Determination of the Number of Light Neutrino Species

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D E T E R M I N A T I O N OF THE N U M B E R OF LIGHT N E U T R I N O SPECIES

ALEPH Collaboration

D. DECAMP, B. DESCHIZEAUX, J.-P. LEES, M.-N. MINARD Laboratoire de Physique des Particules (LAPP), F-74019.4 nnecy-le- Vieux Cedex, France

J.M. CRESPO, M. DELFINO, E. FERNANDEZ 1, M. MARTINEZ, R. MIQUEL, M.L. MIR, S. ORTEU, A. PACHECO, J.A. PERLAS, E. TUBAU

Laboratorio de Fisica de Altas Energias. Universidad Autonoma de Barcelona. E-08193 Bellaterra (Barcelona). Spain 2

M.G. CATANES1, M. DE PALMA, A. FARILLA, G. IASELLI, G. MAGGI, A. MASTROGIACOMO, S. NATALI, S. NUZZO, A. RANIERI, G. RASO, F. ROMANO, F. RUGGIERI, G. SELVAGGI, L. SILVESTRIS, P. TEMPESTA, G. ZITO

INFN, Sezione di Bari e Dipartimento di Fisica dell' Universita. 1-70126 Bari. Italy

Y. CHEN, D. HUANG, J. LIN, T. RUAN, T. WANG, W. WU, Y. XIE, D. XU, R. XU, J. ZHANG, W. ZHAO

Institute o f High-Energy Physics. Academia Sinica, Beijing. P.R. China

H. ALBRECHT 3, F. BIRD, E. BLUCHER, T. CHARITY, H. DREVERMANN, LI. GARRIDO, C. GRAB, R. HAGELBERG, S. HAYWOOD, B. JOST, M. KASEMANN, G. KELLNER,

J. KNOBLOCH, A. LACOURT, 1. LEHRAUS, T. LOHSE, D. LUKE 3, A. MARCHIORO, P. MATO, J. MAY, V. MERTENS, A. MINTEN, A. MIOTTO, P. PALAZZI, M. PEPE-ALTARELLI,

F. RANJARD, J. RICHSTEIN 4 A. ROTH, J. ROTHBERG 5, H. ROTSCHEIDT, W. VON RUDEN, D. SCHLATTER, R. ST.DENIS, M. TAKASHIMA, M. TALBY, H. TAUREG, W. TEJESSY,

H. WACHSMUTH, S. WHEELER, W. WIEDENMANN, W. WITZELING, J. WOTSCHACK European Organization for Nuclear Research (CERN). CH- 1211 Geneva 23. Switzerland

Z. AJALTOUNI, M. BARDADIN-OTWINOWSKA, A. FALVARD, P. GAY, P. HENRARD, J. JOUSSET, B. MICHEL, J-C. MONTRET, D. PALLIN, P. PERRET, J. PRAT, J. PRORIOL, F. PRULHII~RE

Laboratoire de Physique Corpusculaire, UniversitP Blaise Pascal. Clermont-Ferrand. F-63177 A ubibre. France

H. BERTELSEN, F. HANSEN, J.R. HANSEN, J.D. HANSEN, P.H. HANSEN, A. LINDAHL, B. MADSEN, R. MOLLERUD, B.S. NILSSON, G. PETERSEN

Niels Bohr Institute, DK-2100 Copenhagen, Denmark 6

E. SIMOPOULOU, A. VAYAKI

Nuclear Research Center Demokritos (NRCD), Athens, Greece

J. BADIER, D. BERNARD, A. BLONDEL, G. B O N N E A U D , J. BOUROTTE, F. BRAEMS, J.C. BRIENT, M.A. CIOCCI, G. FOUQUE, R. GUIRLET, P. MINI~, A. ROUGI~, M. RUMPF, H. VIDEAU, I. VIDEAU i D. ZWlERSKI

Laboratoire de Physique Nucl~aire Hautes Energies, F.cole Polytechnique. F-91128 Palaiseau Cedex. France

0370-2693/89/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

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Volume 231, number 4 PHYSICS LETTERS B 16 November 1989 D.J. C A N D L I N

Department o f Physics. University o f l:,'dinburgh. Edinburgh ElI9 3JZ. UK 7

A. CONTI, G. PARRINI

Dtpartimento di Fisica, Universita di Firenze, 1-50125 Florence, Italy

M. CORDEN, C. G E O R G I O P O U L O S , J.H. G O L D M A N , M. IKEDA, D. LEVINTHAL s J. L A N N U T T I , M. MERMIKIDES, L. SAWYER

High-Energy Particle Physics Laboratory, Florida State University, Tallahassee, FL 32306, USA 9.~o.JJ

A. ANTONELLI, R. BALDINI, G. BENCIVENNI, G. BOLOGNA, F. BOSSI, P. CAMPANA,

G. CAPON, V. CHIARELLA, G. DE NINNO, B. D ' E T T O R R E - P I A Z Z O L I , G. FELICI, P. LAURELLI, G. M A N N O C C H I , F. MURTAS, G.P. MURTAS, G. NICOLETTI, P. PICCHI, P. Z O G R A F O U Laboratori Nazionali delI' INFN (LNF-INFN), 1-00044 Frascati. Italy

B. ALTOON, O. BOYLE, A.J. FLAVELL, A.W. HALLEY, I. TEN HAVE, J.A. HEARNS, I.S. HUGHES, J.G. LYNCH, D.J. MARTIN, R. O'NEILL, C. RAINE, J.M. SCARR, K. SMITH ~, A.S. T H O M P S O N Department o f Natural Philosophy, University o f Glasgow, Glasgow G I 2 8QQ, UK 7

B. BRANDL, O. BRAUN, R. GEIGES, C. G E W E N I G E R 1, p. HANKE, V. HEPP, E.E. KLUGE, Y. MAUMARY, M. PANTER, A. PUTZER, B. RENSCH, A. STAHL, K. TITTEL, M. WUNSCH lnstitut.Far Hochenergiephysik. Universitiit Heidelberg. D- 6900 Heidelberg, FRG ~ 2

G.J. BARBER, A.T. BELK, R. BEUSELINCK, D.M. BINNIE, W. CAMERON ~, M. CATTANEO, P.J. DORNAN, S. DUGEAY, R.W. FORTY, D.N. GENTRY, J.F. HASSARD, D.G. MILLER, D.R. PRICE, J.K. SEDGBEER, I.R. TOMALIN, G. TAYLOR

Department o f Physics. Imperial College, London S W 7 2BZ, UK 7

P. GIRTLER, D. K U H N , G. R U D O L P H

lnstitut J~r I£xperimentalphysik. Universitiit Innsbruck, A-6020 Innsbruck, Austria

T.J. BRODBECK, C. BOWDERY i A.J. FINCH, F. FOSTER, G. HUGHES, N.R. KEEMER, M. NUTTALL, B.S. R O W L I N G S O N , T. SLOAN, S.W. SNOW

Department o f Physics, University o f Lancaster. Lancaster LA 1 4YB. UK ~

T. BARCZEWSKI, L.A.T. BAUERDICK, K. K L E I N K N E C H T , D. POLLMANN ~3, B. RENK, S. ROEHN, H.-G. SANDER, M. SCHMELLING, F. STEEG

Institut l~r Physik, Universitgit Mainz, D-6500 Mainz. FRG t2

J.-P. ALBANESE, J.-J. AUBERT, C. B E N C H O U K , A. BONISSENT, F. ETIENNE, R. NACASCH, P. PAYRE, B. P I E T R Z Y K i, Z. Q1AN

Centre de Physique des Particules, Facultk des Sciences de Luminy. F- 13288 Marseille, France

W. BLUM, P. CATTANEO, M. COMIN, B. D E H N I N G , G. COWAN, H. DIETL,

M. FERNANDEZ-BOSMAN, D. HAUFF, A. JAHN, E. LANGE, G. LLITJENS, G. LUTZ,

W. MANNER, H.-G. MOSER, Y. PAN, R. RICHTER, A. SCHWARZ, R. SETTLES, U. STIEGLER, U. STIERLIN, G. STIMPFL 14, j. THOMAS, G. WALTERMANN

Max-Planck-lnstitut J~r Physik und Astrophysik. Werner-Heisenberg-lnstitut l~r Physik. D-8000 Munich, FRG ~2

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J. B O U C R O T , O. C A L L O T , A. C O R D I E R , M. D A V I E R , G. DE B O U A R D , G. G A N I S , J.-F. G R I V A Z , Ph. H E U S S E , P. J A N O T , V. JOURNI~, D.W. K I M , J. LEFRAN(~OIS, D. L L O Y D - O W E N ,

A.-M. L U T Z , P. M A R O T T E , J.-J. V E I L L E T

Laboratoire de l'Accdldrateur Lin~aire. Universit~ de Paris-Sud, F-91405 Orsay Cedex, France

S.R. A M E N D O L I A , G. BAGLIESI, G. B A T I G N A N I , L. BOSISIO, U. B O T T I G L I , C. B R A D A S C H I A , 1. F E R R A N T E , F. F I D E C A R O , L. FO~, ~, E. F O C A R D I , F. F O R T I , A. GIASSI, M.A. G I O R G I , F. L I G A B U E , A. L U S I A N I , E.B. M A N N E L L I , P.S. M A R R O C C H E S I , A. M E S S I N E O , F. PALLA, G. S A N G U I N E T T I , S. S C A P E L L A T O , J. S T E I N B E R G E R , R. T E N C H I N I , G. T O N E L L I , G. T R I G G I A N I

Dtpartimento di Fisica dell' Universiti~, INFN Sezione di Pisa, e Scuola Normale Superiore, 1-56010 Pisa, Italy

J.M. C A R T E R , M.G. G R E E N , A.K. M c K E M E Y , P.V. M A R C H , T. M E D C A L F , M.R. S A I C H , J. S T R O N G ~, R.M. T H O M A S , T. W I L D I S H

Department of Physics. Royal Holloway & Bedford New College, University o f London. Surrey TW20 OEX, UK 7

D.R. B O T T E R I L L , R.W. C L I F F T , T.R. E D G E C O C K , M. E D W A R D S , S.M. F I S H E R , D.L. HILL, T.J. J O N E S , G. M c P H E R S O N , M. M O R R I S S E Y , P.R. N O R T O N , D.P. S A L M O N , G.J. T A P P E R N , J.C. T H O M P S O N , J. H A R V E Y

tligh-Energy Physics Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OXI 10QX. UK ~

B. B L O C H - D E V A U X , P. COLAS, C. K L O P F E N S T E I N , E. LANt~ON, E. L O C C I , S. L O U C A T O S , L. MIRABITO, E. MONNIER, P. PEREZ, F. PERRIER, B. PIGNARD, J. R A N D E R , J.-F. R E N A R D Y , A. R O U S S A R I E , J.-P. S C H U L L E R , R. T U R L A Y

D~partement de Physique des Particules El~mentaires, CEN-Saclay. F- 91191 Gtf-sur- Yvette Cedex. France

J.G. A S H M A N , C.N. B O O T H , F. C O M B L E Y , M. D I N S D A L E , J. M A R T I N , D. P A R K E R , L.F. T H O M P S O N

Department o f Physics, University of Sheffield. SheffieM $3 7RH, UK 7

S. B R A N D T , H. B U R K H A R D T , C. G R U P E N , H. M E I N H A R D , E. N E U G E B A U E R , U. S C H A F E R , H. S E Y W E R D , K. S T U P P E R I C H

Fachbereich Physik. Universitiit Siegen, D-5900 Siegen, FRG t2

B. G O B B O , F. LIELLO, E. M I L O T T I , F. R A G U S A 15, L. R O L A N D I l Dipartimento di Fisica. UniversitiJ di Trieste e INFN Sezione di Trieste, 1-34127 Trieste, Italy

L. B E L L A N T O N I , J.F. B O U D R E A U , D. C I N A B R O , J.S. C O N W A Y , D.F. C O W E N , Z. F E N G , J.L. H A R T O N , J. H I L G A R T , R.C. J A R E D t6, R.P. J O H N S O N , B.W. L E C L A I R E , Y.B. PAN, T. P A R K E R , J.R. PATER, Y. S A A D I , V. S H A R M A , J.A. W E A R , F.V. W E B E R , SAU LAN W U , S.T. X U E and G. Z O B E R N I G

Department o f Physics, University of Wisconsin. Madison. W15 3 706, USA ~ r

Received 12 October 1989

The cross-section for e+e - ~ hadrons in the vicinity of the Z boson peak has been measured with the ALEPH detector at the CERN Large Electron Positron collider. LEP. Measurements of the Z mass, Mz= (91.174"+0.070) GeV, the Z width Fz= ( 2.68 ± 0.15 ) GeV, and of the peak hadronic cross-section, tll~ k = ( 29.3 _+ 1.2 )nb. are presented. Within the constraints of the standard electroweak model, the number of light neutrino species is found to be Nv = 3.27 + 0.30. This result rules out the possibility of a fourth type of light neutrino at 98% CL.

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Volume 231, n u m b e r 4 PHYSICS LETTERS B 16 November 1989

I. Introduction

The Z boson was discovered in 1983 at the Pl) collider at C E R N [ 1,2 ]. Detailed studies o f its properties can now be performed in e+e - collisions at LEP. In the standard electroweak model [3], the Z boson is expected to decay with comparable probability into all species o f fermions that are kinematicaily al- lowed. The decay rate o f the Z into light, neutral, penetrating particles such as neutrinos, that would otherwise escape detection, can be measured through an increase in the total width Fz. Detection o f additional neutrino species would put in evidence additional fermion families, even if the masses o f their charged partners are inaccessible at presently avail- able energies. One additional species o f neutrino would result in an increase o f 6.6% in Fz. Further- more the peak cross-section to any detectable final state f is very sensitive to this change and would de- crease by 13% for one more neutrino species. This cross-section can be expressed in terms o f Fz and o f the partial widths, F~, F~, Fr, o f t h e Z into e+e -, neu- trinos, and the final state f a s

0"~ oak _~_ 12n F~fl) _{- -} _{( 1 --~rad) ~ o'O( 1 --~rad) ,} ₍₁₎

Present address: CERN, CH-1211 Geneva 23, Switzerland. 2 Supported by CAICYT, Spain.

3 Permanent address: DESY, D-2000 H a m b u r g 52, FRG. 4 Present address: Lecroy, Geneva, Switzerland.

5 O n leave of absence from University o f Washington, Seattle, WA 98195, USA.

6 Supported by the Danish Natural Science Research Council. 7 Supported by the U K Science and Engineering Research

Council.

Supported by SLOAN fellowship, contract BR 2703. 9 Supported by the US Department o f Energy, contract DE-

FG05-87ER40319.

to Supported by the NSF, contract PHY-8451274.

t t Supported by the US Department o f Energy, contract DE- FCOS-85ER250000.

~2 Supported by the Bundesministerium flit Forsehung und Tcchnologie.

~3 Permanent address: Institut fiir Physik, Univcrsit/it Dort- m u n d , D-4600 D o r t m u n d 50, FRG.

~4 Present address: FSU, Tallahassee, FL 32306, USA. ~5 Present address: INFN, Sezione di Milano, 1-20133 Milan,

Italy.

~6 Permanent address: LBL, Berkeley, CA 94720, USA. ~7 Supported by the US Department of Energy, contract DE-

AC02-76ER00881.

with

Fz=NvFv+ 3F~ + Fhad , (2)

where N,, is the n u m b e r o f light neutrino species. In the particular case where f comprised mostly had- ronie final states, uncertainties in the overall scale o f partial widths, such as those related to the lack o f knowledge o f the top quark mass and more generally to electroweak radiative effects, as well as uncertainties in the ratio o f h a d r o n i c to leptonic partial widths, largely cancel in this formula. The Q E D initial state radiative correction, c~raa, is quite large, but has been calculated to an accuracy believed to be better than 0.5% by several authors [4].

Cross-sections are measured by taking the ratio o f the n u m b e r o f selected Z decays, hereafter referred to as hadronic events, to the number o f small angle e+e - events from the well calculable Bhabha scattering process, hereafter referred to as luminosity events.

2. D e s c r i p t i o n o f the A L E P H detector

The data presented here have been collected with the A L E P H detector during the first three weeks o f running at LEP, from 20 September to 9 October 1989. Guided by earlier measurements o f the Z mass by the C D F [ 5 ] and Markll [ 6 ] Collaborations, data were collected mainly at the Z peak and in the near vicinity o f it. Integrated luminosities o f 64 nb-~ at the peak and 88 n b - t on the sides o f the peak were recorded.

A detailed description o f the A L E P H detector is in preparation [ 7 ]. The principal c o m p o n e n t s relevant for this measurement are:

- The Inner Tracking Chamber, ITC, and 8-layer cylindrical drift chamber with sense wires parallel to the beam axis from 13 cm to 29 cm in radius. Tracks with polar angles from 14 ° to 166 ° traverse all 8 layers. - The large cylindrical Time Projection Chamber, TPC, extending from an inner radius o f 31 cm to an outer radius o f 180 cm over a length o f 4.4 m. Up to 21 space coordinates are recorded for tracks with polar angles from 47 ° to 133 °. Requiring 4 coordinates, tracks are reconstructed down to 15 °.

- The Electromagnetic Calorimeter, ECAL, a lead wire-chamber sandwich. The cathode readout is sub- divided into a total o f 73 728 projective towers. Each 522

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tower o f a b o u t i ° × 1 ° solid angle is read out in three stacks o f 10, 23 and 12 layers (respectively 4, 9 and 9 r a d i a t i o n lengths). The signals from the 45 wire planes o f each o f the 36 m o d u l e s are also read out. The two e n d c a p s cover p o l a r angles from 11 ° to 40 ° and 140 ° to 169° and the barrel covers polar angles from 40 ° to 140 ° .

- The superconducting, solenoidal coil, p r o v i d i n g a magnetic field o f 1.5 T.

- The H a d r o n C a l o r i m e t e r , HCAL, c o m p r i s e d o f 23 layers o f s t r e a m e r tubes interleaved in the iron o f the magnet return yoke, read out by a total o f 4608 pro- j e c t i v e towers. Signals from each o f the tubes are also read out. The modules are rotated in a z i m u t h by ~ 2 ° with respect to the electromagnetic calorimeter so that inactive zones do not align in the two calorimeters. The two e n d c a p s and the barrel o f the h a d r o n calo- r i m e t e r cover polar angles down to 6 : .

- The Small Angle T r a c k i n g chamber, SATR, with 9 planes o f drift tubes, covering angles from 40 to 90 mrads, for precise m e a s u r e m e n t o f small angle electron tracks.

- The L u m i n o s i t y C a l o r i m e t e r , LCAL, s i m i l a r in its construction and read-out to ECAL, e x t e n d i n g from 50 to 180 mrads, p r o v i d i n g energy a n d p o s i t i o n measurement o f the showers p r o d u c e d by l u m i n o s i t y events.

H a d r o n i c events were triggered by two indepen- dent first level triggers: ( i ) an E C A L - b a s e d trigger, requiring a total energy o f 6 GeV d e p o s i t e d in the ECAL barrel or 3 GeV in either o f the E C A L e n d c a p s or 1 GeV in both in coincidence; ( i i ) an I T C - H C A L coincidence, for p e n e t r a t i n g charged particles, requiring 6 ITC wire planes a n d 4 to 8 planes o f H C A L tubes in the same a z i m u t h a l region.

L u m i n o s i t y events were also triggered in two ways: ( i ) a c o i n c i d e n c e o f 15 GeV d e p o s i t e d in LCAL on one side with 10 GeV d e p o s i t e d on the other side, without requiring a z i m u t h a l correlation: ( i i ) a single arm r e q u i r e m e n t o f 32 GcV d e p o s i t e d in either side o f LCAL. In a d d i t i o n , prescaled single arm triggers with 10 and 15 GeV thresholds were recorded to pro- vide an e s t i m a t e o f the b e a m related background.

All types o f events were processed s i m u l t a n e o u s l y through the same trigger system (with the same dead- t i m e ) , through the same d a t a acquisition and recon- struction programs. In o r d e r to ensure that the events were c o u n t e d d u r i n g the same life-time, the list o f e n -

abled triggers and the status o f each o f the relevant s u b d e t e c t o r s was recorded with each event. Events were accepted only when both ECAL and L C A L were running and when both ECAL and L C A L triggers were enabled. A possible bias could come from the few d a t a acquisition failures that occurred d u r i n g the run. A careful investigation o f all the events before, during, and after each failure revealed a possible but small loss o f 0.4% o f the h a d r o n i c events. This check could be d o n e for the events that were m i s h a n d l e d because their trigger p a t t e r n was always recorded and the trigger patterns o f hadronic and luminosity events are unique enough to allow an e s t i m a t e o f the losses in each category.

3. Event selection

To p r o v i d e a check, two i n d e p e n d e n t event selections were used. The first one selected h a d r o n i c Z decays only, and was based on T P C tracks. The second one selected decays o f the Z into h a d r o n s as well as z pairs, and was based on c a l o r i m e t r i c energy. T h e event samples o v e r l a p p e d to the extent o f 95%. The efficiencies o f both m e t h o d s were very close to unity and systematic uncertainties in these efficiencies were less than 1%. Altogether 3112 events were r e t a i n e d in the track selection and 3320 events in the calorime- tric selection.

3. 1. Selection with TPC tracks

H a d r o n i c Z decays (a typical event is shown in fig. 1 ) were selected on the basis o f charged tracks only, requiring at least 5 charged tracks. The energy sum o f all charged tracks was required to be at least 10% o f the centre-of-mass energy. Tracks were required to have a p o l a r angle larger than 18.2 =, to be reconstructed from at least 4 T P C c o o r d i n a t e s , and to orig- inate from a 2 cm r a d i u s 20 cm long c y l i n d e r a r o u n d the n o m i n a l b e a m position. The p e r f o r m a n c e o f the T P C was in r e m a r k a b l e a g r e e m e n t with expectations. In o r d e r to estimate the acceptance, a c o m p l e t e s i m u l a t i o n o f the e+e - - . h a d r o n s process was per- l b r m e d , including initial state r a d i a t i o n effects and h a d r o n i z a t i o n . The p r o p e r t i e s that are relevant for acceptance calculation, total charged-particle energy, track multiplicity, sphericity d i s t r i b u t i o n and p o l a r

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Volume 231, number 4 PHYSICS LETTERS B 16 November 1989 I ~ T " ~ U • D^L.I 09-09-2¢ 2 2 : G ] I%n-4081 £ v t - ] O ( ~ I i ~ ~r1,~l.li~/,a " ¢ b l t l - 50804 Tbtt~o$COSOBO0 • b) - - - _ _ = _ - - -

-q

Fig. 1. A hadronic Z decay in the ALEPH detector: (a) x - y view;

(b) r - z view.

angle d i s t r i b u t i o n , are in good agreement with the s i m u l a t i o n , as shown in fig. 2. The efficiency o f this selection m e t h o d is 0.975 + 0.006, on the peak; the error corresponds to assigning a conservative 20% energy scale u n c e r t a i n t y near the cut. U n c e r t a i n t i e s in hadronization models were reduced to a very small

i 200 o 16C ('4 12C f.O ~ 8c b J ~ 4c I I I I I I I I I ( 0 ) • DATA - - M C " ENERGY (GeV)

25o? ~

200P i 150h lOOb 50t.- ALEPH I I I I I I I I I 1 • DATA f - - M C " 12 20 28 6 CHARGED MULTIPLICITY 400

~

30C .~, 200 100

[()c

' ' ' • DATA - - M C " 0.05 0.15 SPHERICITY (~) ... • D A T A 800 - - MC" 600 ~ 4 0 0 200 I I I * I * n ~ * 0.1 0.3 0.5 0.7 0.9 [cos "dl

Fig. 2. Properties of charged tracks in hadronic events and comparison with simulations. In each plot, the solid points represent data and the lines represent the simulation normalized to the data: (a) distribution of the charged-track energy sum per event; (b) charged-track multiplicity distribution; (c) sphericity distribution for events where the sphericity axis had a polar angle such that I cos O,p, I < 0.8; and (d) polar angle distribution for charged tracks.

level by using the measured sphericity distribution for the acceptance calculation. Due to initial state radiation, this efficiency varies slightly on the side o f the Z peak by up to - 0 . 0 0 3 .

C o n t a m i n a t i o n of the sample of hadronic events by ~ pairs from Z decays was estimated to be (5.1 + 1.5 ) events, a n d in fact three events compati- ble with that hypothesis were found in the sample. C o n t a m i n a t i o n by b e a m - g a s interactions was estimated from the n u m b e r of events found passing the selection cuts except for the l o n g i t u d i n a l vertex position: about one event is expected. Finally the back- g r o u n d from e+e - --,e+e - + h a d r o n s ( " t w o - p h o t o n " e v e n t s ) was calculated to be about 15 pb, represent- ing a c o n t a m i n a t i o n of 0.5X 10 -3 to the peak cross- section.

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3.2. Selection using the calorimeters

The aim o f this m e t h o d was to select h a d r o n i c and x events. The basic r e q u i r e m e n t was that the total ca- l o r i m e t r i c energy be a b o v e 20 GeV, as well as either >/6 GeV in the E C A L barrel or at least 1.5 GeV in each ECAL endcap. These r e q u i r e m e n t s reduce both t w o - p h o t o n events a n d m u o n - p a i r events to a negli- gible level. Large-angle e ÷ e - events were rejected on the basis o f their characteristic tight energy clusters in the electromagnetic calorimeter. For the few events with no tracks at all, cuts were a p p l i e d to e l i m i n a t e cosmic rays: a t i m i n g cut and a cut on the m i n i m a l n u m b e r ( 2 ) o f clusters a b o v e 3 GeV in ECAL. On the basis o f M o n t e Carlo s i m u l a t i o n as well as the scanning o f events the resulting selection efficiency is 0.994 + 0.005 for h a d r o n i c events and 0.60 + 0.05 for T events, giving a c o m b i n e d

crhad+a~

efficiency o f 0.974 + 0.006. C o n t a m i n a t i o n by events o t h e r than h a d r o n i c or "t was e s t i m a t e d to be less than 0.4%.

T h e two event s a m p l e s were c o m p a r e d event by event. Differences were well u n d e r s t o o d given the different characteristics o f the two selections.

y[cm] 1 I,C 30 20 10 x [cm]

Fig. 3. End view of the Luminosity Calorimeter, showing the tower limits; the shaded area represent the towers excluded from the fiducial area.

3.3. Trigger e.[l~ciency

The trigger efficiency was m e a s u r e d by counting events where one or both o f the E C A L and I T C - H C A L triggers occurred: it was found to be 100% for the E C A L trigger and 87% for the I T C - H C A L trigger, giving an overall efficiency o f 100%.

4. Determination of the luminosity

L u m i n o s i t y events were selected on the basis o f the energy d e p o s i t e d in the L C A L towers. N e i g h b o r i n g towers containing more than 50 MeV were j o i n e d into clusters, giving energy and position o f the shower. Events were required to have a shower reconstructed on each side o f LCAL.

In o r d e r to m i n i m i z e the d e p e n d e n c e o f the acceptance upon precise knowledge o f the b e a m p a r a m e - ters, an a s y m m e t r i c selection was p e r f o r m e d : on one side (e.g., the e ÷ side ), showers were required to have m o r e than h a l f o f their energy d e p o s i t e d in a fiducial v o l u m e (fig. 3) excluding the towers situated at the edge o f the detector; the total energy d e p o s i t on that

side was also required to be larger than 55% o f the b e a m energy; on the o t h e r side ( e - s i d e ) , only a total energy d e p o s i t i o n o f m o r e than 44% o f b e a m energy was required. The respective roles o f the e + and e - sides were interchanged in every o t h e r event. Finally, the difference in azimuth, A0, between the e ÷ and the e - was required to be larger than 170 :.

The accepted cross-section was calculated using a first o r d e r event g e n e r a t o r [8 ] with a full s i m u l a t i o n o f the detector. The value o b t a i n e d for the cross-section for Bhabha scattering into the region within these cuts was found to be (31.12 _+ 0.45exp + 0.3 l,h) nb, at a centre-of-mass energy o f 91.0 GeV and for a Z mass o f 91.0 GeV. the first error represents the effect o f uncertainties in the s i m u l a t i o n , calibration, and po- sitioning o f the apparatus; the second one represents the possible error due to neglecting higher o r d e r ra- d i a t i v e effects, and the u n c e r t a i n t y in the p h o t o n v a c u u m p o l a r i z a t i o n [9 ]. Properties o f the luminosity events are shown in fig. 4, and c o m p a r e d with the simulation. The o p t i m u m energy resolution has not yet been o b t a i n e d , but the acceptance is quite insen- sitive to this. O t h e r p r o p e r t i e s are in good a g r e e m e n t with expectations. The d i s p l a c e m e n t s o f the b e a m

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Volume 231, n u m b e r 4 PHYSICS LETTERS B 16 November 1989 ALEPH I (b) . . . .

8001-= DATA

?J 1--15001-" DATA

1000

'°°1

t

I

,.:5, 50 164 172 ENERGY (GeV) A~ "Fable I

Summao' of systematic errors in the luminosity measurement. transverse and longitudinal shower profile + 0.005

energy scale + 0.002

energy resolution and cell-to-cell calibration + 0.007 external alignment and beam parameters + 0.002 internal alignment and inner radius _+ 0.010 description o f material + 0.005 higher order radiative effects _* 0.01

--0.02 total uncertainty: I ! ! i ! I (c)

• DATA

- - M C 3OO 200 ,v, I00 60 1DO 140 'd

Fig. 4. Properties o f luminosity events passing selection criteria and comparison with simulations. In each plot, the solid points represent data and the lines represent the simulation normalized to the data: (a) shower energy distribution for events passing the tight fiducial cut; (b) azimuthal separation A¢~ o f the two oppo- site clusters; (c) polar angle distribution.

were m e a s u r e d using l u m i n o s i t y events t h e m s e l v e s and corrected for.

The b e a m - r e l a t e d b a c k g r o u n d c o n t a m i n a t i o n was e s t i m a t e d from single arm, prescalcd triggers. These triggers were c o m b i n e d into artificial d o u b l e a r m events. The n u m b e r o f such c o m b i n a t i o n s passing the selection cuts was n o r m a l i z e d to the n u m b e r o f real c o i n c i d e n c e s with A 0 < 9 0 :. This b a c k g r o u n d sub- traction was p e r f o r m e d for each run and was o f the o r d e r o f 1% o r smaller.

T h e c o n t a m i n a t i o n by physics sources such as e + c - --*T'/or e+e - ~ c + e - fP has been e s t i m a t e d to bc less than 2 X 10 - 3 . The interference o f the Z ex- change diagram with the purely Q E D contribution has been taken into account when d e t e r m i n i n g the resonance p a r a m e t e r s .

The trigger efficiency was m e a s u r e d for each d a t a t a k i n g p e r i o d by c o m p a r i n g the n u m b e r o f events passing the selection criteria that set the single a r m trigger, the c o i n c i d e n c e trigger, o r both. lnefficien-

cies in the trigger were traced d o w n to faulty elec- tronic channels. Efficiencies vary with the run, rang- ing from 0.98 to 1.00 with an average value o f 0.997 + 0.002.

A s u m m a r y o f the l u m i n o s i t y systematic errors is given in table 1. A relatively small n o r m a l i z a t i o n error was possible mainly as a result o f the excellent spatial resolution o f the c a l o r i m e t e r ( ~ 3 0 0 / a m for electrons near the tower b o u n d a r i e s ) , the good back- g r o u n d c o n d i t i o n s d e l i v e r e d by the machine, the a v a i l a b i l i t y o f a r e d u n d a n t set o f triggers and prog- ress in the theoretical calculations [ 10]. The relative n o r m a l i z a t i o n u n c e r t a i n t y o f cross-section measure- mcnts at different energies comes mostly from differences in b a c k g r o u n d c o n d i t i o n s and trigger efficiency. These uncertainties were taken into account in the statistical error.

5. Determination of the Z resonance parameters

The n u m b e r o f Z events, o f l u m i n o s i t y events, to- gether with the cross-sections are given in table 2 for the two event selection methods.

Two different fits were p e r f o r m e d to the data. In the first fit, the basic p a r a m e t e r s o f the Z resonance, its mass Mz, width Fz, and Q E D corrected peak cross- section

at)_ 12nl'~cFr

Mz r~

are cxtracted with little model dependence. In the second fit. the constraints from the standard model

are applied to determine

Mz

and

Nv.

The three parameter fit was performed using com-

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Table 2

Event numbers and cross-section as a function of centre-of-mass energy. The overall systematic error of ± 2% in the cross-sections is not included.

Energy Selection from TPC tracks Selection by calorimeters GeV

Nh,d Ntumi 0"hi d (nb ) Nhi d + N, Nlumi Grh& d + e, ( n b )

89.263 120 443 9.00± 0.92 134 450 9.89 ± 0.97 90.265 406 715 18.43 ± 1.14 445 736 19.62 ± 1.17 91.020 656 668 31.29 ± 1.72 678 669 32.28 ± 1.76 91.266 1156 1295 28.16± 1.14 1243 1309 29.96± 1,19 92.260 258 377 21.11 ± 1.72 268 374 22.10 ± 1.78 92.519 125 247 15.52 ± 1.71 142 260 16.75 ± 1,76 93.264 391 883 13.36 ± 0.82 410 889 13.92 ± 0.84

purer programs by Burgers [ 11 ] and Borelli ct al. [12], folding a Breit-Wigner resonance (with s-de- pendent width ) with the second order exponentiated initial state radiation spectrum. Results from fitting with the two programs were in good agreement with each other. These approximate programs agree ade- quately with complete electroweak calculations (see D.Y. Bardin et al., in ref. [4] and ref. [ 1 3 ] ) for centre-of-mass energies within + 2.5 GeV of the peak. These fits yield the values for the mass and width of the Z boson shown in table 3.

The data points and the result of the fit are shown in fig. 5 for the track selected events.

The error in Mz does not yet include the uncertainty in the mean e÷e - collision energy. This error was dctermined by the LEP division [ 14] on thc basis of measurements of uncertainties in the magnetic field integrals and in the orbit positions to be 5 × 10 -4 or 45 MeV. Including this uncertainty, we find Mz = (91.174 + 0.055¢,~ _+ 0.045LEP) GeV. (3) The value agrees with the two previous best measurements, refs [ 5,6 ], but the uncertainty is smaller by a factor of 2.

Mz is effectively uncorrelated with the other two parameters. The correlation between Fz and a ° for the track selected data sample is shown in fig. 6.

In the standard model,

Fee, Fh,d

and Fv are calculable with a small uncertainty of about + 1% due to (i) electroweak radiative effects involving unknown

b i ALEPH - - BEST FIT ,,-.,, . . . N. = 2 'i! " - - " N, = 3 ," 2 " , " , 819 i 911 i 913 ENERGY (GeV)

Fig. 5. The cross-section for e + e - ~ h a d r o n s as a function of centre-of-mass energy and result of the three parameter fit.

Table 3

Mz (GeV) I:z (GeV) o ° ( n b ) ~r ~-.k ( n b ) hadronic events 9 I. 178 _+ 0.055 2.66 ± 0.16 39.1 + 1.6 29.3 + 1.2 hadronic + ~ events 91.170 + 0.054 2.70 + 0.15 40.9 +_ 1.7 30.5 +_ 1.3 combined 91.174 + 0.054 2.68 + 0.15

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Volume 23 I, number 4 PHYSICS LETTERS B 16 November 1989 3.1 ALEPH 2,g 90°/, C L 2.8 6 8 % C L 2.7 ~ 2.6 2.5 2.4 N, 2.3 N, = 3 ~" N , ~ 2 3~6 ' 4~o ' 4~ ' 4e o. (~)

Fig. 6. The total width versus the peak hadronic cross-section, with 68% and 90% CL experimental contours. The standard model prediction for 2, 3 or 4 species of neutrinos is also shown, with its theoretical error.

Table 4

Standard model partial widths of the Z in MeV for the measured value of Mz; ct, = 0.12 + 0.02 and sin20, = 0.230 + 0.006 have been used as input.

F~ (MeV) 83.5___ 0.5

Fv (MeV) 166.5_+ 1.0

Fh,a(MeV) 1737 _+22

cancel in a °. The u n c e r t a i n t y in Fhad, related to the Q C D correction, cancels in part in a°; the resulting u n c e r t a i n t y

Aa°/a°=O.4AFhad/Fz=O.O03

is much s m a l l e r than the effect p r o d u c e d by a single n e u t r i n o family,

Aa°/a°=-0.13.

C o m b i n i n g these errors in q u a d r a t u r e one finds

N v = 3 . 2 7 + 0 . 3 0 . ( 5 )

The hypothesis N ~ = 4 is ruled out at 98% c o n f i d e n c e level. This m e a s u r e m e n t i m p r o v e s in a decisive way u p o n p r e v i o u s d e t e r m i n a t i o n s o f the n u m b e r o f neu- t r i n o species from the UA 1 [ 16 ] and UA2 [ 17 ] ex- periments, from PEP [ 18 ] and P E T R A [ 19 ], from cosmological [ 20 ] or a s t r o p h y s i c a l [ 21 ] arguments, as well as from a s i m i l a r d e t e r m i n a t i o n at the Z peak

[221.

The d e m o n s t r a t i o n that there is a third n e u t r i n o c o n f i r m s that the "t neutrino is distinct from the e a n d ~t neutrinos. The absence o f a fourth light n e u t r i n o indicates that the q u a r k - l e p t o n families are closed with the three which are a l r e a d y known, except for the possibility that higher o r d e r families have neutrinos with masses in excess o f ~ 30 GeV.

particles, such as the t o p quark; ( i i ) the value o f the strong coupling c o n s t a n t o~s. The s t a n d a r d m o d e l pre- d i c t i o n s for the partial widths are given in table 4. The s t a n d a r d m o d e l p r e d i c t i o n s for a ° and F z assum- ing 2, 3, and 4 species o f light n e u t r i n o s are shown in fig. 6. T h e value N v = 3 is preferred. M o r e precise in- f o r m a t i o n on Nv is c o n t a i n e d in the peak cross-section a °.

If the partial widths are taken from standard model predictions, a two p a r a m e t e r fit can be p e r f o r m e d , leaving Mz and Nv as only free p a r a m e t e r s . This fit, p e r f o r m e d with the p r o g r a m s o f refs. [ 12,15 ] on the two d a t a samples, leaves the value o f M z unchanged. The result for .~'~ is

Nv= 3.27 + 0.24star + 0.16sys + 0.05th , ( 4 ) where the errors c o m i n g from statistics, e x p e r i m e n - tal s y s t e m a t i c s and theoretical u n c e r t a i n t y are shown separately. T h e o r e t i c a l u n c e r t a i n t i e s due to electroweak r a d i a t i v e effects consist mostly in a change o f the overall scale o f the partial widths, and largely

Acknowledgement

We would like to express our g r a t i t u d e and a d m i - ration to our colleagues o f the LEP division for the t i m e l y and beautiful o p e r a t i o n o f the machine. We thank the Technical C o o r d i n a t o r , Pierre Lazeyras, and the technical staff o f the A L E P H C o l l a b o r a t i o n for their excellent work. We would like to d e d i c a t e this p a p e r to the m e m o r y o f those who d i e d d u r i n g LEP construction: Luigi Barito, M i l o u d Ferras, Luigi Filippi, Fr6d6ric Mouly, Noel Piccini and Franqois Pierrus. T h o s e o f us from n o n - m e m b e r countries thank C E R N for its hospitality.

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[3] S.L. Glashow, Nucl. Phys. 22 ( 1961 ) 579; S. Weinberg, Phys. Rev. Lett. 19 (1967) 1264;

A. Salam, Elementary particle theory, ed. N. Svartholm (Almquist and Wiksell, Stockholm, 1968) p. 367. [4] For references see D.Y. Bardin et al., Z-line-shape group,

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[10]D.Y. Bardin et al., Monte Carlo working group, Proc. Workshop of Z physics at LEP, CERN report 89-08. [ 11 ] Computer program ZAPP, courtesy of G. Burgers. [12]A. Borelli, M. Consoli, L. Maiani and R. Sisto, preprint

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[ 17 ] UA2 Collab., R. Ansari et al., Phys. Lett. B 186 ( 1987 ) 440. [18] MAC Collab.. W.T. Ford et al., Phys. Rev. D 19 (1986)

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