application/xmlSearch for 3- and 4-body decays of the scalar top quark in [formula omitted] collisions at [formula omitted] 1.8 TeVDØ CollaborationV.M. AbazovB. AbbottA. AbdesselamM. AbolinsV. AbramovB.S. AcharyaD.L. AdamsM. AdamsS.N. AhmedG.D. AlexeevA. AltonG.A. AlvesE.W. AndersonY. ArnoudC. AvilaV.V. BabintsevL. BabukhadiaT.C. BaconA. BadenS. BaffioniB. BaldinP.W. BalmS. BanerjeeE. BarberisP. BaringerJ. BarretoJ.F. BartlettU. BasslerD. BauerA. BeanF. BeaudetteM. BegelA. BelyaevS.B. BeriG. BernardiI. BertramA. BessonR. BeuselinckV.A. BezzubovP.C. BhatV. BhatnagarM. BhattacharjeeG. BlazeyF. BlekmanS. BlessingA. BoehnleinN.I. BojkoT.A. BoltonF. BorcherdingK. BosT. BoseA. BrandtR. BreedonG. BriskinR. BrockG. BrooijmansA. BrossD. BuchholzM. BuehlerV. BuescherV.S. BurtovoiJ.M. ButlerF. CanelliW. CarvalhoD. CaseyH. Castilla-ValdezD. ChakrabortyK.M. ChanS.V. ChekulaevD.K. ChoS. ChoiS. ChopraD. ClaesA.R. ClarkL. ConeyB. ConnollyW.E. CooperD. CoppageS. Crépé-RenaudinM.A.C. CummingsD. CuttsH. da MottaG.A. DavisK. DeS.J. de JongM. DemarteauR. DeminaP. DemineD. DenisovS.P. DenisovS. DesaiH.T. DiehlM. DiesburgS. DoulasL.V. DudkoS. DuensingL. DuflotS.R. DugadA. DuperrinA. DyshkantD. EdmundsJ. EllisonJ.T. EltzrothV.D. ElviraR. EngelmannS. EnoG. EppleyP. ErmolovO.V. EroshinJ. EstradaH. EvansV.N. EvdokimovD. FeinT. FerbelF. FilthautH.E. FiskF. FleuretM. FortnerH. FoxS. FuS. FuessE. GallasA.N. GalyaevM. GaoV. GavrilovR.J. Genik IIK. GenserC.E. GerberY. GershteinG. GintherB. GómezP.I. GoncharovH. GordonK. GounderA. GoussiouN. GrafP.D. GrannisJ.A. GreenH. GreenleeZ.D. GreenwoodS. GrinsteinL. GroerS. GrünendahlS.N. GurzhievG. GutierrezP. GutierrezN.J. HadleyH. HaggertyS. HagopianV. HagopianR.E. HallC. HanS. HansenJ.M. HauptmanC. HebertD. HedinJ.M. HeinmillerA.P. HeinsonU. HeintzM.D. HildrethR. HiroskyJ.D. HobbsB. HoeneisenJ. HuangY. HuangI. IashviliR. IllingworthA.S. ItoM. JaffréS. JainR. JesikK. JohnsM. JohnsonA. JonckheereH. JöstleinA. JusteW. KahlS. KahnE. KajfaszA.M. KalininD. KarmanovD. KarmgardR. KehoeA. KhanovA. KharchilavaB. KlimaW. KoJ.M. KohliA.V. KostritskiyJ. KotcherB. KothariA.V. KozelovE.A. KozlovskyJ. KraneM.R. KrishnaswamyP. KrivkovaS. KrzywdzinskiM. KubantsevS. KuleshovY. KulikS. KunoriA. KupcoV.E. KuznetsovG. LandsbergW.M. LeeA. LeflatF. LehnerC. LeonidopoulosJ. LiQ.Z. LiJ.G.R. LimaD. LincolnS.L. LinnJ. LinnemannR. LiptonA. LucotteL. LuekingC. LundstedtC. LuoA.K.A. MacielR.J. MadarasV.L. MalyshevV. ManankovH.S. MaoT. MarshallM.I. MartinA.A. MayorovR. McCarthyT. McMahonH.L. MelansonM. MerkinK.W. MerrittC. MiaoH. MiettinenD. MihalceaN. MokhovN.K. MondalH.E. MontgomeryR.W. MooreY.D. MutafE. NagyF. NangM. NarainV.S. NarasimhamN.A. NaumannH.A. NealJ.P. NegretA. NomerotskiT. NunnemannD. O'NeilV. OguriB. OlivierN. OshimaP. PadleyK. PapageorgiouN. ParasharR. PartridgeN. ParuaA. PatwaO. PetersP. PétroffR. PiegaiaB.G. PopeE. PopkovH.B. ProsperS. ProtopopescuM.B. PrzybycienJ. QianR. RajaS. RajagopalanP.A. RapidisN.W. ReayS. ReucroftM. RidelM. RijssenbeekF. RizatdinovaT. RockwellC. RoyonP. RubinovR. RuchtiB.M. SabirovG. SajotA. SantoroL. SawyerR.D. SchambergerH. SchellmanA. SchwartzmanE. ShabalinaR.K. ShivpuriD. ShpakovM. ShupeR.A. SidwellV. SimakV. SirotenkoP. SlatteryR.P. SmithG.R. SnowJ. SnowS. SnyderJ. SolomonY. SongV. Sorı́nM. SosebeeN. SotnikovaK. SoustruznikM. SouzaN.R. StantonG. SteinbrückD. StokerV. StolinA. StoneD.A. StoyanovaM.A. StrangM. StraussM. StrovinkL. StutteA. SznajderM. TalbyW. TaylorS. Tentindo-RepondS.M. TripathiT.G. TrippeA.S. TurcotP.M. TutsR. Van KootenV. VanievN. VarelasF. Villeneuve-SeguierA.A. VolkovA.P. VorobievH.D. WahlZ.-M. WangJ. WarcholG. WattsM. WayneH. WeertsA. WhiteD. WhitesonD.A. WijngaardenS. WillisS.J. WimpennyJ. WomersleyD.R. WoodQ. XuR. YamadaP. YaminT. YasudaY.A. YatsunenkoK. YipS. YoussefJ. YuM. ZanabriaX. ZhangH. ZhengB. ZhouZ. ZhouM. ZielinskiD. ZieminskaA. ZieminskiV. ZutshiE.G. ZverevA. ZylberstejnPhysics Letters B 581 (2004) 147-155. doi:10.1016/j.physletb.2003.12.001journalPhysics Letters BCopyright © unknown. Published by Elsevier B.V.Elsevier B.V.0370-26935813-419 February 20042004-02-19147-15514715510.1016/j.physletb.2003.12.001http://dx.doi.org/10.1016/j.physletb.2003.12.001doi:10.1016/j.physletb.2003.12.001http://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

PLB20502S0370-2693(03)01855-010.1016/j.physletb.2003.12.001Experiments

Fig. 1

Distributions after initial selection cuts for the total background (open histogram), the sum of the total background and the expected 4-body decay stop signal for mt̃(mχ̃01)=120 (60) GeV in the light sneutrino scenario (shaded histogram), and the data (points) of (a) the transverse energy of the electron, (b) the transverse energy of the muon, (c) the missing transverse energy, (d) the transverse energy of any jets present, (e) the difference in azimuthal angle between the two leptons, (f) the absolute value of the sum in η of the two leptons, and (g) the smallest lepton to jet distance in the event when at least one jet is reconstructed, (h) the distance between the lepton and jet that have not been used in (g), when two jets are reconstructed. For the final selection, all events having distances in (g) or (h) above 1.5 are rejected.

Fig. 2

Cross-section upper limit as a function of mt̃ for mχ̃01=40, 50 and 60 GeV, in the W exchange scenario. The 3-body decay limits are shown as dashed lines, the 4-body decay limits as solid lines. The results of this analysis are compared to the CDF limit on the t̃→bχ̃+1 2-body decay assuming a light χ̃+1 (mχ̃+1=90 GeV) and subsequent decay χ̃+1→Wχ̃01 with mχ̃01=40 GeV. The expected NLO cross-section is also shown (the error band is obtained by varying the factorization scale μ). The renormalization scale is taken to be equal to μ.

Fig. 3

Cross-section upper limit in the light sneutrino scenario as a function of mt̃, for the 3-body decay with mχ̃01<mν̃=60,80 GeV as established in Ref. [6], and for the 4-body decay with mχ̃01=50, 60 GeV and mν̃=mt̃−mb. The 3-body decay limits are shown as dashed lines, the 4-body decay limits as solid lines. The expected NLO cross-section is also shown (the error band is obtained by varying the factorization scale μ).

Fig. 4

Excluded regions in the (mt̃,mχ̃01) plane for the t̃→bℓνχ̃01 decay channel in the MSSM, assuming 3- or 4-body decay with a light sneutrino mass equal, respectively, to mχ̃01 and mt̃−mb. The chargino mass is assumed to be mχ̃+1=140 GeV. The 3-body decay result was established in Ref. [6] and is compared to the LEP 1 (invisible width) and LEP 2 (t̃→bℓν̃) results under the same assumption (mν̃=mχ̃01). The results of this analysis are also compared to the exclusion limits obtained for the t̃→cχ̃01 decay channels at LEP 2 and at the Tevatron by the CDF collaboration, and for the t̃→bℓνχ̃01 decay channel at LEP 2 by the ALEPH collaboration.

Table 1

Cross-sections for the background processes, expected numbers of events surviving the final selection criteria for an integrated luminosity of 108.3 pb−1, number of events selected in the eμE̷T data sample, and expected 4-body decay stop signal assuming mt̃(mχ̃01)=120 (60) GeV in the light sneutrino scenario and in the W-exchange scenario

ProcessCross-section (pb)Number of events after selection“QCD”–4.3±0.3Z→ττ1.700.5±0.1WW0.692.8±0.3tt̄0.400.4±0.1Total background–8.0±0.8Data–6t̃t̃̄ (light sneutrino scenario with mν̃=mt̃−mb)1.004.9±0.89t̃t̃̄ (W-exchange scenario)0.111.0±0.18Search for 3- and 4-body decays of the scalar top quark in pp̄ collisions at s= 1.8 TeV

DØ Collaboration

V.M.

Abazov

Abbott

Abdesselam

Abolins

Abramov

B.S.

Acharya

D.L.

Adams

S.N.

Ahmed

G.D.

Alexeev

Alton

G.A.

Alves

E.W.

Anderson

Arnoud

Avila

V.V.

Babintsev

Babukhadia

T.C.

Bacon

Baden

Baffioni

Baldin

P.W.

Balm

Banerjee

Barberis

Baringer

Barreto

J.F.

Bartlett

Bassler

Bauer

Bean

Beaudette

Begel

Belyaev

S.B.

Beri

Bernardi

∗

gregorio@in2p3.fr

Bertram

Besson

Beuselinck

V.A.

Bezzubov

P.C.

Bhat

Bhatnagar

Bhattacharjee

Blazey

Blekman

Blessing

Boehnlein

N.I.

Bojko

T.A.

Bolton

Borcherding

Bos

Bose

Brandt

Breedon

Briskin

Brock

Brooijmans

Bross

Buchholz

Buehler

Buescher

V.S.

Burtovoi

J.M.

Butler

Canelli

Carvalho

Casey

Castilla-Valdez

Chakraborty

K.M.

Chan

S.V.

Chekulaev

D.K.

Cho

Choi

Chopra

Claes

A.R.

Clark

Coney

Connolly

W.E.

Cooper

Coppage

Crépé-Renaudin

M.A.C.

Cummings

Cutts

da Motta

G.A.

Davis

S.J.

de Jong

Demarteau

Demina

Demine

Denisov

S.P.

Denisov

Desai

H.T.

Diehl

Diesburg

Doulas

L.V.

Dudko

Duensing

Duflot

S.R.

Dugad

Duperrin

Dyshkant

Edmunds

Ellison

J.T.

Eltzroth

V.D.

Elvira

Engelmann

Eno

Eppley

Ermolov

O.V.

Eroshin

Estrada

Evans

V.N.

Evdokimov

Fein

Ferbel

Filthaut

H.E.

Fisk

Fleuret

Fortner

Fox

Fuess

Gallas

A.N.

Galyaev

Gao

Gavrilov

R.J.

GenikII

Genser

C.E.

Gerber

Gershtein

Ginther

Gómez

P.I.

Goncharov

Gordon

Gounder

Goussiou

Graf

P.D.

Grannis

J.A.

Green

Greenlee

Z.D.

Greenwood

Grinstein

Groer

Grünendahl

S.N.

Gurzhiev

Gutierrez

N.J.

Hadley

Haggerty

Hagopian

R.E.

Hall

Han

Hansen

J.M.

Hauptman

Hebert

Hedin

J.M.

Heinmiller

A.P.

Heinson

Heintz

M.D.

Hildreth

Hirosky

J.D.

Hobbs

Hoeneisen

Huang

Iashvili

Illingworth

A.S.

Ito

Jaffré

Jain

Jesik

Johns

Johnson

Jonckheere

Jöstlein

Juste

Kahl

Kahn

Kajfasz

A.M.

Kalinin

Karmanov

Karmgard

Kehoe

Khanov

Kharchilava

Klima

J.M.

Kohli

A.V.

Kostritskiy

Kotcher

Kothari

A.V.

Kozelov

E.A.

Kozlovsky

Krane

M.R.

Krishnaswamy

Krivkova

Krzywdzinski

Kubantsev

Kuleshov

Kulik

Kunori

Kupco

V.E.

Kuznetsov

Landsberg

W.M.

Lee

Leflat

Lehner

Leonidopoulos

Q.Z.

J.G.R.

Lima

Lincoln

S.L.

Linn

Linnemann

Lipton

Lucotte

Lueking

Lundstedt

Luo

A.K.A.

Maciel

R.J.

Madaras

V.L.

Malyshev

Manankov

H.S.

Mao

Marshall

M.I.

Martin

A.A.

Mayorov

McCarthy

McMahon

H.L.

Melanson

Merkin

K.W.

Merritt

Miao

Miettinen

Mihalcea

Mokhov

N.K.

Mondal

H.E.

Montgomery

R.W.

Moore

Y.D.

Mutaf

Nagy

Nang

Narain

V.S.

Narasimham

N.A.

Naumann

H.A.

Neal

J.P.

Negret

Nomerotski

Nunnemann

O'Neil

Oguri

Olivier

Oshima

Padley

Papageorgiou

Parashar

Partridge

Parua

Patwa

Peters

Pétroff

Piegaia

B.G.

Pope

Popkov

H.B.

Prosper

Protopopescu

M.B.

Przybycien

Qian

Raja

Rajagopalan

P.A.

Rapidis

N.W.

Reay

Reucroft

Ridel

Rijssenbeek

Rizatdinova

Rockwell

Royon

Rubinov

Ruchti

B.M.

Sabirov

Sajot

Santoro

Sawyer

R.D.

Schamberger

Schellman

Schwartzman

Shabalina

R.K.

Shivpuri

Shpakov

Shupe

R.A.

Sidwell

Simak

Sirotenko

Slattery

R.P.

Smith

G.R.

Snow

Snyder

Solomon

Song

Sorı́n

Sosebee

Sotnikova

Soustruznik

Souza

N.R.

Stanton

Steinbrück

Stoker

Stolin

Stone

D.A.

Stoyanova

M.A.

Strang

Strauss

Strovink

Stutte

Sznajder

Talby

Taylor

Tentindo-Repond

S.M.

Tripathi

T.G.

Trippe

A.S.

Turcot

P.M.

Tuts

Van Kooten

Vaniev

Varelas

Villeneuve-Seguier

A.A.

Volkov

A.P.

Vorobiev

H.D.

Wahl

Z.-M.

Wang

Warchol

Watts

Wayne

Weerts

White

Whiteson

D.A.

Wijngaarden

Willis

S.J.

Wimpenny

Womersley

D.R.

Wood

Yamada

Yamin

Yasuda

Y.A.

Yatsunenko

Yip

Youssef

Zanabria

Zhang

Zheng

Zhou

Zielinski

Zieminska

Zieminski

Zutshi

E.G.

Zverev

Zylberstejn

aUniversidad de Buenos Aires, Buenos Aires, ArgentinabLAFEX, Centro Brasileiro de Pesquisas Fı́sicas, Rio de Janeiro, BrazilcUniversidade do Estado do Rio de Janeiro, Rio de Janeiro, BrazildInstitute of High Energy Physics, Beijing, PR ChinaeUniversidad de los Andes, Bogotá, ColombiafCharles University, Center for Particle Physics, Prague, Czech RepublicgInstitute of Physics, Academy of Sciences, Center for Particle Physics, Prague, Czech RepublichUniversidad San Francisco de Quito, Quito, EcuadoriInstitut des Sciences Nucléaires, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, FrancejCPPM, IN2P3-CNRS, Université de la Méditerranée, Marseille, FrancekLaboratoire de l'Accélérateur Linéaire, IN2P3-CNRS, Orsay, FrancelLPNHE, Universités Paris VI and VII, IN2P3-CNRS, Paris, FrancemDAPNIA/Service de Physique des Particules, CEA, Saclay, FrancenUniversität Mainz, Institut für Physik, Mainz, GermanyoPanjab University, Chandigarh, IndiapDelhi University, Delhi, IndiaqTata Institute of Fundamental Research, Mumbai, IndiarCINVESTAV, Mexico City, MexicosFOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The NetherlandstUniversity of Nijmegen/NIKHEF, Nijmegen, The NetherlandsuJoint Institute for Nuclear Research, Dubna, RussiavInstitute for Theoretical and Experimental Physics, Moscow, RussiawMoscow State University, Moscow, RussiaxInstitute for High Energy Physics, Protvino, RussiayLancaster University, Lancaster, UKzImperial College, London, UKaaUniversity of Arizona, Tucson, AZ 85721, USAabLawrence Berkeley National Laboratory and University of California, Berkeley, CA 94720, USAacUniversity of California, Davis, CA 95616, USAadCalifornia State University, Fresno, CA 93740, USAaeUniversity of California, Irvine, CA 92697, USAafUniversity of California, Riverside, CA 92521, USAagFlorida State University, Tallahassee, FL 32306, USAahFermi National Accelerator Laboratory, Batavia, IL 60510, USAaiUniversity of Illinois at Chicago, Chicago, IL 60607, USAajNorthern Illinois University, DeKalb, IL 60115, USAakNorthwestern University, Evanston, IL 60208, USAalIndiana University, Bloomington, IN 47405, USAamUniversity of Notre Dame, Notre Dame, IN 46556, USAanIowa State University, Ames, IA 50011, USAaoUniversity of Kansas, Lawrence, KS 66045, USAapKansas State University, Manhattan, KS 66506, USAaqLouisiana Tech University, Ruston, LA 71272, USAarUniversity of Maryland, College Park, MD 20742, USAasBoston University, Boston, MA 02215, USAatNortheastern University, Boston, MA 02115, USAauUniversity of Michigan, Ann Arbor, MI 48109, USAavMichigan State University, East Lansing, MI 48824, USAawUniversity of Nebraska, Lincoln, NE 68588, USAaxColumbia University, New York, NY 10027, USAayUniversity of Rochester, Rochester, NY 14627, USAazState University of New York, Stony Brook, NY 11794, USAbaBrookhaven National Laboratory, Upton, NY 11973, USAbbLangston University, Langston, OK 73050, USAbcUniversity of Oklahoma, Norman, OK 73019, USAbdBrown University, Providence, RI 02912, USAbeUniversity of Texas, Arlington, TX 76019, USAbfRice University, Houston, TX 77005, USAbgUniversity of Virginia, Charlottesville, VA 22901, USAbhUniversity of Washington, Seattle, WA 98195, USA∗Corresponding author.1

Visitor from University of Zurich, Zurich, Switzerland.

Visitor from Institute of Nuclear Physics, Krakow, Poland.

Editor: L. Rolandi

Abstract

We have searched for the signature of 3- and 4-body decays of pair-produced scalar top quarks (stop) in the inclusive final state containing an electron, a muon, and significant missing transverse energy using a sample of pp̄ events corresponding to 108.3 pb−1 of data collected with the DØ detector at Fermilab. The search is done in the framework of the minimal supersymmetric standard model assuming that the neutralino (χ̃01) is the lightest supersymmetric particle and is stable. No evidence for a signal is found and we derive cross-section upper limits as a function of stop (t̃) and neutralino masses in different decay scenarios leading to the bℓνχ̃01 final state.

Supersymmetry (SUSY) [1] is a hypothetical symmetry between bosons and fermions that could lead to an extension of the standard model (SM). SUSY predicts additional elementary particles with quantum numbers identical to those of the SM, except for their spins which differ by a half unit. Their masses must also differ since no evidence has been found for new particles with masses equal to those of the SM. In several SUSY models, the large mass of the top quark induces a strong mixing between the supersymmetric partners of the two chirality states of the top quark leading naturally to two physical states of very different mass [2]. The lightest stop, denoted t̃ in this Letter, could therefore be significantly lighter than the other squarks rendering it a particularly auspicious choice for a direct search.The production of a pair of stops at the Tevatron proceeds through gluon fusion or quark–antiquark annihilation, and its cross-section, for a given stop mass (mt̃), is known at next-to-leading order (NLO) with a precision of 8% [3]. The phenomenology of stop decays depends on the assumptions made in the SUSY model. In the framework of the minimal supersymmetric standard model (MSSM) [4] with R-parity [5] conservation, the lightest SUSY particle (LSP) is stable. In a previous publication [6] we performed this search assuming that the scalar neutrino (sneutrino, ν̃) is the LSP and derived exclusion limits reaching higher stop masses than those of previous similar searches [7–9]. In this Letter we assume that the neutralino is the LSP.We consider alternative scenarios to what has been done in most of the searches at the CERN LEP collider [8,9] or at the Fermilab Tevatron [7,10–12]. Those studies searched for the 2-body decays, t̃→cχ̃01 or t̃→bχ̃+1 (where χ̃+1 is the lightest chargino of the MSSM); it has been recently realized [13] that even if the t̃→bχ̃+1 decay is kinematically forbidden, as will be assumed in the following, the t̃→cχ̃01 channel may not be the dominant one for stop masses accessible at LEP or the Tevatron (mt̃≳90 GeV) when the ratio of the two vacuum expectation values of the Higgs fields is not large (tanβ≲5) [14]. The 3-body decays t̃→bWχ̃01 and/or t̃→bℓν̃ could be kinematically allowed, and if not, the corresponding 4-body decays t̃→bff′̄χ̃01 (where ff′̄ originate from the decay of the virtual W boson produced by t̃→bχ̃+1 followed by χ̃+1→Wχ̃01) and t̃→bℓνχ̃01 (with νχ̃01 from the decay of the virtual sneutrino33

The same final state can be obtained via a charged slepton, but this channel is disfavored [15] and is therefore neglected in the following.

produced by χ̃+1→ν̃ℓ) are generally allowed, i.e., when mt̃⩾mχ̃01+mb+mℓ. When the 3-body decay bℓν̃ is kinematically allowed, the subsequent decay of the ν̃ has no influence on the kinematics. In this case we quote the results established in Ref. [6].The experimental signature for 3- and 4-body decays of a t̃t̃̄ pair consists of two b quarks, two fermions, and missing transverse energy. Since our search is based on the presence of charged leptons in the final state, we have access only to the case where the fermion f (f′) is a neutral (charged) lepton. The final states of all these 3- and 4-body decays are thus identical (bℓνχ̃01). The underlying process depends on the SUSY parameters, and can be a mixture of the described processes. In the following, the analysis is performed assuming the complete dominance of each of these four cases in turn, and will be referred to as 3- or 4-body decay in the “W” or “light ν̃” exchange scenario. We assume that the leptonic branching ratios are equal in each lepton family.In our search, the leptons can be e, μ or τ, but τ leptons are considered only if they decay into eνν̄ or μνν̄. We place no requirements on the presence of jets and use only the eμE̷T signature since it has less background than the eeE̷T or μμE̷T channels. The missing transverse energy (E̷T) represents the measured imbalance in transverse energy due to the escaping neutrinos and neutralinos, and is obtained experimentally from the vector sum of the transverse energy measured in the calorimeter and in the muon spectrometer system. The event sample corresponds to 108.3 pb−1 of data collected by the DØ experiment at Fermilab during the Run I of the Tevatron.A detailed description of the DØ detector and its triggering system can be found in Ref. [16]. This analysis is mainly based on three subsystems: the uranium/liquid-argon calorimeter for identifying electron candidates and measuring electromagnetic and hadronic energies; the inner detector for tracking charged particles and to differentiate photons from electrons; and the muon spectrometer to identify and measure the required muon.The data and pre-selection criteria are identical to those published in Ref. [6], however for the new channels considered in this analysis (W-exchange scenario, and 4-body decay in the light sneutrino scenario), we apply a stricter final event selection. The initial selection requires events to have one or more isolated electrons with transverse energy ETe>15 GeV, and one or more isolated muons with ETμ>15 GeV, and E̷T>20 GeV. A lepton is isolated if its distance in the η–ϕ plane from the closest jet is greater than 0.5, where η and ϕ are the standard pseudorapidity and azimuthal angle variables. Jets are found using a cone algorithm with a radius of 0.5 in the η–ϕ plane. Events are also required to satisfy 15°<Δϕeμ<165° and Σηeμ<2.0, where Δϕeμ and Σηeμ are two kinematic quantities which are used to increase the rejection of the SM background [17] and are defined as: Δϕeμ≡|ϕe−ϕμ|, where ϕℓ is the azimuthal angle of the lepton ℓ, and Σηeμ≡|ηe+ημ|, where ηℓ is its pseudorapidity. The distributions of these kinematic quantities after these initial requirements (which correspond to the final selection criteria of Ref. [6]) are shown in Fig. 1(a)–(c), (e), (f)

The final event selection of this analysis uses the following additional requirements: if the event has one jet with transverse energy greater than 15 GeV, we require that the distance in the η–ϕ plane Dηϕl1,j1<1.5. Dηϕl1,j1 is defined as the smaller of the two distances between the highest energy jet and each of the two leptons. If the event has two or more jets with transverse energy greater than 15 GeV, we require in addition that the second distance Dηϕl2,j2 <1.5. Dηϕl2,j2 is defined as the distance between the second highest energy jet and the lepton that was not used to define Dηϕl1,j1. These requirements reduces the SM background by about a factor of two and removes only a small part (<5%) of the signal in the kinematic domain of the present analysis.44

These requirements were not applied in Ref. [6] since in the t̃→bℓν̃ 3-body decay, the jets are in average more distant from the leptons and the selection requirements would remove a larger fraction of signal events.

The distributions of the transverse energy of any associated jets, Dηϕl1,j1 and Dηϕl2,j2 are shown in Fig. 1(d), (g) and (h), before applying these requirements.The dominant SM processes that result in the eμE̷T signature are, in order of decreasing importance: (i) multi-jet processes (called “QCD” in the following) with one jet misidentified as an electron and one true muon originating from another jet (muon misidentification in our final sample is negligible); (ii) Z→ττ→eμνν̄νν̄; (iii) WW→eμνν̄; (iv) tt̄→eμνν̄jj. The Drell–Yan process (DY) →ττ→eμνν̄νν̄ contributes less than 0.02 events after the final event selection. The QCD background is determined using the data, following the procedure described in Ref. [18]. The other SM backgrounds are estimated using MC samples processed through the full data analysis chain.For simulation of the signal, we use the spythia[19] event generator with its standard hadronization and fragmentation functions and the cteq3m [20] parton distribution functions. The stop decay is generated using comphep [21]. Detector simulation is performed using the fast DØ simulation/reconstruction program, which agrees with reference samples passed through the full DØ analysis chain. The t̃t̃̄ samples are simulated for stop masses varying between 80 and 145 GeV and for neutralino masses varying between 30 and 85 GeV. The chargino mass is set equal to 140 GeV, to prevent the possibility of 2-body decay. The samples are produced separately for the W-exchange and for the light sneutrino scenarios. In the light sneutrino scenario, the mass of the sneutrino is varied between 40 and 80 GeV for the 3-body decay, and is set to mt̃−mb for the 4-body decay (the number of selected signal events slightly increases when the virtual sneutrino mass is increased, and we make a conservative choice).The expected cross-sections for the background processes and the numbers of events passing the final selection are given in Table 1

, and compared to the expected 4-body decay stop signal for mt̃=120 GeV and mχ̃01=60 GeV in the light sneutrino and W-exchange scenarios. The efficiency for selecting the signal varies between 1% and 4% and is largest for high stop masses and low neutralino masses. The most significant sources of uncertainties on the number of signal events passing the selection criteria are given in Ref. [6] and combine to approximately 18%. The total systematic error for the background is about 10%. This error is dominated by the uncertainty on the QCD background (7%) and on the cross-sections for the background processes (10–17%).

The agreement between the number of observed events and the expected SM background allows us to set cross-section upper limits on stop pair production. We make the assumption that all non-SM processes, except the ones specifically searched for, can be neglected. This translates to more conservative limits. The 95% confidence level (C.L.) limits are obtained using a Bayesian approach [22] that takes statistical and systematic uncertainties into account.The two main scenarios that we study are dependent on the sneutrino mass: if mν̃ is large (mν̃≳2mW) the decay χ̃+1→ℓν̃ can be neglected, and only the decay χ̃+1→Wχ̃01 contributes significantly, leading to the so-called W-exchange scenario. Otherwise, the decay χ̃+1→ℓν̃ plays a significant role, and is assumed to be dominant in the so-called light sneutrino scenario, as is the case, for instance, if mν̃≲mW [17]. Experimentally the light sneutrino scenario has an advantage since leptons are always present in the final state, while this is the case for only about one-third of the stops decaying via W-exchange. The exact proportion of the two scenarios depends on the MSSM parameters; we treat them separately, assuming 100% branching ratio in each mode.Cross-section limits in the W-exchange scenario are shown in Fig. 2

for three different neutralino masses, mχ̃01=40, 50 and 60 GeV. Even at low mχ̃01 and mt̃, the limits are about a factor of two higher than the expected cross-section, so this 4-body decay scenario cannot be excluded with these data. The limits for the 3-body decay (i.e., when mt̃>mW+mb+mχ̃01) are also shown, but are about an order of magnitude larger than the expected cross-section. Our results are compared to those of the CDF Collaboration [7] obtained assuming t̃→bχ̃+1 followed by χ̃+1→ff′̄χ̃01 via a virtual W boson, with mχ̃+1=90 GeV and mχ̃01=40 GeV.

Upper limits on the cross-section in the light sneutrino scenario are shown in Fig. 3

assuming mχ̃01⩽mν̃=60,80 GeV for the 3-body decay, and mχ̃01=50,60 GeV for the 4-body decay where mν̃=mt̃−mb. The limits are stronger than those obtained for the W-exchange scenario since two charged leptons are always present in the final state. The limits are below the expected cross-section for some part of the (mt̃,mχ̃01) plane: for instance, for mχ̃01=50 GeV the 4-body decay scenario is excluded for 90≲mt̃≲120 GeV. The limits for the 3-body decay are stronger, extending to mt̃=140 GeV for mχ̃01=60 GeV.

The resulting exclusion contours for the light sneutrino scenario are displayed in Fig. 4

in the (mt̃,mχ̃01) plane assuming 3- or 4-body decay with a light sneutrino mass equal, respectively, to mχ̃01 and mt̃−mb. The results obtained by CDF [11] assuming 100% branching ratio for t̃→cχ̃01 and at LEP [23], in the cχ̃01 and t̃→bℓν̃ channels, are also shown. ALEPH has recently reported the first search at for 4-body decays of the stop [9]. Their limit, when assuming 100% branching ratio for t̃→bℓνχ̃01, is about 95 GeV for mχ̃01≃ 75 GeV, and is also shown in Fig. 4. It is slightly lower when no assumptions on the branching ratio and on the t̃t̃Z coupling are made. All these limits indicate that all decays of stops having masses lower than approximately 115 GeV are strongly constrained when the neutralino mass is lighter than approximately 50 GeV.

In conclusion, our analysis places new cross-section limits on stop pair production as a function of the stop and neutralino masses by considering the 3- and 4-body decays of the stop, i.e., taking into account the possibility that the loop-induced t̃→cχ̃01 decay is negligible when the bχ̃+1 decay is not kinematically allowed: if the sneutrino is of comparable mass to the stop or lighter, the existence of a stop with a mass smaller than approximately 120 GeV is excluded for mχ̃01≲50 GeV. If the sneutrino mass is smaller than 60 GeV, the mass exclusion domain extends up to a stop mass of 140 GeV. Without assumptions on the sneutrino mass, no exclusion domain can be set in the light sneutrino scenario, and we thus provide new cross-section upper limits on stop pair production in the W-exchange scenario up to mt̃=140 GeV.

Acknowledgements

We thank the staffs at Fermilab and collaborating institutions, and acknowledge support from the Department of Energy and National Science Foundation (USA), Commissariat à L'Energie Atomique and CNRS/Institut National de Physique Nucléaire et de Physique des Particules (France), Ministry for Science and Technology and Ministry for Atomic Energy (Russia), CAPES and CNPq (Brazil), Departments of Atomic Energy and Science and Education (India), Colciencias (Colombia), CONACyT (Mexico), Ministry of Education and KOSEF (Korea), CONICET and UBACyT (Argentina), The Foundation for Fundamental Research on Matter (The Netherlands), PPARC (United Kingdom), Ministry of Education (Czech Republic), A.P. Sloan Foundation, and the Research Corporation.

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