Evidence for t\bar{t}γProduction and Measurement of σ_t\bar{t}γ/ σ_t\bar{t}
aa r X i v : . [ h e p - e x ] J u l Evidence for t ¯ tγ Production and Measurement of σ t ¯ tγ /σ t ¯ t T. Aaltonen, B. ´Alvarez Gonz´alez v , S. Amerio, D. Amidei, A. Anastassov, A. Annovi, J. Antos, G. Apollinari, J.A. Appel, A. Apresyan, T. Arisawa, A. Artikov, J. Asaadi, W. Ashmanskas, B. Auerbach, A. Aurisano, F. Azfar, W. Badgett, A. Barbaro-Galtieri, V.E. Barnes, B.A. Barnett, P. Barria cc , P. Bartos, M. Bauce aa , G. Bauer, F. Bedeschi, D. Beecher, S. Behari, G. Bellettini bb , J. Bellinger, D. Benjamin, A. Beretvas, A. Bhatti, M. Binkley ∗ , D. Bisello aa , I. Bizjak gg , K.R. Bland, B. Blumenfeld, A. Bocci, A. Bodek, D. Bortoletto, J. Boudreau, A. Boveia, B. Brau a , L. Brigliadori z , A. Brisuda, C. Bromberg, E. Brucken, M. Bucciantonio bb , J. Budagov, H.S. Budd, S. Budd, K. Burkett, G. Busetto aa , P. Bussey, A. Buzatu, C. Calancha, S. Camarda, M. Campanelli, M. Campbell, F. Canelli , A. Canepa, B. Carls, D. Carlsmith, R. Carosi, S. Carrillo k , S. Carron, B. Casal, M. Casarsa, A. Castro z , P. Catastini, D. Cauz, V. Cavaliere cc , M. Cavalli-Sforza, A. Cerri f , L. Cerrito q , Y.C. Chen, M. Chertok, G. Chiarelli, G. Chlachidze, F. Chlebana, K. Cho, D. Chokheli, J.P. Chou, W.H. Chung, Y.S. Chung, C.I. Ciobanu, M.A. Ciocci cc , A. Clark, G. Compostella aa , M.E. Convery, J. Conway, M.Corbo, M. Cordelli, C.A. Cox, D.J. Cox, F. Crescioli bb , C. Cuenca Almenar, J. Cuevas v , R. Culbertson, D. Dagenhart, N. d’Ascenzo t , M. Datta, P. de Barbaro, S. De Cecco, G. De Lorenzo, M. Dell’Orso bb , C. Deluca, L. Demortier, J. Deng c , M. Deninno, F. Devoto, M. d’Errico aa , A. Di Canto bb , B. Di Ruzza, J.R. Dittmann, M. D’Onofrio, S. Donati bb , P. Dong, M. Dorigo, T. Dorigo, K. Ebina, A. Elagin, A. Eppig, R. Erbacher, D. Errede, S. Errede, N. Ershaidat y , R. Eusebi, H.C. Fang, S. Farrington, M. Feindt, J.P. Fernandez, C. Ferrazza dd , R. Field, G. Flanagan r , R. Forrest, M.J. Frank, M. Franklin, J.C. Freeman, Y. Funakoshi, I. Furic, M. Gallinaro, J. Galyardt, J.E. Garcia, A.F. Garfinkel, P. Garosi cc , H. Gerberich, E. Gerchtein, S. Giagu ee , V. Giakoumopoulou, P. Giannetti, K. Gibson, C.M. Ginsburg, N. Giokaris, P. Giromini, M. Giunta, G. Giurgiu, V. Glagolev, D. Glenzinski, M. Gold, D. Goldin, N. Goldschmidt, A. Golossanov, G. Gomez, G. Gomez-Ceballos, M. Goncharov, O. Gonz´alez, I. Gorelov, A.T. Goshaw, K. Goulianos, A. Gresele, S. Grinstein, C. Grosso-Pilcher, R.C. Group , J. Guimaraes da Costa, Z. Gunay-Unalan, C. Haber, S.R. Hahn, E. Halkiadakis, A. Hamaguchi, J.Y. Han, F. Happacher, K. Hara, D. Hare, M. Hare, R.F. Harr, K. Hatakeyama, C. Hays, M. Heck, J. Heinrich, M. Herndon, S. Hewamanage, D. Hidas, A. Hocker, W. Hopkins g , D. Horn, S. Hou, R.E. Hughes, M. Hurwitz, U. Husemann, N. Hussain, M. Hussein, J. Huston, G. Introzzi, M. Iori ee , A. Ivanov o , E. James, D. Jang, B. Jayatilaka, E.J. Jeon, M.K. Jha, S. Jindariani, W. Johnson, M. Jones, K.K. Joo, S.Y. Jun, T.R. Junk, T. Kamon, P.E. Karchin, Y. Kato n , W. Ketchum, J. Keung, V. Khotilovich, B. Kilminster, D.H. Kim, H.S. Kim, H.W. Kim, J.E. Kim, M.J. Kim, S.B. Kim, S.H. Kim, Y.K. Kim, N. Kimura, M. Kirby, S. Klimenko, K. Kondo, D.J. Kong, J. Konigsberg, A.V. Kotwal, M. Kreps, J. Kroll, D. Krop, N. Krumnack l , M. Kruse, V. Krutelyov d , T. Kuhr, M. Kurata, S. Kwang, A.T. Laasanen, S. Lami, S. Lammel, M. Lancaster, R.L. Lander, K. Lannon u , A. Lath, G. Latino cc , I. Lazzizzera, T. LeCompte, E. Lee, H.S. Lee, J.S. Lee, S.W. Lee w , S. Leo bb , S. Leone, J.D. Lewis, C.-J. Lin, J. Linacre, M. Lindgren, E. Lipeles, A. Lister, D.O. Litvintsev, C. Liu, Q. Liu, T. Liu, S. Lockwitz, N.S. Lockyer, A. Loginov, D. Lucchesi aa , J. Lueck, P. Lujan, P. Lukens, G. Lungu, J. Lys, R. Lysak, R. Madrak, K. Maeshima, K. Makhoul, P. Maksimovic, S. Malik, G. Manca b , A. Manousakis-Katsikakis, F. Margaroli, C. Marino, M. Mart´ınez, R. Mart´ınez-Ballar´ın, P. Mastrandrea, M. Mathis, M.E. Mattson, P. Mazzanti, K.S. McFarland, P. McIntyre, R. McNulty i , A. Mehta, P. Mehtala, A. Menzione, C. Mesropian, T. Miao, D. Mietlicki, A. Mitra, H. Miyake, S. Moed, N. Moggi, M.N. Mondragon k , C.S. Moon, R. Moore, M.J. Morello, J. Morlock, P. Movilla Fernandez, A. Mukherjee, Th. Muller, P. Murat, M. Mussini z , J. Nachtman m , Y. Nagai, J. Naganoma, I. Nakano, A. Napier, J. Nett, C. Neu, M.S. Neubauer, J. Nielsen e , L. Nodulman, O. Norniella, E. Nurse, L. Oakes, S.H. Oh, Y.D. Oh, I. Oksuzian, T. Okusawa, R. Orava, L. Ortolan, S. Pagan Griso aa , C. Pagliarone, E. Palencia f , V. Papadimitriou, A.A. Paramonov, J. Patrick, G. Pauletta ff , M. Paulini, C. Paus, D.E. Pellett, A. Penzo, T.J. Phillips, G. Piacentino, E. Pianori, J. Pilot, K. Pitts, C. Plager, L. Pondrom, K. Potamianos, O. Poukhov ∗ , F. Prokoshin x , A. Pronko, F. Ptohos h , E. Pueschel, G. Punzi bb , J. Pursley, A. Rahaman, V. Ramakrishnan, N. Ranjan, I. Redondo, P. Renton, M. Rescigno, F. Rimondi z , L. Ristori , A. Robson, T. Rodrigo, T. Rodriguez, E. Rogers, S. Rolli, R. Roser, M. Rossi, F. Rubbo, F. Ruffini cc , A. Ruiz, J. Russ, V. Rusu, A. Safonov, W.K. Sakumoto, Y. Sakurai, L. Santi ff , L. Sartori, K. Sato, V. Saveliev t , A. Savoy-Navarro, P. Schlabach, A. Schmidt, E.E. Schmidt, M.P. Schmidt ∗ , M. Schmitt, T. Schwarz, L. Scodellaro, A. Scribano cc , F. Scuri, A. Sedov, S. Seidel, Y. Seiya, A. Semenov, F. Sforza bb , A. Sfyrla, S.Z. Shalhout, T. Shears, P.F. Shepard, M. Shimojima s , S. Shiraishi, M. Shochet, I. Shreyber, A. Simonenko, P. Sinervo, A. Sissakian ∗ , K. Sliwa, J.R. Smith, F.D. Snider, A. Soha, S. Somalwar, V. Sorin, P. Squillacioti, M. Stancari, M. Stanitzki, R. St. Denis, B. Stelzer, O. Stelzer-Chilton, D. Stentz, J. Strologas, G.L. Strycker, Y. Sudo, A. Sukhanov, I. Suslov, K. Takemasa, Y. Takeuchi, J. Tang, M. Tecchio, P.K. Teng, J. Thom g , J. Thome, G.A. Thompson, E. Thomson, P. Tipton, P. Ttito-Guzm´an, S. Tkaczyk, D. Toback, S. Tokar, K. Tollefson, T. Tomura, D. Tonelli, S. Torre, D. Torretta, P. Totaro ff , M. Trovato dd , Y. Tu, F. Ukegawa, S. Uozumi, A. Varganov, F. V´azquez k , G. Velev, C. Vellidis, M. Vidal, I. Vila, R. Vilar, J. Viz´an, M. Vogel, G. Volpi bb , P. Wagner, R.L. Wagner, T. Wakisaka, R. Wallny, S.M. Wang, A. Warburton, D. Waters, M. Weinberger, W.C. Wester III, B. Whitehouse, D. Whiteson c , A.B. Wicklund, E. Wicklund, S. Wilbur, F. Wick, H.H. Williams, J.S. Wilson, P. Wilson, B.L. Winer, P. Wittich g , S. Wolbers, H. Wolfe, T. Wright, X. Wu, Z. Wu, K. Yamamoto, J. Yamaoka, T. Yang, U.K. Yang p , Y.C. Yang, W.-M. Yao, G.P. Yeh, K. Yi m , J. Yoh, K. Yorita, T. Yoshida j , G.B. Yu, I. Yu, S.S. Yu, J.C. Yun, A. Zanetti, Y. Zeng, and S. Zucchelli z (CDF Collaboration † ) Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China Argonne National Laboratory, Argonne, Illinois 60439, USA University of Athens, 157 71 Athens, Greece Institut de Fisica d’Altes Energies, ICREA, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain Baylor University, Waco, Texas 76798, USA Istituto Nazionale di Fisica Nucleare Bologna, z University of Bologna, I-40127 Bologna, Italy University of California, Davis, Davis, California 95616, USA University of California, Los Angeles, Los Angeles, California 90024, USA Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia Joint Institute for Nuclear Research, RU-141980 Dubna, Russia Duke University, Durham, North Carolina 27708, USA Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA University of Florida, Gainesville, Florida 32611, USA Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy University of Geneva, CH-1211 Geneva 4, Switzerland Glasgow University, Glasgow G12 8QQ, United Kingdom Harvard University, Cambridge, Massachusetts 02138, USA Division of High Energy Physics, Department of Physics,University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland University of Illinois, Urbana, Illinois 61801, USA The Johns Hopkins University, Baltimore, Maryland 21218, USA Institut f¨ur Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany Center for High Energy Physics: Kyungpook National University,Daegu 702-701, Korea; Seoul National University, Seoul 151-742,Korea; Sungkyunkwan University, Suwon 440-746,Korea; Korea Institute of Science and Technology Information,Daejeon 305-806, Korea; Chonnam National University, Gwangju 500-757,Korea; Chonbuk National University, Jeonju 561-756, Korea Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA University of Liverpool, Liverpool L69 7ZE, United Kingdom University College London, London WC1E 6BT, United Kingdom Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Institute of Particle Physics: McGill University, Montr´eal, Qu´ebec,Canada H3A 2T8; Simon Fraser University, Burnaby, British Columbia,Canada V5A 1S6; University of Toronto, Toronto, Ontario,Canada M5S 1A7; and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3 University of Michigan, Ann Arbor, Michigan 48109, USA Michigan State University, East Lansing, Michigan 48824, USA Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia University of New Mexico, Albuquerque, New Mexico 87131, USA Northwestern University, Evanston, Illinois 60208, USA The Ohio State University, Columbus, Ohio 43210, USA Okayama University, Okayama 700-8530, Japan Osaka City University, Osaka 588, Japan University of Oxford, Oxford OX1 3RH, United Kingdom Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, aa University of Padova, I-35131 Padova, Italy LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA Istituto Nazionale di Fisica Nucleare Pisa, bb University of Pisa, cc University of Siena and dd Scuola Normale Superiore, I-56127 Pisa, Italy University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA Purdue University, West Lafayette, Indiana 47907, USA University of Rochester, Rochester, New York 14627, USA The Rockefeller University, New York, New York 10065, USA Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, ee Sapienza Universit`a di Roma, I-00185 Roma, Italy Rutgers University, Piscataway, New Jersey 08855, USA Texas A&M University, College Station, Texas 77843, USA Istituto Nazionale di Fisica Nucleare Trieste/Udine,I-34100 Trieste, ff University of Trieste/Udine, I-33100 Udine, Italy University of Tsukuba, Tsukuba, Ibaraki 305, Japan Tufts University, Medford, Massachusetts 02155, USA University of Virginia, Charlottesville, VA 22906, USA Waseda University, Tokyo 169, Japan Wayne State University, Detroit, Michigan 48201, USA University of Wisconsin, Madison, Wisconsin 53706, USA Yale University, New Haven, Connecticut 06520, USA (Dated: September 6, 2018)Using data corresponding to 6 . − of p ¯ p collisions at √ s = 1 .
96 TeV collected by the CDF IIdetector, we present a cross section measurement of top-quark pair production with an additionalradiated photon, t ¯ tγ . The events are selected by looking for a lepton ( ℓ ), a photon ( γ ), significanttransverse momentum imbalance ( E T ), large total transverse energy, and three or more jets, with atleast one identified as containing a b quark ( b ). The t ¯ tγ sample requires the photon to have 10 GeVor more of transverse energy, and to be in the central region. Using an event selection optimizedfor the t ¯ tγ candidate sample we measure the production cross section of t ¯ t ( σ t ¯ t ), and the ratio ofcross sections of the two samples. Control samples in the dilepton+photon and lepton+photon+ E T ,channels are constructed to aid in decay product identification and background measurements. Weobserve 30 t ¯ tγ candidate events compared to the standard model expectation of 26 . ± . t ¯ tγ cross section ( σ t ¯ tγ ) to be 0 . ± .
08 pb, and the ratio of σ t ¯ tγ to σ t ¯ t to be0 . ± . t ¯ tγ production, we observe a probability of 0.0015 of the backgroundevents alone producing 30 events or more, corresponding to 3.0 standard deviations. PACS numbers: 13.85.Rm, 12.60.Jv, 13.85.Qk, 14.80.Ly ∗ Deceased † With visitors from a University of Massachusetts Amherst,Amherst, Massachusetts 01003, b Istituto Nazionale di Fisica Nu-cleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy, c University of California Irvine, Irvine, CA 92697, d University ofCalifornia Santa Barbara, Santa Barbara, CA 93106 e Universityof California Santa Cruz, Santa Cruz, CA 95064, f CERN,CH-1211 Geneva, Switzerland, g Cornell University, Ithaca, NY 14853, h University of Cyprus, Nicosia CY-1678, Cyprus, i University Col-lege Dublin, Dublin 4, Ireland, j University of Fukui, Fukui City,Fukui Prefecture, Japan 910-0017, k Universidad Iberoamericana,Mexico D.F., Mexico, l Iowa State University, Ames, IA 50011, m University of Iowa, Iowa City, IA 52242, n Kinki University,Higashi-Osaka City, Japan 577-8502, o Kansas State University,
The standard model (SM) [1] of particle physics makessuccessful predictions of the production rates of physicsprocesses that span many orders of magnitude. Data
Manhattan, KS 66506, p University of Manchester, Manchester M139PL, England, q Queen Mary, University of London, London, E14NS, England, r Muons, Inc., Batavia, IL 60510, s Nagasaki In-stitute of Applied Science, Nagasaki, Japan, t National ResearchNuclear University, Moscow, Russia, u University of Notre Dame,Notre Dame, IN 46556, v Universidad de Oviedo, E-33007 Oviedo,Spain, w Texas Tech University, Lubbock, TX 79609, x UniversidadTecnica Federico Santa Maria, 110v Valparaiso, Chile, y YarmoukUniversity, Irbid 211-63, Jordan, gg On leave from J. Stefan Insti-tute, Ljubljana, Slovenia, from p ¯ p collisions collected at the Tevatron have beenused to verify many of these predictions [2]. As a testof the SM, we measure the ratio of production cross sec-tions of t ¯ tγ to t ¯ t . The ratio allows for the cancellation ofsystematic effects, and is a more sensitive test of the SMthan the measurement of the production cross section of t ¯ tγ alone. While current data is not sufficient to studythem in detail, the t ¯ tγ coupling parameters are sensitiveto some new physics models [3], and will be better mea-sured in the future.Top quarks are dominantly produced in pairs, withboth top quarks decaying to a W boson and a b quarknearly 100% of the time. Their decays are classified asdileptonic if both W bosons decay to leptons, semilep-tonic if only one W boson decays to leptons, and hadronicif neither W boson decays to leptons. Selection for the t ¯ tγ events in a semileptonic channel (including τ leptonicdecays) was performed using 6 . − of integrated lumi-nosity from p ¯ p collisions at √ s = 1.96 TeV collected usingthe CDF II detector [4]. In order to isolate non-hadronic t ¯ tγ production, we require: a high-transverse-momentum( p T ) [5] lepton ( ℓ ) identifed as either an electron ( e ) or amuon ( µ ), a photon ( γ ), a b -tagged jet ( b ), missing trans-verse energy ( E T ), large total transverse energy ( H T ),and three or more jets. With these selection criteria, t ¯ tγ dominates SM predictions [6]. The total transverse en-ergy, H T , is the scalar sum of the transverse energy ofelectrons, muons, jets, photons, and E T identified in theevent. Furthermore, we select top-quark pair production( t ¯ t ) events by using nearly the same selection as t ¯ tγ , butwithout the photon requirement. Using similar event se-lection ensures that many systematic uncertainties cancelwhen we measure the cross section ratio of t ¯ tγ to t ¯ t .The semileptonic cross section of t ¯ tγ has been mea-sured to be 0.15 ± − [6]. Using thebranching ratio of W decays to leptons (0.324) [7], thiscorresponds to a production cross section of 0.34 ± t ¯ t production is well-measuredat 7.70 ± t ¯ t and t ¯ tγ cross sections with similar event selection hasnot been performed.The CDF II detector is a cylindrically symmetric mag-netic spectrometer designed to study p ¯ p collisions at theFermilab Tevatron. Here we briefly describe the com-ponents relevant for this analysis. Tracking systems areused to measure the momenta of charged particles andto assist lepton identification. A multi-layer system ofsilicon strip detectors [9] which identifies tracks in the r - φ and r - z views [5], and the central outer tracker(COT) [10], are contained in a superconducting solenoidthat generates a 1.4 T magnetic field. The COT is a 3.1m long open-cell drift chamber capable of making up to96 measurements of each charged particle in the pseudo-rapidity region | η | < ± ◦ stereo superlayers with 12wires each. For high-momentum tracks, the COT trans-verse momentum resolution is σ pT /p T ≃ − . Segmented calorimeters with towers arranged in a pro-jective geometry, each tower consisting of an electromag-netic and a hadronic compartment [11], cover the region | η | < | η | < ∼ .
0, where the centralelectromagnetic shower system (CES) makes profile mea-surements at shower maximum with finer spatial resolu-tion than the calorimeter. Electrons are reconstructed inthe central electromagnetic calorimeter (CEM) with an E T [5] resolution of σ ( E T ) /E T ≃ p E T / GeV ⊕ . η − φ space of radius R = p (∆ φ ) + (∆ η ) = 0 .
4. The jet energy resolutionis approximately σ ≃ . × E T (GeV) + 1 GeV [12] ( i.e. b quark ( b hadrons)are identified by exploiting the long b -hadron lifetime(c τ b ≈ µ m). The tracks originating from the resultingdisplaced vertex are used by the SecVtx [13] algorithmto identify the b hadron. The algorithm works in the re-gion | η | <
2, defined by the silicon system coverage. Jetsthat are identified as coming from b hadrons are said tobe b -tagged.Muons ( µ ) are identified using the central muon(CMU), the central muon upgrade (CMP), and the cen-tral muon extension (CMX) systems [14], which coverthe detector region | η | < < | η | < p ¯ p [15].A three-level online event selection system (trigger) [4]selects events with a high- p T lepton in the central region.The trigger system selects electron candidates from clus-ters of energy in the central electromagnetic calorimeter.Electrons are distinguished from photons by requiring aCOT track associated with the clusters. The muon trig-ger requires a COT track that extrapolates to a tracksegment (“stub”) in the muon detectors.A muon candidate passing our selection criteria musthave: a well-measured track in the COT, energy de-posited in the calorimeter consistent with minimum-ionization expectations, a muon stub in both the CMUand CMP, or in the CMX, consistent with the extrap-olated COT track, and COT timing consistent with atrack from a p ¯ p collision.An electron candidate passing our selection criteriamust have: a high-quality track with p T > . E T , unless E T >
100 GeV, in which case the p T threshold is set to25 GeV, a good transverse shower profile that matchesthe extrapolated track position, a lateral sharing of en-ergy in the two calorimeter towers containing the electronshower consistent with that expected for an electromag-netic (EM) shower, and minimal leakage into the hadroncalorimeter [16].Photon candidates are required to have E γT >
10 GeV,no track with p T > p T < p T of 0.35 GeV due tothe magnetic field curling up lower p T particles. Thephotons are only reconstructed in the CEM and have | η | < E T deposited in the calorimetertowers in a cone in η − ϕ space [5] of R = 0 . E T dueto the photon or lepton is subtracted. The remaining E T is required to be less than 2 . . × ( E T -20 GeV) for a photon, or less than 10% of the E T forelectrons or p T for muons. In addition, for photons, thesum of the p T of all tracks in the cone must be less than2 . . × E T .Missing transverse energy, E T , is calculated from theobserved calorimeter-tower energies in the region | η | < . E T fornon-uniform calorimeter response [18] for jets with un-corrected E T >
15 GeV and η <
2, and for muons with p T >
20 GeV.Events for the analysis are selected by requiring a cen-tral e or µ with E ℓT >
20 GeV originating less than 60 cmalong the beam-line from the detector center and pass-ing the criteria listed above. We further require eventsto have at least one of the following objects: a jet with E jetT >
15 GeV, E T >
20 GeV, an additional lepton, ora central γ with E T >
10 GeV.The first measurement we perform is in the t ¯ t sig-nal sample, which requires an event to contain: E T >
20 GeV, a lepton, a b -tagged jet, H T >
200 GeV, N jets ≥ b -tagged jet) , and transverse massof the lepton and E T to be greater than 20 GeV forthe electron channel, and 10 GeV for the muon channel.Transverse mass for the E T and lepton is defined as: q E ℓ,T × 6 E T − E ℓ,x × 6 E T x − E ℓ,y × 6 E T y ). The selec-tion criteria is inclusive, so if an event contains an addi-tional lepton or a photon it is also accepted as a signalevent. The highest- p T lepton determines if the event isan electron or muon event.Events in the t ¯ tγ signal sample are selected by re-quiring E T >
20 GeV, a lepton, a b -tagged jet, H T >
200 GeV, N jets ≥ b -tagged jet.), and aphoton with E T >
10 GeV. For all photons we requirethe χ of the CES shower profile be less than 20. Tofurther suppress backgrounds, photons with E T between10 and 25 GeV must have a χ of the CES shower pro-file less than 6; we discuss how χ is calculated below.Similar to the t ¯ t analysis, the selection is inclusive. Theselection criteria are identical to the previous t ¯ tγ crosssection measurement [6], with the exception of the low- E T photon χ requirements. The primary difference between the t ¯ t and t ¯ tγ selec-tion, other than the photon selection, is the requirementof a transverse mass selection for t ¯ t . In the t ¯ t selection,the low-transverse-mass region is not well-modeled withbackground estimation methods. The t ¯ tγ sample doesnot suffer from this deficiency, and we do not use thetransverse mass selection criterion to keep acceptance ofsignal events high, this behavior is also seen in the ℓγ E T control sample described below.Control samples are identified by selecting events witha lepton, a photon, and E T >
20 GeV ( ℓγ E T ), or twooppositely charged same-flavor central leptons, a photon,and a three-body mass consistent with the Z boson ( ℓℓγ ).These control samples are used to define the above CES χ selection for photons.The χ value of photons is based on the lateral showershapes observed in the CES compared to that predictedfrom a sample of test-beam electrons. Using the con-trol samples we identify an additional selection criterionon photons [20].It should be noted that the t ¯ tγ samplecontains 30 events and is a subset of the 8276 events inthe ℓγ E T sample. The ℓℓγ sample contains 1344 events.While the samples are not independent, optimizing pho-ton identification selection criteria using the ℓγ E T sam-ple should be minimally affected by the presence of t ¯ tγ events.The dominant SM sources of events with a lepton, pho-ton, and significant E T , not including particle misidenti-fications, are t ¯ tγ production and W γ +heavy flavor (HF),in which a W boson decays leptonically ( ℓν ) and a pho-ton is radiated from an initial-state or final-state quark,the W boson, or a charged final-state lepton [21]. In thispaper, HF includes: c ¯ c , b ¯ b , and c . Similarly, for events inthe t ¯ t selection, the dominant source of events is due to t ¯ t production and W +HF production.The production of t ¯ tγ events with semileptonic anddileptonic decays, as well as the SM background ofsingle-top events and associated production of a W γ +HFis estimated from leading-order (LO) matrix-elementMonte Carlo simulations (MC) event generator
Mad-Graph [22]. Events for all production and decaysof t ¯ t , W W, W Z, and ZZ signals are generated with pythia [23]. The production of W +HF, as well as Z + b ¯ b ,and Z → τ τ decays are generated with alpgen [24].Then the events are processed with the same reconstruc-tion and analysis codes used for the data. Backgroundsfrom ZZ are estimated to be negligible to the t ¯ tγ signa-ture.Initial state radiation is simulated by the pythia shower Monte Carlo simulation code tuned so as to re-produce the underlying event [25]. All of the generatedsamples are then passed through a full simulation of thedetector, then reconstructed with the same reconstruc-tion code used for the data.The expected contributions from t ¯ t , W + HF , single-top, Z + b ¯ b , and Z → τ τ production to the t ¯ t search aregiven in Table I, and the expected contributions from t ¯ tγ and W γ + HF production to the t ¯ tγ search are given inTable II. Additional contributions from misidentificationbackgrounds, described below, are also shown in the Ta-bles. Figure 1 shows kinematic distributions for the t ¯ t sample, and Figs. 2 and 3 show distributions for eventsin the t ¯ tγ sample. There is good agreement between dataand standard-model predictions. We show the data andbackground predictions combined for both electron andmuon events; there is good agreement in both channelsindividually, as shown in [20].High p T photons are copiously produced in hadron jetsinitiated by a scattered quark or gluon. In the t ¯ tγ sam-ple, the number of events in which a jet is misidentifiedas a photon (jet faking photon) is estimated by removingthe isolation requirements on the photon. We fit the re-sulting isolation distribution using signal and backgroundtemplates obtained from data. The signal template isconstructed using electrons from Z /γ ∗ → ee events, anda background template is made from a QCD enrichedsample [20, 26].The expected number of events in which an electronis misidentified as a photon in the t ¯ tγ signature is de-termined by measuring the electron E T spectrum in the ℓ E T b + e +large H T , and ≥ ee or eµ , t ¯ t and di-boson decays), and then multiplying by the probabilitythat an electron is misidentified as a photon. The latteris measured in data using Z /γ ∗ → ee events that aremisreconstructed as Z /γ ∗ → eγ .To estimate the number of b -tagged jets that are inreality mistagged light-quark jets (mistags), each jet inthe ℓγ E T + ≥ H T sample is weighted (GeV) T Lepton E
50 100 150 200 E ve n t s / G e V ) m Data(e+ (7.6 pb)ttMisc (a) (GeV) T E E ve n t s / G e V (b) (GeV) T Jet 1 E
100 200 E ve n t s / G e V (c) (GeV) T H E ve n t s / G e V (d) FIG. 1. The distributions for events in the t ¯ t sample(points) of a) the E T of the lepton; b) the missing transverseenergy E T ; c) the E T of the highest E T jet; and d) the totaltransverse energy H T . The histograms show the expectedSM contributions from top production ( t ¯ t ), andmiscellaneous backgrounds (Misc), which include: dibosonproduction, single top, W +HF and Z +HF production aswell as jets misidentified as leptons (QCD), andmisidentified b tags. TABLE I. Summary for predicted and observed events inthe t ¯ t ( ℓ E T b + H T >
200 GeV + N jets ≥
3) signal sample.
Predicted and Observed t ¯ t EventsSM Source eb E T µb E T ( e + µ ) b E T t ¯ t ±
180 1080 ±
140 2500 ± W W ± ± ± W Z . ± . . ± . . ± . ZZ . ± . . ± . . ± . W bb ±
34 146 ±
24 348 ± W cc ±
23 94 ±
17 221 ± W c ±
13 61 ± ± ±
10 59 ± ± ± ± ± Z → ℓℓ ) b ¯ b ± ± ± Z → τ τ ± ± ± ±
29 214 ±
17 572 ± ±
38 20 ± ± ±
196 1790 ±
146 4420 ± t ¯ t sample, asimilar procedure is used. Each jet in the signal sam-ples has a corresponding probability to be identified as a b -tagged jet. In all cases, however, the resulting predic-tion is overestimated because we count as mistags, eventswhich have true heavy flavor jets ( i.e. events due to t ¯ t events may be mistagged, but they will be accounted for (GeV) T Lepton E
20 40 60 80 100120140160180 E ve n t s / G e V ) m Data(e+ g tt +HF g WMisc (a) (GeV) T E E ve n t s / G e V (b) (GeV) T B-jet E
50 100 150 200 250 E ve n t s / G e V (c) (GeV) T Photon E
20 40 60 80 100 120 E ve n t s / G e V (d) FIG. 2. The distributions for events in the t ¯ tγ sample(points) in a) the E T of the lepton; b) the missing transverseenergy, E T ; c) the E T of the b jet; and d) the E T of thephoton. The histograms show the expected SM contributionsfrom radiative top production ( t ¯ tγ ), W γ production withheavy flavor (HF), and miscellaneous backgrounds (Misc),which include:
W W and
W Z production as well as jets, τ leptons, electrons, and jets misidentified as photons, jetsmisidentified as leptons (QCD), and misidentified b tags. (GeV) T H E ve n t s / G e V ) m Data(e+ g tt +HF g WMisc (a)
N Jets E ve n t s (b) FIG. 3. The distributions for events in the t ¯ tγ sample(points) of a) the total transverse energy H T , the sum of thetransverse energies of the lepton, photon, jets and E T , forthe t ¯ tγ events; b) the total number of jets . The histogramsshow the expected SM contributions from radiativetop-quark pair production ( t ¯ tγ ), W γ production with heavyflavor (
W γ +HF), and miscellaneous backgrounds (Misc),which include: SM
W W and
W Z production as well as jets, τ leptons, electrons, and jets misidentified as photons, jetsmisidentified as leptons (QCD), and misidentified b tags. in the MC). The fraction’s denominator is computed byfinding the total number of ℓγ E T ≥ t ¯ t ana-logue) events. Its numerator is the difference betweenthe denominator and the number of events in the sam-ple with b -tagged jets predicted by MC simulations. Thefraction is the amount of events that have no true heavyflavor content relative to the size of ℓγ E T ≥ t ¯ t analogue) sample; it is used to scale our mistag estimate.This scaled background estimate removes events whichcontain actual heavy flavor content from the mistag to- TABLE II. Summary for t ¯ tγ ( ℓγ E T b + H T >
200 GeV + N jets ≥ e.g. , W W was not generated with an associated photon.)Backgrounds from ZZ are found to be negligible. Predicted and Observed t ¯ tγ Candidate EventsSM Source eγb E T µγb E T ( e + µ ) γb E T t ¯ tγ ( semilep ) 5 . ± .
10 5 . ± .
97 11 . ± . t ¯ tγ ( dilep. ) 1 . ± .
27 1 . ± .
24 2 . ± . W cγ +0.07–0 +0.07–0 +0.09–0 W ccγ +0.05–0 . ± .
05 0 . ± . W bbγ . ± .
07 0 . ± .
05 0 . ± . W Z . ± .
05 0 . ± .
05 0 . ± . W W . ± .
03 0 . ± .
03 0 . ± . . ± .
10 0 ± .
10 0 . ± . . ± .
14 0 . ± .
14 0 . ± . τ → γ fake 0 . ± .
08 0 . ± .
05 0 . ± . γ . ± .
76 1 . ± .
56 7 . ± . . ± .
37 1 . ± .
32 2 . ± . . ± .
38 0 . ± .
02 0 . ± . ee E T b , e → γ . ± .
19 – 0 . ± . µe E T b , e → γ – 0 . ± .
11 0 . ± . . ± . . ± . . ± . E T <
20 GeVare fit to the sum of MC backgrounds and a scaled “non-electron” signal. The QCD background is the sum of thescaled “non-electron” events in the E T >
20 GeV regionexpected from the fit.To avoid double counting, the total background yieldsare corrected by removing the predicted number of eventswith two objects misidentified. Each of the aforemen-tioned data-driven background estimates accounts fora background process where one object in the eventis misidentified. Events with two misidentified objectswould be counted in a pair of background estimates. Inthe t ¯ t sample, double counting is accounted for by re-moving the QCD background from the mistagging back-ground, and vice versa.The background from tau leptons, which decayto hadrons, which decay to photons is a back-ground estimated from the t ¯ t MC sample by selecting τ → hadrons → γ events using MC information.The t ¯ t event detection efficiency and acceptance arecalculated using the MC simulation which has all decaysof t ¯ t . The uncertainty on the t ¯ t cross section is dominatedby systematic uncertainties. The t ¯ tγ event detection effi-ciency and acceptance are calculated using both semilep-tonic, and dileptonic decays of the pair of top quarksin the decay. The total t ¯ tγ cross section is then calcu-lated assuming that t ¯ tγ has the same branching ratio tosemileptonic and dileptonic decays as t ¯ t pair production.The uncertainty in the t ¯ tγ cross section measurement isdominated by statistics.Systematic uncertainties have been calculated by vary-ing detector efficiencies and resolutions within known un-certainties and evaluating the change in our measure-ments. These uncertainties are added in quadraturewhen independent, and summed when positively (or neg-atively) correlated. The largest uncertainties, given indescending order, are due to luminosity, b -hadron taggingefficiencies and, for the t ¯ tγ sample, photon identification.We observe 30 t ¯ tγ candidate events compared toan expectation of 26 . ± .
4. We observe 4429 t ¯ t events, with an expectation of 4420 ± t ¯ t background esti-mate and the number of observed events is due to SM t ¯ t production, we measure the t ¯ t cross section to be7 . ± .
20 (stat) ± .
68 (sys) ± .
46 (lum) pb. Thetheoretical production cross section of t ¯ t at the Tevatronis 7 . +0.00–0.32 +0.36–0.27 pb [27]. The first uncertainty comesfrom scale uncertainty around µ = m top , and the secondis due to parton distribution function uncertainties.If one assumes that t ¯ tγ is not allowed in the SM, andthere are no new physics processes contributing to thissample, the probability that the background events alonewill produce 30 or more events is 0.0015 (3.0 standard de-viations). This is the first experimental evidence for t ¯ tγ production. Assuming the difference between the back-ground estimate and the number of observed events isdue to SM t ¯ tγ production, we measure the t ¯ tγ cross sec-tion to be 0 . ± .
07 (stat) ± .
04 (sys) ± .
01 (lum) pb.The t ¯ tγ event detection efficiency and acceptance are cal-culated using the MC sample requiring at least one W boson decaying leptonically. The acceptance times ef-ficiency, using both semileptonic and dileptonic modes,for this t ¯ tγ signal is 0.015 ± t ¯ tγ (sum of all three lepton flavors) cross sec-tion σ t ¯ tγ = 0 . ± .
011 pb is obtained from lead-ing order (LO)
MadGraph semileptonic cross section σ t ¯ tγ = 0 . σ NLO /σ LO = 0 . t ¯ tγ is thus σ totalt ¯ tγ = 0.17 ± t ¯ tγ and t ¯ t is measured to be ℜ = 0 . ± .
009 which agreeswith the SM prediction of ℜ = 0.024 ± . t ¯ tγ and t ¯ t . When measuring ℜ many of the systematic un-certainties nearly cancel, such as those due to: leptonidentification, b hadron identification, jet energy scale,and luminosity uncertainties. However, other system-atic uncertainties do not cancel out completely such as:QCD systematic uncertainties, and photon identificationand acceptance uncertainties. The total systematic un-certainties combine to less than 10% however the statisti- cal uncertainty is the dominant contribution to the totaluncertainty.In conclusion, we have performed a search for t ¯ tγ ,which is the dominant standard model process that pro-duces the event signature of lepton + photon + E T + b -jets with large total transverse energy and N jets ≥ t ¯ tγ cross section σ t ¯ tγ = 0 . ± .
08 pb, and the ratio of production crosssections of t ¯ tγ to t ¯ t = 0 . ± . Acknowledgments
We thank the Fermilab staff and the technical staffs ofthe participating institutions for their vital contributions.Uli Baur, Frank Petriello, Alexander Belyaev, EdwardBoos, Lev Dudko, Tim Stelzer, and Steve Mrenna wereextraordinarily helpful with the SM predictions. Thiswork was supported by the U.S. Department of Energyand National Science Foundation; the Italian IstitutoNazionale di Fisica Nucleare; the Ministry of Education,Culture, Sports, Science and Technology of Japan; theNatural Sciences and Engineering Research Council ofCanada; the National Science Council of the Republic ofChina; the Swiss National Science Foundation; the A.P.Sloan Foundation; the Bundesministerium f¨ur Bildungund Forschung, Germany; the Korean World Class Uni-versity Program, the National Research Foundation ofKorea; the Science and Technology Facilities Council andthe Royal Society, UK; the Institut National de PhysiqueNucleaire et Physique des Particules/CNRS; the RussianFoundation for Basic Research; the Ministerio de Cien-cia e Innovaci´on, and Programa Consolider-Ingenio 2010,Spain; the Slovak R&D Agency; the Academy of Finland;and the Australian Research Council (ARC). [1] S.L. Glashow, Nucl. Phys. ,094019, (2001).[4] D. Acosta et al. (CDF Collaboration), Phys. Rev. D ,032001 (2005).[5] The CDF coordinate system of r , ϕ , and z is cylindrical,with the z -axis along the proton beam. The pseudora-pidity is η = − ln(tan( θ/ p T = p sin θ and E T = E sin θ ,respectively. We use the convention that “momentum”refers to pc and “mass” to mc . The coordinate y pointsup and down, and x completes the right-handed system.[6] T. Aaltonen et al. (CDF Collaboration), Phys. Rev. D , 011102(R) (2009).[7] K. Nakamura et al. (Particle Data Group), J. Phys. G 37, 075021 (2010).[8] T. Aaltonen et al. (CDF Collaboration), Phys. Rev. Lett , 012001 (2010).[9] A. Sill et al. , Nucl. Instrum. Methods A , 1 (2000); A.Affolder et al. , Nucl. Instrum. Methods A , 84 (2000);C.S. Hill, Nucl. Instrum. Methods A , 1 (2000).[10] A. Affolder et al. , Nucl. Instrum. Methods A , 249(2004).[11] S. Kuhlmann et al. , Nucl. Instrum. Methods A , 39,(2004);S. Bertolucci et al. , Nucl. Instrum. Methods A , 301(1988).[12] F. Abe et al. , (CDF Collaboration) Phys. Rev. Lett. ,1104 (1992).[13] D. Acosta et al. (CDF Collaboration) Phys. Rev. D, ,052003 (2005).[14] The CMU consists of a central barrel of gas proportionalwire chambers in the region | η | < .
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