15June 1995

PHYSICS LETTERS B

Physics Letters B 352 (1995) 479-486

The forward-backward asymmetry for charm quarks at the Z pole

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, M.-N. Minard a, P. Odier a, B. Pietrzyka, F. Ariztizabal b, M. Chmeissani b, J.M. Crespo b, I. Efthymiopoulos b, E. Fernandez b, M. Fernandez-Bosman b, V. Gaitan b, Ll. Garrido b,15, M. Martinez b, S. Orteu b, A. Pacheco b, C. Padilla b, F. Palla b, A. Pascual b, J.A. Perlas b,

F. Sanchezb, F. Teubertb, D. Creanza’, M. de PalmaC, A. FarillaC, M. GironeC, G. Iasellic, G. Maggi ‘v3, N. Marinelli ‘, S. Natali ‘, S. Nuzzo ‘, A. Ranieri ‘, G. Raso ‘, F. Roman0 ‘, F. RuggieriC, G. Selvaggi ‘, L. Silvestris ‘, P. TempestaC, G. Zito ‘, X. Huang d, J. Lin d, Q. Ouyangd, T. Wangd, Y. Xied, R. Xud, S. Xued, J. Zhang d, L. Zhangd, W. Zhaod,

G. Bonvicini e, M. Cattaneo e, P. Comas e, P. Coyle e, H. Drevermann e, A. Engelhardt e, R.W. Forty e, M. Franke, R. Hagelberg e, J. Harvey e, R. Jacobsen e,24, P. Janot e, B. Jost e,

J. Knobloche, I. Lehraus e, M. Maggie, C. Markouep23, E.B. Martin e, P. Mato e, H. Meinharde, A. Mintene, R. Miquel e, T. Oest e, P. Palazzi e, J.R. Pater e, J.-F. Pusztaszeri e, F. Ranjarde,

P Rensinge, L. Rolandi e, D. Schlatter e, M. Schmellinge, 0. Schneider e, W. Tejessy e, I.R. Tomalin e, A. Venturi e, H. Wachsmuthe, W. Wiedenmanne, T. Wildishe, W. Witzelinge,

J. Wotschack e, Z. Ajaltouni f, M. Bardadin-Otwinowskaf*2, A. Barres f, C. Boyer f, A. Falvard f, P. Gay f, C. Guicheney f, P. Henrard f, J. Jousset f, B. Michel f, S. Monteil f,

J-C. Montret f, D. Pallin f, P. Perret f, F. Podlyski f, J. Proriol f, J.-M. Rossignol f, F. Saadi f, T. Fearnley s, J.B. Hansen g, J.D. Hansen s, J.R. Hansens, P.H. Hansen s, B.S. Nilssons, A. Kyriakis h, E. Simopoulou h, I. Siotis h, A. Vayaki h, K. Zachariadou h, A. Blonde1 i~21,

G. Bonneaud i, J.C. Brient i, P. Bourdon’, L. Passalacqua i, A. Rouge i, M. Rumpf i, R. Tanaka i, A. Valassi i, M. Verderi i, H. Videau i, D.J. Candlinj, M.I. Parsonsj, E. Focardi k, G. Parrini k,

M. Corden e, M. Delfino e*12, C. Georgiopoulos e, D.E. Jaffe e, A. Antonelli m, G. Bencivenni m, G. Bolognam,4, F. Bossi m, P. Campana m, G. Capon m, F. Cerutti m, V. Chiarella m, G. Felici m, P. Laurelli m, G. Mannocchi m,S, F. Murtas m, G.P. Murtas m, M. Pepe-Altarelli m, S.J. Dorris n,

A.W. Halley “, I. ten Have “TV, I.G. Knowles”, J.G. Lynch”, W.T. Morton”, V. O’Shea”, C. Raine “, P. Reeves “, J.M. Scat-r “, K. Smith “, M.G. Smith “, AS. Thompson “, F. Thomson n, S. Thorn *, R.M. Tumbull”, U. Becker O, 0. Braun O, C. Geweniger O, G. Graefe O, P. Hanke O,

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D.J. Colling P, P.J. Dornan P, N. Konstantinidis P, L. Moneta P, A. Moutoussi P, J. Nash P,

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480 ALEPH Collaboration/Physics Letters B 352 (1995) 479-486

G. San Martin P, J.K. Sedgbeerr, A.M. Stacey P, G. Dissertori 4, P. Girtlerq, E. Kneringerq, D. Kuhn q, G. Rudolph q, C.K. Bowdery r, T.J. Brodbeck r, P. Colrain r, G. Crawford r,

A.J. Finch’, F. Foster’, G. Hughes r, T. Sloan r, E.P. Whelan r, MI. Williams r, A. Galla”, A.M. Greene ‘, K. Kleinknecht ‘, G. Quast ‘, J. Raab ‘, B. Renk s, H.-G. Sanders, R. Wanke s,

C. Zeitnitz ‘, J.J. Aubert t, A.M. Bencheikh t, C. Benchouk t, 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, P. Payre t,

D. Rousseau t, M. Talby t, I. Abt “, R. Assmann “, C. Bauer “, W. Blum “, D. Brown u,24, H. Diet1 “, F. DydakUy21, C. Gotzhein “, K. Jakobs “, H. Kroha “, G. Liitjens “, G. Lutz “,

W. Manner “, H.-G. Moser “, R. Richter “, A. Rosado-Schlosser “, R. Settles “, H. Seywerd “, U. Stierlin “y2 , 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. Kim “,i9, F. Le Diberder “, J. Lefranqois “, 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. FOB w,l, 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 w,

R. Tenchini w, G. Tonelli w,26, G. Triggiani w, C. Vannini w, P.G. Verdini w, J. Walsh w, A.P. Betteridge x, G.A. Blairx, L.M. Bryant x, Y. Gao ‘, M.G. Green ‘, D.L. Johnson ‘,

T. Medcalf x, L1.M. Mir ‘, P. Perrodo ‘, J.A. Strong ‘, V. Bertin Y, D.R. Botterill Y, R.W. Clifft Y, T.R. Edgecock Y, S. Haywoody, M. Edwards Y, P. Maley Y, P.R. Nortony, J.C. Thompson Y,

B. Bloch-Devaux”, P. Colas ‘, H. Duarte ‘, S. Emery ‘, W. Kozanecki ‘, E. Langon ‘, M.C. Lemaire ‘, E. Locci z, B. Marx z, P. Perez ‘, J. Rander z, J.-F. Renardy ‘, A. Rosowsky ‘, A. Roussarie z, J.-P. Schuller ‘, J. Schwindling ‘, D. Si Mohand ‘, A. Trabelsi ‘, B. Vallage ‘,

R.P. Johnson aa, H.Y. Kim aa, A.M. Litke aa, M.A. McNeil aa, G. Taylor aa, A. Beddall ab, C.N. Booth ab, R. Boswell ab, S. Cartwright ab, F. Combley ab, I. Dawson ab, A. Koksal ab, M. Lethoab, W.M. Newton ab, C. Rankin ab, L.F. Thompson ab, A. Biihrer ac, S. Brandt ac,

G. Cowan ac, E. Feigl ac, C. Grupen ac, G. Lutters ac, J. Minguet-Rodriguez “, F. Rivera ac,2s, P. SaraivaaC, L. Smolikac, F. StephanaC, L. Bosisio ad, R. Della Marinaad, G. Ganis ad,

G. Giannini ad, B. Gobbo ad, L. Pitis ad, F. Ragusa ad*20, J. Rothberg ae, S. Wasserbaech ae, S.R. Armstrong af, L. Bellantoni af, P. Elmer af, Z. Feng af, D.P.S. Ferguson af, Y.S. Gao af,

S. Gonzalez af, J. Grahl af, J.L. Harton af, O.J. Hayes af, H. Hu af, P.A. McNamara III af, J.M. Nachtman af, W. Orejudos af, Y.B. Pan af, Y. Saadi , . af M. Schmitt af, I.J. Scott af,

V. Sharma af, J.D. Turk af, A.M. Walsh af, F.V. Weber af,l, Sau Lan Wu af, X. Wu af, J.M. Yamartinoaf, M. Zheng af, G. Zobernig af

a Laboratoire de Physique des Particules (LAPP), INZP3-CNRS, 74019 Annecy-le-Keux Cedex, France h lnstitut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain’

c Dipartimento di Fisica, INFN Sezione di Bari, 70126 Bari, Italy d Institute of High-Energy Physics, Academia Sinica, Beijing, People’s Republic of China a

e European Laboratory for Particle Physics (CERN). 1211 Geneva 23, Switzerland t Laboratoire de Physique Corpusculaire, Universith Blaise Pascal, IN2P3-CNRS. Clermont-Ferrand, 63177 Aubiere, France

s Niels Bohr Institute, 2100 Copenhagen, Denmark’

ALEPH Collaboration/Physics Letters B 352 (I 995) 479-486 481

h 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, Universita di Firenze, INFN Sezione di Firenze, 50125 Firenze, Italy r Supercomputer Computations Research Institute, Florida State University, Tallahassee, FL 32306-40.52, USA ‘%I4

m Laboratori Nazionali dell’INFN (LNF-INFN). 00044 Frascati, Italy

n Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ. United Kingdom lo 0 Institut fiir Hochenergiephysik, Universitiit Heidelberg, 69120 Heidelberg, Fed. Rep. Germany’”

v Department of Physics, Imperial College, London SW7 ZBZ United Kingdom lo 9 Institut fiir Experimentalphysik, Universitiit Innsbruck, 6020 Innsbruck, Austria I8

r Department of Physics, University of Lancaster, Lancaster LA1 4YB, United Kingdom to s Institut ftir Physik, Universitiit Mainz, 55099 Mainz, Fed. Rep. Germany I6

’ Centre de Physique des Particules, Fact& des Sciences de Luminy, INZP3-CNRS, 13288 Marseille, France ’ Max-Planck-lnstitut fiir Physik, Werner-Heisenberg-lnstitut, 80805 Miinchen, Fed. Rep. Germany t6

’ Laboratoire de I’Accfltrateur Lineaire, Universite de Paris-Sud, INZP3-CNRS, 91405 Orsay Cedex, France w Dipartimento di Fisica dell’lmiversita. INFN Sezione di Piss, e Scuola Normale Superiore, 56010 Pisa, Italy

’ Department of Physics. Royal Holloway & Bedford New College, University of London, Surrey TW20 OEX, United Kingdom lo Y Particle Physics Dept., Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX1 1 OQX. United Kingdom ‘9

’ CEA, DAPNIA/Service de Physique des Particules, CE-Saclay. 91191 Gif-sur-Yvette Cedex, France ” ” Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz. CA 95064, USA 22

ah Department of Physics. University of She#eld, Shefield S3 7RH, United Kingdom ‘O ” Fachbereich Physik, Universitiit Siegen, 57068 Siegen, Fed. Rep. Germany I6

a’ Dipartimento di Fisica, Universita di Trieste e INFN Sezione di Trieste, 34127 Trieste, Italy a~ Experimental Elementary Particle Physics, University of Washington, WA 98195 Seattle, USA

‘t Department of Physics. University of Wisconsin, Madison, WI 53706, USA ”

Received 18 April 1995 Editor: L. Montanet

Abstract

From 1.4 million hadronic Z decays collected by the ALEPH detector at LEP, an enriched sample of Z -+ CC events is extracted by requiring the presence of a high momentum D**. The charm quark forward-backward charge asymmetry at the Z pole is measured to be AZ; = (8.0 f 2.4)% corresponding to an effective electroweak mixing angle of sin20kff =

0.2302 f 0.0054.

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

Lecce, Italy.

4 Also Istituto di Fisica Generale, Universita di Torino, Torino,

Italy.

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

7 Supported by CICYT, Spain.

s Supported by the National Science Foundation of China.

9 Supported by the Danish Natural Science Research Council.

lo Supported by the UK Particle Physics and Astronomy Research Council.

” Supported by the US Department of Energy, contract DE-ACO2-

76EROO88 1. I2 On leave from Universitat Autonoma de Barcelona, Barcelona,

Spain.

l3 Supported by the US Department of Energy, contract DE-FGOS-

92ER40742.

l4 Supported by the US Department of Energy, contract DE-FCOS- 85ER250000.

I5 Permanent address: Universitat de Barcelona, 08208 Barcelona,

Spain.

t6 Supported by the Bundesministerium fiir Forschung und Tech- nologie, Germany.

” Supported by the Direction des Sciences de la Matitre, C.E.A.

I8 Supported by Fonds zur Fiirderung der wissenschafthchen

Forschung, Austria. ” Permanent address: Kangnung National University, Kangnung,

Korea.

2o Now at Dipartimento di Fisica, Universita di Milano. Milano,

Italy.

” Also at CERN. 1211 Geneva 23, Switzerland.

482 ALEPH Collaboration /Physics Letters B 352 (1995) 479-486

1. Introduction

The existence of both vector and axial contributions

to the coupling of the Z boson to fermions leads to the

prediction of a forward-backward charge asymmetry

(denoted ALB) in the production of fermion pairs ff.

In the Standard Model, the differential cross-section, expressed as a function of the angle 0 between the out-

going fermion and the incoming electron directions,

is:

s K ( 1 + cos28 + t . AA . cos8) .

At the Z pole, for unpolarized e+e- beams, the

asymmetry term in the Born approximation is related to the effective couplings through the expression:

AOJ = 2 2gVegAe %VfgAf

FB 4 (& + $Ae) (&f-t- SL) . (1)

Therefore, the measurement of the forward-backward

asymmetry for each produced fermion type constitutes an important test of the Standard Model. In this pa- per, a measurement of the asymmetry in charm quark

production and the corresponding electroweak mixing

angle sin20hff is described. Decays of the Z into D** mesons produced in the

Z -+ qii process are used to tag charm and beauty

quarks. Essentially all charmed mesons are expected to be produced either directly from the hadronization

of charm quarks in the process Z -+ CC or from the

decay of b hadrons produced in Z --) bb events, with approximately equal rates. The selection of high mo- mentum D** candidates removes a large amount of

background events and enriches the charmed-meson

sample to a purity of about 80% in D**‘s originating

from the Z + CC process.

The decay channel D*+ --+ Don’ (the 7~+ being labeled n-T hereafter) is used to select D*+‘s. Three

Do decay channels have been considered (charge- conjugate modes are implied throughout this paper) :

22 Supported by the US Department of Energy, grant DE-FGOS-

92ER40689. 2.7 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 lstituto di Matematica e Fisica, Universita di Sassari,

Sassari, Italy.

(i) Do --f K-W+ ;

(ii) Do -+ K-n-+~+r- ;

(iii) Do -+ K-rr+r’.

This paper is an update of Ref. [ 11, with statistics increased by more than a factor of 3 to 1.4 million hadronic Z’s collected by the ALEPH detector [ 21

from 1991 to 1993. Center-of-mass energies within 1

GeV of the Z peak were used with an average of 91.24

GeV. The channel (iii) is selected differently, the ?r”

not being directly used in the Do reconstruction. The Do’s containing a soft n-c’ originating mainly from a p’ decay are tagged in a way which makes use of the kinematics of this decay. This new selection increases the number of selected Do -+ K-&?r” by a factor of 2.

2. Selection of the D*+ Decays

Hadronic Z’s are selected as described in [ I].

The channels D*+ --t r,fK-&’ and D*+ -+

rfK-&n-+r- are reconstructed as follows. The in-

variant mass of a system combining 2 tracks (Do + K-n-+) or 4 tracks (Do + K-r+?r+n-) with rel- evant mass assignments is required to be within 30

MeVlc2 of the Do mass. This is equivalent to roughly 2 standard deviations of the invariant mass resolution.

A low momentum pion (P?,: < 4.2 GeV/c) is added

to the system and a D*+ candidate is kept if AM =

MsfrYJ - MDo satisfies 143.5 MeV/c2 < AM <

14j.5 MeV/c2. A cut XE = ED*+/E~~~ greater than 0.5 is imposed to enrich the sample in charm events against b decays and other backgrounds.

The scalar nature of the De is exploited in order

to remove a large fraction of the remaining back- ground by requiring 1 cos 8*] 5 0.8, where 8’ is the angle between the K track (Do + K-lr+ case) or the sphericity axis of the four decay products (Do -+ K-T+T+T- case) and the Do direction, in the Do

rest frame. The background behaviour is reproduced by select-

ing fake Do candidates in a mass range above the Do mass from 2.00 GeV/c2 to 2.26 GeV/c2, which pass the other selection criteria. For the D*+ -+ rrfK-r+ channel, the background mass-difference spectrum is normalized to the data upper side spectrum (AM >

ALEPH Collaboration/Physics Letters B 352 (1995) 479-486 483

2, 350

> J L\1 ALEPH

. lIlato

izma Background

r-7 Monte-Carlo

0 0.13 0.135 0.14 0.145 015 0155 0.16 0.165 0.17 0175 (

0.4 = M(Knn)-M(KR) (W/c

Fig. 1. Mass-difference distribution, for XE(KWQ$) > 0.5. The KQT maSs is requited to be between 1.8345 and 1.8945 GeV/c*. The background, estimated from the upper side-band, is shown. The arrows indicate the selection cuts.

160 MeV/c2), giving an estimate of the number of background events in the selected AM region (Fig. 1). In the D*+ -t rr+K-&&rr- channel, the experi- mental spectrum has an additional contribution due to a K- /rTT- inversion in the Do or coming from partially reconstructed Do --+ K-&n-+r-, where one of the four Do tracks is taken from the combinatorial back- ground. This additional contribution gives a tail in the upper side of the mass-difference spectrum and forms a broad enhancement in the region of the D* peak, leading to a double counting of events containing a D*. These partially reconstructed events allow a cor- rect estimation of the quark direction, just as well as fully reconstructed Do -+ K-rr+rr+r- contribution. The number of background events for this channel is derived from a fit of the experimental spectrum (Fig. 2) with two contributions, one for the D* signal and the second for the pure combinatorial background, the shape of these two contributions being given by the Monte Carlo simulation. Finally, for each event, only the candidate with the mass closest to the Do mass is kept. This further selection criterion reduces the signal sample by ( 18 f 1 )%. The selection leads to samples containing 1022 f 33 D*+ + 7.rzK-7~.+ and 2035 f68 D*+ ---f v+K-T+T+~~-. 9

5

8 8 800 ALEPH P 1 700 . Data

m Background B 600 “I 2

I Monte-Carlo

500

300

200

100

0 0.1, 0.135 0.14 0.145 0.15 0.155 0.16 0.165 0.17 0.175 0.16

h&A = tA(Knnnn)-M(Knnn) (Gev/c’)

Fig. 2. Mass-difference distribution, for XE(K~U~%W~) > 0.5. The Klrrrr mass is required to be between 1.8345 and 1.8945 GeV/c* and all Do candidates are kept. The pure combinatorial background, estimated as described in the text, is shown. The arrows indicate the selection cuts.

The Do’s from D*+ -+ rrzK-7rfro are not fully reconstructed. Leaving out the 9, the Do is selected by requiring a K?r invariant mass in the range from 1.5 GeV/c2 to 1.7 GeV/c2, populated mainly by the Do --t K-p+ contribution to the Do -+ K-rrf7ro channel, with the ?r” emitted backward with respect to the p flight direction. The selection is the same as that of D*+ -+ s-~K-& channel, except that the XE cut is lowered to 0.42 and the range for AM is enlarged to the interval 141 MeV/c2 < AM < 152 MeV/c2, to take into account the 4-momentum carried by the rr’. The mass-difference distribution is shown in Fig. 3. The contribution from background events in the se- lected signal region is evaluated in the same way as for the D*+ --) n-,fK-W+ channel, from fake Do can- didates in a mass window above the Do mass. This se- lection leads to a sample containing 2439 f 73 D*+ -+ 7rzK-?rfro. Table 1 summarizes the statistics of the three samples available for the asymmetry measure- ment.

484 ALEPH Collaboration/Physics Letters B 352 (1995) 479-486

100 1 50 1

0 L-- 0.13 0.135

AM = M(Knn)-M(h) (C&/c:‘)

Fig. 3. Mass-difference distribution for the channel

D” -+ K-rr+rr’ , with X~(Krrrr~) > 0.42. The Krr mass is re- quired to be between 1.S and 1.7 GeV/c*. The background, esti-

mated from the side band, is shown. The signal shape predicted

by Monte Carlo is added, demonstrating that the background size

is correctly estimated by the upper side-band normalization. The

arrows indicate the selection cuts.

Table 1

Summary table for the three channels used in the asymmetry mea-

surement. The error is the sum in quadrature of the statistical error

and the systematic error coming from the background estimation.

channel candidates D* signal

D*+ --+ rr+K-r+

$K-,r+n+n-

1068 1022 f 33

D’f +

$K-?r+rrO

3542 2035 f 68

D*+ -+ s 3940 2439 f 73

3. Forward-backward asymmetry measurement

The thrust axis is used to define the production an-

gle Othrust of the qq pair with respect to the e- beam axis. It is computed using both charged and neutral particles and oriented according to the reconstructed D* direction. The charge Q of the D* gives the charm quark sign and the cosine of the c-quark production

angle is estimated by cos 0 = Q x cos 19th~~~. The charm quark asymmetry is extracted from the observed angu- lar distributions by means of an unbinned maximum likelihood fit. For each decay mode, the angular dis- tribution includes three contributions: charm, beauty

and background, each of them being of the form:

(l+~os~~+~~A~c~~e).

The function is calculated for each D*+ candidate,

taking into account the asymmetry and the proportion

of each physical source to the sample. The distribu- tions from the three samples are considered simulta-

neously, the sum of the log-likelihood functions being maximized to obtain the best estimate of the only un-

known parameter Ah. The fraction of D*+ in each

sample is deduced from the mass difference spectrum

(see Section 2)) and the relative abundance of charm and beauty in the D’ signal is taken from the Monte

Carlo with the relative probability for a D* to origi- nate from a beauty rather than a charm quark set to

the measured value Pb_D**/Pc+Da* = 0.87::$ [ I]. This charm purity, which depends basically on the X, cut, is found to be the same in each of the three sam- ples and equal to (79 * 3) %. The proportions of each kind of event in the samples are deduced from these fraction numbers.

The background in the signal region is purely combinatorial and in these events the asymmetry is

expected to be very small. An experimental back-

ground asymmetry is extracted from the Do side-band

samples. The side-bands include also partially re- constructed charm decays giving a small positive

asymmetry. The measured background asymmetry AK’ = ( 1.45 f 0.91 cstat)) % is compatible with these

expectations, and is used in the fit with an error of

100% of its value included in the systematic errors. The beauty asymmetry is diluted by B” - Do mix-

ing, and by F originating from b -+ CW W --f Es. The effective b asymmetry can be expressed as the true b asymmetry corrected by two dilution factors depend-

ing on parameters xtix and xEs:

Ab& Fn =A&.(1-2~,,,&(1-2~~~).

The ALEPH measurement from semileptonic b- decays, corrected from B” - B” mixing: AkB = (8.1 f 1.7)% [3], was used to fix the b quark asymmetry in the analysis. In the D** sample, the contributions from By and beauty baryons are small, leading to a xhx different from that for semileptonic b-decays. To evaluate xtix in the D** sample, the World average value Xd = 0.183 f 0.017 [ 51 is used and a value xs > 0.4 is derived from the ALEPH 161

ALEPH Collaboration/Physics Letters B 352 (I 995) 479-486 485

Table 2 Sources of systematic errors on the measured asymmetry

Background normalization Background asymmetry b,c relative contribution b asymmetry

Xmix X?S

csyst

0.22 % 0.93 % 0.07 % 0.29 % 0.16 % 0.08 %

1.02 %

and OPAL [7] limits on B, oscillations. The mix-

ing value in the analysed sample is then deduced to

be xhx = 0.16 f 0.04, The b hadrons fractions in

b-+D ** decays being given by the Monte-Carlo

simulation. The decay b -+ cW, W -+ Es has an expected rate

of about 14% [ 81, and produces three types of final

states:

(a) b -+ (cF)X;

(b) b -+ D’*‘X- s 1

(c) b -+ D’*‘X. 9

where the X states do not contain the E from the decay W ---t ES. Only decays of type (c) produce D* states with the wrong sign. The rate of charged D*‘s in type

(c) decays was evaluated to be (25 f 25)%; this as-

sumes (50 f 50) % vector meson production, half on it in charged states. Using CLEO and ARGUS mea- surements for decays (a) and (b) [ 91, an estimate of Xrs = 0.025 f 0.025 is obtained.

The simultaneous fit yields:

Ah = (6.99f 2.05(stat.) f l.O2(syst.))%

The contributions to systematics are listed in Table

2. The dominant error is due to the background. A change of +l% in the input beauty asymmetry Ah would result in a change of -0.17% on AC&.

Fig. 4 shows the combined angular distribution after

background subtraction and acceptance corrections, together with the result of the fit. Corrections sum-

marized in Table 3 are applied to the measurement to extract the pole asymmetry. The first correction arises from the fact that the average centre-of-mass energy is slightly above the Z pole. The main correction comes from QED initial state contribution which lowers the available energy, leading to an effect of opposite sign.

-1.0 -0.5 0.0 0.5 1 .o Q*COSQ,,,.,

Fig. 4. Distribution of the signed cosine angle of the thrust axis. The distribution is corrected for acceptance; the combinatorial background and beauty contributions are subtracted. A function (solid line) corresponding to the fitted charm asymmetry is su- perimposed.

Table 3 Corrections to the measured c-quark asymmetry

correction

v’?s, # MZ QED initial state QED final state QCD final state y exchange and interference

-0.0023 io.0104 to.0001 +0.0023 -0.0007

QED final state radiation, y exchange and yZ interfer- ence give negligible corrections to the asymmetry. No systematic error was assign to the corrections, which

have been calculated using the MIZA program [lo] where QCD final state correction is estimated from

[ 11 I, giving:

A”‘C=(80f24)% FE . . .

Finally, expressing the effective couplings as a func-

tion of sin20eff - ’ w (= s ( 1 - gVe/gAe) [ 1411, including appropriate c-quark vertex corrections from the Stan- dard Model [ lo], leads to:

sin*8: = 0.2302 f 0.0054

486 ALEPH Collaboration /Physics Letters B 352 (1995) 479-486

4. Conclusion

A sample of 5496 D* of which 80% originate from

Z -+ cE events has been used to measure the forward-

backward asymmetry in charm quark production. The

asymmetry is found to be A!$ = (8.0 I-t 2.4) %, which is in agreement with results obtained from D* samples by DELPHI [ 121 and OPAL [ 131. The effective elec-

troweak mixing angle resulting from this measurement sin28F = 0.2302 & 0.0054 is in good agreement with

the various determinations of this quantity performed

at LEP [ 141.

Acknowledgements

We wish to thank our colleagues from the acceler-

ator divisions for the successful operation of LEF? We are grateful to the engineers and technicians in all our

institutions for their contribution towards ALEPH’s

success. Those of us from non-member countries also thank CERN for its hospitality.

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