application/xmlSearch for the lightest scalar top quark in events with two leptons in [formula omitted] collisions at [formula omitted]DØ CollaborationV.M. AbazovB. AbbottM. AbolinsB.S. AcharyaM. AdamsT. AdamsE. AguiloM. AhsanG.D. AlexeevG. AlkhazovA. AltonG. AlversonG.A. AlvesM. AnastasoaieL.S. AncuT. AndeenB. AndrieuM.S. AnzelcM. AokiY. ArnoudM. ArovM. ArthaudA. AskewB. ÅsmanA.C.S. Assis JesusO. AtramentovC. AvilaF. BadaudL. BagbyB. BaldinD.V. BandurinP. BanerjeeS. BanerjeeE. BarberisA.-F. BarfussP. BargassaP. BaringerJ. BarretoJ.F. BartlettU. BasslerD. BauerS. BealeA. BeanM. BegalliM. BegelC. Belanger-ChampagneL. BellantoniA. BellavanceJ.A. BenitezS.B. BeriG. BernardiR. BernhardI. BertramM. BesançonR. BeuselinckV.A. BezzubovP.C. BhatV. BhatnagarG. BlazeyF. BlekmanS. BlessingK. BloomA. BoehnleinD. BolineT.A. BoltonE.E. BoosG. BorissovT. BoseA. BrandtR. BrockG. BrooijmansA. BrossD. BrownX.B. BuN.J. BuchananD. BuchholzM. BuehlerV. BuescherV. BunichevS. BurdinT.H. BurnettC.P. BuszelloP. CalfayanS. CalvetJ. CamminM.A. Carrasco-LizarragaE. CarreraW. CarvalhoB.C.K. CaseyH. Castilla-ValdezS. ChakrabartiD. ChakrabortyK.M. ChanA. ChandraE. CheuD.K. ChoS. ChoiB. ChoudharyL. ChristofekT. ChristoudiasS. CihangirD. ClaesJ. ClutterM. CookeW.E. CooperM. CorcoranF. CoudercM.-C. CousinouS. Crépé-RenaudinV. CuplovD. CuttsM. ĆwiokH. da MottaA. DasG. DaviesK. DeS.J. de JongE. De La Cruz-BureloC. De Oliveira MartinsK. DeVaughanF. DéliotM. DemarteauR. DeminaD. DenisovS.P. DenisovS. DesaiH.T. DiehlM. DiesburgA. DominguezT. DorlandA. DubeyL.V. DudkoL. DuflotS.R. DugadD. DugganA. DuperrinS. DuttJ. DyerA. DyshkantM. EadsD. EdmundsJ. EllisonV.D. ElviraY. EnariS. EnoP. ErmolovH. EvansA. EvdokimovV.N. EvdokimovA.V. FerapontovT. FerbelF. FiedlerF. FilthautW. FisherH.E. FiskM. FortnerH. FoxS. FuS. FuessT. GadfortC.F. GaleaC. GarciaA. Garcia-BellidoV. GavrilovP. GayW. GeistW. GengC.E. GerberY. GershteinD. GillbergG. GintherB. GómezA. GoussiouP.D. GrannisH. GreenleeZ.D. GreenwoodE.M. GregoresG. GrenierPh. GrisJ.-F. GrivazA. GrohsjeanS. GrünendahlM.W. GrünewaldF. GuoJ. GuoG. GutierrezP. GutierrezA. HaasN.J. HadleyP. HaefnerS. HagopianJ. HaleyI. HallR.E. HallL. HanK. HarderA. HarelJ.M. HauptmanJ. HaysT. HebbekerD. HedinJ.G. HegemanA.P. HeinsonU. HeintzC. HenselK. HernerG. HeskethM.D. HildrethR. HiroskyT. HoangJ.D. HobbsB. HoeneisenM. HohlfeldS. HossainP. HoubenY. HuZ. HubacekV. HynekI. IashviliR. IllingworthA.S. ItoS. JabeenM. JaffréS. JainK. JakobsC. JarvisR. JesikK. JohnsC. JohnsonM. JohnsonD. JohnstonA. JonckheereP. JonssonA. JusteE. KajfaszD. KarmanovP.A. KasperI. KatsanosV. KaushikR. KehoeS. KermicheN. KhalatyanA. KhanovA. KharchilavaY.N. KharzheevD. KhatidzeT.J. KimM.H. KirbyM. KirschB. KlimaJ.M. KohliJ.-P. KonrathA.V. KozelovJ. KrausT. KuhlA. KumarA. KupcoT. KurčaV.A. KuzminJ. KvitaF. LacroixD. LamS. LammersG. LandsbergP. LebrunW.M. LeeA. LeflatJ. LellouchJ. LiL. LiQ.Z. LiS.M. LiettiJ.K. LimJ.G.R. LimaD. LincolnJ. LinnemannV.V. LipaevR. LiptonY. LiuZ. LiuA. LobodenkoM. LokajicekP. LoveH.J. LubattiR. Luna-GarciaA.L. LyonA.K.A. MacielD. MackinR.J. MadarasP. MättigA. MagerkurthP.K. MalH.B. MalbouissonS. MalikV.L. MalyshevY. MaravinB. MartinR. McCarthyM.M. MeijerA. MelnitchoukL. MendozaP.G. MercadanteM. MerkinK.W. MerrittA. MeyerJ. MeyerJ. MitrevskiR.K. MommsenN.K. MondalR.W. MooreT. MoulikG.S. MuanzaM. MulhearnO. MundalL. MundimE. NagyM. NaimuddinM. NarainH.A. NealJ.P. NegretP. NeustroevH. NilsenH. NogimaS.F. NovaesT. NunnemannD.C. O'NeilG. ObrantC. OchandoD. OnoprienkoN. OshimaN. OsmanJ. OstaR. OtecG.J. Otero y GarzónM. OwenP. PadleyM. PangilinanN. ParasharS.-J. ParkS.K. ParkJ. ParsonsR. PartridgeN. ParuaA. PatwaG. PawloskiB. PenningM. PerfilovK. PetersY. PetersP. PétroffM. PetteniR. PiegaiaJ. PiperM.-A. PleierP.L.M. Podesta-LermaV.M. PodstavkovY. PogorelovM.-E. PolP. PolozovB.G. PopeA.V. PopovC. PotterW.L. Prado da SilvaH.B. ProsperS. ProtopopescuJ. QianA. QuadtB. QuinnA. RakitineM.S. RangelK. RanjanP.N. RatoffP. RenkelP. RichM. RijssenbeekI. Ripp-BaudotF. RizatdinovaS. RobinsonR.F. RodriguesM. RominskyC. RoyonP. RubinovR. RuchtiG. SafronovG. SajotA. Sánchez-HernándezM.P. SandersB. SanghiG. SavageL. SawyerT. ScanlonD. SchaileR.D. SchambergerY. ScheglovH. SchellmanT. SchliephakeS. SchlobohmC. SchwanenbergerA. SchwartzmanR. SchwienhorstJ. SekaricH. SeveriniE. ShabalinaM. ShamimV. SharyA.A. ShchukinR.K. ShivpuriV. SiccardiV. SimakV. SirotenkoP. SkubicP. SlatteryD. SmirnovG.R. SnowJ. SnowS. SnyderS. Söldner-RemboldL. SonnenscheinA. SopczakM. SosebeeK. SoustruznikB. SpurlockJ. StarkV. StolinD.A. StoyanovaJ. StrandbergS. StrandbergM.A. StrangE. StraussM. StraussR. StröhmerD. StromL. StutteS. SumowidagdoP. SvoiskyA. SznajderA. TanasijczukW. TaylorB. TillerF. TissandierM. TitovV.V. TokmeninI. TorchianiD. TsybychevB. TuchmingC. TullyP.M. TutsR. UnalanL. UvarovS. UvarovS. UzunyanB. VachonP.J. van den BergR. Van KootenW.M. van LeeuwenN. VarelasE.W. VarnesI.A. VasilyevP. VerdierL.S. VertogradovM. VerzocchiD. VilanovaF. Villeneuve-SeguierP. VintP. VokacM. VoutilainenR. WagnerH.D. WahlM.H.L.S. WangJ. WarcholG. WattsM. WayneG. WeberM. WeberL. Welty-RiegerA. WengerN. WermesM. WetsteinA. WhiteD. WickeM.R.J. WilliamsG.W. WilsonS.J. WimpennyM. WobischD.R. WoodT.R. WyattY. XieC. XuS. YacoobR. YamadaW.-C. YangT. YasudaY.A. YatsunenkoH. YinK. YipH.D. YooS.W. YounJ. YuC. ZeitnitzS. ZelitchT. ZhaoB. ZhouJ. ZhuM. ZielinskiD. ZieminskaA. ZieminskiL. ZivkovicV. ZutshiE.G. ZverevPhysics Letters B 675 (2009) 289-296. doi:10.1016/j.physletb.2009.04.039journalPhysics Letters BCopyright © 2009 Elsevier B.V. All rights reserved.Elsevier B.V.0370-26936753-418 May 20092009-05-18289-29628929610.1016/j.physletb.2009.04.039http://dx.doi.org/10.1016/j.physletb.2009.04.039doi:10.1016/j.physletb.2009.04.039http://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

S300.2

PLB25780S0370-2693(09)00443-210.1016/j.physletb.2009.04.039Elsevier B.V.Experiments

Fig. 1

Distributions (eμ channel) of (a) E̸T after preselection, (b) Δϕ(e,E̸T) and (c) Δϕ(μ,E̸T) after Emu 1, (d) Δϕ(e,E̸T)+Δϕ(μ,E̸T) after Emu 2, (e) HT and (f) ST after Emu 3, for observed events (dots), expected background (filled areas), and signal expectations for Signal A (solid line) and Signal B (dashed line).

Fig. 2

Distributions (ee channel) of (a) the number of jets after the preselection, (b) E̸T after Dielec 2, (c) the dielectron invariant mass and (d) E̸T after Dielec 3, (e) HT and (f) ST after Dielec 5, for observed events (dots), expected background (filled areas), and signal expectations for Signal A (solid line) and Signal B (dashed line).

Fig. 3

The 95% C.L. exclusion contour in the sneutrino mass versus stop mass plane. Shaded areas represent the kinematically forbidden region and the LEP I [22] and LEP II [4] exclusions. The dashed and continuous lines represent, respectively, the expected and observed 95% C.L. exclusion limit for this analysis. The band surrounding the observed limit denotes the effect of the uncertainty on the stop production cross section.

Table 1

Numbers of events observed in data and expected from SM background processes and the two signal samples A and B at the various stages of the analysis in the eμ channel. The quoted uncertainties are statistical only.

Total SMBackground contributionsSelectionDataBackgroundZ/γ∗→τ+τ−tt¯DibosonInstrumentalSignal ASignal BPreselection735736±1545829.760.618834.0±126.3±0.7Emu 1106106±52323.538.72110.6±0.719.4±0.6Emu 27177±45.920.036.2158.4±0.717.6±0.6Emu 36165±40.716.434.5136.0±0.616.1±0.5Table 2

Numbers of observed events in data and expected yields from SM background processes for the twelve ST and HT bins in the eμ channel. The quoted uncertainties are statistical only.

ST (GeV)HT (GeV)0–7070–120>120DataSMDataSMDataSM0–1510.3±0.31513±21219±215–6010.09±0.164.2±0.9118±160–12000.06±0.111.6±0.689±1>12000.01±0.0500.9±0.467±1Table 3

Numbers of events observed in data and expected from SM background processes and the two signal samples A and B at the various stages of the analysis in the ee channel. The quoted uncertainties are statistical only.

Total SMBackground contributionsSelectionDataBackgroundZ/γ∗→e+e−Z/γ∗→τ+τ−tt¯DibosonInstrumentalSignal ASignal BPreselection2775725419±872481012014.123.445210.7±0.512.7±0.3Dielec 162786335±3861432914.212.61364.8±0.410.6±0.3Dielec 2192200±51661112.13.9123.0±0.38.9±0.2Dielec 3142152±41229.311.43.55.82.6±0.38.0±0.2Dielec 41516.0±0.66.70.58.40.220.170.6±0.14.7±0.2Dielec 51212.2±0.43.00.58.40.120.160.6±0.14.6±0.2Table 4

Numbers of observed events in data and expected yields from SM background processes for the four ST and HT bins in the ee channel. The quoted uncertainties are statistical only.

ST (GeV)HT (GeV)45–150>150DataSMDataSM15–6011.9±0.321±0.1>6033.3±0.266±0.2Search for the lightest scalar top quark in events with two leptons in pp¯ collisions at s=1.96 TeV

DØ Collaboration

V.M.

Abazov

Abbott

Abolins

B.S.

Acharya

Adams

Aguilo

Ahsan

G.D.

Alexeev

Alkhazov

Alton

Alverson

G.A.

Alves

Anastasoaie

L.S.

Ancu

Andeen

Andrieu

M.S.

Anzelc

Aoki

Arnoud

Arov

Arthaud

Askew

Åsman

A.C.S. Assis

Jesus

Atramentov

Avila

Badaud

Bagby

Baldin

D.V.

Bandurin

Banerjee

Barberis

A.-F.

Barfuss

Bargassa

Baringer

Barreto

J.F.

Bartlett

Bassler

Bauer

Beale

Bean

Begalli

Begel

Belanger-Champagne

Bellantoni

Bellavance

J.A.

Benitez

S.B.

Beri

Bernardi

Bernhard

Bertram

Besançon

Beuselinck

V.A.

Bezzubov

P.C.

Bhat

Bhatnagar

Blazey

Blekman

Blessing

Bloom

Boehnlein

Boline

T.A.

Bolton

E.E.

Boos

Borissov

Bose

Brandt

Brock

Brooijmans

Bross

Brown

X.B.

N.J.

Buchanan

Buchholz

Buehler

Buescher

Bunichev

Burdin

T.H.

Burnett

C.P.

Buszello

Calfayan

Calvet

Cammin

M.A.

Carrasco-Lizarraga

Carrera

Carvalho

B.C.K.

Casey

Castilla-Valdez

Chakrabarti

Chakraborty

K.M.

Chan

Chandra

Cheu

D.K.

Cho

Choi

Choudhary

Christofek

Christoudias

Cihangir

Claes

Clutter

Cooke

W.E.

Cooper

Corcoran

Couderc

M.-C.

Cousinou

Crépé-Renaudin

Cuplov

Cutts

Ćwiok

da Motta

Das

Davies

S.J.

de Jong

De La Cruz-Burelo

De Oliveira Martins

DeVaughan

Déliot

Demarteau

Demina

Denisov

S.P.

Denisov

Desai

H.T.

Diehl

Diesburg

Dominguez

Dorland

Dubey

L.V.

Dudko

Duflot

S.R.

Dugad

Duggan

Duperrin

Dutt

Dyer

Dyshkant

Eads

Edmunds

Ellison

V.D.

Elvira

Enari

Eno

Ermolov

Evans

Evdokimov

V.N.

Evdokimov

A.V.

Ferapontov

Ferbel

Fiedler

Filthaut

Fisher

H.E.

Fisk

Fortner

Fox

Fuess

Gadfort

C.F.

Galea

Garcia

Garcia-Bellido

Gavrilov

Gay

Geist

Geng

C.E.

Gerber

Gershtein

Gillberg

Ginther

Gómez

Goussiou

P.D.

Grannis

Greenlee

Z.D.

Greenwood

E.M.

Gregores

Grenier

Ph.

Gris

⁎

gris@clermont.in2p3.fr

J.-F.

Grivaz

Grohsjean

Grünendahl

M.W.

Grünewald

Guo

Gutierrez

Haas

N.J.

Hadley

Haefner

Hagopian

Haley

Hall

R.E.

Hall

Han

Harder

Harel

J.M.

Hauptman

Hays

Hebbeker

Hedin

J.G.

Hegeman

A.P.

Heinson

Heintz

Hensel

Herner

Hesketh

M.D.

Hildreth

Hirosky

Hoang

J.D.

Hobbs

Hoeneisen

Hohlfeld

Hossain

Houben

Hubacek

Hynek

Iashvili

Illingworth

A.S.

Ito

Jabeen

Jaffré

Jain

Jakobs

Jarvis

Jesik

Johns

Johnson

Johnston

Jonckheere

Jonsson

Juste

Kajfasz

Karmanov

P.A.

Kasper

Katsanos

Kaushik

Kehoe

Kermiche

Khalatyan

Khanov

Kharchilava

Y.N.

Kharzheev

Khatidze

T.J.

Kim

M.H.

Kirby

Kirsch

Klima

J.M.

Kohli

J.-P.

Konrath

A.V.

Kozelov

Kraus

Kuhl

Kumar

Kupco

Kurča

V.A.

Kuzmin

Kvita

Lacroix

Lam

Lammers

Landsberg

Lebrun

W.M.

Lee

Leflat

Lellouch

Q.Z.

S.M.

Lietti

J.K.

Lim

J.G.R.

Lima

Lincoln

Linnemann

V.V.

Lipaev

Lipton

Liu

Lobodenko

Lokajicek

Love

H.J.

Lubatti

Luna-Garcia

A.L.

Lyon

A.K.A.

Maciel

Mackin

R.J.

Madaras

Mättig

Magerkurth

P.K.

Mal

H.B.

Malbouisson

Malik

V.L.

Malyshev

Maravin

Martin

McCarthy

M.M.

Meijer

Melnitchouk

Mendoza

P.G.

Mercadante

Merkin

K.W.

Merritt

Meyer

Mitrevski

R.K.

Mommsen

N.K.

Mondal

R.W.

Moore

Moulik

G.S.

Muanza

Mulhearn

Mundal

Mundim

Nagy

Naimuddin

Narain

H.A.

Neal

J.P.

Negret

Neustroev

Nilsen

Nogima

S.F.

Novaes

Nunnemann

D.C.

O'Neil

Obrant

Ochando

Onoprienko

Oshima

Osman

Osta

Otec

G.J.

Otero y Garzón

Owen

Padley

Pangilinan

Parashar

S.-J.

Park

S.K.

Park

Parsons

Partridge

Parua

Patwa

Pawloski

Penning

Perfilov

Peters

Pétroff

Petteni

Piegaia

Piper

M.-A.

Pleier

P.L.M.

Podesta-Lerma

V.M.

Podstavkov

Pogorelov

M.-E.

Pol

Polozov

B.G.

Pope

A.V.

Popov

Potter

W.L. Prado

da Silva

H.B.

Prosper

Protopopescu

Qian

Quadt

Quinn

Rakitine

M.S.

Rangel

Ranjan

P.N.

Ratoff

Renkel

Rich

Rijssenbeek

Ripp-Baudot

Rizatdinova

Robinson

R.F.

Rodrigues

Rominsky

Royon

Rubinov

Ruchti

Safronov

Sajot

Sánchez-Hernández

M.P.

Sanders

Sanghi

Savage

Sawyer

Scanlon

Schaile

R.D.

Schamberger

Scheglov

Schellman

Schliephake

Schlobohm

Schwanenberger

Schwartzman

Schwienhorst

Sekaric

Severini

Shabalina

Shamim

Shary

A.A.

Shchukin

R.K.

Shivpuri

Siccardi

Simak

Sirotenko

Skubic

Slattery

Smirnov

G.R.

Snow

Snyder

Söldner-Rembold

Sonnenschein

Sopczak

Sosebee

Soustruznik

Spurlock

Stark

Stolin

D.A.

Stoyanova

Strandberg

M.A.

Strang

Strauss

Ströhmer

Strom

Stutte

Sumowidagdo

Svoisky

Sznajder

Tanasijczuk

Taylor

Tiller

Tissandier

Titov

V.V.

Tokmenin

Torchiani

Tsybychev

Tuchming

Tully

P.M.

Tuts

Unalan

Uvarov

Uzunyan

Vachon

P.J.

van den Berg

Van Kooten

W.M.

van Leeuwen

Varelas

E.W.

Varnes

I.A.

Vasilyev

Verdier

L.S.

Vertogradov

Verzocchi

Vilanova

Villeneuve-Seguier

Vint

Vokac

Voutilainen

Wagner

H.D.

Wahl

M.H.L.S.

Wang

Warchol

Watts

Wayne

Weber

Welty-Rieger

Wenger

Wermes

Wetstein

White

Wicke

M.R.J.

Williams

G.W.

Wilson

S.J.

Wimpenny

Wobisch

D.R.

Wood

T.R.

Wyatt

Xie

Yacoob

Yamada

W.-C.

Yang

Yasuda

Y.A.

Yatsunenko

Yin

Yip

H.D.

Yoo

S.W.

Youn

Zeitnitz

Zelitch

Zhao

Zhou

Zhu

Zielinski

Zieminska

Zieminski

Zivkovic

Zutshi

E.G.

Zverev

aUniversidad de Buenos Aires, Buenos Aires, ArgentinabLAFEX, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, BrazilcUniversidade do Estado do Rio de Janeiro, Rio de Janeiro, BrazildUniversidade Federal do ABC, Santo André, BrazileInstituto de Física Teórica, Universidade Estadual Paulista, São Paulo, BrazilfUniversity of Alberta, Edmonton, Alberta, and Simon Fraser University, Burnaby, British Columbia, and York University, Toronto, Ontario and McGill University, Montreal, Quebec, CanadagUniversity of Science and Technology of China, Hefei, People's Republic of ChinahUniversidad de los Andes, Bogotá, ColombiaiCenter for Particle Physics, Charles University, Prague, Czech RepublicjCzech Technical University, Prague, Czech RepublickCenter for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech RepubliclUniversidad San Francisco de Quito, Quito, EcuadormLPC, Université Blaise Pascal, CNRS/IN2P3, Clermont, FrancenLPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, FranceoCPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, FrancepLAL, Université Paris-Sud, IN2P3/CNRS, Orsay, FranceqLPNHE, IN2P3/CNRS, Universités Paris VI and VII, Paris, FrancerCEA, Irfu, SPP, Saclay, FrancesIPHC, Université Louis Pasteur, CNRS/IN2P3, Strasbourg, FrancetIPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, France and Université de Lyon, Lyon, FranceuIII. Physikalisches Institut A, RWTH Aachen University, Aachen, GermanyvPhysikalisches Institut, Universität Bonn, Bonn, GermanywPhysikalisches Institut, Universität Freiburg, Freiburg, GermanyxInstitut für Physik, Universität Mainz, Mainz, GermanyyLudwig-Maximilians-Universität München, München, GermanyzFachbereich Physik, University of Wuppertal, Wuppertal, GermanyaaPanjab University, Chandigarh, IndiaabDelhi University, Delhi, IndiaacTata Institute of Fundamental Research, Mumbai, IndiaadUniversity College Dublin, Dublin, IrelandaeKorea Detector Laboratory, Korea University, Seoul, Republic of KoreaafSungKyunKwan University, Suwon, Republic of KoreaagCINVESTAV, Mexico City, MexicoahFOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The NetherlandsaiRadboud University Nijmegen/NIKHEF, Nijmegen, The NetherlandsajJoint Institute for Nuclear Research, Dubna, RussiaakInstitute for Theoretical and Experimental Physics, Moscow, RussiaalMoscow State University, Moscow, RussiaamInstitute for High Energy Physics, Protvino, RussiaanPetersburg Nuclear Physics Institute, St. Petersburg, RussiaaoLund University, Lund, and Royal Institute of Technology and Stockholm University, Stockholm, and Uppsala University, Uppsala, SwedenapLancaster University, Lancaster, United KingdomaqImperial College, London, United KingdomarUniversity of Manchester, Manchester, United KingdomasUniversity of Arizona, Tucson, AZ 85721, USAatLawrence Berkeley National Laboratory and University of California, Berkeley, CA 94720, USAauCalifornia State University, Fresno, CA 93740, USAavUniversity of California, Riverside, CA 92521, USAawFlorida State University, Tallahassee, FL 32306, USAaxFermi National Accelerator Laboratory, Batavia, IL 60510, USAayUniversity of Illinois at Chicago, Chicago, IL 60607, USAazNorthern Illinois University, DeKalb, IL 60115, USAbaNorthwestern University, Evanston, IL 60208, USAbbIndiana University, Bloomington, IN 47405, USAbcUniversity of Notre Dame, Notre Dame, IN 46556, USAbdPurdue University Calumet, Hammond, IN 46323, USAbeIowa State University, Ames, IA 50011, USAbfUniversity of Kansas, Lawrence, KS 66045, USAbgKansas State University, Manhattan, KS 66506, USAbhLouisiana Tech University, Ruston, LA 71272, USAbiUniversity of Maryland, College Park, MD 20742, USAbjBoston University, Boston, MA 02215, USAbkNortheastern University, Boston, MA 02115, USAblUniversity of Michigan, Ann Arbor, MI 48109, USAbmMichigan State University, East Lansing, MI 48824, USAbnUniversity of Mississippi, University, MS 38677, USAboUniversity of Nebraska, Lincoln, NE 68588, USAbpPrinceton University, Princeton, NJ 08544, USAbqState University of New York, Buffalo, NY 14260, USAbrColumbia University, New York, NY 10027, USAbsUniversity of Rochester, Rochester, NY 14627, USAbtState University of New York, Stony Brook, NY 11794, USAbuBrookhaven National Laboratory, Upton, NY 11973, USAbvLangston University, Langston, OK 73050, USAbwUniversity of Oklahoma, Norman, OK 73019, USAbxOklahoma State University, Stillwater, OK 74078, USAbyBrown University, Providence, RI 02912, USAbzUniversity of Texas, Arlington, TX 76019, USAcaSouthern Methodist University, Dallas, TX 75275, USAcbRice University, Houston, TX 77005, USAccUniversity of Virginia, Charlottesville, VA 22901, USAcdUniversity of Washington, Seattle, WA 98195, USA⁎Corresponding author.1

Visitor from Augustana College, Sioux Falls, SD, USA.

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Editor: H. Weerts

Abstract

We report results of a search for the pair production of the lightest supersymmetric partner of the top quark, t˜1, using a data set corresponding to an integrated luminosity of 1 fb−1 collected by the DØ detector at a pp¯ center-of-mass energy of 1.96 TeV at the Fermilab Tevatron collider. Both scalar top quarks are assumed to decay into a b quark, a charged lepton and a scalar neutrino. The search is performed in the electron plus muon and dielectron final states. The signal topology consists of two isolated leptons, missing transverse energy, and jets. We find no evidence for this process and exclude regions of parameter space in the framework of the minimal supersymmetric standard model.

PACS

14.80.Ly12.60.Jv13.85.Rm

Supersymmetric theories [1] predict for every Standard Model (SM) particle the existence of a superpartner that differs by half a unit of spin. The top quark would have two scalar partners, t˜L and t˜R, corresponding to its left- and right-handed states. Mixing between t˜L and t˜R, being proportional to the top quark mass mt, may lead to a possible large mass splitting between the physical states t˜1 and t˜2. Hence, the lightest supersymmetric partner of the top quark, t˜1, might be light enough to be produced at the Fermilab Tevatron collider.In this Letter we present a search for scalar top (stop) pair production in a data sample corresponding to an integrated luminosity of 1 fb−1 collected at a center-of-mass energy of 1.96 TeV with the DØ detector during Run II of the Fermilab Tevatron pp¯ collider. The phenomenological framework is the minimal supersymmetric standard model (MSSM) with R-parity conservation. We assume that BR(t˜1→bℓν˜)=1, where ν˜ is the scalar neutrino (sneutrino). Among possible stop decays [2], this final state is one of the most attractive; in addition to a b quark, it benefits from the presence of a lepton with high transverse momentum with respect to the beam axis (pT). The sneutrino is either the lightest supersymmetric particle (LSP) or decays invisibly: ν˜→νχ˜10 or νG˜ where the lightest neutralino, χ˜10, or the gravitino, G˜, is the LSP. We suppose an equal sharing among lepton flavors and consider t˜1t˜¯1→bb¯ℓℓ′ν˜ν˜ final states, with ℓℓ′=e±μ∓ (eμ channel) and ℓℓ′=e+e− (ee channel). The signal topology consists of two isolated leptons, missing transverse energy (E̸T), coming mainly from undetected sneutrinos, and jets. A search for stop pair production in the eμ and μμ(t˜1t˜¯1→bb¯μμν˜ν˜) channels has previously been performed by the DØ Collaboration [3] using a data set corresponding to a luminosity of 428 pb−1. The eμ sample in [3] is a subset of the data sample used in this analysis. Searches for stop pair production in the bb¯ℓℓ′ν˜ν˜ final state have been reported by the ALEPH, L3, and OPAL Collaborations [4].The DØ detector [5] comprises a central tracking system surrounded by a liquid-argon/uranium sampling calorimeter and muon detectors. Charged particles are reconstructed using multi-layer silicon detectors and eight double layers of scintillating fibers in a 2 T magnetic field produced by a superconducting solenoid. After passing through the calorimeter, muons are detected in the muon system comprising three layers of tracking detectors and scintillation counters. Events containing electrons or muons are selected for offline analysis by an online trigger system. A combination of single electron (ee channel) and dilepton (eμ channel) triggers is used to tag the presence of electrons and muons based on their energy deposition in the calorimeter, hits in the muon detectors, and tracks in the tracking system.In pp¯ collisions, stops are pair-produced via quark–antiquark annihilation and gluon fusion. The t˜1 pair production cross section, σt˜1t˜¯1, depends primarily on mt˜1, with only a weak dependence on other MSSM parameters. At s=1.96 TeV, σt˜1t˜¯1 at next-to-leading-order (NLO), calculated with prospino[6], ranges from 15 pb to 0.5 pb for 100⩽mt˜1⩽180 GeV. These cross sections are estimated using CTEQ6.1M parton distribution functions (PDF) [7,8] and equal renormalization and factorization scales μr,f=mt˜1. A theoretical uncertainty of about 18% is estimated due to scale and PDF choice.Three-body decays of the stop are simulated using comphep[9] and pythia[10] for parton-level generation and hadronization, respectively. We consider a range of stop mass values from 100 to 200 GeV in steps of 10 GeV. The range of sneutrino masses explored extends from 40 to 140 GeV in steps of 10 to 20 GeV. For each choice of [mt˜1, mν˜], 10,000 events are generated. Background processes are simulated using the pythia and alpgen[11] Monte Carlo (MC) generators. alpgen is interfaced with pythia for parton showering and hadronization. The MC samples use the CTEQ6L PDF and are normalized using next-to-leading order cross sections [12–14]. All generated events are passed through the full simulation of the detector geometry and response based on geant[15]. MC events are then reconstructed and analyzed with the same software as used for the data.The signal topology depends both on mt˜1 and on the mass difference Δm=mt˜1−mν˜. The pT of the leptons and b quarks decrease with smaller values of Δm and E̸T values are correlated with mt˜1 and Δm. For both eμ and ee channels, the two signal points [mt˜1,mν˜]=(140,110) GeV and (170,90) GeV, referred to respectively as “Signal A” and “Signal B” in the following, are chosen to illustrate the effect of the selections for low mt˜1 and low Δm (Signal A) and for high mt˜1 and high Δm (Signal B).The main SM background processes mimicking the signal signature are Z/γ∗→τ+τ−, WW, WZ, ZZ, and tt¯ (eμ and ee decay channels), Z/γ∗→e+e− (ee channel), and instrumental background (eμ and ee channels). All but the latter are estimated using MC simulations.Electrons are identified as clusters of energy in calorimeter cells in a cone of size R≡(Δϕ)2+(Δη)2=0.4 where ϕ is the azimuthal angle and η the pseudorapidity.1111

The pseudorapidity η is defined as η=−ln[tan(θ/2)], with θ being the polar angle with respect to the proton beam direction.

Electron candidates are required to have a large fraction of their energy deposited in the electromagnetic layers of the calorimeter. The clusters are required to be isolated from hadronic energy depositions. The calorimeter isolation variable I=[Etot(0.4)−EEM(0.2)]/EEM(0.2) is less than 0.15, where Etot(0.4) is the total transverse shower energy in a cone of radius R=0.4 and EEM(0.2) is the electromagnetic energy in a cone R=0.2. The clusters are also required to have a spatially-matched track in the central tracking system with pT larger than 8 GeV, and to have a shower shape consistent with that of an electron. Electrons are also required to satisfy identification criteria combined in a likelihood variable and based on multivariate discriminators derived from calorimeter shower shape and track variables. Only central electrons (|η|<1.1) with transverse energy with respect to the beam axis (ET) measured in the calorimeter larger than 15 GeV are considered.Muons are reconstructed by finding tracks pointing to hit patterns in the muon system. Non-isolated muons are rejected by requiring the sum of the transverse momenta of tracks inside a cone of radius R=0.5 around the muon direction to be less than 4 GeV, and the sum of transverse energy in the calorimeter in a hollow cone of size 0.1<R<0.4 around the muon to be less than 4 GeV. To reject cosmic ray muons, requirements on the time of arrival of the muon at the various scintillator layers in the muon system are made. Muons with |η|<2 and pT>8 GeV are considered.Jets are reconstructed from the energy deposition in the calorimeter towers using the Run II cone algorithm [16] with a radius Rcone≡(Δϕ)2+(Δy)2=0.5, where y is the rapidity.1212

The rapidity y is defined as y=12ln[(E+pZ)/(E−pZ)].

Jet energies are calibrated to the particle level using correction factors primarily derived from the transverse momentum balance in photon plus jets events. Only jets with pT>15 GeV and |η|<2.5 are considered. The E̸T is calculated using all calorimeter cells and is corrected for the jet and electromagnetic energy scales and for the momentum of selected muons.In each event, the best primary vertex is selected from all reconstructed primary vertices as the one with the smallest probability of originating from a minimum bias interaction [17]. Its longitudinal position with respect to the detector center, z, is restricted to |z|<60 cm to ensure efficient reconstruction. The leptons in an event are required to be isolated from each other (R(ℓ,ℓ′)>0.5) and from a jet (R(ℓ,jet)>0.5).The instrumental background is due to either misidentified electrons or muons, mismeasured E̸T, or electrons or muons from multijet processes that pass the lepton isolation requirements presented above. Data samples dominated by instrumental background are selected by inverting the muon isolation requirements or the electron-likelihood cut (eμ channel) or both electron-likelihood criteria (ee channel). The normalization factors for those samples are estimated from observed events. In the eμ channel, an exponential fit is performed to the E̸T distribution in the range E̸T<35 GeV, after subtraction of the MC estimates of the non-instrumental backgrounds, in events containing one electron and one muon. In the ee channel, the normalization is performed using both electron ET shapes in events containing two electrons in a domain where the instrumental background has a large contribution.The integrated luminosity [18] of the eμ data sample is 1100±67 pb−1. Events are preselected with the requirement that they contain one electron and one muon. To remove a large part of the instrumental background as well as events coming from Z/γ∗→τ+τ−, selections on the E̸T [Fig. 1

(a)] and on the E̸T significance, S(E̸T), defined as the ratio of the E̸T in an event to its estimated uncertainty given the expected resolutions on the pT measurements for the selected leptons and jets, are applied:(Emu 1)E̸T>30 GeV,S(E̸T)>4.

At this stage, the instrumental and Z/γ∗→τ+τ− events comprise a large part (41%) of the total background. In these processes, reconstructed leptons are correlated with the E̸T, giving rise to higher event populations at high and low values of the azimuthal angle difference between the leptons and E̸T, with a low value of the angular difference for one lepton being correlated with a high value for the other. As there is a higher background contribution at low values of the angular distributions [Figs. 1(b) and 1(c)], we require:(Emu 2)Δϕ(μ,E̸T)>0.4 rad,Δϕ(e,E̸T)>0.4 rad.To reduce the Z/γ∗→τ+τ− background, selections on the transverse mass of the muon and E̸T, MT(μ,E̸T)[19], and of the electron and E̸T, MT(e,E̸T), are applied. To further reduce this background, we use the azimuthal angular differences between the leptons and the missing energy, Δϕ(μ,E̸T) and Δϕ(e,E̸T), which should be large [Fig. 1(d)]. We require:(Emu 3)MT(μ,E̸T)>20 GeV,MT(e,E̸T)>20 GeV,Δϕ(μ,E̸T)+Δϕ(e,E̸T)>2.9 rad.The number of events surviving at each analysis step for the data, for each background component, and for the two signal samples A and B are summarized in Table 1

. After all selections, the WW, tt¯, and instrumental background contributions dominate. To separate the signal from these backgrounds, two topological variables are used: ST, defined as the scalar sum of the muon pT, the electron pT, and the E̸T; and HT, defined as the scalar sum of the transverse momenta of all the jets. WW and instrumental backgrounds populate low values of HT and ST while top quark pairs have large values for both variables. The signal distribution depends on the stop mass and on the mass difference Δm, with low values of Δm having low values of HT and ST [Figs. 1(e) and 1(f)]. Rather than selecting events using these two variables, the numbers of events predicted for signal and background are compared to the observed numbers in twelve [ST,HT] bins (Table 2

) when extracting limits on the cross section for the eμ channel.

The integrated luminosity of the ee data sample is 1043±64 pb−1. At preselection, two electrons are required. Z/γ∗→e+e− events account for 94% of the total background. While the signal is characterized by the presence of jets originating from the hadronization of b quarks, the Z/γ∗→e+e− background owes the presence of jets to gluons from initial state radiation which hadronize into softer jets, resulting in a lower multiplicity of jets. To keep sensivity to low Δm signals while rejecting substantial background, we require at least one jet [Fig. 2

(a)]:(Dielec 1)N(jets)⩾1. To reject contributions from both the instrumental and Z/γ∗→e+e− backgrounds, cuts on the E̸T and on its significance are performed:(Dielec 2)E̸T>15 GeV,S(E̸T)>5. At this stage of the analysis, the Z/γ∗→e+e− sample is still dominant [Fig. 2(b)] and give rise to higher event populations at high values of the azimuthal angle difference between the two electrons. To remove these events, the following selection is applied:(Dielec 3)Δϕ(ee)<3 rad.

To increase the search sensitivity in this channel, we take advantage of the presence of jets originating from the fragmentation of long-lived b quarks in the signal. A neural network (NN) tagging tool [20] for heavy flavor that combines information from several lifetime-based b-taggers to maximize the b quark tagging efficiency is used for this purpose. At least one jet in the event is required to be b-tagged (Dielec 4) by satisfying a given NN selection. The b quark tagging operating point preserves high efficiency for the detection of b jets (≈66%) with a ≈3% probability for a light parton jet to be mistakenly tagged. This point maximizes the sensitivity of the analysis for stop masses of 130–140 GeV and for low Δm. At this stage, most of the surviving Z/γ∗→e+e− events have a dielectron mass in the vicinity of the Z boson resonance and low E̸T values [Figs. 2(c) and 2(d)]. To further suppress this background while preserving the signal, a cut in the plane [M(e,e),E̸T] is applied. This selection is optimized for low Δm signals and is defined by:(Dielec 5)M(e,e)∉[75,105] GeVif E̸T<30 GeV.The selections applied in the ee channel are summarized in Table 3

along with the number of events surviving at each step for the data, for each background component, and for the two signal samples A and B. Compared to the eμ channel, the estimated yields of tt¯, Z/γ∗→τ+τ− and diboson backgrounds are lower at the preselection stage. This is explained mainly by the threshold values of pT and η used to identify electrons and muons. A slight excess of observed events is seen at the preselection level and is due to Z/γ∗→e+e− events having no jets and for which the boson transverse momentum is lower than 20 GeV. For these events, the parton showering implemented in the MC generators used in this analysis gives inaccurate results. The tt¯ background dominates in the final stage of the selection. Four bins in HT and ST [Figs. 2(e) and 2(f) and Table 4

] are considered to separate the signal from the SM background.

For both eμ and ee channels, signal efficiencies, defined with respect to the numbers of events in the relevant channels, reach a value of 10% for large mass differences but decrease to values lower than 0.1% for Δm<20 GeV.The expected numbers of background and signal events depend on several measurements and parametrizations which each introduce a systematic uncertainty. The main sources of uncertainty that are common to eμ and ee channels and affect both the backgrounds and the signal consist of: electron identification and reconstruction efficiency (5% for the background, between 2% and 10% for the signal), jet energy calibration (3% for the background, between 2% and 11% for the signal), jet identification efficiency and energy resolution (2% for the background, between 3% and 17% for the signal), luminosity (6.1%) [18], trigger efficiency (2%). The following systematic uncertainties related to the background only are considered: instrumental background modeling (5% in the eμ channel and 18% in the ee channel) and PDF (5% for diboson and 15% for tt¯ and Z/γ∗ processes). In addition, the eμ channel is affected by a systematic uncertainty related to the muon identification and reconstruction efficiency (2% for the background, between 2% and 5% for the signal). In the ee channel, an uncertainty coming from HF tagging is applied (2% for the background, between 2% and 5% for the signal). These systematic uncertainties (except those for the luminosity and the instrumental background) are obtained by varying sequentially, before any selection, each concerned quantity within one standard deviation. For each channel, the systematic uncertainty on the instrumental background is estimated by varying the fit parameters within one standard deviation of their uncertainty. Higher systematic uncertainties are observed for signal samples with low mt˜1 and low Δm which give rise to higher event populations at low values of the pT of the leptons and b quarks.No evidence for t˜1 production is observed after applying all selections for the eμ and ee data sets. No overlap is expected or observed between the two samples. We combine the numbers of expected signal and background events and their corresponding uncertainties, and the number of observed events in data from the twelve bins of the eμ channel (Table 2) and the four bins of the ee channel (Table 4) to calculate upper limits on the signal production cross section at the 95% C.L. for various signal points using the modified frequentist approach [21]. This method employs a likelihood-ratio (LLR) test-statistic, computed under the background-only (LLRb) or signal plus background (LLRs+b) hypotheses. Simulated pseudo-experiments assuming Poisson statistics and including the effect of systematic uncertainties are generated and distributions for LLRb and LLRs+b are obtained. By integrating the corresponding LLR distributions up to the LLR value observed in data, confidence levels CLb and CLs+b are derived. The stop cross section is varied until the ratio CLs=CLs+b/CLb equals 0.05, which defines the 95% C.L. upper limit for the cross section for a given [mt˜1, mν˜] point. The intersection of the obtained cross section limit with the theoretical prediction for the cross section as a function of mt˜1 and mν˜ yields the corresponding exclusion point in the [mt˜1, mν˜] plane. In this calculation, all systematic uncertainties except the ones related to the instrumental background modeling and the PDF are considered as fully correlated between signal and background. The theoretical uncertainty of the stop signal cross section Δσt˜1t˜¯1 is estimated by adding in quadrature the variations corresponding to the PDF uncertainty and the change in renormalization and factorization scale by a factor of two around the nominal value. Limits are estimated for nominal (σt˜1t˜¯1), minimal (σt˜1t˜¯1−Δσt˜1t˜¯1) and maximal (σt˜1t˜¯1+Δσt˜1t˜¯1) cross section values. We choose not to correlate uncertainties between signal and background so that the cross section limits can also be applied to other models or calculations.

Fig. 3

shows the excluded region as a function of the scalar top quark and sneutrino masses, for nominal (continuous line) and for both minimal and maximal (band surrounding the line) values of σt˜1t˜¯1, corresponding to the estimated theoretical uncertainty. For larger mass differences between the stop and the sneutrino, a stop mass lower than 175 GeV is excluded. A sensitivity up to Δm=60 GeV is observed for stop masses of 150 GeV. Combining the search in the ee final state with the eμ channel extends the final sensitivity by approximately 5 GeV for large mass differences. The observed limit is within one standard deviation of the expected limit for mt˜1⩾150 GeV and within two standard deviations for mt˜1⩽150 GeV.

In summary, we presented the results of a search for the pair production of the lightest scalar top quark which decays into bℓν˜. Events with an electron and a muon or with two electrons have been considered in this analysis. No evidence for the lightest stop is observed in this decay, leading to a 95% C.L. exclusion in the [mt˜1, mν˜] plane. The largest stop mass excluded is 175 GeV for a sneutrino mass of 45 GeV, and the largest sneutrino mass excluded is 96 GeV for a stop mass of 140 GeV.

Acknowledgements

We thank the staffs at Fermilab and collaborating institutions, and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom and RFBR (Russia); CNPq, FAPERJ, FAPESP and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Korea); CONICET and UBACyT (Argentina); FOM (The Netherlands); STFC (United Kingdom); MSMT and GACR (Czech Republic); CRC Program, CFI, NSERC and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); CAS and CNSF (China); and the Alexander von Humboldt Foundation (Germany).

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