Search for heavy lepton pair production in e+e− collisions at centre-of-mass energies of 130 and 136 GeV

19 September 1996

Physics Letters B 384 (1996) 439-448

PHYSICS LETTERS 6

Search for heavy lepton pair production in efe- collisions at centre-of-mass energies of 130 and 136 GeV

ALEPH Collaboration

D. Buskulic a, I. De Bonis a, D. Decampa, P Gheza, C. Goya, J.-P Lees a, A. Lucotte a, M.-N. Minard a, J.-Y. Nief a, l? Odier a, B. Pietrzyka, M.P Casado b, M. Chmeissani b,

J.M. Crespo b, M. Delfino, b, I. Efthymiopoulos by20, E. Fernandez b, M. Fernandez-Bosman b, Ll. Garridoba15, A. Juste b, M. Martinez b, S. Orteu b, C. Padilla b,

I.C. Park b, A. Pascual b, J.A. Perlas b, I. Riu b, F. Sanchez b, F. Teubert b, A. Colaleo ‘, D. CreanzaC, M. de PalmaC, G. Gelao ‘, M. Girone ‘, G. Iaselli ‘, G. Maggi c,3, M. Maggi ‘,

N. Marinelli ‘, S. Nuzzo ‘, A. Ranieri ‘, G. Raso ‘, F. Ruggieri ‘, G. Selvaggi ‘, L. Silvestris c, P TempestaC, G. Zito ‘, X. Huangd, J. Lin d, Q. Ouyangd, T. Wangd, Y. Xied, R. Xu d,

S. Xue d, J. Zhang d, L. Zhang d, W. Zhao d, R. Alemany e, A.O. Bazarko e, M. Cattaneo e, P. Comas e, l? Coyle e, H. Drevermann e, R.W. Forty e, M. Franke, R. Hagelberg e,

J. Harvey e, P Janot e, B. Jost e, E. Kneringere, J. Knobloche, I. Lehraus e, G. Lutters e, E.B. Martine, P. Mato e, A. Mintene, R. Miquel e, L1.M. Mir e,2, L. Monetae, T. Oest e,l, A. Pacheco e, J.-F. Pusztaszeri e, F. Ranjard e, P. Rensing e,25, L. Rolandi e, D. Schlatter e,

M. Schmelling e~24, M. Schmitt”, 0. Schneider”, W. Tejessy e, I.R. Tomalin e, A. Venturi”, H. Wachsmuthe, A. Wagner e, Z. Ajaltouni f, A. Bar&s f, C. Boyerf, 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. Podlyskif, J. Proriolf, P. Rosnet f, J.-M. Rossignolf, T. Fearnleys,

J.B. Hansen s, J.D. Hansen g, J.R. Hansen s, PH. Hansen s, B.S. Nilsson g, B. Rensch s, A. Wa2n2nens, A. Kyriakis h, C. Markou h, E. Simopoulou h, A. Vayaki h, K. Zachariadou h,

A. Blondeli, J.C. Brient’, A. Rougei, M. Rumpf’, A. Valassii,6, H. Videaui,21, E. Focardij,21, G. Parrinij, M. Corden k, C. Georgiopoulos k, D.E. Jaffe k, A. Antonellie,

G. Bencivenni e, G. BolognaeT4, F. Bossi e, P. Campana “, G. Capon e, D. Casper e, V. Chiarellaj, G. Felicie, P. Laurellie, G. Mannocchi e35, F. Murtase, G.P. Murtas e,

L. Passalacqua e, M. Pepe-Aharelli e, L. Curtis m, S.J. Dorris m, A.W. Halley m, I.G. Knowles m, J.G. Lynch m, V. O’Shea”, C. Raine m, P Reeves m, J.M. Starr m,

K. Smith m, P Teixeira-Dias m, A.S. Thompson m, F. Thomson m, S. Thorn m, R.M. Turnbullm, U. Becker n, C. Geweniger “, G. Graefe n, P. Hanke n, G. Hansper n,

V Hepp n, E.E. Kluge n, A. Putzer”, M. Schmidt “, J. Sommer n, K. Tittel n, S. Werner n,

0370-2693/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved.

PII SO370-2693(96)00928-S

440 ALEPH Collaboration/Physics Letters B 384 (1996) 439-448

M. Wunsch”, D. Abbaneo O, R. Beuselinck’, D.M. Binnie O, W. Cameron’, P.J. Dornan’, P. Morawitz O, A. Moutoussi O, J. Nash O, J.K. Sedgbeer O, A.M. Stacey O, M.D. Williams O,

G. Dissertorip, P. Girtlerr, D. Kuhn”, G. Rudolphp, A.P. Betteridgeq, C.K. Bowderyq, P Colrain 4, G. Crawfordq, A.J. Finch 4, F. Foster 4, G. Hughes 9, T. Sloan 4, E.P. Whelan 4,

M.I. Williams 4, A. Galla r, A.M. Greener, C. Hoffmannr, K. Kleinknecht r, G. Quast r, B. Renk r, E. Rohne ‘, H.-G. Sander r, P. van Gemmeren ‘, C. Zeitnitz ‘, J.J. Aubert sy21,

A.M. Bencheikh”, C. Benchouks, A. Bonissent ‘, G. BujosaS, D. Calvet s, J. Carr s, C. Diaconu ‘, N. KonstantinidisS, l? PayreS, D. Rousseau ‘, M. Talby s, A. Sadouki s,

M. Thulasidas ‘, A. TilquinS, K. Trabelsi s, M. Aleppo, t, F. Ragusat121, I. Abt *, R. Assmann u, C. Bauer ‘, R. Berlich “, W. Blum”, V. Biischer u, H. Diet1 u, F. Dydak u,21, G. Ganis “, C. Gotzhein ‘, K. Jakobs “, H. KrohaU, G. Ltitjens ‘, G. Lutz “, W. Manner “,

H. -G. Moser”, R. RichterU, A. Rosado-Schlosser”, S. Schael”, R. Settles “, H. Seywerd “, R. St. Denis “, H. Stenzel”, W. Wiedenmann”, G. Wolf”, J. Boucrot “, 0. Callot “,

A. Cordier”, M. Davier “, L. Duflot “, J.-F. Grivaz”, Ph. Heusse “, A. Hacker “, A. Jacholkowska”, M. Jacquet “, D.W. Kimv>19, F. Le Diberder”, J. Lefran~oisV,

A.-M. Lutz”, I. Nikolic”, H.J. Park”,19, M.-H. Schune”, S. Simian”, J.-J. Veillet’, I. Videau “, D. Zerwas “, 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, A. Giassi w, A. Gregorio w, F. Ligabue w, A. Lusiani w,

P.S. Marrocchesi w, A. Messineo w, F. Palla w, G. Rizzo w, G. Sanguinetti w, A. Sciaba w, P. Spagnolo w, J. Steinberger w, R. Tenchini w, G. Tonelli w,26, C. Vannini w, P.G. Verdini w,

J. WalshW, G.A. BlairX, L.M. Bryant”, F. Cerutti ‘, J.T. Chambers ‘, Y. Gao x, M.G. Green x, T. Medcalf ‘, P. PerrodoX, J.A. Strong’, J.H. von Wimmersperg-Toeller ‘, D.R. Botterill Y,

R.W. Clifft Y, T.R. Edgecock Y, S. Haywood Y, P Maley Y, P.R. Norton Y, J.C. Thompson Y, A.E. Wright Y, B. Bloch-Devaux ‘, P. Colas ‘, S. Emery ‘, W. Kozanecki ‘, E. LanGon z,

M.C. Lemaire ‘, E. Locci ‘, B. Marx z, l? Perez ‘, J. Rander z, J.-F. Renardy z, A. Roussarie z, J.-P. Schuller ‘, J. Schwindling”, A. Trabelsi ‘, B. Vallage ‘, S.N. Blackaa, J.H. Dannaa, R.P. Johnson aa, H.Y. Kima”, A.M. Litkeaa, M.A. McNeil%, G. Taylora”, C.N. Boothab, R. Boswellab, C.A.J. Brewab, S. Cartwrightab, F. Combleyab, A. Koksalab, M. Lethoab, W.M. Newtonab, J. Reeve ab, L.F. Thompson ab, A. Bbhrer ac, S. Brandt ac, G. Cowanac,

C. Grupenac, P. Saraiva”, L. Smolikac, F. StephanaC, M. Apollonioad, L. Bosisio ad, R. Della Marinaad, G. Giannini ad, B. Gobbo ad, G. Musolino ad, J. Putz =, J. Rothberg ae,

S. Wasserbaechae, R.W. Williamsa”, S.R. Armstrongaf, P. Elmeraf, Z. Fengaf,12, D.P.S. Ferguson af, Y.S. Gao af,23, S. Gonzalez &, J. Grahl af, T.C. Greening &, O.J. Hayes &, H. Huaf, PA. McNamara IITaf, J.M. Nachtmanaf, W. Orejudos af, Y.B. Panaf, Y. Saadiaf,

I.J. Scottaf, A.M. Wa.lshaf,27, Sau Lan Wu af, X. Wuaf, J.M. Yamartinoaf, M. Zheng af, G. Zobemig af

a Labomtoire de Physique des Particules (LAPP), IN2P3-CNRS, 74019 Annecy-le-Weux Cedex, France b Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain’

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

ALEPH Collaborafion/ Physics Letters B 384 (1996) 439-448 44

e European Laboratory for Particle Physics (CERN), 1211 Geneva 23, Switzerland f Laboratoire de Physique Corpuscutaire, Vniversite’ Blaise Pascal, lN2P3-CNRS, Clermont-Ferrand, 63177 Aubiere, France

g Niels Bohr Institute, 2100 Copenhagen, Denmark9 h Nuclear Research Center Demokritos (NRCD), Athens, Greece

i Laboratoire de Physique Nucleaire et des Hautes Energies, Ecole Polytechnique, lNZP3.CNRS, 91128 palajseau C&x, France

i Dipartimento di Fisica, Vniversita di Firenze, INFN Sezione di Firenze, 50125 Firenze, Italy k Supercomputer Computations Research Institute, Florida State University, Tallahassee, FL 32306-4052, VSA l3.14

e Laboratori Nazionali dell’lNFN (LNF-INFN), 00044 Frascati, Italy m Department of Physics and Astronomy, University of Glasgow, Glasgow GI2 8&Q, United Kingdom ‘0

n lnstitut fir Hochenergiephysik, Universitlit Heidelberg, 69120 Heidelberg, Germany16 a Department of Physics, Imperial College, London SW7 2BZ, United Kingdom lo

P lnstitut fir Experimentalphysik, Universitiit Innsbruck, 6020 Innsbruck, Austria l8 4 Department of Physics, University of Lancaster Lancaster LA1 4YB, United Kingdom lo

t lnstitut fiir Physik, Vniversitiit Mainz. 55099 Mainz, Germany16 ’ Centre de Physique des Particules, Fact&e’ des Sciences de Luminy, lN2P3-CNRS, 13288 Marseille, France

t Dipartimento di Fisica. Universita di MiZano e INFM Sezione di Milano, 20133 Milano, Italy. u Max-Planck-lnstitut fur Physik, Werner-Heisenberg-lnstitut, 80805 Miinchen, Germany I6

v Laboratoire de I’ttcce’le’rateur Line’aire, Universite’ de Paris-Sud, lN2P3CNRS, 91405 Orsay Cedex, France w Dipartimento di Fisica dell’llniversitri, INFN Sezione di Pisa, e Scuola Normale Saperiore, 56010 Pisa, Italy

x 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 I OQX, United Kingdom lo

Z CEA, DAPNbVService de Physique des Particules, CE-Saclay, 91 I91 Gif-sur-Yvette Cedex, France l7

aa Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA” ab Department of Physics, University of Shefield, Sheffield S3 7RH, United Kingdom lo

ac Fachbereich Physik, Vniversitiit Siegen, 57068 Siege& Germany I6 * Dipartimento di Fisica, Universita di Trieste e 1NFN 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 I1

Received 19 June 1996 Editor: K. Winter

Abstract

A search for pair production of new heavy leptons has been performed assuming different scenarios for the mixing of the new particles with Standard Model leptons. No candidate events were found in a data sample corresponding to an integrated luminosity of 5.6 pb-’ collected by the ALEPH detector at centre-of-mass energies of 130 and 136 GeV. New limits on production cross-sections and on masses of sequential leptons were obtained which significantly extend the mass regions excluded at LEPl. For instance, charged heavy leptons with masses below 63.5 GeV/c2 are excluded at 95% C.L. for mass differences to the associated neutral lepton of more than 7 GeV/c2.

’ Now at DESY, Hamburg, Germany. v Supported by the Danish Natural Science Research Council.

2 Supported by Direction General de Investigation Cientifica y lo Supported by the UK Particle Physics and Astronomy Research

Tecnica, Spain. Council.

s Now at Dipartimento di Fisica, Universim di Lecce, 73100

Lecce, Italy.

l1 Supported by the US Department of Energy, grant DE-FG0295

ER40896.

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

Italy.

l2 Now at The Johns Hopkins University, Baltimore, MD 21218,

USA.

5 Also Istituto di Cosmo-Geofisica de1 C.N.R., Torino, Italy. 6 Supported by the Commission of the European Communities,

contract ERBCHBICT941234. 7 Supported by CICYT, Spain. 8 Supported by the National Science Foundation of China.

I3 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,


1. Introduction

A variety of models suggest the existence of ad-

ditional leptons beyond the three known generations of the Standard Model [ 11. They can occur as weak

isosinglets or in new isodoublets, of which the sim- plest example is a fourth family of heavy leptons with standard quantum numbers (sequential leptons) . The existence of mirror fermions, i.e. new fermions with righthanded weak isodoublets and lefthanded isosinglets, is proposed in models restoring left-right sym-

metry. Singlet leptons are predicted in a number of extensions of the Standard Model, two well known of which are the models based on SO( 10) or the super-

string inspired E6 group. Assuming a non-suppressed coupling to the Z-

boson, all these new leptons have already been re- stricted by measurements of the partial Z-widths to have masses above rnz /2 [ 21. Precision electroweak measurements indicate that additional generations are allowed only if almost mass degenerate, and further- more suggest that more than one such generation is unlikely [ 31. Therefore there is room for additional weak isodoublets or isosinglets at masses above 45 GeV/c*.

The successful running of LEP at energies of 130

and 136 GeV at the end of 1995 (referred to as LEP1.5) opens new possibilities to search for such new leptons [ 41. This analysis covers many of the predictions discussed above by searching for the pair production of new heavy leptons and taking into ac-

Spain.

l6 Supported by the Bundesministerium fiir Forschung und Tech- nologie, Germany.

l7 Supported by the Direction des Sciences de la Mat&e, C.E.A.

I8 Supported by Fonds zur Fiirderung der wissenschaftlichen Fomchung, Austria.

I9 Permanent address: Kangnung National University, Kangnung,

Korea.

‘O Now at CERN, 1211 Geneva 23, Switzerland. ** Also at CERN, 1211 Geneva 23, Switzerland. ** Supported by the US Department of Energy, grant DE-FG03- 92ER40689.

23 Now at Harvard University, Cambridge, MA 02138, USA. 24 Now at Max-Plank-Instittit fur Kernphysik, Heidelberg, Germany.

zs Now at Dragon Systems, Newton, MA 02160, USA. 26 Also at Istituto di Matematica e Fisica, Universita di Sassari, Sassari, Italy,

27 Now at Rutgers University, Piscataway, NJ 08855-0849, USA.

count a possible mixing between the new leptons and the Standard Model leptons.

2. Decay channels and cross sections

The production of heavy leptons is dominated by reactions containing the s-channel exchange of a virtual Z/y (pair production). Single or pair production of neutral heavy leptons through virtual W-exchange

in the t-channel is also possible, provided the new lepton mixes with the electron. The mixing enters the cross-section for single (pair) production as an additional factor I&T/~ ( \V,~V]~). The square of the mixing matrix element Vex is bound from low-energy data to

values less than 0.005 [ 51. Given the small amount of data collected at LEPl.5, this limit cannot be improved [ 61. For this reason, the present analysis concentrates on the search for pair production of new heavy leptons. Three different scenarios for the heavy lepton decay modes are considered:

(i>

(ii)

(iii)

Pair production of charged heavy leptons (I,) with subsequent charged current decay into neutral heavy leptons (N), which have a lifetime

long enough to escape undetected (i.e. /VI2 5 lo-“). This channel (L -+ NW*) will be re-

ferred to throughout the paper by the symbolic notation L --f N. Pair production of charged heavy leptons decaying via mixing into light neutrinos (L -+ qW*, referred to as L + ZQ> . The decay length of the charged lepton is assumed to be less than 1 cm, limiting the sensitivity for this scenario to 1 VI2 2 lo-lo. This covers the several orders of magni- tude to the upper limit of 0.02, valid for mixing

between new leptons and any known lepton [ 51. Pair production of neutral heavy leptons with an arbitrary admixture of the three light flavours (N + lW*, referred to as N + I). As above, the neutral heavy lepton is expected to decay within 1 cm ( IV12 2 lo-“).

Cascade decays (N + L -+ vl or L + N + 1) are not considered, as the mass limits set in this analysis are already close to the kinematical limit independent of the mass of the isopartner.

The cross sections expected for sequential leptons were calculated as a function of the particle masses. For heavy neutral leptons both Dirac and Majorana

ALEPH Collaboration/Physics Letters B 384 (1996) 439-448 443

neutrino types were considered, the main difference being the dependence of the cross section on the ve- locity /3 = dw of the heavy lepton:

where cro is the standard neutrino cross section ( CTO = 4.4 pb at fi = 136 GeV). Initial state radiation has

been taken into account by means of the REMT package [7], modified to account for the a2 part of the spectrum. In general, these cross sections are relatively

large (i.e. a few pb) up to the kinematical limit.

3. The ALEPH detector

A thorough description of the ALEPH detector can be found in Ref. [8], and an account of its performance as well as a description of the standard analysis

algorithms in Ref. [ 91. Briefly, the tracking system consists of a newly in-

stalled silicon vertex detector, a cylindrical drift chamber and a large time projection chamber (TPC), all situated in a 1.5 T magnetic field provided by a super- conducting solenoidal coil. Between the TPC and the

coil, a highly granular electromagnetic calorimeter is used to identify electrons and photons and to measure

their energy. Complemented by luminosity calorime- ters, the coverage is hermetic down to 24 mrad from the beam axis. The iron return yoke is instrumented to provide a measurement of the hadronic energy and, together with external chambers, muon identification.

All this information is combined in an energy flow algorithm [9] which supplies the analysis programs with a list of objects, categorised as charged particles, of which some are identified electrons and muons, as photons and as neutral hadrons.

4. Signal and background simulations

To design selection criteria and evaluate efficiencies, signal events were generated with the PYTHIA 5.7 [ lo] Monte Carlo at mass points between 45 GeV/c2 and the kinematical limit of 68 GeV/c2. This generator simulates pair production of heavy leptons in Z/y-exchange. The leptons are

then allowed to decay via charged current either to their isopartners or to the Standard Model fermions.

A large amount of these signal events was processed through the full detector simulation and reconstruc-

tion programs, especially in the mass regions close to the resulting limit. Efficiencies at intermediate points were determined by interpolation using a simplified detector simulation.

Samples of all the major background processes have been generated using the full detector simu-

lation, whereby the number of background events corresponds to at least five times the integrated luminosity. The PYTHIA package was used to simulate all processes e+e- + ff(r) excluding the electron channel, which was treated separately with the UNI- BAB program [ 111. Background from four-fermion final states was produced using PYTHIA and the FEMSV generator [ 121, supplemented by a sample of W-pairs generated with the LPWW02 package [ 131. The PHGTO2 event generator [ 141 was used to simulate all backgrounds from yy-interactions,

5. Selection criteria

The most discriminating signature of both scenarios containing neutral leptons in the final state (i.e. L -+ N and L --+ zq) is the missing energy carried away by the undetected neutrinos. The third scenario (N --+ 1) is characterised by two isolated leptons plus the decay products of the two virtual W-bosons. To account for

the different signatures and W-decay modes, i.e. two hadronic (h-h), one hadronic and one leptonic (h- 1) as well as two leptonic W-decays (l-l), different sets of cuts were optimised for the various final state topologies.

5.1. Charged lepton decaying into heavy, stable neutrino

As the two neutral heavy leptons escape undetected, the very distinct signature of this scenario is large missing energy. In the limit of a vanishing mass difference between charged and neutral heavy lepton (Am = rnL - mN>, the signal efficiency is limited by the trigger efficiency and the overwhelming background from yy-events. To extend the search to the smallest possible mass difference, cuts were optimised sepa-

444 ALEPH Collaboration/ Physics Letters B 384 (1996) 439-448

Table 1

Cuts applied to select heavy charged leptons decaying into stable

heavy neutral leptons. There are four different sets of cuts opti-

mised for different W-decay modes and mass differences between

charged and neutral lepton. No energy within 12“ around the beam

is required for all topologies; the variables are defined in the text.

W-decay modes Arn = rrrr - “ZN

h-h h-l 1-l h-h, h-l

large all small

MviSI 6 0.10-0.25 0.04-0.25 0.01-0.25 0.01-0.10

N+Lls& >4 >2 =2 >2

py (GeV/c) >4 >3 >4 >3

EJ_ ( GeV) > 15 > 12 >4

@miss > 2o” > 4o”

pp ( GeV/c) < 15 < 10 <5

Q $&t ( GeV,c)

< 165’ < 150° < 160“

> 1.2 > 0.5 > 2.1

Nl,pton 21

Single 7 ( GeV) < 10

rately for small mass differences (most efficient for Am < 10 GeV/c2) and large mass differences (Am > 10 GeV/c2).

Taking into account the different decay modes of the two virtual W-bosons (cf. previous paragraph),

this results in six different final state topologies. For small mass differences it was found to be sufficient not to search explicitly for leptons in the h-l topologies, so that the two cases h-l and h-h could be combined into a single search. Similarly, a single set of cuts was developed to search for two leptonic W-decays independent of the mass difference. The cuts designed to select signal events in these four topologies are summarised in Table 1 and will be described in the following paragraphs.

After restricting the visible mass to a certain range, which is defined according to the mass difference and the number of neutrinos in the final state, the remaining background is dominated by yy-events. Although the optimal positions of the cuts vary with final state topology and mass difference, the quantities used to further suppress background are very similar for all the four topologies (cf. Table 1). The special case of two leptonically decaying W-bosons is treated separately in the sense that here at least one electron or muon must have been identified and no photons with energies above 10 GeV are allowed in the event.

Exploiting the hermeticity of the detector, most of the yy-background can be rejected by demanding

missing transverse momentum pp in combination with the requirement that no objects be detected in a cone around the beam with a half-angle of 12’. After requiring a minimum transverse energy El, defined as the scalar sum of the transverse momenta of all objects, further suppression is obtained by cutting on the direction of the missing momentum: a minimum

angle @miss between beam and missing momentum vector as well as a maximum missing momentum component in beam direction ppss rejects background

from yy-events with no significant efficiency loss for

the signal. The background left at this point comes from yy ---f

rr and yy -+ CC events, which pass the initial selection because of the additional missing momentum car-

ried away by neutrinos from r- or c-decays. As this missing momentum is always close to detected particles, two more cuts exploiting this characteristic are applied. The particles detected in an event are clus- tered using the Durham algorithm [ 1.51 to exactly two jets, which are then projected onto the plane transverse to the beam direction. The minimum Durham distance k’;iss’@ between the missing transverse momentum vector and the two projected jet momenta is

calculated. To define the projected acoplanarity an-

gle @‘aC0p a thrust axis is determined with the transverse momenta of all charged particles and then used to di- vide the event into two hemispheres. Q,, is defined as the angle between the two hemisphere momenta in the transverse plane. Requiring a minimum k$ssjet and a maximum projected acoplanarity removes most of the remaining background.

Searching for the scenario L -+ N, events are selected if they satisfy at least one of these sets of cuts.

5.2. Charged lepton decaying into light neutrino

For this scenario, a single set of cuts was designed to search for the topology with two hadronic W-decays as well as one hadronic and one leptonic W-decay. The third possibility of two leptonic W-decays is treated with a second set of cuts.

Since in this case the neutrinos escaping undetected are Standard Model neutrinos with zero mass, the amount of missing energy is somewhat smaller compared to the previous scenario. Therefore, background from radiative returns to the Z becomes more impor- tant.

ALEPH Collaboration/ Physics Lefters B 384 (1996) 439-448 445

b) I

- - 10 10 90 loo 110 120 130 MO 150 160 170 180 0 10 20 30 40 5o 60

ky (Gev)

b io i5 io 25 30

Fig. 1. Distribution of discriminating variables k’fss, Waco, k’?“” and ~4 (see text) : points represent data, the histogram is the Monte Carlo prediction with absolute normalisation, and the hatched histogram shows the signal distribution with arbitrary normalisation. Only a subset of cuts has been applied to increase statistics. Under- and overflow are added to the first and last bin contents, respectively. The arrow indicates the position of the cut used to select L --f IQ (a, b) and N --f I (c, d) topologies.

Searching for the two topologies h-h and h-l, signal events should have at least five charged tracks and a visible mass between 20%,,& and 8O%fi. Back- ground from both ‘yy- and QCD-events can be suppressed by requiring no detected objects within 12” around the beam axis and a minimum transverse imbalance of 17%. The transverse imbalance is defined as the ratio of missing transverse momentum and total visible energy. Radiative returns to the Z are further suppressed by rejecting events containing a photon with an energy of more than 25 GeV as well as events with missing momentum component in beam direction of more than 40 GeV/c. The background left after these cuts comes from b- or c-quark pair production (with subsequent semileptonic decays) as well as rr- events. As in the previous section, the r-background is suppressed by requiring a projected acoplanarity angle of less than 165”. This cut is applied in low mul-

tiplicity events only (less than eight charged tracks). Heavy quark production is rejected by requiring the missing momentum to be isolated: there should be no energy flow object within 14” around the missing momentum vector, and the transverse missing momentum should have a component Izp of at least 10 GeV/c transverse to the closest projected jet momentum (cf. Fig. la).

In the search for heavy charged leptons decaying into four neutrinos and two leptons, events with visible masses between 5%& and 60%& are selected. The event should contain exactly two charged tracks with the sum of their energies exceeding 70% of the visible energy. At Ieast one of the tracks should be identified as an electron or muon. Background from yy- interactions is then suppressed effectively by requiring a minimum transverse energy of 12 GeV. Most of the remaining background from 7r is rejected as described

446 ALEPH Collaboration/ Physics Letters B 384 (1996) 439-448

in Section 5.1: the events should have a projected acoplanarity angle of less than 165” (cf. Fig. lb) and missing momentum k q,, transverse to the closest jet of more than 10 GeV/c.

5.3. Neutral lepton decaying into light, charged lepton

Common background to all topologies in this scenario are radiative returns to the Z. The presence of the energetic ISR photon is used to reject most of this

background: events containing photons with energy greater than 25 GeV and events with an angle @miss between missing momentum and beam axis of less

than 17” are not selected. A set of cuts was designed to search for neutral

heavy lepton decays with one hadronic and one leptonic W-decay, with the goal of retaining substantial efficiency for the case of two hadronic W-decays, too. These events should have at least seven charged tracks and visible masses in the range 20%,/X to 95%&. After determining the isolation angle between identified electrons or muons and their closest object, events are selected if the sum of the momentum components

ky transverse to the closest object, calculated for the two most isolated leptons, exceeds 5.5 GeV/c (cf. Fig. lc). Here neutral objects within a cone of 3” around the lepton momentum are assumed to be bremsstrahlung photons and therefore ignored.

The background left after these cuts consists of four- fermion final states with two quarks and two leptons. As two of these fermions usually form a low mass pair, requiring a minimum separation between all fermions suppresses this background. This is done by requiring the differential 3-jet-rate y4 (as calculated with the Durham algorithm) to be greater than 0.006 (cf.

Fig. Id). In addition the event should satisfy the con- dition kysS > 2 GeV/c (cf. previous sections) if the visible mass exceeds 85%&

Searching for neutral leptons in the l-l topology, events with visible masses between lo%& and SO%& are selected. There should be at least three, but not more than 12 charged tracks, with a total energy of more than 50% of the visible energy. Most of the background is then rejected by looking for isolated electrons or muons, requiring that the sum of the two largest isolation angles be greater than 25”, again ignoring bremsstrahlung within a cone of

Table 2

Selection efficiencies (in %) for masses (in GeV/s) close to

the limits set in this analysis.

Process

L-+N

L + VI

N’7

N+e,p

fnL lnN

65 60

55 50 65 50

65 -

- 65 - 6.5

Efficiency

16% 1

2011

58f2

61 f 1

603x2

69 z!c 1

3” as explained above. Remaining background with

r-leptons in the final state is suppressed by selecting events with a projected acoplanarity angle of less than 175’ (less than 170” if the visible mass is below

55%fi) and cutting out events where the sum of the invariant masses of the two jets (cf. Section 5.1) is below 16 GeV/c2.

6. Results

The efficiencies corresponding to the search strate- gies described in the previous section are shown in Table 2. In addition to the statistical contribution of up to 2.5%, the errors reflect small systematic uncer- tainties from lepton identification and missing-energy resolution.

Using the selection criteria described in Section 5, no candidate events were found in the ALEPH data taken in the high energy runs for integrated luminosities of about 2.9 pbb’ at 130 GeV and 2.7 pb-r at 136 GeV. This is consistent with the expectations for the background of 1.9 f 0.4( stat.) events, summed

over all channels. Most of these events come from four-fermion processes and yy-interactions. Uncer- tainties inherent to the PHOTO2 generator have not

been taken into account.

6.1. Limits on cross sections

Assuming Poisson statistics for the number of expected events and conservatively reducing the efficiencies and luminosities by one standard deviation, the maximal signal cross-sections consistent with the data at 95% confidence level were determined. These cross-

ALEPH Collaboration/Physics Letters B 384 (1996) 439-448 447

mL (Gev) mL WV) Fig. 2. Limits on the cross section for pair production of charged leptons decaying into a) stable heavy neutrinos and b) Standard Model

neutrinos; cross sections above the limits are excluded at 9.5% confidence level. The shaded area is forbidden kinematically. The dotted

curve in b) shows the cross section expected for a new sequential charged Iepton.

- ALEPHlimit ......I Dime (sequeatlal) - MajoraM (seqoentlp1)

Fig. 3. Limits on the cross section for pair production of neutral

leptons decaying into r-leptons (upper solid line) or into electrons

or muons (lower solid line); cross sections above the limits are

excluded at 95% confidence level. The dashed and dotted curves represent the expectations for sequential Dirac and Majorana neu-

trinos, respectively.

sections are calculated at a centre-of-mass energy of 136 GeV. The data taken at 130 GeV were included using the Standard Model prediction for the energy evolution of the cross-section. The limits extracted in this way are presented in Figs. 2 and 3 and compared to the cross-sections expected for a fourth generation of heavy leptons.

For neutral heavy leptons decaying into light charged leptons, the limits are shown separately for decays into r-Ieptons and decays into electrons or muons (Fig. 3).

6.2. Mass limits

Using the cross-sections calculated in the Stan-

dard Model extended by a fourth generation (cf. Section 2), mass limits have been derived from the non-observation of candidate events:

(i)

(ii)

(iii)

The production of charged heavy Ieptons that decay into stable neutral heavy leptons is consistent with the data at 95% confidence level for

mL > 63.5 GeV/c2 (for /VI2 5 10-12,

PnL - mN > 7 GeV/c2).

Limits for charged heavy leptons decaying into Standard Model neutrinos were extended to

mL > 65 GeV/c2 (for IV12 2 lo-“).

Pair production of heavy Dirac neutrinos mixing with taus (electrons or muons) has been con- strained at 95% confidence level to

mN > 63 GeV/c2 (63.6 GeV/c2)

for /VI2 2 10-t*.

The production of Majorana neutrinos is consistent with the data for

mN > 54.3 GeV/c2 (55.2 GeV/c2)

for (VI* 2 lo-lo.

The mass region excluded in the context of the first scenario (L -+ N) is displayed in the plane mL versus mN in Fig. 4.


55

50

q, (Gev)

Fig. 4. Mass limit for the charged lepton in a new sequential doublet

as derived from the search for L + N. The area within the hatched

border is excluded at 95% confidence level for IV1’ 5 lo-I*.

The shaded region is kinematically forbidden.

7. Conclusions

The data taken by ALEPH in the fall of 1995 were used to search for the pair production of new heavy leptons. From the non-observation of candidate events, limits on cross-sections and on the masses of new sequential leptons have been extracted. These limits significantly extend the mass region excluded by measurements at LEPI and are more constraining than those recently published by the L3 Collaboration [ 161.

Acknowledgements

We are grateful to D. Choudhury for useful dis- cussions. We wish to thank and congratulate our col- leagues from the accelerator divisions for having been so fast and efficient in bringing up and operating LEP in this new energy regime. We are indebted to the en- gineers and technicians in all our institutions for their contribution to the excellent performance of ALEPH. Those of us from non-member states thank CERN for its hospitality.

References

[ 1 J For a recent review see e.g.: A. Djouadi, .I. Ng and T.G. Rizzo, New Ptiicles and Interactions, SLAC-PUB-95-6772,

to appear as a chapter in Electroweak Symmetry Breaking

and Beyond the Standard Model, edited by T. Barlow et al.,

World Scientific.

[2] ALEPH Collab., D. Decamp et al., Phys. Lett. B 236 (1990)

511;

DELPHI Collab., P. Abreu et al., Phys. Lett. B 274 (1992)

230;

L3 Collab., B. Adeva et al., Phys. Lett. B 251 (1990) 321;

OPAL Collab., M.Z. Akrawy et al., Phys. Lett. B 247 ( 1990)

448.

[31 P. Langacker, Precision Experiments, Grand Unification, and

Compositeness, Proc. of SUSY’95, hep-ph/9511207.

I41 G. Bhattachatyya and D. Choudhury, Nucl. Phys. B 468

(1996) 59.

[51 E. Nardi, E. Roulet and D. Tommasini, Phys. Lett. B 344

(1995) 225.

[61 ALEPH Collab., D. Buskulic et al., Search for excited leptons in ALEPH at 130-140GeV, in preparation.

[71 EA. Bercnds and R. Kleiss, Nucl. Phys. B 260 (1985) 32.

[ 81 ALEPH Collab., D. Decamp et al., Nucl. Instr. Meth. A 294 (1990) 121.

[91 ALEPH Collab., D. Buskulic et al., Nucl. Instr. Meth. A 360 (1995) 481.

[ 101 T. Sjiistrand, Comput. Phys. Comm. 82 (1994) 74.

[ 11 I H. Anlauf et al., Comput. Phys. Comm. 79 (1994) 466.

[ 121 J.M. Hilgart, R. Kleiss and F. Le Diberder, Comput. Phys.

Comm. 75 (1993) 191.

[ 131 R. Miquel and M. Schmitt, Four-Fermion Production

Through Resonating Boson Pairs, CERN-PPE/95-109, to

appear in Z. Phys. C.

[ 141 ALEPH Collab., D. Buskulic et al., Phys. Lett. B 313 (1993)

509.

[151 Report of Hard QCD Working Group, in Proc. Durham

Workshop on Jet Studies at LEP and HERA, J. Phys. G17

(1991) 1537.

[ 161 L3 Collab., M. Acciarri et al., Search for Unstable Sequential Neutral and Charged Heavy Leptons in e+e- Annihilation

at fi = 130 and 136 GeV, CERN-PPE/96-38, submitted to Phys. Lett. B.

Documents

Search for heavy lepton pair production in e+e− collisions at centre-of-mass energies of 130 and 136 GeV