Measurement of the Bs0 lifetime and production rate with Ds−ℓ+ combinations in Z decays

2 November 1995

EISEVIER Physics Letters B 361(1995) 221-233

Measurement of the Bt lifetime and production combinations in Z decays

rate with D; l-t

ALEPH Collaboration

D. Buskulic a, D. Casper a, I. De Bonis a, D. Decamp a, P. Ghez a, C. Goy a, J.-P Lees a,

PHYSICS LETTERS B

A. Lucotte a, M.-N. Minard a, I? Odier a, B. Pietrzyk’, F. Ariztizabal ‘, M. Chmeissani ‘, J.M. Crespo b, I. Efthymiopoulos b, E. Fernandezb, M. Fernandez-Bosman b, V. Gaitan b,

Ll. Garridobp15, M. Martinezb, S. Orteu b, A. Pacheco b, C. Padilla b, F. Palla b, A. Pascual b, J.A. Perlas b, F. Sanchez b, F. Teubertb, A. Colaleo ‘, D. CreanzaC, M. de Palma ‘,

A. FarillaC, G. Gelao c, M. Girone ‘, G. Iaselli ‘, G. Maggi c93, M. Maggi ‘, N. Marinelli ‘, S. Natali ‘, S. Nuzzo c, A. Ranieri ‘, G. Raso ‘, F. Roman0 ‘, F. RuggieriC, G. Selvaggi”,

L. Silvestris ‘, P. Tempesta”, G. Zito’, X. Huangd, J. Lind, Q. Ouyang d, T. Wang d, Y. Xie d, R. Xu d, S. Xue d, J. Zhang d, L. Zhang d, W. Zhao d, G. Bonvicinie, M. Cattaneo e,

P. Comas e, P. Coyle e, H. Drevermanne, A. Engelhardt e, R.W. Forty e, M. Frank e, R. Hagelberge, J. Harvey e, R. Jacobsen e124, l? Janot e, B. Jost e, J. Knobloch”, I. Lehraus e, C. Markou e,23, E.B. Martine, P. Matoe, H. Meinharde, A. Mintene, R. Miquele, T. Oest e,

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J.B. Hansen g, J.D. Hansen s, J.R. Hansen s, PH. Hansen g, B.S. Nilsson s, A. Kyriakis h, E. Simopoulou h, I. Siotish, A. Vayaki h, K. Zachariadou h, A. Blonde1 i*21, G. Bonneaud’,

J.C. Brient i, P Bourdon’, L. Passalacqua’, A. Rouge i, M. Rumpf i, R. Tanaka i, A. Valassi i131, M. Verderi’, H. Videau’, D.J. Candlinj, M.I. Parsonsj, E. Focardik,

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G. Felici m, P. Laurelli m, G. Mannocchi m,5, F. Murtasm, G.P. Murtas m, M. Pepe-Altarelli”, S.J. Dorris”, A.W. Halley”, I. ten Havenv6, I.G. Knowles”, J.G. Lynch”, W.T. Morton”,

V. O’Shea”, C. Raine”, I? Reeves”, J-M. Scar-r”, K. Smith”, M.G. Smith”, A.S. Thompson n, F. Thomson n, S. Thorn n, R.M. Turnbull”, U. Becker O, 0. Braun O,

C. Geweniger O, G. Graefe O, P. Hanke’, V. Hepp O, E.E. Kluge O, A. PutzerO, B. Rensch O,

0370.2693/95/.W9.50 @ 1995 Elsevier Science B.V. All rights reserved SSDIO370-2693(95)01173-O

222 ALEPH Collaboration/Physics Letters B 361 (1995) 221-233

M. Schmidt O, J. Sommer O, H. Stenzel O, K. Tittel O, S. Werner O, M. Wunsch O, R. Beuselinck p, D.M. Binnie p, W. Cameron P, D.J. Colling P, P.J. Doman P,

N. Konstantinidis P, L. Monetar, A. Moutoussi p, J. Nash P, G. San Martin P, J.K. Sedgbeerp, A.M. Stacey P, G. Dissertori 9, l? Girtler q, E. Kneringer 9, D. Kuhn 9, G. Rudolph q, C.K. Bowdery r, T.J. Brodbeck ‘, P. Colrain ‘, G. Crawford r, A.J. Finch r, F. Foster r,

G. Hughes r, T. Sloan’, E.P. Whelan’, M.I. Williams’, A. GallaS, A.M. Greene s, K. KleinknechtS, G. Quast ‘, J. Raab ‘, B. Renk s, H.-G. Sanders, R. WankeS,

P. van Gemmeren ‘, C. Zeitnitz ‘, J.J. Aubert t, A.M. Bencheikh t, C. Benchouk’, A. Bonissent t*21, G. Bujosa t, D. Calvet t, J. Carr t, C. Diaconu t, F. Etienne t,

M. Thulasidas t, D. Nicod t, l? Payre t, D. Rousseau t, M. Talby t, I. Abt “, R. Assmann “, C. Bauer “, W. Blum”, D. Brown u,24, H. Diet1 “, F. DydakU,21, G. Ganis “, C. Gotzhein”,

K. Jakobs “, H. Kroha”, G. Liitjens “, G. Lutz”, W. Manner “, H.-G. Moser “, R. Richter “, A. Rosado-Schlosser “, S. Schael “, R. Settles “, H. Seywerd”, U. Stierlin”p2, R. St. Denis “, G. Wolf “, R. Alemany “, J. Boucrot “, 0. Callot “, A. Cordier “, F. Courault “, M. Davier “,

L. Duflot “, J.-F. Grivaz “, Ph. Heusse “, M. Jacquet “, D.W. Kimvy19, F. Le Diberder’, J. Lefransois “, A.-M. Lutz “, G. Musolino “, I. Nikolic “, H.J. Park “, I.C. Park “,

M.-H. Schune’, S. Simian”, J.-J. Veillet “, I. Videau “, D. Abbaneo w, P Azzurri w, G. Bagliesi w, G. Batignani w, S. Bettarini w, C. Bozzi w, G. Calderini w, M. Carpinelli w,

M.A. Ciocci w, V. Ciulli w, R. Dell’Orso w, R. Fantechi w, I. Ferrante w, L. Foaw*‘, F. Forti w, A. Giassi w, M.A. Giorgi w, A. Gregorio w, F. Ligabue w, A. Lusiani w, P.S. Marrocchesi w, A. Messineo w, G. Rizzo w, G. Sanguinetti w, A. Sciaba w, P. Spagnolo w, J. Steinberger”,

R. Tenchini w, G. Tonelli w,26, G. Triggiani w, C. Vannini w, P.G. Verdini w, J. Walsh w, A.P. Betteridge ‘, G.A. BlairX, L.M. Bryant”, F. Cerutti ‘, Y. Gao ‘, M.G. Green ‘, D.L. Johnson x, T. Medcalf ‘, L1.M. Mir ‘, P Perrodo ‘, J.A. Strong ‘, V. Bertin y,

D.R. Botterill Y, R.W. Clifft Y, T.R. Edgecocky, S. HaywoodY, M. Edwards Y, P. Maley y, P.R. Norton Y, J.C. Thompson Y, B. Bloch-Devaux ‘, P. Colas ‘, H. Duarte ‘, S. Emery ‘,

W. Kozanecki z, E. LanGon z, MC. Lemaire ‘, E. Locci ‘, B. Marx ‘, P. Perez ‘, J. Rander ‘, J.-F. Renardy ‘, A. Rosowsky ‘, A. Roussarie z, J.-P Schuller ‘, J. Schwindling”,

D. Si Mohand z, A. Trabelsi z, B. Vallage ‘, R.P. Johnson aa, H.Y. Kim aa, A.M. Litke aa, M.A. McNeil aa, G. Taylor =, A. Beddall ab, C.N. Booth ab, R. Boswell ab, S. Cartwrightab,

F. Combley ab, I. Dawsonab, A. Koksal ab, M. Lethoab, W.M. Newton ab, C. Rankin ab, L.F. Thompson ab, A. Bijhrer ac, S. Brandt ac, G. CowanaC, E. FeiglaC, C. Grupen”,

G. Lutters ac, J. Minguet-Rodriguez ac, F. Rivera ac*25, P SaraivaaC, L. Smolik ac, F. Stephan ac, M. Apollonio ad, L. Bosisio ad, R. Della Marinaad, G. Gianniniad, B. Gobbo ad,

F. Ragusa ad,20, J. Rothberg ae, S. Wasserbaech ae, S.R. Armstrong af, L. Bellantoni af*30, P. Elmer af, 2. Feng af, D.l?S. Ferguson af, Y.S. Gao af, S. Gonzalez af, J. Grahl af, J.L. Harton af728, O.J. Hayesaf, H. Hu af, P.A. McNamara IIIaf, J.M. Nachtman af, W. Orejudos af, Y.B. Panaf, Y. Saadiaf, M. Schmittaf, I.J. Scottaf, V. Sharmaafv29,

J.D. Turk af, A.M. Walsh af, Sau Lan Wu af, X. Wu af, J.M. Yamartino af, M. Zheng af, G. Zobemig af

ALEPH Collaboration/Physics Letters B 361 (1995) 221-233 223

a Laboratoire de Physique des Particuies (LAPP), IN2P3-CNRS, 74019 Annecy-le-View Cedex, France b fnstitut de Fisica d’Altes Energies, Vniversitat Autonoma de Barcelona, 08193 Be&terra (Barcelona), Spain’

c Dipartimento di Fisica, INFN Sezione di Bari, 70126 Bari, Italy

* Institute of High-Energy Physics, Academia Sinica, Beijing, People’s Republic of China s e European Laboratory for Particle Physics (CERN), 1211 Geneva 23, Switzerland

t Laboratoire de Physique Corpuscuiaire, Vniversite’ Biaise Pascal, IN2P3-CNRS, Clermont-Ferrand, 63177 AubiPre, France s Niels Bohr Institute, 2100 Copenhagen, Denmark9

’ Nuclear Research Center Demokritos (NRCD). Athens Greece i Laboratoire de Physique Nucleaire et des Hautes Energies, Ecole Polytechnique. IN2P3-CNRS, 91128 Palaiseau Cedex, France

j Department of Physics, University of Edinburgh, Edinburgh EH9 3JZ United Kingdom lo k Dipartimento di Fisica, Vniversitd di Firenze, INFN Sezione di Firenze, 50125 Firenze, Italy

e Supercomputer Computations Research Institute, Florida State University, Tallahassee, FL 323064052. USA 13*14 m Laboratori Nazionali dell’INFN (LNF-INFN), 00044 Frascati, Italy

n Department of Physics and Astronomy, University of Glasgow, Giasgow G12 8QQ. Vnited Kingdom lo ’ Institut fiir Hochenergiephysik, Vniversitiit Heidelberg. 69120 Heidelberg, Germany l6

v Department of Physics, Imperial College, London SW7 2BZ United Kingdom to 4 Institut fiir Experimentalphysik. Vniversitlit Innsbruck, 6020 Innsbruck Austria ‘~3

r Department of Physics, University of Lancaster: Lancaster LA1 4YB. United Kingdom ‘O ’ Institut fiir Physik., Vniversitat Mainz, 55099 Mainz, Germany I6

’ Centre de Physique des Pamcules, Faculte des Sciences de Luminy, IN2P3-CNRS, 13288 Marseille. France u Max-Planck-lnstitut fiir Physik, Werner-Heisenberg-lnstitut, 80805 Miinchen. Germany16

’ loboratoire de I’Accele’rateur Lineaire. Vniversite de Paris-Sud, IN2P3-CNRS. 91405 Orsay Cedex, France w Dipariimento di Fisica dell’Vniversita, INFN Sezione di Pisa, e Scuola Normale Superiore. 56010 Pisa, italy

’ Department of Physics, Royal Holloway & Bedford New College, University of London, Surrey TW20 OEX, United Kingdom lo v Particle Physics Dept.. Rutherford Appleton laboratory, Chilton, Didcot. Oxon OX11 OQX, United Kingdom lo

’ CEA, DAPNbVService de Physique des Particules, CE-Saciay, 91191 Gif-sur-Yvette Cedex, Francell aa Institute for Particle Physics, University of Cahfomia at Santa Cruz, Santa Cruz, CA 95064, USA 22

ab Department of Physics, University of Shejield, Sheftield S3 7Rl-L United KingdomlO ac Fachbereich Physik Vniversitilt Siegen, 57048 Siegen, Germany I6

ad Dipartimento di Fisica, Vniversitd di Trieste e INFN Sezione di Trieste, 34127 Trieste, Italy ae Experimental Elementary Particle Physics, University of Washington, WA 98195 Seattle, USA

af Department of Physics, University of Wisconsin, Madison, WI 53706, USA II

Received 18 August 1995 Editor: K. Winter

Abstract

The lifetime of the Bf meson is measured in approximately 3 million hadronic Z decays accumulated using the ALEPH detector at LEP from 1991 to 1994. Seven different DC decay modes were reconstructed and combined with an opposite sign lepton as evidence of semileptonic Bf decays. Wo hundred and eight DltS candidates satisfy selection criteria designed to ensure precise proper time reconstruction and yield a measured By lifetime of r( BP) = 1.59 ‘_“d.:‘, (stat) f 0.03 (syst) ps . Using a larger, less constrained sample of events, the product branching ratio is measured to be Br(g -+ Bf) . Br(By + D;!+vX) = 0.82 f 0.09 (stat) ?i”d.lA (syst) %.

’ Now at CERN, 1211 Geneva 23, Switzerland. 2 Deceased. 3 Now at Dipartimento di Fisica, Universita di L.ecce, 73100

Lecce, Italy. 4 Also Istituto di Fisica Generale, Universita di Torino, Torino,

Italy. s Also lstituto di Cosmo-Geofisica de1 C.N.R., Torino, Italy. 6 Now at TSM Business School, Enschede, The Netherlands.

’ Supported by CICYT, Spain. 8 Supported by the National Science Foundation of China. g Supported by the Danish Natural Science Research Council.

lo Supported by the UK Particle Physics and Astronomy Research Council. l1 Supported by the US Department of Energy, grant DEFGO295-

ER40896. I2 On leave from Universitat Autonoma de Barcelona, Barcelona,

224 ALEPH Collaboration/Physics Letters B 361 (1995) 221-233

1. Introduction

Measurements of the individual B meson lifetimes provide a well-defined test of the theory of b hadron decays. The lifetimes of the Bi and Bt are expected to be equal to within a few percent and the B- lifetime is expected to be approximately 5% greater than the neutral B meson [ 11.

This letter presents the lifetime of the By meson measured via reconstructed semileptonic decays, B; + D,C+yX 32 The current measurement im- proves upon the pr&ious ALEPH result [2] with more than four times the number of observed D;a+ combinations by increasing the number of reconstructed D; decay channels from two to seven and making use of approximately 3 million hadronic Z

Spain. I3 Supported by the US Department of Energy, contract DE-FGOS-

92ER40742. I4 Supported by the US Department of Energy, contract DEFCOS-

85ER250000. I5 Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain. I6 Supported by the Bundesministerium fiir Forschung und Tech-

nologie, Germany. I7 Supported by the Direction des Sciences de la Mat&, C.E.A. I8 Supported by Fonds zur Fijrderung der wissenschaftlichen

Forschung, Austria. I9 Permanent address: Kangnung National University, Kangnung, Korea. 2o Now at Dipartimento di Fisica, Universith di Milano, Milano, Italy. 21 Also at CERN, 1211 Geneva 23, Switzerland. z Supported by the US Department of Energy, grant DE-FG03- 92ER40689. 23 Now at University of Athens, 157-71 Athens, Greece. 24 Now at Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA. 25 Partially supported by Colciencias, Colombia. 26 Also at Istituto di Matematica e Fisica, Universita di Sassari, Sassari, Italy. *’ Now at Schuster Laboratory, University of Manchester, Manch- ester Ml3 9PL, UK. ** Now at Colorado State University, Fort Collins, CO 80523, USA. 2g Now at University of California at San Diego, La Jolla, CA 92093, USA. 30 Now at Fermi National Accelerator Laboratory, Batavia, IL 605 10, USA. 31 Supported by the Commission of the European Communities, contract ERBCHBICT941234. O2 In this paper charge conjugate modes are always implied and “lepton” refers to electrons and muons.

decays recorded from 1991 to 1994. The increased

D,C+ yield is also used to improve ALEPH’s previous measurement [ 3 I of the product branching ratio, Br(6 -+ Bf) . Br(BF + D,e+vX), which is essential to Bf -@ oscillation measurements [4-61, and for determining the various b hadron fractions [ 71. The following sections describe the ALEPH detector, the DC and By candidate selection, the proper time reconstruction, the lifetime fit and the extraction of the product branching ratio.

2. The ALEPH detector

The ALEPH detector and its performances are described in detail elsewhere [8,9] and only a brief overview of the apparatus is given here. A high resolution vertex detector (VDET) consisting of two layers of double-sided silicon microstrip detectors sur- rounds the beam pipe. The inner layer is 6.5 cm from the beam axis and covers 85% of the solid angle and the outer layer is at an average radius of 11.3 cm and covers 69%. The spatial resolution for the r$ and z, projections (transverse to and along the beam axis respectively) is 12 pm at normal incidence. The vertex detector is surrounded by a drift chamber (ITC) with eight coaxial wire layers with an outer radius of 26 cm and by a time projection chamber (TPC) that measures up to 21 three-dimensional points per track at radii between 30 cm and 180 cm. These detectors are immersed in an axial magnetic field of 1.5 T and together measure the momentum of charged particles with a resolution a(p)/p = 6 x lo-“p, @ 0.005 (p in GeV/c). The resolution of the three-dimensional impact parameter in the transverse and longitudinal view for tracks having information from all track- ing detectors and two VDET hits can be parameter- ized as u = 25p.m + 95pmlp (p in GeVlc). The TPC also provides up to 338 measurements of the specific ionization of a charged track (dE/dx). The TPC is surrounded by an electromagnetic calorimeter of lead/proportional-chamber construction segmented into 0.9“ x 0.9” projective towers and read out in three sections in depth, with energy resolution a(E) /E = 0. 18/a + 0.009 (E in GeV) . The iron return yoke of the magnet is instrumented with streamer tubes to form a hadron calorimeter, with a thickness of over 7 interaction lengths and is surrounded by two addi-

ALEPH Collaboration/ Physics Lefters 3 3611199S) 221-233 225

tional double-layers of streamer tubes to aid in muon identification. An algorithm combines all these measurements to provide a determination of the energy flow [ 91 with a precision on the measurable total energy of (T(E) = 0.6dm + 0.6 GeV.

The selection of hadronic events is based on charged tracks and is described elsewhere [ 101. The interaction point is reconstructed on an event-by-event basis using the constraint of the average beam spot posi- tion [ 1 1 ] . The resolution is 85 ,rtm for 2 4 b& events, projected along the sphericity axis of the event.

3. Sources of D,e correlations

The five sources of D,C correlations considered are (i) lJf --) D,C+VX ,

(ii) B --) Dz*)-D(*)X followed by D(*’ + C+vX, (iii) B + D;X,-!?v , (iv) kinematic reflections in the D; + K*‘K-

(KtK-) channels due to D- + K*‘mTT- ( K~?T- ) , and

(v) combinatorial background. These different sources of signal and background

have been extensively discussed previously [ 2,3,12] and will simply be reviewed here. Only two decays, Bf ---t D,*-C+Y and Bf -+ D;C+v , are considered to contribute to By --+ D;f?vX as the higher order Cs resonances decay to D(*)K final states while the Df - + D;r decay dominates [ 131. The kinematics of the B -+ Dz*)-D(*)X background are simulated by

z -+ DL*)-D(*)+(r) decays but the contributions from B- , Bf and b baryon decays are included in the rate estimate. The branching ratio of the as-yet unob- served B --t D;X,l+v process is estimated to be less than 0.36% at 90% C.L. The assumptions and rates used to derive these estimates are discussed in Sec- tion 8. The reflection background is due to misinter- pretation of a charged pion as a charged kaon and is discussed in Section 4.1.

4. The Bf + D;.PvX candidate selection

Candidates for Bf -+ D;e+vX are selected from hadronic 2 decays where a D; and a lepton are found in the same hemisphere as defined by the thrust axis. The lepton identification in ALEPH is described in

detail in Ref. [ 141. In this paper the requirement that the dE/dx information be available for electron candidates is dropped.

The D; is reconstructed in five hadronic decay modes, &r- , K*OK- , &T+T-IT~ , K*OK*- and KfK- , and two semileptonic decay modes, 4e- Y and 4,~~v. The selection criteria for the first two modes are similar to those used in the previous Bf lifetime measurement [ 21. The next two modes suf- fer from larger combinatorial background and require more involved selection criteria. The final hadronic mode, D; -+ KfK- , is contaminated by the kinematic reflections from D- -+ Kf7r- requiring more stringent cuts be applied to the charged kaon candidate. The following sections describe the details of the D; selection.

4.1. Kinematic criteria for hadronic 0; decays

Each D; candidate must carry at least 15 % of the beam energy. For all the channels the momentum of the charged kaon candidate must be at least 1.5 GeVlc to ensure good kaon and pion dEldx identification. The charged kaon candidates specific ionization, when available, should fulfil x,, + XK < 1 [ 21, where XH is the difference between the measured and expected ionization for the particle hypothesis H in terms of the expected resolution. Pion candidates, ex- cept those from the e , should have ]xr] < 3 if the dEldx information is available, and a momentum greater than 0.5 GeVlc. The 4 , K*+(O) and Kt are reconstructed in the 4 4 K+K- , K*+(O) --+ K”(-)& and e -+ v+?r- modes and are required to be within 9, 50 and 12 MeV/c* of the nominal $J , K*+(O) and Kf masses [ 131, respectively, according to the reso- lutions and natural widths. The details of the momentum requirements for each hadronic decay mode are summarized in Table 1.

For the &r-and K*OK-modes, ( cos /3* 1 > 0.4 is required, where 8* is the center-of-mass decay angle of the 4 or K*’ relative to its flight direction. This cut exploits the fact that the distribution of 8* is proportional to cos* 8* for the decay of the pseudoscalar DC into a vector (9 or K*O) and a pseudoscalar, To reduce the larger combinatorial background for K*‘K-, 1 cos A*1 < 0.8 is also required where A* is the angle of the K*’ in the D; center-of-mass relative to the D; flight direction. For the &r+~-r- mode, the

226 ALEPH Collaboration/ Physics Letters B 361 (1995) 221-233

Table 1 The momentum cuts in GeV/c for each hadronic D; decay mode. Additional details are described in the text.

dr- K*‘K- K;K- qhr+~-~- K*OK*-

@ > 4.0 PK. > 4.0 p,$, > 2.0 p$$ > 3.5 “@ > 2.0 pr > 1.0 pK- > 2.0 pK- > 3.0

pr > I.0

momentum of at least one pion is required to exceed 1.5 GeVlc and the DC must carry at least 20% of the beam energy.

The dE/dx information is required to be available for the kaon from the DC in the K*‘K- and eK- modes to reduce the kinematic reflection background from D- -+ K*‘?T- and D- --+ e7r- , respectively. In addition the kinematic reflection background to DC -+ QK- fr om D- + Kzrr- is rendered negligible by requiring Pk < 5 GeV/c or ,$‘k < 0. The kinematic reflection background to D; -+ KfK- from A; --f Kfp is also rendered negligibIe by requiring the invariant mass to be incompatible with the A; when the kaon candidate is given the proton mass.

4.2. Kinematic criteria for the semileptonic 0; decays

The 4 is reconstructed as in the &T- mode. The momentum of the lepton from the D; must be p,(,) > 3 (2) GeVlc and satisfy the lepton identification criteria cited earlier [ 141. Additional electron candidates are identified as tracks satisfying the standard electron criteria with momenta between 1 and 2 GeV/c where the dE/dx information must be available and inconsistent with the proton hypothesis ( IxP] > 2.0) to eliminate hadron contamination. At 1 GeV/c the electron and proton ionization is indistinguishable.

To unambiguously assign the two leptons (one from the Bf , the other from the D; ), M( qb, f&, )

(M( 4, .t,z ) ) must be at least 15 MeV/c’ less (more) than the nominal D; mass. If the two leptons are of the same flavour, their invariant mass must be inconsistent33 with the J/e and I,#; in addition, the mass of electron-positron pairs must be greater

‘3Specifically. IM(@‘)) - M(p+~-)l > 100 MeV/c* and M(I,&))-M(e+e-) > 150MeV/c20rM(e+e-)-Mt~(“) > 100 MeV/$ are required.

than 100 MeV to exclude photon conversions. The B -+ Dz*)-D(*)X background is further reduced by requiring the missing energy in the hemisphere, estimated using an energy flow technique [9 1, to be greater than 10 GeV or the momentum of the lepton from the Bf decay to be greater that 5 GeVlc.

4.3. @ selection

The tracks forming D; candidates are required to form a vertex in three dimensions and at least two of the charged D; decay products must have at least one rd and t VDET hit and at least one ITC hit. For the D; --f KtK- mode, only the charged kaon track should fulfil this requirement. The DC vertex probability must be greater than 0.01% and the calculated uncertainty on the D; decay length must be less than 1 .O mm.

These D; candidates are in turn used to form a three-dimensional vertex with a candidate lepton with a momentum of at least 3 GeV/c and three- dimensional VDET information and an ITC hit to create the Bf candidates. The D;Cf vertex probability should be greater than 0.01% and the uncertainty on the Bf decay length must be less than 0.5 mm. The requirements on the tracks used to form the vertices and on the vertex probabilities are somewhat different from the previous analysis [ 23. Based on studies of the Monte Carlo simulation, these selection criteria give a 210 pm resolution on the By decay length for the hadronic D; decays. This resolution is similar to Ref. [ 21 with a slight reduction in the tails of the distributions. The Bf decay length resolution is approximately 20% worse for the semileptonic D; decay modes.

The reconstructed Bf momentum for the D; + &r+n--7r- mode must be at least 50% of the beam energy in order to reduce the greater combinatorial background. The D;P invariant mass should be in the range 3.0 to 5.5 GeV/c2 (for D; -+ @-v the lower bound is 2.5 GeV/c2 because of the additional neutrino). This cut considerably reduces the B -+ Dz*)-D(*)X and B -+ D;X,e+v backgrounds [ 31.

The requirement that Lo7 /+,, ) > -0.5, where

Lr,_ is the distance between the reconstructed Bf and D,’ vertices projected along the D; direction, reduces the combinatorial background by approximately 50%

AL.EPH Collaboration/ Physics Letters B 361 fl995) 221-233 221

Table 2 The second and third columns contain the Dp or qb mass resolution and mass acceptance window about the nominal D; or 4 mass for each mode. The signal detection efficiency is shown in the last column where the uncertainty is due to Monte Carlo statistics.

D; decay Mass resolution Mass cut D;!+vX mode (MeV/$) (MeVls) efficiency (%)

W- 7.0 f 15 9.7fO.l K’OK- 7.0 f 15 6.2 f 0.1 K;K- 10.0 f 20 2.3 f 0.1 Cp?r+rr-W- 5.0 f 10 5.7 f 0.1 K&K*- 7.0 f 15 3.4 f 0.1 4e-v 4.7 f 6 7.6 f 0.2

dP-v 4.7 f 6 5.5 f 0.2

and retains about 90% of the Bf signal while inducing a very small bias on the Bf decay length measurement. From a study of simulated Bf semileptonic decays, this bias was determined to be -19 f 3 pm. The bias was taken to be -20 pm with a 100% systematic uncertainty. For the average reconstructed By momentum of 33 GeV/c, this leads to a bias of -0.01 f 0.01 ps on the By lifetime.

5. Sample composition

To determine the relative contributions of the signal and background processes in each channel, the D; (4) mass spectra are fitted with a Gaussian (Breit- Wigner) to represent the signal and a second degree polynomial to represent the combinatorial background for the hadronic (semileptonic) D, decay modes. The masses are fixed to their nominal values [ 131 and the widths are fixed to their expected values determined from simulated semileptonic BP decays (Table 2). For the D; --+ K**K- channel, the fraction of the reflection background from D- --+ K*Ov- decays is fitted using a shape determined from the simulation An additional Gaussian at the nominal D- mass represents the contribution of the Cabibbo-suppressed hadronic decays of the D-. The fitted mass spectra for the hadronic and semileptonic D; decay modes are shown in Fig. 1. The estimated contribution of the three main components within the mass windows (Ta- ble 2) is shown in Table 3 for each mode. A small correction has been applied to the qSe_ v yield to account for an estimated background of 0.9 f 0.2 events

z 0

0 20

0 1.7 1.8 1.9 2 2.1 2.2

D, candidate LOSS GeV/c’

:u 10

1 o+-

M(K+K-) G&/c’-

Fig. 1. (a) The mass distribution of the D; candidates for DcP+ combinations for hadronic D; decays, and (b) the K+K- mass distribution for semileptonic D; decays, showing the fits described in the text. The shaded histograms represent the like-sign mass spectra.

Table 3 Tbe estimated contributions to the yield for each decay mode. The estimated contribution of one D- 4 K*‘a- event to the K”‘K-mode is not shown in the table but is included in the totals.

D; decay mode

&r- K’OK- qK- &r+lr-?r- K*oK’-

de-v

k-v

DF!+vX D!*)-D(*)X Combinatorial Total signal background background per mode

36.7 3.6 6.7 47 47.0 4.3 23.6 76

9.3 0.8 5.9 16 9.8 0.9 9.3 20 9.8 0.9 4.3 15

16.6 0.9 7.5 25 4.9 0.2 3.9 9

MTAL 134.2 11.6 61.3 208

due to hadrons misidentified as electrons. The small contribution of the B + D;X,Pv decay is considered as a systematic uncertainty only. The contribution to the DLL+ yield from Bf + D;~+vX followed by r+ --+ @vV is estimated to be less than one event.

The reconstruction efficiencies for the signal and the mass resolution for each D; decay mode are

228 ALEPH Cokboration /Physics Letters B 361(1995) 22 I-233

listed in Table 2. The reconstruction efficiency for the B + D,(*)-D(*)X background relative to the BF -+ D;!+v signal for the hadronic and semileptonic D; decays is determined to be 15.4 ic 1.4 % and 8.4 * 1.2 %, respectively, where the uncertainties are due to the Monte Carlo statistics. For the B ---t D;X,@v background, the relative efficiencies for the hadronic and semileptonic D; decays are 54.2 & 1.8 % and 39.0 f 3.6 %, respectively.

Approximately 134 of the 208 D;P combinations are attributed to Bi + D;C+vX decays.

6. Proper time reconstruction

The measured proper time of the Bf is given by

t, = MB+B~

PB:,

where Ma: = 5375 f 6 MeV/c* [ 131 is the mass of the Bf , Lap is the distance between the primary vertex and the D;!+ vertex projected on the DC!+ direction and pa! is the By momentum computed from the reconstructed D; and lepton momenta and the neutrino energy estimated using the missing energy technique of Ref. [ 21. Monte Carlo simulation shows that the Bf boost resolution for the majority of the events is 5% as described in Ref. [ 21.

The temporal resolution is parametrized by a sum of two Gaussians and analytically convolved with the decay time distribution. The explicit expression for the probability density function for the Bf signal is

where r,? is the Bz lifetime, pi is the fractional contribution of the ith Gaussian, Si is the offset and ai is the proper time resolution measured with the simulation. A scale factor S corrects for the differences in proper time resolution between the data and the simulation.

A separate parametrization of the proper time resolution for both the Bf --+ D;C+V and Bi -+ Dz?+y decays for each of the seven D[

decay modes is determined from Monte Carlo simulation with an input Bf lifetime of 1.50 ps.

To determine S the negative part of the Lup/pnf distribution is fitted with two Gaussians for “fake” D,C combinations in events where the uds content is en- hanced to 50% by requiring the tracks in the hemisphere opposite to the Bf candidate to be consistent with zero lifetime [ 111. “Fake” D,& are candidates from either the like-sign D,e combinations, the unlike- sign D,e sideband or a D, formed from the 4 or K* sidebands as defined in Section 7. The ratio of the data to the simulation for the proper time resolution is measured to be S = 1.2 f 0.1.

Extensive analysis of simulated data with generated Bf lifetimes of 1 .O, 1.5 and 2.0 ps has shown that the parametrization of the temporal resolution as a sum of Gaussians yields an average bias of -1.0 f 0.5% on the fitted lifetime. This bias arises from neglecting the dependence of the proper time resolution on the proper time itself in the formulation of B ( tm; 7,).

7. Lifetime fit

The likelihood function to be maximized for the lifetime fit is

(1)

fr is the relative fraction of Bf -+ D;PYX events in the DC or 4 peak, f2 is the relative fraction of B -+ DJ”)-D(*)X events, rd is the effective lifetime of the simulated B ---f D,(*)-D(*)X events, fs is the relative fraction of reflection background for D; + K*OK- , 76 is the average b lifetime, f4 is the relative fraction of combinatorial background, B( t,,,) is the proper time distribution of the combinatorial background, and N is the number of D;e+ candidates. B( t,,,) is estimated from the proper time distribu-

tions of the candidates in the D; and + mass “side-

ALEPH Collaboration/Physics Letters B 361(1995) 221-233 229

0

Propertime (PS) I

v) 500 (b) !t a

In 6 400 t

+\

-’ -‘- ’ -2 0 2 4 6 a 10

Proper time (PSI

Fig. 2. The proper time distributions of (a) the D;e+ events in the D; and 4 mass window showing the fitted contributions of the signal and the combinatorial and B + D!*)-D(*)X backgrounds as described in the text. (b) The fit to the proper time distribution of the events in the D;e+ sideband and the like-sign events.

band”. For D,f , .P ( qbl* l? ) candidates, the sideband extends from 2.05 to 2.30( 1.021 to 1.120) GeV/c2. For the like-sign candidates, Df , C* (@*e*) , the lower limit of the sideband is reduced to 1.95 (0.997) GeV/c2. The proper time distributions for the hadronic and semileptonic D; decays are separately parametrized by the sum of two Gaussians and two exponentials (one for t, < 0 and one for r,,, > 0).

Although not explicitly noted in Eq. ( 1 ), the fi are calculated separately for each D; decay mode as described in Section 5.

The measured lifetime from the unbinned maximum likelihood fit for the entire data sample is 7( B,) = 1.56 +0.17 _o.,5 ps (statistical uncertainty only) without correct- ing for the small biases due the parametrization of the resolution and the cut on Lb- /o( I,,- ) . The proper time distribution and fit are shown in Fig. 2.

7.1. Systematic uncertainties and corrections

The individual contributions to the systematic uncertainty on the Bf lifetime are summarized in Table

Table 4 The components of the estimated systematic uncertainty on the measured Bf lifetime. Additional details are described in the text.

Source Uncertainty ( ps)

Combinatorial background +0.013 -0.016

E -+ D!*)-D(*)X background 1-0.023 -0.016 B + D;X,@v background +0.001 -0.009 Proper time resolution parametrization bias +0.008 -0.008 Parametrization of proper time resolution +o.ots -0.015 Lo-/u(Lo-) > -0.5 bias to.010 -0.010

Bf’boost reiolution +0.010 -0.010 Other +0.006 -0.005

Total in quadrature f0.035 -0.033

4 and discussed below. The systematic uncertainty due to the combinato-

rial background is estimated by varying the calculated signal fraction of all decay modes by one standard deviation and by varying the proper time distribution of the combinatorial background, B( t,,,) . The systematic uncertainty due to B( tm) is estimated by alter- ing the parametrization (using a single Gaussian and two exponentials or by forcing the two Gaussians to have identical means), by using the unlike-sign or the like-sign sideband only, or by halving the width of the sidebands. In addition, a simultaneous fit to the signal and sidebands was performed similarly to the previous ALEPH measurement [ 21. In this fit, the proper time distributions of the hadronic and semileptonic D; sidebands were fitted separately with two non-zero lifetime components and a zero lifetime component.

The uncertainty due to the B --) D,(*)-D(*)X background is estimated by varying both the calculated yield and the rd lifetime within their estimated uncertainties. For simulated B -+ Di*j-D(*)X events, rd is fitted as 1.90 f 0.20 ps, which is longer than the input b hadron lifetime of 1.50 ps due to the under- estimation of the boost. To account for the unknown multiplicity of “X” in the B decay, the uncertainty has been increased to 10.30 ps. The uncertainty due to the B --) D;X,C+y decays is estimated by assuming that the rate is equal to the upper limit quoted earlier and varying the effective lifetime. The effective lifetime fitted for simulated B ---) DC X,~+Y is 1.75 f 0.20 ps which also is longer than the input b hadron lifetime due to the under-estimated boost.

The lifetime is corrected by +0.016 f 0.008 ps

230 AL.EPH Collaboration /Physics Letters B 361 (1995) 221-233

for the bias observed in simulated By -+ D;t+vX events when they are fitted with the parametrization of the proper time resolution. An additional uncertainty arises due to the statistical precision in the parametrization of the proper time resolution and is estimated by varying the parameters by one standard deviation of their fitted values.

The lifetime is also corrected for the bias due to Lo-/g(L,-) > -0.5 by +O.OlO f 0.010 ps which w& described in Section 4.3.

As in the previous measurement [ 2 1, the relative fractions of the tails of the reconstructed Bf boost were varied by f 20% to estimate the uncertainty due to the neutrino energy resolution. This yields an uncertainty of &O.OlO ps. The Bi momentum distribution was modified in the simulation and resulted in a negligible uncertainty in the lifetime.

The uncertainties due to following additional small effects are added in quadrature and are labeled as “Other” in Table 4: - Monte Carlo studies show that if the lifetime to

be measured differs substantially from the lifetime, rsen, used to create the parametrization of the proper time resolution (Section 6), then the measured lifetime will be slightly biased towards rgen. This bias is treated as an additional systematic uncertainty by fitting the lifetime with parameters derived from different rgen of 1.0, 1.5 and 2.0 ps.

- The scale factor for the proper time resolution is varied: S= 1.2f0.1.

- The estimated relative production rate of Bt + Df-Cfv to Bf + D;.!?v is approximated by the measured B -+ D(*)CV rates [ 13 J to be 2.3 f 0.6.

- The mass of the Bf meson is varied by 6 MeV/c2

1131. - rb is varied by 0.05 ps.

The final result of the maximum likelihood fit for the Bf lifetime, including the measurement biases, is

r(Bf) = 1.59 ‘r_od.\: (stat) f 0.03 (syst) ps .

8. Product branching ratio measurement

From yields of D;e+ combinations, it is possible to calculate the product branching ratio Br( b ---f Bv) * Br(Bf -+ D;e+yX), which can be used to estimate the production rate of Bz mesons in Z --$ bb decay.

Due to the sensitivity of such a measurement to the reconstruction efficiency, estimated from simulation, all requirements not essential to a yield measurement were removed: the requirements on the vertex probability, VDET and ITC assignments, the Bf and D; decay length resolution and the cut Lo: /u( Lo; ) > -0.5. The number of events within the mass acceptance windows (Table 2) and background estimates for the 3.1 million hadronic 2 decays analysed are de- tailed in Table 5. The reconstruction efficiency estimates for signal and background processes are listed in Table 6. A likelihood function is constructed, taking into account the expected backgrounds in each channel. All channels are then simultaneously fitted for the product branching ratio.

The background processes are estimated as follows. The combinatorial background in each channel is estimated from the sidebands of the mass spectrum, fitted with a second-order polynomial, as described in Section 5. The background due to real DC mesons paired with misidentified hadrons or non-prompt leptons (“Accidental Combinations” in Tables 5 and 8) is expected to be small. In the &r- mode, it is estimated from a fit to the wrong-sign mass spectrum, then scaled by the ratio of right-sign to wrong-sign sideband events. As the other channels carry significantly less weight in the fit, this background in the other channels is scaled from the #n-- channel. This yields a conservative error common to all modes. The retlec- tion background is fitted directly in the mass spectrum of the K*OK- decay mode, as described in Section 5.

The background due to B -+ Di*)-D(*)X , (DC*) -+ efvX) is estimated from the product of the inclusive branching ratio, Br(B -+ D;X), measured at the Y(4S) [ 15,161, and the average rate for secondary leptons in b decays, Br( b 3 c + e>, measured at LEP [ 211. Combining these rates thus includes the contributions from the By and from b

baryons. The measured relative rate of two-body decays, T(B -+ D,(*)-DC*)) /F(B + D,X) [ 15,161, is used to calculate the efficiencies for this background (Table 7).

The background due to B -+ D,X,e+v is calculated as follows. The fraction of B + D;X decays in which the D; is not produced from the W* (“lower vertex production”) was recently measured to

be flower = 0.172 f 0.083, and a limit set at flow,, < 0.31 at 90% C.L.[ 171. Combined with the inclu-

Table 5

ALEPH Collaboration/Physics Letters B 361 (I 995) 221-233 231

Event yields and background estimates. The estimated contribution of 10.2 f 5.1 reflection background events to the K*OK- mode is not shown in the table but is included in the totals.

Mode Yield

e- 145 f 12.0 K*‘K- 257 f 16.0 4e-v 102 f 10.1 +I-- v 53 f 1.3 I$oK_ 111 f 10.5 &r+?r--rr- 180f 13.4 K&K*- 55 f 7.4

Total 903 f 30.0

Combin.

48.2 f 1.6 148.6 f 2.9 44.4 f 1.8 27.1 f 1.5 83.8 f 2.5

184.3 f 2.7 42.6 f 1.8

579.6 f 5.8

Di*)-D(*)X

15.3 f 1.8 12.0 f 1.4 4.6 f 0.7 3.0 f 0.5 6.9 f 0.8 5.1 f 0.1 2.7 f 0.3

50.1 f 2.6

Accid.

2.5 f 8.1 2.8 f 9.0 1.5 f 4.8 0.7 f 2.1 0.7 f 2.3 0.0 f 0.4 0.3f 1.0

8.5 f 27.7

D;x,e+v

8.2 f 4.0 70.8 f 15.2 6.4 f 3.1 77.0 f 19.6 3.4f 1.7 48.1 f 11.5 2.4f 1.2 19.2f 7.8 3.7 f 1.8 15.9 f 11.2 3.1 f 1.5 -13.0 f 13.8 1.5 f 0.7 1.9 f 1.1

28.7 f 14.0 225.9 f 44.0

Signal

Table 6 Reconstruction efficiencies for signal and physics background processes. The uncertainties are due to Monte Carlo statistics.

Mode 13: -+ D;e+vX B + D;*)-D(*)x B + D;X,e+V

4r- 16.5 f 0.1% 3.7 f 0.3 % 9.7 f 0.2 % K*OK- 10.1 f 0.2 % 2.3 f 0.2 8 5.9 f 0.2 8 qbe-v 17.9 f 0.3 % 2.1 f 0.2% 7.5 f 0.5 % &-v 13.4 f 0.3 8 1.4f 0.2% 5.3 & 0.5 % K;K- 10.6 f 0.2% 2.4 -f 0.2 % 6.2 f 0.2 % r$?r+rr-rr- 12.1 f0.2% 2.1 f 0.2 8 7.1 f0.26 K&K’- 5.9 f 0.1% 1.3f0.18 3.5 f 0.1 %

Table 7 Rates used in the calculation of the product Br(b -+ Bf) Br(Bi + D;e+vX) and in estimating background process contributions. Additional details are given in the text.

Quantity Value Source

Rb Br(D; -+ &rr- ) l-(D; + K’OK-) / r(D; --+ @-) T(D; -+ @e-v) /r(D; - @-) r(D; 4 K;K-) / T(D; -+ drr-) IyD, -+&T+?f-?r-) /r(D, -t&r--) ;jIl;l;~)) /UK + 9K)

Br(Kf --t rr+rr- ) Br(B -+ D;X) - r(B --t D:*‘-D’*‘) /r(B --+ D;X) Br(b -+ evx) Br(b-+cde) UB -+ JTWI,, vertex / UB 4 D;X)

0.2204 f 0.0020 1201 3.5 f 0.4 8 [I31 0.95 f 0.10 [131 0.54 f 0.05 Ll31 1.01 f 0.16 1131 0.51 f 0.12 1131

1.6% 0.6 1131 49.1 f 0.9 8 1131

68.61 f 0.28 % 1131 10.46 f 0.73 8 115,161 52.8f6.1 % 115,161

11.08 f0.31 % [211 8.22 f 0.46 % 1211 0.172 f 0.083 I171

sive DC production rate, Br(B --+ D;X) [ 15,161, and the inclusive b semileptonic decay rate, Br( b --f X~V) [ 211, the rate and limit for this background are

estimated to be Br(B + D;X,L+Y) = 0.20 f 0.10 % and < 0.36% at 90% C.L., respectively. These estimates are consistent with an earlier search [ 181 and

232 ALEPH Collaboration/Physics Letters B 361 11995) 221-233

Table 8 Sources of systematic uncertainty in percent on the product brancb- ing ratio Br(z - Bf) Br(Bt --t D,J?vX).

Source

Br(D; -+ #P- ) Accidental combination background B -+ D;X,e+u background Other branching ratios B --t Di*)-D(*)X background Combinatorial background Reflection background

Rh Efficiency estimates (simulation)

Total

Uncertainty (%)

i-0.106 -0.085 i-0.030 -0.097 +a.045 -0.045 f0.036 -0.039 f0.016 -0.016 40.016 -0.015 +a.013 -0.012 $0.008 -0.007 +o.Oos -0.005

+0.128 -0.144

theoretical estimates [ 191. The calculated Bi + D;e+vX signal for each mode

after subtraction of the estimated backgrounds is given in Table 5. Using the number of observed events, the estimated reconstruction efficiencies and background contributions, and the branching ratios in Table 7, the product branching ratio is simultaneously fitted to all seven decay modes. The result is

Br(& 4 Bf) . Br(Bf -+ D,C+VX)

= 0.82 f 0.09 (stat) T”.i3 (syst) % , 0.14

which is the sum of the electron and muon decay channels divided by two 34. The specific contributions to the systematic uncertainty are listed in Table 8.

9. Conclusion

The Bf has been reconstructed in the semileptonic decay mode By + D;C+VX using seven different D; decay modes in approximately 3 million hadronic Z decays. The result of an unbinned, maximum likelihood fit to the Bz lifetime for the observed 208 D,!’ combinations is

r(By) = 1.59 ?:,\: (stat) f 0.03 (syst) ps ,

34 The product branching ratio quoted in Ref. (7 ] was calculated neglecting B -+ D; XSefv ; including this process, f( b --f Bf ) is 1 l.Of1.2~?& % and f(b - B:) is 38.8f1.3&2.1%. The baryon

fraction remains unchanged, f( b 4 Aa) = 11.5 f 2.2 f 3.4 %.

in agreement with the results from Ref. [ 121. An al-

ternative and independent ALEPH measurement using D, -hadron correlations [ 241 found r(By) = 1.61 ?$$t (stat) zi,\i (syst) ps using data from 1991 to 1993. The two ALEPH results can be combined to give

r(B;) = 1.59 :‘)d; ps ,

which is consistent with the Bi lifetime 1.58 f 0.06 ps [25], as expected from theoretical considera- tions [ 11.

The product branching ratio is measured to be

Br(& -+ Bi) . Br(Bf -+ D;C+VX)

=0.82 f0.09 (stat) zt,:i (syst) %,

in agreement with previous measurements [ 3,22,23].

Acknowledgements

It is a pleasure to thank our colleagues in the accelerator divisions of CERN for the excellent perfor- mance of the LEP accelerator. Thanks are also due to the technical personnel of the collaborating institu- tions for their support in constructing and maintaining the ALEPH experiment. Those of us not from member states wish to thank CERN for its hospitality.

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