Article
Reference
Measurement of the b jet cross section in events with a Z boson in pp collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
A measurement of the inclusive bottom jet cross section is presented for events containing a Z boson in pp collisions at s√=1.96 TeV using the Collider Detector at Fermilab. Z bosons are identified in their electron and muon decay modes, and b jets with ET>20 GeV and |η|
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the b jet cross section in events with a Z boson in pp collisions at s√=1.96 TeV. Physical Review. D , 2006, vol. 74, no. 03, p. 032008
DOI : 10.1103/PhysRevD.74.032008
Available at:
http://archive-ouverte.unige.ch/unige:38358
Disclaimer: layout of this document may differ from the published version.
Measurement of the b jet cross section in events with a Z boson in p p collisions at p s
1:96 TeV
A. Abulencia,23D. Acosta,17J. Adelman,13T. Affolder,10T. Akimoto,55M. G. Albrow,16D. Ambrose,16S. Amerio,43 D. Amidei,34A. Anastassov,52K. Anikeev,16A. Annovi,18J. Antos,1M. Aoki,55G. Apollinari,16J.-F. Arguin,33 T. Arisawa,57A. Artikov,14W. Ashmanskas,16A. Attal,8F. Azfar,42P. Azzi-Bacchetta,43P. Azzurri,46N. Bacchetta,43 H. Bachacou,28W. Badgett,16A. Barbaro-Galtieri,28V. E. Barnes,48B. A. Barnett,24S. Baroiant,7V. Bartsch,30G. Bauer,32
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Y. D. Oh,27T. Okusawa,41R. Oldeman,29R. Orava,22K. Osterberg,22C. Pagliarone,46E. Palencia,11R. Paoletti,46 PHYSICAL REVIEW D74,032008 (2006)
V. Papadimitriou,16A. A. Paramonov,13B. Parks,39S. Pashapour,33J. Patrick,16G. Pauletta,54M. Paulini,12C. Paus,32 D. E. Pellett,7A. Penzo,54T. J. Phillips,15G. Piacentino,46J. Piedra,44L. Pinera,17K. Pitts,23C. Plager,8L. Pondrom,59 X. Portell,3O. Poukhov,14N. Pounder,42F. Prakoshyn,14A. Pronko,16J. Proudfoot,2F. Ptohos,18G. Punzi,46J. Pursley,24
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A. Sedov,48S. Seidel,37Y. Seiya,41A. Semenov,14L. Sexton-Kennedy,16I. Sfiligoi,18M. D. Shapiro,28T. Shears,29 P. F. Shepard,47D. Sherman,21M. Shimojima,55M. Shochet,13Y. Shon,59I. Shreyber,36A. Sidoti,44P. Sinervo,33
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P. Wilson,16B. L. Winer,39P. Wittich,16S. Wolbers,16C. Wolfe,13T. Wright,34X. Wu,19S. M. Wynne,29A. Yagil,16 K. Yamamoto,41J. Yamaoka,52T. Yamashita,40C. Yang,60U. K. Yang,13Y. C. Yang,27W. M. Yao,28G. P. Yeh,16J. Yoh,16
K. Yorita,13T. Yoshida,41G. B. Yu,49I. Yu,27S. S. Yu,16J. C. Yun,16L. Zanello,51A. Zanetti,54I. Zaw,21F. Zetti,46 X. Zhang,23J. Zhou,52and S. Zucchelli5
(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
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
9University of California, San Diego, La Jolla, California 92093, USA
10University of California, Santa Barbara, Santa Barbara, California 93106, USA
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, USA
14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
15Duke University, Durham, North Carolina 27708
16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
17University of Florida, Gainesville, Florida 32611, USA
18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
19University of Geneva, CH-1211 Geneva 4, Switzerland
20Glasgow University, Glasgow G12 8QQ, United Kingdom
21Harvard University, Cambridge, Massachusetts 02138, USA
22Division of High Energy Physics, Department of Physics, University of Helsinki, FIN-00014, Helsinki, Finland and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
23University of Illinois, Urbana, Illinois 61801, USA
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
A. ABULENCIAet al. PHYSICAL REVIEW D74,032008 (2006)
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
31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8 and University of Toronto, Toronto, Canada M5S 1A7
34University of Michigan, Ann Arbor, Michigan 48109, USA
35Michigan State University, East Lansing, Michigan 48824, USA
36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
37University of New Mexico, Albuquerque, New Mexico 87131, USA
38Northwestern University, Evanston, Illinois 60208, USA
39The Ohio State University, Columbus, Ohio 43210, USA
40Okayama University, Okayama 700-8530, Japan
41Osaka City University, Osaka 588, Japan
42University of Oxford, Oxford OX1 3RH, United Kingdom
43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
48Purdue University, West Lafayette, Indiana 47907, USA
49University of Rochester, Rochester, New York 14627, USA
50The Rockefeller University, New York, New York 10021, USA
51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
52Rutgers University, Piscataway, New Jersey 08855, USA
53Texas A&M University, College Station, Texas 77843, USA
54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
55University of Tsukuba, Tsukuba, Ibaraki 305, Japan
56Tufts University, Medford, Massachusetts 02155, USA
57Waseda University, Tokyo 169, Japan
58Wayne State University, Detroit, Michigan 48201, USA
59University of Wisconsin, Madison, Wisconsin 53706, USA
60Yale University, New Haven, Connecticut 06520, USA (Received 22 May 2006; published 23 August 2006)
A measurement of the inclusive bottom jet cross section is presented for events containing aZboson in ppcollisions atps
1:96 TeVusing the Collider Detector at Fermilab.Zbosons are identified in their electron and muon decay modes, and b jets with ET>20 GeV and jj<1:5 are identified by reconstructing a secondary decay vertex. The measurement is based on an integrated luminosity of about 330 pb1. A cross section times branching ratio ofZbjets BZ!‘‘ 0:930:36 pbis found, whereBZ!‘‘is the branching ratio of theZboson orinto a single flavor dilepton pair (e or) in the mass range between 66 and116 GeV=c2. The ratio ofbjets to the total number of jets of any flavor in the Z sample, within the same kinematic range as the b jets, is 2:360:92%. Here, the uncertainties are the quadratic sum of statistical and systematic uncertainties. Predictions made with next- to-leading order QCD agree, within experimental and theoretical uncertainties, with these measurements.
DOI:10.1103/PhysRevD.74.032008 PACS numbers: 13.60.Hb
I. INTRODUCTION
The measurement of the Zb jet production cross section provides an important test of quantum chromody- namics (QCD) calculations [1]. The cross section is sensi- tive to thebquark density in the proton and thus tests the
perturbative calculations of this quantity. A precise knowl- edge of the b quark density is essential to accurately predict the production of particles that couple strongly to bquarks including Higgs bosons (h) within supersymme- try models (gb!hb,bb!h) [2,3] or single top produc- tion [4] within the standard model (qb!q0t and MEASUREMENT OF THEbJET CROSS SECTION IN. . . PHYSICAL REVIEW D74,032008 (2006)
gb!Wt). TheZbjet cross section is also an important test of the background predictions to standard model Higgs boson production in association with a Z boson, ZH! Zbb[5].
TheZcross section [6–8] andZjets cross section [9]
have been measured at the Tevatron. Next-to-leading order (NLO) QCD calculations are found to describe the data. In this paper, the first measurement of thebjet cross section for events with a Zboson using the Collider Detector at Fermilab (CDF) [10] is reported. A similar measurement has been made recently by the D0 collaboration [11].
The dominant production diagrams are gb!Zb and qq !Zbb: in NLO calculations they contribute about 65% and 35%, respectively. At present thebquark density is derived from the gluon distribution function [12] and agrees well with the available measurements of the con- tribution to the proton structure function F2 for Q2<
1000 GeV2 [13], where Q2 is the momentum transfer squared. The measurement reported in this paper is sensi- tive to parton densities at higher values withQ2 approxi- mately equal to the square of theZmass (MZ).
The analysis uses Run IIpp collision data from CDF taken up to September 2004 at a center of mass energy of
s
p 1:96 TeV. The measurement is made by searching for pairs of electrons or muons with an invariant mass consistent with MZ and jets which contain a displaced secondary vertex consistent with the decay of a long-lived bottom hadron. The light (u,d,s, and gluon) and charm (c) jets remaining after this vertex requirement are distin- guished from thebjets using the mass distribution of the charged particles forming the secondary vertex. This tech- nique exploits the larger mass of the b quark compared with light andcquarks. TheZcross section is defined to include the irreducible Drell-Yan contribution !‘‘ within the dilepton invariant mass range 66< M‘‘<
116 GeV=c2. Note that this cross section is numerically only 0.4% higher than the inclusiveZcross section inde- pendent of the mass range [6]. The Zbjet production cross section is defined to be proportional to the number of bjets with jet transverse energyEjetT >20 GeVand pseu- dorapidityjjetj<1:5contained in events with aZboson.
In Sec. II a brief description of the CDF detector is given, and in Sec. III the Monte Carlo simulation is de- scribed. Section IV summarizes the event selection and the background sources. In Sec. V the fraction ofbjets within the data sample is determined. In Sec. VI the method to measure the cross section is described, and in Sec. VII the sources of systematic uncertainties are discussed. The results of the measurement are given in Sec. VIII, and a conclusion is presented in Sec. IX.
II. THE CDF II DETECTOR
The CDF II detector is described in detail elsewhere [10]. It is a general purpose, nearly hermetic detector
situated around the pp collision point. A coordinate sys- tem is used, in whichis the polar angle with respect to the proton beam direction, is the azimuthal angle and ln tan=2is the pseudorapidity. The transverse energy and transverse momentum of a particle is defined asET EsinandpT psin, respectively, where E is the en- ergy measured by the calorimeter andpis the momentum measured in the tracking system. The missing transverse energy vector is defined asE6~T P
iEiTn~i, wheren~iis a unit vector that points from the interaction vertex to the center of the ith calorimeter tower in the transverse plane and EiT is the transverse energy of the ith tower. The quantity E6~T is the magnitude of E6 T, which is corrected for all identified muons in an event [7].
The transverse momenta of charged particles are mea- sured by an eight-layer silicon strip detector [14–16] and the central outer tracker (COT), a 96-layer drift chamber [17] located inside a solenoid that provides a 1.4 T mag- netic field. The innermost layer of the silicon detector is located on the beryllium beampipe at a radius of 1.5 cm, and the outermost layer is located at 28 cm. The silicon detector provides tracking in the pseudorapidity region jj<2, with partial coverage up to jj<2:8.
The single hit resolution is about11m. Located outside of the silicon detector, the COT is a 3.1-m long, open-cell drift chamber with an active tracking region extending radially from 41 to 137 cm. The COT provides coverage for jj<1. For tracks with pT>1:5 GeV=cand silicon hits the resolution on the impact parameter is about34m [18], including the transverse size of the beam of about 25m.
Located outside the solenoid, a segmented sampling calorimeter is used for the measurement of particle ener- gies. The central part of the calorimeter covers the region jj<1:1[19,20], and the forward part of the calorimeter consists of two identical detectors covering 1:1<jj<
3:6 [21]. The central calorimeter uses lead-scintillator sampling in the electromagnetic compartment and steel- scintillator sampling in the hadronic compartment. It is instrumented with proportional strip and wire chambers [central electromagnetic shower maximum detector (CES)]. They are located at a depth of about six radiation lengths where the lateral profile of electromagnetic show- ers is expected to be maximal and have a segmentation of 1.5 cm. The forward calorimeter uses lead-scintillator sampling for the electromagnetic compartment and iron- scintillator for the hadronic compartment. Further details about the calorimeters can be found in Ref. [22].
Drift chambers, located outside the central hadron cal- orimeters and behind 60 cm of iron shielding, detect muons withjj<0:6[23]. Additional drift chambers and scintil- lation counters detect muons in the regions0:6<jj<1:0 and 1:0<jj<1:5. Gas Cherenkov counters [24] mea- sure the average number of pp inelastic collisions per bunch crossing to determine the luminosity.
A. ABULENCIAet al. PHYSICAL REVIEW D74,032008 (2006)
III. MONTE CARLO SIMULATION
A Monte Carlo simulation is used to correct for ineffi- ciencies due to the selection requirements and detector effects. The Monte Carlo generator PYTHIA v6.2 [25] is used to generate the Drell-Yan signal and the background processes, using CTEQ5L parton density functions [26].
An underlying event model that describes the interactions of the spectator partons and initial state QCD radiation has been included in the generation. This model has been tuned to describe the Tevatron data [22,27]. The decays of theb hadrons are generated by the Monte Carlo generator QQ v9.1 [28]. The CDF detector response is simulated using a
GEANTbased detailed detector simulation [29,30].
The Drell-Yan Monte Carlo samples are normalized to the next-to-next-to-leading-order (NNLO) QCD cross sec- tion of 251.3 pb [31] for66< M‘‘<116 GeV=c2, so that comparisons with the data can be made. For tt and ZZ processes, which contribute to the background, the NLO QCD cross sections are used for the normalization:tt 6:77 pb[32] andZZ1:4 pb[33].
Simulated events are reconstructed in the same manner as the data events, and the same event selection criteria are applied.
IV. EVENT SELECTION AND BACKGROUND A. Event selection
Zbosons are detected in their decays into two electrons or two muons with an invariant mass of the two leptons M‘‘(‘eor) between 66 and116 GeV=c2. The trigger requirements and the lepton selection follow closely those described in detail in Ref. [7].
Electrons are triggered by requiring a cluster of electro- magnetic energy with ET>18 GeV and jj<1:1 matched to a track withpT>10 GeV=c. Further require- ments are made on position matching and the shower shape in the CES. At least one trigger electron candidate is required. The second electron candidate can either be in the central or forward calorimeter, and looser identification criteria are imposed. For forward electron candidates no matching track is required. All electron candidates are required to be isolated from other calorimeter energy de- posits [7].
Muons are triggered by requiring a track with pT >
18 GeV=c and jj<1:0 and a track segment in the muon chambers that matches the extrapolated position of the track. At least one muon candidate that satisfies the trigger requirements is required. The other muon candidate is not required to have signals in the muon chambers. All muon candidates are required to have a calorimeter energy deposit consistent with that of a muon and to be isolated from other energy depositions [6,7]. The two highestpT muon candidates are required to have opposite electric charges.
Candidate Z boson events are selected if 66< M‘‘<
116 GeV=c2. A total of 27 659 candidate events are ob- served in the electron channel and 15 698 events in the muon channel.
Having selected an event with aZboson candidate, jets withEjetT >20 GeVandjjetj<1:5are searched for. Jets are defined by a cone jet algorithm with a cone sizeR
2 2
p 0:7[22]. The jet energy is corrected to the hadron level energy. The hadron level energy is defined to include all particles from thepp collision within the jet cone, including particles from the hard scatter, multiple parton-parton interactions, and beam remnants. The jet energy is also corrected for particles produced in additional pp interactions, reconstructed in the same bunch crossing.
The jets are not corrected to the parton level to be inde- pendent of the Monte Carlo modeling of this correction.
Events must satisfy either E6 T<25 GeV or HT<
150 GeV, where HT is the scalar sum of E6 T and the transverse energies of all leptons and jets in the event [34]. This requirement reduces background fromttevents in which thetdecays toWband both theW bosons decay leptonically (W!l) by about 80%, while reducing the signal by only 4%, as determined from Monte Carlo simulation.
Abjet is defined as any jet that has at least onebhadron within a cone of 0.7 around the jet axis. In this analysis ab jet is identified through the presence of a displaced vertex within the jet arising from the decay of the long-lived bottom hadron. The algorithm used was optimized for the measurement of the top quark production cross section [35] but found to give adequate efficiency and purity for the present analysis. A jet that has a reconstructed dis- placed vertex is called a ‘‘b tagged’’ jet. The displaced vertex algorithm uses a two-pass approach to find a sec- ondary vertex. In the first pass an attempt is made to reconstruct a secondary vertex using one track withpT>
1:0 GeV=cand two or more additional tracks withpT>
0:5 GeV=c, and all tracks are required to have an impact parameter significanced0=d0>2:5. Here,d0is the mini- mum distance between the track and the primary vertex in the plane transverse to the beam direction and has uncer- taintyd0. If the first pass is unsuccessful, a second pass is made using two tracks withpT>1:5 GeV=cfor one track, pT>1:0 GeV=c for the other and d0=d0>3 for both.
The jet is labeled as a tag if the transverse displacement significance jL2Dj=L2D>7:5. Here,L2D is the distance from the primary vertex to the secondary vertex in the plane transverse to the beam direction projected onto the jet axis, andL2Dis the estimated uncertainty. The distance L2D is defined as positive if the angle between the trans- verse displacement and the jet direction is less than 90 , and as negative otherwise. Apositive taghas positiveL2D, and anegative taghas negativeL2D.
Thebtagging efficiency is determined as a function of EjetT andjet from a separate data sample of about 28 000 MEASUREMENT OF THEbJET CROSS SECTION IN. . . PHYSICAL REVIEW D74,032008 (2006)
dijet events, where one of the jets has a reconstructed semileptonicborcdecay [35]. The ratio of the efficiency in data to that in Monte Carlo simulation is found to be 0:910:06. The average databtagging efficiency for jets in this analysis is332%.
In total, 115 tagged jets are selected, 60 in the electron channel and 55 in the muon channel. This compares with a Monte Carlo estimate of 69 in the electron and 45 in the muon channel. The Monte Carlo estimate includes the Drell-Yan contribution, which has been scaled by the factors obtained in the fit to the secondary vertex mass (see Sec. V), and the backgrounds listed in Sec. IV B. Out of the 115 tagged jets 16 are negatively tagged compared with a Monte Carlo estimate of 16.7. One event contains two positively taggedbjets compared with a Monte Carlo expectation of 1.46.
B. Backgrounds
Backgrounds toZbproduction can arise from mis- identified leptons, from genuine leptons andbjets coming from other processes, or from light jets orc jets that are misidentified asbjets. The first two background sources are discussed in this section, and the latter is discussed in Sec. V.
The background in which one or both reconstructed electrons in the Z!ee channel are misidentified from other particles in the final state is estimated from the data. The probability that a jet will pass all electron identification criteria is determined from several inclusive jet samples. These samples have negligible prompt elec- tron content. The probabilities are parameterized as a function of EjetT and are on average 0.1% for central jets and 1.5% for forward jets. Because an associated track is not required for a forward electron, the misidentification probability is much higher than for central electrons.
A sample of events in which exactly one trigger electron is reconstructed is now taken. The trigger electron is paired with any other jet in the event such that the invariant mass of the electron and the jet lies within theZmass window.
The jet energy is taken at the electromagnetic calorimeter energy scale i.e. the correct energy scale for an electron or photon. A weight, which equals the jet misidentification probability, is assigned to the jet. The total background to inclusive Z production is then the sum of all weights. If there is more than one jet in the event that forms an invariant mass within the mass window of the Z, each combination is used. Background distributions are derived by weighting the electronjets distributions with the weights. The background for Zjets (Zbjets) is the sum of those weights for events which contain at least one jet (bjet) in addition to the one paired with the electron.
Using this method, a background contribution of3:1 1:5btagged jets is estimated within theZmass window.
Background in the Z! channel in which the electric charges of the two muon candidates are uncorre-
lated is estimated from events with two reconstructed muon candidates that have the same electric charge [7].
This background comes from events in which one or both muon candidates arise from hadron decays or events in which hadrons in the final state are misidentified as muons.
Because of the low statistics of the events with abtagged jet, the fractional contribution of this background is esti- mated as the observed ratio of generic jets (i.e., any jet regardless of which quark flavor or gluon it originated from) with a like-sign Z candidate to those with an unlike-sign Z candidate. The number of b tagged jets from this background source is estimated to be 0:24 0:12. This number is in agreement with the zero tagged jets with a like-sign Zcandidate observed in the data.
The background from other processes is estimated from Monte Carlo simulations. The production of tt pairs is found to contribute0:250:05(0:240:05) tagged jets
2) ( GeV/c Mll
50 60 70 80 90 100 110 120 130
)2 Events / ( GeV/c
1 10 102 103
2) ( GeV/c Mll
50 60 70 80 90 100 110 120 130
)2 Events / ( GeV/c
1 10 102 103
CDF data Drell-Yan MC Background
FIG. 1. The invariant mass of the dilepton pair for the sample with jets withEjetT >20 GeVandjjetj<1:5compared with the expectation from signal and background sources. The Drell-Yan Monte Carlo has been normalized to the luminosity of the data sample assuming the NNLO Drell-Yan cross section.
2) (GeV/c Mll
50 60 70 80 90 100 110 120 130
)2 Events / ( 4 GeV/c
0 5 10 15 20 25 30 35 40 45
2) (GeV/c Mll
50 60 70 80 90 100 110 120 130
)2 Events / ( 4 GeV/c
0 5 10 15 20 25 30 35 40
45 CDF data
Drell-Yan MC Background
FIG. 2. The invariant mass of the dilepton pair for the sample with positively tagged jets withEjetT >20 GeVandjjetj<1:5 compared with the expectation from signal and background sources. The Drell-Yan Monte Carlo has been scaled by the factors determined in Sec. V.
A. ABULENCIAet al. PHYSICAL REVIEW D74,032008 (2006)
to the electron (muon) channel. The production ofZZ is found to contribute0:360:07(0:280:06) tagged jets to the electron (muon) channel. Backgrounds from other processes such asWW,WZ,bbproduction orZ! are estimated to be negligible.
The invariant mass of the dilepton pair is shown in Fig.1 for events with at least one generic jet withEjetT >20 GeV andjjetj<1:5. The data are compared with the expected Drell-Yan and estimated background contributions. Good agreement is observed nearMZ and in the tails. Figure2 shows the dilepton invariant mass for events with at least one positively tagged jet in the same kinematic range. The data are well modeled by the simulation, and a clear Z signal is also observed in this sample.
V. FRACTION OFbJETS
The fraction ofb jets in the tagged jet sample is esti- mated by performing a fit to the invariant mass of all charged tracks attached to the secondary vertexMS [36].
On averagebjets have a largerMSthancjets or light jets due to the larger mass ofbhadrons. TheMSdistribution for both positively and negatively tagged jets is used, in order to better discriminate between heavy and light jets, since jets with a genuine secondary vertex overwhelmingly have positive tags, whereas jets with a false secondary vertex may have positive or negative tags. In addition using the negative tags in the fit allows a better separation of the charm and light quark contributions that have similarMS distribution for positively tagged jets.
The MS distributions for positively and negatively tagged data jets are shown in Fig. 3. The distributions of MS for b, c, and light jet events are taken from the Monte Carlo simulation. The Monte Carlo light jet distri- bution has been corrected using data as described below.
It is important to ensure that the simulation models the MSdistributions well. TheMSdistributions are affected by the tracking efficiency, the charged particle multiplicity of thebhadron decay, and the fraction of jets with twoborc hadrons. Systematic uncertainties have been assigned to each of these contributions (see Sec. VII), and the MS distribution in a Monte Carlo simulation of b jets has been compared with the dijet sample used to determined thebefficiency (see Sec. IV). The simulation is found to describe the data within the uncertainties quoted [37].
The number of positive tags in light quark jets is larger than the number of negative tags due to long-lived parti- cles, such as K0S and, and due to vertices produced by nuclear interactions of particles with material in their path [35]. The current simulation may not describe these effects accurately. In order to investigate these effects, a sample of approximately 13 000jet data events is taken, with jets in the same kinematic range as for theZbjets selection.
This sample was chosen since it has higher statistics than and a similar event topology as theZbjets sample. The MS distribution of thejet data is obtained separately
for positive and negative tags. The fraction of light,c, and b jets in each sample is determined using Monte Carlo distributions as templates and performing a likelihood fit, similar to the one described below, in which the normal- ization of each Monte Carlo template is allowed to vary. It is found that the ratio of positive to negative tags for light jets in the data is 1.49, compared with the expectation from the Monte Carlo of 1.93. To correct for the difference between the data and the Monte Carlo simulation, the Monte Carlo light negative tagged jets are increased by a factor of 1:93=1:491:30. This reweighted template shape is taken for the central value, and the full difference from unity,30%, is taken as the systematic uncertainty.
A binned maximum likelihood fit using Poisson statis- tics is performed to the MS distributions of the positively and negatively tagged jets in theZsample for rangeMS<
3:51 GeV=c2for positively (negatively) tagged jets. The range is chosen in order to have enough statistics for the fit.
The Monte Carlo distributions are taken as templates and
2) (GeV/c MS
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
)2 Jets / (0.5 GeV/c
0 5 10 15 20 25 30 35 40
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0 5 10 15 20 25 30 35
40 CDF data
light jets c jets b jets
=1.96 TeV s
>20 GeV
jet
ET
|<1.5 ηjet
|
(a) Positive Tags
2) (GeV/c MS
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
)2 Jets / (0.5 GeV/c
0 2 4 6 8 10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0 2 4 6 8
10 CDF data
light jets c jets b jets
=1.96 TeV s
>20 GeV
jet
ET
|<1.5 ηjet
|
(b) Negative Tags
FIG. 3 (color online). The mass at the secondary vertex,MS, for (a) positively and (b) negatively tagged jets with EjetT >
20 GeV and jjetj<1:5. The non-Drell-Yan background has been subtracted from the data. The data are compared with the sum of the light, c and b Monte Carlo templates after being scaled by the factors l, c, and b, respectively. The open white area represents the light quark template, the lightly shaded are the c quark template and the dark shaded are the b quark template.
MEASUREMENT OF THEbJET CROSS SECTION IN. . . PHYSICAL REVIEW D74,032008 (2006)
scaled by factors b, c, and l for b, c, and light jets, respectively. The data distribution is used after subtraction of the non-Drell-Yan background as estimated in Sec. IV B. The quantity b is thus the number of fitted reconstructed signalbjets in data divided by the number in the simulation. Only statistical errors from the data and Monte Carlo are used in the fit. The fit takes into account the Monte Carlo statistical errors using the method de- scribed in Ref. [38]. The fit gives values of b0:93 0:29, c 1:690:94, and l1:360:53. The corre- lation coefficient between band cis0:68and between
band lis 0.10..
The number ofbjets in theZsample after subtracting the background contributions is estimated to beNDataZ bjets 4514. A check is performed by fitting only positively tagged jets, and good agreement is obtained with a value of b0:950:31.
VI. CROSS SECTION
The inclusivebjet cross section Zbjets is pro- portional to the number ofbjets withEjetT >20 GeVand jjetj<1:5and is defined for dilepton masses66< M‘‘<
116 GeV=c2. The branching fraction BZ!‘‘ is defined for a single lepton flavor. All the cross sections and cross section ratios presented are fully corrected for detector response and resolution and presented at the had- ron level. No corrections are made for the underlying event or for hadronization effects (see Sec. IV). They are defined for jets with a cone size of 0.7 that include a b hadron within this cone.
A ratio method is used to extract the cross section. In doing so use can be made of the uncertainties estimated for the inclusive Z cross section measurement [7] for the lepton and trigger selection. First a measurement is made of the ratio of theZbjet cross section to the totalZcross section:
Zbjets
Z NDataZbjets=Zbjets NDataZ=Z ; (1)
whereNDataZbjetsis the fitted number ofbjets (see Sec. V) andNDataZis the total number of events with a lepton pair in the mass range66< M‘‘<116 GeV=c2 in the data. In both cases the number of data events is taken after subtraction of the background contributions (see Sec. IV B). The efficiencies of the Zbjets and the Z boson selections are Zbjet 7:7% and Z 27%, respectively. They are determined from PYTHIA
Monte Carlo simulation and are corrected for any differ- ences from the data. The ratio Zbjets=Zis also determined usingHERWIGv6.5 [39] and a similar result is obtained (0.286 for HERWIG compared with 0.285 for
PYTHIA).
The cross section is then calculated as Zbjets BZ!‘‘ Zbjets
Z CDFZ
BZ!‘‘; (2) where CDFZ BZ!‘‘ 254:93:3stat:
4:6syst: 15:2lum:pb is the CDF measurement of the Z production cross section times branching fraction for a single lepton flavor [6].
A measurement is made also of the ratio of the Zb jets cross section to the Z generic jets cross section in order to measure the fraction of jets that contain at least onebhadron. TheZgeneric jets cross section is propor- tional to the number of generic jets withEjetT >20 GeVand jjetj<1:5. The ratio of theZbjets toZgeneric jets cross section is obtained from the data as
Zbjets
Zjets NDataZbjets=Zbjets NDataZjets=Zjets ; (3) whereNDataZjetsis the number of generic data jets for events with a lepton pair within theZmass window after
(GeV)
jet
ET
0 10 20 30 40 50 60 70 80 90 100
Jets /(10 GeV)
0 500 1000 1500 2000 2500 3000
(GeV)
jet
ET
0 10 20 30 40 50 60 70 80 90 100
0 500 1000 1500 2000 2500
3000 CDF data
Drell-Yan MC Background
=1.96 TeV s
>20 GeV
jet
ET
|<1.5 ηjet
|
ηjet
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Jets / (0.2 units pseudorapidity)
0 100 200 300 400 500 600 700
ηjet
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Jets / (0.2 units pseudorapidity)
0 100 200 300 400 500 600
700 CDF data
Drell-Yan MC Background
=1.96 TeV s
>20 GeV
jet
ET
|<1.5 ηjet
|
FIG. 4. The EjetT and jet distributions for generic jets with EjetT >20 GeVandjjetj<1:5. The Drell-Yan Monte Carlo has been scaled such that the total number of jets in the simulation is the same as in the data.
A. ABULENCIAet al. PHYSICAL REVIEW D74,032008 (2006)
