application/xmlFlavour independent search for neutral Higgs bosons 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. RoscaC. RosenbleckS. Rosier-LeesS. RothJ.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 583 (2004) 14-27. doi:10.1016/j.physletb.2003.12.039journalPhysics Letters BCopyright © unknown. Published by Elsevier B.V.Elsevier B.V.0370-26935831-211 March 20042004-03-1114-27142710.1016/j.physletb.2003.12.039http://dx.doi.org/10.1016/j.physletb.2003.12.039doi:10.1016/j.physletb.2003.12.039http://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

S300.3

PLB20560S0370-2693(04)00029-210.1016/j.physletb.2003.12.039Experiments

Fig. 1

Distributions for the e+e−→hZ search in the four-jet final state of: (a) |cosΘ2B|, (b) m5C, (c) γtriple, (d) LhZ. The points indicate data collected at s > 203 GeV, the open histograms represent the expected background and the hatched histograms stand for a mh=110 GeV signal expected for ξ2×B(h→hadrons)=1, multiplied by a factor of 10. The arrow in (d) indicates the position of the cut.

Fig. 2

Distributions of the signal over background ratio for events selected in the e+e−→hZ search by the (a) four-jet, (b) two jets and missing energy and (c) two jets and two lepton analyses. The points indicate data collected at s>203 GeV, the open histograms represent the expected background and the hatched histograms stand for a mh=110 GeV signal expected for ξ2×B(h→hadrons)=1. Only events with s/b>0.05 are shown.

Fig. 3

Distributions for the e+e−→hZ search in the two jet and missing energy final state of: (a) mmis, (b) sinΨ, (c) neural network output and (d) mqq. The points indicate data collected at s>203 GeV, the open histograms represent the expected background and the hatched histograms stand for a mh=110 GeV signal expected for ξ2×B(h→hadrons)=1, multiplied by a factor of 10.

Fig. 4

Distributions for the e+e−→hA search of: (a) cosΘW+, (b) −logy34, (c) |cosΘT|, (d) LhA. The points indicate the data, the open histograms represent the expected background and the hatched histograms stand for a (mh,mA)=(60,80) GeV signal expected for η2×B(hA→hadrons)=1, multiplied by a factor of 10. The arrow in (d) indicates the position of the cut.

Fig. 5

Distributions of the signal over background ratio for events selected by the e+e−→hA search for different (mh,mA) mass hypotheses: (a) (75, 75) GeV, (b) (65, 75) GeV, (c) (60, 80) GeV, (d) (55, 85) GeV, (e) (50, 90) GeV and (f) (45, 95) GeV. The points indicate the data, the open histograms represent the expected background and the hatched histograms stand for a signal of the given (mh,mA) hypothesis expected for η2×B(hA→hadrons)=1.

Fig. 6

The 95% confidence level upper limit on ξ2×Br(h→hadrons) as a function of mh. The solid line indicates the observed limit and the dashed line stands for the median expected limit. The shaded areas show the 1σ and 2σ intervals centered on the median expected limit. The observed and expected limits on mh for ξ2×Br(h→hadrons)=1 are also shown.

Fig. 7

The 95% confidence level upper limit on the quantity η2×Br(hA→hadrons) as a function of mh+mA for different values of the difference |mA−mh|: (a) 0 GeV, (b) 10 GeV, (c) 20 GeV, (d) 30 GeV, (e) 40 GeV and (f) 50 GeV. The solid lines indicate the observed limits and the dashed lines stand for the median expected limits. The shaded areas show the 1σ and 2σ intervals centered on the median expected limits.

Table 1

Effective centre-of-mass energies and corresponding integrated luminosities, L

s (GeV)188.6191.6195.6199.5201.5203.8205.1206.3206.6208.0Lpb−1176.429.783.782.837.07.668.166.963.78.2Table 2

Final states of the e+e−→hZ and e+e−→hA processes and topologies under study

ProcessProcesse+e−→hZe+e−→hAh→bb̄,cc̄,ggZ→qq̄,νν̄,ℓ+ℓ−h→bb̄,cc̄,ggA→bb̄,cc̄,ggFinal stateTopologyFinal stateTopologybb̄qq̄, cc̄qq̄, ggqq̄Four jetsbb̄bb̄, bb̄cc̄Four jetsbb̄νν̄, cc̄νν̄, ggνν̄Two jets and missing energybb̄gg, cc̄cc̄bb̄ℓ+ℓ−, cc̄ℓ+ℓ−, ggℓ+ℓ−Two jets and two leptonscc̄gg, ggggTable 3

Numbers of selected candidates, ND, expected background events, NB, and expected signal events, NS, for different mh hypotheses in the e+e−→hZ search. The selection efficiencies, ε, are also given. The numbers of signal events are quoted for the h decay mode corresponding to the lowest efficiency and are computed assuming ξ2×B(h→hadrons)=1. Only events with a signal over background ratio greater than 0.05 are considered

e+e−→hZh→hadronsh→hadronsZ→qq̄Z→νν̄mh (GeV)NDNBNSε (%)NDNBNSε (%)6013561336172434033.148.8637013631295122438984.040.660809389661044520920132.2569058458571.24518318121.65310036035539.9467469.912.25011012612711.8461816.43.548h→hadronsZ→ℓ+ℓ−Combinedmh (GeV)NDNBNSε (%)NDNBNS604949.428.24914451419249704352.824.15014951432187806163.219.25112081230155905661.313.0508238271061002418.45.84745844357.911034.21.64214714816.9Table 4

Number of selected candidates, ND, expected background events, NB, and expected signal events, NS, and selection efficiencies, ε, for different (mh,mA) hypotheses in the e+e−→hA search. The numbers of signal events are quoted for the h and A decay modes corresponding to the lowest efficiencies and are computed assuming η2×B(h→hadrons)=1. Only events with a signal over background ratio greater than 0.05 are considered

e+e−→hA(mh,mA) (GeV)NDNBNSε (%)(50, 50)11411084.441(50, 70)22021156.236(50, 90)22323929.228(70, 70)24422339.940(70, 90)9695.85.711(90, 90)1011.430.64Flavour independent search for neutral Higgs bosons 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

Rosenbleck

Rosier-Lees

Roth

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

Supported also 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 Lab. 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

A flavour independent search for the CP-even and CP-odd neutral Higgs bosons h and A is performed in 624 pb−1 of data collected with the L3 detector at LEP at centre-of-mass energies between 189 and 209GeV. Higgs boson production through the e+e−→Zh and the e+e−→hA processes is considered and decays of the Higgs bosons into hadrons are studied. No significant signal is observed and 95% confidence level limits on the hZZ and hAZ couplings are derived as a function of the Higgs boson masses. Assuming the Standard Model cross section for the Higgs-strahlung process and a 100% branching fraction into hadrons, a 95% confidence level lower limit on the mass of the Higgs boson is set at 110.3 GeV.

Introduction

One of the goals of the LEP program is the search for Higgs bosons, the particles postulated by the Standard Model of the electroweak interactions [1], and some of its extensions, to explain the mechanism [2] which gives the elementary particles their observed masses.At the centre-of-mass energies, s, at which the LEP e+e− collider was operated, the Standard Model Higgs boson, H, is predicted to decay dominantly into b quarks. For a large part of the parameter space of the Minimal Supersymmetric Standard Model (MSSM) [3], decays of neutral Higgs bosons into b quarks are also predicted to be dominant. Experimental searches for the Higgs bosons predicted both in the Standard Model and in the MSSM exploit this feature through sophisticated flavour tagging techniques. No significant signal was found at LEP either for the Standard Model Higgs boson [4–7] or for neutral Higgs bosons of the MSSM [5,8,9].In some extensions of the Standard Model, decays of the Higgs bosons into bb̄ pairs are strongly suppressed to the benefit of other decay modes such as cc̄, gg or τ+τ−. For instance, this occurs for specific parameters of the two Higgs doublet model [10] or the MSSM [11], as well as for some composite models [12]. It is hence important to investigate such scenarios with dedicated experimental analyses in which the information about the flavour of the Higgs boson decay products is not used, reducing the model dependence of the conventional Higgs searches.This Letter describes the search for hadronic decays of the light CP-even Higgs boson, h, and of the CP-odd Higgs boson, A, using data collected by the L3 detector [13] at LEP. Production of a h boson in association with a Z boson, Higgs-strahlung, and pair-production of the h and A bosons, are considered: e+e−→hZ,e+e−→hA. The tree level cross sections of these processes are related to the cross section of the Standard Model Higgs boson production through the Higgs-strahlung process, σHZSM, as [10]: σhZ=ξ2σHZSM,σhA=η2λ̃σHZSM, where λ̃ is a p-wave suppression factor, which depends on s and on the Higgs boson masses, mh and mA. The hZZ and hAZ couplings relative to the HZZ coupling of the Standard Model are defined as ξ=ghZZ/gHZZSM and η=ghAZ/gHZZSM. In the following, these couplings are not fixed to any prediction but rather considered as free parameters, reducing the model dependence of the analysis.2

Data and Monte Carlo samples

An integrated luminosity of 624 pb−1 of data, collected at s=189–209GeV, is analysed. The data are grouped into several subsamples according to their s value, as listed in Table 1

The cross section of the Higgs-strahlung process in the Standard Model is calculated using the HZHA generator [14]. For efficiency studies, Monte Carlo samples are generated using PYTHIA [15] for the two production mechanisms and for each of the decay modes h→bb̄, cc̄ and gg, A→bb̄, cc̄ and gg. Several Higgs mass hypotheses are considered and 2000 events are generated in each case. For the e+e−→hZ process, mh ranges in steps of 10 GeV from 60 to 100 GeV, and in steps of 1 GeV from 100 to 120 GeV. For the e+e−→hA process, mh and mA range from 40 to 110 GeV in steps of 10 GeV.For background studies, the following Monte Carlo programs are used: KK2f [16] for e+e−→qq̄(γ) and e+e−→τ+τ−, PYTHIA for e+e−→ZZ and e+e−→Ze+e− and YFSWW [17] for e+e−→W+W−. EXCALIBUR [18] is used for four-fermion final states not covered by these generators. Hadron production in two-photon interactions is simulated with PYTHIA and PHOJET [19]. The number of simulated events for the most important background channels is more than 100 times the number of expected events.The L3 detector response is simulated using the GEANT program [20], which models the effects of energy loss, multiple scattering and showering in the detector. The GHEISHA program [21] is used to simulate hadronic interactions. Time dependent detector inefficiencies, monitored during data taking, are also taken into account.3

Analysis procedures

Three different decay modes are considered for the h and A bosons: bb̄, cc̄ and gg. Table 2

summarises the different signal signatures and the investigated topologies. Three topologies cover the possible final states of the e+e−→hZ process. They correspond to the decay of the Z boson into hadrons, neutrinos or charged leptons, associated to the hadrons from the h decay. They give rise to events with four hadronic jets, two hadronic jets and missing energy and two hadronic jets and two charged leptons, respectively. A single topology consisting of four hadronic jets, covers all final states of the e+e−→hA process.

Analyses in all channels proceed from a preselection of high multiplicity hadronic events which suppresses copious backgrounds from two-photon interactions, lepton-pair production and pair-production of gauge bosons which decay into leptons. A selection based on kinematic cuts, neural networks or likelihoods is then applied to further discriminate the signal from the background. Finally, discriminant variables which depend on the Higgs mass hypothesis are built to separate signal and background. Their distributions are studied to test the presence of a signal and to probe the ξ and η couplings as a function of mh and mA. Events are ordered as a function of the signal over background ratio and only events with this ratio greater than 0.05 are retained.4

Search for e+e−→hZ

The three analyses used in the search for e+e−→hZ are similar to those used in the search for the Standard Model Higgs boson [4], with the exception that no b quark identification is used.4.1

Four jets

If both the h and the Z bosons decay into hadrons, the signature is four hadronic jets. The invariant mass of two of them has to be compatible with the mass of the Z boson, mZ. The dominant background comes from hadronic decays of pair-produced gauge bosons and from the e+e−→qq̄(γ) process.After a preselection of high multiplicity events [4], events are resolved into four jets using the DURHAM algorithm [22] and a kinematic fit imposing four-momentum conservation is performed. A likelihood, LhZ, is built [4] from the following variables:

•the maximum energy difference between any two jets,

•the minimum jet energy,

•the parameter y34 of the DURHAM algorithm for which the event is resolved from three into four jets,

•the minimum opening angle between any two jets,

•the event sphericity,

•the absolute value of the cosine of the polar angle, Θ2B, for the di-jet system most compatible with the production of a pair of gauge bosons,

•the mass from a kinematic fit imposing four-momentum conservation and equal di-jet masses, m5C,

•the maximal triple-jet boost, γtriple, defined as the maximum three-jet boost obtained from the four possibilities to construct a one-jet against three-jet configuration in a four-jet event.

Fig. 1

shows the distributions of |cosΘ2B|, m5C, γtriple and LhZ for data collected at s > 203 GeV, the expected background and a signal with mh=110GeV. Events are retained for which the value of LhZ exceeds a threshold, around 0.6, optimised separately for each s and mh hypothesis.

For each of the three possible jet pairings, the quantity χ2hZ=(Σ−(mh+mZ))2/σΣ2+(Δ−|mh−mZ|)2/σΔ2 is calculated [4], where Σ and Δ are the di-jet mass sum and difference, while σΣ and σΔ are the corresponding resolutions. The pairing which minimises χ2hZ is chosen and the corresponding value is used as the final discriminant variable. Fig. 2(a)

presents the distributions of the signal over background ratio in the χ2hZ variable for selected data and Monte Carlo events. Table 3

lists the numbers of selected and expected events for different mh hypotheses.

4.2

Two jets and missing energy

The signature for h decays into hadrons and Z decays into neutrinos is a pair of high multiplicity jets, large missing energy and a missing mass, mmis, consistent with mZ. The dominant backgrounds are the e+e−→qq̄(γ) process, W pair-production in which only one W decays into hadrons and Z pair-production with a Z decaying into hadrons and the other into neutrinos.High multiplicity hadronic events are selected with a visible energy, Evis, such that 0.25<Evis/s<0.70. Events with isolated photons of energy greater than 20GeV are rejected. The events are forced into two jets using the DURHAM algorithm and the di-jet mass is required to be greater than 40 GeV to suppress background from two-photon interactions. Events from the e+e−→qq̄(γ) process are suppressed by requiring mmis>60GeV. In addition, the polar angle, θ, of the missing momentum must satisfy |cosθ|<0.9 and the energy deposited in the very forward calorimeters is required to be less than 20 GeV. Finally, the sine of the angle Ψ between the beam axis and the plane spanned by the directions of the two jets must be greater than 0.025. Fig. 3(a) and (b)

presents the distributions of mmis and sinΨ for data collected at s>203GeV, expected background and a signal with mh=110 GeV, when all other cuts are applied.

A neural network [23] is built from the following variables:

•Evis,

•mmis,

•sinΨ,

•the longitudinal missing momentum,

•the transverse missing momentum,

•the absolute value of the cosine of the angle between the two jets in the plane transverse to the beam direction,

•the event thrust,

•the sum of the jet opening angles after forcing the event into a three-jet configuration.

The distributions of the output of the neural network are presented in Fig. 3(c). Fig. 3(d) shows the distributions of the hadronic mass mqq, calculated with a fit which imposes mmis=mZ. These two variables are combined into a final discriminant, whose distributions are presented in Fig. 2(b) in terms of the signal over background ratio. Table 3 lists the numbers of selected events for different mh hypotheses.4.3

Two jets and two leptons

Different signal topologies correspond to h decays into hadrons and Z decays into electrons and muons or into tau leptons. For decays into electrons and muons, the signature is a pair of well-isolated leptons with mass close to mZ and two hadronic jets. In the case of tau leptons, events with four jets are expected, where two of the jets are narrow, of low multiplicity, and of unit charge. The dominant background is due to Z-pair production followed by the hadronic decay of one Z and the decay into leptons of the other.The event selection is identical to that used for the same final states of the Standard Model Higgs search [4]. After this selection, a kinematic fit is applied which imposes four-momentum conservation and constrains the di-lepton mass to mZ. The mass of the hadronic system after the fit is used as a discriminant to test different mh hypotheses. Its distributions in terms of the signal over background ratio are presented in Fig. 2(c). The yield of this selection is presented in Table 3.5

Search for e+e−→hA

The pair-production of h and A bosons gives rise to high multiplicity events with four hadronic jets. The largest backgrounds are the pair-production of W and Z bosons which decay into hadrons and the e+e−→qq̄(γ) process. High multiplicity events are selected, subjected to a kinematic fit which enforces four-momentum conservation and forced into four jets with the DURHAM algorithm. A neural network [24] is used to separate genuine four-jet events from events most likely due to fermion-pair production.For each (mh,mA) hypothesis, a likelihood, LhA, is built [8] to separate the signal from the background from W- and Z-pair production. It uses the following variables:

•the maximum energy difference between any two jets,

•the minimum jet energy,

•the probabilities of kinematic fits which impose four-momentum conservation together with the hypotheses of W- or Z-pair production,

•the cosine of the polar angle of the di-jet system which best fits the hA pair-production hypothesis,

•the cosine of the polar angle, ΘW+, at which the positive charged77

Charge assignment is based on jet-charge techniques [25].

boson is produced for the di-jet system which best fits the W-pair production hypothesis,

•y34,

•the absolute value of the cosine of the polar angle, ΘT, of the thrust axis.

Fig. 4

shows the distributions of the last three variables and of LhA for data, the expected background and the signal corresponding to the Higgs boson mass hypothesis (mh,mA)=(60,80) GeV. A cut on LhA is applied, which depends on s and on the (mh,mA) hypothesis, typically around 0.2. The remaining events are tested for consistency with a given (mh,mA) hypothesis by means of the variable χ2hA [8], defined analogously to χ2hZ. The pairing which minimises the value of χ2hA is chosen. For each event and each (mh,mA) hypothesis, the value of the signal over background ratio of the variable χ2hA is calculated. The distributions of these ratios are presented in Fig. 5

for different mass hypotheses.

Table 4

reports the numbers of observed events, expected background and expected signal events for several Higgs boson mass hypotheses, together with selection efficiencies.

Results

Table 3 shows the result of the combination of the different channels of the e+e−→hZ search. The observed number of events agrees with the Standard Model expectations. No significant excess is observed either in the e+e−→hZ search or in the e+e−→hA search, which is summarised in Table 4. Limits on the ξ and η couplings are extracted as a function of mh and mA from the distributions of the signal over background ratios derived from the final discriminant variables. The log-likelihood ratio technique [7] is used for the combination of the different channels of the e+e−→hZ search and to derive all the limits. For each final state, among the three possible decays of the h and A bosons into bb̄, cc̄ and gg, the case with the lowest efficiency is considered.Several sources of systematic uncertainties are investigated and their impact on the signal efficiency and the determination of the background level is assessed. The limited Monte Carlo statistics affects the signal by around 2% and the background by around 5%, depending on the final state. The selection criteria are varied within the resolution of the corresponding variables yielding an uncertainty from the selection procedure around 2% on the signal and from 3% to 6% on the background. Lepton identification criteria contribute to this source with an additional 1% for the signal and 2% for the background. The expected background level has an uncertainty up to 5%, depending on the final state, due to the uncertainty in the calculation of the cross sections of background processes.Particular care is payed to validate the accuracy of the simulation of gluon jets. A reference sample of three-jet events, from the e+e−→qq̄g(γ) process, is selected and the jet with the smallest energy in the rest frame of the hadronic system is taken as the gluon jet. The distributions of the most important gluon jet characteristics such as jet broadening, boosted sphericity and charged track multiplicity are compared for data and Monte Carlo samples. The latter, for instance, is found to be on average overestimated by the simulations and is not considered as input to the likelihoods and the neural networks. From this comparison, an additional systematic uncertainty is assigned as 1.5% for the signal and 2% for the background.The overall systematic uncertainties depend on the search channel and are estimated to range between 2% and 4% for the signal efficiencies and between 4% and 8% for the background levels. They are included in the derivation of the limits. For ξ2=1 they lower the sensitivity to mh by about 0.8 GeV and for η2=1 and mh=mA by about 0.7 GeV.

Fig. 6

shows the 95% confidence level (CL) upper limit on ξ2×B(h→hadrons) as a function of mh. The expected limit and the 68.3% and 95.4% probability bands expected in the absence of a signal are also displayed and denoted as 1σ and 2σ, respectively. For ξ2×B(h→hadrons)=1, i.e., for a cross section equivalent to the Standard Model one and a Higgs boson decaying into hadrons, a 95% CL lower limit of 110.3GeV is set on mh. The expected limit is 108.7GeV.

Fig. 7

shows the 95% CL upper limit on η2×B(hA→hadrons) as a function of mh+mA for several values of |mh−mA|. The expected limits and the 1σ and 2σ probability bands in absence of a signal are also shown. The observed limits for η=1 are between 120 and 140GeV, as expected. An excess of 2.9σ is observed around 135GeV for the mh=mA hypothesis. A similar behaviour is also observed in the search for charged Higgs bosons [26]. This excess is mainly due to data at low values of s. At higher energies and for larger integrated luminosities it does not scale with the cross section expected for a e+e−→hA signal. It is hence ascribed to a statistical fluctuation.

In conclusion, a flavour independent search for h and A bosons produced through Higgs-strahlung or in pairs and decaying into hadrons, shows no evidence of a signal and further constrains the scenario of Higgs bosons light enough to have been produced at LEP.

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