Measurement of the B_s⁰ Lifetime in Fully and Partially Reconstructed B_s⁰ -&gt; D_s^-(ϕπ⁻)X Decays in <em>pp</em> Collisions at √s=1.96  TeV

Article

Reference

Measurement of the B

Lifetime in Fully and Partially Reconstructed B

-> D

(ϕπ

)X Decays in pp Collisions at √s=1.96  TeV

CDF Collaboration

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

Abstract

We present a measurement of the B0s lifetime in fully and partially reconstructed B0s→D−s(ϕπ−)X decays in 1.3  fb−1 collected in pp collisions at s√=1.96  TeV by the CDF II detector at the Fermilab Tevatron. We measure τ(B0s)=1.518±0.041(stat)±0.027(syst)  ps. The ratio of this result and the world average B0 lifetime yields τ(B0s)/τ(B0)=0.99±0.03, which is in agreement with recent theoretical predictions.

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

Lifetime in Fully and Partially Reconstructed B

-> D

(ϕπ

)X Decays in pp Collisions at √s=1.96  TeV.

Physical Review Letters , 2011, vol. 107, no. 27, p. 272001

DOI : 10.1103/PhysRevLett.107.272001

Available at:

http://archive-ouverte.unige.ch/unige:38683

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

1 / 1

Measurement of the B

Lifetime in Fully and Partially Reconstructed B

! D

ð

ÞX Decays in pp Collisions at ffiffiffi

p s

¼ 1:96 TeV

T. Aaltonen,²¹B. A´ lvarez Gonza´lez,^9,wS. Amerio,^41aD. Amidei,³²A. Anastassov,³⁶A. Annovi,¹⁷J. Antos,¹² G. Apollinari,¹⁵J. A. Appel,¹⁵A. Apresyan,⁴⁶T. Arisawa,⁵⁶A. Artikov,¹³J. Asaadi,⁵¹W. Ashmanskas,¹⁵B. Auerbach,⁵⁹ A. Aurisano,⁵¹F. Azfar,⁴⁰W. Badgett,¹⁵A. Barbaro-Galtieri,²⁶V. E. Barnes,⁴⁶B. A. Barnett,²³P. Barria,^44c,44aP. Bartos,¹² M. Bauce,^41b,41aG. Bauer,³⁰F. Bedeschi,^44aD. Beecher,²⁸S. Behari,²³G. Bellettini,^44b,44aJ. Bellinger,⁵⁸D. Benjamin,¹⁴ A. Beretvas,¹⁵A. Bhatti,⁴⁸M. Binkley,^15,aD. Bisello,^41b,41aI. Bizjak,^28,aaK. R. Bland,⁵B. Blumenfeld,²³A. Bocci,¹⁴ A. Bodek,⁴⁷D. Bortoletto,⁴⁶J. Boudreau,⁴⁵A. Boveia,¹¹B. Brau,^15,bL. Brigliadori,^6b,6aA. Brisuda,¹²C. Bromberg,³³ E. Brucken,²¹M. Bucciantonio,^44b,44aJ. Budagov,¹³H. S. Budd,⁴⁷S. Budd,²²K. Burkett,¹⁵G. Busetto,^41b,41aP. Bussey,¹⁹

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(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, 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

272001-2

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, I-00185 Roma, 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

58University of Wisconsin, Madison, Wisconsin 53706, USA

59Yale University, New Haven, Connecticut 06520, USA (Received 10 March 2011; published 29 December 2011)

We present a measurement of theB⁰s lifetime in fully and partially reconstructedB⁰s !DsðÞX decays in1:3 fb¹collected inppcollisions at ffiffiffi

ps

¼1:96 TeVby the CDF II detector at the Fermilab Tevatron. We measure ðB⁰_sÞ ¼1:5180:041ðstatÞ 0:027ðsystÞps. The ratio of this result and the world average B⁰ lifetime yields ðB⁰_sÞ=ðB⁰Þ ¼0:990:03, which is in agreement with recent theoretical predictions.

DOI:10.1103/PhysRevLett.107.272001 PACS numbers: 14.40.Nd, 13.25.Hw

In the spectator model of heavy hadron decay, the life- times of allbhadrons are equal, independent of the flavor of the lighter quarks bound to the b quark. Using the heavy-quark expansion [1,2] in the calculation of the width, spectator quark interactions enter in higher order ð_QCD=mbÞ³terms wherembis the mass of thebquark and _QCD is the energy scale of the QCD interactions within the hadron. This leads to the lifetime hierarchy ðB⁰sÞ ffi ðB⁰Þ< ðB^þÞ. Theoretical results predictðB^þÞ=ðB⁰Þ ¼ 1:060:02 and ðB⁰sÞ=ðB⁰Þ ¼1:000:01 [3,4]. The world averages for the corresponding experimental num- bers are 1:0710:009 and 0:9650:017, respectively

[5]. The precision of our knowledge of theB⁰_s lifetime is much less than for theB⁰andB^þlifetimes, and therefore, a more precise measurement would be useful, both in gen- eral and for comparison with theoretical calculations.

Such a measurement is especially warranted since the agreement on the lifetime ratio between theory and experi- ment is only fair.

In this Letter, we present a measurement of theB⁰s life- time in flavor-specific decay modes. The data come from pp collisions at ffiffiffi

ps

¼1:96 TeVat the Fermilab Tevatron.

This analysis is based on an integrated luminosity of 1:3 fb¹ collected by the CDF II detector between

February 2002 and November 2006. This sample yields more than 1100 fully reconstructed (FR) B⁰s!Ds^þ candidates with Ds ! and !K^þK after on- line and off-line selection [6]. In addition, the sample reconstructed as B⁰s!Ds^þ includes partially recon- structed (PR)B⁰s candidates that are used in this lifetime measurement and more than double the number of B⁰_s candidates available for analysis. One such PR decay is B⁰s !Ds^þwith^þ!^þ0where the⁰is not recon- structed. The inclusion of PR decays introduces an uncertainty in the momentum measurement of a given candidate. However, a correction to the proper decay time has been estimated, and the total uncertainty on the lifetime measurement is improved by the use of the PR final states.

The CDF II detector is described in detail in Ref. [7]. The detector elements relevant for this analysis are the silicon vertex detectors [8–10] and the central drift chamber (COT) [11]. The silicon detectors consist of 7 or 8 layers of microstrip silicon sensors covering the pseudorapidity [12] rangejj<2:0. The COT is an open cell drift cham- ber coveringjj<1:0. Both the COT and silicon vertex detectors are immersed in a uniform 1.4 T axial magnetic field with the field axis parallel to the proton beam.

A data sample enriched in hadronicBdecays is selected with a three-level trigger system that searches for tracks displaced from the primary vertex [13]. At level 1, patterns of hits in the COT are identified as tracks by the extremely fast tracker (XFT) [14]. At level 2, the silicon vertex trigger [15] associates a set of silicon hits with the XFT tracks and improves track measurement precision. The trigger re- quires each event to contain a pair of charged particle tracks, each having transverse momentum pT 2 GeV=cand transverse impact parameterd0 in the range d02 ½120m;1 mm, whered0is defined as the distance of closest approach between the particle trajectory and the beam line, measured in the transverse plane. The opening angle between the tracks’ trajectories ( in the plane transverse to the beam) must be between 2and 90, and their intersection must be at least200mfrom the inter- action point, as measured in the plane transverse to the beam direction. At level 3, track reconstruction is per- formed entirely in software, with the full precision of the tracking system available, and the level 1 and 2 require- ments are confirmed. These trigger requirements preferen- tially select events containing long-lived particles and sculpt the proper time distribution of the particles that are accepted for analysis. As the background rate of this trigger requires prescaling at higher instantaneous lumi- nosities, CDF also employs two more restrictive triggers that require the tracks in the trigger pair to have opposite charges, individual p_T 2ð2:5Þ GeV=c, and the scalar sump_T 5:5ð6:5Þ GeV=c.

We reconstruct B⁰_s !D_s^þ candidates (where B⁰_s and Ds imply B⁰s candidates and Ds candidates) by

first identifying D_s !ðKK^þÞ from tracks with p_T>350 MeV=cusing the invariant mass require- ments jmðKK^þÞ 1020:5j<7:5 MeV=c² and jmðKK^þÞ 1968:3j<20 MeV=c². The D_s daugh- ter tracks must satisfy a three-dimensional vertex fit. We then combine eachD_s with a positively charged track with p_T>1:0 GeV=c to form a B⁰_s !D_s^þ candidate and require the pair to satisfy an additional three-dimensional vertex fit. We do not constrain the mass of theorD_s in this fit. The decay length of theB⁰_sis measured with respect to the event’s primary vertex and must satisfy requirements on the following quantities: the decay length of the B⁰_s projected along the transverse momentum, L_xyðB⁰_sÞ>

450m, and its significance, LxyðB⁰sÞ=LxyðB⁰sÞ>5; the transverse distance between theB⁰sandDs decay points is greater than 0; the transverse impact parameter of the B⁰s, jd0ðB⁰_sÞj<60m; and the significance of the longitudinal impact parameter, jz0ðB⁰sÞ=z0ðB⁰sÞj<3. Both fits for the B⁰_s andD_s vertices must have reasonable goodness-of-fit values when considering only the track parameters mea- sured in the transverse plane.

To further separate B⁰s mesons from backgrounds with similar topologies, we require the transverse momentum of the B⁰s, pTðB⁰sÞ>5:5 GeV=c, and the angular separation between theffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDs and the from theB⁰s, RðD_s; BÞ ¼

ðÞ²þ ðÞ²

p <1:5. We require the isolation of theB⁰s

to be greater than 0.5, defined as pTðB⁰sÞ divided by the scalar sum of the transverse momenta of all the tracks in a cone ofR <1around theB⁰s. We tighten the requirement on the mass of the Ds (jm1968:3j<12 MeV=c²) and veto Ds candidates consistent with D!D⁰ where the D⁰ decays toK^þ0. TheD veto is accomplished by taking the Ds daughter tracks (KK^þ), assigning the pion mass to the negative kaon, and requiring m¼ mðK^þÞ mðK^þÞ>180 MeV=c². We also re- quire that the decay contain two reconstructed tracks sat- isfying the level 2 trigger requirements.

The simulated data samples used in this analysis consist of singlebhadrons generated byBGENERATOR[16,17] with pT spectra consistent with next-to-leading-order QCD and decayed withEVTGEN[18]. Full detector and trigger simu- lations are performed. The simulated B candidates are reconstructed with the same procedure and the same se- lection as the data candidates. We reweight the simulated sample to match the data distributions for pTðBÞ and trigger mixture.

The lifetime of the B⁰s meson is determined from two sequential fits. The first is a fit to the invariant mass distribution of candidates reconstructed as Ds^þ and is used to determine the fractions of the total number of events found in the various decay modes. These fractions are fixed inputs to the second fit, which is a fit to the proper decay time distribution of the candidates. The uncertainties on the fractions returned by the mass fit are treated as sources of systematic uncertainty.

272001-4

The mass fit is an unbinned maximum likelihood fit to the invariant mass of the candidate reconstructed asD_s^þ with m^rec_B 2 ½4:85;6:45GeV=c². The mass fit compo- nents can be characterized as coming from one of three possible sources: singlebhadrons, real-D_s þtrack back- ground, and fake-D_s þtrack background. The mass probability distribution functions (PDFs) for singlebhad- rons were obtained from simulation, with an additional small shift and resolution smearing to bring the simulated B⁰s peak central value and width into agreement with data.

The single-bmodes were separated in the fit as follows:

B⁰s!Ds^þðnÞ, B⁰s!DsK, B⁰s!Ds^þ, B⁰s! Ds ^þ,B⁰s!D^ðÞs X,B⁰=B!DX,B⁰ !D^ðÞs ^þþ D^ðÞs K^þ, and _b !_cX. The DsK=Ds^þ ratio was constrained to the results of Ref. [19].

Real-Ds þtrack backgrounds consist of a real Ds, produced promptly or from a b-hadron decay, plus an additional track produced in the event. The mass PDF for these events is obtained from an auxiliary fit to the wrong- sign sample, which consists of data events reconstructed as D_sand sideband subtracted in theD_s mass. The mass PDF for the fake-D_s þtrack background is obtained from an auxiliary fit to theDs sidebands. The results of the mass fit are shown in Fig.1 with various modes combined for plotting only. The real-Ds þtrack background and fake-Ds þtrack background components are drawn to- gether as the ‘‘combinatorial background.’’

For the lifetime fit, the variable of interest is the proper decay time, defined as ct ðLxyðB⁰sÞm^rec_B Þ=pTðB⁰sÞ. The reconstructed massm^rec_B is used instead of the world aver- ageB⁰_s mass. A salient feature of this analysis is the treat- ment of partially reconstructedB⁰smesons as signal events that contribute to the lifetime measurement. Since in the partially reconstructed casesLxyðB⁰sÞ,m^rec_B , andpTðB⁰sÞare

extracted from candidates that are missing particles after reconstruction or have the wrong mass assignment for a daughter particle, a multiplicative correction factor K to the decay time is needed. K is defined as K¼

½p_TðB⁰_sÞm^true_B =½p^true_T ðB⁰_sÞm^rec_B cos _PR, where _PR is the angle in thex-yplane between the true momentum of the B⁰_s and the momentum of the partially reconstructed B⁰_s. Because the ratiom^rec_B =p_TðB⁰_sÞis numerically very close to the ratiom^true_B =p^true_T ðB⁰_sÞ, this choice ofctdefinition forces the Kfactor distributions to be centered near K¼1with widths of a few percent. The K factor distributions are determined with simulation.

The lifetime of the B⁰_s meson is determined from an unbinned likelihood fit to theB⁰_s candidates with invariant masses in the range½5:00;5:45 GeV=c². There are three main types of lifetime fit components that will be described in the following paragraphs: fully reconstructed B⁰s, par- tially reconstructedB⁰s, and backgrounds. The treatment of each component depends on its decay structure and whether it can provide information about theB⁰s lifetime.

Fully reconstructed modes where all of theB⁰s daughter particles are included with the correct mass assignment in the construction of the B⁰s candidate are the first type of lifetime fit component. The only FR mode in this analysis is theDs^þ. The core functional form of the FR PDF is an exponential with decay constantcðB⁰_sÞconvoluted with a Gaussian resolution function with width:

PFRðctÞ ¼1

ce^ct0=ðcÞ_t⁰ 1 ffiffiffiffiffiffiffi 2

p e^{ðctct0Þ2=22Þ}

effðctÞ:

(1) A multiplicative ‘‘efficiency curve’’ accounts for the trig- ger and analysis selection criteria:

effðctÞ ¼X³

i¼1

NiðctiÞ²e^ct=ðciÞ ðiÞ:

The shape parameters (,i,Ni, andi) of the PDF are determined in a fit to a simulated B⁰s sample where the lifetime used for generation is known. All the parameters for the PDF are then fixed and onlyðB⁰sÞis varied in the final fit to the data. As we depend on the simulation of the displaced-track trigger, we use a data sample of J=c ! ^þdecays collected with a dimuon trigger to assess the accuracy of this assumption and assign a ‘‘trigger simula- tion’’ systematic uncertainty based on these studies. The partially reconstructed,PHOTOS-modeledD_sðnÞdecays [20] are combined with the FR D_s, as the momentum carried by the photon is small and their lifetime distribution is extremely close to the FR one. This simplification is considered as a possible source of systematic uncertainty.

Partially reconstructed modes either neglectB⁰sdaughter particles in the construction of theB⁰s candidate or assign them an incorrect mass.B⁰s!DsK^þ,Ds^þ,Ds ^þ, and other decay modes partially reconstructed under the Ds^þ hypothesis can also contribute to the B⁰s lifetime

2) m(B) (GeV/c

4.8 5 5.2 5.4 5.6 5.8 6 6.2 6.4

2candidates/10 MeV/c

0 50 100 150 200 250

data fit

(FR)

sπ

→ D Bs

X (PR) Ds s→ B

(bkgd) Λb 0,

+,B B

comb. bkgd

FIG. 1 (color online). Mass distribution for candidates recon- structed asB⁰s !Ds^þ with fit projections overlaid.

measurement. The PR PDF is similar to the FR PDF of Eq.

(1) with an additional convolution with theKfactor distri- bution for each mode. There are separate efficiency curve parameters for each mode, again determined from fits to simulated events.

The backgrounds in the lifetime fit can either come from decays ofbhadrons other than theB⁰s(e.g.,B⁰=B ! DX, B⁰!DsX, and _b!_cX), or they can come from real-Ds þtrack and fake-Ds þtrack combinations.

The PDFs for the former modes are derived from fits to simulatedB⁰,B, and_bsamples, while the ones for the latter combinatorial backgrounds come from the two proxies available: the B⁰s upper sideband taken from the m^rec_B interval ½5:7;6:4GeV=c² and the Ds sidebands.

The Ds sidebands are taken from the m^rec_B interval

½5:0;5:45GeV=c² and the m^rec_D interval ½1:924;1:939 [

½1:999;2:014 GeV=c². Both proxies contain a mixture of fake-Ds þtrack events and real-Ds þtrack events, where a real Ds can be poorly reconstructed. All the background shape parameters are fixed in the final lifetime fit.

The analysis procedure was tested extensively on three control samples: B⁰!D^þ with D!K^þ, B⁰ !D^þ withD!D⁰ andD⁰ !K^þ, and B^þ!D^0þwithD⁰!K^þbefore performing theB⁰_s fits. Furthermore, the lifetime fit of theB⁰_s was performed using ablindapproach, i.e., by determining the statistical and systematic uncertainties without knowledge of the fit result itself. Good agreement with the world average values [5] of theB⁰ andB^þlifetimes was found.

The lifetime of ðB⁰sÞ ¼1:5180:041ðstatÞps is ob- tained from the full fit. The fit results are plotted in Fig.2. The results of the fits performed separately in the FR mass region (1:4560:067 ps) and PR mass region (1:5440:051 ps) agree with each other at a level of1:0.

We use a Monte Carlo technique to assess the systematic uncertainties. For each source of systematic uncertainty, we generate 1000 simulated experiments with the number of events in each experiment Poisson distributed around the number of events in data. The simulated experiments are generated with a nonstandard lifetime fit configuration (where the PDFs or numbers of events in the various modes are modified to account for the systematic effect) and fit with the default configuration. The mean biases returned from the fits to the simulated experiments (retgen) are used to set the size of the systematic uncertainties. We consider several sources of systematic uncertainty: combi- natorial background fraction, modeling of backgrounds from single b-hadron decays, effect of reweighting the full simulations to match the data, modeling of the trigger bias as a function ofct, off-line–on-line impact parameter correlation, accuracy of the trigger simulation, and detector alignment. Table I contains the final list of systematic uncertainties for this measurement. The largest contribu- tion comes from the uncertainty on the total amount of combinatorial background and the amount of promptly produced real-Ds background.

The displaced-track trigger, in addition to modifying the accepted decay length distribution from a simple exponen- tial to the form in Eq. (1), alters the expected mixture of mass eigenstatesB⁰_s;LandB⁰_s;Hin the flavor-specificB⁰_s! Ds^þ decay by preferentially selecting the longer-lived B⁰_s;H. The size of the imbalance can be calculated using the parameters of the efficiency curve and the world average of 1=¼1:47 ps[5]. Our result can be corrected back to a flavor-specific lifetime measurement with ðB⁰_sÞ ¼ 0:11ð=Þ² ps. Given the world average =¼ 0:092^þ0:051_0:054 [5], the correction would be smaller than our statistical and systematic uncertainties. Therefore, we do not correct the central value or assess an additional system- atic uncertainty.

In summary we have measured the B⁰_s lifetime using both fully reconstructedB⁰_s !D_sðÞ^þand partially reconstructedB⁰_s !D_sðÞXdecay modes, in a sample

FIG. 2 (color online). Distribution ofctfor candidates recon- structed asB⁰s !DsðÞ^þ with fit projection overlaid.

TABLE I. Summary of sources of systematic uncertainty for theB⁰s!DsðÞXlifetime fit. The total uncertainty is calcu- lated assuming the individual contributions are uncorrelated.

Description Value (ps)

Background modeling and fractions 0.019

Fixed single-bbackgroundct 0.003

Reweighting forpT and trigger 0.012

Lifetime contribution ofDradiative tail 0.002 Efficiency curve parametrization 0.002

Trigger simulation 0.014

Impact parameter correlation 0.003

Detector alignment 0.003

Total systematic uncertainty 0.027

272001-6

with 1:3 fb¹ of integrated luminosity. We measure ðB⁰_sÞ ¼1:5180:041ðstatÞ 0:027ðsystÞps, which is consistent with theoretical expectations [3,4].

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.

This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean World Class University Program, the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, UK; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; the Academy of Finland; and the Australian Research Council (ARC).

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.

dVisitor from University of California Irvine, Irvine CA 92697, USA.

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.

jVisitor from University College Dublin, Dublin 4, Ireland.

kVisitor from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017.

lVisitor from Universidad Iberoamericana, Mexico D.F., Mexico.

mVisitor from Iowa State University, Ames, IA 50011, USA.

nVisitor from University of Iowa, Iowa City, IA 52242, USA.

oVisitor from Kinki University, Higashi-Osaka City, Japan 577-8502.

pVisitor from Kansas State University, Manhattan, KS 66506, USA.

qVisitor from University of Manchester, Manchester M13 9PL, United Kingdom.

rVisitor from Queen Mary, University of London, London, E1 4NS, United Kingdom.

sVisitor from Muons, Inc., Batavia, IL 60510, USA.

tVisitor from Nagasaki Institute of Applied Science, Nagasaki, Japan.

uVisitor from National Research Nuclear University, Moscow, Russia.

vVisitor from University of Notre Dame, Notre Dame, IN 46556, USA.

wVisitor from Universidad de Oviedo, E-33007 Oviedo, Spain.

xVisitor from Texas Tech University, Lubbock, TX 79609, USA.

yVisitor from Universidad Tecnica Federico Santa Maria, 110v Valparaiso, Chile.

zVisitor from Yarmouk University, Irbid 211-63, Jordan.

aaOn leave from J. Stefan Institute, Ljubljana, Slovenia.

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