is constrained to the known K⁰_S mass [3]. In the B⁰_s !J=cK⁰ analysis, we reconstruct the K⁰ candidate from the combination of a and a K. If twoK⁰ candidates are reconstructed with the same tracks, with the only difference that the kaon and pion hypotheses are interchanged, we select the K⁰ candidate whose mass is closer to the pole value of 896 MeV=c². We perform a kinematic fit of each B candidate where the final-state tracks are constrained to come from a common decay point and the invariant mass of the muon pair is constrained to the known J=c ^mass [3]. These preliminary selection criteria for B⁰ and B⁰_s candidates are listed in Table I. Additional selection criteria optimized for the individual channels are de- scribed in Secs. III and IV.

Simulated samples of B⁰ and B⁰_s decays are used to optimize event selection, model signal distributions, and TABLE I. Selection criteria forB⁰!J=cKcandidates andB⁰s !J=cKcandidates, whereK

representsK⁰ orK_S⁰.

Variable (Units) B⁰s !J=cK⁰ B⁰s!J=cK_S⁰

B⁰=B⁰s candidate four-track fit² <50

B⁰=B⁰s candidate four-track fit probability >10⁵ B⁰=B⁰s candidate transverse momentumpT ðGeV=cÞ >6 >4 B⁰=B⁰s candidate impact parameter (m) <50

B⁰=B⁰s candidate transverse decay length significanceLxy= >2

J=c candidate mass (GeV=c²) >3:05 >2:8

<3:15 <3:3 J=c candidate 3-dimensional two-track fit² <30 <30

Kcandidate mass (GeV=c²) >0:55 >0:55

<0:846 <0:846 Kcandidate 3-dimensional two-track fit² <30 <20 Kcandidate transverse decay lengthLxy(cm) >0:5 transverse momentumpT ðGeV=cÞ >1:5 >1:5 between the two muons (radians) <2:25 <2:25

1charge2charge ¼ 1 ¼ 1

zin the beam line between the two(cm) <5 <5

transverse momentumpTðGeV=cÞ >0:5

83,

assess systematic uncertainties. For our default Monte Carlo simulation (MC) samples, we generate single bhadrons according to the predicted next-to-leading order QCD calculation [15]. For systematic studies, we also generate single b hadrons according to momentum and rapidity spectra measured by CDF [7]. These hadrons are then decayed using theEVTGENpackage [16] and fed into a

GEANTsimulation of the CDF detector [17]. The simulated data are then processed and reconstructed in the same manner as the detector data. In the case of J=cK⁰ mode, it is necessary to specify the polarization parameters in the simulation. For both B⁰ and B⁰s, we use trans- versity basis [18] polarization amplitudesjA0j²¼0:6and jA_?j² ¼0:22, which are similar to the PDG values of jA0j² ¼0:5710:008 and jA_?j²¼0:220:013 [3].

For systematic acceptance studies, MC samples with other polarization values were generated.

In all of the MC samples generated, and throughout the analyses presented below, we assume that there is no CP violation inB⁰smixing or decay. We also assume that equal numbers ofB⁰andB⁰mesons, as well as equal numbers of B⁰s and B⁰s mesons, are produced in the pp collisions.

In this untagged analysis that does not distinguish B! J=cK_S⁰ from B !J=cK_S⁰, the observed yield is unaf- fected byCPviolation provided that equal numbers ofB andB mesons are produced att¼0

III.B⁰_s!J=cKð892Þ⁰ ANALYSIS

We optimize the selection criteria to provide the highest likelihood for evidence of this mode. This is done by maximizing S=ð1:5þ ffiffiffiffi

pB

Þ, whereSrefers to the number of signal events andBis the number of background events in the signal region. Reference [19] demonstrates that this quantity is well suited for discovery. For the signal sample, aB⁰_s !J=cK⁰ MC sample is used. For the background sample, we use J=cK⁰ candidate events from data with the requirement that the reconstructed candidate massM_B falls in the range 5:6 GeV=c²< MB<5:8 GeV=c². This

‘‘upper sideband’’ region contains events kinematically similar to the combinatorial background in the signal re- gion and is not contaminated by residual signal events. We avoid using the sideband below theB⁰ peak because it is contaminated with partially reconstructedBdecays such as B⁰ !J=cK⁰⁰. We optimize simultaneously over the transverse momenta pTðÞ and pTðK^þÞ, the B⁰s trans- verse decay lengthLxyðB⁰_sÞ, and theB⁰s decay kinematic-fit probability. The final cuts we use are pTðÞ>

1:5 GeV=c, pTðK^þÞ>1:5 GeV=c, LxyðB⁰sÞ>300m, and fit probability greater than10⁵.

Particle identification using specific ionization (dE=dx) in the COT was evaluated to further separateK⁰ !K^þ from ^þ and K^þK backgrounds. Although further background reduction could be achieved, the corresponding reduction in signal efficiency rendered particle identifica- tion unprofitable, and we choose not to use it.

We determine theB⁰_s andB⁰ yields using a binned like- lihood fit in the candidate masses. We model the signal contributions with templates composed of three Gaussians obtained from fits to B⁰ MC. The two dominant, narrow Gaussians model detector resolution effects and also ac- count for cases where the identities of theandKfrom the K⁰ decay are interchanged. As mentioned above, some events are identified where a single pair of tracks passes the selection requirements under both the -K andK- hy- potheses. In those cases, we reconstruct the event using both sets of=Kassignments and then choose a candidate whose particle assignment yields a reconstructed mass that is closer to the nominalKð892Þmass. This technique ensures that candidates are not used twice. Approximately 10% of B!J=cK⁰ events are reconstructed with the incorrect -K assignment. These events peak at the B masses, but have a significantly broader width. A wide Gaussian models misreconstructed signal events and other non-Gaussian resolution effects. The relative contribu- tions, means, and widths of each Gaussian are fixed in the fit. TheB⁰s templates used in the fit are identical toB⁰ templates, except for a shift of86:8 MeV=c² in the mean value of the three Gaussians. This value corresponds to the known [3,20] mass difference betweenB⁰sandB⁰. The MC slightly underestimates the mass resolution, so the widths of the two narrow Gaussians are multiplied by a scale factor common to the B⁰ and B⁰s templates, which is allowed to float in the fit. The scale factor is not applied to the third Gaussian since the resolution effects are neg- ligible compared to the other effects. Moreover, a common mass shift is added to the means of all Gaussian templates to account for a possible mass mismodeling in the MC.

This mass shift is floating in the fit.

The B⁰_s!J=cK⁰ analysis has three primary back- ground contributions: events with random track combina- tions (combinatorics), partially reconstructed b hadrons, andB⁰s!J=cdecays. Combinatorial background arises from sources such as a real J=c plus two other tracks, where theJ=c could be either prompt or coming from aB decay. Another source arises from false J=c ^candidates reconstructed from misidentified hadrons. The combinato- rial background is nonpeaking and accurately modeled in the fit with an exponential function.

Backgrounds from partially reconstructed b hadrons come from multibody decays where a , K, or is not reconstructed, for example, the decay mode B⁰! J=cK⁰⁰. We fit this background with two ARGUS functions [21], one for partially reconstructedB⁰ and an- other for partially reconstructedB⁰_s. The ARGUS function parametrization form < m0 is

fðmÞ ¼N1

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1m² m²0

s

e^Cm2=m20; (4) wherem0is the mass cutoff,Cthe decay constant, andN1

is the normalization. The function is set to zero for

m > m0. The ARGUS function for partially reconstructed B⁰ has a fixed mass cutoff of mðB⁰Þ mð⁰Þ ¼ 5:140 GeV=c², and the function for partially reconstructed B⁰_s has a fixed mass cutoff of mðB⁰_sÞ mð⁰Þ ¼ 5:220 GeV=c². The decay constants of the two functions are constrained to be the same, and the normalizations are independent. Each ARGUS function is convoluted with a Gaussian having a width of 12 MeV=c² to account for detector resolution effects.

Since it is possible forB⁰s!J=c candidates to pass theJ=cK⁰ reconstruction criteria,B⁰s !J=cmust be considered as a background. We use a template consisting of two Gaussians, extracted from simulation, to model this background in theJ=cK⁰ fit, where both Gaussians are primarily modeling detector resolution effects. We fix to the template values the widths, means, and relative contri- butions from each Gaussian in the final fit. We multiply the constant width of the narrower Gaussian by the same scale factor used in the signal templates. We constrain theB⁰_s ! J=c contribution in the J=cK⁰ fit by measuring the yield of B⁰_s !J=c in the data using selection criteria efficient for reconstructing B⁰_s !J=c. We then use simulation to calculate the fraction of thoseJ=cevents that would satisfy theJ=cK⁰selection.

We perform a binned log likelihood fit to theJ=cK invariant mass distribution using the templates for signals and the background functions described above. The mass distributions in data forJ=cK⁰ candidates and the final fit appear in Fig. 1. The yields for B⁰ !J=cK⁰ and B⁰s !J=cK⁰ signal are 9530110 and 15125, re- spectively. The ratioNðB⁰_s !J=cK⁰Þ=NðB⁰ !J=cK⁰Þ is0:01590:0022ðstatÞ.

We determine the statistical significance of the B⁰s ! J=cK⁰ signal by fitting the mass distribution without the B⁰_s contribution (background-only hypothesis). For like- lihood L, we interpret 2 logLas a ² distribution. We use ² with 1 degree of freedom to determine that the probability of background fluctuations producing a com- parable or greater signal is8:910¹⁶or8:0. This is the first observation of theB⁰s !J=cK⁰ decay.

We consider several sources of systematic uncertainty in the measured ratio of NðB⁰_s!J=cK⁰Þ=NðB⁰! J=cK⁰Þ. The modeling of the B⁰ and B⁰_s signal peaks can influence the ratio. To quantify the effect of the mis- modeling, we repeat the fit using two Gaussian templates instead of three for the signal. The fit value of NðB⁰sÞ=NðB⁰Þis shifted by710⁴.

We vary the input mass difference betweenB⁰andB⁰sin the templates within its uncertainty of 0:7 MeV=c². The difference in NðB⁰s !J=cK⁰Þ=NðB⁰!J=cK⁰Þ with the alternate templates is 210⁵. This is sufficiently small that we ascribe no systematic uncertainty due to the mass difference uncertainty.

The modeling of the combinatorial background is an- other source of systematic uncertainty. To explore the sensitivity to the choice parameterization, we use a power function (fðmÞ ¼km , with k and free parameters) instead of an exponential. The overall fit quality with the power function is similar to that obtained using an expo- nential model. We assign the difference in relative yield as a background modeling systematic uncertainty of 210⁴ on the relative branching ratio.

In the likelihood fit, we allow the combinatorial back- ground contribution to float. We performed a study to evaluate how the ratio of yields depends upon the specific, arbitrary choice of the fit range. We compare the main fit, which allows the combinatorial background to float over the entire fit range, to a control case where the combina- torial contribution is fitted in the upper sideband and ex- trapolated to the full mass range prior to the final fit.

Because of the difference in the result from these two methods, we include a systematic uncertainty of 0.0050 on theNðB⁰_s !J=cK⁰Þ=NðB⁰!J=cK⁰Þratio.

Several sources contribute to an uncertainty in theB⁰s! J=ccontribution. While there is 30% uncertainty in the branching ratio, the dominant uncertainty arises from the uncertainty in theB⁰_s !J=ctemplate, given that we rely on MC to derive this. To perform a conservative assess- ment of this uncertainty, we repeated the fit while doubling the fraction ofB⁰s!J=ccandidates. The resulting shift

2) mass (GeV/c π

ψ K J/

5.0 5.5 6.0

2 candidates per 3 MeV/c

0 200 400 600 800

1000 (a)

2) mass (GeV/c π

ψ K J/

5.0 5.5 6.0

2 candidates per 3 MeV/c

0 200 400 600 800 1000

2) mass (GeV/c π

ψ K J/

5.1 5.2 5.3 5.4 5.5

2 candidates per 3 MeV/c

0 10 20 30 40 50 60 70 80

Data Total Fit

K*

ψ

→ J/

B0

K*

ψ

→ J/

Bs

φ ψ

→ J/

Bs

Combinatorial Partially Reconstructed B

(b)

2) mass (GeV/c π

ψ K J/

5.1 5.2 5.3 5.4 5.5

2 candidates per 3 MeV/c

0 10 20 30 40 50 60 70 80

FIG. 1 (color online). (a) Invariant mass distribution in data for J=cK⁰candidates and fit including the different contributions.

(b) We enlarge the distribution in the signal region for more detail.

83,

of 210⁴ is assigned as the uncertainty in the B⁰s ! J=ccontribution.

We add the different systematic uncertainty contribu- tions, summarized in TableII, in quadrature, resulting in a final value of NðB⁰_s!J=cK⁰Þ=NðB⁰ !J=cK⁰Þ of 0:01590:0022ðstatÞ 0:0050ðsystÞ.

IV.B⁰s!J=cK⁰_SANALYSIS

The B⁰s!J=cK⁰_S decay has several differences com- pared to theB⁰_s!J=cK⁰ decay. It contains aK_S⁰, which has a relatively long lifetime ofc¼2:68 cm. We use the displacement between the reconstructed K⁰_S decay point and the reconstructedBdecay point in the event selection to reduce backgrounds such asB⁰_s !J=c. Finally, as in the B⁰ system, we expect the B⁰s !J=cK_S⁰ signal to be smaller than that of theB⁰s!J=cK⁰ mode. Therefore, we use a neural network (NN) technique to take full advantage of all the kinematic variables and their correla- tions. We use theNEUROBAYES[22] NN package. The NN provides an output value close toþ1for signal-like events and near1for backgroundlike events.

We train the NN using simulated B⁰_s MC events as a signal sample. We use data from the upper sideband in the J=cK_S⁰ candidate mass distribution, well separated from the signal region, as a background training sample. We use as inputs for the NN the quantities listed in TableIII. These input quantities are chosen as variables with good discrimi- nating power which, alone or in combination, do not bias the mass spectrum. After the training, the NN achieves strong discrimination between signal and background, as shown in Fig.2(a).

As in theB⁰_s!J=cK⁰ analysis, we optimize the se- lection by maximizing S=ð1:5þ ffiffiffiffi

pB

Þ. The signal S is modeled using B⁰_s MC events in the reconstructed mass range 5:350 GeV=c²< M_B<5:400 GeV=c². The back- ground B is modeled using J=cK_S⁰ candidates in data populating the mass range 5:430 GeV=c²< MB<

5:480 GeV=c². The figure of merit suggests a cut value in the NN response of 0.88, as shown in Fig.2(b).

The fitting technique is similar to the B⁰s !J=cK⁰ analysis. We obtain the yields ofB⁰!J=cK⁰_SandB⁰s ! J=cK_S⁰ signals in a binned likelihood fit to the invariant mass distribution. We again model the B⁰ and B⁰s signal

contributions with three Gaussian templates obtained from fitting B⁰ !J=cK_S⁰ MC and use the mass difference between B⁰_s and B⁰ for the formation of the B⁰_s! J=cK⁰_S template. The two major sources of background in this analysis are combinatorial background and partially reconstructed b-hadron decays. We model these with the same functional forms used in theB⁰ !J=cK⁰ analysis.

However, we include only one ARGUS function because the contribution of partially reconstructedB⁰_sis negligible.

An additional background in this analysis is_b⁰ !J=c decays where thepfrom thedecay is assumed to be a. In order to suppress the_b⁰contribution, we apply a cut to the angular variable cosð_K⁰

S;2Þ, where_K⁰

S;2 is the angle between theK⁰_ScandidatepTin the lab frame and the lower p_T pion (2) in theK⁰_Scenter-of-mass frame. Cutting out events with cosð_K⁰

S;2Þ<0:75 removes 99.8% of the _b⁰ while retaining 86% of the B⁰s. The residual _b⁰ contamination is less than one event and is neglected.

The invariant mass distribution forJ=cK_S⁰and the fit result including the different contributions are shown in Fig.3.

TABLE II. Systematic uncertainties for the ratio of yields. All numbers in percent.

Source ^NðB_NðB^0s0!J=_!J=^c_c^K_K⁰0^ÞÞ ð%Þ ^NðB_NðB^0s0!J=_!J=^c_c^K_K^0S0Þ

SÞ ð%Þ

Signal modeling 4.4 4.6

Mass difference betweenB⁰ andB⁰s 0.1 0.1

Combinatorial background modeling 1.3 5.6

Combinatorial background contribution 31.4 5.6

B⁰s !J=ccontribution 1.3

Total 31.8 9.2

TABLE III. Variables used as input in the NN training.

Input variables in the NN

B⁰=B⁰s candidate transverse momentum B⁰=B⁰s candidate four-track decay point fit B⁰=B⁰s candidate proper decay length B⁰=B⁰s candidate impact parameter J=c candidate transverse momentum J=c candidate mass

J=c candidate proper decay length J=c candidate impact parameter K⁰_Scandidate transverse momentum K⁰_Scandidate mass

K⁰_Scandidate proper decay length K⁰Scandidate impact parameter transverse momentum impact parameter transverse momentum impact parameter

cosine of the helicity angle inJ=c rest frame