Measurement of the Λ_b⁰ Lifetime in Λ_b⁰ -&gt; <em>J/ψΛ⁰</em> in <em>pp</em> Collisions at s√=1.96  TeV

Article

Reference

Measurement of the Λ

Lifetime in Λ

-> J/ψΛ

in pp Collisions at s√=1.96  TeV

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We report a measurement of the Λ0b lifetime in the exclusive decay Λ0b→J/ψΛ0 in pp collisions at s√=1.96  TeV using an integrated luminosity of 1.0  fb−1 of data collected by the CDF II detector at the Fermilab Tevatron. Using fully reconstructed decays, we measure τ(Λ0b)=1.593+0.083−0.078(stat)±0.033(syst)  ps. This is the single most precise measurement of τ(Λ0b) and is 3.2σ higher than the current world average.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the Λ

Lifetime in Λ

-> J/ψΛ

in pp Collisions at s√=1.96  TeV. Physical Review Letters, 2007, vol. 98, no. 12, p.

122001

DOI : 10.1103/PhysRevLett.98.122001

Available at:

http://archive-ouverte.unige.ch/unige:38378

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

1 / 1

Measurement of the

Lifetime in

! J=

in p p Collisions at p s

1:96 TeV

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I. Zaw,²¹X. Zhang,²³J. Zhou,⁵²and S. Zucchelli⁵

(CDF Collaboration)

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

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

4Baylor University, Waco, Texas 76798, USA

5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

6Brandeis University, Waltham, Massachusetts 02254, USA

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

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

9University of California, San Diego, La Jolla, California 92093, USA

10University of California, Santa Barbara, Santa Barbara, California 93106, USA

11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

15Duke University, Durham, North Carolina 27708, USA

16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

17University of Florida, Gainesville, Florida 32611, USA

18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

19University of Geneva, CH-1211 Geneva 4, Switzerland

20Glasgow University, Glasgow G12 8QQ, United Kingdom

21Harvard University, Cambridge, Massachusetts 02138, USA

22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

23University of Illinois, Urbana, Illinois 61801, USA

122001-2

24The Johns Hopkins University, Baltimore, Maryland 21218, USA

25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

27Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;

Seoul National University, Seoul 151-742, Korea;

and SungKyunKwan University, Suwon 440-746, Korea

28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

29University of Liverpool, Liverpool L69 7ZE, United Kingdom

30University College London, London WC1E 6BT, United Kingdom

31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;

and University of Toronto, Toronto, Canada M5S 1A7

34University of Michigan, Ann Arbor, Michigan 48109, USA

35Michigan State University, East Lansing, Michigan 48824, USA

36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

37University of New Mexico, Albuquerque, New Mexico 87131, USA

38Northwestern University, Evanston, Illinois 60208, USA

39The Ohio State University, Columbus, Ohio 43210, USA

40Okayama University, Okayama 700-8530, Japan

41Osaka City University, Osaka 588, Japan

42University of Oxford, Oxford OX1 3RH, United Kingdom

43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

48Purdue University, West Lafayette, Indiana 47907, USA

49University of Rochester, Rochester, New York 14627, USA

50The Rockefeller University, New York, New York 10021, USA

51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

52Rutgers University, Piscataway, New Jersey 08855, USA

53Texas A&M University, College Station, Texas 77843, USA

54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

55University of Tsukuba, Tsukuba, Ibaraki 305, Japan

56Tufts University, Medford, Massachusetts 02155, USA

57Waseda University, Tokyo 169, Japan

58Wayne State University, Detroit, Michigan 48201, USA

59University of Wisconsin, Madison, Wisconsin 53706, USA

60Yale University, New Haven, Connecticut 06520, USA (Received 11 September 2006; published 19 March 2007)

We report a measurement of the⁰_b lifetime in the exclusive decay⁰_b!J= ⁰inpp collisions at s

p 1:96 TeVusing an integrated luminosity of1:0 fb¹of data collected by the CDF II detector at the Fermilab Tevatron. Using fully reconstructed decays, we measure ⁰_b 1:593^0:083_0:078stat 0:033systps. This is the single most precise measurement of⁰_band is3:2higher than the current world average.

DOI:10.1103/PhysRevLett.98.122001 PACS numbers: 13.30.Eg

The weak decay of quarks depends on fundamental parameters of the standard model, including elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix which describe mixing between quark families. Extraction of these parameters from weak decays is complicated since observed quarks are not free but are confined within color- singlet hadrons, as described by quantum chromodynamics (QCD). An essential tool used in this extraction is the heavy quark expansion (HQE) technique [1]. In HQE, the total decay width of a heavy hadron is expressed as an

expansion in inverse powers of the heavy quark massm_q. Lifetime ratios of b-flavored hadrons are predicted to be unity throughO1=m_b, andO1=m²_bcorrections are small (&2%) [2]. Detailed analysis ofO1=m³_bcorrections to the lifetime ratio [3], summed with theO1=m²_bcorrections, leads to an expected value of_B^0b0’0:94. This theoretical prediction has been in poor agreement with measurements for more than a decade [4–8]. Recent calculations includ- ing next-to-leading-order QCD and O1=m⁴_b corrections

lower the predictions to 0:900:05[9] and 0:860:05 [10], respectively. The ⁰_b lifetime world average of ⁰_b 1:2300:074 ps, corresponding to a ratio of

⁰_b

B⁰0:8040:049 [11], is consistent with the lower end of the theory predictions. In this Letter, we report a measurement of⁰_bconsistent with the higher end of the theory predictions.

We measure ⁰_b in the fully reconstructed decay ⁰_b!J= ⁰, with J= ! and ⁰!p. Charge conjugate modes are implied throughout. Our data sample consists of1:0 fb¹ ofpp collisions at

ps 1:96 TeVcollected by the CDF II detector at the Fermilab Tevatron. This is the first measurement of⁰_busing fully reconstructed decays that is competitive with the best previous measurements, which are based on semileptonic decays. As compared to fully reconstructed decays, mea- surements using partially reconstructed semileptonic de- cays have additional uncertainties due to the missing energy of the unobserved neutrino and the modeling of background from otherb-flavored baryons.

As a check of our method, we also measureB⁰using a sample ofB⁰ !J= K_S⁰ decays, which has larger yield than the ⁰_b sample. This decay channel is topologically similar to⁰_b!J= ⁰, since bothK⁰_Sand⁰decay with large displacement from the b-hadron-decay vertex. The analysis procedure for⁰_bis developed usingB⁰ !J= K⁰_S only and checked using otherb-meson decays containing a J= ! in the final state. The ⁰_blifetime was not extracted until all procedures were established including the systematic uncertainty estimate.

The components of the CDF II detector relevant to this analysis are described briefly here; a more complete de- scription can be found elsewhere [12]. Charged particles are reconstructed using an open-cell drift chamber called the central outer tracker (COT) [13] and 7 layers of silicon microstrip detectors with radii between 2.4 and 28 cm [14].

These are immersed in a 1.4 T solenoidal magnetic field and cover the rangejj 1, whereis the pseudorapidity defined as ln tan=2 and is the polar angle [15]. Four layers of planar drift chambers (CMU) [16]

detect muons with transverse momentum p_T >

1:4 GeV=c within jj<0:6. Additional chambers and scintillators (CMX) [17] cover0:6<jj<1:0for muons withp_T>2:0 GeV=c.

A sample ofJ= !candidates, collected using a dimuon trigger, is selected to begin the reconstruction of ⁰_b and B⁰ candidates. At level 1 of a three-level trigger system, the eXtremely Fast Tracker (XFT) [18] uses COT information to fit tracks. Those tracks with p_T >

1:52:0 GeV=c are extrapolated to the CMU (CMX) chambers and compared with the positions of muon- chamber tracks. The event passes level 1 if two or more XFT tracks are matched to muon-chamber tracks.

Opposite-charge and opening-angle requirements are im- posed at level 2. At level 3, full tracking information is used

to reconstruct J= ! candidates. Events with a candidate in the mass range 2.7 to4:0 GeV=c²are accepted at level 3 and recorded for further analysis.

Tracks corresponding to two triggered muon candidates are constrained to originate from a common vertex to make aJ= !candidate. To ensure a high-quality vertex for the lifetime measurement, each muon track is required to have at least 3 axial hits in the silicon system. The reconstructed invariant mass is required to be in the range 3:014< M<3:174 GeV=c². This corre- sponds to approximately 4, whereis the rms of the reconstructed signal and is dominated by experimental resolution [19].

We construct K⁰_S! and⁰ !p candidates from pairs of oppositely charged tracks fit to a common vertex. Since manyK⁰_Sand⁰ decays occur outside some layers of the silicon system due to their long lifetime, their tracks are not required to have silicon hits. We suppressK_S⁰ and ⁰ cross contamination by rejecting K_S⁰ (⁰) candi- dates with proton-pion (dipion) invariant mass in the range [1.1085, 1.1235] 0:481 75;0:5115GeV=c².

The B⁰ and ⁰_b candidates are reconstructed by as- sociating J= candidates with K_S⁰ or ⁰ candidates in each event. We choose further selection requirements that

optimize S=

SB

p , where S and B are the numbers of signal and background events, respectively. In the op- timization procedure,Sis estimated using a Monte Carlo simulation, while B is estimated using the b-hadron invariant mass sidebands, which are chosen to exclude the data used in the lifetime fits to avoid any potential bias.

The selection requirements resulting from the opti- mization are described below. We require 0:473<

M<0:523 GeV=c² andp_T>1:5 GeV=cforK_S⁰candi- dates. For ⁰ candidates, we require 1:107< M_p<

1:125 GeV=c² and p_T>2:6 GeV=c. We require L^V_xy⁰=

L^Vxy⁰ >6 for K⁰_S and L^V_xy⁰=

L^Vxy⁰>4 for ⁰, where L^V_xy⁰ is defined as the distance from the J= vertex to the V⁰K⁰_S;⁰ vertex projected onto the V⁰ transverse momentum vector and _LV0

xy is its estimated uncertainty.

Both B⁰ and ⁰_b candidates are required to have p_T>

4:0 GeV=c. Finally, the ² of ab-hadron kinematic fit is required to be less than 26 for 5 degrees of freedom. This fit finds the best b-hadron decay vertex and momentum subject to the constraints that the muon tracks originate from a common vertex, the K⁰_S or ⁰ daughter tracks originate from a common vertex with combined momen- tum pointing back in three dimensions to the dimuon vertex, and the invariant mass of the two muons is equal to the world average J= mass [11]. The invariant mass distributions of the B⁰ and ⁰_b candidates pass- ing these requirements are shown in Fig. 1. The yields are NB⁰!J= K_S⁰ 337688stat and N⁰_b!J= ⁰ 53838stat.

122001-4

We determineB⁰and⁰_bfrom the distribution of proper decay time t given by ctL^b_xy= ^b_T L^b_xycM_b=p^b_T, where L^b_xy is the distance traveled by each b-hadron candidate along the direction of its transverse momentum p^b_T and ^b_T p^b_T=cM_b is the transverse boost, whereM_bis the world average mass of thebhadron [11]. Since theJ= vertex occurs at the same point as theb hadron decay and is well determined, it is used as the b-hadron decay vertex. Thebhadron is assumed to origi- nate from the average beamline determined on a run-by-

run basis using inclusive jet data. The primary vertex for a given event is the x-y position of this beam line at the average z coordinate of the muon tracks at their closest approach to the beam line.

The lifetimes are extracted using the maximum like- lihood method. The likelihood functionLis multivariate, and is constructed from the products of single variable probability density functions describing the distributions of the invariant massm_i,ct_i, and their respective estimated resolutions^m_i and^ct_i . It is given by

L Y^N

i1

1f_BP^ct_Sct_ij^ct_iP_S^ctct_iP^m_Sm_ij^m_i f_BP^ctBct_ij^ct_iPB^ctct_iP^mBm_ij^m_i;

where N is the number of events in the b-hadron mass window,f_Bis the background fraction, andP^ct,P^ct, and P^m are probability density functions forct,^ct, and mass, respectively. The mass resolution probability distributions P^mm_i do not appear in L because they are equal for signal and background, within the available statistics.

Since this is not true for the ct resolution distributions, P^ct must be included inL[20].

The mass distribution is modeled as the sum of a Gaussian signal and linear background, where the Gaussian width ^m_i is scaled by a parameter to account for misestimation of the mass resolutions. Thectdistribu- tion is modeled by the sum of five components, all con- voluted with a Gaussian resolution function with a scale factor parameter for the ^ct_i : a positive exponential (e^cti=c=c) for the signal, a-function representing the zero-lifetime component, one negative and two positive exponentials accounting for mismeasured decay vertices and background from other heavy-flavor decays. The rela- tive contribution of each of the background components is determined by the fit. The^ctdistribution is modeled by a Gaussian convoluted with an exponential for both signal and background.

We fit over the mass range [5.170, 5.390] and 5:521;5:721GeV=c² for B⁰ and ⁰_b, respectively.

These ranges provide a sufficient sideband to constrain the background shape while avoiding regions where the mass distribution has complex structure. For both B⁰ and ⁰_b, we require ^m_i <20 MeV=c² and fit over the range 2000;4000minctand 0;100 min^ct. We max- imize the likelihood to determine the best values of all fit model parameters, including the signal lifetimescB⁰ 456:8^9:0_8:9 m and c⁰_b 477:6^25:0_23:4 m. Fit projec- tions are shown in Fig.2.

Systematic uncertainties come from four main sources:

fitting procedure and model, primary vertex determination, alignment of detector elements, and K_S⁰ or ⁰ pointing requirement in the B⁰ or ⁰_b kinematic fit. The fitting bias is determined using a simple Monte Carlo simulation in which events are distributed according to the fit model.

2) Mass (GeV/c

0

Ks

ψ J/

5.1 5.15 5.2 5.25 5.3 5.35 5.4 5.45

Entries / 5 MeV

0 100 200 300 400 500 600 700 800

900 _Data

Signal + Background

No ct cut

µm ct > 200

(a)

2) Mass (GeV/c Λ0

ψ J/

5.45 5.5 5.55 5.6 5.65 5.7 5.75 5.8

Entries / 10 MeV

0 50 100 150 200 250 300

Data

Signal + Background

No ct cut

µm ct > 200

(b)

FIG. 1 (color online). Invariant mass distribution of (a)B⁰!J= K⁰_S and (b)⁰_b!J= ⁰ candidates. The distri- butions with ct >200 m, where tis the proper decay time, illustrate that the majority of backgrounds originate from the primary interaction vertex. The distributions are fit to the sum of a Gaussian signal and linear background.

The bias was found to be less than0:4mand0:5mfor B⁰ and⁰_b, respectively. The systematic uncertainties due to ct resolution and mass resolution are estimated by including additional Gaussian components to their respec- tive parts of the model in separate fits to the data and observing the deviations from the nominal result. We estimate the contribution from our mass background model by fitting with a uniform rather than linear background shape. The systematic uncertainty due to thectbackground model is estimated by fitting with two or four background exponentials convoluted with the resolution function and fitting with two, three, or four background exponentials without convolution. We study a possible mass dependence in thectbackground shape by separately fitting forB⁰(⁰_b) lifetime in the following low and high mass regions:

[5.170, 5.3225] ([5.521, 5.651]) and [5.2375, 5.390]

5:591;5:721 GeV=c². The observed shifts are consis- tent with the statistical differences of the two samples for both modes. We use the average shift of 1.9 and4:1mfor B⁰ and⁰_b, respectively, as an estimate of the systematic uncertainty due to a mass-dependent ct background. We estimate the systematic uncertainty due to our^ctand^m distribution models by the observed shift between simple Monte Carlo events generated with the data distributions but fit with our model compared with simple Monte Carlo events both generated and fit with our model. Using the same simple Monte Carlo technique, we estimate the sys- tematic uncertainty due to a correlation between thectand ^ctfor the background by generating simple Monte Carlo events with the correlation observed in the data and fitting with our baseline model where this correlation is absent.

We estimate the systematic uncertainty due to our pri- mary vertex determination by comparing different choices of thezcoordinate used to evaluate the run-averaged beam line. We estimate uncertainties due to any residual mis- alignments of the silicon detector using Monte Carlo samples generated with radial displacements of individual sensors (internal alignment) and relative translation and

rotation of the silicon detector with respect to the COT (global alignment). We also study the resolution and bias on theV⁰pointing to theJ= vertex in data. If these were strongly ct dependent, the kinematic fit quality require- ment could bias the b-hadron lifetime. We observe no lifetime bias and assign uncertainties of 0:6m for B⁰ and5:4mfor⁰_bbased on the statistical precision of our study.

The systematic uncertainties are summarized in TableI.

We obtain total systematic uncertainties of4:9mforB⁰ and9:9mfor⁰_b by adding the individual uncertainties in quadrature.

A number of cross checks on the analysis procedure are performed. We measure B and B⁰ lifetimes that are statistically consistent with the world average values in

TABLE I. Systematic uncertainties (inm) for the measure- ment of cB⁰ and c⁰_b. The total uncertainties are the individual uncertainties added in quadrature.

Source cB⁰ c⁰_b

Fitter Bias 0.4 0.5

Fit Model:

ctResolution 3.1 5.5

Mass Signal 0.7 2.3

Mass Background 0.1 0.1

ctBackground 0.5 0.7

^ctDistribution Modeling 0.1 0.2

^mDistribution Modeling 0.6 0.2

Mass-ctBackground Correlation 1.9 4.1

ct-^ctBackground Correlation 0.3 1.3

Primary Vertex Determination 0.2 0.3

Alignment:

Silicon Detector (Internal) 2.0 2.0

Silicon Detector/COT (Global) 2.2 3.2

V⁰ Pointing 0.6 5.4

Total 4.9 9.9

µm) ct ( -2000 -1000 0 1000 2000 3000 4000

mµEntries / 40

1 10 102 103

Data Signal Background Signal+Background

(a)

µm) ct ( -2000 -1000 0 1000 2000 3000 4000

mµEntries / 40

1 10 102

Data Signal Background Signal+Background

(b)

µm) ct ( σ 0 10 20 30 40 50 60 70 80 90 100

mµEntries / 5

0 50 100 150 200 250 300 350 400 450

Data Signal Background Signal+Background

(c)

FIG. 2 (color online). (a)ctfit projection for B⁰!J= K_S⁰ candidates; and (b) ctand (c) ^ct fit projections for ⁰_b!J= ⁰ candidates.

122001-6

the following decay modes: B⁰ ! ⁰K⁰_S, B⁰ !

0K⁰K⁰!K, B ! ⁰K, and B ! KK!K⁰_S, with ⁰! and ⁰ !

. We search for unexpected lifetime dependence on the V⁰ and b-hadron kinematics, data-taking period, number of tracks in the event, and use of silicon hits onV⁰ daughter and muon tracks; no dependence is observed.

Finally, we determine the lifetime using two alternative techniques which give results consistent with our baseline fit: act-only binned fit applied to sideband-subtracted data and a fit to the mass distribution in ct bins which is insensitive to thectbackground shape.

In summary, we measure

⁰_b 1:593^0:083_0:078stat 0:033systps:

As a cross check, we also measure B⁰ 1:524 0:030stat 0:016syst ps which is consistent with the world average B⁰ 1:5300:009 ps. Our measure- ment of ⁰_b is consistent with the D0 result in the same channel [6] at the1:7level and is the first measure- ment using a fully reconstructed mode that reaches a precision comparable with the previous best measurements based on semileptonic decays of the⁰_b. It is also compa- rable in precision to the current world average, but is3:2 higher [11]. Forming a ratio with the world average B⁰ lifetime, we determine

⁰_b

B⁰ 1:0410:057statsyst:

This ratio is consistent with the higher end of the theory predictions [3,9,10].

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 Foun- dation; the A. P. Sloan Foundation; the Bundes- ministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, UK;

the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic

Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme under Contract No. HPRN-CT- 2002-00292; and the Academy of Finland.

*Visiting scientist from Universidad Autonoma de Madrid, 28049 Madrid, Spain.

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