Article
Reference
First Measurement of the b -Jet Cross Section in Events with a W Boson in pp Collisions at s√=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
The cross section for jets from b quarks produced with a W boson has been measured in pp collision data from 1.9 fb−1 of integrated luminosity recorded by the CDF II detector at the Tevatron. The W+b-jets process poses a significant background in measurements of top quark production and prominent searches for the Higgs boson. We measure a b-jet cross section of 2.74±0.27(stat)±0.42(syst) pb in association with a single flavor of leptonic W boson decay over a limited kinematic phase space. This measured result cannot be accommodated in several available theoretical predictions.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . First Measurement of the b -Jet Cross Section in Events with a W Boson in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2010, vol. 104, no. 13, p. 131801
DOI : 10.1103/PhysRevLett.104.131801
Available at:
http://archive-ouverte.unige.ch/unige:38643
Disclaimer: layout of this document may differ from the published version.
1 / 1
First Measurement of the b-Jet Cross Section in Events with a W Boson in p p Collisions at ffiffiffi
p s
¼ 1:96 TeV
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X. Zhang,25Y. Zheng,9,eand S. Zucchelli6b,6a (CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3University of Athens, 157 71 Athens, Greece
4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
5Baylor University, Waco, Texas 76798, USA
6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy
6bUniversity of Bologna, I-40127 Bologna, Italy
7Brandeis University, Waltham, Massachusetts 02254, USA
8University of California, Davis, Davis, California 95616, USA
9University of California, Los Angeles, Los Angeles, California 90024, USA
10University of California, San Diego, La Jolla, California 92093, USA
11University of California, Santa Barbara, Santa Barbara, California 93106, USA
12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
17Duke University, Durham, North Carolina 27708, USA
18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
19University of Florida, Gainesville, Florida 32611, USA
20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
21University of Geneva, CH-1211 Geneva 4, Switzerland
131801-2
22Glasgow University, Glasgow G12 8QQ, United Kingdom
23Harvard University, Cambridge, Massachusetts 02138, USA
24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
25University of Illinois, Urbana, Illinois 61801, USA
26The Johns Hopkins University, Baltimore, Maryland 21218, USA
27Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
28Center for High Energy Physics, Kyungpook National University, Daegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
Sungkyunkwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;
Chonnam National University, Gwangju, 500-757, Korea;
Chonbuk National University, Jeonju 561-756, 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, Que´bec, Canada H3A 2T8;
Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6;
University of Toronto, Toronto, Ontario, Canada M5S 1A7;
and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3
35University of Michigan, Ann Arbor, Michigan 48109, USA
36Michigan State University, East Lansing, Michigan 48824, USA
37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
38University of New Mexico, Albuquerque, New Mexico 87131, USA
39Northwestern University, Evanston, Illinois 60208, USA
40The Ohio State University, Columbus, Ohio 43210, USA
41Okayama University, Okayama 700-8530, Japan
42Osaka City University, Osaka 588, Japan
43University of Oxford, Oxford OX1 3RH, United Kingdom
44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44bUniversity of Padova, I-35131 Padova, Italy
45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy
47bUniversity of Pisa, I-56127 Pisa, Italy
47cUniversity of Siena, I-56127 Pisa, Italy
47dScuola Normale Superiore, I-56127 Pisa, Italy
48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
49Purdue University, West Lafayette, Indiana 47907, USA
50University of Rochester, Rochester, New York 14627, USA
51The Rockefeller University, New York, New York 10021, USA
52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy
52bSapienza Universita` di Roma, I-00185 Roma, Italy
53Rutgers University, Piscataway, New Jersey 08855, USA
54Texas A&M University, College Station, Texas 77843, USA
55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy
55bUniversity of Trieste/Udine, I-33100 Udine, Italy
56University of Tsukuba, Tsukuba, Ibaraki 305, Japan
57Tufts University, Medford, Massachusetts 02155, USA
58Waseda University, Tokyo 169, Japan
59Wayne State University, Detroit, Michigan 48201, USA
60University of Wisconsin, Madison, Wisconsin 53706, USA
61Yale University, New Haven, Connecticut 06520, USA
62Center for High Energy Physics, Kyungpook National University, Daegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
Sungkyunkwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;
Chonnam National University, Gwangju, 500-757, Korea (Received 16 September 2009; published 2 April 2010)
PRL104,131801 (2010) P H Y S I C A L R E V I E W L E T T E R S 2 APRIL 2010
data from1:9 fb1 of integrated luminosity recorded by the CDF II detector at the Tevatron. TheWþ b-jets process poses a significant background in measurements of top quark production and prominent searches for the Higgs boson. We measure ab-jet cross section of2:740:27ðstatÞ 0:42ðsystÞpbin association with a single flavor of leptonicW boson decay over a limited kinematic phase space. This measured result cannot be accommodated in several available theoretical predictions.
DOI:10.1103/PhysRevLett.104.131801 PACS numbers: 14.70.Fm, 13.85.Ni, 14.65.Fy
The measurement of associated production of aWboson and one or more jets frombquarks, herein referred to as Wþb-jet production, provides an important test of quan- tum chromodynamics (QCD). The understanding of this process and its description by current theoretical calcula- tions are important since it is the largest background to the search for the standard model Higgs boson via WH pro- duction with decayH!bb[1,2], to measurements of top quark properties via single [3,4] and pair production [5–7]
with decay t!Wb, and to some searches for physics beyond the standard model [8].
Theoretical predictions for vector boson production with associatedbjets have a large uncertainty. Summed fixed- order QCD calculations for WþbbþN-jets production are available for up toN¼4additional light flavor jets and take into account b-quark mass effects [9]. The next-to- leading order (NLO) calculations forWþb-jets produc- tion in the 1-jet and 2-jet multiplicities show an enhance- ment over LO up to a factor of 2 for certain diagrams [10–
12]. In order to minimize the impact of the Wþb-jets theoretical uncertainty in top quark property measurements and searches forWHproduction, the theoretical prediction for the cross section ofWþb-jets production is not used in the evaluation of background estimates. Instead, the prediction from theory for the ratio of the event yields from Wþb jets and Wþ inclusive jets, corrected to match what is measured in data control samples, is scaled to the observed cross section of Wþ jets in data. The systematic uncertainty on theWþb-jets yield, driven by imprecise knowledge in the fraction of jets frombproduc- tion, is approximately 30%–40% [1–7]. These uncertain- ties are very large compared to the small expected cross sections of the processes mentioned above. We therefore wish to directly measure theWþb-jet cross section with sufficient precision to improve those background estima- tions. In addition, such a measurement will provide an important constraint on the theoretical predictions.
Finally this measurement is a complement to other Tevatron measurements of vector boson plus heavy flavor jet production [13–16].
In this Letter, we describe a measurement of the b-jet cross section in events with affiffiffi W boson inpp collisions at ps
¼1:96 TeV from a data sample corresponding to an integrated luminosity L¼1:9 fb1 acquired by the Collider Detector at Fermilab (CDF II) [5]. We select events that are consistent with the electronic or muonic
decay of aWboson and contain one or two jets. Among the jets in these selected events, we seek those that originate fromb-quark production. Hadrons that contain abquark have a relatively long lifetime of1:6 ps, and a large mass of 5:3 GeV=c2 [17]. We exploit the B hadron’s long lifetime by examining the charged particles within each jet and attempting to reconstruct a common origin for their trajectories that is well displaced from the primary pp interaction location. The distance between the primary and secondary vertices corresponds to the trajectory through which the relativistically boosted Bhadron trav- eled during its lifetime. The technique is commonly known as vertexbtagging.
Thentagtagged jets in the selected sample are not purely frombjets. Charm hadrons and certain light flavor hadrons have an appreciable lifetime, and hence jets containing these hadrons can be tagged despite not originating from b-quark production. Also, the finite resolution of the CDF tracking system can allow for spurious displaced vertices.
In order to reduce contamination from charm and light flavor or gluon jets, the requirements on the quality of the secondary decay vertex have been optimized for this measurement. Further, we exploit the B hadron’s large mass by examining the invariant mass of the charged particles forming the secondary decay vertex (vertex mass, Mvert). Vertex mass is correlated with the mass of the parent hadron and partially discriminates between the possible jet flavors to yield the b-jet fraction, fb. The number ofbjets from other processes,nbbkgjets, is estimated for top quark pair, single top quark, diboson, and multijet production. The acceptanceAbWþbjets is defined with respect to the restricted region of phase space defined below. The b-jet identification efficiency, btag, and the event trigger efficiencies,, are calibrated with data. The cross section for b jets times the branching fraction for one flavor of W !‘decay is defined as
bjetsBðW !‘Þ ¼ ntagfbnbbkgjets
Pi¼e;ðLAbWþbjetsbtagÞi (1) where the sum is over the electron and muon channels.
It is important to note that we quote our result as a jet- level cross section in order to avoid a model-dependent correction on the number ofbjets per event that would be required to convert our result into an event-level cross section. Further the result is defined in a restricted region 131801-4
of phase space for the kinematics and multiplicity of the outgoing particles in order to make comparisons with theoretical predictions that minimize extrapolation outside the experimentally accessible region. We define this re- stricted region of phase space as coincident with our analy- sis selection criteria, namely, to events that contain one or two hadron-level jets withET >20 GeVandjj<2:0, an electron or muon withpT>20 GeV=candjj<1:1, and a neutrino with pT >25 GeV=c [18]. We compute the theoretical predictions with these requirements imposed as well.
The data used in this measurement come from the general purpose CDF II detector [19] operating at Fermilab’s Tevatron collider. Detailed descriptions of the various subdetectors new for Run II can be found else- where [20–24]. The data are collected with a charged lepton trigger that requires an electron (muon) candidate withjj<1:0(1.1) andET>18 GeV(pT>18 GeV=c).
The identified charged lepton and the large missing trans- verse energy, E6 T, from the undetected neutrino provide background suppression compared to hadronicW decays.
In offline event selection we require a single reconstructed electron (muon) withET>20 GeV(pT>20 GeV=c) that is well isolated from other activity in the calorimeter, and E6 T>25 GeV. A cone-algorithm-based jet reconstruction with cone size R¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2þ2
p ¼0:4 is used; jet re- construction and calibration are described elsewhere [25].
We require exactly one or two jets withET>20 GeVand jj<2:0. Events consistent with cosmic rays,Z!‘þ‘, photon conversions, and multijet QCD production are re- jected [26]. In data from1:9 fb1of integrated luminosity, there are 175 712 events satisfying theW selection and jet requirements.
Theb-tagging criteria have been designed for this mea- surement to obtain a significantly higher purity forbjets and thus reduce the overall systematic uncertainty from the model of the vertex mass distribution. With respect to the default CDF vertexbtagging [5], this optimized algorithm reduces the rate for false positives of jets from light quark flavors (u,d,s) and gluons by a factor of 10 and charm by a factor of 4 at the expense of a 50% reduction in efficiency forbjets. To be considered forbtagging, charged particle tracks within the jet cone are required to have pT >
0:5 GeV=c, and impact parameter significance d0
d0>3:5, where the impact parameter d0 is the distance of closest approach of the particle track to the location of the primary pp interaction in the transverse plane with respect to the beam axis, andd0 is its uncertainty. Particles must also have an impact parameter less than 0.15 cm, originate from within 2 cm of the primary pp interaction location in the z coordinate [18], and have at least a minimum number of hits from the silicon tracking detectors. These requirements reduce contamination from interactions with detector material, multiplepp interactions, and misrecon- struction, respectively. A common decay vertex is sought
among subsets of these selected particles, and if one is found that contains three or more particles, then the decay length in the transverse plane, L2D, is calculated as the projection along the jet axis of the displacement of the secondary vertex with respect to the primary pp interac- tion location. The vertex is required to have decay length significance L2D
L2D>7:5, and pseudo-c L2DpMT;vertvertc<
1:0 cm, where the invariant mass Mvert and transverse momentum pT;vert of the vertex are calculated from the constituent particles. Note that the mass of each particle is set to the charged pion mass. Any vertices consistent with K0S and decay, and nuclear interactions in the detector material are rejected. The sign of the vertex tag is deter- mined by the position of the vertex with respect to the jet direction; those on the same (opposite) hemisphere as the jet direction are called positively (negatively) tagged.
Among the events satisfying our event selection,ntag ¼ 943 jets are found to be positively tagged. The flavor composition of the positively tagged sample is determined through a maximum likelihood fit of the distribution of the vertex mass in the data. Simulated distributions forband charm jets are formed from standard model processes that are major contributors to the selected event sample.
Sources of bjets includeWþbjets, which is simulated by the Monte Carlo event generator ALPGENversion 2.1.0 [27] withCTEQ5Lparton densities [28] andPYTHIAversion 6.325 for hadronization [29];ttand diboson production are simulated withPYTHIAversion 6.216, and single top quark production byMADEVENTversion 4.2.11 [30]. The yields of tagged jets from these processes are determined from the simulated samples scaled to the latest theoretical cross sections [31–33] with the event selection requirements applied.
We check the simulation model of b jets against an independent data sample from double-tagged dijet events collected with a singlepT>9 GeV=cmuon trigger. One jet is required to contain the muon, presumably from semi- leptonic B hadron decay. The other tagged jet in these events is a sample whose b-jet purity is estimated to be above 99%. This sample is used to validate the model of the b-jet vertex mass; the agreement between simulation and data is shown in Fig.1. We use the difference between simulation and data to estimate the systematic uncertainty from theb-jet model.
Vertex tags of jets from charm hadrons are primarily due toWþc-jets production, which is simulated withALPGEN. Positive vertex tags of light flavor jets are modeled with a simulation of inclusive jet production fromPYTHIA. We use negatively tagged jets in the data as an alternative model for light flavor. This second light flavor model is used in the vertex mass fit to assess the impact of light flavor model choice on the result.
The maximum likelihood fit of the vertex mass data distribution, shown in Fig. 2, is used to extract two pa- rameters: the fraction of jets from bottom hadronsfb, and PRL104,131801 (2010) P H Y S I C A L R E V I E W L E T T E R S 2 APRIL 2010
the fraction of jets from charmed hadrons fc, where the fraction of jets from light flavors is fLF1fbfc. The best fit is fb¼0:710:05ðstatÞ corresponding to 67044ðstatÞ tagged jets from bottom hadrons. From simulated experiments with flavor compositions similar to the data, we confirmed that our vertex mass fit procedure returns results consistent with the assumed background content. These simulated experiments indicated that the systematic uncertainties on the model of theb, charm, and light flavor vertex mass distributions manifest themselves
respectively, on the fitted b-jet fraction.
This yield ofbjets includes our signal but also contains a contribution from other processes with jets fromb-quark production. We use simulated samples and the theoretical predictions for production rates oftt[31], single top quark [32], and diboson processes (WZ, WW, andZZ) [33] in order to estimate a contribution of 15221b jets from these processes. This includes a small contribution of 7:30:8jets fromWþb-jets production withW !, which is treated as a background. Sources of systematic uncertainty in the background yield of taggedbjets include the uncertainty in theb-jet tagging efficiency in the data (a relative 6% uncertainty on all tagged b-jet yields), the uncertainty on the top quark and diboson predicted cross sections (a relative uncertainty of 10% onttand diboson, and 30% on single top yields, which translate to an overall 2% uncertainty on bjetsB) and the uncertainty in the accumulated CDF luminosity (a relative 6% on all yields).
We estimate a contribution of258bjets from QCD multijet production, where mismeasured jets pass the lep- ton identification requirements and result in sufficientE6 T. As this background is difficult to model with simulation, a complementary data sample was collected with the same highpT electron trigger, but where the electron candidate failed at least two of the identification criteria [34]. This provides both a model of theE6 T distribution, which is used to estimate the rate of QCD multijet background above our selection [34], and a vertex mass distribution, which is used to determine the fraction of tagged jets from bottom had- rons. The model for tagged jets from multijet production is statistics limited; we recover statistics by relaxing theE6 T
requirement and perform the vertex mass distribution fit for E6 T>15and 20 GeV as well as the defaultE6 T>25 GeV and use all three results to determine the fittedbfraction from multijet production. The uncertainties on the QCD multijet taggedb-jet background come from the modeling of theE6 Tdistribution for the overall multijet normalization (a relative 30% uncertainty, which translates to a 1%
uncertainty on bjetsB), and the spread in the fittedb fraction from vertex mass distribution fits from the differ- entE6 Tthresholds (a relative 25% uncertainty, which trans- lates to a 1% uncertainty onbjetsB).
After subtracting the background ofnbbkgjets¼17722, we have a yield of49348ðstatÞtaggedbjets fromWþb production. We define the acceptance, AbWþbjets, of our se- lection with respect to a restricted region of kinematic phase space, as defined earlier. The phase space restrictions are applied to the outgoing leptonicWdaughters and jets in the simulatedWþbproduction ALPGENevents. Hadron- level jets are defined bySPARTYJET[35] as a collection of simulated final state particles that have been clustered using the same cone algorithm as in the jet reconstruction.
A hadron-level jet is said to be bmatched if it has R <
0:4with respect to a bquark in the simulated event. The FIG. 2 (color online). Maximum likelihood fit of the vertex
mass for tagged jets in the selected data sample.
FIG. 1 (color online). Comparison of the vertex mass distribu- tions of taggedbjets from a simulated sample and from data.
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matching considersb-quark candidates after showering but before hadronization. The denominator of the acceptance is the number ofb-matched hadron-level jets in simulated Wþb-jet events that pass the phase space requirements as given earlier; the numerator is the number ofb-matched reconstructed jets in simulatedWþb-jet events that pass the phase space requirements and, in addition, the event selection described above through the jet multiplicity re- quirement. The weighted average acceptance over the electron and muon channels is found to be 0:680:03, where the sources that dominate the systematic uncertainty are the jet energy calibration (3%), the factorization and renormalization scale (3%), and the dependence of event kinematics on the parton distribution functions (2%).
For clarity, we separate theb-tag efficiency and several data-based corrections from the acceptance. The b-tag efficiency is the ratio of the number of b-tagged recon- structed b-matched jets to the number of reconstructed b-matched jets in the simulated Wþb-jet events that have passed the event selection and phase space require- ments:b;simtag ¼0:1770:001ðstatÞ. This value needs to be corrected by a factor of 0:880:01ðstatÞ 0:05ðsystÞ, which quantifies the discrepancy in tag efficiency between simulation and data [36]. The correctedb-tag efficiency is thenbtag¼0:1560:009. The final correction factoris the average over all triggers of the product of the following three terms determined from data: the fraction of events that happen in the luminous region well contained by the CDF detector, with primarypp interaction within 60 cm of the center of the detector along the beam line, 0:963 0:003; the efficiency of the trigger,0:9430:004; and the correction factor for charged lepton identification effi- ciency,0:9690:004.
Having obtained all of the information needed as input to Eq. (1), we measure theb-jet cross section to bebjets BðW !‘Þ ¼2:740:27ðstatÞ 0:42ðsystÞpb with a W boson decaying to a single leptonic flavor within the restricted kinematic phase space defined earlier. The over- all relative uncertainty on the measurement is 18%. This uncertainty is dominated by the uncertainty in the b-jet vertex mass model (a relative 8% onbjetsB), the tag efficiency (6%), and the luminosity (6%). The results in the electron and muon channels were examined independently as a cross check and are consistent.
Finally, we have determined the theoretical prediction of bjetsB, using our kinematic definition above, at lead- ing order from PYTHIA and at summed fixed order from
ALPGEN. ThePYTHIAprediction is 1.10 pb and theALPGEN
prediction is 0.78 pb, assuming aQ2 scale ofM2Wþp2T;W; these predictions are factors of 2.5–3.5 lower than our result. These are important comparisons given the wide use of these programs in the generation of simulated phys- ics events at the Tevatron and LHC experiments. A NLO calculation of bjetsB has recently been completed [37]; their prediction of 1:220:14ðsystÞpb is also low
with respect to the measured value. Further study is under- way to examine the differential cross section as a function of jet kinematics and compare to LO, summed fixed-order and NLO predictions.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Founda- tion; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Science and Technology Facilities Council and the Royal Society, UK; the Institut National de Physique Nucleaire et Physique des Particules/CNRS;
the Russian Foundation for Basic Research; the Ministerio de Educacio´n y Ciencia and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; and the Academy of Finland.
aDeceased
bVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA.
cVisitor from Universiteit Antwerpen, B-2610 Antwerp, Belgium.
dVisitor from University of Bristol, Bristol BS8 1TL, United Kingdom.
eVisitor from Chinese Academy of Sciences, Beijing 100864, China.
fVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.
gVisitor from University of California Irvine, Irvine, CA 92697, USA.
hVisitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA.
iVisitor from Cornell University, Ithaca, NY 14853, USA.
jVisitor from University of Cyprus, Nicosia CY-1678, Cyprus.
kVisitor from University College Dublin, Dublin 4, Ireland.
lVisitor from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
mVisitor from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017.
nVisitor from Kinki University, Higashi-Osaka City, Japan 577-8502.
oVisitor from Universidad Iberoamericana, Mexico D. F., Mexico.
pVisitor from University of Iowa, Iowa City, IA 52242, USA.
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9PL, England.
sVisitor from Nagasaki Institute of Applied Science, Nagasaki, Japan.
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uVisitor from University de Oviedo, E-33007 Oviedo, Spain.
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yVisitor from Bergische Universita¨t Wuppertal, 42097 Wuppertal, Germany.
zOn leave from J. Stefan Institute, Ljubljana, Slovenia.
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