Measurement of the <em>B_c⁺</em> Meson Lifetime Using the Decay Mode <em>B_c⁺</em> -&gt; <em>J/ψe⁺ν_e</em>

Article

Reference

Measurement of the B

Meson Lifetime Using the Decay Mode B

-> J/ψe

ν

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al .

Abstract

We present a measurement of the B+c meson lifetime in the decay mode B+c→J/ψe+νe using the Collider Detector at Fermilab II detector at the Fermilab Tevatron Collider. From a sample of about 360  pb−1 of pp collisions at s√=1.96  TeV, we reconstruct J/ψe+ pairs with invariant mass in the kinematically allowed range 4

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

Meson Lifetime Using the Decay Mode B

-> J/ψe

ν

. Physical Review Letters, 2006, vol. 97, no.

01, p. 012002

DOI : 10.1103/PhysRevLett.97.012002

Available at:

http://archive-ouverte.unige.ch/unige:38348

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

1 / 1

Measurement of the B

Meson Lifetime Using the Decay Mode B

! J= e

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(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

012002-2

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 9 March 2006; published 7 July 2006)

We present a measurement of theBc meson lifetime in the decay modeBc !J= ee using the Collider Detector at Fermilab II detector at the Fermilab Tevatron Collider. From a sample of about 360 pb¹ of pp collisions at ps

1:96 TeV, we reconstruct J= e pairs with invariant mass in the kinematically allowed range4< M_{J= e}<6 GeV=c². A fit to the decay-length distribution of 238 signal events yields a measuredB_c meson lifetime of0:463^0:073_0:065stat 0:036systps.

DOI:10.1103/PhysRevLett.97.012002 PACS numbers: 14.40.Nd, 13.20.He

TheB_c meson is the only known meson consisting of two heavy quarks of different flavor: a charm quark and a bottom antiquark. It provides a unique test of heavy-quark dynamics, since the bound state can be treated using the same nonrelativistic expansion that successfully describes bothccandbbfamilies. However, unlikeccandbbstates, the B_c meson decays only via weak interactions, thus having a measurable lifetime. The lifetime of the B_c meson is expected to be about 2 – 3 times smaller than the B meson lifetime if one assumes three major decay subprocesses [1,2]: b quark decay with the c quark as a

spectator, cquark decay with the b quark as a spectator, and bc annihilation decays. In the B meson case, the dominant decay subprocess is the bquark decay with the uquark as a spectator. An early measurement from CDF [3] found a B_c lifetime consistent with predictions. More precise measurements will determine the relative impor- tance of the three decay subprocesses and provide insight into the strong dynamics of heavy quarks. Here we report a new B_c meson lifetime measurement using the decay mode B_c !J= e_e, where charge-conjugate modes are implied, from a sample of360 pb¹ collected during

2002 – 2004 with the CDF II detector [4] at the Fermilab Tevatron Collider at a center of mass energy of 1.96 TeV.

The new measurement is about 2.5 times more precise than the previous one.

The B_c !J= e_e reconstruction starts with J= ! candidates selected based upon a two-muon topol- ogy by the CDF trigger system [5]. TheJ= candidates are further purified during offline reconstruction by vertex constraining the pairs and by selecting the pairs with momentum transverse to the beam line p_TJ= >

3 GeV=c. Then each J= candidate with a reconstructed mass within50 MeV=c² of its nominal value is combined with an electron to form aB_c candidate.

Electron identification uses both specific ionization (dE=dx) information from the central outer tracker (COT) and calorimeter shower information from the cen- tral electromagnetic calorimeter (CEM). The logarithm of the ratio of the measured dE=dx value from a charged particle to that expected for an electron, Z_e lndE=dx lndE=dx_predict, is compared to its standard deviation_Ze. The expecteddE=dxand_Zeare functions of the particle charge, momentum, and the multiplicity of associated COT hits. Electron candidates are required to have Z_e=_Ze>1:3 to reject hadrons (=K=p) while remaining efficient for true electrons. Samples of electrons, pions, kaons, and protons selected from collision data are used to determine the dE=dx identification efficiencies listed in TableI. These control samples come from photon conversions !ee and from hadron decays K⁰_s ! , D⁰ !K, and ⁰ !p. The hadrons sur- viving the Z_e=_Ze selection are mainly pions, which are rejected using calorimeter shower shape information.

The calorimeter shower shape of a charged particle with p_T>2 GeV=c is obtained by extrapolating its track re- constructed in the COT into the calorimeter to match shower clusters there [3]. The probabilities for a particle to have a shower shape consistent with being an electron or a hadron are calculated using the distributions of shower energy and shower cluster profiles for the electron and hadron samples described above. We first define the proba-

bility for a charged particle to be an electron based on shower shape by the ratio between its probability to be an electron and the sum of its probabilities to be an electron or a hadron. We then obtain a cumulative probability distri- bution of the ratio using the electron sample and impose a selection on electron candidates at a 70% probability value.

To calculate the average probability for hadrons to pass this requirement, as listed in Table I, the control samples of

=K=pparticles are mixed using fractions predicted by a

PYTHIA Monte Carlo (MC) simulation of B!J= X events [6]. In addition, electrons found to originate from photon conversion !ee are removed from consid- eration as J= e candidates [3]. Overall, the electron identification using combined dE=dx and calorimeter in- formation has an efficiency of 60% for true electrons, while hadrons have a probability to pass the selection lower than 0.15%.

A B_c candidate is a J= e pair with transverse mo- mentum p_{TJ= e}>5 GeV=c and invariant mass 4<

M_{J= e}<6 GeV=c². The upper bound onM_{J= e} is simply the kinematic limit from the mass of B_c meson [7]. The lower bound is set higher than the kinematic limit ofM_J=

to reduce the background from B_c semileptonic decays other than the exclusive decay B_c !J= e_e to a few percent [1,3]. The opening angle between the J= and electron momenta in the transverse plane must be within 90to reduce genericbbbackground that produces aJ=

and an electron from differentbhadrons. Finally, the tracks of the three daughter particles,, ande are fit to a common vertex, and theB_c decay length in the transverse plane L_xy is calculated as the projection of the displace- ment of the B_c vertex from the primary vertex onto the momentum of the J= e system. The primary vertex position is obtained from run-by-run averages using samples of prompt tracks.

Before making a lifetime measurement, we first estab- lish the B_c signal in the J= e pairs. The background pairs from prompt decays are removed by imposing a selection of L_xy=_Lxy>3. The M_{J= e} distribution of J= e pairs with L_xy=_Lxy>3 is shown in Fig. 1.

TABLE I. Electron identification efficiencies (percent) as functions of particlep_T(GeV=c) usingdE=dxand a calorimeter (Cal) for electron and hadrons. The dE=dx results are averages of positively and negatively charged particles. The calorimeter results for hadrons (h) are the weighted averages of,K, andp. The conversion-finding efficiency_conv is also listed.

p_T 2 – 3 3– 4 4 –5 5– 6 >6

dE=dx:e 91:30:1 91:40:2 90:70:2 90:50:3 89:50:2

dE=dx: 16:40:1 23:80:1 32:00:1 39:00:2 49:40:2

dE=dx:K 2:090:09 2:350:07 3:290:09 4:40:1 9:50:2

dE=dx:p 3:380:03 2:140:04 2:540:07 3:00:1 4:90:2

Cal:e 68:50:3 68:80:5 68:90:8 68:91:1 67:61:0

Cal:e 67:90:4 69:50:5 69:60:7 68:11:2 68:50:9

Cal:h 0:770:04 0:370:04 0:370:03 0:290:04 0:130:04

Cal:h 0:640:04 0:370:02 0:270:03 0:250:04 0:210:04

_conv 49:81:4 55:02:2 56:53:3 61:54:5 69:23:4

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Within the 4< M_{J= e}<6 GeV=c² window, 203 candi- dates are found with a total of background 8814 as listed in Table II. The background is classified into four groups: (i) background with a falseJ= , background with a correctly identified J= but a wrong electron candidate which can be either (ii) a misidentified hadron (false electron) or (iii) an electron from photon toeeconver- sion, and (iv) background from decays of otherBhadrons (bb). The number of false J= backgrounds is estimated using pairs with an invariant mass outside the 50 MeV=c² window. To estimate the false electron back- ground, we use a sample of J= -track pairs passing the same selection as the J= e pairs, including thedE=dx requirement and the requirement to point to the CEM fiducial region, but without any selection based on calo- rimeter information. The size of the contribution is esti- mated from a weighted counting of the J= -track pairs with the weights taken as the averaged probabilities in Table I for hadrons to pass the electron selection using the calorimeter. We derive the residual conversion electron contribution from the rate of identified photon conversions together with the conversion-finding efficiency listed in TableI. The conversion-finding efficiency, defined as the

fraction of the identified electrons with their conversion partners in the kinematic acceptance of the CDF detector, is estimated from a MC simulation. Finally, the contribu- tion from decays resulting frombbproduction is estimated using a PYTHIA MC sample with relative rates of flavor creation, flavor excitation, and gluon splitting tuned to the Tevatron data [6,8]. The number of B_c signal events is found to be 11516stat 14syst. For comparison, there are 287259 B !J= K events in the data sample corresponding to the same integrated luminosity.

The selection of B is the same as for B_c except for electron identification on the K. We find the production rate of B_c relative to that of B, B_cBB_c ! J= Xe_e =BBB!J= K , in the kinematic range p_T>4 GeV=c and rapidity jyj<1 [7] to be 0:2820:038stat 0:035syst 0:065acceptance.

The first error is statistical, the second covers the system- atic uncertainty ofB_c signal excess counting, and the third pertains to the estimated detector acceptance ratio correc- tion A_B=A_Bc 4:421:02from MC simulation where theB_c p_T spectrum, its lifetime values, and decay modes are the major sources of uncertainty. This new production ratio result agrees with the earlier CDF measurement [3].

Having established a clear B_c signal inJ= ecombi- nations, we measure the B_c meson lifetime in a larger sample of 783 events, selected with the same criteria as above, but without theL_xy selection. We estimate the net signal excess in this sample to be 238. The background sources and their contributions are listed in Table II.

Contributions from false electrons, conversion electrons, bb, and false J= are estimated as described earlier. The number of additional prompt-decay events is extracted directly from the lifetime fit.

The Lorentz-invariant proper decay time of aB_c event is its decay lengthL_xywith a Lorentz boostp_T

Bc=M_Bc in the transverse plane, whereM_Bcis the mass ofB_c meson and p_TBc its transverse momentum. The B_c mass is as- sumed to have a value of6:271 GeV=c²[2]. Because of the missing neutrino, we cannot directly calculate the boost factor from the lab system to theB_c rest frame. We can, however, calculate the B_c lifetime in the center of mass frame of the J= e pair, ct⁰L_xyM_{J= e}=p_{TJ= e}, which provides the best estimator of the B_c proper decay time in the absence of the neutrino momentum. The true proper B_c decay time is given by ctL_xyp_T

Bc=M_Bc Kct⁰, whereKis the residual correction factor depending on the momenta of the missing neutrino. In MC simulations, K can be calculated K p_{TJ= e}=p_TBcM_Bc=M_{J= e}cos, where is the angle between the vectors of p_{TJ= e} and p_T

Bc. The K distributions, as shown in Fig. 2, always includeK1and decrease in width asM_{J= e} gets closer to the value ofB_c mass when the neutrino’s momentum is minimal.

An unbinned maximum-likelihood fit [3,4] is used to extract theB_c meson lifetime. In the fit,t⁰and its event-by-

2) (GeV/c

ψ e

MJ/

0 2 4 6 8 10 12

2Events / 0.5 GeV/c

0 10 20 30 40 50 60 70 80

FIG. 1 (color online). M_{J= e}distribution ofJ= epairs (dots with error bars) with L_xy=_Lxy>3 together with the expected shape (dashed line) from a sum of a MC signal and the estimated background (solid lines) which is mostly fromb-hadron decays.

The falseJ= is already removed in the distribution.

TABLE II. Numbers of J= e pairs and estimated back- grounds. The error listed is the sum of statistical and systematic errors.

AllL_xy L_xy=_Lxy>3

FalseJ= 164:09:1 24:53:5

False electron 110:219:0 15:42:5

Conversion electron 67:434:8 14:57:8

bb 63:018:5 33:611:4

Prompt decay 141:732:0

Total background 54555 8814

ObservedJ= epairs 783 203

event error_t⁰ _LxyM_{J= e}=cp_{TJ= e}are the input varia- bles. The likelihood function has the form [9]Ft⁰; _t⁰ 1P5

1f_biFst⁰; _t⁰Ps_t⁰ P5

1f_biFbit⁰; _t⁰Pbi_t⁰, where Fst⁰; _t⁰ is the lifetime probability density func- tion (PDF) for a pure B_c signal, f_bi andFbit⁰; _t⁰ are fractions of the five background contributions and their lifetime PDFs, and Ps_t⁰and Pb_it⁰ are the PDFs of the_t⁰ for signal and backgrounds. The lifetime PDF for the B_c signal is an exponential lifetime distribution con- voluted with theKdistribution and a Gaussian resolution function. The prompt background lifetime PDF is assumed to have zero lifetime with a Gaussian resolution function.

The PDFs for other backgrounds are described by a sum of a Gaussian distribution centered at zero and two pairs of positive and negative exponential lifetime functions with noK correction applied. The initial parameters for these background PDFs are obtained from fits to the background samples as shown in Fig.3. The obtained results are used to constrain the corresponding parameters in the final lifetime fit. The constraints are imposed by multiplying the like- lihood function with Gaussian functions of the appropriate mean and width. Similarly, the background fractions f_bi are also constrained in the B_c meson lifetime fit to the estimated values in TableII.

In Fig.4, thect⁰distribution from the 783B_c candidates is shown with the fit result superimposed. We findc _Bc 139²²₂₀ m. The sources of systematic uncertainty on the lifetime fit are now considered, and their magnitudes are estimated. The effect of theK distribution uncertainty on theB_c lifetime fit is estimated using alternativeK distri- butions obtained from MC simulations with different p_T spectrum, mass, and lifetime values, andB_c meson decay modes. The p_T spectrum is changed from that derived using a theoretical calculation [10] to that of the inclusive decayB!J= X[4]. The mass and lifetime in the MC cal- culation are varied in the rangesM_Bc6:2–6:4 GeV=c² and _Bc0:4–0:7 ps [1,2]. The B_c decay considered in the MC calculation is varied from the exclusive B_c !

J= e_ealone to that of an inclusive decay table predicted in Ref. [1]. We found the change on theB_c lifetime fit as c _Bc 2:8m. The uncertainty related to back- ground lifetime shapes is estimated from investigating the p_T dependence of the electron identification and the conversion-finding efficiencies, from using false J=

events from different samples with or without an electron nearby, and from changing fractions ofbbevents originat- ing from the three main production mechanisms according to a study using CDF data [8]. The estimated effect is 9:2m. The uncertainty related to theL_xy calculation and its error distribution is found to be4:7mfrom the uncertainty in the silicon detector alignment, by using an alternative functional form describing the L_xy resolution that includes an additional Gaussian and symmetric ex- ponential tails, and by using alternative decay-length er-

µm)

e (

ψ

/ pT J/

ψe

MJ/

Lxy

-1000 -500 0 500 1000 1500 2000

mµEvents / 50

10-1

1 10 102

FIG. 4 (color online). ct⁰distribution from 783B_c candidates.

The points with error bars are data points and the solid line is the fit result. The dashed line is theB_c signal and the dotted line is the background.

10-2

10-1

1 10 (a)

1 10 102

(b)

10-2

10-1

1 10 (c)

10-2

10-1

1 10 (d)

µm)

e (

ψ

/ pT J/

ψe

M J/

Lxy

-2000 0 2000 4000 -2000 0 2000 4000

mµEvents / 50

FIG. 3. ct⁰ PDF obtained from fits to background samples (points with error bars) for (a) false J= (solid lines), (b)bb (solid lines), (c) false electron (hatched area), and (d) conversion electron (hatched area). In (c) and (d), the solid lines are the sum of false J= (shaded area) and a false or conversion electron.

The false J= fraction and lifetime shape in (c) and (d) are constrained to that obtained from sideband events.

0 0.05 0.1

(a)

0 0.05

0.1 (b)

0 0.5 1 1.5 2

0 0.05 0.1 0.15 (c)

0 0.5 1 1.5 2

0 0.1 0.2 0.3 (d)

K distribution

Arbitrary unit

FIG. 2. Distribution ofKforM_{J= e}within (a) 4 – 4.5, (b) 4.5–

5.0, (c) 5.0 –5.5, and (d)5:5–6:0 GeV=c².

012002-6

ror distributions. The fitting procedure is also checked using MCct⁰ distributions similar to that of the B_c can- didates, and there is no bias found. Adding all the esti- mated systematic errors in quadrature, we find c _Bc 139²²₂₀stat 11syst mor _Bc 0:463^0:073_0:065stat 0:036systps.

In conclusion, from an unbinned maximum-likelihood fit to the decay-length distribution of 238 signal events of B_c !J= e_e, the B_c meson lifetime is found to be 0:463^0:073_0:065stat 0:036syst ps, which is about one- third of the B meson lifetime of 1:6710:018 ps [7].

This agrees with theoretical models [1,2] in which all the three major decay subprocesses, the two spectator pro- cesses (b-quark and c-quark) and the bc annihilation, play important roles in theB_c decays.

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 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, United Kingdom; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential Programme under Contract no. HPRN- CT-2002-00292; and the Academy of Finland.

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