Article
Reference
Measurement of the tt production cross section in pp collisions at s√=1.96 TeV using lepton plus jets events with semileptonic B decays
to muons
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al .
Abstract
We present a measurement of the tt¯ production cross section using 194 pb−1 of CDF II data using events with a high transverse momentum electron or muon, three or more jets, and missing transverse energy. The measurement assumes 100% t→Wb branching fraction.
Events consistent with tt decay are found by identifying jets containing heavy-flavor semileptonic decays to muons. The dominant backgrounds are evaluated directly from the data. Based on 20 candidate events and an expected background of 9.5±1.1 events, we measure a production cross section of 5.3±3.3+1.3−1.0 pb, in agreement with the standard model.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the tt production cross section in pp collisions at s√=1.96 TeV using lepton plus jets events with semileptonic B decays to muons. Physical Review. D , 2005, vol. 72, no. 03, p. 032002
DOI : 10.1103/PhysRevD.72.032002
Available at:
http://archive-ouverte.unige.ch/unige:38296
Disclaimer: layout of this document may differ from the published version.
Measurement of the t t production cross section in p p collisions at p s
1:96 TeV using lepton plus jets events with semileptonic B decays to muons
D. Acosta,16J. Adelman,12T. Affolder,9T. Akimoto,54M. G. Albrow,15D. Ambrose,15S. Amerio,42D. Amidei,33 A. Anastassov,50K. Anikeev,15A. Annovi,44J. Antos,1M. Aoki,54G. Apollinari,15T. Arisawa,56J.-F. Arguin,32 A. Artikov,13W. Ashmanskas,15A. Attal,7F. Azfar,41P. Azzi-Bacchetta,42N. Bacchetta,42H. Bachacou,28W. Badgett,15 A. Barbaro-Galtieri,28G. J. Barker,25V. E. Barnes,46B. A. Barnett,24S. Baroiant,6G. Bauer,31F. Bedeschi,44S. Behari,24
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F. Zetti,44J. Zhou,50and S. Zucchelli4
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
5Brandeis University, Waltham, Massachusetts 02254, USA
6University of California at Davis, Davis, California 95616, USA
7University of California at Los Angeles, Los Angeles, California 90024, USA
8University of California at San Diego, La Jolla, California 92093, USA
9University of California at Santa Barbara, Santa Barbara, California 93106, USA
10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
14Duke University, Durham, North Carolina 27708
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, USA
21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00044, Helsinki, Finland
22Hiroshima University, Higashi-Hiroshima 724, Japan
23University of Illinois, Urbana, Illinois 61801, USA
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
27Center for High Energy Physics, Kyungpook National University, Taegu 702-701 Korea;
Seoul National University, Seoul 151-742 Korea;
and SungKyunKwan University, Suwon 440-746 Korea
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
32Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
33University of Michigan, Ann Arbor, Michigan 48109, USA
34Michigan State University, East Lansing, Michigan 48824, USA
35Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
36University of New Mexico, Albuquerque, New Mexico 87131, USA
37Northwestern University, Evanston, Illinois 60208, USA
38The Ohio State University, Columbus, Ohio 43210, USA
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, University of Padova, I-35131 Padova, Italy
43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
44Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
46Purdue University, West Lafayette, Indiana 47907, USA
47University of Rochester, Rochester, New York 14627, USA
48The Rockefeller University, New York, New York 10021, USA
49Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University di Roma ‘‘La Sapienza’’, I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52Texas Tech University, Lubbock, Texas 79409, USA
53Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
54University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Tufts University, Medford, Massachusetts 02155, USA
56Waseda University, Tokyo 169, Japan
57Wayne State University, Detroit, Michigan 48201, USA
58University of Wisconsin, Madison, Wisconsin 53706, USA
59Yale University, New Haven, Connecticut 06520, USA (Received 1 June 2005; published 9 August 2005)
We present a measurement of thettproduction cross section using194 pb1 of CDF II data using events with a high transverse momentum electron or muon, three or more jets, and missing transverse energy. The measurement assumes 100%t!Wbbranching fraction. Events consistent withttdecay are found by identifying jets containing heavy-flavor semileptonic decays to muons. The dominant back- grounds are evaluated directly from the data. Based on 20 candidate events and an expected background of 9:51:1 events, we measure a production cross section of 5:33:31:31:0pb, in agreement with the standard model.
DOI:10.1103/PhysRevD.72.032002 PACS numbers: 13.85.Ni, 13.85.Qk, 14.65.Ha
I. INTRODUCTION
Top quark pair production in the standard model pro- ceeds via either quark-antiquark annihilation or gluon- gluon fusion. At the Fermilab Tevatron collider, with a center-of-mass energy of 1.96 TeV , quark-antiquark anni- hilation is expected to dominate. For a top mass of
175 GeV=c2, the calculated cross section is 6:70:70:9pb [1], and is approximately 0.2 pb smaller for each 1 GeV=c2 increase in the value of the top mass over the range170 GeV=c2< Mtop<190 GeV=c2.
Measurements of the cross section for top quark pair production provide a test of QCD, as well as the standard MEASUREMENT OF THEttPRODUCTION CROSS. . . PHYSICAL REVIEW D72,032002 (2005)
model decay t!Wb. Nonstandard model production mechanisms, such as the production and decay of a heavy resonance into tt pairs [2], could enhance the measured cross section. Nonstandard model top quark decays, such as the decay into supersymmetric particles [3], could sup- press the measured value, for which at!Wbbranching fraction of nearly 100% is assumed.
In this paper we report a measurement of thettproduc- tion cross section inpp collisions atps
1:96 TeVwith the CDF II detector at the Fermilab Tevatron. The standard model decayt!Wbof the top quark results in a final state from tt production of two W bosons and two bottom quarks. We select events consistent with a decay of one of theW bosons to an electron or muon plus a neutrino, both of which have large momentum transverse to the beam direction (PT). We refer to these highPT electrons or muons as the ‘‘primary lepton.’’ The neutrino is unde- tected and results in an imbalance in transverse momen- tum. The imbalance is labeled ‘‘missingET’’ (E6 T) since it is reconstructed based on the flow of energy in the calo- rimeter [4]. The otherWboson in the event decays to a pair of quarks. The two quarks from theWboson and the twob quarks from the top decays hadronize and are observed as jets of charged and neutral particles. This mode is referred as W plus jets. We take advantage of the semileptonic decay of b hadrons to muons to identify final-state jets that result from hadronization of the bottom quarks. Such a technique, called ‘‘soft-lepton tagging’’ (SLT), is effective in reducing the background to thettsignal fromW boson produced in association with several hadronic jets with large transverse momentum. The production cross section is measured in events with three or more jets and at least one SLT tagged jet.
This measurement is complementary to other measure- ments from CDF II, which use secondary vertex tagging, kinematic fitting, or a combination of the two [5–7]. A forthcoming paper [8] will present a combined cross sec- tion measurement based on the result of these four analyses.
Previous measurements [9] from run I at the Tevatron have measured production cross sections statistically con- sistent with the standard model prediction. This and other run II measurements are made at a slightly higher center- of-mass energy (1.96 TeV vs 1.8 TeV) and with nearly twice as much integrated luminosity.
The organization of this paper is as follows: Section II reviews the detector systems relevant to this analysis. The trigger and event selection, the data and the Monte Carlo samples and the SLT tagging algorithm are described in Sec. III. The estimate of the background is presented in Sec. IV. The acceptance and thettevent tagging efficiency are described in Sec. V. The evaluation of the systematic uncertainties on the measurement is presented in Sec. VI.
Thettproduction cross section measurement and the con- clusions are presented in Secs. VII and VIII.
II. THE CDF II DETECTOR
The CDF II detector is described in detail in [10], only the components relevant to this measurement are summa- rized here. The CDF II detector is a nearly azimuthally and forward-backward symmetric detector designed to study pp interactions at the Fermilab Tevatron. It consists of a magnetic spectrometer surrounded by calorimeters and muon chambers. An elevation view of the CDF II detector is shown in Fig. 1.
Charged particles are tracked inside a 1.4 T solenoidal magnetic field by an 8-layer silicon strip detector covering radii from 1.5 to 28 cm, followed by the central outer tracker (COT), an open-cell drift chamber that provides up to 96 measurements of charged-particle position over the radial region from 40 to 137 cm. The 96 COT mea- surements are arranged in 8 ‘‘superlayers’’ of 12 sense wires each alternating between axial and 2 stereo. The COT and the silicon detectors track charged particles for jj<1andjj<2, respectively.
Surrounding the tracking system are electromagnetic and hadronic calorimeters, used to measure charged and neutral particle energies. The electromagnetic calorimeter is a lead-scintillator sandwich and the hadronic calorimeter is an iron-scintillator sandwich. Both calorimeters are seg- mented in azimuth and polar angle to provide directional information for the energy deposition. The segmentation varies with position on the detector and is15in azimuth by 0.1 units of in the central region (jj<1:1).
Segmentation in the plug region (1:1<jj<3:6) is7:5 up to jj<2:1, and 15 for jj>2:1in azimuth, while ranging from 0.1 to 0.64 units of in pseudorapidity (a nearly constant 2:7 change in polar angle). The electro- magnetic calorimeters are instrumented with proportional and scintillating strip detectors that measure the transverse profile of electromagnetic showers at a depth correspond- ing to the shower maximum.
Outside the central calorimeter are four layers of muon drift chambers coveringjj<0:6(CMU). The calorimeter provides approximately 1 m of steel shielding. Behind an additional 60 cm of steel in the central region sit an addi- tional four layers of muon drift chambers (CMP) arranged in a box-shaped layout around the central detector. Central muon extension (CMX) chambers, which are arrayed in a conical geometry, provide muon detection for the region 0:6<jj<1with four to eight layers of drift chambers, depending on polar angle. All the muon chambers measure the coordinates of hits in the drift direction, x, via a drift time measurement and a calibrated drift velocity.
The CMU and the CMX also measure the longitudinal coordinate, z. The longitudinal coordinate is measured in the CMU by comparing the height of pulses, encoded in time-over-threshold, at opposite ends of the sense wire. In the CMX, the conical geometry provides a small stereo angle from which the z coordinate of track segments can be determined. Reconstructed track
segments have a minimum of three hits, and a maximum of four hits in the CMU and the CMP, and 8 hits in the CMX.
III. DATA SAMPLE AND EVENT SELECTION In this section we describe the collision data and the Monte Carlo samples used in this analysis. Section III A outlines the trigger system used for the initial selection of events from thepp collisions. Section III B describes the Monte Carlo samples used for acceptance and background studies. The selection of the Wjets data sets from the triggered data samples is presented in Sec. III C. The tt signal is extracted from the Wjets events through the identification of candidatebhadron semileptonic decays to muons. The algorithm for identifying these decays is sum- marized in Sec. III D, and its application to theWjets data set is described in Sec. III E.
This analysis is based on an integrated luminosity of 19411 pb1 [11] collected with the CDF II detector between March 2002 and August 2003 (175 pb1 with the CMX detector operational).
A.pp collision data
CDF II employs a three-level trigger system, the first two consisting of special purpose hardware and the third consisting of a farm of commodity computers. Triggers for this analysis are based on selecting high transverse mo- mentum electrons and muons. The electron sample is triggered as follows: At the first trigger level events are selected by requiring a track withPT>8 GeV=cmatched to an electromagnetic calorimeter tower withET>8 GeV and little energy in the hadronic calorimeter behind it. At the second trigger level, calorimeter energy clusters are assembled and the track found at the first level is matched to an electromagnetic cluster with ET>16 GeV. At the third level, offline reconstruction is performed and an electron candidate with ET>18 GeV is required. The efficiency of the electron trigger is measured from Z! eedata and found to be96:20:6%[12]. The selection of the muon sample begins at the first trigger level with a track withPT>4 GeV=cmatched to hits in the CMU and the CMP chambers or a track withPT>8 GeV=cmatched to hits in the CMX chambers. At the second level, a track withPT>8 GeV=cis required in the event for about 70%
of the integrated luminosity, while for the remainder, trig- FIG. 1. Elevation view of the CDF II detector.
MEASUREMENT OF THEttPRODUCTION CROSS. . . PHYSICAL REVIEW D72,032002 (2005)
gers at the first level are fed directly to the third level. At the third trigger level, a reconstructed track with PT>
18 GeV=cis required to be matched to the muon chamber hits. The efficiency of the muon trigger, measured from Z!data, is88:70:7%for CMU/CMP muons and 95:40:4%for CMX muons [12].
B. Monte Carlo data sets
The detector acceptance oftt events is modeled using
PYTHIAv6.2 [13] andHERWIGv6.4 [14]. These are leading- order event generators with parton showering to simulate radiation and fragmentation effects. The generators are used with the CTEQ5L parton distribution functions [15].
Decays ofbandchadrons are modeled usingQQv9.1 [16].
Estimates of backgrounds from diboson production (WW, WZ,ZZ) are derived using theALPGENgenerator [17] with parton showering provided by HERWIG. The background from single top production (e.g. W !tb) is simulated usingPYTHIA. Samples of the remaining backgrounds are derived directly from the data as described in Sec. IV.
The detector simulation reproduces the response of the detector to particles produced inpp collisions. The same detector geometry database is used in both the simulation and the reconstruction, and tracking of particles through matter is performed withGEANT3[18]. The drift model for the COT uses a parametrization of aGARFIELDsimulation [19] with parameters tuned to match COT collider data.
The calorimeter simulation uses theGFLASH[20] parame- trization package interfaced with GEANT3. The GFLASH
parameters are tuned to test beam data for electrons and high-PT pions and checked by comparing the calorimeter energy of isolated tracks in the collision data to their momenta as measured in the COT. Further details of the CDF II simulation can be found in [21].
C.Wjets data set
From the inclusive lepton data set produced by the electron and muon triggers described in Sec. III A, we select events with an isolated electron ET (muon PT) greater than 20 GeV and E6 T>20 GeV. The isolation I of the electron or muon is defined as the calorimeter
transverse energy in a cone of R
22 p <
0:4 around the lepton (not including the lepton energy itself ) divided by theET(PT) of the lepton. We requireI <
0:1. TheWjets data set is categorized according to the number of jets with ET>15 GeV and jj<2:0. The decay ofttpairs gives rise to events with typically at least three such jets, while the W plus one or two jet samples provide a control data set with little signal contamination.
Jets are identified using a fixed-cone algorithm with a cone size of 0.4 and are constrained to originate from the pp collision vertex. Their energies are corrected to account for detector response variations in , calorimeter gain instability, and multiple interactions in an event. A
complete description of Wjets event selection is given in [6].
TheWjets data set consists mainly of events of direct production of W bosons with multiple jets. This amounts also to the largest background tottsignal. As a first stage of background reduction, we define a total event energy,HT, as the scalar sum of the electronETor muonPT, the event E6 T and jet ET for jets with ET>8 GeV and jj<2:4.
Because of the large mass of the top quark, a ttevent is expected to have a largeHTcompared to aWplus three or more jets event, as illustrated in Fig. 2. We studied the expected amount of signal (S) and background (B) as a function ofHT using thePYTHIAMonte Carlo program to model the signalHTdistribution. Data is used to model the backgroundHTdistribution. We optimized the selection of events by imposing a minimum HT requirement which
maximizes S=
SB
p . We select events with HT>
200 GeV, rejecting approximately 40% of the background while retaining more than 95% of thettsignal.
There are 337Wplus three or more jet events withHT>
200 GeV in 194 pb1 of data, 115 fromW ! candi- dates and 222 fromW!ecandidates.
Even after theHTcut, the expectedS:BinWplus three or more jet events is only of order1:3. To further improve the signal to background ratio, we identify events with one or more b-jets by searching inside jets for semileptonic decays ofbhadrons into muons.
D. Soft lepton tagging algorithm
Muon identification at CDF proceeds by extrapolating tracks found in the central tracker, through the calorimeter to the muon chambers, and matching them to track seg- ments reconstructed in the muon chambers. Matching is done in the following observables: extrapolated position
[GeV]
HT
0 50 100 150 200 250 300 350 400 450 500
stinU yrartibrA
t t W+jets
niB wolfrevO
FIG. 2 (color online). HT distributions, normalized to unity, fortt(solid line) andWjets (dotted line)PYTHIAMonte Carlo events with three or more jets after the event selection described in the text.
along the muon chamber drift direction (x), the longitudi- nal coordinate along the chamber wires (z) when such information is available, and the extrapolated slope com- pared to the slope of the reconstructed muon chamber track segment (L). Tracks are paired with muon chamber track segments based on the best match in x for those track segments that are within 50 cm of an extrapolated COT track. In what follows we refer to the difference between the extrapolated and measured positions in xandzasdx and dz, respectively, and the extrapolated and measured slope asdL. The distributions of these variables over an ensemble of events are referred to as the matching distri- butions. In addition to selection based ondxand dz, the standard muon identification also requires consistency with minimum ionizing energy deposition in the calorimeters.
However, in order to retain sensitivity for muons embedded in jets, the muon SLT algorithm makes full usage of the muon matching information without any requirement on the calorimeter information. The algorithm starts with high-quality reconstructed tracks in the COT, selected by requiring at least 24 axial and 24 stereo hits on the track.
Some rejection for pion and kaon decays in flight is achieved by requiring that the impact parameter of the reconstructed track be less than 3 mm with respect to the beam line. The track is also required to originate within 60 cm inzof the center of the detector. Only tracks passing these cuts and extrapolating within3PTinxoutside of the muon chambers, wherePTis the multiple scattering width, are considered as muon candidates. Also, when a track extrapolates to greater than 3PT in x inside the muon chambers, but no muon chamber track segment is found, the track is rejected and not allowed to be paired to other muon chamber track segments.
Candidate muons are selected with the SLT algorithm by constructing a quantity L, based on a comparison of the measured matching variables with their expectations. To construct L we first form a sum, Q, of individual 2 variables
QXn
i1
Xii2
2i ; (1)
wherei andiare the expected mean and width of the distribution of matching variableXi. The sum is taken over n selected variables, as described below. L is then con- structed by normalizingQaccording to
L Qn varQ
p ; (2)
where the variance varQ is calculated using the full covariance matrix for the selected variables. The normal- ization is chosen to makeLindependent of the number of variables n; note that the distribution of L tends to a
Gaussian centered at zero and with unitary width, for n sufficiently large. The correlation coefficients between each pair of variables are measured from J= ! data. The calculation proceeds by comparing the variance of the sum with the sum of the variances of each pair of2 variables in Eq. (1). Since the values of the matching variables are either positive or negative, according to the local coordinate system, separate correlation coefficients are used for pairs that have same-sign and opposite-sign values.
The selected variables are the full set of matching var- iables, x; z; L in the CMU, CMP and CMX with the following two exceptions: The CMP chambers do not provide a measurement of the longitudinal coordinate z, and matching in L is not included for track segments in the muon chambers that have only three hits. Because of their significantly poorer resolution, track segments recon- structed in the CMU chambers with three hits are not used.
Note that a muon that traverses both the CMU and the CMP chambers yields two sets of matching measurements inx andLand onezmatching measurement, and are referred as CMUP muons. All available matching variables are used in the calculation of L for a given muon candidate. By placing an appropriate cut onL, background is preferen- tially rejected because hadrons have broader matching distributions than muons since the track segments in the muon chambers from hadrons are generally a result of leakage of the hadronic shower.
The widths of the matching distributions that enter into L are a combination of intrinsic resolution of the muon chambers and multiple scattering. The multiple scattering term varies inversely withPTand is dominant at lowPT. The expected widths of the matching distributions are based on measurements of muons from J= decays at lowPT(see Fig. 3) andWandZboson decays at highPT. The mean values [i in Eq. (1)] are typically zero, except for a small offset in the CMUdz. We parametrize the widths as a function of up to three variables:PT,and . These variables describe to first order the effects of multiple scattering in the detector. For the CMU detector, PT is sufficient since the material traversed by a muon candidate is approximately homogenous inand. The widths are parametrized with a second-order polynomial in 1=PTwith an exponential term that describes thePTrange below3 GeV=c. For the CMP detector we parametrize the widths as functions of PT andto take into account the rectangular shape of the absorber outside the central calo- rimeter. For the CMX detector we use PT, and to account for a number of irregularities in the amount of absorber between0:6and1:0. The measurement of the widths of the matching distributions as functions of PT, overlayed with their fits, are shown in Fig. 4.
Figure 5 (left) shows an example of the distribution ofL from J= decays. The number of variables used varies from two to five. Figure 5 (right) shows the efficiency of MEASUREMENT OF THEttPRODUCTION CROSS. . . PHYSICAL REVIEW D72,032002 (2005)
dx [cm]
-20 -15 -10 -5 0 5 10 15 20
stinU yrartibrA
≤ 3.0 PT
≤ 2.5
≤ 6.0 PT
≤ 5.0
dx [cm]
-20 -15 -10 -5 0 5 10 15 20
stinU yrartibrA
) [cm]
θ dz sin(
-30 -20 -10 0 10 20 30
stinU yrartibrA
≤ 3.0 PT
≤ 2.5
≤ 6.0 PT
≤ 5.0
) [cm]
θ dz sin(
-30 -20 -10 0 10 20 30
stinU yrartibrA
[rad]
φL
d
-0.2 -0.1 0 0.1 0.2
stinU yrartibrA
≤ 3.0 PT
≤ 2.5
≤ 6.0 PT
≤ 5.0
[rad]
φL
d
-0.2 -0.1 0 0.1 0.2
stinU yrartibrA
FIG. 3 (color online). Examples of muon matching distributions for the CMU in (left to right) drift direction,x, the longitudinal coordinate, z, and angle,L. For each variable we show twoPT ranges,2:5PT3 GeV=c(wide distribution) and5PT 6 GeV=c(narrow distribution). For the longitudinal coordinatez, we plot the product ofdzbysin&, which is the projection orthogonal to the direction of flight.
[GeV/c]
PT
1 10
]mc[ xdσ
0 5 10 15 20 25
[GeV/c]
PT
1 10
]mc[ xdσ
0 5 10 15 20 25
CMU CMP CMX
µ-
µ+
→ ψ J/
ν µ
→ W
µ-
µ+
→ γ* 0/ Z
3 5 7 20 40
[GeV/c]
PT
1 10
]dar[ Lφdσ
0 0.05 0.1 0.15 0.2 0.25
[GeV/c]
PT
1 10
]dar[ Lφdσ
0 0.05 0.1 0.15 0.2 0.25
CMU CMP CMX
µ-
µ+
→ ψ J/
ν µ
→ W
µ-
µ+
→ γ* 0/ Z
3 5 7 20 40
[GeV/c]
PT
1 10
]mc[ )θ(nis zdσ
0 2 4 6 8 10 12 14 16 18 20
[GeV/c]
PT
1 10
]mc[ )θ(nis zdσ
0 2 4 6 8 10 12 14 16 18 20
CMU CMX
µ-
µ+
→ ψ J/
ν µ
→ W
µ-
µ+
→ γ* 0/ Z
3 5 7 20 40
FIG. 4 (color online). Width of the matching variable distributions (dx;dzsin&;dL) vsPTfor each of the muon chambers. The data points, fromJ= ,W andZdecays, are fit to the parametrizations described in the text.
the SLT algorithm as a function ofLfromJ= data. The efficiency plateaus at about 85% forjLj 3:5.
E. Event tagging
In this analysis we seek to identify semileptonic decays of bhadrons inside jets inttevents. The transverse mo- mentum spectrum of these muons, covering a broad range from a fewGeV=cto over40 GeV=c, is shown in Fig. 6 from thePYTHIAMonte Carlo sample. Within theWjets data set defined in Sec. III C, we isolate a subset of events with at least one ‘‘taggable’’ track. A taggable track is defined as any track, distinct from the primary lepton, passing the track quality requirements described in Section III D, withPT>3 GeV=c, withinR <0:6of a jet axis and pointing to the muon chambers to within a3 multiple scattering window (theof the multiple scatter- ing window is defined as the dx shown in Fig. 3). The
z-coordinate of the track at the origin must be within 5 cm of the reconstructed event vertex (the vertex reconstruction is described in detail in [5]). Jets are considered ‘‘SLT tagged’’ if they contain a taggable track, which is also attached to a track segment in the muon chambers and the resulting muon candidate hasjLj<3:5.
A potentially large background arises fromJ= decay and sequential, double semileptonic b!c!s decay, resulting in one lepton from thebdecay and an oppositely charged lepton from the c decay. Therefore, events are rejected if the primary lepton is of opposite charge to a SLT muon tag and the invariant mass of the pair is less than 5 GeV=c2. Similarly, events are also rejected if the primary lepton is a muon that together with an oppositely charged SLT muon tag forms an invariant mass between 8 and 11 GeV=c2 or 70 and 110 GeV=c2, consistent with an or a Z particle, respectively. The sequential decay cut and the andZremoval reduce thettacceptance by less than 1%.
Events passing all event selection cuts that have at least one taggable track are referred to as the ‘‘pretag’’ sample.
There are 319 pretag events with three or more jets, 211 in which the primary lepton is an electron and 108 in which it is a muon. Out of these events we find 20 events with a SLT tag, 15 in which the primary lepton is an electron and 5 in which it is a muon. This set of events is the tt candidate sample from which we measure the tt production cross section in Sec. VII.
IV. BACKGROUNDS
In this section we describe the evaluation of background events in the ttcandidates sample. The background con- tributions are mostly evaluated directly from the data. The dominant background in this analysis is from W plus jets events where one jet produces an SLT tag. The estimate of this background relies on our ability to predict the number of such SLT tags starting from the pretag sample. The L
-4 -2 0 2 4 6 8
Arbitrary Units
|L| cut
0 0.5 1 1.5 2 2.5 3 3.5 4
Efficiency [%]
50 55 60 65 70 75 80 85 90 95 100
FIG. 5. Left: distribution ofLfromJ= decays. Right: the SLT efficiency as a function of thejLjcut, as measured fromJ= decay.
[GeV/c]
µ
P T
0 5 10 15 20 25 30 35 40
stinU yrartibrA niB wolfrevO
MC t Muon from B decay in t
µ X ν µ
→ B
µ X ν µ
→
→ D B
FIG. 6 (color online). PTdistribution of muons frombhadron decays inPYTHIA Monte Carlo top events. The circles are all muons frombhadron decays. The triangles are directB!X decays and the squares are sequential B!D!X (where
‘‘B’’ indicates abhadron).
MEASUREMENT OF THEttPRODUCTION CROSS. . . PHYSICAL REVIEW D72,032002 (2005)
prediction is based on the probability for a given track in a jet to yield an SLT tag, and is measured in(jets events.
We then evaluate the systematic uncertainty on the W jets background estimate by testing the predictive power of the measured probabilities in a variety of data samples. The Wjets background evaluation and its systematic uncer- tainty is described in Sec. IVA. AfterWjets production, the next largest background is due to QCD multijet events.
The evaluation of the QCD multijet contribution also relies on tagging probabilities measured in (jets events.
However, we must account for the possible difference between the tagging probabilities for the QCD events that populate thettcandidate sample because theE6 T often comes from a mismeasured jet and not from a neutrino.
The evaluation of the QCD background is described in Sec. IV B. An additional small source of background is due to Drell-Yan events and is estimated from the data and described in Sec. IV C. The remaining background contri- butions are relatively small and are evaluated using Monte Carlo samples, as described in Sec. IV D.
A. Backgrounds fromWjets
W plus jets events enter the signal sample either when one of the jets is ab-jet or ac-jet with a semileptonic decay to a muon, or a light quark jet is misidentified as containing a semileptonic decay (‘‘mistagged’’). We refer to these background events as Wheavy flavor and W
“fakes”, respectively. Wheavy flavor events include Wbb, Wcc and Wc production. One way of estimating
these backgrounds would be to use a Monte Carlo program, such as ALPGENto predict the Wheavy flavor compo- nent, and the data to predict the Wfakes (because the data provides a more reliable measure of mistags than the simulation). However, to avoid double-counting, this would require an estimate of mistags that is uncontami- nated by tags from heavy flavor. Instead we have chosen to estimate both background components directly from the data, and we test the accuracy of the prediction as de- scribed below. We measure the combinedWheavy fla- vor and Wfakes background by constructing a ‘‘tag matrix’’ that parametrizes the probability that a taggable track with a givenPT,and, in a jet withET>15 GeV, will satisfy the SLT tagging requirement described in Sec. III E. The variablesandare measured at the outer radius of the COT with respect to the origin of the CDF II coordinate system. The tag matrix is constructed using jets in(jets events with one or more jets. The tag probabil- ity is approximately 0.7% per taggable track, and includes tags from both fakes and heavy flavor. The tag rate as a function of each of the matrix parameters (integrated over the remaining two) is shown in Fig. 7. The features in the tag rate vsandplots are a result of calorimeter gaps and changes in the thickness of the absorber before the muon chambers. The matrix is binned to take account of these variations. The bottom right plot shows the tag rate as the function of thejLjcut for each muon category. The tag rate is higher for the CMP-only muons due to the smaller amount of absorber material that results from cracks in the
P [GeV/c]
0 10 20 30 40 50
Tag Rate [%]
0 0.2 0.4 0.6 0.8 1 1.2 1.4
T
-1 -0.5 0 0.5 1
Tag Rate [%]
0 0.2 0.4 0.6 0.8 1 1.2 1.4
η
[rad]
-3 -2 -1 0 1 2 3
Tag Rate [%]
0 0.2 0.4 0.6 0.8 1 1.2 1.4
φ 0 0.5 1 1.5 |L| cut2 2.5 3 3.5 4
Tag Rate [%]
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
CMP-only CMU-only CMUP CMX
FIG. 7. Tag rates in the photon plus jets sample as a function of each of the tag-rate matrix parameters. Each plot is integrated over the two parameters not shown. The bottom right plot shows the tag rate as the function of thejLjcut for each of the muon categories.
