application/xmlSearch for scalar leptons and scalar quarks at LEPL3 CollaborationP. AchardO. AdrianiM. Aguilar-BenitezJ. AlcarazG. AlemanniJ. AllabyA. AloisioM.G. AlviggiH. AnderhubV.P. AndreevF. AnselmoA. ArefievT. AzemoonT. AzizP. BagnaiaA. BajoG. BaksayL. BaksayS.V. BaldewS. BanerjeeSw. BanerjeeA. BarczykR. BarillèreP. BartaliniM. BasileN. BatalovaR. BattistonA. BayF. BecattiniU. BeckerF. BehnerL. BellucciR. BerbecoJ. BerdugoP. BergesB. BertucciB.L. BetevM. BiasiniM. BigliettiA. BilandJ.J. BlaisingS.C. BlythG.J. BobbinkA. BöhmL. BoldizsarB. BorgiaS. BottaiD. BourilkovM. BourquinS. BracciniJ.G. BransonF. BrochuJ.D. BurgerW.J. BurgerX.D. CaiM. CapellG. Cara RomeoG. CarlinoA. CartacciJ. CasausF. CavallariN. CavalloC. CecchiM. CerradaM. ChamizoY.H. ChangM. ChemarinA. ChenG. ChenG.M. ChenH.F. ChenH.S. ChenG. ChiefariL. CifarelliF. CindoloI. ClareR. ClareG. CoignetN. ColinoS. CostantiniB. de la CruzS. CucciarelliJ.A. van DalenR. de AsmundisP. DéglonJ. DebreczeniA. DegréK. DehmeltK. DeitersD. della VolpeE. DelmeireP. DenesF. DeNotaristefaniA. De SalvoM. DiemozM. DierckxsensC. DionisiM. DittmarA. DoriaM.T. DovaD. DuchesneauM. DudaB. EchenardA. ElineA. El HageH. El MamouniA. EnglerF.J. EpplingP. ExtermannM.A. FalaganS. FalcianoA. FavaraJ. FayO. FedinM. FelciniT. FergusonH. FesefeldtE. FiandriniJ.H. FieldF. FilthautP.H. FisherW. FisherI. FiskG. ForconiK. FreudenreichC. FurettaYu. GalaktionovS.N. GanguliP. Garcia-AbiaM. GataullinS. GentileS. GiaguZ.F. GongG. GrenierO. GrimmM.W. GruenewaldM. GuidaR. van GulikV.K. GuptaA. GurtuL.J. GutayD. HaasD. HatzifotiadouT. HebbekerA. HervéJ. HirschfelderH. HoferM. HohlmannG. HolznerS.R. HouY. HuB.N. JinL.W. JonesP. de JongI. Josa-Mutuberrı́aD. KäferM. KaurM.N. Kienzle-FocacciJ.K. KimJ. KirkbyW. KittelA. KlimentovA.C. KönigM. KopalV. KoutsenkoM. KräberR.W. KraemerA. KrügerA. KuninP. Ladron de GuevaraI. LaktinehG. LandiM. LebeauA. LebedevP. LebrunP. LecomteP. LecoqP. Le CoultreJ.M. Le GoffR. LeisteM. LevtchenkoP. LevtchenkoC. LiS. LikhodedC.H. LinW.T. LinF.L. LindeL. ListaZ.A. LiuW. LohmannE. LongoY.S. LuC. LuciL. LuminariW. LustermannW.G. MaL. MalgeriA. MalininC. MañaJ. MansJ.P. MartinF. MarzanoK. MazumdarR.R. McNeilS. MeleL. MerolaM. MeschiniW.J. MetzgerA. MihulH. MilcentG. MirabelliJ. MnichG.B. MohantyG.S. MuanzaA.J.M. MuijsB. MusicarM. MusyS. NagyS. NataleM. NapolitanoF. Nessi-TedaldiH. NewmanA. NisatiT. NovakH. NowakR. OfierzynskiG. OrgantiniI. PalC. PalomaresP. PaolucciR. ParamattiG. PassalevaS. PatricelliT. PaulM. PauluzziC. PausF. PaussM. PedaceS. PensottiD. Perret-GallixB. PetersenD. PiccoloF. PierellaM. PioppiP.A. PirouéE. PistolesiV. PlyaskinM. PohlV. PojidaevJ. PothierD. ProkofievJ. QuartieriG. Rahal-CallotM.A. RahamanP. RaicsN. RajaR. RamelliP.G. RancoitaR. RanieriA. RasperezaP. RazisD. RenM. RescignoS. ReucroftS. RiemannK. RilesB.P. RoeL. RomeroA. RoscaS. Rosier-LeesS. RothC. RosenbleckJ.A. RubioG. RuggieroH. RykaczewskiA. SakharovS. SaremiS. SarkarJ. SalicioE. SanchezC. SchäferV. SchegelskyH. SchopperD.J. SchotanusC. SciaccaL. ServoliS. ShevchenkoN. ShivarovV. ShoutkoE. ShumilovA. ShvorobD. SonC. SougaP. SpillantiniM. SteuerD.P. SticklandB. StoyanovA. StraessnerK. SudhakarG. SultanovL.Z. SunS. SushkovH. SuterJ.D. SwainZ. SzillasiX.W. TangP. TarjanL. TauscherL. TaylorB. TelliliD. TeyssierC. TimmermansSamuel C.C. TingS.M. TingS.C. TonwarJ. TóthC. TullyK.L. TungJ. UlbrichtE. ValenteR.T. Van de WalleR. VasquezV. VeszpremiG. VesztergombiI. VetlitskyD. VicinanzaG. ViertelS. VillaM. VivargentS. VlachosI. VodopianovH. VogelH. VogtI. VorobievA.A. VorobyovM. WadhwaQ. WangX.L. WangZ.M. WangM. WeberP. WienemannH. WilkensS. WynhoffL. XiaZ.Z. XuJ. YamamotoB.Z. YangC.G. YangH.J. YangM. YangS.C. YehAn. ZaliteYu. ZaliteZ.P. ZhangJ. ZhaoG.Y. ZhuR.Y. ZhuH.L. ZhuangA. ZichichiB. ZimmermannM. ZöllerPhysics Letters B 580 (2004) 37-49. doi:10.1016/j.physletb.2003.10.010journalPhysics Letters BCopyright © unknown. Published by Elsevier B.V.Elsevier B.V.0370-26935801-229 January 20042004-01-2937-49374910.1016/j.physletb.2003.10.010http://dx.doi.org/10.1016/j.physletb.2003.10.010doi:10.1016/j.physletb.2003.10.010http://vtw.elsevier.com/data/voc/oa/OpenAccessStatus#Full2014-01-01T00:14:32ZSCOAP3 - Sponsoring Consortium for Open Access Publishing in Particle Physicshttp://vtw.elsevier.com/data/voc/oa/SponsorType#FundingBodyhttp://creativecommons.org/licenses/by/3.0/

Journals

S300.2

PLB20326S0370-2693(03)01554-510.1016/j.physletb.2003.10.010Experiments

Fig. 1

Distributions in data and MC of the energy of the most energetic lepton of the (a) scalar lepton searches and (b) single electron analysis. (c) Visible energy and (d) b-tag variable for the scalar quark analysis. Signal events are scaled by the factors indicated in the figures and correspond to (a) Mℓ̃R=90 GeV and Mχ̃10=40 GeV, (b) MẽL=110 GeV and Mχ̃10=50 GeV, (c) and (d) t̃1→cχ̃10 decay for Mt̃R=90 GeV, Mχ̃10=60 GeV.

Fig. 2

Model independent upper limits on the e+e−→ℓ̃Rℓ̃̄R cross section in the Mχ̃01–Mℓ̃ plane, for (a) scalar electrons, (b) scalar muons and (c) scalar taus.

Fig. 3

Model independent upper limits on the (a), (b) and (c) e+e−→t̃1t̃̄1 and (d) e+e−→b̃1b̃̄1 production cross sections multiplied by the branching ratio of the decay mode: (a) t̃1→cχ̃10, (b) t̃1→bℓν̃, (c) t̃1→bτν̃ and (d) b̃1→bχ̃10.

Fig. 4

Regions of the plane Mχ̃10–Mℓ̃R excluded in the MSSM for (a) scalar electrons, (b) scalar muons and (c) scalar taus.

Fig. 5

Absolute ẽR mass limit as a function of (a) tanβ and (b) m0.

Fig. 6

Regions excluded in the planes (a), (b) and (c) Mχ̃10–Mt̃1 and (d) Mχ̃10–Mb̃1. The MSSM decay modes: (a) t̃1→cχ̃10, (b) t̃1→bℓν̃, (c) t̃1→bτν̃ and (d) b̃1→bχ̃10 are studied. Different values of the mixing angles are considered.

Fig. 7

(a) MSSM exclusion limits in the Mχ̃10–Mq̃ plane for degenerate scalar quarks decaying via q̃→qχ̃10. (b) Excluded regions in the Mg̃–Mq̃ plane. The dark shaded area is excluded by the search for scalar quarks of the first two families, assuming mass degeneracy among different flavours and between left- and right-handed scalar quarks. The light shaded area illustrates indirect limits on the gluino mass, derived from the chargino, neutralino and scalar lepton searches. The regions excluded by the CDF and D0 Collaborations [8] are valid for tanβ=4 and μ=−400 GeV. The exclusions obtained by the UA1 and UA2 Collaborations [28] are also shown.

Table 1

Summary of the investigated processes, decay modes and studied topologies

ProcessDecay modeTopologye+e−→ℓ̃Rℓ̃̄Rℓ̃R→χ̃10ℓAcoplanar leptonse+e−→ẽRẽLẽL,R→χ̃10eSingle electrone+e−→b̃b̃̄b̃→χ̃10bAcoplanar b-jetse+e−→t̃t̃̄t̃→χ̃10cAcoplanar jetse+e−→t̃t̃̄t̃→ν̃bℓAcoplanar jets and leptonse+e−→q̃q̃̄q̃→χ̃10qAcoplanar jetsTable 2

Results of the scalar lepton analysis: number of observed events, ND, SM background expectations, NSM, and efficiencies, ε, at s=205 GeV for the scalar electron, muon and tau selections at low (Z<10 GeV), medium (10 GeV<Z<30 GeV) and high ΔM (Z>30 GeV) for different values of the scalar lepton masses

ẽμ̃τ̃Mẽ=94 GeVMμ̃=90 GeVMτ̃=80 GeVNDNSMε (%)NDNSMε (%)NDNSMε (%)Low ΔM798410151138293172703Medium ΔM19254546475214612429High ΔM5053351081055712212329Table 3

Results of the scalar quark analysis: number of observed events, ND, SM background expectations, NSM, and efficiencies, ε, for a 90 GeV scalar quark at very low (5–10 GeV), low (10–20 GeV), medium (20–40 GeV) and high ΔM (⩾40 GeV) at s=205 GeV

t̃1→cχ̃10t̃1→bℓν̃t̃1→bτν̃b̃1→bχ̃10NDNSMε (%)NDNSMε (%)NDNSMε (%)NDNSMε (%)Very low ΔM2321.61822.2511.3613.813Low ΔM13.12200.41401.61612.322Medium ΔM41.33621.41820.52321.542High ΔM11.91510.71330.72521.621Table 4

Summary of the number of observed data events, ND, and SM background expectations, NSM, for all the studied topologies

ProcessDecay modeNDNSMe+e−→ẽẽ̄ẽ→χ̃10e143153e+e−→μ̃μ̃̄μ̃→χ̃10μ269253e+e−→τ̃τ̃̄τ̃→χ̃10τ410381e+e−→ẽRẽLẽL,R→χ̃10e4544.6e+e−→b̃b̃̄b̃→χ̃10b67.7e+e−→t̃t̃̄t̃→χ̃10c2926.5e+e−→t̃t̃̄t̃→ν̃bℓ44.0e+e−→t̃t̃̄t̃→ν̃bτ53.9Search for scalar leptons and scalar quarks at LEP

L3 Collaboration

Achard

Adriani

Aguilar-Benitez

Alcaraz

Alemanni

Allaby

Aloisio

M.G.

Alviggi

Anderhub

V.P.

Andreev

Anselmo

Arefiev

Azemoon

Aziz

Bagnaia

Bajo

Baksay

S.V.

Baldew

Banerjee

Sw.

Banerjee

Barczyk

Barillère

Bartalini

Basile

Batalova

Battiston

Bay

Becattini

Becker

Behner

Bellucci

Berbeco

Berdugo

Berges

Bertucci

B.L.

Betev

Biasini

Biglietti

Biland

J.J.

Blaising

S.C.

Blyth

G.J.

Bobbink

Böhm

Boldizsar

Borgia

Bottai

Bourilkov

Bourquin

Braccini

J.G.

Branson

Brochu

J.D.

Burger

W.J.

Burger

X.D.

Cai

Capell

Cara Romeo

Carlino

Cartacci

Casaus

Cavallari

Cavallo

Cecchi

Cerrada

Chamizo

Y.H.

Chang

Chemarin

Chen

G.M.

Chen

H.F.

Chen

H.S.

Chen

Chiefari

Cifarelli

Cindolo

Clare

Coignet

Colino

Costantini

de la Cruz

Cucciarelli

J.A.

van Dalen

de Asmundis

Déglon

Debreczeni

Degré

Dehmelt

Deiters

della Volpe

Delmeire

Denes

DeNotaristefani

De Salvo

Diemoz

Dierckxsens

Dionisi

Dittmar

Doria

M.T.

Dova

Duchesneau

Duda

Echenard

Eline

El Hage

El Mamouni

Engler

F.J.

Eppling

Extermann

M.A.

Falagan

Falciano

Favara

Fay

Fedin

Felcini

Ferguson

Fesefeldt

Fiandrini

J.H.

Field

Filthaut

P.H.

Fisher

Fisk

Forconi

Freudenreich

Furetta

Yu.

Galaktionov

S.N.

Ganguli

Garcia-Abia

Gataullin

Gentile

Giagu

Z.F.

Gong

Grenier

Grimm

M.W.

Gruenewald

Guida

van Gulik

V.K.

Gupta

Gurtu

L.J.

Gutay

Haas

Hatzifotiadou

Hebbeker

Hervé

Hirschfelder

Hofer

Hohlmann

Holzner

S.R.

Hou

B.N.

Jin

L.W.

Jones

de Jong

Josa-Mutuberrı́a

Käfer

Kaur

M.N.

Kienzle-Focacci

J.K.

Kim

Kirkby

Kittel

Klimentov

A.C.

König

Kopal

Koutsenko

Kräber

R.W.

Kraemer

Krüger

Kunin

Ladron de Guevara

Laktineh

Landi

Lebeau

Lebedev

Lebrun

Lecomte

Lecoq

Le Coultre

J.M.

Le Goff

Leiste

Levtchenko

Likhoded

C.H.

Lin

W.T.

Lin

F.L.

Linde

Lista

Z.A.

Liu

Lohmann

Longo

Y.S.

Luci

Luminari

Lustermann

W.G.

Malgeri

Malinin

Maña

Mans

J.P.

Martin

Marzano

Mazumdar

R.R.

McNeil

Mele

Merola

Meschini

W.J.

Metzger

Mihul

Milcent

Mirabelli

Mnich

G.B.

Mohanty

G.S.

Muanza

A.J.M.

Muijs

Musicar

Musy

Nagy

Natale

Napolitano

Nessi-Tedaldi

Newman

Nisati

Novak

Nowak

Ofierzynski

Organtini

Pal

Palomares

Paolucci

Paramatti

Passaleva

Patricelli

Paul

Pauluzzi

Paus

Pauss

Pedace

Pensotti

Perret-Gallix

Petersen

Piccolo

Pierella

Pioppi

P.A.

Piroué

Pistolesi

Plyaskin

Pohl

Pojidaev

Pothier

Prokofiev

Quartieri

Rahal-Callot

M.A.

Rahaman

Raics

Raja

Ramelli

P.G.

Rancoita

Ranieri

Raspereza

Razis

Ren

Rescigno

Reucroft

Riemann

Riles

B.P.

Roe

Romero

Rosca

Rosier-Lees

Roth

Rosenbleck

J.A.

Rubio

Ruggiero

Rykaczewski

Sakharov

Saremi

Sarkar

Salicio

Sanchez

Schäfer

Schegelsky

Schopper

D.J.

Schotanus

Sciacca

Servoli

Shevchenko

Shivarov

Shoutko

Shumilov

Shvorob

Son

Souga

Spillantini

Steuer

D.P.

Stickland

Stoyanov

Straessner

Sudhakar

Sultanov

L.Z.

Sun

Sushkov

Suter

J.D.

Swain

Szillasi

X.W.

Tang

Tarjan

Tauscher

Taylor

Tellili

Teyssier

Timmermans

Samuel C.C.

Ting

S.M.

Ting

S.C.

Tonwar

Tóth

Tully

K.L.

Tung

Ulbricht

Valente

R.T.

Van de Walle

Vasquez

Veszpremi

Vesztergombi

Vetlitsky

Vicinanza

Viertel

Villa

Vivargent

Vlachos

Vodopianov

Vogel

Vogt

Vorobiev

A.A.

Vorobyov

Wadhwa

Wang

X.L.

Wang

Z.M.

Wang

Weber

Wienemann

Wilkens

Wynhoff

Xia

Z.Z.

Yamamoto

B.Z.

Yang

C.G.

Yang

H.J.

Yang

S.C.

Yeh

An.

Zalite

Yu.

Zalite

Z.P.

Zhang

Zhao

G.Y.

Zhu

R.Y.

Zhu

H.L.

Zhuang

Zichichi

Zimmermann

Zöller

aIII. Physikalisches Institut, RWTH, D-52056 Aachen, Germany11

Supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie.

bNational Institute for High Energy Physics, NIKHEF, and University of Amsterdam, NL-1009 DB Amsterdam, The NetherlandscUniversity of Michigan, Ann Arbor, MI 48109, USAdLaboratoire d'Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux cedex, FranceeInstitute of Physics, University of Basel, CH-4056 Basel, SwitzerlandfLouisiana State University, Baton Rouge, LA 70803, USAgInstitute of High Energy Physics, IHEP, 100039 Beijing, China66

Supported by the National Natural Science Foundation of China.

hUniversity of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, ItalyiTata Institute of Fundamental Research, Mumbai (Bombay) 400 005, IndiajNortheastern University, Boston, MA 02115, USAkInstitute of Atomic Physics and University of Bucharest, R-76900 Bucharest, RomanialCentral Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary22

Supported by the Hungarian OTKA fund under Contract Nos. T019181, F023259 and T037350.

mMassachusetts Institute of Technology, Cambridge, MA 02139, USAnPanjab University, Chandigarh 160 014, IndiaoKLTE-ATOMKI, H-4010 Debrecen, Hungary33

Also supported by the Hungarian OTKA fund under Contract No. T026178.

pDepartment of Experimental Physics, University College Dublin, Belfield, Dublin 4, IrelandqINFN-Sezione di Firenze and University of Florence, I-50125 Florence, ItalyrEuropean Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, SwitzerlandsWorld Laboratory, FBLJA Project, CH-1211 Geneva 23, SwitzerlandtUniversity of Geneva, CH-1211 Geneva 4, SwitzerlanduChinese University of Science and Technology, USTC, Hefei, Anhui 230 029, China6vUniversity of Lausanne, CH-1015 Lausanne, SwitzerlandwInstitut de Physique Nucléaire de Lyon, IN2P3-CNRS, Université Claude Bernard, F-69622 Villeurbanne, FrancexCentro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, E-28040 Madrid, Spain44

Also supported by the Comisión Interministerial de Ciencia y Tecnologı́a.

yFlorida Institute of Technology, Melbourne, FL 32901, USAzINFN-Sezione di Milano, I-20133 Milan, ItalyaaInstitute of Theoretical and Experimental Physics, ITEP, Moscow, RussiaabINFN-Sezione di Napoli and University of Naples, I-80125 Naples, ItalyacDepartment of Physics, University of Cyprus, Nicosia, CyprusadUniversity of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The NetherlandsaeCalifornia Institute of Technology, Pasadena, CA 91125, USAafINFN-Sezione di Perugia and Università Degli Studi di Perugia, I-06100 Perugia, ItalyagNuclear Physics Institute, St. Petersburg, RussiaahCarnegie Mellon University, Pittsburgh, PA 15213, USAaiINFN-Sezione di Napoli and University of Potenza, I-85100 Potenza, ItalyajPrinceton University, Princeton, NJ 08544, USAakUniversity of Californa, Riverside, CA 92521, USAalINFN-Sezione di Roma and University of Rome, “La Sapienza”, I-00185 Rome, ItalyamUniversity and INFN, Salerno, I-84100 Salerno, ItalyanUniversity of California, San Diego, CA 92093, USAaoBulgarian Academy of Sciences, Central Laboratory of Mechatronics and Instrumentation, BU-1113 Sofia, BulgariaapThe Center for High Energy Physics, Kyungpook National University, 702-701 Taegu, Republic of KoreaaqPurdue University, West Lafayette, IN 47907, USAarPaul Scherrer Institut, PSI, CH-5232 Villigen, SwitzerlandasDESY, D-15738 Zeuthen, GermanyatEidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, SwitzerlandauUniversity of Hamburg, D-22761 Hamburg, GermanyavNational Central University, Chung-Li, Taiwan, ROCawDepartment of Physics, National Tsing Hua University, Taiwan, ROC5

Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.

Editor: L. Rolandi

Abstract

Scalar partners of quarks and leptons, predicted in supersymmetric models, are searched for in e+e− collisions at centre-of-mass energies between 192 and 209 GeV at LEP. No evidence for any such particle is found in a data sample of 450 pb−1. Upper limits on their production cross sections are set and lower limits on their masses are derived in the framework of the Minimal Supersymmetric Standard Model.

Introduction

The minimal supersymmetric extension of the Standard Model (MSSM) [1,2] postulates a scalar partner, f̃L,R, for each weak eigenstate of Standard Model (SM) fermions fL,R. Generally, the left, f̃L, and right, f̃R, eigenstates mix to form mass eigenstates. This mixing is an unitary transformation of the f̃R and f̃L states, parameterised by a mixing angle, θLR. Since the off-diagonal elements of the sfermion mass matrix are proportional to the SM partner mass, the mixing is expected to be relevant only for scalar fermions of the third family: the scalar top, t̃L,R, the scalar bottom, b̃L,R, and the scalar tau, τ̃L,R. The lightest scalar quarks are denoted as t̃1 and b̃1.The R-parity is a quantum number which distinguishes SM particles from supersymmetric particles. If R-parity is conserved, supersymmetric particles are pair-produced and the lightest supersymmetric particle, assumed hereafter to be the lightest neutralino, χ̃10, is stable. In addition, the χ̃10 is weakly-interacting and hence escapes detection. R-parity conservation is assumed in the following, which implies that the decay chain of pair-produced supersymmetric particles always contains, besides the relevant SM particles, at least two invisible neutralinos. The typical signature of the production of scalar leptons and scalar quarks is the presence of leptons or jets in events with missing energy and momentum. The difference between the masses of the scalar fermion and the χ̃10, ΔM, determines the kinematic of the event.The pair-production of scalar fermions in e+e− interactions proceeds through the s-channel γ or Z exchange. For scalar electrons, the production cross section is typically enhanced by the t-channel exchange of a neutralino.At LEP energies, all scalar fermions, but the scalar top, decay into their SM partners mainly via f̃→χ̃10f. Cascade decays, such as f̃→χ̃20f→χ̃10Z∗f are also possible and may dominate in some regions of the MSSM parameter space. According to the values of the scalar top mass and couplings, four channels can become dominant among the possible scalar top decays: t̃1→cχ̃10, bνℓℓ̃, bℓν̃ℓ and bχ̃+1. The additional decay into bχ̃01ff̄′ which can originate six-fermion final states is not considered [3]. This topology is indirectly covered by searches in the framework of R-parity violation, which revealed no excess [4]. In the following, for the t̃→ν̃bℓ decay, scalar neutrinos are assumed to be lighter than charged scalar leptons. For this decay, ΔM refers to the mass difference between the scalar top and scalar neutrino masses.The supersymmetric partners of the right-handed leptons, ℓ̃R, are generally expected to be lighter than their left-handed counterparts and are considered in the following. If the mass difference between the right-handed scalar electron and the lightest neutralino is very small the search for e+e−→ẽRẽR has little sensitivity. The e+e−→ẽRẽL process is then considered. The left-handed scalar electron, too heavy to be produced in pairs, decays into an energetic electron, while the electron from the right-handed scalar electron decay remains often invisible, leading to a ‘single electron’ topology.Scalar leptons and scalar quarks are searched for at centre-of-mass energies, s, up to 209 GeV. The present study supersedes previous L3 limits on scalar lepton [5] and scalar quark production [6] obtained at lower s. Searches for scalar fermions were also reported by other experiments at LEP [7] and at the TEVATRON [8]. Table 1

summarises the investigated processes and decay modes together with the studied topology.

Data samples and Monte Carlo simulation

The data used in the present analysis were collected with the L3 detector [9] at LEP and correspond to an integrated luminosity of 450.5 pb−1 at s=192–209 GeV. Two average centre-of-mass energies are considered in the following: 196 and 205 GeV, with corresponding integrated luminosities of 233.2 and 217.3 pb−1.SM processes are simulated with the following Monte Carlo (MC) generators: PYTHIA [10] for e+e−→qq̄(γ), e+e−→Ze+e− and e+e−→ZZ, EXCALIBUR [11] for e+e−→W±e∓ν, KORALZ [12] for e+e−→μ+μ−(γ) and e+e−→τ+τ−(γ), BHWIDE [13] for e+e−→e+e−(γ) and KORALW [14] for e+e−→W+W−. Two-photon interaction processes are simulated using DIAG36 [15] for e+e−→e+e−ℓ+ℓ− and PHOJET [16] for e+e−→e+e− hadrons, requiring at least 3 GeV for the invariant mass of the two-photon system. The number of simulated events for each background process is more than 100 times the data statistics, except for two-photon processes for which the MC statistics amounts to about 7 times that of the data.Signal events for scalar leptons are generated with the SUSYGEN [17] MC program, for scalar lepton masses, Mℓ̃, ranging from 45 GeV up to the kinematic limit, and for values of ΔM varying between 3 GeV and Mℓ̃−1 GeV. For scalar quarks, a generator [18] based on PYTHIA is used. Scalar quark masses vary from 45 GeV up to the kinematical limit and Mχ̃10 varies from 1 GeV to Mt̃1−3 GeV and to Mb̃1−7 GeV, for scalar top and bottom, respectively. The t̃1→bℓν̃ and t̃1→bτν̃ channels are generated with ν̃ mass ranging from the 43 GeV limit [19] up to Mt̃1−8 GeV. In total, about 180 samples are generated, each with at least 1000 events.The response of the L3 detector is simulated using the GEANT package [20]. It takes into account effects of energy loss, multiple scattering and showering in the detector materials and in the beam pipe. Hadronic interactions are simulated with the GHEISHA program [21]. Time-dependent detector inefficiencies are monitored during data taking and reproduced in the simulation.3

Event selection

3.1

Analysis procedure

Besides the common signature of missing momentum in the direction transverse to the beam axis, signals from supersymmetric particles are further specified according to the number of leptons or the multiplicity of hadronic jets in the final state.Signatures of scalar leptons are simple and the final states mostly contain just two acoplanar leptons of the same generation. To account for the three lepton flavours, three different selections are performed. For scalar electrons and muons a pair of electrons or muons is required in the event, respectively, while scalar taus are selected as low-multiplicity events with electrons or muons or with narrow jets. Events from the t̃1→cχ̃10 and b̃1→bχ̃10 processes contain two high-multiplicity acoplanar jets originated by c or b quarks. In addition, two charged leptons are present when both scalar top quarks decay via t̃1→bℓν̃.An optimization procedure is devised [5] which maximizes signal efficiency and background rejection by varying simultaneously all cuts for a given process. The signal topology depends on ΔM and therefore the optimization is repeated for different values of ΔM. Details of the selections performed for each topology are given in the following.3.2

Acoplanar leptons

Scalar leptons are searched for in events with two isolated leptons of the same flavour. The lepton identification and isolation criteria follow those used at lower s[22]. An electron is isolated if the calorimetric energy deposition in a 10° cone around its direction is less than 2 GeV. Muon isolation requires an energy below 2 GeV in the cone between 5° and 10° around the muon direction. A tau is isolated if the energy deposition in the cone between 10° and 20° around its direction is less than 2 GeV and less than 50% of the tau energy. Furthermore, the energy deposition in a cone between 20° and 30° must be less than 60% of the tau energy.The large background from two-photon interactions is rejected with cuts on the lepton transverse momentum, the visible mass, Mvis, the transverse missing momentum, PmissT, the energy deposited at low polar angle, E30, and the sine of the polar angle of the missing momentum, sinθmiss. Acoplanarity and acollinearity cuts together with upper bounds on the visible energy, Evis, reduce the background from W boson and fermion pair-production. After these cuts, the distributions of selection variables for data and Monte Carlo are in good agreement, as shown in Fig. 1(a)

for the energy of the most energetic lepton, E1.

The final selections are optimised for each scalar lepton flavour, using a set of parameterized cuts (Evis, PmissT, Mvis, E1) together with fixed cuts (acoplanarity, acollinearity and sinθmiss). The parameterised cuts depend on Z=(ΔM/Mℓ̃)×Ebeam, to reflect the dependence of the final state topologies on ΔM and Mℓ̃. Ebeam is the beam energy. The variables used for each selection are described in Ref. [5].The selection efficiencies for scalar lepton pair-production, the number of candidates in data and the SM expectations are given in Table 2

for three ΔM regions.

3.3

Single electron

The single-electron analysis requires one or two identified electrons. Cuts on Evis and sinθmiss are applied in order to reject background from two-photon interactions. At least one electron with energy greater than 5 GeV is required. The electron energy has to be less than 65 GeV to reject photon conversion from the e+e−→νν̄γ process when the two tracks are not resolved. If two electrons are selected, their acoplanarity must be between 10° and 160° and the energy of the second electron must be less than 5 GeV to suppress background from W pair-production. To remove events with additional activity in the detector, the difference between the total energy and the energy of the most energetic electron must be less than 5 GeV. In addition, a cut PmissT>15 GeV is applied. If no second electron of at least 100 MeV is detected, this cut is released to PmissT>10 GeV. Fig. 1(b) compares data and MC for the energy of the most energetic electron, the remaining background originates from four-fermion final states. Signal efficiencies vary from 3% at ΔM=MẽL−Mχ̃10=5 GeV up to 60% for ΔM=60 GeV.3.4

Acoplanar jets

The search for scalar quarks decaying into quarks and neutralino is based on events with two high-multiplicity acoplanar jets. The DURHAM algorithm [23] is used for the clustering of hadronic jets. A common preselection is applied [6] which is based on: Evis, the calorimetric cluster multiplicity, PmissT, E30 and sinθmiss. After this preselection, the data agree well with the SM expectations, as depicted in Fig. 1(c) and (d).Four selections are optimised for scalar top quarks and four for scalar bottom quarks. They depend on ΔM and cover the regions 5–10, 10–20, 20–40 GeV and above 40 GeV. Lower cuts on Evis/s and PmissT/s separate the signal from the two-photon background, whereas an upper cut on Evis/s removes events from four-fermion final states. A cut on sinθmiss also rejects the two-photon background. Cuts on the jet widths and on the absolute value of the projection of the total momentum of the jets onto the direction perpendicular to thrust, computed in the transverse plane, further suppress the two-photon as well as W+W− and qq̄(γ) backgrounds.For the scalar bottom selection, b-quark identification in the final state is enforced by an additional cut on the event b-tagging variable [6], Db-tag.The expected signal efficiencies at various ΔM values are given in Table 3

together with the observed number of events and the SM background expectations.

3.5

Acoplanar jets and leptons

A selection of events with two acoplanar jets and one or two isolated leptons complements the scalar top searches in presence of the t̃1→bℓν̃ decay. Large values of the Db-tag variables are required for the two jets and additional cuts on Evis/s reject part of the two-photon and four-fermion events. Lower cuts on the energy of the leptons suppress background from two-photon interactions at low ΔM and the qq̄(γ) final state at medium ΔM. At high ΔM, an upper cut on the lepton energy reject four-fermion events. This selection covers the ΔM region above the limit Mν̃>43 GeV.The expected signal efficiencies for scalar top detection are given in Table 3 together with data counts and the SM background expectations, for various ΔM values.4

Results

4.1

Cross section limits

As discussed above and summarized in Table 4

, no excess with respect to the Standard Model expectations is observed in the data. Upper limits on the production cross section are therefore derived combining these results with those obtained at lower s[5,6]. This combination scales the signal cross sections with s and the limits refer to s=205 GeV.

Figs. 2 and 3

show the 95% confidence level (CL) upper bounds on the production cross sections as a function of the scalar fermion masses and of the neutralino mass. The case of right-handed scalar leptons and of the lightest scalar quarks is considered. These limits include [24] the systematics effects discussed below.

4.2

Systematic uncertainties

Systematic uncertainties on the signal efficiency for scalar lepton searches and on all background predictions are dominated by Monte Carlo statistics. They are smaller than 5%. The main systematic uncertainties on the scalar quark signal selection efficiency arise from uncertainties on the production mechanism, hadronisation and decay of the scalar quark [6]. These uncertainties are in the range from 7 to 18% for scalar top, with the highest uncertainty in the very low ΔM region. For scalar bottom, the highest uncertainty is about 10% and is observed in the very low and high ΔM regions.5

Interpretations in the MSSM

In the MSSM, with grand unification assumptions [25], the masses and couplings of the supersymmetric particles as well as their production cross sections are described [2] in terms of five parameters: tanβ, the ratio of the vacuum expectation values of the two Higgs doublets, M2≃0.81×m1/2, the gaugino mass parameter, μ, the Higgsino mixing parameter, m0, the common mass for scalar fermions at the GUT scale and A0, the trilinear coupling in the scalar fermion sector. We investigate the following MSSM parameter space: 1⩽tanβ⩽60,0⩽M2⩽2000 GeV,−2000⩽μ⩽2000 GeV,0⩽m0⩽500 GeV,−1000<A0<1000 GeV. The limits on the production cross section for scalar leptons and scalar quarks discussed above are translated into exclusion regions in the MSSM parameter space. To derive these limits, we optimise the event selection for each point in the MSSM parameter space by choosing the combination of selections which provides the highest sensitivity for each process. This sensitivity is derived by calculating at each point the production cross sections and the decay branching fractions of scalar leptons and scalar quarks. For the latter, the mixing angle θLR is also considered. A point of the MSSM parameter space is excluded if any of these calculated cross sections exceeds its corresponding experimental limit. Mass lower limits are derived as the lowest value for the mass of a particle over all points which are not excluded.5.1

Limits on scalar lepton masses

Fig. 4(a)–(c)

shows the exclusion contours in the Mχ̃10–Mℓ̃R plane obtained by considering only the reaction e+e−→ℓ̃Rℓ̃̄R for μ=−200 GeV and tanβ=2. These exclusions hold for tanβ⩾2 and |μ|⩾200.

Under these assumptions, 95% CL lower limits on the masses of scalar leptons are derived as 94.4 GeV for scalar electrons with ΔM>10, 86.7 GeV for scalar muons with ΔM>10 and 78.3 GeV for scalar taus with ΔM>15 GeV.The limiting factor towards an absolute limit on the scalar electron mass is the lack of detection efficiency for very small ΔM values. This is overcome, in the constrained MSSM, by using the e+e−→ẽRẽL process. The searches for acoplanar electrons and single electrons are combined to derive a lower limit on MẽR as a function of tanβ and for any value of m0, M2 and μ as shown in Fig. 5(a)

. For tanβ<1 the mass difference between ẽL and ẽR decreases, reducing the sensitivity of the single electron search. As an example, Fig. 5(b) shows the limit as a function of m0 for a fixed value of tanβ. For tanβ⩾1, the 95% CL lower limit for the lightest scalar electron, independent of the MSSM parameters, is MẽR⩾71.3 GeV. Assuming a common mass for the scalar leptons at the GUT scale, this limit holds for the lightest scalar muon, μ̃R, as well.

5.2

Limits on scalar quark masses

Fig. 6(a)

shows the excluded t̃1 mass region as a function of Mt̃1 and Mχ̃10 at cosθLR=1 and cosθLR=0.57 for the t̃1→cχ̃10 decay. The second value of the mixing angle corresponds to a vanishing contribution of the Z exchange in the s-channel production. For this decay mode, scalar top masses below 95 GeV are excluded at 95% CL under the assumptions cosθLR=1 and ΔM=15–25 GeV. For the same values of ΔM and in the most pessimistic scenario of cosθLR=0.57, the 95% CL mass limit is 90 GeV. The region in which the t̃1→bWχ̃10 decay is kinematically accessible and becomes the dominant decay mode, is indicated. This decay is not considered in this analysis.

Fig. 6(b) shows the scalar top mass regions which are excluded if the dominant three-body decay t̃1→bℓν̃ is kinematically accessible. Equal branching fractions for the decays into e, μ or τ are assumed and 95% CL mass lower limits are derived as 96 and 93 GeV for cosθLR=1 and cosθLR=0.57, respectively. The corresponding exclusion limits for the scalar top decay t̃1→bτν̃ are shown in Fig. 6(c). Mass lower limits at 95% CL in the range 93–95 GeV are obtained, assuming ΔM>15 GeV.Fig. 6(d) shows the region excluded as a function of Mb̃1 and Mχ̃10 considering the b̃1→bχ̃10 decay for cosθLR=1 and cosθLR=0.39. The latter value corresponds to a vanishing contribution of the Z exchange in the s-channel production. Scalar bottom masses below 95 GeV are excluded at 95% CL assuming cosθLR=1 and ΔM=15–25 GeV. For cosθLR=0.39, the 95% CL mass lower limit is 81 GeV.For scalar quarks of the first two generations, the same selection efficiencies are assumed as for the t̃1→cχ̃01 decay because of the similar event topologies. The cross section limits given in Fig. 3(a) are then interpreted in terms of degenerate scalar quark masses. Fig. 7(a)

shows the scalar quark mass lower limits as a function of the χ̃10 mass. Two scenarios are considered: left- and right-handed scalar quark degeneracy or only right-handed scalar quark production. In the first case, with four degenerate scalar quark flavours, the 95% CL mass limit is 99.5 GeV at for ΔM>10 GeV. In the case of only right-handed scalar quark production, the 95% CL mass lower limit is 97 GeV. Regions excluded in the hypotheses that all scalar quarks but the scalar top are degenerate are also shown.

Assuming gaugino unification at the GUT scale, the results for the four degenerate scalar quarks are reinterpreted on the plane of the scalar quark and gluino masses, as shown in Fig. 7(b). In addition, gaugino unification [25] allows a transformation of the absolute limit on M2, obtained from the chargino and neutralino [26] as well as scalar lepton searches, into a lower limit on the gluino mass, also shown in Fig. 7(b). The ISAJET program [27] is used for the calculation of the exclusion contours. For tanβ=4, gluino masses up to about 270–310 GeV are excluded at 95% CL.In conclusion, no evidence for the production of scalar lepton and quarks is observed in the data set collected by the L3 experiment at LEP. Stringent upper limits on the cross sections for the production of these scalar particles are derived, which correspond to lower mass limits in the MSSM.

References

[1]

Yu.A.

Golfand

E.P.

LikhtmanSov. Phys. JETP

1971

323

D.V.

Volkhov

V.P.

AkulovPhys. Lett. B

1973

109

Wess

ZuminoNucl. Phys. B

1974

Fayet

FerraraPhys. Rep. C

1977

249

Salam

StrathdeeFortschr. Phys.

1978

[2]

H.P.

NillesPhys. Rep.

110

1984

H.E.

Haber

G.L.

KanePhys. Rep.

117

1985

BarbieriNuovo Cimento

1988

[3]

Bohem

Djouadi

MambriniPhys. Rev. D

2000

0950006

and references therein

[4]L3 Collaboration

Achard

Phys. Lett. B

524

2002

[5]L3 Collaboration

Acciarri

Phys. Lett. B

471

1999

280

and references therein

[6]L3 Collaboration

Acciarri

Phys. Lett. B

471

1999

308

and references therein

[7]ALEPH Collaboration

Barate

Phys. Lett. B

537

2002

ALEPH Collaboration

Barate

Phys. Lett. B

526

2002

206

ALEPH Collaboration

Barate

Phys. Lett. B

544

2002

DELPHI Collaboration

Abreu

Eur. Phys. J. C

2000

DELPHI Collaboration

Abreu

Phys. Lett. B

496

2000

OPAL Collaboration

Abbiendi

Phys. Lett. B

545

2002

272

OPAL Collaboration

Abbiendi

Eur. Phys. J. C

2000

[8]CDF Collaboration

Abe

Phys. Rev. D

1997

1357

D0 Collaboration

Abachi

Phys. Rev. Lett.

1995

618

[9]L3 Collaboration

Adeva

Nucl. Instrum. Methods A

289

1990

Chemarin

Nucl. Instrum. Methods A

349

1994

345

Acciarri

Nucl. Instrum. Methods A

351

1994

300

Basti

Nucl. Instrum. Methods A

374

1996

293

I.C.

Brock

Nucl. Instrum. Methods A

381

1996

236

Adam

Nucl. Instrum. Methods A

383

1996

342

[10]

PYTHIA version 5.722 is used: T. Sjöstrand, preprint CERN-TH/7112/93, revised August 1995

SjöstrandComput. Phys. Commun.

1994

Sjöstrand

hep-ph/0001032

[11]EXCALIBUR Monte Carlo

F.A.

Berends

Kleiss

PittauComput. Phys. Commun.

1995

437

[12]KORALZ version 4.02 is used

Jadach

B.F.L.

Ward

Wa̧sComput. Phys. Commun.

1994

503

[13]BHWIDE version 1.01 is used

Jadach

Placzek

B.F.L.

WardPhys. Lett. B

390

1997

298

[14]KORALW version 1.33 is used

Jadach

Comput. Phys. Commun.

1996

216

Jadach

Phys. Lett. B

372

1996

289

[15]DIAG36 Monte Carlo

F.A.

Berends

P.H.

Daverveldt

KleissNucl. Phys. B

253

1985

441

[16]PHOJET version 1.05 is used

EngelZ. Phys. C

1995

203

Engel

RanftPhys. Rev. D

1996

4244

[17]SUSYGEN Monte Carlo

Katsanevas

Comput. Phys. Commun.

112

1998

227

[18]Modified version of the OPAL MC generator for scalar quarks production

Accomando

Altarelli

Sjöstrand

Zwirner

Physics at LEP2

vol. 2

1996CERNGeneva

343

CERN 96-01

[19]

Hagiwara

Phys. Rev. D

2002

010001

[20]

GEANT version 3.15 is used: R. Brun, et al., preprint CERN DD/EE/84-1, revised 1987

[21]

H. Fesefeldt, RWTH Aachen Report PITHA 85/2, 1985

[22]L3 Collaboration

Acciarri

Eur. Phys. J. C

1998

207

[23]

Bethke

Nucl. Phys. B

370

1992

310

and references therein

[24]

R.D.

Cousins

V.L.

HighlandNucl. Instrum. Methods A

320

1992

331

[25]

L.E.

IbanezPhys. Lett. B

118

1982

Barbieri

Ferrara

SavoyPhys. Lett. B

119

1982

343

[26]

L3 Collaboration, M. Acciarri, et al., Search for charginos and neutralinos in e+e− collisions up to s=209 GeV, in preparation

[27]

Baer

Hewett

ZeppenfeldProceedings of the Workshop on Physics at Current Accelerators and Supercolliders1993Argonne National LaboratoryArgonne, IL

[28]UA1 Collaboration

Albajar

Phys. Lett. B

198

1987

261

UA2 Collaboration

Alitti

Phys. Lett. B

235

1990

363