Evidence for <em>ttγ</em> production and measurement of σ_{<em>tt</em>γ}/σ_<em>tt</em>

Article

Reference

Evidence for ttγ production and measurement of σ

/σ

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al.

Abstract

Using data corresponding to 6.0  fb−1 of pp collisions at s√=1.96  TeV collected by the CDF II detector, we present a cross section measurement of top-quark pair production with an additional radiated photon in the central region with 10 GeV or more of transverse energy ttγ.

The events are selected by looking for a lepton (ℓ or μ), a photon (γ), significant transverse momentum imbalance (E̸T), large total transverse energy, and three or more jets, with at least one identified as containing a b quark (b). Using an event selection optimized for the ttγ candidate sample, we also measure the cross section of tt (σtt). We measure the ttγ cross section (σttγ) to be 0.18±0.08  pb, and the ratio of σttγ to σtt to be 0.024±0.009. We observe a probability of 0.0015 (3.0 standard deviations) of the background (non-ttγ events alone producing 30 events or more.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Evidence for ttγ production and measurement of σ

/σ

. Physical Review. D , 2011, vol. 84, no. 03, p. 031104

DOI : 10.1103/PhysRevD.84.031104

Available at:

http://archive-ouverte.unige.ch/unige:38707

Disclaimer: layout of this document may differ from the published version.

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Evidence for t t production and measurement of

=

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PHYSICAL REVIEW D84,031104(R) (2011)

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S. S. Yu,¹⁵J. C. Yun,¹⁵A. Zanetti,^52aY. Zeng,¹⁴and S. Zucchelli^6b,6a

(CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.

3University of Athens, 157 71 Athens, Greece

4Institut de Fisica d’Altes Energies, ICREA, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

5Baylor University, Waco, Texas 76798, USA

6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy

6bUniversity of Bologna, I-40127 Bologna, Italy

7University of California, Davis, Davis, California 95616, U.S.A.

8University of California, Los Angeles, Los Angeles, California 90024, U.S.A.

9Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

10Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, U.S.A.

11Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, U.S.A.

aDeceased

bWith visitors from University of Massachusetts, Amherst, Amherst, MA 01003, USA.

cWith visitors from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.

dWith visitors from University of California, Irvine, Irvine, CA 92697, USA.

eWith visitors from University of California, Santa Barbara, Santa Barbara, CA 93106, USA.

fWith visitors from University of California, Santa Cruz, Santa Cruz, CA 95064, USA.

gWith visitors from CERN, CH1211 Geneva, Switzerland.

hWith visitors from Cornell University, Ithaca, NY 14853, USA.

iWith visitors from University of Cyprus, Nicosia CY-1678, Cyprus.

jWith visitors from University College Dublin, Dublin 4, Ireland.

kWith visitors from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017.

lWith visitors from Universidad Iberoamericana, Mexico D. F., Mexico.

mWith visitors from Iowa State University, Ames, IA 50011, USA.

nWith visitors from University of Iowa, Iowa City, IA 52242, USA.

oWith visitors from Kinki University, Higashi-Osaka City, Japan 577-8502.

pWith visitors from Kansas State University, Manhattan, KS 66506, USA.

qWith visitors from University of Manchester, Manchester M13 9PL, England.

rWith visitors from Queen Mary, University of London, London, E1 4NS, England.

sWith visitors from Muons, Inc., Batavia, IL 60510, USA.

tWith visitors from Nagasaki Institute of Applied Science, Nagasaki, Japan.

uWith visitors from National Research Nuclear University, Moscow, Russia.

vWith visitors from University of Notre Dame, Notre Dame, IN 46556, USA.

wWith visitors from Universidad de Oviedo, E-33007 Oviedo, Spain.

xWith visitors from Texas Tech University, Lubbock, TX 79609, USA.

yWith visitors from Universidad Tecnica Federico Santa Maria, 110v Valparaiso, Chile.

zWith visitors from Yarmouk University, Irbid 211-63, Jordan.

aaOn leave from J. Stefan Institute, Ljubljana, Slovenia.

T. AALTONENet al. PHYSICAL REVIEW D84,031104(R) (2011)

031104-2

12Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia

13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

14Duke University, Durham, North Carolina 27708, U.S.A.

15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

16University of Florida, Gainesville, Florida 32611, USA

17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

18University of Geneva, CH-1211 Geneva 4, Switzerland

19Glasgow University, Glasgow G12 8QQ, United Kingdom

20Harvard University, Cambridge, Massachusetts 02138, U.S.A.

21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

22University of Illinois, Urbana, Illinois 61801, U.S.A.

23The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.

24Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany

25Center 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

26Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, U.S.A.

27University of Liverpool, Liverpool L69 7ZE, United Kingdom

28University College London, London WC1E 6BT, United Kingdom

29Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

30Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.

31Institute of Particle Physics McGill University, Montre´al, Que´bec, 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, V6T 2A3, Canada

32University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

33Michigan State University, East Lansing, Michigan 48824, U.S.A.

34Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

35University of New Mexico, Albuquerque, New Mexico 87131, USA

36Northwestern University, Evanston, Illinois 60208, USA

37The Ohio State University, Columbus, Ohio 43210, U.S.A.

38Okayama University, Okayama 700-8530, Japan

39Osaka City University, Osaka 588, Japan

40University of Oxford, Oxford OX1 3RH, United Kingdom

41aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

41bUniversity of Padova, I-35131 Padova, Italy

42LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

43University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A.

44aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy

44bUniversity of Pisa, I-56127 Pisa, Italy

44cUniversity of Siena, I-56127 Pisa, Italy

44dScuola Normale Superiore, I-56127 Pisa, Italy

45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

46Purdue University, West Lafayette, Indiana 47907, U.S.A.

47University of Rochester, Rochester, New York 14627, U.S.A.

48The Rockefeller University, New York, New York 10065, U.S.A.

49aIstituto Nazionale di Fisica Nucleare, I-00185 Roma, Italy

49bSapienza Universita` di Roma, I-00185 Roma, Italy

50Rutgers University, Piscataway, New Jersey 08855, U.S.A.

51Texas A&M University, College Station, Texas 77843, U.S.A.

52aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, I-33100 Udine, Italy

52bUniversity of Trieste/Udine, I-33100 Udine, Italy

53University of Tsukuba, Tsukuba, Ibaraki 305, Japan

54Tufts University, Medford, Massachusetts 02155, U.S.A.

55University of Virginia, Charlottesville, Virginia 22906, U.S.A.

56Waseda University, Tokyo 169, Japan

57Wayne State University, Detroit, Michigan 48201, U.S.A.

EVIDENCE FORttPRODUCTION AND. . . PHYSICAL REVIEW D84,031104(R) (2011)

58University of Wisconsin, Madison, Wisconsin 53706, U.S.A.

59Yale University, New Haven, Connecticut 06520, U.S.A.

(Received 24 June 2011; published 31 August 2011) Using data corresponding to 6:0 fb¹ of pp collisions at ffiffiffi

ps

¼1:96 TeV collected by the CDF II detector, we present a cross section measurement of top-quark pair production with an additional radiated photon in the central region with 10 GeV or more of transverse energytt. The events are selected by looking for a lepton (‘or), a photon (), significant transverse momentum imbalance (6E_T), large total transverse energy, and three or more jets, with at least one identified as containing abquark (b). Using an event selection optimized for thettcandidate sample, we also measure the cross section oftt(_tt). We measure thettcross section (tt) to be0:180:08 pb, and the ratio oftttottto be0:0240:009. We observe a probability of 0.0015 (3.0 standard deviations) of the background (non-ttevents alone producing 30 events or more.

DOI:10.1103/PhysRevD.84.031104 PACS numbers: 13.85.Rm, 12.60.Jv, 13.85.Qk, 14.80.Ly

The standard model (SM) [1] of particle physics makes successful predictions of the production rates of physics processes that span many orders of magnitude. Data from pp collisions collected at the Tevatron have been used to verify many of these predictions [2]. As a test of the SM, we measure the ratio of production cross sections ofttto tt. The ratio allows for the cancellation of systematic effects, and is a more sensitive test of the SM than the measurement of the production cross section oftt alone.

While current data is not sufficient to study them in detail, the tt coupling parameters are sensitive to some new physics models [3], and will be better measured in the future.

Top quarks are dominantly produced in pairs, with both top quarks decaying to a W boson and a b quark nearly 100% of the time. Their decays are classified as dileptonic if bothWbosons decay to leptons, semileptonic if only one W boson decays to leptons, and hadronic if neither W boson decays to leptons. Selection for thettevents in a semileptonic channel (including leptonic decays) was performed using 6:0 fb¹ of integrated luminosity from ppcollisions at ffiffiffi

ps

¼1:96 TeVcollected using the CDF II detector [4]. In order to isolate nonhadronic tt produc- tion, we require a high-transverse-momentum (p_T) [5]

lepton (‘) identifed as either an electron (e) or a muon (), a photon (), ab-tagged jet (b), missing transverse energy (6ET), large total transverse energy (HT), and three or more jets. With these selection criteria,tt dominates SM predictions [6]. The total transverse energy,H_T, is the scalar sum of the transverse energy of electrons, muons, jets, photons, and6E_T identified in the event. Furthermore, we select top-quark pair production (tt) events by using nearly the same selection as tt , but without the photon requirement. Using similar event selection ensures that many systematic uncertainties cancel when we measure the cross section ratio oftttott.

The semileptonic cross section oftthas been measured to be0:150:08 pbusing data corresponding to an inte- grated luminosity of 1:9 fb¹ [6]. Using the branching

ratio ofW decays to leptons (0.324) [7], this corresponds to a production cross section of0:340:18 pb. The cross section ofttproduction is well measured at7:700:52 pb [8]. However, a measurement of both tt and tt cross sections with similar event selection has not been performed.

The CDF II detector is a cylindrically symmetric mag- netic spectrometer designed to studypp collisions at the Fermilab Tevatron. Here we briefly describe the compo- nents relevant for this analysis. Tracking systems are used to measure the momenta of charged particles and to assist lepton identification. A multilayer system of silicon strip detectors [9] which identifies tracks in therandrz views [5], and the central outer tracker (COT) [10], are contained in a superconducting solenoid that generates a 1.4 T magnetic field. The COT is a 3.1 m long open-cell drift chamber capable of making up to 96 measurements of each charged particle in the pseudorapidity regionjj<1 [5]. Sense wires are arranged in 8 alternating axial and2 stereo superlayers with 12 wires each. For high-momentum tracks, the COT transverse momentum resolution is pT=p²_T ’0:0015 GeV¹.

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 jj<3:6. In this analysis, we select photons and electrons from the central region,jj&1:0, where the central elec- tromagnetic shower system (CES) makes profile measure- ments at shower maximum with finer spatial resolution than the calorimeter. Electrons are reconstructed in the central electromagnetic calorimeter (CEM) with an ET

[5] resolution of ðE_TÞ=E_T ’13:5%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E_T=GeV

p 1:7%.

Jets are identified using the hadronic and electromagnetic calorimeter using a cone in space of radius R¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðÞ²þ ðÞ²

p ¼0:4. The jet energy resolution is approximately ’0:1ETðGeVÞ þ1 GeV [12] (i.e., 2.5 GeV for a 15 GeV jet).

Jets containing a hadron with a b quark (b hadrons) are identified by exploiting the long b-hadron lifetime

T. AALTONENet al. PHYSICAL REVIEW D84,031104(R) (2011)

031104-4

(c_b450m). The tracks originating from the resulting displaced vertex are used by theSECVTX[13] algorithm to identify thebhadron. The algorithm works in the region jj<2, defined by the silicon system coverage. Jets that are identified as coming fromb hadrons are said to beb tagged.

Muons () are identified using the central muon (CMU), the central muon upgrade (CMP), and the central muon extension (CMX) systems [14], which cover the detector regionjj<1.

Luminosity is measured using C˘ erenkov luminosity counters in the range3:7<jj<4:7. The uncertainty in the luminosity has been estimated to be 6%, where 4.4%

comes from the acceptance and operation of the luminosity monitor, and 4% comes from the uncertainty on the inelas- tic cross section ofpp [15].

A three-level online event selection system (trigger) [4]

selects events with a high-pT 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 a COT track associated with the clusters. The muon trigger requires a COT track that extrapolates to a track segment (‘‘stub’’) in the muon detectors.

A muon candidate passing our selection criteria must have a well-measured track in the COT, energy deposited in the calorimeter consistent with minimum-ionization expectations, a muon stub in both the CMU and CMP, or in the CMX, consistent with the extrapolated COT track, and COT timing consistent with a track from a pp collision.

An electron candidate passing our selection criteria must have a high-quality track with pT >0:5ET, unless ET >

100 GeV, in which case thepTthreshold is set to 25 GeV, a good transverse shower profile that matches the extrapo- lated track position, a lateral sharing of energy in the two calorimeter towers containing the electron shower consis- tent with that expected for an electromagnetic (EM) shower, and minimal leakage into the hadron calorimeter [16].

Photon candidates are required to haveE_T >10 GeV, no track withp_T >1 GeVand at most one track withp_T <

1 GeV, pointing at the EM cluster, good profiles in both transverse dimensions at shower maximum, and minimal leakage into the hadron calorimeter [16]. The detected tracks have a minimum p_T of 0.35 GeV due to the mag- netic field curling up lowerp_T particles. The photons are only reconstructed in the CEM and havejj<1:0.

To reduce background from photons or leptons that originate from decays of hadrons produced in jets, both the photon and the lepton in each event are required to be

‘‘isolated.’’ TheETdeposited in the calorimeter towers in a cone in’space [5] ofR¼0:4around the photon or lepton position is summed, and theETdue to the photon or lepton is subtracted. The remaining ET is required to be less than2:0 GeVþ0:02 ðET20 GeVÞfor a photon, or less than 10% of theE_Tfor electrons orp_Tfor muons. In

addition, for photons, the sum of thep_Tof all tracks in the cone must be less than2:0 GeVþ0:005E_T.

Missing transverse energy, 6E_T, is calculated from the observed calorimeter-tower energies in the region jj<3:6 [17]. Corrections are then made to the 6E_T for nonuniform calorimeter response [18] for jets with uncor- rected ET >15 GeV and <2, and for muons with pT>20 GeV.

Events for the analysis are selected by requiring a central e or with E^‘_T>20 GeV originating less than 60 cm along the beam line from the detector center and passing the criteria listed above. We further require events to have at least one of the following objects: a jet with E^jet_T >

15 GeV, 6E_T >20 GeV, an additional lepton, or a central withE_T>10 GeV.

The first measurement we perform is in the tt signal sample, which requires an event to contain6E_T>20 GeV, a lepton, a b-tagged jet, H_T>200 GeV, N_jets 3 [19]

(including the b-tagged jet), and transverse mass of the lepton and 6E_T to be greater than 20 GeV for the electron channel, and 10 GeV for the muon channel. Transverse massffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffifor the 6E_T and lepton is defined as

2ðE_‘;T 6E_TE_‘;x 6E_TxE_‘;y 6E_TyÞ q

. The selection criteria is inclusive, so if an event contains an additional lepton or a photon it is also accepted as a signal event. The highest-pT lepton determines if the event is an electron or muon event.

Events in thettsignal sample are selected by requiring 6E_T>20 GeV, a lepton, a b-tagged jet, H_T>200 GeV, N_jets3(including theb-tagged jet), and a photon with ET>10 GeV. For all photons we require the²of the CES shower profile be less than 20. To further suppress back- grounds, photons with ET between 10 and 25 GeV must have a²of the CES shower profile less than 6; we discuss how² is calculated below. Similar to thettanalysis, the selection is inclusive. The selection criteria are identical to the previous ttcross section measurement [6], with the exception of the low-E_Tphoton²requirements.

The primary difference between thettandttselection, other than the photon selection, is the requirement of a transverse mass selection fortt. In thettselection, the low- transverse-mass region is not well-modeled with back- ground estimation methods. Thett sample does not suffer from this deficiency, and we do not use the transverse mass selection criterion to keep acceptance of signal events high, this behavior is also seen in the ‘6ET control sample described below.

Control samples are identified by selecting events with a lepton, a photon, and6E_T>20 GeV(‘6E_T), or two oppo- sitely charged same-flavor central leptons, a photon, and a three-body mass consistent with theZboson (‘‘). These control samples are used to define the above CES ² selection for photons.

The ² value of photons is based on the lateral shower shapes observed in the CES compared to that predicted

EVIDENCE FORttPRODUCTION AND. . . PHYSICAL REVIEW D84,031104(R) (2011)

from a sample of test-beam electrons. Using the control samples, we identify an additional selection criterion on photons [20]. It should be noted that the tt sample contains 30 events and is a subset of the 8276 events in the ‘6E_T sample. The ‘‘ sample contains 1344 events. While the samples are not independent, optimizing photon identification selection criteria using the ‘6E_T sample should be minimally affected by the presence of tt events.

The dominant SM sources of events with a lepton, photon, and significant6ET, not including particle misiden- tifications, are tt production and Wþheavy flavor (HF), in which aW boson decays leptonically (‘) and a photon is radiated from an initial-state or final-state quark, the W boson, or a charged final-state lepton [21]. In this paper, HF includescc,bb, andc. Similarly, for events in thettselection, the dominant source of events is due tott production andWþHFproduction.

The production of tt events with semileptonic and dileptonic decays, as well as the SM background of single-top events and associated production of a WþHF is estimated from leading-order (LO) matrix- element Monte Carlo (MC) simulations event generator

MADGRAPH[22]. Events for all production and decays oftt, WW,WZ, andZZsignals are generated withPYTHIA[23].

The production of WþHF, as well as Zþbb, and Z! decays are generated with ALPGEN [24]. Then the events are processed with the same reconstruction and analysis codes used for the data. Backgrounds from ZZare estimated to be negligible to thettsignature.

Initial-state radiation is simulated by thePYTHIAshower MC simulation code tuned so as to reproduce the under- lying event [25]. All of the generated samples are then passed through a full simulation of the detector, then reconstructed with the same reconstruction code used for the data.

The expected contributions from tt, WþHF, single- top, Zþbb, and Z! production to the ttsearch are given in TableI, and the expected contributions fromtt and WþHF production to the tt search are given in Table II. Additional contributions from misidentification backgrounds, described below, are also shown in the Tables. Figure1 shows kinematic distributions for the tt sample, and Figs.2and3show distributions for events in thett sample. There is good agreement between data and SM predictions. We show the data and background pre- dictions combined for both electron and muon events; there is good agreement in both channels individually, as shown in [20].

Highp_T photons are copiously produced in hadron jets initiated by a scattered quark or gluon. In thettsample, the number of events in which a jet is misidentified as a photon (jet faking photon) is estimated by removing the isolation requirements on the photon. We fit the resulting isolation distribution using signal and background

templates obtained from data. The signal template is con- structed using electrons from Z⁰=!ee events, and a background template is made from a QCD-enriched sam- ple [20,26].

The expected number of events in which an electron is misidentified as a photon in thett signature is determined by measuring the electronET spectrum in the‘6ETbþeþ largeHT, and 3 jets sample (events of this type are TABLE I. Summary for predicted and observed events in thett (‘6E_TbþH_T>200 GeVþNjets3) signal sample.

Predicted and ObservedttEvents

SM Source eb6E_T b6E_T ðeþÞb6E_T

tt 1420180 1080140 2500330

WW 294 223 517

WZ 8:61:1 6:50:9 15:12:0

ZZ 1:30:2 1:00:1 2:30:3

Wbb 20334 14624 34858

Wcc 12723 9417 22140

Wc 8513 619 14723

Single-top (s-ch.) 7610 598 13518 Single-top (t-ch.) 669 507 11616 ðZ!‘‘Þbb 313 222 535

Z! 68 98 1411

Mistags 35829 21417 57246

QCD 22238 203 24040

Total Predicted 2630196 1790146 4420340

Observed 2720 1709 4429

TABLE II. Summary for tt (‘6E_TbþH_T>200 GeVþ Njets3). Monte Carlo samples listed in the table are given as they were generated (e.g., WW was not generated with an associated photon.) Backgrounds from ZZ are found to be negligible.

Predicted and ObservedttCandidate Events

SM Source eb6ET b6ET ðeþÞb6ET

tt(semilep.) 5:981:10 5:210:97 11:192:04 tt(dilep.) 1:470:27 1:270:24 2:740:50 Wc 0^þ0:07₀ 0^þ0:07₀ 0^þ0:09₀ Wcc 0^þ0:05₀ 0:050:05 0:050:07 Wbb 0:150:07 0:060:05 0:210:08 WZ 0:050:05 0:050:05 0:090:06 WW 0:060:03 0:060:03 0:110:03 Single-Top (s-chan) 0:090:10 00:10 0:090:13 Single-Top (t-chan) 0:140:14 0:130:14 0:270:19 !fake 0:200:08 0:100:05 0:290:09 Jet faking 5:751:76 1:791:56 7:542:53 Mistags 1:470:37 1:020:32 2:500:51 QCD 0:380:38 0:020:02 0:400:38 ee6E_Tb,e! 0:940:19 0:940:19 e6E_Tb,e! 0:490:11 0:490:11 Total Predicted 16:72:2 10:31:9 26:93:4

Observed 17 13 30

T. AALTONENet al. PHYSICAL REVIEW D84,031104(R) (2011)

031104-6

created from dileptonic,eeore,ttand diboson decays), and then multiplying by the probability that an electron is misidentified as a photon. The latter is measured in data using Z⁰= !ee events that are misreconstructed as Z⁰=!e.

To estimate the number of b-tagged jets that are in reality mistagged light-quark jets (mistags), each jet in

the ‘6ETþ 3jets and high-HT sample is weighted by a mistag rate. The mistag rate per jet is measured using a large inclusive-jet sample. For the tt sample, a similar procedure is used. Each jet in the signal samples has a corresponding probability to be identified as ab-tagged jet.

In all cases, however, the resulting prediction is overesti- mated because we count as mistags, events which have true heavy flavor jets (i.e., events due to tt events may be mistagged, but they will be accounted for in the MC).

The fraction’s denominator is computed by finding the total number of‘6E_T 3jets (orttanalogue) events. Its numerator is the difference between the denominator and the number of events in the sample with b-tagged jets predicted by MC simulations. The fraction is the amount of events that have no true heavy flavor content relative to the size of‘6E_T 3jets (orttanalogue) sample; it is used to scale our mistag estimate. This scaled background esti- mate removes events which contain actual heavy flavor content from the mistag total; the scale factor is nearly 60%.

The background due to events in which a jet is misidentified as a lepton (QCD) is estimated using

‘‘nonelectrons.’’ Nonelectrons are jets which are kinemati- cally similar to electrons, but which fail a pair of selection criteria normally passed by electrons (regardless of energy) such as EM shower-shape, energy over momentum, or isolation. Nonelectrons play the role of leptons in our model of the QCD background. The remaining object selection is unchanged. In each signature, a template of signal events in data with 6ET <20 GeV are fit to the sum of MC backgrounds and a scaled nonelectron signal. The QCD background is the sum of the scaled nonelectron events in the 6ET >20 GeV region expected from the fit.

To avoid double counting, the total background yields are corrected by removing the predicted number of events with two objects misidentified. Each of the aforementioned

(GeV) Lepton ET

50 100 150 200

Events/10 GeV ²⁰⁰

400 600

800 Data(e+µ)

(7.6 pb) t t Misc

(a)

(GeV) ET

0 100 200 300 400 500

Events/20 GeV

500 1000 1500

(b)

(GeV) Jet 1 ET

100 200

Events/20 GeV

500 1000

(c)

(GeV) HT

0 200 400 600 800 1000

Events/50 GeV

500 1000

(d)

FIG. 1 (color online). The distributions for events in the tt sample (points) of (a) the E_T of the lepton; (b) the missing transverse energy6E_T; (c) theE_Tof the highestE_Tjet; and (d) the total transverse energyH_T. The histograms show the expected SM contributions from top production (tt), and miscellaneous backgrounds (Misc), which include diboson production, single- top,WþHF, andZþHFproduction as well as jets misidenti- fied as leptons (QCD), and misidentifiedbtags.

(GeV) HT

0 100 200 300 400 500 600 700 800

Events/50 GeV

0 5

10 Data(e+µ)

γ t t

γ+HF W Misc

(a)

N Jets

0 2 4 6 8 10

Events

0 5 10

15 (b)

FIG. 3 (color online). The distributions for events in the tt sample (points) of (a) the total transverse energyH_T, the sum of the transverse energies of the lepton, photon, jets and6E_T, for the ttevents; (b) the total number of jets. The histograms show the expected SM contributions from radiative top-quark pair pro- duction (tt),Wproduction with heavy flavor (WþHF), and miscellaneous backgrounds (Misc), which include SMWW and WZ production as well as jets, leptons, electrons, and jets misidentified as photons, jets misidentified as leptons (QCD), and misidentifiedbtags.

(GeV) Lepton ET

20 40 60 80 100120140160180

Events/20 GeV

0 5 10 15

µ) Data(e+

γ t t

γ+HF W Misc

(a)

(GeV) ET

0 50 100 150 200 250 300 350 400

Events/20 GeV

0 5 10

(b)

(GeV) B-jet ET

50 100 150 200 250

Events/20 GeV

0 5 10

(c)

(GeV) Photon ET

20 40 60 80 100 120

Events/10 GeV

0 5 10 15

20 (d)

FIG. 2 (color online). The distributions for events in the tt sample (points) in (a) the E_T of the lepton; (b) the missing transverse energy,6E_T; (c) theE_T of thebjet; and (d) theE_T of the photon. The histograms show the expected SM contributions from radiative top production (tt),Wproduction with heavy flavor (HF), and miscellaneous backgrounds (Misc), which in- clude WW and WZ production as well as jets, leptons, electrons, and jets misidentified as photons, jets misidentified as leptons (QCD), and misidentifiedbtags.

EVIDENCE FORttPRODUCTION AND. . . PHYSICAL REVIEW D84,031104(R) (2011)

data-driven background estimates accounts for a back- ground process where one object in the event is misidenti- fied. Events with two misidentified objects would be counted in a pair of background estimates. In thettsample, double counting is accounted for by removing the QCD background from the mistagging background, and vice versa.

The background from tau leptons, which decay to had- rons, which decay to photons is a background estimated from the tt MC sample by selecting !hadrons! events using MC information.

The tt event detection efficiency and acceptance are calculated using the MC simulation which has all decays oftt. The uncertainty on thettcross section is dominated by systematic uncertainties. Thett event detection effi- ciency and acceptance are calculated using both semilep- tonic, and dileptonic decays of the pair of top quarks in the decay. The totaltt cross section is then calculated assum- ing thattthas the same branching ratio to semileptonic and dileptonic decays as tt pair production. The uncer- tainty in thett cross section measurement is dominated by statistics.

Systematic uncertainties have been calculated by vary- ing detector efficiencies and resolutions within known uncertainties and evaluating the change in our measure- ments. These uncertainties are added in quadrature when independent, and summed when positively (or nega- tively) correlated. The largest uncertainties, given in descending order, are due to luminosity, b-hadron tagging efficiencies and, for the tt sample, photon identification.

We observe 30 tt candidate events compared to an expectation of 26:93:4. We observe 4429 tt events, with an expectation of4420340. Assuming the differ- ence between the non-tt background estimate and the number of observed events is due to SM tt production, we measure thettcross section to be7:620:20ðstatÞ 0:68ðsysÞ 0:46ðlumÞpb. The theoretical production cross section of tt at the Tevatron is 7:08^þ0:00_0:32^þ0:36_0:27 pb [27]. The first uncertainty comes from scale uncertainty around¼mtop, and the second is due to parton distri- bution function uncertainties.

If one assumes thattt is not allowed in the SM, and there are no new physics processes contributing to this sample, the probability that the background events alone will produce 30 or more events is 0.0015 (3.0 standard deviations). This is the first experimental evidence fortt production. Assuming the difference between the back- ground estimate and the number of observed events is due to SMttproduction, we measure thettcross section to be 0:180:07ðstatÞ 0:04ðsysÞ 0:01ðlumÞ pb. The tt event detection efficiency and acceptance are calcu- lated using the MC sample requiring at least oneWboson decaying leptonically. The acceptance times efficiency, using both semileptonic and dileptonic modes, for this

tt signal is 0:0150:002. The uncertainty on the mea- sured cross section is dominated by the statistical uncer- tainties associated with the small number of events observed. A theoretical value for the nonhadronic decays oftt(sum of all three lepton flavors) cross section_tt¼ 0:0710:011 pbis obtained from the leading-order (LO)

MADGRAPH semileptonic cross section _tt¼0:0726 pb multiplied by a K-factor to find the next to leading-order (NLO); K-factor ¼_NLO=_LO¼0:977[28]. The next to leading-order theoretical total cross section forttis thus ^total_tt ¼0:170:03 pb.

The ratio between the production cross sections oftt andttis measured to be< ¼0:0240:009which agrees with the SM prediction of < ¼0:0240:005, obtained from theoretical predictions of the cross sections of tt andtt. When measuring<many of the systematic uncer- tainties nearly cancel, such as those due to lepton identi- fication, b hadron identification, jet energy scale, and luminosity uncertainties. However, other systematic uncer- tainties do not cancel out completely such as QCD system- atic uncertainties, and photon identification and acceptance uncertainties. The total systematic uncertainties combine to less than 10%, however, the statistical uncertainty is the dominant contribution to the total uncertainty.

In conclusion, we have performed a search for tt , which is the dominant SM process that produces the event signature of leptonþphotonþE_Tþb-jets with large to- tal transverse energy andNjets3. We find that the num- bers of events observed are consistent with SM predictions.

We obtain att cross sectiontt¼0:180:08 pb, and the ratio of production cross sections of tt to tt¼ 0:0240:009.

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.

Uli Baur, Frank Petriello, Alexander Belyaev, Edward Boos, Lev Dudko, Tim Stelzer, and Steve Mrenna were extraordinarily helpful with the SM predictions. This work was supported by the U. S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P.

Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean World Class University Program, the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, UK; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; the Academy of Finland; and the Australian Research Council (ARC).

T. AALTONENet al. PHYSICAL REVIEW D84,031104(R) (2011)

031104-8

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EVIDENCE FORttPRODUCTION AND. . . PHYSICAL REVIEW D84,031104(R) (2011)