FERMILAB-PUB-15-013-E Measurement of the ratio of inclusive cross sections σ(pp¯ Z +2 b jets)/σ(pp¯ Z + 2 jets) in pp¯ collisions at √s = 1.96 TeV → → V.M. Abazov,31 B. Abbott,67 B.S. Acharya,25 M. Adams,46 T. Adams,44 J.P. Agnew,41 G.D. Alexeev,31 G. Alkhazov,35 A. Altona,56 A. Askew,44 S. Atkins,54 K. Augsten,7 C. Avila,5 F. Badaud,10 L. Bagby,45 B. Baldin,45 D.V. Bandurin,73 S. Banerjee,25 E. Barberis,55 P. Baringer,53 J.F. Bartlett,45 U. Bassler,15 V. Bazterra,46 A. Bean,53 M. Begalli,2 L. Bellantoni,45 S.B. Beri,23 G. Bernardi,14 R. Bernhard,19 I. Bertram,39 M. Besanc¸on,15 R. Beuselinck,40 P.C. Bhat,45 S. Bhatia,58 V. Bhatnagar,23 G. Blazey,47 S. Blessing,44 K. Bloom,59 5 A. Boehnlein,45 D. Boline,64 E.E. Boos,33 G. Borissov,39 M. Borysoval,38 A. Brandt,70 O. Brandt,20 R. Brock,57 1 A. Bross,45 D. Brown,14 X.B. Bu,45 M. Buehler,45 V. Buescher,21 V. Bunichev,33 S. Burdinb,39 C.P. Buszello,37 0 E. Camacho-P´erez,28 B.C.K. Casey,45 H. Castilla-Valdez,28 S. Caughron,57 S. Chakrabarti,64 K.M. Chan,51 2 A. Chandra,72 E. Chapon,15 G. Chen,53 S.W. Cho,27 S. Choi,27 B. Choudhary,24 S. Cihangir,45 D. Claes,59 v J. Clutter,53 M. Cookek,45 W.E. Cooper,45 M. Corcoran,72 F. Couderc,15 M.-C. Cousinou,12 D. Cutts,69 o N A. Das,71 G. Davies,40 S.J. de Jong,29,30 E. De La Cruz-Burelo,28 F. D´eliot,15 R. Demina,63 D. Denisov,45 S.P. Denisov,34 S. Desai,45 C. Deterrec,41 K. DeVaughan,59 H.T. Diehl,45 M. Diesburg,45 P.F. Ding,41 0 A. Dominguez,59 A. Dubey,24 L.V. Dudko,33 A. Duperrin,12 S. Dutt,23 M. Eads,47 D. Edmunds,57 2 J. Ellison,43 V.D. Elvira,45 Y. Enari,14 H. Evans,49 V.N. Evdokimov,34 A. Faur´e,15 L. Feng,47 T. Ferbel,63 ] F. Fiedler,21 F. Filthaut,29,30 W. Fisher,57 H.E. Fisk,45 M. Fortner,47 H. Fox,39 S. Fuess,45 P.H. Garbincius,45 x A. Garcia-Bellido,63 J.A. Garc´ıa-Gonz´alez,28 V. Gavrilov,32 W. Geng,12,57 C.E. Gerber,46 Y. Gershtein,60 e - G. Ginther,45,63 O. Gogota,38 G. Golovanov,31 P.D. Grannis,64 S. Greder,16 H. Greenlee,45 G. Grenier,17 p Ph. Gris,10 J.-F. Grivaz,13 A. Grohsjeanc,15 S. Gru¨nendahl,45 M.W. Gru¨newald,26 T. Guillemin,13 G. Gutierrez,45 e h P. Gutierrez,67 J. Haley,68 L. Han,4 K. Harder,41 A. Harel,63 J.M. Hauptman,52 J. Hays,40 T. Head,41 [ T. Hebbeker,18 D. Hedin,47 H. Hegab,68 A.P. Heinson,43 U. Heintz,69 C. Hensel,1 I. Heredia-De La Cruzd,28 2 K. Herner,45 G. Heskethf,41 M.D. Hildreth,51 R. Hirosky,73 T. Hoang,44 J.D. Hobbs,64 B. Hoeneisen,9 J. Hogan,72 v M. Hohlfeld,21 J.L. Holzbauer,58 I. Howley,70 Z. Hubacek,7,15 V. Hynek,7 I. Iashvili,62 Y. Ilchenko,71 5 R. Illingworth,45 A.S. Ito,45 S. Jabeenm,45 M. Jaffr´e,13 A. Jayasinghe,67 M.S. Jeong,27 R. Jesik,40 P. Jiang,4 2 3 K. Johns,42 E. Johnson,57 M. Johnson,45 A. Jonckheere,45 P. Jonsson,40 J. Joshi,43 A.W. Jung,45 A. Juste,36 5 E. Kajfasz,12 D. Karmanov,33 I. Katsanos,59 M. Kaur,23 R. Kehoe,71 S. Kermiche,12 N. Khalatyan,45 A. Khanov,68 0 A. Kharchilava,62 Y.N. Kharzheev,31 I. Kiselevich,32 J.M. Kohli,23 A.V. Kozelov,34 J. Kraus,58 A. Kumar,62 . 1 A. Kupco,8 T. Kurˇca,17 V.A. Kuzmin,33 S. Lammers,49 P. Lebrun,17 H.S. Lee,27 S.W. Lee,52 W.M. Lee,45 X. Lei,42 0 J. Lellouch,14 D. Li,14 H. Li,73 L. Li,43 Q.Z. Li,45 J.K. Lim,27 D. Lincoln,45 J. Linnemann,57 V.V. Lipaev,34 5 1 R. Lipton,45 H. Liu,71 Y. Liu,4 A. Lobodenko,35 M. Lokajicek,8 R. Lopes de Sa,45 R. Luna-Garciag,28 : A.L. Lyon,45 A.K.A. Maciel,1 R. Madar,19 R. Magan˜a-Villalba,28 S. Malik,59 V.L. Malyshev,31 J. Mansour,20 v i J. Mart´ınez-Ortega,28 R. McCarthy,64 C.L. McGivern,41 M.M. Meijer,29,30 A. Melnitchouk,45 D. Menezes,47 X P.G. Mercadante,3 M. Merkin,33 A. Meyer,18 J. Meyeri,20 F. Miconi,16 N.K. Mondal,25 M. Mulhearn,73 E. Nagy,12 r a M. Narain,69 R. Nayyar,42 H.A. Neal,56 J.P. Negret,5 P. Neustroev,35 H.T. Nguyen,73 T. Nunnemann,22 J. Orduna,72 N. Osman,12 J. Osta,51 A. Pal,70 N. Parashar,50 V. Parihar,69 S.K. Park,27 R. Partridgee,69 N. Parua,49 A. Patwaj,65 B. Penning,45 M. Perfilov,33 Y. Peters,41 K. Petridis,41 G. Petrillo,63 P. P´etroff,13 M.-A. Pleier,65 V.M. Podstavkov,45 A.V. Popov,34 M. Prewitt,72 D. Price,41 N. Prokopenko,34 J. Qian,56 A. Quadt,20 B. Quinn,58 P.N. Ratoff,39 I. Razumov,34 I. Ripp-Baudot,16 F. Rizatdinova,68 M. Rominsky,45 A. Ross,39 C. Royon,15 P. Rubinov,45 R. Ruchti,51 G. Sajot,11 A. Sa´nchez-Herna´ndez,28 M.P. Sanders,22 A.S. Santosh,1 G. Savage,45 M. Savitskyi,38 L. Sawyer,54 T. Scanlon,40 R.D. Schamberger,64 Y. Scheglov,35 H. Schellman,48 C. Schwanenberger,41 R. Schwienhorst,57 J. Sekaric,53 H. Severini,67 E. Shabalina,20 V. Shary,15 S. Shaw,41 A.A. Shchukin,34 V. Simak,7 P. Skubic,67 P. Slattery,63 D. Smirnov,51 G.R. Snow,59 J. Snow,66 S. Snyder,65 S. So¨ldner-Rembold,41 L. Sonnenschein,18 K. Soustruznik,6 J. Stark,11 D.A. Stoyanova,34 M. Strauss,67 L. Suter,41 P. Svoisky,67 M. Titov,15 V.V. Tokmenin,31 Y.-T. Tsai,63 D. Tsybychev,64 B. Tuchming,15 C. Tully,61 L. Uvarov,35 S. Uvarov,35 S. Uzunyan,47 R. Van Kooten,49 W.M. van Leeuwen,29 N. Varelas,46 E.W. Varnes,42 I.A. Vasilyev,34 A.Y. Verkheev,31 L.S. Vertogradov,31 M. Verzocchi,45 M. Vesterinen,41 D. Vilanova,15 P. Vokac,7 H.D. Wahl,44 M.H.L.S. Wang,45 J. Warchol,51 G. Watts,74 M. Wayne,51 J. Weichert,21 L. Welty-Rieger,48 2 M.R.J. Williamsn,49 G.W. Wilson,53 M. Wobisch,54 D.R. Wood,55 T.R. Wyatt,41 Y. Xie,45 R. Yamada,45 S. Yang,4 T. Yasuda,45 Y.A. Yatsunenko,31 W. Ye,64 Z. Ye,45 H. Yin,45 K. Yip,65 S.W. Youn,45 J.M. Yu,56 J. Zennamo,62 T.G. Zhao,41 B. Zhou,56 J. Zhu,56 M. Zielinski,63 D. Zieminska,49 and L. Zivkovic14 ∗ (The D0 Collaboration ) 1LAFEX, Centro Brasileiro de Pesquisas F´ısicas, Rio de Janeiro, Brazil 2Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil 3Universidade Federal do ABC, Santo Andr´e, Brazil 4University of Science and Technology of China, Hefei, People’s Republic of China 5Universidad de los Andes, Bogot´a, Colombia 6Charles University, Faculty of Mathematics and Physics, Center for Particle Physics, Prague, Czech Republic 7Czech Technical University in Prague, Prague, Czech Republic 8Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 9Universidad San Francisco de Quito, Quito, Ecuador 10LPC, Universit´e Blaise Pascal, CNRS/IN2P3, Clermont, France 11LPSC, Universit´e Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France 12CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France 13LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France 14LPNHE, Universit´es Paris VI and VII, CNRS/IN2P3, Paris, France 15CEA, Irfu, SPP, Saclay, France 16IPHC, Universit´e de Strasbourg, CNRS/IN2P3, Strasbourg, France 17IPNL, Universit´e Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universit´e de Lyon, Lyon, France 18III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany 19Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany 20II. Physikalisches Institut, Georg-August-Universita¨t G¨ottingen, G¨ottingen, Germany 21Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 22Ludwig-Maximilians-Universit¨at Mu¨nchen, Mu¨nchen, Germany 23Panjab University, Chandigarh, India 24Delhi University, Delhi, India 25Tata Institute of Fundamental Research, Mumbai, India 26University College Dublin, Dublin, Ireland 27Korea Detector Laboratory, Korea University, Seoul, Korea 28CINVESTAV, Mexico City, Mexico 29Nikhef, Science Park, Amsterdam, the Netherlands 30Radboud University Nijmegen, Nijmegen, the Netherlands 31Joint Institute for Nuclear Research, Dubna, Russia 32Institute for Theoretical and Experimental Physics, Moscow, Russia 33Moscow State University, Moscow, Russia 34Institute for High Energy Physics, Protvino, Russia 35Petersburg Nuclear Physics Institute, St. Petersburg, Russia 36Instituci´oCatalanadeRecerca iEstudisAvanc¸ats (ICREA)andInstitutdeF´ısicad’AltesEnergies(IFAE),Barcelona, Spain 37Uppsala University, Uppsala, Sweden 38Taras Shevchenko National University of Kyiv, Kiev, Ukraine 39Lancaster University, Lancaster LA1 4YB, United Kingdom 40Imperial College London, London SW7 2AZ, United Kingdom 41The University of Manchester, Manchester M13 9PL, United Kingdom 42University of Arizona, Tucson, Arizona 85721, USA 43University of California Riverside, Riverside, California 92521, USA 44Florida State University, Tallahassee, Florida 32306, USA 45Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA 46University of Illinois at Chicago, Chicago, Illinois 60607, USA 47Northern Illinois University, DeKalb, Illinois 60115, USA 48Northwestern University, Evanston, Illinois 60208, USA 49Indiana University, Bloomington, Indiana 47405, USA 50Purdue University Calumet, Hammond, Indiana 46323, USA 51University of Notre Dame, Notre Dame, Indiana 46556, USA 52Iowa State University, Ames, Iowa 50011, USA 53University of Kansas, Lawrence, Kansas 66045, USA 54Louisiana Tech University, Ruston, Louisiana 71272, USA 55Northeastern University, Boston, Massachusetts 02115, USA 56University of Michigan, Ann Arbor, Michigan 48109, USA 3 57Michigan State University, East Lansing, Michigan 48824, USA 58University of Mississippi, University, Mississippi 38677, USA 59University of Nebraska, Lincoln, Nebraska 68588, USA 60Rutgers University, Piscataway, New Jersey 08855, USA 61Princeton University, Princeton, New Jersey 08544, USA 62State University of New York, Buffalo, New York 14260, USA 63University of Rochester, Rochester, New York 14627, USA 64State University of New York, Stony Brook, New York 11794, USA 65Brookhaven National Laboratory, Upton, New York 11973, USA 66Langston University, Langston, Oklahoma 73050, USA 67University of Oklahoma, Norman, Oklahoma 73019, USA 68Oklahoma State University, Stillwater, Oklahoma 74078, USA 69Brown University, Providence, Rhode Island 02912, USA 70University of Texas, Arlington, Texas 76019, USA 71Southern Methodist University, Dallas, Texas 75275, USA 72Rice University, Houston, Texas 77005, USA 73University of Virginia, Charlottesville, Virginia 22904, USA 74University of Washington, Seattle, Washington 98195, USA (Dated: January 15, 2015) We measure the ratio of cross sections, σ(pp¯→Z+2 b jets)/σ(pp¯→Z+2 jets), for associated production of a Z boson with at least two jets with transverse momentum pjet > 20 GeV and T pseudorapidity|ηjet|<2.5. This measurement uses datacorresponding to an integrated luminosity of9.7fb−1 collectedbytheD0experimentinRunIIofFermilab’sTevatronppCollideratacenter- of-mass energy of 1.96 TeV. Themeasured integrated ratio of 0.0236±0.0032(stat)±0.0035(syst) isinagreementwithpredictionsfromnext-to-leading-orderperturbativeQCDandtheMonteCarlo eventgenerators pythia and alpgen. PACSnumbers: 12.38.Qk,13.85.Qk,14.65.Fy,14.70.Hp Studies of Z boson production in association with a ThisarticlepresentstheratioofZ+2bjetstoZ+2 jets bottom and an anti-bottom quark provide important inclusiveproductioncrosssectionsandisanextensionof tests of the predictions of perturbative quantum chro- the previousD0 measurementsutilizing similar eventse- modynamics (pQCD) [1, 2]. A good theoretical descrip- lections. The measurementofthe ratiobenefits fromthe tion of this process is essential since it forms a major cancellation of many systematic uncertainties, such as background for a variety of physics processes, including theuncertaintyinluminosityandthoserelatedtolepton standard model (SM) Higgs boson production in associ- and jet identification, allowing a more precise compari- ation with a Z boson, ZH(H b¯b) [3], and searches for sonwith theory. The remainingsystematic uncertainties → supersymmetric partners of the b quark [4]. arisefromthedifferencesbetweenbjetsandlightjets. In the following, light-quark flavor (u, d, s) and gluon jets The ratio of Z+b jet to Z+jet production cross sec- are referred to as “light jets”. The Z +2 b jet produc- tions, for events with at least one jet, has been previ- tioncrosssectionshavebeenmeasuredatCMS [12]and ously measured by the CDF [5, 6] and D0 [7–9] collabo- ATLAS [13] at √s = 7 TeV. The current measurement rationsusingRunIIdata. TheATLAS[10]andCMS[11] is based on the complete Run II data sample collected collaborations have also studied Z +b jet production at by the D0 experiment at the Fermilab Tevatron pp col- √s=7 TeV. lider at a center-of-mass energy of √s = 1.96 TeV, and corresponds to an integrated luminosity of 9.7 fb−1. We first briefly describe the main components of the ∗with visitors from aAugustana College, Sioux Falls, SD, USA, D0RunIIdetector[14,15]relevanttothisanalysis. The bThe University of Liverpool, Liverpool, UK, cDESY, Hamburg, D0 detectorhas acentraltrackingsystemconsistingofa Germany, dUniversidad Michoacana de San Nicolas de Hidalgo, Morelia, MexicoeSLAC, MenloPark, CA, USA, fUniversityCol- silicon microstrip tracker (SMT) [16] and a central fiber legeLondon, London, UK,gCentro deInvestigacion enComputa- tracker (CFT), both located within a 1.9 T supercon- cion-IPN,MexicoCity,Mexico,hUniversidadeEstadualPaulista, ducting solenoidal magnet, with designs optimized for S˜ao Paulo, Brazil, iKarlsruher Institut fu¨r Technologie (KIT) - tracking and vertexing at pseudorapidities η < 3 Steinbuch Centre forComputing (SCC), D-76128 Karlsruhe, Ger- | det| many,jOfficeofScience,U.S.DepartmentofEnergy,Washington, and ηdet < 2.5, respectively [17]. A liquid argon and D.C.20585, USA,kAmericanAssociationfortheAdvancement of urani|um|calorimeter has a central section (CC) covering Science,Washington,D.C.20005,USA,lKievInstituteforNuclear pseudorapidities η . 1.1, and two end calorimeters Research, Kiev, Ukraine, mUniversity of Maryland, College Park, | det| Maryland 20742, USA and nEuropean Orgnaization for Nuclear (EC) that extend coverage to ηdet 4.2, with all three | |≈ Research(CERN),Geneva, Switzerland housedinseparatecryostats[18]. Anoutermuonsystem, 4 at η <2,consistsofa layeroftrackingdetectors and withp >15GeVand η <2. Thesemuonsmustpass det T det | | | | scintillation counters in front of 1.8 T toroids, followed a combined tracking and calorimeter isolation require- by two similar layers after the toroids. Luminosity is ment discussed in detail in Ref. [3]. Muons originating measuredusingplasticscintillatorarrayslocatedinfront fromcosmicraysarerejectedbyapplyingtiming criteria of the EC cryostats. The trigger and data acquisition using the hits in the scintillation counters and by limit- systems aredesignedto accommodatethe highinstanta- ing the measured displacement of the muon track with neous luminosities of Run II. respect to the pp interaction vertex [21]. This analysis relies on all components of the D0 de- A total of about 1.2 million Z boson candidate events tector: tracking systems, the liquid-argon sampling are retained in the combined ee and µµ channels with calorimeter,muonsystem,andtheabilitytoidentifysec- the abovelepton selectioncriteria. The Z+2 jet sample ondary vertices [14]. The SMT allows for precise recon- isthenselectedbyrequiringatleasttwojetsintheevent struction of the primary pp interaction vertex and sec- with pjet > 20 GeV and ηjet < 2.5. Jets are recon- T | | ondary vertices [17, 19]. It also enables an accurate structed from energy deposits in the calorimeter using determination of the impact parameter, defined as the an iterative midpoint cone algorithm [22] with a cone distance of closest approach of a track to the primary of radius ∆R = p(∆ϕ)2+(∆y)2 = 0.5 where ϕ is the interaction vertex in the x-y plane. The impact param- azimuthal angle and y is the rapidity. Jet energy is cor- eter measurements of tracks, along with reconstructed rected for detector response, the presence of noise and secondaryvertices,areimportantinputs tothe b-jettag- multiple pp¯interactions. We also correct the jet energy ging algorithm. for the energy of those particles within the reconstruc- Events containing Z bosons decaying to µµ or ee tionconethatisdepositedinthe calorimeteroutsidethe are collected using triggers based on single electrons or cone (and vice versa) [23]. muons. For the off-line selection requirements discussed To suppress background from top-antitop quark (tt¯) below, the triggers have an efficiency of approximately production, events are rejected if the missing transverse 100%forZ eeandmorethan78%forZ µµ decays energyis largerthan60GeV, reducingthe tt¯contamina- → → depending on the transverse momentum of the muon. tionbya factoroftwo. Theseselectioncriteriaretainan TheZ+2 jetsamplerequiresthepresenceofatleasttwo inclusive sample of 20,950 Z +2 jet event candidates in jets in the event, while the Z +2 b jet sample requires the combined ee and µµ channels. at least two b-jet candidates, selected using a b-tagging Processes such as diboson (WW, WZ, ZZ) produc- algorithm [20]. tion can contribute to the background when two lep- An event is selected if it contains a pp interaction ver- tons are reconstructed in the final state. Inclusive dibo- tex, reconstructed from at least three tracks, located son production is simulated with the pythia [24] Monte within 60 cm of the center of the D0 detector along Carlo (MC) event generator. The Z + jet, including the beam axis. The selected events must also contain heavy flavor jets, and tt¯ events are modeled by alp- a Z boson candidate with a dilepton invariant mass gen [25], which generates hard sub-processes includ- 70 <M <110 GeV. ing higher order QCD tree level matrix elements, inter- ℓℓ Dielectron (ee) events are required to have two elec- faced with pythia for parton showering and hadroniza- tronsoftransversemomentum(p )greaterthan15GeV tion. The CTEQ6L1 [26] parton distribution functions T identified through electromagnetic (EM) showers in the (PDFs) areused inall simulations. The crosssections of calorimeter. The showers must have more than 97% of thesimulatedsamplesarethenscaledto thecorrespond- theirenergydepositedintheEMcalorimeter,beisolated inghigher-ordertheoreticalcalculations. Forthediboson from other energy depositions, and have transverse and and Z+2 jet processes, including the Z+b¯b signal pro- longitudinalenergyprofilesconsistentwiththatexpected cessandZ+cc¯production,next-to-leadingorder(NLO) for electrons. At least one electron must be identified in cross section predictions are taken from mcfm [27]. The the CC, with η < 1.1, and a second electron either tt¯cross section is determined from NLO+NNLL (next- det | | in the CC or the EC, 1.5 < η < 2.5. Electron can- to-next-leading log) calculations [28]. To improve the det | | didates in the CC are required to match central tracks modeling of the p distribution of the Z boson, simu- T or have a pattern of hits consistent with the passage of latedZ+2 jeteventsarealsoreweightedtobeconsistent an electron through the central tracker. Electrons in the with the measured p spectrum of Z bosons observedin T ECs are not required to have a track matched to them data [29]. duetodeterioratingtrackingcoveragefor η >2. Due These generated samples are processed through a de- det | | to the lack of trackrequirementin EC regionswe do not tailed detector simulation based on geant [30]. To apply any opposite sign requirement for the dielectron model the effects of detector noise and pile-up events, events. colliderdatafromrandombeamcrossingswith the same The dimuon (µµ) event selection requires two oppo- instantaneous luminosity distribution as for data are su- sitely charged muons detected in the muon system that perimposed on simulated events. These events are then arematchedtoreconstructedtracksinthecentraltracker reconstructedusingthesamealgorithmsasusedfordata. 5 Scale factors, determined from data using independent samples, are applied to account for differences in recon- struction efficiency between data and simulation. The ts 105 DØ, L = 9.7 fb-1 Data energies of simulated jets are corrected, based on their n e (a) Z→ µµ Z+LF flavor, to reproduce the resolution and energy scale ob- v E Z+bb served in data [23]. 104 Z+cc The background contribution from multijet events, in tt which jets are misidentified as leptons, is evaluated from Diboson Multijet data. Thisisperformedusingamultijet-enrichedsample 103 ofeventsthatpassallselectioncriteriaexceptforsomeof the leptonquality requirements. Inthe caseofelectrons, the multijet sample is obtained by inverting the shower 102 shape requirement and relaxing other electron identifi- cation criteria, while for the muon channel, the multijet sample consists of events with muon candidates that fail 10 theisolationrequirements. Thenormalizationofthemul- tijetbackgroundisdeterminedfromasimultaneousfitto the dilepton invariant mass distributions in different jet 70 75 80 85 90 95 100 105 110 multiplicity bins. M (GeV) µµ Figures 1 and 2 show the dileptoninvariantmass and leading jet p distributions in data compared to the ex- T pectations from various processes. The dominant con- tribution comes from Z+light jet production. The non- ts DØ, L = 9.7 fb -1 Data Z15%+,jeatnbdaciksgdrooumnidnaftreadctiboyn minutlthiejeteepcrhoadnuncteiloins.abTohuet ven 105 (b) Z→ ee Z+LF E Z+bb muon channel has a higher purity with a background Z+cc fraction of about 7%. 104 tt This analysis employs a two-step procedure to deter- Diboson mine the b-quark content of jets in the selected events. Multijet First, a b-tagging algorithm is applied to jets to select 103 a sample of Z +2 jet events that is enriched in heavy flavor jets. After b tagging, the relative light, c, and b- 102 quarkcontentisextractedbyfittingtemplatesbuiltfrom adedicateddiscriminantthatprovidesanoptimizedsep- aration between the three components. 10 Jets considered for b-jet tagging are subject to a pre- selection requirement, called taggability, to decouple the 70 75 80 85 90 95 100 105 110 intrinsicperformanceofthe b-jettaggingalgorithmfrom effects related to the track reconstruction efficiency. For M (GeV) ee thispurpose,thejetisrequiredtohaveatleasttwoasso- ciated tracks with p >0.5 GeV, the leading track must T FIG. 1: (color online) The invariant mass in (a) Z have pT >1 GeV, and each track must have at least one µµ and (b) Z ee channels for data and backgrou→nd SMT hit. This requirement has a typical efficiency of → in events with a Z boson candidate and at least two 90% per jet. jets before b tagging is applied. The b-jet tagging algorithm is based on a multivariate analysis (MVA) technique [31]. This algorithm, MVA , bl discriminatesbjetsfromlight-flavorjetsutilizingtherel- atively long lifetime of the b hadrons when compared to rameterofsecondaryvertices,themultiplicityofcharged their lighter counterparts [20]. Events with at least two tracks associated with them, and the Jet Lifetime Prob- jets tagged by this algorithm are considered. ability (JLIP), which is the probability that tracks as- The MVA discriminant combines various properties sociated with the jet originate from the pp interaction bl of the jet and associated tracks to create a continuous vertex [20]. Events are retained for further analysis if output that tends towards unity for b jets and zero for they contain at least two jets with an MVA output bl light jets. Inputs include the number of secondary ver- greaterthan0.15. Aftertheserequirements,241Z+2 jet tices and the charge track multiplicity, invariant mass of eventsare selectedwith atleasttwob-taggedjets, where thesecondaryvertex(M ),decaylengthandimpactpa- only the two highest p tagged jets are examined in the SV T 6 ts 104 DØ, L = 9.7 fb-1 Data DMJL1 D(aØ), L = 9.7 fb-1 DZ+a2ta b jets n ve (a)Z→µµ Z+LF s/d 103 Z+2 c jets E 103 Z+bb nt Z+2 light jets Z+cc e v Sum tt E Diboson 102 Multijet 102 10 10 1 0 20 40 60 80 100 120 140 160 180 200 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 D PjTet1 (GeV) MJL1 JL2 DØ, L = 9.7 fb-1 Data s M t DØ, L = 9.7 fb-1 Data D (b) Z+2 b jets n d 103 ve (b)Z→ ee Z+LF s/ Z+2 c jets E Z+bb nt Z+2 light jets 103 e Z+cc v Sum E tt Diboson 102 Multijet 102 10 10 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 20 40 60 80 100 120 140 160 180 200 D MJL 2 jet1 P (GeV) T FIG. 3: (color online) The one dimensional projection FIG. 2: (color online) The leading jet pT in the (a) onto (a) the highest-pT jet and (b) the second highest- Z µµ and (b) Z ee channels for data and back- p jet D axis of the 2D fit. The distributions of → → T MJL ground in events with a Z boson candidate and at least the b, c, and light jets are normalized by the fractions two jets before b tagging is applied. found from the fit. analysis and the electron and muon channels are com- inputs, M and JLIP: D = 0.5 (M /(5 GeV) SV MJL SV × − bined. The efficiency for tagging two b jets in data is ln(JLIP)/20). 33%. IntheMCcorrectionfactorsareappliedtoaccount To measure the fraction of events with different jet for differences with data [20]. The background contami- flavors in the selected sample, we count the number of nation from diboson, multijet, and top production after events as a function of the D of the two leading jets MJL b-tagging, for the electron and muon channels combined N(D ,D ) and then perform a two dimensional MJL1 MJL2 are 8%, 2% and 15% respectively. binned maximum likelihoodfit to that distribution. The To determine the fraction of events with 2 b jets, a datasamplewithtwoheavy-flavor-taggedjetsisfittedto dedicated discriminant, D , is employed [8, 32]. It templates consisting mainly of 2 b-jet, 2 c-jet, and light MJL is a combination of the two most discriminating MVA flavorjetevents,asobtainedfromalpgen+pythiasim- bl 7 ulatedsamples. Wealsocomparedtheshapesofthetem- the shape of the D templates used in the fit includ- MJL plates from SHERPA simulated samples and found the ingthatdue toMC statisticsofthe samplesusedtocon- templatestobeconsistentforthetwomodels. Beforethe structthetemplates. Theshapeofthetemplatesmaybe fit, all non-Z+jet background contributions, estimated affectedby the choiceofthe bquarkfragmentationfunc- fromsimulatedsamplesaftertheMVA requirement,are tion[33],thebackgroundestimation,thedifferenceinthe bl subtracted from the data leaving 180 Z+2 jet events in shape of the light jet MC template and a template de- the combined ee and µµ channel. Next, we measure the rivedfromalightjetenricheddijetdatasample,andthe jet-flavor fractions in the dielectron and dimuon sam- composition of the charm states used to determine the ples combined, yielding the 2 b jet flavor fraction (f ) charm template shape [8]. It also includes uncertainties bb of 0.64 0.08 (stat.) and the 2 c jet flavor fraction of on production rates of different hadrons and uncertain- ± 0.32 0.08 (stat.). Figure 3 shows the one dimensional ties on branching fractions. These effects are evaluated ± projectionontothehighest-p jetandthesecondhighest- by varying the central values by the corresponding un- T p jet D axis of the 2D fit. certainties, one at a time. The entire analysis chain is T MJL The fraction of 2 b jets measured in the heavy fla- checked for possible biases using a MC closure test and vor enriched sample is combined with the corresponding no significant deviations are observed. The next largest event acceptances to determine the ratio, R, of the cross systematic uncertainty of 5.5% is due to the b-jet iden- sections. tification efficiency. The uncertainty on b-jet energy cal- ibration is 2.6%; it comprises the uncertainties on the jet energy resolution and the jet energy scale. For the σ(Z +2 b jets) N f R= = bb bb Aincl (1) integrated ratio measurement, these uncertainties, when σ(Z+2 jets) N ǫbb × incl tag Abb summed in quadrature, result in a total systematic un- certainty of 14.9%. For the integrated ratio we obtain whereN isthetotalnumberofZ+2 jeteventsbefore incl the tagging requirements, N is the number of Z+2 jet bb R=0.0236 0.0032(stat) 0.0035(syst). (3) events used in the DMJL fit, fbb is the extracted 2 b ± ± jet fraction, and ǫbb is the overall selection efficiency tag To check the stability of the result, the ratio is remea- of D for 2 b jets that combines the efficiencies for MJL sured using a looser(tighter) MVA selection with the taggabilityandMVA discriminant. BothN andN bl bl incl bb lowerlimitontheMVA outputof>0.10(>0.225). The correspond to the number of events that remain after bl looserselectionprovidesincreaseddatastatisticsandthe the contributions from non-Z+jets processes have been tighter selection yields a 2 b enriched sample. The new subtracted from the data. and default ratios are found to be in agreement within The pseudorapidity acceptance for electrons and uncertainties of about 4%. muons is different. In order to quote a combined ra- To validate the tt¯background estimation, we reduce tio for the two channels, we correct to a common lepton thecontributionoftt¯eventsbyrejectingeventswherethe acceptance as follows. The detector acceptances for the scalarsumofalljetp valuesismorethan130GeV.This inclusivejetsampleand2bjetsaredeterminedfromMC T selection reduces the tt¯fraction by an additional factor simulations in the kinematic region that satisfies the p T of2withasignalefficiencyof80%. Thenew anddefault andη requirementsforleptonsandjets. Forthe and bb A ratios are found to be in agreement within systematic calculations, we apply selections for both the elec- incl A uncertainties. tron and muon channels for the fiducial region for the events with two jets and two leptons defined as: InTableI,wepresentthe ratioofintegratedcrosssec- tions, σ(pp¯ Z +2 b jet)/σ(pp¯ Z +2 jet), in the pjet >20 GeV and ηjet <2.5, fiducial regio→n defined in Eq. (2). T→he ratio is compared T | | pℓ >15 GeV and ηℓ <2. (2) to predictions from NLO QCD calculations and two MC T | | generators, pythia and alpgen. The NLO predictions The resultingratioofthe twoacceptancesismeasured use the MSTW2008 PDF set [34] using mcfm with cen- to be / =1.09 0.02 (stat). tral values of renormalization and fragmentation scales incl bb A A ± Using Eq. (1), we obtain the ratio of the Z +2 b jet µ = µ = M . Uncertainties are estimated by varying r f Z crosssectiontotheinclusiveZ+2 jetcrosssectioninthe µ andµ togetherbyafactoroftwo,andareabout15%. r f combinedµµandeechanneltobe0.0236 0.0032(stat). alpgen generates multi-parton final states using tree- ± Several systematic uncertainties cancel when the ra- level matrix elements. When interfaced with pythia, tio σ(Z +2 b jets)/σ(Z+2 jets) is measured. These in- it employs an MLM scheme [35] to match matrix ele- clude uncertainties on the luminosity measurement, lep- ment partons with those after showering in pythia, re- ton trigger efficiency, and lepton and jet reconstruction sulting in an improvement over leading-logarithmic ac- efficiencies. The remaining uncertainties are estimated curacy. The measured ratio is in reasonable agreement separately for the integrated result. The largest system- withMCFMNLOcalculationsconsideringtheuncertain- atic uncertainty of 13.7%comes from the uncertainty on ties on the data and theory. 8 TABLE I: The ratio of integrated cross sections, σ(pp¯ Z+2 b jet)/σ(pp¯ Z+2 jet) together with statistical → → uncertainties (δ ) and total systematic uncertainties (δ ). The column δ shows the total experimental stat syst tot uncertainty obtained by adding δ and δ in quadrature. The last three columns show theoretical predictions stat syst obtained using NLO QCD with scale uncertainties and two MC event generators, pythia and alpgen. σ(pp¯→Z+2 b jet)/σ(pp¯→Z+2 jet) Data ±δstat±δsyst δtot nlo qcd(mstw) pythia alpgen (2.36±0.32±0.35)×10−2 0.47×10−2 (1.76±0.26)×10−2 2.42×10−2 2.21×10−2 In summary, we report the measurement at the Teva- tron of the ratio of integrated cross sections, σ(pp¯ → Z +2 b jet)/σ(pp¯ Z +2 jet), for events with Z ℓℓ → → [1] J. M. Campbell, R. K. Ellis, F. Maltoni, and S. Willen- in a restrictedphase space ofleptons with pℓT >15 GeV, brock, Phys. Rev.D 73, 054007 (2006). |ηℓ| < 2.0 and with two jets limited to pjTet > 20 GeV [2] F. F. Cordero, L. Reina, and D. Wackeroth, Phys. Rev. and ηjet < 2.5. 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