Article
Reference
Searching the inclusive ℓγE̸
T+ b -quark signature for radiative top quark decay and non-standard-model processes
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We compare the inclusive production of events containing a lepton (ℓ), a photon (γ), significant transverse momentum imbalance (E̸T), and a jet identified as containing a b-quark, to SM predictions. The search uses data produced in proton-antiproton collisions at s√=1.96 TeV corresponding to 1.9 fb−1 of integrated luminosity taken with the CDF detector. We find 28 ℓγbE̸T events versus an expectation of 31.0+4.1−3.5 events. If we further require events to contain at least three jets and large total transverse energy, the largest SM source is radiative top-quark pair production, tt+γ. In the data we observe 16 ttγ candidate events versus an expectation from SM sources of 11.2+2.3−2.1. Assuming the difference between the observed number and the predicted non-top-quark total of 6.8+2.2−2.0 is due to SM top-quark production, we estimate the ttγ cross section to be 0.15±0.08 pb.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Searching the inclusive ℓγE̸
T+ b -quark signature for radiative top quark decay and non-standard-model processes. Physical Review. D , 2009, vol. 80, no. 01, p. 011102
DOI : 10.1103/PhysRevD.80.011102
Available at:
http://archive-ouverte.unige.ch/unige:38628
Disclaimer: layout of this document may differ from the published version.
Searching the inclusive ‘ E 6
Tþ b -quark signature for radiative top quark decay and non-standard-model processes
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Th. Muller,26R. Mumford,25P. Murat,17M. Mussini,6b,6aJ. Nachtman,17,pY. Nagai,51A. Nagano,51J. Naganoma,51 PHYSICAL REVIEW D80,011102(R) (2009)
1550-7998=2009=80(1)=011102(10) 011102-1 Ó 2009 The American Physical Society
K. Nakamura,51I. Nakano,40A. Napier,52V. Necula,16J. Nett,55C. Neu,44,xM. S. Neubauer,24S. Neubauer,26 J. Nielsen,28,hL. Nodulman,2M. Norman,9O. Norniella,24E. Nurse,30L. Oakes,42S. H. Oh,16Y. D. Oh,27I. Oksuzian,18 T. Okusawa,41R. Orava,23K. Osterberg,23S. Pagan Griso,44b,44aE. Palencia,17V. Papadimitriou,17A. Papaikonomou,26
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Y. Zheng,8,eand S. Zucchelli6b,6a
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439
3University of Athens, 157 71 Athens, Greece
4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
5Baylor University, Waco, Texas 76798
6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy;
6bUniversity of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254
7University of California, Davis, Davis, California 95616
8University of California, Los Angeles, Los Angeles, California 90024
9University of California, San Diego, La Jolla, California 92093
10University of California, Santa Barbara, Santa Barbara, California 93106
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637
14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510
18University of Florida, Gainesville, Florida 32611
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138
T. AALTONENet al. PHYSICAL REVIEW D80,011102(R) (2009)
23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland, USA
24University of Illinois, Urbana, Illinois 61801
25The Johns Hopkins University, Baltimore, Maryland 21218
26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
27Center 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
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
33Institute 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, Canada V6T 2A3
34University of Michigan, Ann Arbor, Michigan 48109
35Michigan State University, East Lansing, Michigan 48824
36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
37University of New Mexico, Albuquerque, New Mexico 87131
38Northwestern University, Evanston, Illinois 60208
39The Ohio State University, Columbus, Ohio 43210
40Okayama University, Okayama 700-8530, Japan
41Osaka City University, Osaka 588, Japan
42University of Oxford, Oxford OX1 3RH, United Kingdom
44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy;
44bUniversity of Padova, I-35131 Padova, Italy
43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
44University of Pennsylvania, Philadelphia, Pennsylvania 19104
47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy;
47bUniversity of Pisa, I-56127 Pisa, Italy;
47cUniversity of Siena, I-56127 Pisa, Italy;
47dScuola Normale Superiore, I-56127 Pisa, Italy
45University of Pittsburgh, Pittsburgh, Pennsylvania 15260
46Purdue University, West Lafayette, Indiana 47907
jVisitors from University of Cyprus, Nicosia CY-1678, Cyprus
yVisitors from Bergische Universita¨t Wuppertal, 42097 Wuppertal, Germany
nVisitors from Kinki University, Higashi-Osaka City, Japan 577-8502
mVisitors from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017
lVisitors from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
iVisitors from Cornell University, Ithaca, NY 14853, USA
gVisitors from University of CA Irvine, Irvine, CA 92697, USA
kVisitors from University College Dublin, Dublin 4, Ireland
xVisitors from University of VA, Charlottesville, VA 22904, USA
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zVisitors from On leave from J. Stefan Institute, Ljubljana, Slovenia
fVisitors from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy
dVisitors from University of Bristol, Bristol BS8 1TL, United Kingdom
hVisitors from University of CA Santa Cruz, Santa Cruz, CA 95064, USA
cVisitors from Universiteit Antwerpen, B-2610 Antwerp, Belgium
bVisitors from University of MA Amherst, Amherst, MA 01003, USA
eVisitors from Chinese Academy of Sciences, Beijing 100864, China
sVisitors from Nagasaki Institute of Applied Science, Nagasaki, Japan
pVisitors from University of IA, IA City, IA 52242, USA
oVisitors from Universidad Iberoamericana, Mexico D.F., Mexico
rVisitors from University of Manchester, Manchester M13 9PL, England
aDeceased
vVisitors from TX Tech University, Lubbock, TX 79609, USA
uVisitors from University de Oviedo, E-33007 Oviedo, Spain
SEARCHING THE INCLUSIVE‘6ETþb-. . . PHYSICAL REVIEW D80,011102(R) (2009)
011102-3
47University of Rochester, Rochester, New York 14627
48The Rockefeller University, New York, New York 10021
52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy;
52bSapienza Universita` di Roma, I-00185 Roma, Italy
49Rutgers University, Piscataway, New Jersey 08855
50Texas A & University, College Station, Texas 77843
55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, I-33100 Udine, Italy;
55bUniversity of Trieste/Udine, I-33100 Udine, Italy
51University of Tsukuba, Tsukuba, Ibaraki 305, Japan
52Tufts University, Medford, Massachusetts 02155
53Waseda University, Tokyo 169, Japan
54Wayne State University, Detroit, Michigan 48201
55University of Wisconsin, Madison, Wisconsin 53706
56Yale University, New Haven, Connecticut 06520 (Received 3 June 2009; published 30 July 2009)
We compare the inclusive production of events containing a lepton (‘), a photon (), significant transverse momentum imbalance (6ET), and a jet identified as containing ab-quark, to SM predictions. The search uses data produced in proton-antiproton collisions at ffiffiffi
ps
¼1:96 TeVcorresponding to1:9 fb1of integrated luminosity taken with the CDF detector. We find 28‘b6ET events versus an expectation of 31:0þ4:13:5events. If we further require events to contain at least three jets and large total transverse energy, the largest SM source is radiative top-quark pair production, ttþ. In the data we observe 16 tt candidate events versus an expectation from SM sources of11:2þ2:32:1. Assuming the difference between the observed number and the predicted non-top-quark total of6:8þ2:22:0is due to SM top-quark production, we estimate thettcross section to be0:150:08 pb.
DOI:10.1103/PhysRevD.80.011102 PACS numbers: 13.85.Rm, 12.60.Jv, 13.85.Qk, 14.80.Ly
The unknown nature of possible new phenomena in the energy range accessible at the Tevatron collider is the motivation for a search strategy [1–3] that does not focus on current hypothetical models of new physics, but instead tests the standard model (SM) [4]. The emphasis on pre- senting measurements and SM predictions, rather than comparisons with arbitrarily chosen other models, allows a wide net for physics beyond the SM that can be used now by proponents of current models of ‘‘new physics’’ as well as in the future by theorists with new ideas and facts. Here we report the results of a search for events containing a lepton (‘), a photon (), significant transverse momentum imbalance (6ET) [5], and a jet identified as containing a b-quark; i.e. the final-state ‘b6ETþX. This channel, which contains a vector boson and a third-generation quark, is suppressed in the SM, and is consequently sensi- tive to rare new phenomena. The data correspond to an integrated luminosity of1:9 fb1ofpp collisions at ffiffiffi
ps 1:96 TeV, collected using the CDF II detector [7]. This¼ search is an extension of a previous search in the leptonþ photonþXsignature, described in detail in Ref. [8].
A search for SM production of top-quark pairs with an additional photon, tt , is a natural extension of the
‘b6ETþX analysis. By further requiring events to con- tain at least three jets and large total transverse energy (HT) [5], we find that the SM predicts the largest source of events will be top-quark pair production with an additional radiated photon,ttþ. The process is of interest for the direct measurement of the electric charge of the top quark
[9], as well as being another low-cross section search signature in which rare non-SM processes could appear.
The CDF II detector [7] is a cylindrically-symmetric magnetic spectrometer designed to study pp collisions at the Fermilab Tevatron. Here we briefly describe the detec- tor subsystems relevant for the present analysis.
Tracking systems are used to measure the momenta of charged particles and to identify leptons with large trans- verse momenta [5]. A multilayer system of silicon strip detectors [10], which identifies tracks in both the r and rzviews [6], and the central outer tracker (COT) [11], are contained in a superconducting solenoid that generates a magnetic field of 1.4 T. The COT is a 3.1 m long open-cell drift chamber that makes up to 96 measure- ments along the track of each charged particle in the region jj<1. Sense wires are arranged in 8 alternating axial and 2stereo superlayers with 12 wire layers each. For high- momentum tracks, the COT transverse momentum (pT) resolution ispT=p2T ’0:0017 GeV1 [11].
Segmented calorimeters with towers arranged in a pro- jective geometry, each tower consisting of an electromag- netic and an hadronic compartment [12,13], cover the region jj<3:6. In this analysis we select photons and electrons in the central region, jj<1, where a system (CES) with finer spatial resolution is used to make profile measurements of electromagnetic showers at shower maxi- mum [7]. Electrons are reconstructed in the central elec- tromagnetic calorimeter (CEM) with an ET resolution of ðETÞ=ET ’13:5%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ET=GeV
p 2%[12]. Jets are identi-
T. AALTONENet al. PHYSICAL REVIEW D80,011102(R) (2009)
fied in the electromagnetic and hadronic calorimeters using a cone in-space of radius 0.4 [14,15]. The jet energy resolution is approximately ’0:1ET ðGeVÞ þ 1:0 GeV[16].
Muons are identified using the central muon (CMU), the central muon upgrade (CMP), and the central muon exten- sion (CMX) systems [17,18], which cover the kinematic regionjj<1. The CMU uses four layers of planar drift chambers to detect muons with pT>1:4 GeV in the re- gion of jj<0:6. The CMP consists of four additional layers of planar drift chambers located behind 0.6 m of steel outside the magnetic return yoke, and detects muons withpT>2:0 GeV. The CMX detects muons in the region 0:6<jj<1:0with four to eight layers of drift chambers, depending on the polar angle.
We use identification algorithms that exploit the long lifetime (c0450m) of b hadrons to identify jets containing b hadrons. Candidate b-jets are identified through the presence of a secondary decay vertex displaced from the beam line in the regionjj<2[19].
The beam luminosity is measured using two arrays of gas Cherenkov counters, located in the region3:7<jj<
4:7. The total uncertainty on the luminosity has been estimated to be 6%, where 4.4% comes from the accep- tance and operation of the luminosity monitor and 4.0%
from the calculation of the accepted inelastic pp cross section [20].
We use events selected by the online event selection (trigger) system [7] to have a high pT electron or muon in the central region, jj&1:0. The electron trigger re- quires a cluster of energy in the central electromagnetic calorimeter with a COT track pointing at the cluster. The muon trigger requires a COT track that extrapolates to a track segment in the muon chambers.
Inclusive ‘events are selected by requiring a central high-energycandidate and a central high-energyeor candidate originating less than 60 cm along the beam line from the detector center and passing the selection criteria listed below. To reduce background from the decays of hadrons produced in jets, both the photon and the lepton in each event are required to be isolated [21].
An electron candidate must meet the following selection criteria: (a) a high-quality track [22] with pT>0:5ET, unless ET >100 GeV, in which case thepT threshold is set to 20 GeV; (b) a good transverse shower profile that matches the extrapolated track position; (c) a lateral shar- ing of energy in the calorimeter towers containing the electron shower consistent with that expected; and (d) minimal leakage into the hadron calorimeter [23].
A muon candidate must have: (a) a well-measured track in the COT; (b) energy deposited in the calorimeter con- sistent with expectations; (c) a muon track segment in both the CMU and CMP, or in the CMX, consistent with the extrapolated COT track; and (d) COT timing consistent with a track from app collision.
(GeV) Lepton ET
20 40 60 80 100 120 140 160 180 200
Events/20 GeV
0 2 4 6 8 10 12 14 16 18 20
µ) Data(e+
γ t t
γ+HF W
CDF Run II 1.9 fb-1 (a)
Misc
(GeV) Photon ET
20 40 60 80 100 120 140 160 180 200
Events/10 GeV
0 2 4 6 8 10 12 14 16 18
20 (b)
(GeV) b-jet ET
20 40 60 80 100 120 140 160 180
Events/20 GeV
0 2 4 6 8 10 12 14 16
(c)
(GeV) ET
0 20 40 60 80 100 120 140 160
Events/20 GeV
0 2 4 6 8 10 12 14 16 18 20 22
(d)
FIG. 1 (color online). The distributions for events in the‘b6ETsample (points) in (a) theETof the lepton; (b) theETof the photon;
(c) the ET of the most energeticb-jet in an event; and (d) the missing transverse energy. The histograms show the estimated SM contributions from radiative top quark decay (tt),WZproduction,Wproduction with heavy flavor (HF),leptons, electrons, and jets misidentified as photons, mistagged light-quark and gluon jets, and jets misidentified as leptons (QCD).
SEARCHING THE INCLUSIVE‘6ETþb-. . . PHYSICAL REVIEW D80,011102(R) (2009)
011102-5
Photon candidates are required to have no track with pT>1 GeV, and at most one track with pT<1 GeV, pointing at the calorimeter cluster, good profiles in both transverse dimensions at shower maximum, and minimal leakage into the hadron calorimeter [23].
Missing transverse energy (6ET) is calculated from the calorimeter tower energies in the region jj<3:6. Corrections are then made to the 6ET for the position of the reconstructed primary vertex, and for nonuniform calo- rimeter response [24] for jets with uncorrected ET >
15 GeVandjj<2:0, and for muons withPT >20 GeV. The inclusive‘b6ET search is defined by requiring that an event contain a central electron (or muon) with EeTðPTÞ>20 GeV [5], a central photon with ET >
10 GeV, a b-tagged jet with EjetT >15 GeV, and 6ET >
20 GeV[25]. Figures1and2show kinematic distributions for events in the‘b6ET sample.
The dominant SM sources of ‘b6ET events at the Tevatron are tt production and Wþheavy flavor (HF) (Wc, Wcc , Wbb ) in which a W boson decays leptonically (‘) and a photon is radiated from an initial- state quark, theW, or a charged final-state lepton [26]. We use theMADGRAPHmatrix-element event generator [27] to estimate these contributions. Initial-state radiation and parton showering are simulated by the PYTHIA shower code [28] tuned to reproduce the underlying event [29].
The generated particles are then passed through a full simulation of the detector, and these events are then recon- structed with the same reconstruction code used for the data. The expected contributions fromtt andWþHF production to the ‘b6ET and tt searches are given in TablesIandII. A correction for higher-order processes (K- factor) of1:100:15for thett [25] has been applied to the LO MC estimates. We have also applied a K-factor of 2:101:05 for the WþHF [30]. Backgrounds from WW,ZZ, and the production of a single top quark plus a photon are estimated to be negligible.
The background from top decays in which tau leptons are misidentified as photons is estimated from attPYTHIA [28] sample using simulation information to identify tau
leptons and then applying the same analysis selection criteria as for data.
High-pT photons are copiously created from hadron decays in jets initiated by a scattered quark or gluon. In particular, mesons such as the 0 ordecay to multiple photons which may pass the photon selection criteria. To estimate the number of events with a jet misidentified as a photon, we first measure the probability for a jet to be misidentified as a photon, PjetðETÞ, as a function of the measured EjetT , in data samples triggered on jets. We then measure the jet ET in ‘6ETbþjet and ‘E6 Tþ>3jets (HT>200 GeV) samples, respectively, and multiply by PjetðETÞ. An uncertainty of 50% on the number of such events is calculated by using the measured jet spectrum and the upper and lower bounds on theET-dependent misiden- tification rate [26].
Number of Jets
0 1 2 3 4 5 6 7 8 9
Events
0 2 4 6 8 10 12 14
µ) Data(e+
γ t t
γ+HF W
CDF Run II 1.9 fb-1 (a)
Misc
(GeV) HT
0 50 100 150 200 250 300 350 400 450 500
Events/20 GeV
0 1 2 3 4 5 6 7 8 9
(b)
FIG. 2 (color online). The distributions for events in the‘b6ETsample (points) in (a) the total number of jets; (b) the total transverse energyHT for the‘b6ET events. The histograms show the estimated SM contributions from radiative top-quark decay (tt),WZ production,Wproduction with heavy flavor (HF),leptons, electrons, and jets misidentified as photons, mistagged light-quark and gluon jets, and jets misidentified as leptons (QCD).
TABLE I. Summary for the‘b6ETsearch. Backgrounds from WW, ZZ, and single top quark with an additional radiated photon are found to be negligible.
LeptonþPhotonþ 6ETþbEvents
SM Source eb6ET b6ET ðeþÞb6ET
ttsemileptonic 2:060:38 1:520:28 3:580:65 ttdileptonic 1:300:23 1:020:18 2:320:41 Wc 1:580:83 1:510:80 3:091:59 Wcc 0:170:12 0:460:26 0:630:35 Wbb 1:300:67 0:880:46 2:181:11 ZðÞ 0:130:09 0:110:08 0:240:12 WZ 0:080:04 0:010:01 0:090:04 !fake 0:120:04 0:100:03 0:220:05 Jet faking 4:561:92 3:021:19 7:583:11 Mistaggedb-jets 4:110:41 3:540:37 7:650:70 QCD 1:50:8 0:0þ1:00:0 1:5þ1:30:8 ee6ETb,e! 1:500:28 1:500:28 e6ETb,e! 0:450:10 0:450:10 Predicted 18:42:4ðtotÞ 12:6þ1:91:6ðtotÞ 31:0þ4:13:9ðtotÞ
Observed 16 12 28
T. AALTONENet al. PHYSICAL REVIEW D80,011102(R) (2009)
To estimate the probability to mistakenlyb-tag a light jet (a mistag), each jet in the‘6ETþpretagged jet sample is weighted by its mistag rate that is obtained from tagged events in which theb-hadron decay vertex is measured to be on the opposite side of the primary vertex from the direction of the jet, an unphysical geometry. The mistag rate derived from these ‘‘negative’’ tags provides an esti-
mate of the number of false positive tags after a correction for interactions in material in the inner tracking volume and long-lived light-flavor particles. The mistag rate per jet is measured using a large inclusive-jet data sample.
We have estimated the background due to events with jets misidentified as high-pT leptons by studying the total pT of tracks in a cone in -’ space of radius R¼0:4 around the lepton track (track isolation) [8]. We compared the distribution of track isolation in the signal sample to that of theZ0 !eþeandZ0!þdata samples, and to that of the QCD background data sample, which is dominated by light-flavor and gluon jets.
The number of events with an electron misidentified as a photon expected in the ‘b6ET sample is determined by measuring the electronETspectrum in‘e6ETbsamples, and then multiplying by Pe!, the probability of an electron being misidentified as a photon. We determinePe! from Z0 !eþeevents in which one of the electrons radiates a high-ET photon, resulting in an electron-photon system with an invariant mass consistent with that of theZ-boson.
The uncertainties on the numbers of expected events for the‘b6ET search listed in TablesIandIIinclude system- atic and statistical uncertainties. A total uncertainty of 6%
is quoted for the luminosity measurements [20]. The sys- tematics relevant to the ‘b6ET and tt analyses also include a 5% uncertainty on the b-tagging efficiency, and uncertainties on the K-factors of 15% for the tt MC samples and 50% for the WþHF samples. The largest TABLE II. Summary of the expected SM contributions to the
ttsearch. Backgrounds fromWW,ZZ, single top quark with an additional radiated photon are found to be negligible.
tt
SM Source eb6ET b6ET ðeþÞb6ET
ttsemileptonic 1:970:36 1:470:27 3:440:62 ttdileptonic 0:520:10 0:430:08 0:950:17 Wc 0:0þ0:050 0:0þ0:050 0þ0:070 Wcc 0:0þ0:040 0:030:03 0:03þ0:050:03 Wbb 0:130:08 0:020:02 0:150:09 WZ 0:020:02 0:0þ0:020 0:020:02 !fake 0:080:01 0:020:01 0:100:01 Jet faking 2:371:22 1:420:70 3:791:92 Mistaggedb-jets 0:780:20 0:830:22 1:610:31 QCD 0:50:5 0:0þ1:00:0 0:5þ1:10:5 ee6ETb,e! 0:340:11 0:340:11 e6ETb,e! 0:200:06 0:200:06 Predicted 6:71:4ðtotÞ 4:4þ1:30:8ðtotÞ 11:2þ2:32:1ðtotÞ
Observed 8 8 16
(GeV) Lepton ET
20 40 60 80 100 120 140 160 180 200
Events/20 GeV
0 2 4 6 8 10
µ) Data(e+
γ t t
γ+HF W
CDF Run II 1.9 fb-1 (a)
Misc
(GeV) Photon ET
20 40 60 80 100 120 140 160 180 200
Events/10 GeV
0 2 4 6 8 10 12 14
(b)
(GeV) b-jet ET
20 40 60 80 100 120 140 160 180
Events/20 GeV
0 1 2 3 4 5 6 7 8
(c)
(GeV) ET
0 20 40 60 80 100 120 140 160
Events/20 GeV
0 2 4 6 8
10 (d)
FIG. 3 (color online). The distributions for events in thettsample (points) in (a) theETof the lepton; (b) theETof the photon;
(c) theETof the most energeticb-jet in an event; and (d) the missing transverse energy,6ET. The histograms show the estimated SM contributions from radiative top-quark decay (tt),WZproduction,Wproduction with heavy flavor (HF),leptons, electrons, and jets misidentified as photons, mistagged light-quark and gluon jets, and jets misidentified as leptons (QCD).
SEARCHING THE INCLUSIVE‘6ETþb-. . . PHYSICAL REVIEW D80,011102(R) (2009)
011102-7
experimental systematic uncertainty comes from the rate of misidentifying jets as photons, which we estimate to be uncertain to approximately 50%.
We find 28 ‘b6ET events versus an expectation of 31:0þ4:13:5 events. The data agree well with the SM predic- tions, with the precision of the comparison being limited by statistics for the present integrated luminosity.
A second search, forttevents, is constructed by further requiringHT>200 GeV[31] andNjets>2, whereNjetsis the number of jets in the event [15]. We observe 16tt candidate events. Figures3and4show the corresponding kinematic distributions for events in thett subsample. An event display of attcandidate event is shown in Fig.5.
For the ttsearch, the detection efficiency and accep- tance are calculated using MADGRAPH to generate tt events with one leptonicWdecay. As in the‘b6ETsearch, the generated particles are then passed through a full detector simulation of the detector and are then recon-
structed with the same reconstruction code used for the data. We find a SM expectation of11:2þ2:32:1events.
The probability that the backgrounds alone (i.e. assum- ing that there is no SM production of the ttfinal state) will produce 16 or more events, is 1% (2.3 standard devia- tions). Assuming that the difference between the nontop background estimate and the number of observed events is due tott SM production, we estimate thett cross section to be 0:150:08 pb (see Fig. 6). An estimate of the expected semileptonic cross section ðSMÞ ¼0:080 0:011 pbis obtained from the LOMADGRAPHcross section of 0.073 pb, multiplied by a K-factor (NLO=LO) of 1:100:15[25]. The uncertainty on the cross section is dominated by the statistical uncertainties associated with the small number of events observed.
In conclusion, we have performed a search for events containing a lepton, photon,b-quark production, and miss- ing ET, a channel which contains a vector boson and a third-generation quark and is suppressed in the SM. We find no evidence for non-SM production. As an extension
FIG. 5 (color online). Theplot of attcandidate event, in which the energies deposited in the calorimeter towers are displayed in theplane. The reconstructed top-quark mass is 167 GeV; the photonET is 12 GeV.
(pb) σ
10-1
1 10 102
103
104
(Theory) γ
t t
-1) (CDF Run II 1.9 fb γ
t t
(Theory) t
(CDF Run II) t t
t
(CDF Run II) ν
→ l W
ll (CDF Run II)
→ Z
(Theory) ν
→ l W
ll (Theory)
→ Z
FIG. 6 (color online). Estimate of tt compared with SM expectations and other SM cross sections W!‘, Z!‘þ‘ andtt[32].
Number of Jets
0 1 2 3 4 5 6 7 8 9
Events
0 2 4 6 8 10 12 14
µ) Data(e+
γ t t
γ+HF W
CDF Run II 1.9 fb-1 (a)
Misc
(GeV) HT
0 50 100 150 200 250 300 350 400 450 500
Events/20 GeV
0 1 2 3 4 5 6 7 8
(b)
FIG. 4 (color online). The distributions for events in thettsample (points) in (a) the total number of jets; (b) the total transverse energyHT for the‘b6ET events. The histograms show the estimated SM contributions from radiative top-quark decay (tt),WZ production,Wproduction with heavy flavor (HF),leptons, electrons, and jets misidentified as photons, mistagged light-quark and gluon jets, and jets misidentified as leptons (QCD).
T. AALTONENet al. PHYSICAL REVIEW D80,011102(R) (2009)
of this search we have also performed a search for the SM processpp !tt, which is predicted to be the dominant process that produces this signature with at least 3 jets and large total transverse energy HT. Here too we find good agreement with the SM expectations. Although not statis- tically significant, the number of observed tt events is larger than the SM prediction not includingtt production.
Assuming the difference between the observed number and the predicted non-top-quark SM total is due to top-quark production, we estimate thett cross section to be0:15 0:08 pb.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
Uli Baur, Johan Alwall, Fabio Maltoni, Michel Herquet, Zack Sullivan, Stephen Mrenna, Frank Pietrello, and Tim Stelzer were extraordinarily helpful with the SM predic- tions. We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
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 Bundes- ministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; 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; and the Academy of Finland.
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