Physics Letters B 698 (2011) 6–13 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Search for W H associated production in 5.3 fb−1 of pp̄ collisions at the Fermilab Tevatron D0 Collaboration V.M. Abazov ak, B. Abbott bw, B.S. Acharya ae, M. Adams ay, T. Adams aw, G.D. Alexeev ak, G. Alkhazov ao, A. Alton bk,1, G. Alverson bj, G.A. Alves b, L.S. Ancu aj, M. Aoki ax, M. Arov bh, A. Askew aw, B. Åsman ap,aq, O. Atramentov bo, C. Avila i, J. BackusMayes cd, F. Badaud n, L. Bagby ax, B. Baldin ax, D.V. Bandurin aw, S. Banerjee ae, E. Barberis bj, P. Baringer bf, J. Barreto c, J.F. Bartlett ax, U. Bassler s, V. Bazterra ay, S. Beale f,g, A. Bean bf, M. Begalli c, M. Begel bu, C. Belanger-Champagne ap,aq, L. Bellantoni ax, S.B. Beri ac, G. Bernardi r, R. Bernhard x, I. Bertram ar, M. Besançon s, R. Beuselinck as, V.A. Bezzubov an, P.C. Bhat ax, V. Bhatnagar ac, G. Blazey az, S. Blessing aw, K. Bloom bn, A. Boehnlein ax, D. Boline bt, T.A. Bolton bg, E.E. Boos am, G. Borissov ar, T. Bose bi, A. Brandt bz, O. Brandt y, R. Brock bl, G. Brooijmans br, A. Bross ax, D. Brown r, J. Brown r, X.B. Bu ax, M. Buehler cc, V. Buescher z, V. Bunichev am, S. Burdin ar,2, T.H. Burnett cd, C.P. Buszello ap,aq, B. Calpas p, E. Camacho-Pérez ah, M.A. Carrasco-Lizarraga bf, B.C.K. Casey ax, H. Castilla-Valdez ah, S. Chakrabarti bt, D. Chakraborty az, K.M. Chan bd, A. Chandra cb, G. Chen bf, S. Chevalier-Théry s, D.K. Cho by, S.W. Cho ag, S. Choi ag, B. Choudhary ad, T. Christoudias as, S. Cihangir ax, D. Claes bn, J. Clutter bf, M. Cooke ax, W.E. Cooper ax, M. Corcoran cb, F. Couderc s, M.-C. Cousinou p, A. Croc s, D. Cutts by, A. Das au, G. Davies as, K. De bz, S.J. de Jong aj, E. De La Cruz-Burelo ah, F. Déliot s, M. Demarteau ax, R. Demina bs, D. Denisov ax, S.P. Denisov an, S. Desai ax, K. DeVaughan bn, H.T. Diehl ax, M. Diesburg ax, A. Dominguez bn, T. Dorland cd, A. Dubey ad, L.V. Dudko am, D. Duggan bo, A. Duperrin p, S. Dutt ac, A. Dyshkant az, M. Eads bn, D. Edmunds bl, J. Ellison av, V.D. Elvira ax, Y. Enari r, H. Evans bb, A. Evdokimov bu, V.N. Evdokimov an, G. Facini bj, T. Ferbel bs, F. Fiedler z, F. Filthaut aj, W. Fisher bl, H.E. Fisk ax, M. Fortner az, H. Fox ar, S. Fuess ax, T. Gadfort bu, A. Garcia-Bellido bs, V. Gavrilov al, P. Gay n, W. Geist t, W. Geng p,bl, D. Gerbaudo bp, C.E. Gerber ay, Y. Gershtein bo, G. Ginther ax,bs, G. Golovanov ak, A. Goussiou cd, P.D. Grannis bt, S. Greder t, H. Greenlee ax, Z.D. Greenwood bh, E.M. Gregores d, G. Grenier u,v, Ph. Gris n, J.-F. Grivaz q, A. Grohsjean s, S. Grünendahl ax, M.W. Grünewald af, F. Guo bt, G. Gutierrez ax, P. Gutierrez bw, A. Haas br,3, S. Hagopian aw, J. Haley bj, L. Han h, K. Harder at, A. Harel bs, J.M. Hauptman be, J. Hays as, T. Head at, T. Hebbeker w, D. Hedin az, H. Hegab bx, A.P. Heinson av, U. Heintz by, C. Hensel y, I. Heredia-De La Cruz ah, K. Herner bk, M.D. Hildreth bd, R. Hirosky cc, T. Hoang aw, J.D. Hobbs bt, B. Hoeneisen m, M. Hohlfeld z, S. Hossain bw, Z. Hubacek k,s, N. Huske r, V. Hynek k, I. Iashvili bq, R. Illingworth ax, A.S. Ito ax, S. Jabeen by, M. Jaffré q, S. Jain bq, D. Jamin p, R. Jesik as, K. Johns au, M. Johnson ax, D. Johnston bn, A. Jonckheere ax, P. Jonsson as, J. Joshi ac, A. Juste ax,4, K. Kaadze bg, E. Kajfasz p, D. Karmanov am, P.A. Kasper ax, I. Katsanos bn, R. Kehoe ca, S. Kermiche p, N. Khalatyan ax, A. Khanov bx, A. Kharchilava bq, Y.N. Kharzheev ak, D. Khatidze by, M.H. Kirby ba, J.M. Kohli ac, A.V. Kozelov an, J. Kraus bl, A. Kumar bq, A. Kupco l, T. Kurča u,v, V.A. Kuzmin am, J. Kvita j, S. Lammers bb, G. Landsberg by, P. Lebrun u,v, H.S. Lee ag, S.W. Lee be, W.M. Lee ax, J. Lellouch r, L. Li av, Q.Z. Li ax, S.M. Lietti e, J.K. Lim ag, D. Lincoln ax, J. Linnemann bl, V.V. Lipaev an, R. Lipton ax, Y. Liu h, Z. Liu f,g, A. Lobodenko ao, M. Lokajicek l, P. Love ar, H.J. Lubatti cd, R. Luna-Garcia ah,5, A.L. Lyon ax, A.K.A. Maciel b, D. Mackin cb, R. Madar s, R. Magaña-Villalba ah, S. Malik bn, V.L. Malyshev ak, Y. Maravin bg, J. Martínez-Ortega ah, R. McCarthy bt, C.L. McGivern bf, M.M. Meijer aj, A. Melnitchouk bm, D. Menezes az, P.G. Mercadante d, M. Merkin am, A. Meyer w, J. Meyer y, F. Miconi t, N.K. Mondal ae, G.S. Muanza p, 0370-2693/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2011.02.036 http://dx.doi.org/10.1016/j.physletb.2011.02.036 http://www.ScienceDirect.com/ http://www.elsevier.com/locate/physletb http://dx.doi.org/10.1016/j.physletb.2011.02.036 D0 Collaboration / Physics Letters B 698 (2011) 6–13 7 M. Mulhearn cc, E. Nagy p, M. Naimuddin ad, M. Narain by, R. Nayyar ad, H.A. Neal bk, J.P. Negret i, P. Neustroev ao, S.F. Novaes e, T. Nunnemann aa, G. Obrant ao, J. Orduna ah, N. Osman as, J. Osta bd, G.J. Otero y Garzón a, M. Owen at, M. Padilla av, M. Pangilinan by, N. Parashar bc, V. Parihar by, S.K. Park ag, J. Parsons br, R. Partridge by,3, N. Parua bb, A. Patwa bu, B. Penning ax, M. Perfilov am, K. Peters at, Y. Peters at, G. Petrillo bs, P. Pétroff q, R. Piegaia a, J. Piper bl, M.-A. Pleier bu, P.L.M. Podesta-Lerma ah,6, V.M. Podstavkov ax, M.-E. Pol b, P. Polozov al, A.V. Popov an, M. Prewitt cb, D. Price bb, S. Protopopescu bu, J. Qian bk, A. Quadt y, B. Quinn bm, M.S. Rangel b, K. Ranjan ad, P.N. Ratoff ar, I. Razumov an, P. Renkel ca, M. Rijssenbeek bt, I. Ripp-Baudot t, F. Rizatdinova bx, M. Rominsky ax, C. Royon s, P. Rubinov ax, R. Ruchti bd, G. Safronov al, G. Sajot o, A. Sánchez-Hernández ah, M.P. Sanders aa, B. Sanghi ax, A.S. Santos e, G. Savage ax, L. Sawyer bh, T. Scanlon as, R.D. Schamberger bt, Y. Scheglov ao, H. Schellman ba, T. Schliephake ab, S. Schlobohm cd, C. Schwanenberger at, R. Schwienhorst bl, J. Sekaric bf, H. Severini bw, E. Shabalina y, V. Shary s, A.A. Shchukin an, R.K. Shivpuri ad, V. Simak k, V. Sirotenko ax, P. Skubic bw, P. Slattery bs, D. Smirnov bd, K.J. Smith bq, G.R. Snow bn, J. Snow bv, S. Snyder bu, S. Söldner-Rembold at, L. Sonnenschein w, A. Sopczak ar, M. Sosebee bz, K. Soustruznik j, B. Spurlock bz, J. Stark o, V. Stolin al, D.A. Stoyanova an, M. Strauss bw, D. Strom ay, L. Stutte ax, L. Suter at, P. Svoisky bw, M. Takahashi at, A. Tanasijczuk a, W. Taylor f,g, M. Titov s, V.V. Tokmenin ak, Y.-T. Tsai bs, D. Tsybychev bt, B. Tuchming s, C. Tully bp, P.M. Tuts br, L. Uvarov ao, S. Uvarov ao, S. Uzunyan az, R. Van Kooten bb, W.M. van Leeuwen ai, N. Varelas ay, E.W. Varnes au, I.A. Vasilyev an, P. Verdier u,v, L.S. Vertogradov ak, M. Verzocchi ax, M. Vesterinen at, D. Vilanova s, P. Vint as, P. Vokac k, H.D. Wahl aw, M.H.L.S. Wang bs, J. Warchol bd, G. Watts cd, M. Wayne bd, M. Weber ax,7, L. Welty-Rieger ba, A. White bz, D. Wicke ab, M.R.J. Williams ar, G.W. Wilson bf, S.J. Wimpenny av, M. Wobisch bh, D.R. Wood bj, T.R. Wyatt at, Y. Xie ax, C. Xu bk, S. Yacoob ba, R. Yamada ax, W.-C. Yang at, T. Yasuda ax, Y.A. Yatsunenko ak, Z. Ye ax, H. Yin ax, K. Yip bu, S.W. Youn ax, J. Yu bz, S. Zelitch cc, T. Zhao cd, B. Zhou bk, J. Zhu bk, M. Zielinski bs, D. Zieminska bb, L. Zivkovic by a Universidad de Buenos Aires, Buenos Aires, Argentina b LAFEX, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brazil c Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil d Universidade Federal do ABC, Santo André, Brazil e Instituto de Física Teórica, Universidade Estadual Paulista, São Paulo, Brazil f Simon Fraser University, Vancouver, British Columbia, Canada g York University, Toronto, Ontario, Canada h University of Science and Technology of China, Hefei, People’s Republic of China i Universidad de los Andes, Bogotá, Colombia j Charles University, Faculty of Mathematics and Physics, Center for Particle Physics, Prague, Czech Republic k Czech Technical University in Prague, Prague, Czech Republic l Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic m Universidad San Francisco de Quito, Quito, Ecuador n LPC, Université Blaise Pascal, CNRS/IN2P3, Clermont, France o LPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France p CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France q LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France r LPNHE, Universités Paris VI and VII, CNRS/IN2P3, Paris, France s CEA, Irfu, SPP, Saclay, France t IPHC, Université de Strasbourg, CNRS/IN2P3, Strasbourg, France u IPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, France v Université de Lyon, Lyon, France w III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany x Physikalisches Institut, Universität Freiburg, Freiburg, Germany y II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany z Institut für Physik, Universität Mainz, Mainz, Germany aa Ludwig-Maximilians-Universität München, München, Germany ab Fachbereich Physik, Bergische Universität Wuppertal, Wuppertal, Germany ac Panjab University, Chandigarh, India ad Delhi University, Delhi, India ae Tata Institute of Fundamental Research, Mumbai, India af University College Dublin, Dublin, Ireland ag Korea Detector Laboratory, Korea University, Seoul, Republic of Korea ah CINVESTAV, Mexico City, Mexico ai FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands aj Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands ak Joint Institute for Nuclear Research, Dubna, Russia al Institute for Theoretical and Experimental Physics, Moscow, Russia am Moscow State University, Moscow, Russia an Institute for High Energy Physics, Protvino, Russia ao Petersburg Nuclear Physics Institute, St. Petersburg, Russia ap Stockholm University, Stockholm, Sweden aq Uppsala University, Uppsala, Sweden 8 D0 Collaboration / Physics Letters B 698 (2011) 6–13 ar Lancaster University, Lancaster LA1 4YB, United Kingdom as Imperial College London, London SW7 2AZ, United Kingdom at The University of Manchester, Manchester M13 9PL, United Kingdom au University of Arizona, Tucson, AZ 85721, USA av University of California Riverside, Riverside, CA 92521, USA aw Florida State University, Tallahassee, FL 32306, USA ax Fermi National Accelerator Laboratory, Batavia, IL 60510, USA ay University of Illinois at Chicago, Chicago, IL 60607, USA az Northern Illinois University, DeKalb, IL 60115, USA ba Northwestern University, Evanston, IL 60208, USA bb Indiana University, Bloomington, IN 47405, USA bc Purdue University Calumet, Hammond, IN 46323, USA bd University of Notre Dame, Notre Dame, IN 46556, USA be Iowa State University, Ames, IA 50011, USA bf University of Kansas, Lawrence, KS 66045, USA bg Kansas State University, Manhattan, KS 66506, USA bh Louisiana Tech University, Ruston, LA 71272, USA bi Boston University, Boston, MA 02215, USA bj Northeastern University, Boston, MA 02115, USA bk University of Michigan, Ann Arbor, MI 48109, USA bl Michigan State University, East Lansing, MI 48824, USA bm University of Mississippi, University, MS 38677, USA bn University of Nebraska, Lincoln, NE 68588, USA bo Rutgers University, Piscataway, NJ 08855, USA bp Princeton University, Princeton, NJ 08544, USA bq State University of New York, Buffalo, NY 14260, USA br Columbia University, New York, NY 10027, USA bs University of Rochester, Rochester, NY 14627, USA bt State University of New York, Stony Brook, NY 11794, USA bu Brookhaven National Laboratory, Upton, NY 11973, USA bv Langston University, Langston, OK 73050, USA bw University of Oklahoma, Norman, OK 73019, USA bx Oklahoma State University, Stillwater, OK 74078, USA by Brown University, Providence, RI 02912, USA bz University of Texas, Arlington, TX 76019, USA ca Southern Methodist University, Dallas, TX 75275, USA cb Rice University, Houston, TX 77005, USA cc University of Virginia, Charlottesville, VA 22901, USA cd University of Washington, Seattle, WA 98195, USA a r t i c l e i n f o a b s t r a c t Article history: Received 4 December 2010 Received in revised form 9 February 2011 Accepted 10 February 2011 Available online 24 February 2011 Editor: M. Doser Keywords: Tevatron Standard Model Higgs boson Electroweak symmetry breaking We present a search for associated production of Higgs and W bosons in pp̄ collisions at a center of mass energy of √ s = 1.96 TeV in 5.3 fb−1 of integrated luminosity recorded by the D0 experiment. Multivariate analysis techniques are applied to events containing one lepton, an imbalance in transverse energy, and one or two b-tagged jets to discriminate a potential W H signal from Standard Model backgrounds. We observe good agreement between data and expected backgrounds, and set an upper limit of 4.5 (at 95% confidence level and for mH = 115 GeV) on the ratio of the W H cross section multiplied by the branching fraction of H → bb̄ to its Standard Model prediction, which is consistent with an expected limit of 4.8. © 2011 Elsevier B.V. All rights reserved. The only unobserved particle of the Standard Model (SM) is the Higgs boson (H). Its observation would support the hypoth- esis that the Higgs mechanism generates the masses of the weak gauge bosons and accommodates finite masses of fermions through their Yukawa couplings to the Higgs field. The mass of the Higgs boson (mH ) is not predicted by the SM, but the combination of direct searches at the CERN e+e− Collider (LEP) [1] and preci- sion measurements of electroweak parameters constrain mH to 1 Visitor from Augustana College, Sioux Falls, SD, USA. 2 Visitor from The University of Liverpool, Liverpool, UK. 3 Visitor from SLAC, Menlo Park, CA, USA. 4 Visitor from ICREA/IFAE, Barcelona, Spain. 5 Visitor from Centro de Investigacion en Computacion – IPN, Mexico City, Mexico. 6 Visitor from ECFM, Universidad Autonoma de Sinaloa, Culiacán, Mexico. 7 Visitor from Universität Bern, Bern, Switzerland. 114.4 < mH < 185 GeV at the 95% CL [2]. While the region 158 < mH < 175 GeV has been excluded at the 95% CL by a combina- tion of searches at CDF and D0 [3–6], the remaining mass range continues to be probed at the Fermilab Tevatron Collider. The asso- ciated production of a Higgs boson and a leptonically-decaying W boson is among the cleanest Higgs boson search channels at the Tevatron, and provides the largest usable event yield for the de- cay H → bb̄ in the range mH < 135 GeV. Several searches for W H production at a pp̄ center-of-mass energy of √ s = 1.96 TeV have been published. Three of these [7–9] use subsamples (0.17 fb−1, 0.44 fb−1, and 1.1 fb−1) of the data analyzed in this Letter, while three from the CDF Collaboration are based on cumulative sam- ples (0.32 fb−1, 0.95 fb−1 and 2.7 fb−1) of integrated luminos- ity [10–12]. We present a new search using an improved multivariate tech- nique, in 5.3 fb−1 of integrated luminosity collected by the D0 D0 Collaboration / Physics Letters B 698 (2011) 6–13 9 detector. The search selects events with one charged lepton (� = electron, e, or muon, μ), an imbalance in transverse energy (/E T ) that arises from the unobserved neutrino in the W → �ν decay, and either two or three jets, with one or two of these selected as candidate b-quark jets (b-tagged). The channels are separated into independent categories based on the number of b-tagged jets in an event (one or two). Single b-tagged events contain three important sources of backgrounds: (i) multijet events, where a jet is misidentified as an isolated lep- ton, (ii) W boson production in association with c-quark or light- quark jets, and (iii) W boson production in association with two heavy-flavor (bb̄, cc̄) jets. In events with two b-tagged jets, the dominant backgrounds are from W bb̄, tt̄ , and single top-quark pro- duction. The analysis relies on the following components of the D0 de- tector [13]: (i) a central-tracking system, which consists of a silicon microstrip tracker (SMT) and a central fiber tracker (CFT), both located within a 2 T superconducting solenoidal magnet; (ii) a liquid-argon/uranium calorimeter containing electromagnetic, fine hadronic, and coarse hadronic layers, segmented into a central section (CC), covering pseudorapidity |η| < 1.1 relative to the cen- ter of the detector [14], and two end calorimeters (EC) extending coverage to |η| ≈ 4.0, all housed in separate cryostats [15], with scintillators between the CC and EC cryostats providing sampling of developing showers for 1.1 < |η| < 1.4; (iii) a muon system located beyond the calorimetry consisting of layers of tracking detectors and scintillation trigger counters, one before and two after the 1.8 T iron toroids. A 2006 upgrade of the D0 detec- tor added an inner layer of silicon [16] to the SMT and an im- proved calorimeter trigger [17]. The integrated luminosity is mea- sured using plastic scintillator arrays located in front of the EC cryostats at 2.7 < |η| < 4.4. The trigger and data acquisition sys- tems are designed to accommodate high instantaneous luminosi- ties. Events in the electron channel are triggered by a logical OR of several triggers that require an electromagnetic (EM) object or an EM object in conjunction with a jet. Trigger efficiencies are taken into account in the Monte Carlo (MC) simulation through a weighting of events based on an efficiency derived from data, and parametrized as a function of electron η and azimuth φ, and jet transverse momentum pT . We accept events for the muon channel from an inclusive mix- ture of single high-pT muon, jet and muon plus jet triggers. This inclusive trigger approach provides a gain in efficiency relative to the single muon triggers alone. We validate it by comparing events passed by the single muon triggers and find good agreement be- tween data and MC. Events not selected by the single muon trigger are selected by complementary triggers, typically jet triggers. The efficiency of the complementary triggers is modeled as a function of the scalar sum of the pT of jets (HT ) in an event, and is used to weight the MC. We find good agreement between data and MC when combining the single muon and complementary triggers to form the inclusive trigger set. The leading-order (LO) pythia [18] MC generator is used to sim- ulate production of dibosons (W W , W Z , and Z Z ) with inclusive decays, W H → lνbb̄ and Z H → llbb̄ (l = e, μ, or τ ). The contri- bution from Z H events (in which one lepton is not identified) to the total signal corresponds to approximately 5%. Background from W /Z (V ) + jets and tt̄ events is generated with alpgen [19], in- terfaced to pythia for parton showering and hadronization. The alpgen samples are produced in the leading logarithm approxi- mation with the MLM parton-jet matching prescription [19]. The V + jets samples are divided into V + light jets and V + heavy- flavor jets. The V + light jets samples include V jj, V bj, and V cj processes, where j is a light-flavor (u, d or s quark or a gluon) jet, while the V +heavy-flavor samples for V bb̄ and V cc̄ are generated separately. Single top-quark events are generated using comphep [20,21] at next-to-leading order (NLO), with pythia used for par- ton evolution and hadronization. Simulation of both background and signal processes relies on the CTEQ6L1 [22] LO parton dis- tribution functions for all MC events. These events are processed through the full D0 detector simulation based on geant [23], and use the same reconstruction software as used for D0 data. Events from randomly chosen beam crossings with the same instanta- neous luminosity profile as the data are overlaid on the simulated events to reproduce the effect of multiple pp̄ interactions and de- tector noise. The simulated background processes are normalized to their predicted SM cross sections, except for W + jets events, which are normalized to data before applying b-tagging, where contam- ination from any W H signal is expected to be negligible. The W bb̄ (W cc̄) fraction within W + jets predicted by alpgen is in- creased by the Kbb̄/Klp (Kcc̄/Klp ) factor, where Kbb̄ (Kcc̄) is the NLO/LO K -factor for W bb̄ (W cb̄) and Klp is the NLO/LO K -factor for W + (two light partons), as calculated with the mcfm pro- gram [24]. The signal cross sections and branching fractions are calculated at next-to-next-to-leading order (NNLO) and are taken from Refs. [25–29], while the tt̄ , single t , and diboson cross sec- tions are at NLO, and taken from Refs. [30,31], and the mcfm program, respectively. As a cross check, we compare data with the alpgen prediction for W + jets, corrected in such a way that the inclusive W production cross section is equal to its NNLO calcula- tion [32] with MRST2004 NNLO PDFs [33], and we find a relative data/MC normalization factor of 1.0 ± 0.1 for W (at least two jets), where all background contributions other than W + jets were first subtracted from data. Based on the fractions of data events with 0, 1, or 2 b-tagged jets [34], we also observe good agree- ment with our prediction for the fraction of W bb̄ and W cc̄ in W + jets. This analysis is based on a preselection of events with an electron of pT > 15 GeV, with |η| < 1.1 or 1.5 < |η| < 2.5, or a muon of pT > 15 GeV, with |η| < 1.6. Preselected events are also required to have /E T > 20 GeV, either two or three jets with pT > 20 GeV (after correcting jet energies [35]) and |η| < 2.5, and HT > 60 GeV for 2-jet events, or HT > 80 GeV for 3-jet events. The /E T is calculated from the individual calorimeter cells in the EM and fine hadronic layers of the calorimeter, and is corrected for the presence of muons. All energy corrections to electrons and jets (including energy in the coarse-hadronic layers associated with jets) are propagated into the /E T . To suppress multijet background, events with MW T < 40 − 0.5/E T (GeV) are removed, where MW T =√ 2E� T · /E T (1 − cosφ(�, /E T )) is the transverse mass of the W bo- son candidate. Events that contain additional charged leptons iso- lated from jets, with the lepton passing the flavor-dependent pT thresholds pe T > 15 GeV, pμ T > 10 GeV, and pτ T > 10 or 15 GeV de- pending on τ decay channel [36], are rejected to decrease dilepton background from Z boson and tt̄ events. Events must have a recon- structed pp̄ interaction vertex (containing at least three associated tracks) that is located within ±40 cm of the center of the detector in the longitudinal direction. Lepton candidates are identified in two steps. First, each can- didate must pass “loose” identification criteria. For electrons, we require 95% of the energy in a shower to be deposited in the EM section of the calorimeter (isolation from other calorimeter energy depositions), spatial distributions of calorimeter energies consis- tent with those expected for EM showers, and a reconstructed track matched to the EM shower, but isolated from other tracks. A “loose” muon is defined by hits in each layer of the muon sys- tem, scintillator hits in time with a beam crossing (to veto cosmic 10 D0 Collaboration / Physics Letters B 698 (2011) 6–13 rays), a spatial match with a track in the central tracker, and isola- tion relative to jet axies (�R > 0.5) [14] to reject semileptonic de- cays of hadrons. In the second step, the loose leptons are subjected to a more restrictive “tight” selection. Tight electrons must sat- isfy more restrictive calorimeter isolation and EM energy-fraction criteria, and satisfy a likelihood test developed on Z → ee data based on eight quantities characterizing the EM nature of differ- ent particle interactions [37]. Tight muons must satisfy more strict isolation criteria on energy in the calorimeter and on momenta of tracks near trajectories of muon candidates. Inefficiencies intro- duced by lepton-identification and isolation criteria are determined from Z → �� data. The final selections for signal rely on events with only tight leptons, and events with loose leptons but not tight leptons are used to determine the multijet background. Jets are reconstructed using a midpoint cone algorithm [38] with radius 0.5. Identification requirements for jets are based on longitudinal and transverse shower profiles, and minimize the pos- sibility that the jets are caused by noise or spurious depositions of energy. For data taken after the upgrade in 2006, we require that jets in data and in the corresponding simulation have at least two associated tracks emanating from the reconstructed pp̄ interaction vertex. The parameters for jet-identification efficiency, energy cali- bration, and energy resolution are adjusted accordingly in the sim- ulation to match the data. Also, comparison of alpgen with other generators and with data shows small discrepancies in distribu- tions of jet pseudorapidity and dijet angular separations [39]. The data are therefore used to correct the alpgen W + jets and Z + jets MC events through polynomial reweighting functions, parameter- ized by the leading and second-leading jet η, and �R between the two jets of highest pT , that bring these distributions for the total simulated background and in the high-statistics sample of events prior to b-tagging into agreement. Instrumental background and that from semileptonic decays of hadrons, referred to jointly as the multijet background, are esti- mated from data. The instrumental background is significant in the electron channel, where a jet with a high EM fraction can pass electron-identification criteria, or a photon can be misidentified as an electron. In the muon channel, the multijet background is less important and arises mainly from semileptonic decay of heavy- flavor quarks, where the muon passes isolation criteria. To estimate the number of events that contain a jet that passes “tight” lepton selection, we determine the probability f T |L for a “loose” lepton candidate, originating from a jet, to also pass tight identification. This is done in events that pass preselection re- quirements before applying the selection on MW T , i.e., events that contain one loose lepton and two jets, but small /E T (5–15 GeV). The total non-multijet background is estimated from MC and sub- tracted from the data before estimating the contribution from multijet events. For electrons, f T |L is determined as a function of electron pT in three regions of |η| and four of �φ(/E T , e), while for muons it is taken as a function of |η| for two regions of �φ(/E T ,μ). The efficiency for a loose lepton to pass the tight iden- tification (εT |L ) is measured in Z → �� events in data, and is mod- eled as a function of pT for electrons and muons. The estimation of multijet background described in Ref. [37] is used to determine the multijet background directly from data, where each event is as- signed a weight that contributes to the multijet estimation based on f T |L and εT |L as a function of event kinematics. Since f T |L de- pends on /E T , the scale of this estimate of the multijet background must be adjusted when comparing to data with /E T > 20 GeV. Be- fore applying b-tagging, we fit the background templates to the data MW T distribution to obtain the normalizations for the multijet and W + jets backgrounds simultaneously. Efficient identification of b jets is central to the search for W H production. The D0 neural network (NN) b-tagging algorithm [40] Table 1 Summary of event yields for the � + b-tagged jets + /E T final state. Event yields in data are compared with the expected number of ST and DT events in the samples with W boson candidates plus two or three jets, comprised of contribu- tions from simulated diboson pairs (labeled “W Z ” in the table), W /Z + bb̄ or cc̄ (“W bb̄”), W /Z + light-quark jets (“W + l f ”), and top-quark (“tt̄” and “Single t”) production, as well as data-derived multijet background (“MJ”). The quoted uncer- tainties include both statistical and systematic contributions, including correlations between background sources and channels. The expectation for W H signal is given for mH = 115 GeV. W + 2-jet ST W + 2-jet DT W + 3-jet ST W + 3-jet DT W Z 153 ±18 22.5±3.3 33.9 ±4.8 2.6±1.1 W bb̄ 1601 ±383 346±93 358 ±90 48 ±13 W + l f 1290 ±201 57.5±9.2 210 ±35 12.1±1.8 tt̄ 417 ±54 177±35 633 ±96 176 ±35 Single t 203±33 58 ±11 53.6 ±9.1 13.0 ±2.7 MJ 663 ±43 56.5±4.2 186 ±13 12.7±1.0 All Bkg. 4326 ±501 718 ±120 1474 ±160 264 ±44 W H 9.7±0.9 6.5±1.0 2.1 ±0.3 0.8±0.2 Data 4316 709 1463 301 for identifying heavy-flavored jets is based on a combination of seven variables sensitive to the presence of tracks or secondary vertices displaced significantly from the primary vertex. All tagging efficiencies are determined separately for data and for simulated events. We first use a low threshold on the NN output that corre- sponds to a rate of 2.7% for light-flavor jets of pT � 50 GeV that are mistakenly tagged as heavy-flavored jets. If two jets in an event pass this b-tagging requirement, the event is classified as double- b-tagged (DT). Events that are not classified as DT are considered for placement in an independent single-b-tag (ST) sample, which requires exactly one jet to satisfy a more restrictive NN operating point corresponding to a misidentification rate of 0.9%. The effi- ciencies for identifying a jet that contains a b hadron for the two NN operating points are (63 ± 1)% and (53 ± 1)%, respectively, for a jet with a pT of 50 GeV. These efficiencies are determined for “taggable” jets, i.e., jets with at least two tracks, each with at least one hit in the SMT. Simulated events are corrected to have the same fraction of jets satisfying the taggability and b-tagging re- quirements as found in preselected data. The expected event yields following these selection criteria for specific backgrounds and for mH = 115 GeV are compared to the observed number of events in Table 1. Distributions in dijet invari- ant mass for the two jets of highest pT , in 2-jet and 3-jet events are shown for the ST and DT samples in Figs. 1(a)–1(d). The data are adequately described by the sum of the simulated SM pro- cesses and multijet background. The contributions expected from a Higgs boson with mH = 115 GeV, multiplied by a factor of ten, are also shown for comparison. We use a random forest (RF) multivariate technique [41,42] to separate the SM background from signal, and search for an excess, which is expected primarily at large values of RF discriminant. A separate RF discriminant is used for each combination of jet multiplicity (two or three), lepton flavor (e or μ), and number of b-tagged jets (one or two). The 2-jet events are divided into data- taking periods, before and after the 2006 detector upgrade, for a total of twelve separately trained RFs for each chosen Higgs boson mass. Each RF consists of a collection of individual decision trees, with each tree considering a random subset of the twenty kine- matic and topological input variables listed in Table 2. The final RF output is the average over the individual trees. The input vari- ables √ ŝ and �R(dijet, �+ν) each have two solutions arising from the two possibilities for the longitudinal neutrino momentum, as- suming the lepton and /E T (ν) constitute the decay products of an on-shell W boson. The angles θ∗ and χ are described in Ref. [43], and exploit kinematic differences arising from the scalar nature of D0 Collaboration / Physics Letters B 698 (2011) 6–13 11 Fig. 1. (Color online.) Dijet mass distributions for candidate W -boson ST (1 b-tag) events with (a) 2-jets and (b) 3-jets and for DT (2 b-tag) events in (c) and (d), respectively. The distributions in RF discriminant for 2-jet ST and DT events, combined for lepton flavors, are shown in (e) and (f), respectively. The expectation from σ(pp̄ → W H) × B(H → bb̄) for mH = 115 GeV is overlaid, multiplied by a factor of 10. Table 2 List of RF input variables, where j1 ( j2) refers to the jet with the highest (second highest) pT . Variable Definition pT ( j1) Leading jet pT pT ( j2) Sub-leading jet pT E( j2) Sub-leading jet energy �R( j1, j2) �R between jets �φ( j1, j2) �φ between jets �φ( j1, �) �φ between lepton and leading jet pT (dijet system) pT of dijet system m jj Dijet invariant mass pT (�–/E T system) pT of W candidate /E T Missing transverse energy Aplanarity See Ref. [44]√ ŝ Invariant mass of the ν + � + dijet system �R(dijet, � + ν) �R between the dijet system and � + ν system MW T Lepton-/E T transverse mass HT Scalar sum of the transverse momenta of all jets in the event H Z Scalar sum of the longitudinal momenta of all jets in the event cos θ∗ Cosine of angle between W candidate and beam direction in zero-momentum frame cosχ See Ref. [45] the Higgs and the spins of objects in the W bb̄ background. The RF outputs from 2-jet ST and DT events are shown in Figs. 1(e) and 1(f). The dijet mass distribution is especially sensitive to W H pro- duction, and was used previously to set limits on σ(pp̄ → W H) × B(H → bb̄) in Ref. [8]. However, the gain in sensitivity using the RF output as the final discriminant is about 20% for a Higgs mass of 115 GeV, which, in terms of the expected limit on the W H cross section, is equivalent to a gain of about 40% in integrated luminos- ity. The systematic uncertainties that affect the signal and SM back- grounds can be categorized by the nature of their source, i.e., the- oretical (e.g., uncertainty on a cross section), MC modeling (e.g., reweighting of alpgen samples), or experimental (e.g., uncertainty on integrated luminosity). Some of these uncertainties affect only the normalization of the signal or backgrounds, while others also affect the differential distribution of the RF output. Theoretical uncertainties include uncertainties on the tt̄ and single top-quark production cross sections (10% and 12%, respec- tively [30,31]), an uncertainty on the diboson production cross section (6% [24]), and an uncertainty on W + heavy-flavor produc- tion (20%, estimated from mcfm). These uncertainties affect only the normalization of the backgrounds. Uncertainties from modeling that affect the distribution in the RF output include uncertainties on trigger efficiency as derived from data (3–5%), lepton identification and reconstruction effi- ciency (5–6%), reweighting of alpgen MC samples (2%), the MLM matching applied to W /Z + light-jet events (<0.5%), and the sys- tematic uncertainties associated with choice of renormalization and factorization scales in alpgen as well as the uncertainty on the strong coupling constant (2%). Uncertainties on the alpgen renormalization and factorization scales are evaluated by adjust- ing the nominal scale for each, simultaneously, by a factor of 0.5 and 2.0. Experimental uncertainties that affect only the normalization of the signal and SM backgrounds arise from the uncertainty on 12 D0 Collaboration / Physics Letters B 698 (2011) 6–13 Table 3 Expected and observed 95% CL upper limits on the ratio of σ(pp̄ → W H) × B(H → bb̄) to its SM expectation as a function of mH . mH [GeV] 100 105 110 115 120 125 130 135 140 145 150 Expected ratio 3.3 3.6 4.2 4.8 5.6 6.8 8.5 11.5 16.5 23.6 36.8 Observed ratio 2.7 4.0 4.3 4.5 5.8 6.6 7.0 7.6 12.2 15.0 30.4 Fig. 2. (Color online.) Distribution in the output of the RF discriminant for mH = 115 GeV, for the difference between data and background expectation, combined for all channels (both e and μ, ST and DT, and 2-jet and 3-jet), shown with statistical uncertainties. The lightly-shaded region represents the total systematic uncertainty before using constraints from data (referred to as “Pre-Fit” in the legend), while the solid lines represent the total systematic uncertainty after constraining with data (“Post-Fit” in the legend). The darker shaded region represents the SM Higgs signal expectation scaled up by a factor of 5. integrated luminosity (6.1%) [46]. Those that also affect the distri- bution in RF output include jet taggability (3%), b-tagging efficiency (2.5–3% per heavy quark-jet), the light-quark jet misidentification rate (10%), acceptance for jet identification (5%); jet-energy cali- bration and resolution (varies between 15% and 30%, depending on the process and channel). Model in multijet background is lim- ited by the statistical uncertainty of data after tagging (10–20%), which also covers the uncertainty in the flavor dependence of f T |L . The background-subtracted data points for the RF discriminant for mH = 115 GeV, with all channels combined, are shown with their systematic uncertainties in Fig. 2. We observe no excess relative to expectation from SM back- ground, and we set upper limits on the production cross section σ(W H) using the RF outputs from all the channels. The binning of the RF output is adjusted to assure adequate population of back- ground events in each bin. We calculate all limits at the 95% CL using a modified frequentist approach and a Poisson log-likelihood ratio as test statistic [47,48]. The likelihood ratio is studied using pseudoexperiments based on randomly drawn Poisson trials of sig- nal and background events. We treat systematic uncertainties as “nuisance parameters” constrained by their priors, and the best fits of these parameters to data are determined at each value of mH by maximizing the likelihood ratio [49]. Independent fits are performed to the background-only and signal-plus-background hy- potheses. All appropriate correlations of systematic uncertainties are maintained among channels and between signal and back- ground. The systematic uncertainties before and after fitting are indicated in Fig. 2. The log-likelihood ratios for the background- only model and the signal-plus-background model as a function of mH are shown in Fig. 3(a). The upper limit on σ(pp̄ → W H) × B(H → bb̄) at the 95% CL is a factor of 4.5 larger than the SM expectation for mH = 115 GeV, and the corresponding expected upper limit is 4.8. The analysis is repeated for ten other mH values from 100 to 150 GeV; the corre- sponding observed and expected 95% CL limits relative to their SM expectations are given in Table 3 and in Fig. 3(b). Fig. 3. (Color online.) (a) Log-likelihood ratios for the background-only model (LLRB , with 1 and 2 standard deviation bands), signal + background model (LLRS+B ), and observation in data (LLRobs) as a function of mH . (b) 95% CL cross section upper limit (and the corresponding expected limit) on σ(pp̄ → W H) × B(H → bb̄) relative to the SM expectation, as a function of mH . Results are calculated in steps of 5 GeV, and joined by straight lines. In conclusion, � + /E T + 2 or 3-jet events have been analyzed in a search for W H production in 5.3 fb−1 of pp̄ collisions at the Fermilab Tevatron. The yield of single and double b-tagged jets in these events is in agreement with the expected background. We have applied a Random Forest multivariate analysis technique to further separate signal and background. We have set upper limits on σ(pp̄ → W H)× B(H → bb̄) relative to their SM expectation for Higgs masses between 100 and 150 GeV. 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