Article
Reference
Measurement of the forward-backward charge asymmetry from W
→eν production in pp collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We report a measurement of the forward-backward charge asymmetry of electrons from W boson decays in pp¯ collisions at s√=1.96 TeV using a data sample of 170 pb−1 collected by the Collider Detector at Fermilab. The asymmetry is measured as a function of electron rapidity and transverse energy and provides new input on the momentum fraction dependence of the u and d quark parton distribution functions within the proton.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the forward-backward charge asymmetry from W →eν production in pp collisions at s√=1.96 TeV. Physical Review. D , 2005, vol. 71, no. 05, p. 051104
DOI : 10.1103/PhysRevD.71.051104
Available at:
http://archive-ouverte.unige.ch/unige:38304
Disclaimer: layout of this document may differ from the published version.
1 / 1
Measurement of the forward-backward charge asymmetry from W ! e production in pp collisions at
p s
1:96 TeV
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A. Zsenei,18and 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
D. ACOSTAet al. PHYSICAL REVIEW D71,051104 (2005)
051104-2
21The Helsinki Group: Helsinki Institute of Physics, FIN-00014, Helsinki, Finland;
and Division of High Energy Physics, Department of Physical Sciences, University of Helsinki, FIN-00014, 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, University and Scuola Normale Superiore of Pisa, I-56100 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 10 January 2005; published 31 March 2005)
We report a measurement of the forward-backward charge asymmetry of electrons from W boson decays inpp collisions at
ps
1:96 TeV using a data sample of170 pb1 collected by the Collider Detector at Fermilab. The asymmetry is measured as a function of electron rapidity and transverse energy and provides new input on the momentum fraction dependence of theuanddquark parton distribution functions within the proton.
DOI: 10.1103/PhysRevD.71.051104 PACS numbers: 13.38.Be, 13.85.Qk, 14.60.Cd, 14.70.Fm
I. INTRODUCTION
A necessary input for cross section calculations at a hadron collider is an estimate of the momentum distribu- tion of the incoming partons that participate in the hard- scattering process. The probability of finding a parton carrying momentum fractionxwithin the incoming hadron is expressed in the parton distribution function (PDF). At the Tevatron, any cross section calculation will have to integrate over the proton and antiproton PDFs. Presently,
many measurements at the Tevatron have significant un- certainties associated with the choice of PDF. These un- certainties will become more important as the data sets continue to grow. For example, PDF uncertainty is ex- pected to be among the dominant systematic uncertainties in a precision determination of the Wboson mass.
The PDFs are not calculable and must be determined using measurements from a wide range of scattering pro- cesses [1,2]. Measurement of the forward-backward charge
asymmetry inpp!WXprovides important input on the ratio of theuanddquark components of the PDF. Since uquarks carry, on average, a higher fraction of the proton momentum than dquarks [3], a W produced by ud! W tends to be boosted forward, in the proton direction.
Similarly, aWtends to be boosted backward. This results in a nonzero forward-backward charge asymmetry defined as
AyW dW=dyWdW=dyW
dW=dyWdW=dyW; (1) where yW is the rapidity of the W bosons and dW=dyW is the differential cross section for W or W boson production.
Leptonic decays of theW boson, in our caseW !e, provide a high purity sample for measuring this asymme- try. However, because pZ of the neutrino is unmeasured, yW is not directly determined, and we instead measure
Ae de=dede=de
de=dede=de; (2) wheree is the electron pseudorapidity [4]. By assuming theW !edecays are described by the Standard Model VAcouplings,Aeprobes the PDF.
Previous measurements of the asymmetry [5], using 110 pb1 of pp data at
ps
1:8 TeV collected by the Collider Detector at Fermilab (CDF), have provided con- straints on the PDFs for u and d quarks at momentum transfer of Q2 M2W. In this article we describe a new measurement based on data collected with the CDF II detector at
ps
1:96 TeVcorresponding to an integrated luminosity of170 pb1. We measure the asymmetry in two regions of electronETthat probe different ranges ofyWand thus increase sensitivity to the PDFs in the regionx >0:3 where currently they are least constrained.
II. DETECTOR DESCRIPTION
The CDF II detector [6] has undergone a major upgrade since the previous data-taking period. The components relevant to this measurement are described here.
Tracking detectors immersed within a 1.4 T solenoidal magnetic field are used to reconstruct the trajectories (tracks) and measure the momentum of charged particles.
The Central Outer Tracker (COT) is a 3.1 m long open-cell drift chamber which provides track measurements (hits) in 96 layers in the radial range 40 cm< r <137 cm [7].
Closer to the beam, a silicon tracking system [8] provides precise hits from eight layers of sensors spanning1:3 cm<
r <28 cmand extending up to 1.8 m along the beam line.
The COT allows track reconstruction in the rangejj&1.
The silicon detector extends that range tojj&2:5.
Segmented electromagnetic (EM) and hadronic calorim- eters surround the tracking system and measure the energy of particles [9]. The energy of electrons is measured by lead-scintillator sampling calorimeters. In the central re-
gion, jj<1:1, the calorimeters are arranged in a pro- jective barrel geometry and measure EM energy with a resolution of ET=ET2 13:5%2=ETGeV 2%2. In the forward region, 1:2<jj<3:5, the calo- rimeters are arranged in a projective ‘‘end-plug’’ geometry and measure EM energy with a resolution of ET=ET2 14:4%2=ETGeV 0:7%2.
Both central and forward EM calorimeters are instru- mented with finely segmented detectors which measure shower position at a depth where energy deposition by a typical shower reaches its maximum. In the central region we use proportional wire chambers with cathode strip readout; in the forward region shower position is measured by two layers of 5 mm wide scintillating strips with a stereo angle of 45 degrees between them.
III. DATA SETS AND SELECTION
Our signal sample is comprised of W !e candidate events, and a sample of Z0 !ee candidate events is used to calibrate the charge identification. Events of inter- est are initially selected by an online trigger system with differing requirements for the central and forward regions.
For W candidates, the central trigger requires an EM energy cluster with ET>18 GeV and a matching track withpT>9 GeV=c. To avoid any potential charge bias in the track trigger efficiency, we also accept events from a trigger which requires an EM energy cluster with ET>
20 GeV and missing transverse energy (E6 T) of at least 25 GeV, but has no explicit track requirement. The forward trigger for W candidates requires an EM energy cluster with ET>20 GeV and E6 T>15 GeV. A backup trigger drops theE6 T requirement and is used to estimate the QCD jet background contribution. The trigger for Zcandidates requires two EM energy clusters withET>18 GeV.
The criteria used to identify the electron and positron candidates, which are described in detail in Ref. [10] and summarized below, are designed to reject the energy de- posits from photons or QCD jets.
(i) ET>25 GeV.
(ii) FIso<0:1, where FIso additional energy in an
‘‘isolation’’cone, of angular radius R 2 2
p 0:4 centered on the electron, divided by the electron energy.
(iii) It is required that the associated hadronic energy is less than 5% of the EM energy.
(iv) The shower shape in the EM calorimeter and shower maximum detector must be consistent with that observed from test-beam data.
(v) The position along the beam line of the ppcolli- sion, z0, is well reconstructed and jz0j<60 cm [11].
(vi) A track consistent with the position and energy measured in the calorimeter is required.
COT tracks, reconstructed independent of the calorime- ter measurement, can be compared to it in position and
D. ACOSTAet al. PHYSICAL REVIEW D71,051104 (2005)
051104-4
momentum. However, the coverage of the COT is limited
tojj&1. To extend the measurement to higherjj, we
instead use silicon tracks reconstructed by a new calorimeter-seeded algorithm as described below. Two points and a signed curvature define a unique helix. The positions of the electromagnetic shower and of the pp collision provide the two points. The curvature of the trajectory is predicted from the transverse energy measured by the calorimeter. These two points and the curvature are used to generate two seed helices and associated covari- ance matrices, one for each charge hypothesis. Those seed helices are then projected into the silicon detector where hits are attached using a road-based search and requiring at least 4 attached hits with2=dof<8. If silicon tracks are fit for both charge hypotheses, the 2=dof is used to identify the charge with the best fit, and cases with 2=dof<0:5are rejected as ambiguous.
The relative alignment of the silicon detector and the calorimeter is determined using a sample of well identified e with both COT and silicon tracks. To avoid a charge bias from theW charge asymmetry, we explicitly equalize the number of events of each charge used in the alignment for >0and separately for <0. Offsets ofO(1 mm) and rotations ofO(10 mrad) are measured and corrected.
The resulting position resolution in the forward calorimeter is measured to be 1 mrad, consistent with the design expectation.
CandidateW!eevents are required to have exactly one suchecandidate as well asE6 T>25 GeVand trans- verse mass in the range50 GeV=c2< MT<100 GeV=c2. To suppress backgrounds from QCD and Drell-Yan pro- cesses, we require that there be no other EM energy depositions withET >25 GeV. The selected sample con- tains 49 124 central and 28 806 forward events.
IV. MEASUREMENT OF THE CHARGE ASYMMETRY
Directly measured in the experiment and shown in Fig. 1 is the raw, uncorrected, asymmetry. In order to reconstruct
Ae, the measurement needs to be corrected for the effects of charge misidentification and background contri- butions. Thesedependent corrections are applied bin-by- bin, and binning coarser than shown in Fig. 1 is used to reduce the effect of the uncertainty from these corrections.
A. Charge misidentification
The electron identification is constructed, and observed, to have a charge symmetric efficiency. However, resolution effects can lead to misidentification of the charge, which dilutes the asymmetry. Residual misalignments in the sili- con detector and calorimeters could give rise to a bias in the charge identification that would directly bias the asym- metry. We measure the probability of such misidentifica- tion and correct for it. Calling that probabilityf for e andffore, the corrected asymmetry can be computed from the raw asymmetry as A Arawff=1 ff.
We measure f with Z0!ee events where a track matched to one lepton tags the charge of the other.
The tagging leg must have jj<1:5, and COT track information is used if it is available. The average misiden- tification probability i.e., without distinguishing between e and e, is shown as a function of in Fig. 2. The difference between the misidentification probability fore ande is shown in Fig. 3,
B. Background corrections
We correct the measurement for the contributions of three sources of background: QCD jets, Z0 !ee, and W !!!e.
The background contribution from QCD jets faking the W !esignature is measured by comparing the isolation of thee candidate to theE6 T in the event [10]. Electrons from W decays tend to be isolated, i.e., have low Fiso values, while background from QCD jets have larger val- ues. Similarly, W !eevents have largeE6 T while QCD jets have lower values. If there is no correlation between
e
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
η
2.5Uncorrected Asymmetry
-0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4
FIG. 1. The raw, uncorrected, charge asymmetry is plotted as a function of electron.
isolation andE6 Tfor QCD jets, we can measure their shapes in the non-W regions and extrapolate them into the signal region. Studies of these variables demonstrate that they are not correlated if the selection requirements related to the EM shower shape are relaxed. Including those require- ments suppresses events with high values of Fiso, which makes the extrapolation statistically imprecise and de- grades our ability to measure any potential correlation, so we remove them in estimating the QCD jet background.
That results in an overestimate of the background, but it yields a statistically and systematically robust estimate.
This measured upper bound on the background fraction is2%forjj<1and increases to about15%forjj>2.
We correct the raw asymmetry by a factor of 1FQCD, where for the background fraction FQCD we use half the measured upper bound, with uncertainties of50%. Since we have only an upper limit, this choice provides full coverage of the actual value at2.
Z0 !ee events in which one of the leptons is lost represent a small, but asymmetric background [12]. This background contribution is determined with a Monte Carlo calculation using the PYTHIA generator[13], and it corre- sponds to about1%of the signal.W !!!eevents
bias the measured asymmetry because the!decay dilutes the information available in the e direction. This back- ground contribution is about4%of the signal. The number of e ande events predicted for these backgrounds are subtracted from the measured values bin-by-bin in.
Figure 4 shows the fully correctedAe. C.ETdependence
The asymmetry probes a large range ofxfor the parentu anddquarks, from an upper value of approximately 0:5, where valence quarks dominate, down to2103, where sea quarks dominate. Large values ofyWcorrespond to the extreme values ofx. For example, a high-x uquark and a low-xdquark lead toWwith largepZand therefore large yW. TheVAcouplings in theW!edecay cause the e to be preferentially emitted opposite the W flight direction. The electron asymmetry,Ae, is a convolution of these competing production and decay asymmetries, which results in the sign change ofAeat largejej.
Direct sensitivity to the PDF would be improved by reducing the decay asymmetry effect, e.g., by reconstruct- ing the W direction. The unmeasured pZ of the neutrino and the poorE6 T resolution complicate this reconstruction.
e
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
η
2.5-
- f
+f
-0.2 -0.15 -0.1 -0.05 -0 0.05 0.1 0.15 0.2
FIG. 3. The difference in charge misidentification probability ofeande,f f, is plotted as a function of electron.
e
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
η
2.5Charge misid probability
0 0.02 0.04 0.06 0.08 0.1 0.12
FIG. 2. The charge misidentification probability is plotted as a function of electron.
D. ACOSTAet al. PHYSICAL REVIEW D71,051104 (2005)
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However, we can improve the correspondence betweene andyW based on the kinematics of just the electron, which is well measured. The neutrinopZambiguity is a smaller effect for electrons with highET than for those at lowET. We exploit this by separating the asymmetry measurement into bins of electron ET. The size of the statistical and
systematic uncertainties allows two bins,25 GeV< ET<
35 GeVand35 GeV< ET<45 GeV. For a givene, the twoET regions probe different ranges ofyW, and therefore x, and the higherET bin corresponds to a narrower range.
As a result, measuring the asymmetry separately in the two bins allows a finer probe of thexdependence.
e
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
η
2.5Corrected Asymmetry
-0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4
FIG. 4. The fully corrected charge asymmetry is plotted as a function of electron . Both statistical and total (statistical systematic) uncertainties are shown.
| η
e0 0.5 1 1.5 2
|
2.5Corrected Asymmetry
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
< 35 GeV 25 < ET
| η
e0 0.5 1 1.5 2
|
2.5Corrected Asymmetry
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
< 45 GeV 35 < ET
FIG. 5. The measured asymmetry,Ajej, is plotted and predictions from the CTEQ6.1M (solid) and MRST02 (dashed) PDFs are compared using a NLO RESBOS calculation. Both statistical and total (statisticalsystematic) uncertainties are shown. The upper plot is for25< ET<35 GeV. The lower plot is for35< ET<45 GeV.
D. Systematic uncertainties
The corrections for charge misidentification and back- ground contributions are measured and applied separately for eachETbin since they areETdependent. The statistical uncertainty on the charge misidentification correction dominates the systematic uncertainty on the asymmetry measurement. The uncertainty from the QCD jet back- ground correction is small, and the other background un- certainties are negligible.
Detector misalignments can induce an inherent charge bias. Such biases would be naturally corrected by the charge misidentification probabilities measured from the data. Nonetheless, we check the robustness of the charge determination by varying the alignment corrections within their uncertainties and verifying that the resulting changes in the asymmetry are not significant. We also verify that using COT tracks, when they are available, instead of silicon tracks results in no significant difference.
CP invariance requires Ae Ae. The fully corrected data shown in Fig. 4 show no evidence ofCP asymmetry; the level of agreement is characterized by 2=dof9:5=11. The e data are folded together to obtain a more precise measure ofAjej.
These results are most useful as input to future global PDF fits. Such fits use Monte Carlo generators without a full detector simulation. We have studied possible biases introduced by detector effects by comparing the asymme- try from a PYTHIA Monte Carlo generator to the fully simulated results and found no significant effects.
E. Results
The measured asymmetryAjejis listed in Table I and plotted in Fig. 5 for the twoET regions. Predictions from CTEQ [1] and MRST [2] PDFs, which fit to previous CDF results [5], are shown for comparison. Those predictions use a NLO RESBOS Monte Carlo calculation with soft gluon resummation to model the W pT distribution, to which they can be sensitive [14]. Since the previous mea- surements upon which these predictions are based are least constraining forjj>1and do not separate theET depen- dence, inclusion of our results will further constrain future fits and improve the predictions.
ACKNOWLEDGMENTS
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
We thank Pavel Nadolsky for providing the RESBOS predictions. This work was supported by the U.S.
Department of Energy and National Science Foundation;
the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A.P. Sloan Foundation;
the Bundesministerium fuer 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, U.K.; the Russian Foundation for Basic Research; the Comision Interministerial de Ciencia y Tecnologia, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN- CT-20002, Probe for New Physics.
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ln tan"=2is an excellent approximation for the rapid- ity of low mass particles. ETEsin" andpTpsin"
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iEiTni, where ni is the unit vector in the azimuthal plane that points from the beam line to the center of the ith calorimeter tower. The TABLE I. The measured asymmetry values are tabulated in percent with combined statistical and systematic uncertainties.
The listed jej is the event weighted average. Asymmetric uncertainties listed for some values arise because of the Poisson and binomial statistics inherent in the event counting.
jej Ajej
ET>25 25< ET<35 35< ET<45
0:11 3:41:61:5 4:82:0 2:31:9
0:30 6:21:2 7:51:9 6:31:5
0:50 7:51:5 7:51:9 8:81:8
0:70 12:61:3 13:51:8 11:81:7
0:89 12:21:61:4 12:82:3 12:61:71:9
1:09 13:82:3 13:13:5 17:12:9
1:33 16:81:6 17:03:43:0 17:62:4 1:57 13:01:8 7:03:83:6 15:72:2 1:81 2:92:9 11:54:24:5 13:44:44:6
2:04 0:46:25:7 236 281210
2:31 2910 4914 92623
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