Strong angular momentum effects in near-barrier fusion reactions

30 Pages·2017·0.81 MB·English

Measurement of the effective b quark fragmentation
function at the Z resonance
D. Buskulic, D. Casper, I. de Bonis, D. Decamp, P. Ghez, C. Goy, J.P. Lees,
A. Lucotte, M.N. Minard, P. Odier, et al.
To cite this version:
D. Buskulic, D. Casper, I. de Bonis, D. Decamp, P. Ghez, et al.. Measurement of the effective b quark
fragmentationfunctionattheZresonance. PhysicsLettersB,1995,357,pp.699-714. ￿in2p3-00002081￿
HAL Id: in2p3-00002081
https://hal.in2p3.fr/in2p3-00002081
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EUROPEAN LABORATORY FOR PARTICLE PHYSICS (CERN)
CERN{PPE/95{113
20 July 1995
Measurement of the e(cid:11)ective b quark fragmentation
function at the Z resonance
The ALEPH Collaboration 1
Abstract
Using a sample of about 1.46 million hadronic Z decays collected between 1991 and
0 (cid:6)
1993 with the ALEPH detector at LEP, the energy distribution of the B and B mesons
produced at the Z resonance is measured by reconstructing semileptonic decays B!
‘(cid:23)‘D(X) or B! ‘(cid:23)‘D(cid:3)+(X). The charmed mesons are reconstructed through the decay
modes D0 ! K(cid:0)(cid:25)+, D0 !K (cid:0)(cid:25)+(cid:25)(cid:0)(cid:25)+, D+ !K (cid:0)(cid:25)+(cid:25)+ and D(cid:3)+ ! D0(cid:25)+. The neutrino
(cid:3)
energy is estimated from the missing energy in the lepton hemisphere. Accounting for B
and B(cid:3)(cid:3) production, theshape ofthescaled energydistribution x(Eb) formesonscontaining
a b quark is compared to the predictions of di(cid:11)erent fragmentation models. The mean
(b)
value of xE is found to be
(b)
hxE i = 0:715(cid:6)0:007(stat)(cid:6)0:013(syst):
(Submitted to Physics Letters B)
1
See next pages for the list of authors
1
The ALEPH Collaboration
D.Buskulic,D.Casper,I.DeBonis,D.Decamp,P.Ghez,C.Goy,J.-P.Lees, A.Lucotte,M.-N.Minard,
P.Odier, B. Pietrzyk
2 3
LaboratoiredePhysiquedesParticules(LAPP),IN P -CNRS,74019Annecy-le-VieuxCedex,France
F. Ariztizabal, M. Chmeissani, J.M. Crespo, I. Efthymiopoulos, E. Fernandez, M. Fernandez-Bosman,
15
V.Gaitan,Ll.Garrido, M.Martinez,S.Orteu,A.Pacheco,C.Padilla,F.Palla,A.Pascual,J.A.Perlas,
F.Sanchez, F.Teubert
InstitutdeFisicad’AltesEnergies,UniversitatAutonomadeBarcelona,08193Bellaterra(Barcelona),
7
Spain
3
A.Colaleo,D.Creanza, M.de Palma,A. Farilla,G.Gelao,M.Girone,G.Iaselli,G.Maggi, M.Maggi,
N.Marinelli,S.Natali,S.Nuzzo,A.Ranieri,G.Raso,F.Romano,F.Ruggieri,G.Selvaggi,L.Silvestris,
P.Tempesta,G.Zito
Dipartimentodi Fisica,INFN Sezione diBari,70126Bari,Italy
X. Huang,J.Lin,Q. Ouyang,T.Wang,Y. Xie,R.Xu, S.Xue, J. Zhang,L. Zhang,W.Zhao
8
Institute of High-EnergyPhysics, AcademiaSinica,Beijing,The People’s Republic ofChina
G.Bonvicini,M. Cattaneo, P. Comas,P. Coyle,H. Drevermann, A. Engelhardt, R.W.Forty,M. Frank,
24 23
R. Hagelberg, J. Harvey, R. Jacobsen, P. Janot, B. Jost, J. Knobloch, I. Lehraus, C. Markou,
27
E.B. Martin, P. Mato, H. Meinhard, A. Minten, R. Miquel, T. Oest, P. Palazzi, J.R. Pater, J.-
F.Pusztaszeri,F.Ranjard,P.Rensing,L.Rolandi,D.Schlatter,M.Schmelling,O.Schneider,W.Tejessy,
I.R.Tomalin,A.Venturi,H. Wachsmuth,W.Wiedenmann,T.Wildish,W.Witzeling,J.Wotschack
European Laboratory forParticle Physics (CERN), 1211Geneva 23,Switzerland
2
Z. Ajaltouni, M. Bardadin-Otwinowska, A. Barres, C. Boyer, A. Falvard, P. Gay, C. Guicheney,
P.Henrard, J.Jousset, B.Michel,S.Monteil,J-C.Montret,D.Pallin,P.Perret, F.Podlyski,J.Proriol,
J.-M.Rossignol,F. Saadi
2 3
Laboratoire de Physique Corpusculaire, Universit(cid:19)e Blaise Pascal, IN P -CNRS, Clermont-Ferrand,
63177Aubi(cid:18)ere, France
T.Fearnley,J.B. Hansen, J.D.Hansen, J.R.Hansen, P.H.Hansen, B.S. Nilsson
9
NielsBohr Institute, 2100Copenhagen, Denmark
A. Kyriakis,E. Simopoulou,I.Siotis,A.Vayaki,K. Zachariadou
Nuclear Research Center Demokritos(NRCD),Athens, Greece
21
A. Blondel, G.Bonneaud, J.C. Brient, P.Bourdon, L. Passalacqua, A. Roug(cid:19)e,M. Rumpf,R. Tanaka,
31
A. Valassi, M.Verderi, H.Videau
2 3
Laboratoirede PhysiqueNucl(cid:19)eaire etdes Hautes Energies, EcolePolytechnique,IN P -CNRS,91128
PalaiseauCedex, France
D.J.Candlin,M.I.Parsons
10
DepartmentofPhysics, University ofEdinburgh, EdinburghEH9 3JZ, United Kingdom
E. Focardi,G.Parrini
Dipartimentodi Fisica,Universit(cid:18)a diFirenze, INFN Sezione di Firenze, 50125Firenze, Italy
12
M.Corden, M. Del(cid:12)no, C.Georgiopoulos,D.E.Ja(cid:11)e
Supercomputer Computations Research Institute, Florida State University, Tallahassee, FL 32306-
13;14
4052,USA
4
A. Antonelli, G. Bencivenni, G. Bologna, F. Bossi, P. Campana, G. Capon, V. Chiarella, G. Felici,
5
P.Laurelli,G.Mannocchi, F.Murtas, G.P.Murtas, M. Pepe-Altarelli
LaboratoriNazionalidell’INFN(LNF-INFN), 00044Frascati,Italy
2
6
S.J. Dorris, A.W. Halley, I. ten Have, I.G. Knowles, J.G. Lynch, W.T. Morton, V. O’Shea, C. Raine,
P.Reeves, J.M.Scarr, K. Smith,M.G.Smith,A.S.Thompson,F.Thomson,S. Thorn,R.M.Turnbull
10
DepartmentofPhysicsandAstronomy,UniversityofGlasgow,GlasgowG128QQ,UnitedKingdom
U. Becker, O. Braun, C. Geweniger, G. Graefe, P. Hanke, V. Hepp, E.E. Kluge, A. Putzer, B. Rensch,
M.Schmidt,J.Sommer,H. Stenzel, K. Tittel,S. Werner, M.Wunsch
16
Institut fu(cid:127)r Hochenergiephysik, Universit(cid:127)at Heidelberg, 69120Heidelberg, Fed. Rep.ofGermany
R. Beuselinck, D.M. Binnie, W. Cameron, D.J. Colling, P.J. Dornan, N. Konstantinidis, L. Moneta,
A. Moutoussi,J. Nash,G.San Martin,J.K. Sedgbeer, A.M.Stacey
10
DepartmentofPhysics, ImperialCollege,London SW72BZ, United Kingdom
G.Dissertori, P.Girtler,E. Kneringer, D.Kuhn, G.Rudolph
18
Institut fu(cid:127)r Experimentalphysik,Universit(cid:127)at Innsbruck, 6020Innsbruck, Austria
C.K. Bowdery, T.J. Brodbeck, P. Colrain, G. Crawford, A.J. Finch, F. Foster, G. Hughes, T. Sloan,
E.P.Whelan,M.I.Williams
10
DepartmentofPhysics, University ofLancaster, Lancaster LA1 4YB,United Kingdom
A.Galla,A.M.Greene,K.Kleinknecht,G.Quast,J.Raab,B.Renk,H.-G.Sander,R.Wanke,C.Zeitnitz
16
Institut fu(cid:127)r Physik,Universit(cid:127)at Mainz,55099Mainz,Fed.Rep. ofGermany
21
J.J. Aubert, A.M. Bencheikh, C. Benchouk, A. Bonissent, G. Bujosa, D. Calvet, J. Carr, C. Diaconu,
F.Etienne, M.Thulasidas,D.Nicod,P. Payre,D.Rousseau, M. Talby
2 3
Centre de Physique des Particules, Facult(cid:19)e des Sciences de Luminy, IN P -CNRS, 13288 Marseille,
France
24 21
I. Abt, R. Assmann, C. Bauer, W. Blum, D. Brown, H. Dietl, F. Dydak, G. Ganis, C. Gotzhein,
K. Jakobs, H. Kroha, G. Lu(cid:127)tjens, G. Lutz, W. M(cid:127)anner, H.-G. Moser, R. Richter, A. Rosado-Schlosser,
2
S.Schael, R.Settles, H. Seywerd, U. Stierlin, R.St. Denis,G.Wolf
16
Max-Planck-Institutfu(cid:127)rPhysik,Werner-Heisenberg-Institut,80805Mu(cid:127)nchen,Fed.Rep.ofGermany
R. Alemany, J. Boucrot, O. Callot, A. Cordier, F. Courault, M. Davier, L. Du(cid:13)ot, J.-F. Grivaz,
19
Ph.Heusse, M.Jacquet, D.W.Kim, F.Le Diberder,J.Lefran(cid:24)cois,A.-M.Lutz, G.Musolino,I.Nikolic,
H.J.Park, I.C.Park,M.-H.Schune, S. Simion,J.-J. Veillet,I.Videau
2 3
Laboratoire de l’Acc(cid:19)el(cid:19)erateur Lin(cid:19)eaire, Universit(cid:19)e de Paris-Sud, IN P -CNRS, 91405 Orsay Cedex,
France
D. Abbaneo, P. Azzurri, G. Bagliesi, G. Batignani, S. Bettarini, C. Bozzi, G. Calderini, M. Carpinelli,
1
M.A. Ciocci, V. Ciulli,R.Dell’Orso, R. Fantechi, I. Ferrante, L. Fo(cid:18)a, F. Forti,A. Giassi,M.A. Giorgi,
A.Gregorio,F.Ligabue,A.Lusiani,P.S.Marrocchesi, A.Messineo,G.Rizzo,G.Sanguinetti,A.Sciaba(cid:18),
26
P.Spagnolo,J. Steinberger, R.Tenchini,G.Tonelli, G.Triggiani,C. Vannini,P.G.Verdini,J. Walsh
DipartimentodiFisicadell’Universit(cid:18)a,INFNSezionediPisa,eScuolaNormaleSuperiore,56010Pisa,
Italy
A.P. Betteridge, G.A. Blair, L.M. Bryant, F. Cerutti, Y. Gao, M.G. Green, D.L. Johnson, T. Medcalf,
Ll.M.Mir, P.Perrodo, J.A.Strong
DepartmentofPhysics,RoyalHolloway&BedfordNewCollege,UniversityofLondon,Surrey TW20
10
OEX, United Kingdom
V.Bertin, D.R.Botterill,R.W.Cli(cid:11)t,T.R.Edgecock, S.Haywood,M.Edwards, P.Maley,P.R.Norton,
J.C.Thompson
ParticlePhysics Dept.,Rutherford Appleton Laboratory,Chilton,Didcot,OxonOX11 OQX, United
10
Kingdom
3
B. Bloch-Devaux, P. Colas, H. Duarte, S. Emery, W. Kozanecki, E. Lan(cid:24)con, M.C. Lemaire, E. Locci,
B. Marx, P. Perez, J. Rander, J.-F. Renardy,A. Rosowsky,A. Roussarie, J.-P.Schuller, J. Schwindling,
D.Si Mohand,A.Trabelsi,B. Vallage
17
CEA,DAPNIA/ServicedePhysiquedesParticules,CE-Saclay,91191Gif-sur-YvetteCedex,France
R.P.Johnson, H.Y.Kim,A.M.Litke,M.A.McNeil, G.Taylor
22
Institute for Particle Physics, University ofCaliforniaat SantaCruz, Santa Cruz, CA 95064,USA
A. Beddall, C.N. Booth, R. Boswell, S. Cartwright, F. Combley, I. Dawson, A. Koksal, M. Letho,
W.M.Newton, C.Rankin,L.F.Thompson
10
DepartmentofPhysics, University ofShe(cid:14)eld, She(cid:14)eld S3 7RH,United Kingdom
25
A. B(cid:127)ohrer, S. Brandt, G. Cowan, E. Feigl, C. Grupen, G. Lutters, J. Minguet-Rodriguez, F. Rivera,
P.Saraiva,L. Smolik,F. Stephan, P.vanGemmeren
16
Fachbereich Physik,Universit(cid:127)at Siegen, 57068Siegen,Fed. Rep.of Germany
20
M.Apollonio,L.Bosisio,R.DellaMarina,G.Giannini,B. Gobbo,F.Ragusa
Dipartimentodi Fisica,Universit(cid:18)a diTrieste e INFN Sezione diTrieste, 34127Trieste, Italy
J. Rothberg,S. Wasserbaech
ExperimentalElementaryParticle Physics, Universityof Washington,WA 98195Seattle, U.S.A.
30
S.R. Armstrong, L. Bellantoni, P. Elmer, Z. Feng, D.P.S. Ferguson, Y.S. Gao, S. Gonz(cid:19)alez, J. Grahl,
28
J.L.Harton, O.J.Hayes,H.Hu,P.A.McNamaraIII,J.M.Nachtman,W.Orejudos,Y.B.Pan,Y.Saadi,
29
M. Schmitt, I.J. Scott, V. Sharma, J.D. Turk, A.M. Walsh, Sau Lan Wu, X. Wu, J.M. Yamartino,
M.Zheng, G.Zobernig
11
DepartmentofPhysics, University ofWisconsin, Madison,WI53706,USA
1
NowatCERN,1211Geneva 23,Switzerland.
2
Deceased.
3
NowatDipartimento di Fisica, Universit(cid:18)a di Lecce,73100 Lecce,Italy.
4
Also Istituto diFisica Generale, Universit(cid:18)a di Torino,Torino,Italy.
5
Also Istituto diCosmo-Geo(cid:12)sica del C.N.R.,Torino, Italy.
6
NowatTSMBusiness School, Enschede, TheNetherlands.
7
Supported byCICYT,Spain.
8
Supported bytheNational Science Foundation ofChina.
9
Supported bytheDanish Natural Science ResearchCouncil.
10
Supported bytheUKParticle Physics andAstronomy Research Council.
11
Supported bytheUSDepartment ofEnergy,grant DE-FG0295-ER40896.
12
Onleave fromUniversitat Autonomade Barcelona, Barcelona, Spain.
13
Supported bytheUSDepartment ofEnergy,contract DE-FG05-92ER40742.
14
Supported bytheUSDepartment ofEnergy,contract DE-FC05-85ER250000.
15
Permanent address: Universitat deBarcelona, 08208 Barcelona, Spain.
16
Supported bytheBundesministerium fu(cid:127)rForschung und Technologie, Fed. Rep. ofGermany.
17
Supported bytheDirection des Sciences de laMati(cid:18)ere, C.E.A.
18
Supported byFonds zurF(cid:127)orderung der wissenschaftlichen Forschung, Austria.
19
Permanent address: Kangnung National University, Kangnung, Korea.
20
NowatDipartimento di Fisica, Universit(cid:18)a di Milano, Milano, Italy.
21
Also atCERN,1211 Geneva23,Switzerland.
22
Supported bytheUSDepartment ofEnergy,grant DE-FG03-92ER40689.
23
NowatUniversity ofAthens,157-71 Athens,Greece.
24
NowatLawrenceBerkeley Laboratory, Berkeley, CA94720,USA.
25
Partially supported byColciencias, Colombia.
26
Also atIstitutodi Matematica eFisica, Universit(cid:18)a di Sassari, Sassari, Italy.
27
NowatSchusterLaboratory, University ofManchester,Manchester M139PL,UK.
28
NowatColorado StateUniversity, FortCollins, CO80523, USA.
29
NowatUniversity ofCalifornia atSan Diego, LaJolla, CA92093,USA.
30
NowatFermiNational Accelerator Laboratory,Batavia, IL60510,USA.
31
Supported bytheCommission oftheEuropean Communities, contract ERBCHBICT941234.
4
1 Introduction
Themeasurementof the energyspectra of heavy (cid:13)avourhadrons produced inZ decays tests the
hadronizationmechanismofheavyquarksintophysicalstates,butalsohaspracticalimportance
for many b physics measurementsat LEP. The predicted B hadron energy distributions depend
upon a convolution of perturbative QCD (i.e. hard gluon radiation) and the hadronization
mechanismitself. Due to the non-perturbative nature of the latter, precise quantitative predic-
tionsarestillmissing,and phenomenologicalmodelsmustbeusedto describethehadronization
process.
Assuming that the transition amplitude for a fast moving heavy quark Q to fragment into
a hadron H = (Qq(cid:22)) and a light quark q is proportional to the inverse of the energy transfer
(cid:0)1 (cid:0)1
(cid:1)E = (EH + Eq EQ) in the breakup process, the following parametrization has been
(cid:0)
proposed by Peterson et al. [1] for heavy quark fragmentation:
(cid:0)2
H 1 1 (cid:15)Q
DQ(z) 1 : (1)
/ z (cid:0) z (cid:0) 1 z
(cid:18) (cid:19)
(cid:0)
Although other forms of the fragmentation function have been suggested in the literature [2, 3,
4], the shape of Ref. [1] is widely used in the interpretation of experimental results and in the
modelling of b-hadron production in simulation programs. The only parameter of the model,
(cid:15)Q, is expected to be the squared ratio of the e(cid:11)ective light quark mass to the heavy quark
mass
2
mq
(cid:15)Q 2 ; (2)
(cid:25) mQ
and z is de(cid:12)ned as
(E +pk)hadron
z = : (3)
(E +p)quark
Here, (E + p)quark is the sum of the energy and momentum of the quark after accounting
for initial state radiation, gluon bremsstrahlung and photon radiation in the (cid:12)nal state. Un-
fortunately, this variable is not experimentally accessible on an event-by-event basis and the
interpretationof the results in termsof z can therefore only be given in the context of a speci(cid:12)c
model. Results are usually expressed in terms of the scaled energy xE, de(cid:12)ned as the ratio of
the heavy hadron energy to the beam energy
Ehadron
xE = : (4)
Ebeam
This variable, which includes the e(cid:11)ect of photon and hard gluon radiation, is used in the
present analysis.
Todate,theb-quarkfragmentationfunctionattheZ peakhasmostlybeenprobed indirectly
by studies of inclusive lepton spectra in b semileptonic decays [5, 6, 7, 8], by inclusive J=
(cid:22)
production [9] and by charged particlemultiplicitiesin Z bb events[10]. Such measurements
!
usuallyonlyprovideinformationabout themeanenergyfraction xE carriedbytheb (cid:13)avoured
h i
5
hadrons, although some attempts have been made to unfold the shape of the xE distributions
from the lepton momentumspectra [7].
Another promising method [11] uses semileptonic B meson decays B ‘(cid:23)‘D(X) with full
!
reconstruction of the D meson, computingthe missing neutrino energy from energy-momentum
conservation. It has the advantage of directlymeasuring the B energy spectrum,independently
of the fragmentation model, but has been limited, until now, by the small number of events
available.
This paper presents a new measurementof the b-quark fragmentation using about 1400 de-
caysB ‘(cid:23)‘D(X)reconstructedwiththeALEPH detectorat LEP. Itis basedon a data sample
!
of about 1.465 million hadronic Z decays collected between 1991 and 1993. The symbol B rep-
0 (cid:6)
resents either a Bd or a B meson. The symbol D represents any fully reconstructed charmed
0 (cid:6) (cid:3)(cid:6)
mesonandcanbeeitheraD ,aD oraD . Thefollowingcharmedmesondecaychannelshave
0 (cid:0) + (cid:0) + (cid:0) + + (cid:0) + + (cid:3)+ 0 +
been used in the analysis: D K (cid:25) ; K (cid:25) (cid:25) (cid:25) , D K (cid:25) (cid:25) and D D (cid:25) followed
! ! !
0 (cid:0) + (cid:0) + (cid:0) +
by D K (cid:25) or K (cid:25) (cid:25) (cid:25) . The symbol X represents all particles which are not explicitly
!
0
identi(cid:12)ed as originating from the B decay. It can be either a (cid:25) ((cid:13)) from the decay B ‘(cid:23) ‘
!
(cid:3)0 (cid:3)0 0 0 (cid:3)(cid:3) (cid:3)(cid:3)
D with D D (cid:25) ((cid:13)) or an additional (cid:25) produced in the decay B ‘(cid:23)‘D . Here D is
! !
((cid:3))
a generic term covering all the non-resonant decays to D +n(cid:25) or decays to P-wave charmed
((cid:3))
mesons and higher spin states decaying to D (cid:25); unless otherwise speci(cid:12)ed, a mixture of 50%
(cid:3)(cid:3)
narrow resonant and 50% non-resonant or wide resonant states are called D throughout the
paper.
The outline of this paper is as follows. First, a brief description of the ALEPH experiment
is presented. The selection of B ‘(cid:23)‘D(X) events is described in Section 3. The measurement
!
of the B momentum using neutrino energy reconstruction is discussed in Section 4. Section 5
is devoted to the study of b fragmentation, and Section 6 describes the main systematic errors
a(cid:11)ecting this measurement.
0 (cid:6)
While the reconstructed particles are either Bd or B mesons, the quantity relevant for the
study of the b fragmentation is the scaled energy of the (cid:12)rst b-hadron produced in the b-quark
0 (cid:6) (cid:3)
hadronization. For Bd or B states, the (cid:12)rst b-hadron can be either a B, a B or an orbitally
(cid:3)(cid:3) (B) 0
excited B state. For clarity, throughout the paper xE denotes the scaled energy of the Bd
(cid:6) (b) (cid:3) (cid:3)(cid:3)
or B meson, and xE the scaled energy of the (cid:12)rst B, B or B state. The (cid:12)rst b-hadron
(B)
produced in b quark hadronization is called the leading b-hadron. A comparison of the xE
distribution to the predictions of di(cid:11)erent fragmentation models and a reconstruction of the
(b)
xE spectrum after corrections for acceptance, detector resolution and missing particles are
given in Section 5.
2 Detector description and lepton identi(cid:12)cation
A detailed description of the ALEPH detector and of its performance can be found in Ref.
[12, 13]. The charged particles are tracked inside a three-component magnetic spectrometer
6
immersed in the 1.5 T (cid:12)eld from a superconducting solenoid. Closest to the beam pipe is the
vertexdetector(VDET) [16], consistingof double sidedsiliconmicrostripdetectors,arranged in
two cylindricallayersat average radii of 6.3 and 10.8 cm. This detector covers an angular range
cos(cid:18) 0:85 for the inner layer, and cos(cid:18) 0:69 for the outer layer. The spatial resolution
j j(cid:20) j j(cid:20)
forr(cid:30) coordinates is 12 (cid:22)m;for z coordinates, itvariesbetween12 and 22 (cid:22)m,depending on the
polar angle of the charged particle. The vertexdetector is surrounded bya drift chamber(ITC)
witheightaxialwirelayersup toa radius of26 cm,and a timeprojectionchamber(TPC) which
measures up to 21 three dimensional space points per track at radii between 40 and 171 cm.
(cid:0)3 (cid:0)1
Withthis combinedsystem,a transverse momentumresolution (cid:27)(1=pT) = 0:6 10 (GeV=c)
(cid:2)
is achieved. For tracks with associated hits in both layers of the VDET, the impact parameter
resolution is (cid:27) = 25(cid:22)m+95(cid:22)m=p (p in GeV/c), in both the r(cid:30) and the z views. The tracking
systemallows the interactionpoint to be reconstructed on an event-by-eventbasis [17, 18], with
an average resolution of 85(cid:22)m for bb(cid:22) events.
The TPC also provides up to 338 measurementsof the speci(cid:12)c ionization for charged parti-
cles, allowing electrons to be separated from other charged particle species by more than three
standard deviations up to a momentum of 8 GeV/c. In the relativistic rise region, the (cid:25) K
(cid:0)
separation is roughly two standard deviations.
The electromagnetic calorimeter (ECAL) is a lead-proportional chamber sandwich with
0 0
cathode pad readout in 0:9 0:9 projective towers with three segments in depth. Electrons,
(cid:2)
as wellas photons, are identi(cid:12)edby thecharacteristiclongitudinal and transversedevelopments
of their associated showers.
Muons are identi(cid:12)ed by their characteristic penetration pattern in the hadron calorimeter
(HCAL) and in the muon chambers. The HCAL is the iron return yoke of the magnet, instru-
mentedwith 23 layersof streamertubes whichprovidea twodimensionalmeasurementof muon
tracks and of the hadronic shower development. The muon chambers surround the HCAL and
consist of two double layers of streamer tubes, providing three dimensional information on the
position of each hit.
Thetotalvisibleenergyismeasuredwithan energy-(cid:13)owreconstructionalgorithm[13]which
combines all the above measurements and gives a relative resolution of 0:60=pE (E in GeV)
on the total visible energy for hadronic Z decays. This algorithm, which also provides a list
of charged and neutral reconstructed objects, is used in the following analysis to compute the
visible energy in the b-hadron hemisphere.
! ‘(cid:23)
3 Selection of B ‘D(X) events
The decays B ‘(cid:23)‘D(X) are searched for in hadronic events [14] containing at least one iden-
!
ti(cid:12)ed lepton of momentum p > 3 GeV=c. Leptons are identi(cid:12)ed using the standard criteria
of Ref. [14]. The transverse momentum pT of the lepton with respect to the closest jet is
computed as described in [14], where the jets are built using the JADE algorithm [15] with
7
2
ycut = (6 GeV=ps) and where the lepton is excluded from the jet for the pT de(cid:12)nition. A cut
pT > 1 GeV=c is applied to reject background from non-primary b decays.
Events are divided into two hemisphereswith respect to a plane perpendicular to the thrust
(cid:3) 0
axis. The D and D mesondecaysarereconstructed inthe(cid:12)vedecaychannelsD K(cid:25);K(cid:25)(cid:25)(cid:25),
!
(cid:6) (cid:3)(cid:6) 0 (cid:6) 0
D K(cid:25)(cid:25) and D D (cid:25) with D K(cid:25);K(cid:25)(cid:25)(cid:25), by combining tracks within the lepton
! ! !
hemisphere. In all the decay channels, the D momentum is required to exceed 5 GeV=c to
(cid:3)(cid:6) 0 (cid:6) 0
reduce the combinatorial background, except for the channel D D (cid:25) (D K(cid:25)) where
(b!) !
this cut is lowered to 3 GeV=c to improve the e(cid:14)ciency at small xE .
Chargedkaon candidatesmusthavethesamechargeasthelepton,asexpectedforasemilep-
tonic B decay. If the dE=dx measurement is available ( 90% of the tracks), the ionization
(cid:24)
(cid:6)
of the K must be within two standard deviations of the expected value. For all the decay
modes, a cut pK > 1:5 GeV=c is applied to the charged kaon momentum. The most energetic
pion must have a momentum p(cid:25) > 1 GeV=c and the others (for K(cid:25)(cid:25)(cid:25) and K(cid:25)(cid:25) decays) are
required to satisfy p(cid:25) > 0:5 GeV=c.
(cid:3)(cid:6) 0 (cid:6)
In the K(cid:25) and K(cid:25)(cid:25)(cid:25) channels, D decaying to D (cid:25) are identi(cid:12)ed by combining a pion
(cid:25)s of momentum above 200 MeV=c with any K(cid:25) or K(cid:25)(cid:25)(cid:25) track combination. The track
combinations for which the mass di(cid:11)erence m(K(cid:25)(cid:25)s) m(K(cid:25)) or m(K(cid:25)(cid:25)(cid:25)(cid:25)s) m(K(cid:25)(cid:25)(cid:25)) is
(cid:0) (cid:0)
2 (cid:3)(cid:6) 0 (cid:3)(cid:6)
within 3.5 MeV=c of the m(D ) m(D ) mass di(cid:11)erence are called D candidates, while
(cid:6) (cid:0)
0 (cid:3)(cid:6)
the remaining track combinations are called D candidates. The D candidates are excluded
0 (cid:3)0
from the D sample, which also includes B decays to D , since the latter are not explicitly
(cid:3)(cid:6) 0 (cid:6)
identi(cid:12)ed. With these criteria, less than 10% of the total number of D D (cid:25)s decays are
!
0
incorrectly assigned to the D sample.
To reduce the combinatorial background and improve the D mass resolution, the sec-
(cid:3)(cid:6)
ondary vertices from B and D decays are reconstructed for all D decay channels except D
!
0 (cid:6) 0 1
D (cid:25) (D K(cid:25)) . The B mesons produced at ps mZ have a typical decay length of 2.6
! ’
mm[19], which can be measured with an average resolution of 280 (cid:22)m; this is used to di(cid:11)eren-
tiate tracks originating from the B or D decay point from those originating from the primary
vertex. At least one VDET hit in both the r(cid:30) and z views is required for the lepton, the kaon
and at least one (cid:25) of momentump(cid:25) > 1 GeV=c. A common D vertex is sought for the K and
2
the pion(s) and a cut on the vertex (cid:31) probability is applied. If a D vertex is identi(cid:12)ed, a
common vertex for the lepton and the D candidate is searched for. This B vertex is required
to lie between the event-by-event reconstructed interaction point and the D vertex. For the
0 0 (cid:6) (cid:6)
channels B ‘(cid:23)‘D X (D K(cid:25)(cid:25)(cid:25)) and B ‘(cid:23)‘D X (D K(cid:25)(cid:25)), a further reduction of the
! ! ! !
background is obtained by requiringthe D vertexto lieat least 1 mmaway fromthe interaction
point.
The D mass distributions for the di(cid:11)erent channels are shown in Fig. 1. The resolution
on the reconstructed D mass (see Table 1) is determined by (cid:12)tting the K(cid:25), K(cid:25)(cid:25) or K(cid:25)(cid:25)(cid:25)
1
The statistics used for that channel is 1.66 million Z hadronic decays; it includes data taken before the
installationof the ALEPH vertex detector.
8