Article
Reference
First Measurement of the Ratio of Central-Electron to Forward-Electron W Partial Cross Sections in pp Collisions at
s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al .
Abstract
We present a measurement of σ(pp→W)×B(W→eν) at s√=1.96 TeV, using electrons identified in the forward region (1.2
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . First Measurement of the Ratio of Central-Electron to Forward-Electron W Partial Cross Sections in pp Collisions at s√=1.96 TeV.
Physical Review Letters , 2007, vol. 98, no. 25, p. 251801
DOI : 10.1103/PhysRevLett.98.251801
Available at:
http://archive-ouverte.unige.ch/unige:38403
Disclaimer: layout of this document may differ from the published version.
1 / 1
First Measurement of the Ratio of Central-Electron to Forward-Electron W Partial Cross Sections in p p Collisions at
p s
1:96 TeV
A. Abulencia,24J. Adelman,13T. Affolder,10T. Akimoto,55M. G. Albrow,17S. Amerio,43D. Amidei,35A. Anastassov,52 K. Anikeev,17A. Annovi,19J. Antos,14M. Aoki,55G. Apollinari,17T. Arisawa,57A. Artikov,15W. Ashmanskas,17 A. Attal,3A. Aurisano,42F. Azfar,42P. Azzi-Bacchetta,43P. Azzurri,46N. Bacchetta,43W. Badgett,17A. Barbaro-Galtieri,29
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A. Zanetti,54I. Zaw,22X. Zhang,24J. 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
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, USA
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
18University of Florida, Gainesville, Florida 32611, USA
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, USA
251801-2
23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
24University of Illinois, Urbana, Illinois 61801, USA
25The Johns Hopkins University, Baltimore, Maryland 21218, USA
26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
27High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
28Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
SungKyunKwan University, Suwon 440-746, Korea
29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
30University of Liverpool, Liverpool L69 7ZE, United Kingdom
31University College London, London WC1E 6BT, United Kingdom
32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
34Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
University of Toronto, Toronto, Canada M5S 1A7
35University of Michigan, Ann Arbor, Michigan 48109, USA
36Michigan State University, East Lansing, Michigan 48824, USA
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
43Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, University of Padova, 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 17 February 2007; published 20 June 2007) We present a measurement ofpp!W BW!eat
ps
1:96 TeV, using electrons identified in the forward region (1:2<jj<2:8) of the CDF II detector, in223 pb1of data. We measureB 279613stat9590syst 162lumpb. Combining this result with a previous CDF measurement obtained using electrons in the central region (jj&1), we present the first measurement of the ratio of central-electron to forward-electronWpartial cross sectionsRexp0:9250:006stat 0:032syst, consistent with theoretical predictions using Coordinated Theoretical-Experimental Project on QCD (CTEQ) and Martin-Roberts-Stirling-Thorne (MRST) parton distribution functions.
DOI:10.1103/PhysRevLett.98.251801 PACS numbers: 13.38.Be, 12.38.Qk, 13.85.Qk, 14.70.Fm
The cross section forW boson production inpp colli- sions has been computed at next-to-leading order (NLO) [1] and next-to-next-to-leading order (NNLO) [2] in the strong coupling constant s. Experimental results can be used to test the calculation of higher-order QCD contribu- tions and the parton distribution functions (PDFs) of the
proton. The PDFs describe the momentum distributions of the elementary constituents of the colliding hadrons and are obtained from various parametrized fits to many sets of experimental data. Their uncertainties affect precision measurements like the masses and production cross sec- tions of theWboson and the top quark, at both the Tevatron
and the LHC [3]. Moreover, accurate PDF modeling of the pseudorapidity [4] of the lepton from W decay is re- quired for the use ofWproduction as a luminosity monitor, an attractive option at the LHC [5].
The momentum fractions carried by the partons in col- liding hadrons determine the momentum distribution of the W boson. TheW boson momentum parallel to the proton beam direction cannot be measured inpp collisions, since the longitudinal momentum of the neutrino from the W decay is not measured. A previous measurement of the forward-backward charge asymmetry of leptons fromWs has been used to obtain some input on the momentum fraction dependence of the u and d quark PDFs within the proton [6]. Independent measurements of theW cross section with central and forward leptons provide additional sensitivity to theWrapidityyW(Fig.1) and are a novel way to constrain the PDFs. We present the first attempt to constrain PDFs using the ratio ofW boson cross sections measured with central and forward electrons. The largest experimental uncertainty, due to luminosity, cancels in this ratio. We compare our measurement to the theoretical pre- dictions obtained with two of the most commonly used PDF sets.
The W cross section measurement presented in this Letter is obtained using 22313 pb1 of data collected by the CDF II detector during run II of the Tevatron at
s
p 1:96 TeV.Wbosons are identified by their decays to electrons in the forward region (1:2<jj<2:8), from which we obtain the inclusive cross section times branch- ing fractionpp !W BW !e.
Previous run II results onW production, based on elec- trons withjj&1, were reported by both the CDF and D0 Collaborations [7,8]. In run I, at
ps
1:8 TeV, D0 re- ported a measurement based on electrons atjj<1:1and 1:5<jj<2:5[9], without separating the central from the forward regions.
The CDF II detector is described in detail elsewhere [10]. Tracking detectors inside a 1.4 T solenoidal magnetic field reconstruct the charged particles’ trajectories (tracks) and measure their momenta. The silicon tracking system
(SVX) [11] provides precise measurement points from up to 8 radial layers of strip sensors. Outside the SVX is the central drift chamber (COT), which provides track mea- surements (hits) in 96 radial layers [12]. The COT allows full track reconstruction in the range jj<1. The SVX extends the track reconstruction capability up tojj ’2:8.
Outside the tracking system, electromagnetic (EM) and hadronic (HAD) calorimeters measure the energy of show- ering particles [13]. In the forward region, the position of the EM shower is measured by two layers of scintillating strips (PES) [14]. The first layer of the forward EM calo- rimeter is used as a preshower detector [13]. Gas Cherenkov counters are used to determine the luminosity, with an uncertainty of 5.8% [15,16]. The trigger system has three levels [17,18]. Data used in this analysis are selected by a trigger requiring missing transverse energy 6ET>
15 GeV and an EM cluster in the forward calorimeter withET>20 GeV.
The offline selection of candidate W decays begins by requiring an energy cluster withET >20 GeVin the fidu- cial region of the forward calorimeter at 1:2<jj<2:8.
The ratio of hadronic to electromagnetic energy deposition must be small: EHAD=EEM<0:05. The EM cluster is re- quired to be isolated [19]. The neutrino from theWdecay is identified by requiring6ET>25 GeV.
To reduce the large remaining background, we compare the location and the energy deposition of the EM clusters in the calorimeter to projections of three-dimensional tracks independently reconstructed by the tracking detectors. Our track sample is dominated by tracks seeded by the SVX [20]. Typically, tracks in the region 1:2<jj<1:6 have COT hit information, while those at largerjjdo not. The candidate events are required to have at least one track that extrapolates to the EM cluster shower centroid in the PES detector within 3 cm in the x and y coordinates. The selection is optimized by using a Z!ee data sample where one electron is detected in the central calorimeter [19] and the other one in the forward calorimeter (Z!ee control sample). The probability for matching a track to an EM cluster detected in the forward region inZ!eeevents is 49:20:5%. Thezcoordinate of the track intersection with the beam axis must be within 60 cm from the detector center. Finally, electron candidates must satisfyE=p <2.
After all requirements the sample contains 48 165 events.
The kinematic and geometric acceptance (A) for W! eevents is determined using thePYTHIAevent generator [21] and a full simulation of the CDF II detector based on theGEANTsimulation package [22]. We extractAyWfrom the simulation and convolve it with a NNLO calculation of d=dyW [23]. We compute the central value of the accep- tance using the MRST 2001 next-to-next-to-leading-log (NNLL) PDF set [24] (in analogy with theW cross section measurement in the central region [19] ) and find A 0:25670:0002. Two different sets of next-to-leading- log (NLL) PDFs are available (MRST01E and CTEQ6.1
W Boson Rapidity
0 0.5 1 1.5 2 2.5 3
Acceptance (arb. normalization)
0 0.1 0.2 0.3 0.4 0.5
0.6 η
| < 2.8) η Forward Region (1.2 < |
FIG. 1. Acceptance, obtained from simulation, as a function of the W boson rapidity, AyW. ‘‘Forward Region’’ refers to this measurement (electron pseudorapidity 1:2<jj<2:8) while
‘‘Central Region’’ refers to the analysis reported in [7] (jj<
1). The two analyses sample different regions ofyW.
251801-4
[25] ). To encode uncertainties in the PDF data and dis- agreements between measurements included in the fits, the PDF sets provide eigenvectors formed in the space of the fit parameters (20 eigenvectors for CTEQ and 15 for MRST).
For each eigenvector, two complete PDF sets are provided corresponding to the changes in each direction of that eigenvector. To estimate the uncertainty due to the choice of the PDFs we convolveAyW with the NLO d=dyW [19] for each PDF central value and 1 eigenvalue.
Using the CTEQ6.1 eigenvector basis set, we obtain a con- tribution to the acceptance uncertainty of1:7;1:3%.
This value is roughly twice that obtained using the MRST01E PDF set. We use the difference between the NNLO and NLOd=dyW calculations to estimate the ac- ceptance uncertainty due to higher-order QCD corrections (0:47%). The other uncertainties on A are described below.
The vector sum of the energy of hadrons recoiling against the W boson enters the calculation of 6ET. We tune the detector response to these hadrons by applying scale factors and offsets to the components of their summed energy parallel and perpendicular to the lepton momentum vector. We obtain a systematic uncertainty of 0:35% on A by taking a variation corresponding to 3 standard deviations in the tuning parameters.
The energy scale and resolution modeling of electrons are calibrated withZ!eeevents and result in an uncer- tainty of0:24%. The uncertainty on the scale as a func- tion ofET is determined using theE=pdistribution. The simulation models theETdependence well, and we include a0:26%uncertainty onAdue to the statistical limitations of the constraint.
We vary the amount of material that an electron passes through by1=3of a radiation length, based on measure- ments of electron energy deposition in the preshower de- tector. The resulting contribution to the acceptance uncertainty is0:71%.
Differences in primary vertex reconstruction efficiency between data and simulation contribute less than 0.1% to the acceptance uncertainty. Finally, we vary the parameters of the PYTHIA model which influence the W boson pT distribution [19] within the constraints of a CDF run IZ boson measurement and find the corresponding acceptance uncertainty to be less than 0.1%.
Electron identification, track matching, and E=p effi- ciencies are measured using the Z!eecontrol sample.
The track matching efficiency is corrected to account for the small kinematic differences of the Z electrons with respect to those from W decay and the distribution of electrons coming from Ws. These efficiencies contribute the largest experimental uncertainty to the cross section measurement and are limited by the Z statistics and the understanding of the background in the Z!eesample.
The relative uncertainties on the cross section measure- ment from electron identification, track matching, andE=p
are 2.0%, 1.1%, and 1.0%, respectively. The trigger effi- ciency is also measured from data, using independent triggers, and results in a relative uncertainty of 0.4%. The overall efficiency is reported in TableI.
Backgrounds fall into two categories: multijet events, where one jet mimics an isolated high-pT electron and another jet is mismeasured in the calorimeters causing 6ET, and electroweak backgrounds,Z!eeandW!.
The multijet background is estimated from data. Multijet events are characterized by significant energy in the cone around the electron and small 6ET [19]. We assume that these two variables are not correlated and estimate the number of background events in the signal region using control regions defined by either low6ET or high energy in the isolation cone. We vary the cuts on 6ET and isolation that define the control region and obtain a relative system- atic uncertainty of 50% on the multijet background esti- mate of 1.8%. We check this calculation by determining the fraction of jets that pass our electron criteria and applying this fraction to multijet events with large 6ET. We obtain good agreement.
The electroweak backgrounds are estimated using simu- lation. We separately calculate the fraction ofZ!eeand W !events passing our selection. These fractions are then normalized to data using the theoretical value for the ratio of Z=W[2] and assuming lepton universality for the W!decays. Background fractions from these processes are estimated to be 2.2% and 0.9% forW ! andZ!ee, respectively.
We show theMT [4] distribution for both the signal and background contributions in Fig. 2. The sum of signal simulation and background matches the data well.
From the number of selected events, the luminosity of the sample, the acceptance, efficiencies, and backgrounds [19] (see TableI), we measure the inclusive cross section to be B279613stat9590syst 162lumpb, con- sistent with previous CDF results and with theoretical predictions [19]. The correct PDF must give the same total W cross section for central and forward electrons, within statistical and systematic uncertainties. It follows that the ratio of partial cross sectionspBA, whereAis the kinematic and geometric acceptance, is equal to the true ratio of acceptances for the two regions. This experi- mental ratio can then be compared with acceptance ratios predicted by any set of PDFs. The cross section based on TABLE I. Geometric and kinematic acceptance, overall effi- ciency, and expected number of background events.
Background multijet 84657stat 423syst
BackgroundZ!ee 4175stat
BackgroundW! 107012stat
AcceptanceA 0:25670:0002stat0:00510:0042syst EfficiencyTOT 0:28630:0042stat0:00600:0061syst
central electrons, using the same PDF as used above for the forward-electron measurement [19], is B2771 14stat 47syst pb, after removing uncertainties due to PDFs, luminosity, renormalization scale, and NLO/NNLO effects. The resultingp, measured for reconstructed elec- trons withET>25 GeV,jj&1[7], and6ET>25 GeVis cenp 6643stat 11systpb. In the forward region pforET>20 GeV,1:2<jj<2:8, and6ET>25 GeVis forp 7183stat 21systpb. All systematic uncer- tainties except those due to PDF and to NLO/NNLO effects are assigned top. Most of the luminosity uncertainty for the overlapping data-taking period cancels in the ratio, and we assign a 1% systematic due to time-dependent lumi- nosity uncertainty. All other uncertainties are uncorrelated.
The experimental ratio is Rexpcenp =forp 0:925 0:006stat 0:032syst. We compute also the central- to-forward ratio of acceptances Rth, obtained with two different PDF sets (CTEQ6.1 and MRST01E) at NLO level. For CTEQ6.1 the ratio isRth0:9240:0230:030PDF 0:004 (NLO/NNLO) and for MRST01E Rth 0:9410:0100:012PDF 0:004 (NLO/NNLO), where ‘‘PDF’’
indicates the uncertainty obtained by varying the eigenval- ues relative to a given PDF set.
Figure 3 shows the experimental ratio of partial cross sections (solid triangles) compared to the CTEQ6.1 (upper plot) and MRST01E (lower plot) acceptance ratios (solid circle and square). The data measurement is independent of PDFs. The ratios of acceptances are also computed varying each PDF eigenvalue by 1 (giving 40 values for CTEQ6.1 and 30 for MRST01E, shown as open circles and squares). Data and both PDF sets agree within uncer- tainties, though the central values for MRST01E and CTEQ6.1 are slightly shifted with respect to each other.
The CTEQ6.1 has a larger uncertainty and some of the individual 1 eigenvalues show a sizeable deviation from the central value. This is notably the case for eigen- vector 1 (PDF eigenvalues 1 and 2 in Fig.3), in which the
dominant contribution is due to the u-valence quark, and eigenvector 3 (PDF eigenvalues 5 and 6), in which the most important contribution is due to thed-valence quark. These eigenvectors impact W boson measurements at the Teva- tron, in particular, theWmass measurement. Another large variation is visible for PDF eigenvector 5 (eigenvalues 9 and 10) in which the dominant contribution is due to sea quarks and gluons. This eigenvector is important for theW rapidity distribution at the LHC.
Recently, a calculation of Rthat NNLO became avail- able, which takes into account the spin correlation between electron and neutrino and the experimental selection of this analysis. The authors findRth0:92660:0019, in good agreement with our measurement [26].
In summary, we have measured theWinclusive produc- tion cross section with electrons identified at large pseu- dorapidities (1:2<jj<2:8) to be B2796 13stat9590syst 162lum pb. We have measured a par- tial cross section using forward electrons BA 7183stat 21syst pb and the ratio of central- electron to forward-electron partial cross sections Rexp 0:9250:006stat 0:032syst.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
This work was supported by the US 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.
0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96
0 5 10 15 20 25 30 35 40
CTEQ6.1 PDF
Data
± 1 σ eigenvalues NLO theoretical expectation
PDF eigenvalue
Central/forward acceptance ratio
0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97
0 5 10 15 20 25 30
MRST01E PDF
Data
± 1 σ eigenvalues NLO theoretical expectation
PDF eigenvalue
Central/forward acceptance ratio
FIG. 3. The ratio of central-to-forward acceptances, as a func- tion of the 1eigenvalue of CTEQ6.1 (top) and MRST01E (bottom) PDF sets. Dashed lines separate eigenvectors.
FIG. 2 (color online). MT distribution forW!ecandidates (䊉). The histogram is the sum of the expected background and the predicted (from simulation) signal spectrum.
251801-6
Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, UK; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme under Contract No. HPRN-CT- 2002-00292; and the Academy of Finland.
aVisiting scientist from University of Athens, 15784 Athens, Greece.
bVisiting scientist from University of Bristol, Bristol BS8 1TL, United Kingdom.
cVisiting scientist from University Libre de Bruxelles, B-1050 Brussels, Belgium.
dVisiting scientist from Cornell University, Ithaca, NY 14853, USA.
eVisiting scientist from University of Cyprus, Nicosia CY- 1678, Cyprus.
fVisiting scientist from University College Dublin, Dublin 4, Ireland.
gVisiting scientist from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
hVisiting scientist from University of Heidelberg, D-69120 Heidelberg, Germany.
iVisiting scientist from Universidad Iberoamericana, Mexico D.F., Mexico.
jVisiting scientist from University of Manchester, Manchester M13 9PL, United Kingdom.
kVisiting scientist from Nagasaki Institute of Applied Science, Nagasaki, Japan.
lVisiting scientist from University de Oviedo, E-33007 Oviedo, Spain.
mVisiting scientist from University of London, Queen Mary College, London, E1 4NS, United Kingdom.
nVisiting scientist from University of California Santa Cruz, Santa Cruz, CA 95064, USA.
oVisiting scientist from Texas Tech University, Lubbock, TX 79409, USA.
pVisiting scientist from University of California, Irvine, Irvine, CA 92697, USA.
qVisiting scientist from IFIC (CSIC-Universitat de Valencia), 46071 Valencia, Spain.
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