application/xmlSearch for the standard model Higgs boson in the [formula omitted] channel in 9.5 fb−1 of [formula omitted] collisions at [formula omitted]D0 CollaborationV.M. AbazovB. AbbottB.S. AcharyaM. AdamsT. AdamsG.D. AlexeevG. AlkhazovA. AltonG. AlversonA. AskewS. AtkinsK. AugstenC. AvilaF. BadaudL. BagbyB. BaldinD.V. BandurinS. BanerjeeE. BarberisP. BaringerJ.F. BartlettU. BasslerV. BazterraA. BeanM. BegalliL. BellantoniS.B. BeriG. BernardiR. BernhardI. BertramM. BesançonR. BeuselinckP.C. BhatS. BhatiaV. BhatnagarG. BlazeyS. BlessingK. BloomA. BoehnleinD. BolineE.E. BoosG. BorissovT. BoseA. BrandtO. BrandtR. BrockA. BrossD. BrownJ. BrownX.B. BuM. BuehlerV. BuescherV. BunichevS. BurdinC.P. BuszelloE. Camacho-PérezB.C.K. CaseyH. Castilla-ValdezS. CaughronS. ChakrabartiD. ChakrabortyK.M. ChanA. ChandraE. ChaponG. ChenS. Chevalier-ThéryD.K. ChoS.W. ChoS. ChoiB. ChoudharyS. CihangirD. ClaesJ. ClutterM. CookeW.E. CooperM. CorcoranF. CoudercM.-C. CousinouA. CrocD. CuttsA. DasG. DaviesS.J. de JongE. De La Cruz-BureloF. DéliotR. DeminaD. DenisovS.P. DenisovS. DesaiC. DeterreK. DeVaughanH.T. DiehlM. DiesburgP.F. DingA. DominguezA. DubeyL.V. DudkoD. DugganA. DuperrinS. DuttA. DyshkantM. EadsD. EdmundsJ. EllisonV.D. ElviraY. EnariH. EvansA. EvdokimovV.N. EvdokimovG. FaciniL. FengT. FerbelF. FiedlerF. FilthautW. FisherH.E. FiskM. FortnerH. FoxS. FuessA. Garcia-BellidoJ.A. García-GonzálezG.A. García-GuerraV. GavrilovP. GayW. GengD. GerbaudoC.E. GerberY. GershteinG. GintherG. GolovanovA. GoussiouP.D. GrannisS. GrederH. GreenleeG. GrenierPh. GrisJ.-F. GrivazA. GrohsjeanS. GrünendahlM.W. GrünewaldT. GuilleminG. GutierrezP. GutierrezS. HagopianJ. HaleyL. HanK. HarderA. HarelJ.M. HauptmanJ. HaysT. HeadT. HebbekerD. HedinH. HegabA.P. HeinsonU. HeintzC. HenselI. Heredia-De La CruzK. HernerG. HeskethM.D. HildrethR. HiroskyT. HoangJ.D. HobbsB. HoeneisenJ. HoganM. HohlfeldI. HowleyZ. HubacekV. HynekI. IashviliY. IlchenkoR. IllingworthA.S. ItoS. JabeenM. JaffréA. JayasingheM.S. JeongR. JesikP. JiangK. JohnsE. JohnsonM. JohnsonA. JonckheereP. JonssonJ. JoshiA.W. JungA. JusteK. KaadzeE. KajfaszD. KarmanovP.A. KasperI. KatsanosR. KehoeS. KermicheN. KhalatyanA. KhanovA. KharchilavaY.N. KharzheevI. KiselevichJ.M. KohliA.V. KozelovJ. KrausS. KulikovA. KumarA. KupcoT. KurčaV.A. KuzminS. LammersG. LandsbergP. LebrunH.S. LeeS.W. LeeW.M. LeeX. LeiJ. LellouchD. LiH. LiL. LiQ.Z. LiJ.K. LimD. LincolnJ. LinnemannV.V. LipaevR. LiptonH. LiuY. LiuA. LobodenkoM. LokajicekR. Lopes de SaH.J. LubattiR. Luna-GarciaA.L. LyonA.K.A. MacielR. MadarR. Magaña-VillalbaS. MalikV.L. MalyshevY. MaravinJ. Martínez-OrtegaR. McCarthyC.L. McGivernM.M. MeijerA. MelnitchoukD. MenezesP.G. MercadanteM. MerkinA. MeyerJ. MeyerF. MiconiN.K. MondalM. MulhearnE. NagyM. NaimuddinM. NarainR. NayyarH.A. NealJ.P. NegretP. NeustroevH.T. NguyenT. NunnemannJ. OrdunaN. OsmanJ. OstaM. PadillaA. PalN. ParasharV. PariharS.K. ParkR. PartridgeN. ParuaA. PatwaB. PenningM. PerfilovY. PetersK. PetridisG. PetrilloP. PétroffM.-A. PleierP.L.M. Podesta-LermaV.M. PodstavkovA.V. PopovM. PrewittD. PriceN. ProkopenkoJ. QianA. QuadtB. QuinnM.S. RangelK. RanjanP.N. RatoffI. RazumovP. RenkelI. Ripp-BaudotF. RizatdinovaM. RominskyA. RossC. RoyonP. RubinovR. RuchtiG. SajotP. SalcidoA. Sánchez-HernándezM.P. SandersA.S. SantosG. SavageL. SawyerT. ScanlonR.D. SchambergerY. ScheglovH. SchellmanS. SchlobohmC. SchwanenbergerR. SchwienhorstJ. SekaricH. SeveriniE. ShabalinaV. SharyS. ShawA.A. ShchukinR.K. ShivpuriV. SimakP. SkubicP. SlatteryD. SmirnovK.J. SmithG.R. SnowJ. SnowS. SnyderS. Söldner-RemboldL. SonnenscheinK. SoustruznikJ. StarkD.A. StoyanovaM. StraussL. SuterP. SvoiskyM. TakahashiM. TitovV.V. TokmeninY.-T. TsaiK. Tschann-GrimmD. TsybychevB. TuchmingC. TullyL. UvarovS. UvarovS. UzunyanR. Van KootenW.M. van LeeuwenN. VarelasE.W. VarnesI.A. VasilyevP. VerdierA.Y. VerkheevL.S. VertogradovM. VerzocchiM. VesterinenD. VilanovaP. VokacH.D. WahlM.H.L.S. WangR.-J. WangJ. WarcholG. WattsM. WayneJ. WeichertL. Welty-RiegerA. WhiteD. WickeM.R.J. WilliamsG.W. WilsonM. WobischD.R. WoodT.R. WyattY. XieR. YamadaS. YangW.-C. YangT. YasudaY.A. YatsunenkoW. YeZ. YeH. YinK. YipS.W. YounJ.M. YuJ. ZennamoT. ZhaoT.G. ZhaoB. ZhouJ. ZhuM. ZielinskiD. ZieminskaL. ZivkovicPhysics Letters B 716 (2012) 285-293. doi:10.1016/j.physletb.2012.08.034journalPhysics Letters BCopyright © 2012 Elsevier B.V. All rights reserved.Elsevier B.V.0370-2693716219 September 20122012-09-19285-29328529310.1016/j.physletb.2012.08.034http://dx.doi.org/10.1016/j.physletb.2012.08.034doi:10.1016/j.physletb.2012.08.034http://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

S300.4

PLB28823S0370-2693(12)00884-210.1016/j.physletb.2012.08.034Elsevier B.V.Experiments

Fig. 1

(Color online.) The measure S of E̸T significance in the analysis sample without the requirement that S be larger than 5. The data are shown as points with error bars and the background contributions as histograms: dibosons are labeled as “VV,” “V+l.f.” includes (W/Z)+(u,d,s,g) jets, “V+h.f.” includes (W/Z)+(b,c) jets, and “Top” includes pair and single top quark production. The distribution for signal (VH) is scaled by a factor of 500 and includes ZH and WH production for MH=125 GeV.

Fig. 2

(Color online.) Distributions of the dijet mass before b-tagging in the (a) EW control and (b) MJ-enriched samples. The data are shown as points with error bars and the background contributions as histograms: dibosons are labeled as “VV,” “V+l.f.” includes (W/Z)+(u,d,s,g) jets, “V+h.f.” includes (W/Z)+(b,c) jets, and “Top” includes pair and single top quark production.

Fig. 3

(Color online.) Distribution of the MJ DT output after the medium b-tagging requirement in the analysis sample. The distribution for signal (VH), shown for MH=125 GeV, is scaled by a factor of 100 and includes ZH and WH production. The data are shown as points with error bars and the background contributions as histograms: dibosons are labeled as “VV,” “V+l.f.” includes (W/Z)+(u,d,s,g) jets, “V+h.f.” includes (W/Z)+(b,c) jets, and “Top” includes pair and single top quark production.

Fig. 4

(Color online.) Dijet invariant mass in the analysis sample after the multijet veto for events with (a) medium b-tag and (b) tight b-tag. The distributions for signal (VH), which are scaled by a factor of 100 for medium b-tag and 10 for tight b-tag respectively, include ZH and WH production for MH=125 GeV. The data are shown as points with error bars and the background contributions as histograms: dibosons are labeled as “VV,” “V+l.f.” includes (W/Z)+(u,d,s,g) jets, “V+h.f.” includes (W/Z)+(b,c) jets, and “Top” includes pair and single top quark production.

Fig. 5

(Color online.) The SM DT output for the (W/Z)H search with MH=125 GeV following the multijet veto for events with (a) medium b-tag and (b) tight b-tag prior to the fit to data. The distributions for signal (VH) are scaled by a factor of 100 for medium b-tag events and 10 for tight b-tag events, respectively, and include ZH and WH production for MH=125 GeV. The data are shown as points with error bars and the background contributions as histograms: dibosons are labeled as “VV,” “V+l.f.” includes (W/Z)+(u,d,s,g) jets, “V+h.f.” includes (W/Z)+(b,c) jets, and “Top” includes pair and single top quark production.

Fig. 6

(Color online.) Final discriminant distributions expected for a SM VH signal with MH=125 GeV (filled histogram) and observed for background-subtracted data (points with statistical error bars) for the (a) medium and (b) tight b-tag channels. The subtracted background is the result of the profile likelihood fit to the data under the background-only hypothesis. Also shown is the ±1 standard deviation (s.d.) band on the fitted background. No scaling factor is applied to the signal.

Fig. 7

(Color online.) (a) Ratio of the observed (solid black) and expected (dotted red) exclusion limits to the SM production cross section. (b) The observed (solid black) and expected LLRs for the background-only (black dots) and signal+background hypotheses (short red dashes), as well as the LLR expected in the presence of a Higgs boson with MH=125 GeV (long blue dashes). All are shown as a function of the tested value of MH with the green and yellow shaded areas corresponding to the 1 and 2 standard deviation (s.d.) variations around the background-only hypothesis.

Fig. 8

(Color online.) Final discriminant distributions expected for a WZ plus ZZ signal (filled histogram) and observed for background-subtracted data (points with statistical error bars) for the (a) medium and (b) tight b-tag channels. (c) Similarly for the dijet mass distribution, summed over the medium and tight b-tag channels. The subtracted backgrounds are the results of profile likelihood fits to the data under the signal+background hypothesis. Also shown are the ±1 standard deviation (s.d.) bands on the fitted backgrounds. The signal is scaled to the SM cross section.

Table 1

The numbers of expected signal, expected background, and observed data events after the multijet veto, for the pre, medium, and tight b-tag samples. The signal corresponds to MH=125 GeV, “Top” includes pair and single top quark production, and “VV” is the sum of all diboson processes. The uncertainties quoted on the signal and total background arise from the statistics of the simulation and from the sources of systematic uncertainties mentioned in the text.

SampleZHWHW+jetsZ+jetsTopVVMJTotal backgroundObservedPre b-tag18.3±1.816.7±1.6668952558519343144197799535±1254298980Medium b-tag6.7±0.76.1±0.6311210747612372785462±7765453Tight b-tag6.0±0.85.3±0.74432523775661134±1921039Table 2

The expected and observed upper limits measured using 9.5 fb−1 of integrated luminosity on the ZH plus WH production cross section relative to the SM expectation, assuming SM branching fractions, as a function of MH.

mH (GeV)100105110115120125130135140145150Expected2.12.22.42.73.23.95.06.79.213.821.6Observed1.92.32.23.03.54.34.37.28.815.316.8Search for the standard model Higgs boson in the ZH→νν¯bb¯ channel in 9.5 fb−1 of pp¯ collisions at s=1.96 TeV

D0 Collaboration

V.M.

Abazov

Abbott

B.S.

Acharya

Adams

G.D.

Alexeev

Alkhazov

Alton

Alverson

Askew

Atkins

Augsten

Avila

Badaud

Bagby

Baldin

D.V.

Bandurin

Banerjee

Barberis

Baringer

J.F.

Bartlett

Bassler

Bazterra

Bean

Begalli

Bellantoni

S.B.

Beri

Bernardi

Bernhard

Bertram

Besançon

Beuselinck

P.C.

Bhat

Bhatia

Bhatnagar

Blazey

Blessing

Bloom

Boehnlein

Boline

E.E.

Boos

Borissov

Bose

Brandt

Brock

Bross

Brown

X.B.

Buehler

Buescher

Bunichev

Burdin

C.P.

Buszello

Camacho-Pérez

B.C.K.

Casey

Castilla-Valdez

Caughron

Chakrabarti

Chakraborty

K.M.

Chan

Chandra

Chapon

Chen

Chevalier-Théry

D.K.

Cho

S.W.

Cho

Choi

Choudhary

Cihangir

Claes

Clutter

Cooke

W.E.

Cooper

Corcoran

Couderc

M.-C.

Cousinou

Croc

Cutts

Das

Davies

S.J.

de Jong

De La Cruz-Burelo

Déliot

Demina

Denisov

S.P.

Denisov

Desai

Deterre

DeVaughan

H.T.

Diehl

Diesburg

P.F.

Ding

Dominguez

Dubey

L.V.

Dudko

Duggan

Duperrin

Dutt

Dyshkant

Eads

Edmunds

Ellison

V.D.

Elvira

Enari

Evans

Evdokimov

V.N.

Evdokimov

Facini

Feng

Ferbel

Fiedler

Filthaut

Fisher

H.E.

Fisk

Fortner

Fox

Fuess

Garcia-Bellido

J.A.

García-González

G.A.

García-Guerra

Gavrilov

Gay

Geng

Gerbaudo

C.E.

Gerber

Gershtein

Ginther

Golovanov

Goussiou

P.D.

Grannis

Greder

Greenlee

Grenier

Ph.

Gris

J.-F.

Grivaz

⁎

grivaz@lal.in2p3.fr

Grohsjean

Grünendahl

M.W.

Grünewald

Guillemin

Gutierrez

Hagopian

Haley

Han

Harder

Harel

J.M.

Hauptman

Hays

Head

Hebbeker

Hedin

Hegab

A.P.

Heinson

Heintz

Hensel

Heredia-De La Cruz

Herner

Hesketh

M.D.

Hildreth

Hirosky

Hoang

J.D.

Hobbs

Hoeneisen

Hogan

Hohlfeld

Howley

Hubacek

Hynek

Iashvili

Ilchenko

Illingworth

A.S.

Ito

Jabeen

Jaffré

Jayasinghe

M.S.

Jeong

Jesik

Jiang

Johns

Johnson

Jonckheere

Jonsson

Joshi

A.W.

Jung

Juste

Kaadze

Kajfasz

Karmanov

P.A.

Kasper

Katsanos

Kehoe

Kermiche

Khalatyan

Khanov

Kharchilava

Y.N.

Kharzheev

Kiselevich

J.M.

Kohli

A.V.

Kozelov

Kraus

Kulikov

Kumar

Kupco

Kurča

V.A.

Kuzmin

Lammers

Landsberg

Lebrun

H.S.

Lee

S.W.

Lee

W.M.

Lee

Lei

Lellouch

Q.Z.

J.K.

Lim

Lincoln

Linnemann

V.V.

Lipaev

Lipton

Liu

Lobodenko

Lokajicek

Lopes de Sa

H.J.

Lubatti

Luna-Garcia

A.L.

Lyon

A.K.A.

Maciel

Madar

Magaña-Villalba

Malik

V.L.

Malyshev

Maravin

Martínez-Ortega

McCarthy

C.L.

McGivern

M.M.

Meijer

Melnitchouk

Menezes

P.G.

Mercadante

Merkin

Meyer

Miconi

N.K.

Mondal

Mulhearn

Nagy

Naimuddin

Narain

Nayyar

H.A.

Neal

J.P.

Negret

Neustroev

H.T.

Nguyen

Nunnemann

Orduna

Osman

Osta

Padilla

Pal

Parashar

Parihar

S.K.

Park

Partridge

Parua

Patwa

Penning

Perfilov

Peters

Petridis

Petrillo

Pétroff

M.-A.

Pleier

P.L.M.

Podesta-Lerma

V.M.

Podstavkov

A.V.

Popov

Prewitt

Price

Prokopenko

Qian

Quadt

Quinn

M.S.

Rangel

Ranjan

P.N.

Ratoff

Razumov

Renkel

Ripp-Baudot

Rizatdinova

Rominsky

Ross

Royon

Rubinov

Ruchti

Sajot

Salcido

Sánchez-Hernández

M.P.

Sanders

A.S.

Santos

Savage

Sawyer

Scanlon

R.D.

Schamberger

Scheglov

Schellman

Schlobohm

Schwanenberger

Schwienhorst

Sekaric

Severini

Shabalina

Shary

Shaw

A.A.

Shchukin

R.K.

Shivpuri

Simak

Skubic

Slattery

Smirnov

K.J.

Smith

G.R.

Snow

Snyder

Söldner-Rembold

Sonnenschein

Soustruznik

Stark

D.A.

Stoyanova

Strauss

Suter

Svoisky

Takahashi

Titov

V.V.

Tokmenin

Y.-T.

Tsai

Tschann-Grimm

Tsybychev

Tuchming

Tully

Uvarov

Uzunyan

Van Kooten

W.M.

van Leeuwen

Varelas

E.W.

Varnes

I.A.

Vasilyev

Verdier

A.Y.

Verkheev

L.S.

Vertogradov

Verzocchi

Vesterinen

Vilanova

Vokac

H.D.

Wahl

M.H.L.S.

Wang

R.-J.

Wang

Warchol

Watts

Wayne

Weichert

Welty-Rieger

White

Wicke

M.R.J.

Williams

G.W.

Wilson

Wobisch

D.R.

Wood

T.R.

Wyatt

Xie

Yamada

Yang

W.-C.

Yang

Yasuda

Y.A.

Yatsunenko

Yin

Yip

S.W.

Youn

J.M.

Zennamo

Zhao

T.G.

Zhao

Zhou

Zhu

Zielinski

Zieminska

Zivkovic

aLAFEX, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, BrazilbUniversidade do Estado do Rio de Janeiro, Rio de Janeiro, BrazilcUniversidade Federal do ABC, Santo André, BrazildUniversity of Science and Technology of China, Hefei, Peopleʼs Republic of ChinaeUniversidad de los Andes, Bogotá, ColombiafCharles University, Faculty of Mathematics and Physics, Center for Particle Physics, Prague, Czech RepublicgCzech Technical University in Prague, Prague, Czech RepublichCenter for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech RepubliciUniversidad San Francisco de Quito, Quito, EcuadorjLPC, Université Blaise Pascal, CNRS/IN2P3, Clermont, FrancekLPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, FrancelCPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, FrancemLAL, Université Paris-Sud, CNRS/IN2P3, Orsay, FrancenLPNHE, Universités Paris VI and VII, CNRS/IN2P3, Paris, FranceoCEA, Irfu, SPP, Saclay, FrancepIPHC, Université de Strasbourg, CNRS/IN2P3, Strasbourg, FranceqIPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, and Université de Lyon, Lyon, FrancerIII. Physikalisches Institut A, RWTH Aachen University, Aachen, GermanysPhysikalisches Institut, Universität Freiburg, Freiburg, GermanytII. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, GermanyuInstitut für Physik, Universität Mainz, Mainz, GermanyvLudwig-Maximilians-Universität München, München, GermanywFachbereich Physik, Bergische Universität Wuppertal, Wuppertal, GermanyxPanjab University, Chandigarh, IndiayDelhi University, Delhi, IndiazTata Institute of Fundamental Research, Mumbai, IndiaaaUniversity College Dublin, Dublin, IrelandabKorea Detector Laboratory, Korea University, Seoul, Republic of KoreaacCINVESTAV, Mexico City, MexicoadNikhef, Science Park, Amsterdam, The NetherlandsaeRadboud University Nijmegen, Nijmegen, The NetherlandsafJoint Institute for Nuclear Research, Dubna, RussiaagInstitute for Theoretical and Experimental Physics, Moscow, RussiaahMoscow State University, Moscow, RussiaaiInstitute for High Energy Physics, Protvino, RussiaajPetersburg Nuclear Physics Institute, St. Petersburg, RussiaakInstitució Catalana de Recerca i Estudis Avançats (ICREA) and Institut de Física dʼAltes Energies (IFAE), Barcelona, SpainalUppsala University, Uppsala, SwedenamLancaster University, Lancaster LA1 4YB, United KingdomanImperial College London, London SW7 2AZ, United KingdomaoThe University of Manchester, Manchester M13 9PL, United KingdomapUniversity of Arizona, Tucson, AZ 85721, USAaqUniversity of California Riverside, Riverside, CA 92521, USAarFlorida State University, Tallahassee, FL 32306, USAasFermi National Accelerator Laboratory, Batavia, IL 60510, USAatUniversity of Illinois at Chicago, Chicago, IL 60607, USAauNorthern Illinois University, DeKalb, IL 60115, USAavNorthwestern University, Evanston, IL 60208, USAawIndiana University, Bloomington, IN 47405, USAaxPurdue University Calumet, Hammond, IN 46323, USAayUniversity of Notre Dame, Notre Dame, IN 46556, USAazIowa State University, Ames, IA 50011, USAbaUniversity of Kansas, Lawrence, KS 66045, USAbbKansas State University, Manhattan, KS 66506, USAbcLouisiana Tech University, Ruston, LA 71272, USAbdBoston University, Boston, MA 02215, USAbeNortheastern University, Boston, MA 02115, USAbfUniversity of Michigan, Ann Arbor, MI 48109, USAbgMichigan State University, East Lansing, MI 48824, USAbhUniversity of Mississippi, University, MS 38677, USAbiUniversity of Nebraska, Lincoln, NE 68588, USAbjRutgers University, Piscataway, NJ 08855, USAbkPrinceton University, Princeton, NJ 08544, USAblState University of New York, Buffalo, NY 14260, USAbmUniversity of Rochester, Rochester, NY 14627, USAbnState University of New York, Stony Brook, NY 11794, USAboBrookhaven National Laboratory, Upton, NY 11973, USAbpLangston University, Langston, OK 73050, USAbqUniversity of Oklahoma, Norman, OK 73019, USAbrOklahoma State University, Stillwater, OK 74078, USAbsBrown University, Providence, RI 02912, USAbtUniversity of Texas, Arlington, TX 76019, USAbuSouthern Methodist University, Dallas, TX 75275, USAbvRice University, Houston, TX 77005, USAbwUniversity of Virginia, Charlottesville, VA 22904, USAbxUniversity 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.

Visitor from UPIITA-IPN, Mexico City, Mexico.

Visitor from DESY, Hamburg, Germany.

Visitor from SLAC, Menlo Park, CA, USA.

Visitor from University College London, London, UK.

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Visitor from ECFM, Universidad Autonoma de Sinaloa, Culiacán, Mexico.

Visitor from Universidade Estadual Paulista, São Paulo, Brazil.

Editor: L. Rolandi

Abstract

We present a search for the standard model Higgs boson in 9.5 fb−1 of pp¯ collisions at s=1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. The final state considered contains a pair of b jets and is characterized by an imbalance in transverse energy, as expected from pp¯→ZH→νν¯bb¯ production. The search is also sensitive to the WH→ℓνbb¯ channel when the charged lepton is not identified. The data are found to be in good agreement with the expected background. For a Higgs boson mass of 125 GeV, we set a limit at the 95% C.L. on the cross section σ(pp¯→[Z/W]H), assuming standard model branching fractions, that is a factor of 4.3 times larger than the theoretical standard model value, while the expected factor is 3.9. The search is also used to measure a combined WZ and ZZ production cross section that is a factor of 0.94±0.31(stat)±0.34(syst) times the standard model prediction of 4.4 pb, with an observed significance of 2.0 standard deviations.

Introduction

In the standard model (SM) [1], electroweak symmetry breaking is achieved via the introduction of a doublet of scalar fields, of which one degree of freedom remains once the W and Z vector bosons have acquired their masses. This degree of freedom manifests itself as a new scalar particle [2], the Higgs boson (H). Associated ZH production in pp¯ collisions at s=1.96 TeV, with Z→νν¯ and H→bb¯, is among the most sensitive processes in the search for a Higgs boson with mass MH≲135 GeV at the Fermilab Tevatron Collider [3]. The D0 Collaboration published a search for this process based on 5.2 fb−1 of integrated luminosity [4]. In this Letter, an extension of this search to the full Run II dataset is presented. The CDF Collaboration recently reported results from a similar search [5], as well as the ATLAS and CMS Collaborations using pp collisions at 7 TeV at the LHC [6,7]. A lower limit of 114.4 GeV was set on MH by the LEP Collaborations [8], while an upper limit at 127 GeV has been established by the ATLAS and CMS Collaborations [9,10]. These limits and those given below are all defined at the 95% C.L. The ATLAS and CMS Collaborations have also published [9,10] excesses above background expectations at approximately 125 GeV and have recently reported results confirming these excesses at the five standard deviations level [11,12].The final-state topology considered in this search consists of a pair of b jets from H→bb¯ decay and missing transverse energy (E̸T) from Z→νν¯. The search is also sensitive to the WH process when the charged lepton from W→ℓν decay is not identified. The main backgrounds arise from (W/Z)+heavy-flavor jets (jets initiated by b or c quarks), top quark production, and multijet (MJ) events with E̸T arising from mismeasurement of jet energies. A boosted-decision-tree discriminant based on kinematic properties is first used to reject most of the multijet events. Next, jets from candidate Higgs boson decays are required to be identified as b jets. Finally, discrimination between signal and remaining backgrounds is achieved by means of additional boosted decision trees.To validate the techniques used in the search for the Higgs boson, the analysis is also interpreted as a measurement of WZ and ZZ diboson production. The only modification is in the training of the final discriminants, for which a diboson signal is used instead of a Higgs boson signal.2

Data and simulated samples

The D0 detector used for Tevatron Run II (2001–2011) is described in detail in Ref. [13]. Its main components are: a tracking system surrounding the beam pipe, followed by a liquid-argon and uranium sampling calorimeter, and then a muon system. The tracking system is immersed in a 2 T magnetic field provided by a superconducting solenoid and consists of a silicon microstrip tracker followed by a scintillating fiber tracker. The calorimeter is composed of a central and two end sections housed in separate cryostats. Each section is segmented in depth, with four electromagnetic layers followed by up to five hadronic layers. Scintillating tiles provide additional sampling between the cryostats. The muon system consists of tracking and trigger detectors in front of and beyond 1.8 T iron toroids. Online event selection is provided by a three-level trigger system.The data used in this analysis were recorded using triggers designed to select events with jets and E̸T[14]. After imposing data quality requirements, the total integrated luminosity recorded with these triggers is 9.5 fb−1, corresponding to all available Run II data for this analysis.The analysis relies on (i) charged particle tracks, (ii) calorimeter jets reconstructed in a cone of radius 0.5 in y–ϕ space, where y is the rapidity and ϕ the azimuthal angle, using the iterative midpoint cone algorithm [15], and (iii) electrons or muons identified through the association of tracks with electromagnetic calorimeter clusters or with hits in the muon detector, respectively. The E̸T is reconstructed as the negative of the vectorial sum of the transverse components of energy deposits in the calorimeter and is corrected for identified muons. Jet energies are calibrated using primarily transverse energy balance in photon+jet events [16], and these corrections are propagated to the E̸T assessment.Those backgrounds arising from MJ processes with instrumental effects giving rise to E̸T are estimated from data. The remainder of the backgrounds and the signal processes are simulated by Monte Carlo (MC). Events from (W/Z)+jets processes are generated with alpgen[17], interfaced with pythia[18] for initial- and final-state radiation and for hadronization. The pT spectrum of the Z boson is reweighted to match the D0 measurement [19]. The pT spectrum of the W boson is reweighted using the same experimental input, corrected for the differences between the W and ZpT spectra predicted in next-to-next-to-leading order (NNLO) QCD [20]. To simulate tt¯ and electroweak single top quark production, the alpgen and singletop[21] generators, respectively, are interfaced with pythia, while vector boson pair production is generated with pythia. The ZH and WH signal processes are generated with pythia for Higgs boson masses from 100 to 150 GeV in 5 GeV steps. All these simulations use CTEQ6L1 parton distribution functions (PDFs) [22].The absolute normalizations for (W/Z) inclusive production are obtained from NNLO calculations of total cross sections [23] using the MSTW2008 NNLO PDFs [24]. The heavy-flavor fractions in (W/Z)+jets are obtained using mcfm[25] at next-to-leading order (NLO). The diboson cross sections are also calculated with mcfm[27]. Cross sections for pair and single top quark production are taken from Ref. [26]. For signal processes, cross sections are taken from Ref. [28].Signal and background samples are passed through a full geant3-based simulation [29] of the detector response and processed with the same reconstruction program as used for data. Events from randomly selected beam crossings with the same instantaneous luminosity distribution as data are overlaid on simulated events to account for detector noise and contributions from additional pp¯ interactions. Parameterizations of the trigger efficiencies are determined using events collected with independent triggers based on information from the muon detectors. Corrections for residual differences between data and simulation are applied for electron, muon, and jet identification. Jet energy calibration and resolution are adjusted in simulated events to match those measured in data.3

Event selection

A preselection that greatly reduces the overwhelming background from multijet events is performed as follows. The interaction vertex must be reconstructed within the acceptance of the silicon vertex detector and at least three tracks must originate from that vertex. Jets with associated tracks that meet criteria ensuring that the b-tagging algorithm operates efficiently are denoted as “taggable” jets, except for those also identified as hadronic decays of τ leptons [30]. Exactly two taggable jets are required, one of which must be the leading (highest pT) jet in the event; the Higgs candidate is formed from these two jets, denoted jet1 and jet2 (ordered in decreasing pT). These jets must have transverse momentum pT>20 GeV and pseudorapidity |η|<2.5. The two taggable jets must not be back-to-back in the plane transverse to the beam direction: Δϕ(jet1,jet2)<165°. Finally, E̸T>40 GeV is required.Additional selection criteria define four distinct samples: (i) an “analysis” sample used to search for a Higgs boson signal; (ii) an “electroweak (EW) control” sample used to validate the background MC simulation, enriched in W(→μν)+jets events where the jet system has a topology similar to that of the analysis sample; (iii) an “MJ-model” sample, dominated by multijet events, used to model the MJ background in the analysis sample; and (iv) a large “MJ-enriched” sample, used to validate this MJ-modeling procedure.The analysis sample is selected by requiring the scalar sum of the transverse momenta of the two leading taggable jets to be greater than 80 GeV and a measure of the E̸T significance S>5[31]. Larger values of S correspond to E̸T values that are less likely to be caused by fluctuations in jet energies. The S distribution is shown for the analysis sample in Fig. 1

The dominant signal topology is a pair of b jets recoiling against the E̸T due to the neutrinos from Z→νν¯ decay, leading to the direction of the E̸T being at a large angle with respect to the direction of each jet. In contrast, in events from MJ background with fluctuations in jet energy measurement, the E̸T tends to be aligned with a mismeasured jet. A second estimate of the E̸T can be obtained from the missing pT, p̸T, calculated from the reconstructed charged particle tracks originating from the interaction vertex. This variable is less sensitive to jet energy measurement fluctuations. In signal events, p̸T is also expected to point away from both jets, while for MJ background, its angular distribution is expected to be more isotropic. Advantage is taken of these features through the variable D=[Δϕ(p̸T,jet1)+Δϕ(p̸T,jet2)]/2. For signal events, as well as for the non-MJ backgrounds, D>π/2 in the vast majority of events, whereas the MJ background events tend to be symmetrically distributed around π/2. In the analysis sample, D>π/2 is therefore required. To improve the efficiency of this criterion for the (W→μν)H signal with non-identified muons, tracks satisfying isolation criteria are removed from the p̸T computation. The reverse of the D requirement is also used to define the MJ-model sample, as explained below.Events containing an isolated electron or muon with pT>15 GeV are rejected to ensure there is no overlap with the D0 WH search in the lepton+E̸T topology [32].The EW control sample is selected in a similar manner to the analysis sample, except that an isolated muon with pT>15 GeV is required. The multijet content of this sample is rendered negligible by requiring that the transverse mass of the muon and E̸T system is larger than 30 GeV and that the E̸T, calculated taking account of the muon from the W boson decay, is greater than 20 GeV. To ensure similar jet topologies for the analysis and EW control samples, the E̸T, not corrected for the selected muon, is required to exceed 40 GeV. The number of selected events is in good agreement with the SM expectation. All the kinematic distributions are also well described once reweightings of the distributions of Δη(jet1,jet2) and η(jet2) are performed, as suggested by a comparison [33] of data with a simulation of (W/Z)+jets using the sherpa generator [34]. The distribution of the dijet mass in the EW control sample is shown in Fig. 2

(a).

The MJ-model sample, used to determine the MJ background, is selected in the same manner as the analysis sample, except that the requirement D>π/2 is reversed. The small remaining contributions from non-MJ SM background processes in the D<π/2 region are subtracted, and the resulting sample is used to model the MJ background in the analysis sample. The MJ background in the region D>π/2 is normalized by performing a fit of the sum of the MJ and SM backgrounds to the E̸T distribution of the data in the analysis sample.The MJ-enriched sample is used to test the validity of this approach and is defined in the same manner as the analysis sample, except that S<4.5 is now required (see Fig. 1). As a result, the MJ background dominates the entire range of D values, and this sample is used to verify that the events with D<π/2 correctly model those with D>π/2. The distribution of the dijet mass in the MJ-enriched sample is shown in Fig. 2(b).A multivariate b-tagging discriminant, with several boosted decision trees as inputs, is used to select events with one or more b quark candidates. This algorithm is an upgraded version of the neural network b-tagging technique described in Ref. [35]. The new algorithm includes more information related to the lifetime of the jet and results in a better discrimination between b and light (u,d,s,g) jets. It provides an output between 0 and 1 for each jet, with a value closer to one indicating a higher probability that the jet originated from a b quark. The output from the algorithm measured in simulated events is adjusted to match the output measured in dedicated data samples as described in more detail in Ref. [35]. From this continuous output, thirteen operating points (Lb=0,1,…,12) are defined, with b purity increasing with Lb. Jets with Lb=0 are defined as untagged. The typical per-b-jet efficiency and misidentification rate for light-flavor jets are about 80% (50%) and 10% (1%) for the loosest non-zero (tightest) b-tag operating point, respectively.To improve the sensitivity of the analysis, two high signal purity samples are defined from the analysis sample using the variable Lbb=Lb(jet1)+Lb(jet2). The two samples are defined as follows: a tight b-tag sample with Lbb⩾18 and a medium b-tag sample with 11⩽Lbb⩽17. The medium b-tag sample contains events with two loosely b-tagged jets, as well as events with one tightly b-tagged jet and one untagged jet. The signal-to-background ratios for a Higgs boson mass of 125 GeV in the pre, medium, and tight b-tag samples, after applying a multijet veto (defined in the next section), are respectively 0.035%, 0.23%, and 1.00%.4

Analysis using decision trees

A stochastic gradient boosted decision tree (DT) technique is employed, as implemented in the tmva package [36], to improve the discrimination between signal and background processes.First, an “MJ DT” (multijet-rejection DT) is trained to discriminate between signal and MJ-model events before b-tagging is applied. To avoid Higgs boson mass dependence at this stage of the analysis, signal events are not used, and the MJ DT is trained on a sample of (W/Z)+heavy-flavor jets events instead. Variables that provide some discrimination have been chosen for the MJ DT, excluding those strongly correlated to the Higgs mass (such as the dijet mass itself or the ΔR=(Δη)2+(Δϕ)2 between jet1 and jet2).The MJ DT output, which ranges between −1 and +1, is shown in Fig. 3

for the analysis sample after the medium b-tagging requirement. Good agreement is seen between data and the predicted background. A value of the multijet discriminant in excess of −0.3 is required (multijet veto), which removes 93% of the multijet background while retaining 90% of the signal for MH=125 GeV. The numbers of expected signal and background events, as well as the number of observed events, are given in Table 1

after imposing the multijet veto. Dijet mass distributions in the analysis sample after the multijet veto are shown in Fig. 4

for b-tagged events.

Next, to separate signal from the remaining SM backgrounds, two “SM DTs” (SM-background-rejection DTs) are trained for each MH, one in the medium b-tag channel and one in the tight b-tag channel. Some of the MJ DT input variables are used again, but most of the discrimination comes from additional kinematic variables correlated to the Higgs boson mass, of which, as expected, the dijet mass has the strongest discriminating power. The SM DT outputs, which range between −1 and +1, are used as final discriminants. Their distributions are shown in Fig. 5

for MH=125 GeV.

Systematic uncertainties

Experimental uncertainties arise from the integrated luminosity (6%) [37], the trigger simulation (2%), the jet energy calibration and resolution [(1–2)%], jet reconstruction and taggability (3%), the lepton identification (1%), the modeling of the MJ background (25%, which translates into a 1% uncertainty on the total background), and the b-tagging (from 4% for background in the medium b-tag sample to 9% for signal in the tight b-tag sample). In addition to the impact of these uncertainties on the integrated signal and background yields mentioned above, modifications of the shapes of the final discriminants are also considered, when relevant. Correlations among systematic uncertainties in signal and each background are taken into account when extracting the final results.Theoretical uncertainties on cross sections for SM processes are estimated as follows. For (W/Z)+jets production, an uncertainty of 10% is assigned to the total cross sections and an uncertainty of 20% to the heavy-flavor fractions (estimated using mcfm at NLO [25]). For other SM backgrounds, uncertainties are taken from Ref. [26] or using mcfm[27] and range from 6% to 10%. The uncertainties on cross sections for signal (7%) are taken from Ref. [28]. Uncertainties on the shapes of the final discriminants arise from (i) the modeling of (W/Z)+jets, assessed by varying the renormalization and factorization scales and by comparing results from alpgen interfaced with herwig[38] to alpgen interfaced with pythia, and (ii) the choice of PDFs, estimated using the prescription of Ref. [22].6

Limit setting procedure

Agreement is found between data and the predicted background, both in the numbers of selected events (Table 1) and in the distributions of final discriminants (Fig. 5), once systematic uncertainties are taken into account. The modified frequentist CLs approach [39] is used to set limits on the cross section for SM Higgs boson production, where the test statistic is a log-likelihood ratio (LLR) for the background-only and signal+background hypotheses. The result is obtained by summing LLR values over the bins in the final discriminants shown in Fig. 5. The impact of systematic uncertainties on the sensitivity of the analysis is reduced by maximizing a “profile” likelihood function [40] in which these uncertainties are given Gaussian constraints associated with their priors. Fig. 6

shows a comparison of the SM DT distributions expected for a signal with MH=125 GeV and observed for the background-subtracted data. The subtracted background and its uncertainties are the result of the profile likelihood fit to the data under the background-only hypothesis.

Higgs boson search results

The results are given as limits in Table 2

and Fig. 7

(a) and in terms of LLR values in Fig. 7(b). For MH=125 GeV, the observed and expected limits on the combined cross section of ZH and WH production are factors of 4.3 and 3.9 larger than the SM value, respectively, assuming SM branching fractions. In Fig. 7(b), the median expected LLR in the presence of a Higgs boson with a mass of 125 GeV is also shown for comparison.

Diboson search results

The final states arising from the SM production of (Z→νν¯)(Z→bb¯) and (W→ℓν)(Z→bb¯) are the same in particle content and topology as those used for the Higgs boson search reported above when the lepton from W→ℓν is not reconstructed. Evidence for ZZ and WZ production can therefore be used to validate the techniques employed in the Higgs boson search. The only modification to the analysis is in the training of the final discriminants, where ZZ and WZ are now treated as signal with the remaining diboson process, WW, kept as background. A cross section scale factor of 0.94±0.31(stat)±0.34(syst) is measured with respect to the predicted SM value of (4.4±0.3)pb[27], with an observed (expected) significance of 2.0 (2.1) standard deviations.The measurement of the diboson cross section has also been carried out using as final discriminants the distributions of dijet invariant mass (as opposed to the SM DTs) in the medium and tight b-tag samples. A cross section scale factor of 1.08±0.35(stat)±0.39(syst) is measured with respect to the predicted SM value, with an observed (expected) significance of 2.0 (1.9) standard deviations. The expected significance is slightly lower than the one expected with the multivariate analysis, in which additional discrimination is provided by variables such as the angular separation between jets or the event centrality.

Fig. 8

shows the final discriminant distributions in the medium and tight b-tag channels, as well as the dijet mass distribution summed over the medium and tight b-tag channels, for the expected WZ+ZZ signal and for the background-subtracted data. The subtracted backgrounds and their uncertainties are the results of the profile likelihood fits to the data under the signal+background hypothesis.

Summary

We have performed a search for the standard model Higgs boson in 9.5 fb−1 of pp¯ collisions at s=1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. The final state considered contains a pair of b jets and is characterized by an imbalance in transverse energy, as expected from pp¯→ZH→νν¯bb¯ production and decays. The search is also sensitive to the WH→ℓνbb¯ channel when the charged lepton is not identified. The data are found to be in good agreement with the expected background. For a Higgs boson mass of 125 GeV, we set a limit at the 95% C.L. on the cross section σ(pp¯→[Z/W]H), assuming standard model branching fractions, that is a factor of 4.3 larger than the theoretical standard model value, for an expected factor of 3.9.To validate our analysis techniques, we also performed a search for WZ and ZZ production, resulting in a measurement of the combined cross section that is a factor of 0.94±0.31(stat)±0.34(syst) relative to the standard model prediction, with a significance of 2.0 standard deviations.

Acknowledgements

We thank the staffs at Fermilab and collaborating institutions, and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); MON, NRC KI and RFBR (Russia); CNPq, FAPERJ, FAPESP and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); NRF (Korea); FOM (The Netherlands); STFC and the Royal Society (United Kingdom); MSMT and GACR (Czech Republic); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); and CAS and CNSF (China).

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