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. AguiloS.H. AhnM. AhsanG.D. AlexeevG. AlkhazovA. AltonG. AlversonG.A. AlvesM. AnastasoaieL.S. AncuT. AndeenS. AndersonB. AndrieuM.S. AnzelcY. ArnoudM. ArovM. ArthaudA. AskewB. ÅsmanA.C.S. Assis JesusO. AtramentovC. AutermannC. AvilaC. AyF. BadaudA. BadenL. BagbyB. BaldinD.V. BandurinS. BanerjeeP. BanerjeeE. BarberisA.-F. BarfussP. BargassaP. BaringerJ. BarretoJ.F. BartlettU. BasslerD. BauerS. BealeA. BeanM. BegalliM. BegelC. Belanger-ChampagneL. BellantoniA. BellavanceJ.A. BenitezS.B. BeriG. BernardiR. BernhardL. BerntzonI. BertramM. BesançonR. BeuselinckV.A. BezzubovP.C. BhatV. BhatnagarC. BiscaratG. BlazeyF. BlekmanS. BlessingD. BlochK. BloomA. BoehnleinD. BolineT.A. BoltonG. BorissovK. BosT. BoseA. BrandtR. BrockG. BrooijmansA. BrossD. BrownN.J. BuchananD. BuchholzM. BuehlerV. BuescherS. BurdinS. BurkeT.H. BurnettC.P. BuszelloJ.M. ButlerP. CalfayanS. CalvetJ. CamminS. CaronW. CarvalhoB.C.K. CaseyN.M. CasonH. Castilla-ValdezS. ChakrabartiD. ChakrabortyK.M. ChanK. ChanA. ChandraF. CharlesE. CheuF. ChevallierD.K. ChoS. ChoiB. ChoudharyL. ChristofekT. ChristoudiasS. CihangirD. ClaesB. ClémentY. CoadouM. CookeW.E. CooperM. CorcoranF. CoudercM.-C. CousinouS. Crépé-RenaudinD. CuttsM. ĆwiokH. da MottaA. DasG. DaviesK. DeS.J. de JongP. de JongE. De La Cruz-BureloC. De Oliveira MartinsJ.D. DegenhardtF. DéliotM. DemarteauR. DeminaD. DenisovS.P. DenisovS. DesaiH.T. DiehlM. DiesburgA. DominguezH. DongL.V. DudkoL. DuflotS.R. DugadD. DugganA. DuperrinJ. DyerA. DyshkantM. EadsD. EdmundsJ. EllisonV.D. ElviraY. EnariS. EnoP. ErmolovH. EvansA. EvdokimovV.N. EvdokimovA.V. FerapontovT. FerbelF. FiedlerF. FilthautW. FisherH.E. FiskM. FordM. FortnerH. FoxS. FuS. FuessT. GadfortC.F. GaleaE. GallasE. GalyaevC. GarciaA. Garcia-BellidoV. GavrilovP. GayW. GeistD. GeléC.E. GerberY. GershteinD. GillbergG. GintherN. GollubB. GómezA. GoussiouP.D. GrannisH. GreenleeZ.D. GreenwoodE.M. GregoresG. GrenierPh. GrisJ.-F. GrivazA. GrohsjeanS. GrünendahlM.W. GrünewaldJ. GuoF. GuoP. GutierrezG. GutierrezA. HaasN.J. HadleyP. HaefnerS. HagopianJ. HaleyI. HallR.E. HallL. HanK. HanagakiP. HanssonK. HarderA. HarelR. HarringtonJ.M. HauptmanR. HauserJ. HaysT. HebbekerD. HedinJ.G. HegemanJ.M. HeinmillerA.P. HeinsonU. HeintzC. HenselK. HernerG. HeskethM.D. HildrethR. HiroskyJ.D. HobbsB. HoeneisenH. HoethM. HohlfeldS.J. HongR. HooperS. HossainP. HoubenY. HuZ. HubacekV. HynekI. IashviliR. IllingworthA.S. ItoS. JabeenM. JaffréS. JainK. JakobsC. JarvisR. JesikK. JohnsC. JohnsonM. JohnsonA. JonckheereP. JonssonA. JusteD. KäferS. KahnE. KajfaszA.M. KalininJ.R. KalkJ.M. KalkS. KapplerD. KarmanovJ. KasperP. KasperI. KatsanosD. KauR. KaurV. KaushikR. KehoeS. KermicheN. KhalatyanA. KhanovA. KharchilavaY.M. KharzheevD. KhatidzeH. KimT.J. KimM.H. KirbyM. KirschB. KlimaJ.M. KohliJ.-P. KonrathM. KopalV.M. KorablevA.V. KozelovD. KropA. KryemadhiT. KuhlA. KumarS. KunoriA. KupcoT. KurčaJ. KvitaF. LacroixD. LamS. LammersG. LandsbergJ. LazofloresP. LebrunW.M. LeeA. LeflatF. LehnerJ. LellouchJ. LevequeP. LewisJ. LiQ.Z. LiL. LiS.M. LiettiJ.G.R. LimaD. LincolnJ. LinnemannV.V. LipaevR. LiptonY. LiuZ. LiuL. LoboA. LobodenkoM. LokajicekA. LounisP. LoveH.J. LubattiA.L. LyonA.K.A. MacielD. MackinR.J. MadarasP. MättigC. MagassA. MagerkurthN. MakovecP.K. MalH.B. MalbouissonS. MalikV.L. MalyshevH.S. MaoY. MaravinB. MartinR. McCarthyA. MelnitchoukA. MendesL. MendozaP.G. MercadanteM. MerkinK.W. MerrittJ. MeyerA. MeyerM. MichautT. MilletJ. MitrevskiJ. MolinaR.K. MommsenN.K. MondalR.W. MooreT. MoulikG.S. MuanzaM. MuldersM. MulhearnO. MundalL. MundimE. NagyM. NaimuddinM. NarainN.A. NaumannH.A. NealJ.P. NegretP. NeustroevH. NilsenA. NomerotskiS.F. NovaesT. NunnemannV. O'DellD.C. O'NeilG. ObrantC. OchandoD. OnoprienkoN. OshimaJ. OstaR. OtecG.J. Otero y GarzónM. OwenP. PadleyM. PangilinanN. ParasharS.-J. ParkS.K. ParkJ. ParsonsR. PartridgeN. ParuaA. PatwaG. PawloskiB. PenningK. PetersY. PetersP. PétroffM. PetteniR. PiegaiaJ. PiperM.-A. PleierP.L.M. Podesta-LermaV.M. PodstavkovY. PogorelovM.-E. PolP. PolozovA. PompošB.G. PopeA.V. PopovC. PotterW.L. Prado da SilvaH.B. ProsperS. ProtopopescuJ. QianA. QuadtB. QuinnA. RakitineM.S. RangelK. RanjanP.N. RatoffP. RenkelS. ReucroftP. RichM. RijssenbeekI. Ripp-BaudotF. RizatdinovaS. RobinsonR.F. RodriguesC. RoyonP. RubinovR. RuchtiG. SafronovG. SajotA. Sánchez-HernándezM.P. SandersA. SantoroG. SavageL. SawyerT. ScanlonD. SchaileR.D. SchambergerY. ScheglovH. SchellmanP. SchieferdeckerT. SchliephakeC. SchwanenbergerA. SchwartzmanR. SchwienhorstJ. SekaricS. SenguptaH. SeveriniE. ShabalinaM. ShamimV. SharyA.A. ShchukinR.K. ShivpuriD. ShpakovV. SiccardiV. SimakV. SirotenkoP. SkubicP. SlatteryD. SmirnovJ. SnowG.R. SnowS. SnyderS. Söldner-RemboldL. SonnenscheinA. SopczakM. SosebeeK. SoustruznikM. SouzaB. SpurlockJ. StarkJ. SteeleV. StolinA. StoneD.A. StoyanovaJ. StrandbergS. StrandbergM.A. StrangM. StraussE. StraussR. StröhmerD. StromL. StutteS. SumowidagdoP. SvoiskyA. SznajderM. TalbyP. TamburelloA. TanasijczukW. TaylorP. TelfordJ. TempleB. TillerF. TissandierM. TitovV.V. TokmeninT. TooleI. TorchianiT. TrefzgerD. TsybychevB. TuchmingC. TullyP.M. TutsR. UnalanS. UvarovL. UvarovS. UzunyanB. VachonP.J. van den BergB. van EijkR. Van KootenW.M. van LeeuwenN. VarelasE.W. VarnesI.A. VasilyevM. VaupelP. VerdierL.S. VertogradovM. VerzocchiF. Villeneuve-SeguierP. VintP. VokacE. Von ToerneM. VoutilainenM. VreeswijkR. WagnerH.D. WahlL. WangM.H.L.S. WangJ. WarcholG. WattsM. WayneM. WeberG. WeberA. WengerN. WermesM. WetsteinA. WhiteD. WickeG.W. WilsonS.J. WimpennyM. WobischD.R. WoodT.R. WyattY. XieS. YacoobR. YamadaM. YanT. YasudaY.A. YatsunenkoK. YipH.D. YooS.W. YounJ. YuA. ZatserklyaniyC. ZeitnitzD. ZhangT. ZhaoB. ZhouJ. ZhuM. ZielinskiD. ZieminskaA. ZieminskiL. ZivkovicV. ZutshiE.G. ZverevPhysics Letters B 659 (2008) 500-508. doi:10.1016/j.physletb.2007.11.086journalPhysics Letters BCopyright © 2007 Elsevier B.V. All rights reserved.Elsevier B.V.0370-2693659324 January 20082008-01-24500-50850050810.1016/j.physletb.2007.11.086http://dx.doi.org/10.1016/j.physletb.2007.11.086doi:10.1016/j.physletb.2007.11.086http://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

PLB24552S0370-2693(07)01485-210.1016/j.physletb.2007.11.086Elsevier B.V.Experiments

Fig. 1

eμ channel. Distributions of the transverse momenta of the electron (a) and of the muon (b) after preselection cuts; (c) the transverse mass MT(μ,E̸T) after preselection cuts and E̸T>15 GeV and ΔR[(e,μ),jet]>0.5; (d) the angular sum Δϕ(μ,E̸T)+Δϕ(e,E̸T) after the cut (2); (e) ST and (f) HT distributions after the cut (3).

Fig. 2

μμ channel. (a) Δϕ(μ1,E̸T) versus E̸T in simulated Z/γ∗→μμ events; the contour of the cut (5) is shown by the solid line. Distributions of the b jet tagging probability P(jet) (b), the invariant mass of the two most energetic muons (c), and E̸T (d) after preselection cuts.

Fig. 3

For the nominal production cross section, the 95% C.L. excluded regions in the [M(t˜1),M(ν˜)] plane for the observed (full curve) and the average expected (dashed curve) limits are shown; the band surrounding the observed limit represents the lower and upper bounds of the signal cross-section variation. The regions excluded by D0 during Run I [14] and by LEP [19] are also shown.

Table 1

eμ channel. Expected numbers of events in various background and signal channels, and number of observed events in data, at various selection levels. Statistical as well as systematic uncertainties from the JES correction are shown for the total background and signal

SelectionBackground contributionsTotal backgroundDataSignalMultijetZ/γ∗→ℓℓtt¯DibosonPoint APoint BPreselection304.5286.712.428.6632.3±19.5−0.0+0.059665.9±2.4−0.0+0.026.6±0.7−0.0+0.0

(1)

194.4115.410.425.3345.4±15.0−0.7+0.732954.1±2.2−0.0+0.022.7±0.7−0.0+0.0

(2)

8.620.09.121.258.9±3.8−2.2+2.25231.6±1.7−0.0+0.819.0±0.6−0.1+0.0

(3)

5.93.67.420.237.1±2.7−0.9+0.93426.0±1.5−0.0+0.317.3±0.6−0.2+0.2Table 2

eμ channel. Expected numbers of events for total background, signal points A and B, and number of observed events in data, in the twelve [ST,HT] bins. Statistical and JES uncertainties are added in quadrature for the total background and signal points

BinTotal backgroundDataSignalPoint APoint BST∈[0,70[ GeV,HT=02.6±1.117.3±1.00.0±0.0ST∈[70,120[ GeV,HT=09.2±1.2144.8±0.70.2±0.1ST∈[120,…[ GeV,HT=07.7±0.750.8±0.31.8±0.2ST∈[0,70[ GeV,HT∈]0,60]1.9±0.725.2±0.70.0±0.0ST∈[70,120[ GeV,HT∈]0,60]3.6±1.245.3±0.81.2±0.2ST∈[120,…[ GeV,HT∈]0,60]3.0±0.420.6±0.36.3±0.5ST∈[0,70[ GeV,HT∈]60,120]0.4±0.600.6±0.30.0±0.0ST∈[70,120[ GeV,HT∈]60,120]0.7±0.211.2±0.31.3±0.2ST∈[120,…[ GeV,HT∈]60,120]3.6±0.820.1±0.14.3±0.3ST∈[0,70[ GeV,HT∈]120,…[0.0±0.000.0±0.00.0±0.0ST∈[70,120[ GeV,HT∈]120,…[0.8±0.610.0±0.00.4±0.1ST∈[120,…[ GeV,HT∈]120,…[3.7±1.120.1±0.11.7±0.3Table 3

μμ channel. Expected numbers of events in various background and signal channels, and number of observed events in data, at various selection levels. Statistical as well as systematic uncertainties from the JES correction are shown for the total background and signal

SelectionBackground contributionsTotal backgroundDataSignalMultijetϒ(1,2S)Z/γ∗→ℓℓtt¯WWPoint APoint BPreselection3607.6973.123781.75.19.628377.1±348−0.0+0.0287339.8±0.4−0.0+0.041.1±1.5−0.0+0.0

(4)

682.180.83894.95.11.54664.4±97−553+45243378.8±0.4−0.1+0.124.2±1.1−1.9+1.5

(5)

41.80.4155.74.71.1203.7±8−22+522137.5±0.3−0.1+0.212.9±0.8−1.3+1.2

(6)

0.00.06.12.60.08.7±1.6−0.1+1.343.5±0.2−0.0+0.23.4±0.4−0.3+0.4

(7)

0.00.00.12.30.02.9±0.4−0.1+0.113.1±0.2−0.0+0.23.3±0.4−0.3+0.4Table 4

μμ channel. Expected numbers of events for total background, signal points A and B, and number of observed events in data, in the 5 HT bins. Statistical and JES uncertainties are added in quadrature for the total background and signal points

BinTotal backgroundDataSignalPoint APoint BHT∈]0,40] GeV0.11±0.002.0±0.30.5±0.1HT∈]40,80] GeV0.89±0.401.1±0.31.0±0.1HT∈]80,120] GeV0.75±0.000.2±0.10.8±0.1HT∈]120,160] GeV0.56±0.010.0±0.00.4±0.1HT∈]160,…[ GeV0.57±0.000.0±0.00.4±0.1Search 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

S.H.

Ahn

Ahsan

G.D.

Alexeev

Alkhazov

Alton

Alverson

G.A.

Alves

Anastasoaie

L.S.

Ancu

Andeen

Anderson

Andrieu

M.S.

Anzelc

Arnoud

Arov

Arthaud

Askew

Åsman

A.C.S.

Assis Jesus

Atramentov

Autermann

Avila

Badaud

Baden

Bagby

Baldin

D.V.

Bandurin

Banerjee

Barberis

A.-F.

Barfuss

Bargassa

⁎

bargassa@cern.ch

Baringer

Barreto

J.F.

Bartlett

Bassler

Bauer

Beale

Bean

Begalli

Begel

Belanger-Champagne

Bellantoni

Bellavance

J.A.

Benitez

S.B.

Beri

Bernardi

Bernhard

Berntzon

Bertram

Besançon

Beuselinck

V.A.

Bezzubov

P.C.

Bhat

Bhatnagar

Biscarat

Blazey

Blekman

Blessing

Bloch

Bloom

Boehnlein

Boline

T.A.

Bolton

Borissov

Bos

Bose

Brandt

Brock

Brooijmans

Bross

Brown

N.J.

Buchanan

Buchholz

Buehler

Buescher

Burdin

Burke

T.H.

Burnett

C.P.

Buszello

J.M.

Butler

Calfayan

Calvet

Cammin

Caron

Carvalho

B.C.K.

Casey

N.M.

Cason

Castilla-Valdez

Chakrabarti

Chakraborty

K.M.

Chan

Chandra

Charles

✠

Cheu

Chevallier

D.K.

Cho

Choi

Choudhary

Christofek

Christoudias

Cihangir

Claes

Clément

Coadou

Cooke

W.E.

Cooper

Corcoran

Couderc

M.-C.

Cousinou

Crépé-Renaudin

Cutts

Ćwiok

da Motta

Das

Davies

S.J.

de Jong

De La Cruz-Burelo

De Oliveira Martins

J.D.

Degenhardt

Déliot

Demarteau

Demina

Denisov

S.P.

Denisov

Desai

H.T.

Diehl

Diesburg

Dominguez

Dong

L.V.

Dudko

Duflot

S.R.

Dugad

Duggan

Duperrin

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

Ford

Fortner

Fox

Fuess

Gadfort

C.F.

Galea

Gallas

Galyaev

Garcia

Garcia-Bellido

Gavrilov

Gay

Geist

Gelé

C.E.

Gerber

Gershtein

Gillberg

Ginther

Gollub

Gómez

Goussiou

P.D.

Grannis

Greenlee

Z.D.

Greenwood

E.M.

Gregores

Grenier

Ph.

Gris

J.-F.

Grivaz

Grohsjean

Grünendahl

M.W.

Grünewald

Guo

Gutierrez

Haas

N.J.

Hadley

Haefner

Hagopian

Haley

Hall

R.E.

Hall

Han

Hanagaki

Hansson

Harder

Harel

Harrington

J.M.

Hauptman

Hauser

Hays

Hebbeker

Hedin

J.G.

Hegeman

J.M.

Heinmiller

A.P.

Heinson

Heintz

Hensel

Herner

Hesketh

M.D.

Hildreth

Hirosky

J.D.

Hobbs

Hoeneisen

Hoeth

Hohlfeld

S.J.

Hong

Hooper

Hossain

Houben

Hubacek

Hynek

Iashvili

Illingworth

A.S.

Ito

Jabeen

Jaffré

Jain

Jakobs

Jarvis

Jesik

Johns

Johnson

Jonckheere

Jonsson

Juste

Käfer

Kahn

Kajfasz

A.M.

Kalinin

J.R.

Kalk

J.M.

Kalk

Kappler

Karmanov

Kasper

Katsanos

Kau

Kaur

Kaushik

Kehoe

Kermiche

Khalatyan

Khanov

Kharchilava

Y.M.

Kharzheev

Khatidze

Kim

T.J.

Kim

M.H.

Kirby

Kirsch

Klima

J.M.

Kohli

J.-P.

Konrath

Kopal

V.M.

Korablev

A.V.

Kozelov

Krop

Kryemadhi

Kuhl

Kumar

Kunori

Kupco

Kurča

Kvita

Lacroix

Lam

Lammers

Landsberg

Lazoflores

Lebrun

W.M.

Lee

Leflat

Lehner

Lellouch

Leveque

Lewis

Q.Z.

S.M.

Lietti

J.G.R.

Lima

Lincoln

Linnemann

V.V.

Lipaev

Lipton

Liu

Lobo

Lobodenko

Lokajicek

Lounis

Love

H.J.

Lubatti

A.L.

Lyon

A.K.A.

Maciel

Mackin

R.J.

Madaras

Mättig

Magass

Magerkurth

Makovec

P.K.

Mal

H.B.

Malbouisson

Malik

V.L.

Malyshev

H.S.

Mao

Maravin

Martin

McCarthy

Melnitchouk

Mendes

Mendoza

P.G.

Mercadante

Merkin

K.W.

Merritt

Meyer

Michaut

Millet

Mitrevski

Molina

R.K.

Mommsen

N.K.

Mondal

R.W.

Moore

Moulik

G.S.

Muanza

Mulders

Mulhearn

Mundal

Mundim

Nagy

Naimuddin

Narain

N.A.

Naumann

H.A.

Neal

J.P.

Negret

Neustroev

Nilsen

Nomerotski

S.F.

Novaes

Nunnemann

O'Dell

D.C.

O'Neil

Obrant

Ochando

Onoprienko

Oshima

Osta

Otec

G.J.

Otero y Garzón

Owen

Padley

Pangilinan

Parashar

S.-J.

Park

S.K.

Park

Parsons

Partridge

Parua

Patwa

Pawloski

Penning

Peters

Pétroff

Petteni

Piegaia

Piper

M.-A.

Pleier

P.L.M.

Podesta-Lerma

V.M.

Podstavkov

Pogorelov

M.-E.

Pol

Polozov

Pompoš

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

Reucroft

Rich

Rijssenbeek

Ripp-Baudot

Rizatdinova

Robinson

R.F.

Rodrigues

Royon

Rubinov

Ruchti

Safronov

Sajot

Sánchez-Hernández

M.P.

Sanders

Santoro

Savage

Sawyer

Scanlon

Schaile

R.D.

Schamberger

Scheglov

Schellman

Schieferdecker

Schliephake

Schwanenberger

Schwartzman

Schwienhorst

Sekaric

Sengupta

Severini

Shabalina

Shamim

Shary

A.A.

Shchukin

R.K.

Shivpuri

Shpakov

Siccardi

Simak

Sirotenko

Skubic

Slattery

Smirnov

Snow

G.R.

Snow

Snyder

Söldner-Rembold

Sonnenschein

Sopczak

Sosebee

Soustruznik

Souza

Spurlock

Stark

Steele

Stolin

Stone

D.A.

Stoyanova

Strandberg

M.A.

Strang

Strauss

Ströhmer

Strom

Stutte

Sumowidagdo

Svoisky

Sznajder

Talby

Tamburello

Tanasijczuk

Taylor

Telford

Temple

Tiller

Tissandier

Titov

V.V.

Tokmenin

Toole

Torchiani

Trefzger

Tsybychev

Tuchming

Tully

P.M.

Tuts

Unalan

Uvarov

Uzunyan

Vachon

P.J.

van den Berg

van Eijk

Van Kooten

W.M.

van Leeuwen

Varelas

E.W.

Varnes

I.A.

Vasilyev

Vaupel

Verdier

L.S.

Vertogradov

Verzocchi

Villeneuve-Seguier

Vint

Vokac

Von Toerne

Voutilainen

Vreeswijk

Wagner

H.D.

Wahl

Wang

M.H.L.S.

Wang

Warchol

Watts

Wayne

Weber

Wenger

Wermes

Wetstein

White

Wicke

G.W.

Wilson

S.J.

Wimpenny

Wobisch

D.R.

Wood

T.R.

Wyatt

Xie

Yacoob

Yamada

Yan

Yasuda

Y.A.

Yatsunenko

Yip

H.D.

Yoo

S.W.

Youn

Zatserklyaniy

Zeitnitz

Zhang

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, BrazildInstituto de Física Teórica, Universidade Estadual Paulista, São Paulo, BrazileUniversity of Alberta, Edmonton, Alberta, and Simon Fraser University, Burnaby, British Columbia, and York University, Toronto, Ontario, and McGill University, Montreal, Quebec, CanadafUniversity of Science and Technology of China, Hefei, People's Republic of ChinagUniversidad de los Andes, Bogotá, ColombiahCenter for Particle Physics, Charles University, Prague, Czech RepubliciCzech Technical University, Prague, Czech RepublicjCenter for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech RepublickUniversidad San Francisco de Quito, Quito, EcuadorlLaboratoire de Physique Corpusculaire, IN2P3-CNRS, Université Blaise Pascal, Clermont-Ferrand, FrancemLaboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, FrancenCPPM, IN2P3-CNRS, Université de la Méditerranée, Marseille, FranceoLaboratoire de l'Accélérateur Linéaire, IN2P3-CNRS et Université Paris-Sud, Orsay, FrancepLPNHE, IN2P3-CNRS, Universités Paris VI and VII, Paris, FranceqDAPNIA/Service de Physique des Particules, CEA, Saclay, FrancerIPHC, Université Louis Pasteur et Université de Haute Alsace, CNRS, IN2P3, Strasbourg, FrancesIPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, France and Université de Lyon, Lyon, FrancetIII. Physikalisches Institut A, RWTH Aachen, Aachen, GermanyuPhysikalisches Institut, Universität Bonn, Bonn, GermanyvPhysikalisches Institut, Universität Freiburg, Freiburg, GermanywInstitut für Physik, Universität Mainz, Mainz, GermanyxLudwig-Maximilians-Universität München, München, GermanyyFachbereich Physik, University of Wuppertal, Wuppertal, GermanyzPanjab University, Chandigarh, IndiaaaDelhi University, Delhi, IndiaabTata Institute of Fundamental Research, Mumbai, IndiaacUniversity College Dublin, Dublin, IrelandadKorea Detector Laboratory, Korea University, Seoul, South KoreaaeSungKyunKwan University, Suwon, South KoreaafCINVESTAV, Mexico City, MexicoagFOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The NetherlandsahRadboud University Nijmegen/NIKHEF, Nijmegen, The NetherlandsaiJoint Institute for Nuclear Research, Dubna, RussiaajInstitute for Theoretical and Experimental Physics, Moscow, RussiaakMoscow State University, Moscow, RussiaalInstitute for High Energy Physics, Protvino, RussiaamPetersburg Nuclear Physics Institute, St. Petersburg, RussiaanLund University, Lund, and Royal Institute of Technology and Stockholm University, Stockholm, and Uppsala University, Uppsala, SwedenaoPhysik Institut der Universität Zürich, Zürich, SwitzerlandapLancaster 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, OH 73050, USAbwUniversity of Oklahoma, Norman, OH 73019, USAbxOklahoma State University, Stillwater, OH 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.

Visitor from The University of Liverpool, Liverpool, UK.

Fermilab International Fellow.

Visitor from ICN-UNAM, Mexico City, Mexico.

Visitor from II. Physikalisches Institut, Georg-August-University Göttingen, Germany.

Visitor from Helsinki Institute of Physics, Helsinki, Finland.

Visitor from Universität Zürich, Zürich, Switzerland.

✠

Deceased.

Editor: L. Rolandi

Abstract

Data collected by the D0 detector at a pp¯ center-of-mass energy of 1.96 TeV at the Fermilab Tevatron Collider have been used to search for pair production of the lightest supersymmetric partner of the top quark decaying into bℓν˜. The search is performed in the ℓℓ′=eμ and μμ final states. No evidence for this process has been found in data samples of approximately 400 pb−1. The domain in the [M(t˜1),M(ν˜)] plane excluded at the 95% C.L. is substantially extended by this search.

PACS

14.80.Ly12.60.Jv

Supersymmetric theories [1] predict the existence of a scalar partner for each standard model fermion. Because of the large mass of the Standard Model top quark, the mixing between its chiral supersymmetric partners is the largest among all squarks; therefore the lightest supersymmetric partner of the top quark, t˜1 (stop), might be the lightest squark. If the t˜1→bℓν˜ decay channel is kinematically accessible, it will be dominant [2] as long as the t˜1→bχ˜1± and t˜1→tχ˜10 channels are kinematically closed, where χ˜1± and χ˜10 are the lightest chargino and neutralino, respectively. In this Letter we present a search for stop pair production in pp¯ collisions at 1.96 TeV with the D0 detector, where a virtual chargino χ˜± decays into a lepton and a sneutrino, and where the sneutrino ν˜, considered to be the next lightest supersymmetric particle, decays into a neutrino and the lightest neutralino χ˜10; in pp¯ collisions, stop pairs are dominantly produced via the strong interaction in quark–antiquark annihilation and gluon fusion. We use the minimal supersymmetric Standard Model (MSSM) as the phenomenological framework for this search. We assume the branching ratio Br(χ˜1±→ℓν˜)=1 with equal sharing among all lepton flavors, and we consider only cases where ℓ=e,μ. For stop pair production, we consider bb¯ℓℓ′νν¯χ˜10χ˜10 final states with ℓℓ′=e±μ∓ and ℓℓ′=μ+μ− (eμ and μμ channels); the signal topology consists of two isolated leptons, missing transverse energy ( E̸T), and jets. D0 has also searched for scalar top in the charm jet final state [3].The D0 detector [4] comprises a central tracking system surrounded by a liquid-argon sampling calorimeter and a system of muon detectors. Charged particles are reconstructed using a multi-layer silicon detector and eight double layers of scintillating fibers in a 2 T magnetic field produced by a superconducting solenoid. The calorimeter provides hermetic coverage up to pseudo-rapidities |η|≃4 (where η=−log(tan(θ/2)), and where θ is the polar angle with respect to the proton beam direction) in a semi-projective tower geometry with longitudinal segmentation. After passing through the calorimeter, muons are detected in the muon detector comprising three layers of tracking detectors and scintillation counters located inside and outside of 1.8 T iron toroids. Events containing electrons or muons are selected for off-line analysis by a trigger system. A set of dilepton triggers is used to tag the presence of electrons and muons based on their energy deposit in the calorimeter, hits in the muon detectors, and tracks in the tracking system.Three-body decays of the t˜1 are simulated using COMPHEP[5] and PYTHIA[6] for generation and hadronization respectively. Standard Model background processes are simulated using the PYTHIA and ALPGEN[7] Monte Carlo (MC) generators. These MC samples are generated using the CTEQ5L[8] parton distribution functions (PDF); they are normalized using next-to-leading order cross sections [9]. All generated events are passed through the full simulation of the detector geometry and response based on GEANT[10]. MC events are then reconstructed and analyzed with the same programs as used for the data.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 (pT) of tracks inside a cone with ΔR=(Δϕ)2+(Δη)2=0.5 (where ϕ is the azimuthal angle) around the muon direction, and the calorimeter energy in an annulus of size 0.1<ΔR<0.4 around the muon to be less than 4 GeV/c and 4 GeV. Isolated electrons are selected based on their characteristic energy deposition in the calorimeter, their fraction of deposited energy in the electromagnetic portion of the calorimeter and their transverse shower profile inside a cone of radius ΔR= 0.4 around the direction of the electron; furthermore, it is required that a track points to the energy deposition in the calorimeter and that its momentum and the calorimeter energy are consistent with the same electron energy; an “electron-likelihood” is defined as a variable combining information from the energy deposition in the calorimeter and the associated track. Backgrounds from jets and photon conversions are further suppressed by requiring the tracks associated with the muons and electrons to each have at least one hit in the silicon detector. Jets are reconstructed from the energy deposition in calorimeter towers using the Run II cone algorithm [11] with radius ΔR=0.5, and corrected for the jet energy scale (JES) [12]; in this search, jets are considered with pT>15 GeV/c. The E̸T is defined as the energy imbalance of all calorimeter cells in the plane transverse to the beam direction, and is corrected for the JES, the electromagnetic energy scale, and reconstructed muons. All efficiencies are measured with data [13].In both eμ and μμ channels, the signal points [M(t˜1),M(ν˜)]=(110,80) GeV/c2 and (145,50) GeV/c2, respectively referred as “soft” (point A) and “hard” (point B) signals, have been used to optimize the selection of signals of different kinematics because of different Δm=M(t˜1)−M(ν˜). The choice of these points was also motivated by the sensitivity of the D0 search during Run I [14]. The main background processes imitating the signal topology are Z/γ∗, WW, tt¯ production, and multijet background. All but the latter are estimated with MC simulation. The multijet background is estimated from data. In the eμ channel, two samples each dominated by a different multijet background are obtained by inverting the muon isolation requirements, and by inverting the cut on the electron-likelihood; in the μμ channel, such a sample is obtained by selecting same-sign muon events. Factors normalizing each sample to the selection sample are also obtained from data, and applied to the background samples to obtain the multijet background estimation, this, at an early stage of the selection.For the eμ channel, the integrated luminosity [15] of the data sample is (428±28) pb−1. The preselection is concluded by requiring the transverse momenta of the electron and muon (see Fig. 1

(a) and (b)) to be greater than 10 and 8 GeV/c, respectively. In this final state, the data are dominated by the multijet and Z/γ∗→ττ backgrounds. In these processes, poorly reconstructed leptons are correlated with E̸T, giving rise to higher event populations at high and low values of the azimuthal angular difference between the leptons and the E̸T, a low value of the angular difference for one lepton being correlated with a high value of the other. Taking advantage of a higher background contribution at low values of angular distributions, we require(1)Δϕ(μ,E̸T)>0.4,Δϕ(e,E̸T)>0.4. We require E̸T to be greater than 15 GeV to reduce contribution of both the multijet and Z/γ∗→ττ backgrounds. To reject multijet events in which leptons are associated with a jet, we require the two leptons to be at a ΔR distance greater than 0.5 from any reconstructed jet. To further reduce the multijet contribution, we require the z component of the origin of the highest transverse momentum muon track to be within four standard deviations σ from the z component of the primary vertex:(2)E̸T>15 GeV,ΔR[(e,μ),jet]>0.5,|z(μ)−z(p.v.)|<4σ. To reduce the Z/γ∗→ττ background, we cut on low values of the transverse mass of the muon and E̸T (MT(μ,E̸T), see Fig. 1(c)). To further reduce this background, we make use of the correlation between the angular differences Δϕ(μ,E̸T) and Δϕ(e,E̸T), and require their sum (see Fig. 1(d)) to be greater than 2.9:(3)MT(μ,E̸T)>15 GeV/c2,Δϕ(μ,E̸T)+Δϕ(e,E̸T)>2.9. The contributions of different backgrounds, and the expected numbers of signal and observed data events in the eμ final state at different selection levels are summarized in Table 1

. After all selections, the WW (dominating the diboson contribution) and tt¯ contributions are the dominant backgrounds. To separate soft signals such as point A from these backgrounds, we consider the variable ST defined as the scalar sum of the transverse momentum of the muon, the electron, and the E̸T (see Fig. 1(e)). To separate hard signals such as point B from background contributions, we consider the variable HT defined as the scalar sum of the transverse momentum of all jets (see Fig. 1(f)). Rather than cutting on these two variables, the HT and ST spectra predicted for signal and background are compared with the observed spectra in twelve [ST,HT] bins (see Table 2

) when extracting limits on the signal cross section, thus allowing a separation of signals of different kinematics from the WW and tt¯ backgrounds.

For the μμ channel, the integrated luminosity [15] of the data sample is (395±26) pb−1. The selection of the signal in this final state is more challenging because of the strongly dominating Z/γ∗→μμ background. The preselection is concluded by requiring the transverse momenta of the two highest transverse momenta opposite-sign muons to be greater than 8 and 6 GeV/c. While the signal is characterized by the presence of jets originating from the hadronization of b quarks, the Z/γ∗→μμ background owes the presence of jets to initial state radiation gluons which hadronize into softer jets, resulting in a lower multiplicity of jets; the latter is also valid for soft signals such as point A. To keep sensitivity to soft signals while rejecting substantial background, we require at least one jet:(4)N(jets)⩾1. To further remove Z/γ∗→μμ background events, where poorly reconstructed muons correlate with the E̸T, we require the E̸T to be greater than the contour shown on Fig. 2

(a), using a cut parametrized by the following equation:(5)E̸T/GeV>20+|Δϕ(μ1,E̸T)−1.55|9.2, where μ1 is the highest transverse momentum muon. To augment 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. An algorithm based on the lifetime of hadrons calculates the probability P for the tracks of a jet to originate from the primary interaction point [16]. This b jet tagging probability is constructed such that its distribution is uniform for light-flavor jets while peaking at zero for heavy-flavor jets which have a vertex significantly displaced from the primary vertex (Fig. 2(b)). Considering the highest transverse energy jet, we require(6)P(jet)<1%. A cut on the dimuon invariant mass (Fig. 2(c)) in the vicinity of the Z boson resonance only at low E̸T (Fig. 2(d)) further suppresses the Z/γ∗→μμ background while preserving the signal:(7)M(μ,μ)∉[75,120]GeV/c2for E̸T<50 GeV.

Table 3

summarizes the different stages of the signal selection in the μμ channel. The tt¯ background dominates after the selection cuts; five HT bins are considered (see Table 4

) to separate various signal points from this background.

The expected numbers of background and signal events depend on several measurements and parametrizations which each introduce a systematic uncertainty: lepton identification and reconstruction efficiency [(2.6–7)%] [13], trigger efficiency [(3.5–5)%] [13], luminosity [6.1%] [15], multijet background modeling [10%], JES [(4–22)%] [12], jet identification and reconstruction efficiency and resolution [(4–16)%] [13], b jet tagging [(1–11)%] [16], PDF uncertainty affecting the signal efficiency [10%] [17].After applying all selection cuts for eμ and μμ data sets, no evidence for t˜1 production is observed. We combine the number of expected signal and background events and their corresponding uncertainty, and the number of observed events in data from the twelve bins of the eμ selection (Table 2) and the five bins of the μμ selection (Table 4) to calculate upper-limit cross sections for signal production at the 95% C.L. for various signal points using the modified frequentist approach [18]. In this calculation, correlated uncertainties are taken into account; no overlap is expected nor observed between the two samples. Regions for which the calculated cross section upper limit is smaller than the theoretical one are excluded at 95% C.L. Fig. 3

shows the excluded region as a function of the scalar top quark and sneutrino masses, for nominal (solid line) and for both minimal and maximal (band surrounding the line) values of the t˜1t˜¯1 production cross section; the latter variation corresponds to the PDF uncertainty for the signal cross section, quadratically added to the 2μr and μr/2 renormalization scale variations of the t˜1t˜¯1 cross section. Although the numbers of expected and observed events are similar (Tables 1 and 3), their distribution across the bins (Tables 2 and 4) causes the expected cross section limit to be lower than the observed one. For minimal values of the production cross section, the search in the eμ final state individually excludes a stop mass of 176 GeV/c2 for a sneutrino mass of 60 GeV/c2, and a sneutrino mass of 97 GeV/c2 for a stop mass of 130 GeV/c2; the search in the μμ final state, once combined with the eμ final state, extends the final sensitivity by approximately 10 GeV/c2 for small and large mass differences.

In summary, we have searched for the lightest scalar top quark decaying into bℓν˜; events with an electron and a muon, and two muons have been considered for this search. No evidence for the lightest stop is observed in these decays, leading to a 95% C.L. exclusion in the [M(t˜1),M(ν˜)] plane. The largest stop mass excluded is 186 GeV/c2 for a sneutrino mass of 71 GeV/c2, and the largest sneutrino mass excluded is 107 GeV/c2 for a stop mass of 145 GeV/c2; these mass limits are obtained with the most conservative theoretical production cross section, taking into account the PDF uncertainty and the variation of the renormalization scale. This is the most sensitive search for stop decaying into bℓν˜ to date.

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); CAPES, 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); Science and Technology Facilities Council (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); Alexander von Humboldt Foundation; and the Marie Curie Program.

References

[1]

S.P.

Martin

G.L.

KanePerspectives on Supersymmetry1998World ScientificSingapore

hep-ph/9709356

[2]

Hikasa

KobayashiPhys. Rev. D

1987

724

[3]D0 Collaboration

V.M.

Abazov

Phys. Lett. B

645

2007

119

[4]D0 Collaboration

V.M.

Abazov

Nucl. Instrum. Methods A

565

2006

463

[5]

A. Pukhov, et al., User's manual for version 3.3, INP-MSU 98-41/542

[6]

Sjöstrand

Comput. Phys. Commun.

135

2001

238

[7]

M.L.

Mangano

JHEP

0307

2003

001

[8]

H.L.

Lai

Eur. Phys. J. C

2000

375

[9]

Hamberg

W.L.

van Neerven

MatsuuraNucl. Phys. B

359

1991

343

Hamberg

W.L.

van Neerven

MatsuuraNucl. Phys. B

644

2002

403

Erratum

J.M.

Campbell

R.K.

EllisPhys. Rev. D

1999

113006

hep-ph/9905386

Kidonakis

VogtPhys. Rev. D

2003

114014

Baur

E.L.

BergerPhys. Rev. D

1990

1476

[10]

R. Brun, F. Carminati, CERN Program Library Long Writeup W5013, 1993, unpublished

[11]

G.C.

Blazey

Baur

R.K.

Ellis

ZeppenfeldProceedings of the Workshop: QCD and Weak Boson Physics in Run II2000Fermilab-Pub-00/297

[12]D0 Collaboration

V.M.

Abazov

Phys. Rev. D

2007

092001

[13]D0 Collaboration

V.M.

Abazov

Phys. Rev. D

2007

052006

[14]D0 Collaboration

V.M.

Abazov

Phys. Rev. Lett.

2002

171802

[15]

T. Andeen, et al., The D0 experiment's integrated luminosity for Tevatron Run IIa, Fermilab-TM-2365, 2007

[16]

S. Greder, Ph.D. thesis, Fermilab-Thesis-2004-28, 2004

B. Clément, Ph.D. thesis, Fermilab-Thesis-2006-06, 2006

[17]

Pumplin

JHEP

0207

2002

012

Stump

JHEP

0310

2003

046

[18]

JunkNucl. Instrum. Methods A

434

1999

435

[19]

LEPSUSYWG, ALEPH, DELPHI, L3 and OPAL experiments, note LEPSUSYWG/04-02, http://lepsusy.web.cern.ch/lepsusy/Welcome.html

ALEPH Collaboration

Heister

Phys. Lett. B

537

2002