Article
Reference
Search for Charged Higgs Bosons in Decays of Top Quarks in pp Collisions at s√=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We report on the first direct search for charged Higgs bosons decaying into cs in tt events produced by pp collisions at s√=1.96 TeV. The search uses a data sample corresponding to an integrated luminosity of 2.2 fb−1 collected by the CDF II detector at Fermilab and looks for a resonance in the invariant mass distribution of two jets in the lepton+jets sample of tt candidates. We observe no evidence of charged Higgs bosons in top quark decays. Hence, 95% upper limits on the top quark decay branching ratio are placed at B(t→H+b)< 0.1 to 0.3 for charged Higgs boson masses of 60 to 150 GeV/c2 assuming B(H+→cs¯)=1.0. The upper limits on B(t→H+b) are also used as model-independent limits on the decay branching ratio of top quarks to generic scalar charged bosons beyond the standard model.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for Charged Higgs Bosons in Decays of Top Quarks in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2009, vol.
103, no. 10, p. 101803
DOI : 10.1103/PhysRevLett.103.101803
Available at:
http://archive-ouverte.unige.ch/unige:38623
Disclaimer: layout of this document may differ from the published version.
1 / 1
Search for Charged Higgs Bosons in Decays of Top Quarks in p p Collisions at ffiffiffi p s
¼ 1:96 TeV
T. Aaltonen,24J. Adelman,14T. Akimoto,56B. A´ lvarez Gonza´lez,12,rS. Amerio,44b,44aD. Amidei,35A. Anastassov,39 A. Annovi,20J. Antos,15G. Apollinari,18A. Apresyan,49T. Arisawa,58A. Artikov,16W. Ashmanskas,18A. Attal,4
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A. Warburton,34D. Waters,31M. Weinberger,54J. Weinelt,27W. C. Wester III,18B. Whitehouse,57D. Whiteson,46,g A. B. Wicklund,2E. Wicklund,18S. Wilbur,14G. Williams,34H. H. Williams,46P. Wilson,18B. L. Winer,40P. Wittich,18,i
S. Wolbers,18C. Wolfe,14T. Wright,35X. Wu,21F. Wu¨rthwein,10S. Xie,33A. Yagil,10K. Yamamoto,42J. Yamaoka,17 U. K. Yang,14,oY. C. Yang,28W. M. Yao,29G. P. Yeh,18J. Yoh,18K. Yorita,58T. Yoshida,42G. B. Yu,50I. Yu,28S. S. Yu,18
J. C. Yun,18L. Zanello,52b,52aA. Zanetti,55aX. 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
9University 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
22Glasgow University, Glasgow G12 8QQ, United Kingdom
23Harvard University, Cambridge, Massachusetts 02138, USA
101803-2
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
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
54Texas A&M University, College Station, Texas 77843, USA
55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, I-33100 Udine, 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 (Received 8 July 2009; published 4 September 2009)
We report on the first direct search for charged Higgs bosons decaying intocsinttevents produced by ppcollisions at ffiffiffi
ps
¼1:96 TeV. The search uses a data sample corresponding to an integrated luminosity of2:2 fb1collected by the CDF II detector at Fermilab and looks for a resonance in the invariant mass distribution of two jets in the leptonþjets sample ofttcandidates. We observe no evidence of charged Higgs bosons in top quark decays. Hence, 95% upper limits on the top quark decay branching ratio are placed atBðt!HþbÞ< 0.1 to 0.3 for charged Higgs boson masses of 60 to150 GeV=c2 assuming BðHþ!csÞ ¼1:0. The upper limits onBðt!HþbÞare also used as model-independent limits on the decay branching ratio of top quarks to generic scalar charged bosons beyond the standard model.
DOI:10.1103/PhysRevLett.103.101803 PACS numbers: 14.80.Cp, 12.60.Jv, 13.85.Rm, 14.65.Ha
The standard model (SM) is remarkably successful in describing the fundamental particles and their interactions.
Nevertheless, it is an incomplete theory. An important unresolved question is the mechanism of electroweak sym- metry breaking (EWSB). In the SM, a single complex scalar doublet field breaks the symmetry, resulting in mas- sive electroweak gauge bosons and a single observable Higgs boson [1]. To date, the Higgs boson has not been discovered, and consequently the mechanism of EWSB remains in question.
Beyond the SM, many diverse hypotheses with extended Higgs sectors have been proposed to explain EWSB. The simplest extension is a two Higgs-doublet model (2HDM).
The minimal supersymmetric standard model (MSSM) employs the type-II 2HDM, where at leading order one doublet couples to the up-type fermions and the other couples to the down-type fermions [2]. The two Higgs- doublet fields manifest themselves as two charged Higgs bosons (H) and three neutral Higgs bosons (h,H,A).
In 2HDM and MSSM, the top quark is allowed to decay into a charged Higgs boson (Hþ) [3] and a bottom quark.
The tree level branching ratio of top quarks toHþ,Bðt! HþbÞ, is a function of theHþmass (mHþ) andtan. The parametertanis the ratio of vacuum expectation values of the two Higgs doublets. In MSSM, Bðt!HþbÞ also depends on extra parameters related to the masses and couplings of the other supersymmetric particles. The Bðt!HþbÞ is relatively large if tan is small (&1) or large (*15) [4]. At low tan, Hþ predominantly decays intocsfor lowmHþ(&130 GeV=c2) andtb(!Wbb) [5]
for higher mHþ. In the high tan region, the Hþ decays intoþalmost 100% of the time.
At Tevatron collider experiments, Hþ searches have been performed for the Hþ! in tt decays. Some searches placed direct upper limits on Bðt!HþbÞ by taking advantage of the expectation that BðHþ !Þ ¼ 1:0at hightan[6]. Other searches set limits on the MSSM parameter plane (mHþ, tan) using inclusive Hþ decay branching ratios in the MSSM [7]. The various Hþ final states supplement the SMttdecay channels. The previous searches focused on measuring deviations from the SM prediction for the tt production and decay, rather than reconstructingHþ bosons.
In this Letter, we report on the first direct search for Hþ !cs produced in top quark decays by fully recon- structing thecsmass. The final state ofHþ !csis mostly two jets, as is the hadronic decay of theWboson [8] in SM top quark decays. The search is performed by looking for a second peak in the dijet mass spectrum (in addition to that from theW boson) in top quark decays.
In the SM, each top quark decays into a W boson and a b quark exclusively. In this analysis we use the leptonþjetsttsample [9], where in the SM oneWdecays
to quarks (qq0) and the otherW decays toe or. Each final-state quark is assumed to form a hadronic jet; the jets are clustered using a cone algorithm with a cone radius R [¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðÞ2þ ðÞ2
p ] of 0.4 [10]. This leptonþjets sample has a good signal-to-background ratio forttand is ideal for dijet mass analysis.
The CDF II experiment at the Fermilab Tevatron mea- sures the products of proton-antiproton collisions at ffiffiffi
ps 1:96 TeV. The lepton momentum is measured using an¼ eight-layer silicon microstrip detector and a cylindrical drift chamber immersed in a 1.4 T magnetic field. The energies of electrons and jets are measured using calorim- eters with acceptance up to pseudorapidity as ofjj ¼3:6 [11]. Charged particle detectors outside the calorimeter identify muon candidates up to jj ¼1:0. Details of CDF II can be found elsewhere [12].
Leptonþjets tt events are selected by requiring an electron or a muon with pT >20 GeV within jj ¼1 and by requiring missing transverse energy larger than 20 GeV to account for the neutrino [13]. Then, the four most energetic jets (called leading jets) withinjj<2.0 are required to haveET>20 GeVafter jet energy corrections [10]. In addition, at least two of the leading jets are required to contain a long-lived hadron containing a b quark [14] by demanding that these jets contain tracks forming a displaced secondary vertex (called abtag).
The SM processes are regarded as backgrounds for the Hþ search. The largest background isW bosons in SMtt events (92% of the total background). The rest of the SM processes are referred to as non-tt backgrounds. These include Wþjets, multijets,Zþjets, diboson (WW,WZ, ZZ), and single top events. Details of the non-tt back- ground estimation method are given in [14]. Assuming a ttcross section of 6.7 pb [15] and a top quark mass of 175 GeV=c2, we expect152:625:0 events from SMtt production and13:97:5events from non-ttbackgrounds in the2:2 fb1 CDF II data sample.
The mass of theHþcandidate is directly reconstructed using the two jets. The mass resolution is improved by reconstructing the tt event as a whole with a kinematic fitter used for the precision top quark mass measurement described in Ref. [12]. The original kinematic fitter is modified for the Hþ search. In the fitter, the lepton, the missingET(from a neutrino), and the four leading jets are assigned to the decay particles from the ttevent, and the quality of the assignment is evaluated using this2:
2¼ X
k¼jjb;lb
ðMkMtÞ2 t2 þ X
i¼l;4jets
ðpi;fitT pi;measT Þ2 i2 þðMlMWÞ2
W2 þ X
j¼x;y
ðpjUE;fitpjUE;measÞ2 UE2 : (1)
101803-4
The2is minimized by constraining the leptonicW final state (l) to have theWinvariant mass (80:4 GeV=c2) [16]
and both top quark final states (blandbjj) to have the same top quark mass of 175 GeV=c2. No constraint is imposed on the dijet mass of the hadronic boson (jj). In the mass constraints, the transverse energies of the final- state objects (pi;measT ) are allowed to vary within measure- ment uncertainties (i). The unclustered energy (pUE;measj ) is the sum of measured transverse energies not included in the leading jetsETand is used to correct the missingET. In the jet assignment, b-tagged jets are assigned to the bquarks. The jets assigned to thebquarks are calledbjets, and the other two jets are called h jets. If the tt event has more than twob-tagged jets, the jets with the best2 are assigned to b quarks. Then, we reconstruct the mass of the hadronic boson using two h jets with fit energies (pi;fitT ). In this kinematic event reconstruction, only 55% of the SMttevents have correctly matching jets. The wrong jet-parton assignments dominantly come from hard radia- tion jets, which are selected as leading jets and from the falselyb-tagged jets originating from the hadronic decays ofWbosons.
The expected dijet mass distributions ofHþ andW in top quark decays are produced using thePYTHIAgenerator [17] and the full CDF II detector simulation. TheALPGEN
generator [18] with thePYTHIAparton shower simulation is used for non-ttbackgrounds. In the simulation sample, the Hþ is forced to decay solely intocswith zero width and with masses ranging from 60 to150 GeV=c2.
The simulation shows that the reconstructedHþ has a significant low-mass tail, which is predominantly caused by final-state gluon radiation (FSR) from the hadronic decays of the Higgs boson. The hard FSR results in more than four final-state jets in a leptonþjets tt event. To recover the energy loss due to the FSR, the fifth most energetic jet is merged with the closest jet among the four leading ones if the pair has a R distance smaller than 1.0, provided that the fifth most energetic jet hasET >
12 GeV and jj<2:4. Merging the fifth jet results in better jet energy resolution and improves themHþ resolu- tion by approximately 5% in more than four final jet events for the120 GeV=c2 Higgs sample.
In the CDF II data sample of2:2 fb1, we observe 200tt candidates in the leptonþjets decay channel. No signifi- cant excess is observed in the dijet invariant mass of top quark decays. Figure1shows that the observed dijet mass distribution agrees with the SM expectations. Hence, we extract upper limits onBðt!HþbÞusing a binned like- lihood fit on the dijet mass distribution.
The binned likelihood (LH) function is constructed em- ploying Poisson probabilities:
L ¼Y
i
niiei
ni! GðNbkg; NbkgÞ: (2) The probability of finding events in the mass binicomes
from a set of simulated dijet mass distributions ofHþ,W, and non-tt backgrounds. These distributions are called templates. The Poisson probability (Pi) in each bin is computed from the number of observed events, ni, and from the number of expected events, i¼PiHþNHþþ PiWNWþPibkgNbkg, whereNHþ,NW, andNbkgare parame- ters representing the total number of events in each tem- plate category. The minimization oflnLgives the most probable values forNHþ,NW, andNbkg. In the LH fit,NHþ andNW are free to vary, however, the non-ttbackground (Nbkg) is estimated independently and is allowed to vary within its Gaussian uncertainty (Nbkg). Based on the num- ber of events from the LH fit, aBðt!HþbÞis extracted assumingBðHþ !csÞ ¼ 1. In Fig.1, dijet mass distribu- tions of the SM events are normalized by the likelihood fit to the observed dijet mass distribution with Bðt!HþbÞ fixed to 0.
The sources of systematic uncertainty in the extracted Bðt!HþbÞ include uncertainties in the jet energy scale corrections [10], initial state and final-state radiation, mod- eling of the non-ttbackground, choice of event generators in simulation. These systematic sources perturb the shape of the dijet mass and cause a shift in the result of the LH fit.
The shift in the resulting Bðt!HþbÞis estimated using
‘‘pseudoexperiments’’ of the perturbed and unperturbed dijet mass distributions for each systematic source; the pseudoexperiments are generated by the bin-to-bin Poisson fluctuations of the simulated dijet mass distribu- tions. The dominant systematic uncertainty originates from
2] M(dijet) [GeV/c
0 20 40 60 80 100 120 140 160 180 ]2Number of Events/[6 GeV/c
0 5 10 15 20 25 30 35 40
0 20 40 60 80 100 120 140 160 180 0
5 10 15 20 25 30 35 40
Data
b) = 0.1 H+
→ B(t
t in t W+
bkg t non-t
FIG. 1 (color online). Observed dijet mass distribution (crosses) compared with background distributions ofW bosons (filled) and non-tt processes (cross hatched) in CDF II data sample of 2:2 fb1; the background distributions are added on top of each other. An example of the dijet mass distribution from 120 GeV=c2Hþbosons (bold line) is overlaid assumingBðt! HþbÞ ¼0:1, which is about the 95% C.L. upper limit onBðt! HþbÞ.
the choice of event generators in the simulation, unless mHþ is close to mW, in which case the jet energy scale uncertainty dominates. The other systematic uncertainties from data/Monte Carlo differences inb-tagging rates and top quark mass constraints inttreconstruction are negli- gible compared to the uncertainties from the perturbed dijet mass shape.
The individual systematic uncertainties are combined in quadrature. The total systematic uncertainty [Bðt! HþbÞ] is represented by a nuisance parameter which adds to the branching ratio and has a Gaussian prior PDF with width Bðt!HþbÞ. We eliminate this nuisance parameter by Bayesian marginalization [19] and obtain a posterior PDF in Bðt!HþbÞ assuming a uniform prior PDF in0Bðt!HþbÞ 1. The expected upper limits onBðt!HþbÞwith 95% C.L. are derived from a thou- sand pseudoexperiments using the SM backgrounds events for eachmHþ.
The upper limits onBðt!HþbÞ at 95% C.L. show a good agreement between the observation and the SM expectation. The upper limits in Fig.2include the system- atic uncertainty inBðt!HþbÞ. Since the LH fit has very little sensitivity for mHþ mW, the upper limits around
80 GeV=c2 Hþare omitted in the figure. The exact values of the upper limits in Fig.2are listed in TableI.
This analysis can set model-independent limits for anomalous scalar charged boson production in top quark decays. Besides the assumption that a scalar boson de- cays only to cs with zero width, no model-specific pa- rameter is used in this analysis. Therefore any generic charged boson would make a secondary peak in the dijet mass spectrum if it decays into a dijet final state like the Hþ!csin top quark decays. Here, we extend the search below theWboson mass [20] down to60 GeV=c2for any non-SM scalar charged boson produced in top quark de- cays, t!Xþð!udÞb . This process is simulated for the CDF II detector and is similar to Hþ!cs. In the simu- lation, we obtain a better dijet mass resolution for ud decays than for thecsdecays. The difference in the mass resolution comes from the smaller chance of falseb tag- ging from light quark final states ofXþthan thecsdecays, thus resulting in a smaller ambiguity of jet-parton assign- ments in the tt reconstruction. Consequently, the upper limits on Bðt!Xþð!udÞbÞ are lower than the limits on Bðt!Hþð!csÞbÞ regardless of the charged boson mass.
In summary, we have searched for a non-SM scalar charged boson, primarily the charged Higgs boson pre- dicted in the MSSM, in top quark decays using leptonþ jets ttcandidates. This is the first attempt to search for Hþ!csusing fully reconstructed charged Higgs bosons.
In the CDF II data sample of2:2 fb1, we find no evidence of charged Higgs bosons in the dijet mass spectrum of the top quark decays. Hence, upper limits on Bðt!HþbÞ with 95% C.L. are placed at 0.1 to 0.3 assumingBðHþ! csÞ ¼ 1:0for charged Higgs masses of 60 to150 GeV=c2. This analysis also yields conservative upper limits on any non-SM scalar charged boson Xþ production from top quarks. Based on simulation, we find that the upper limits on the branching ratioBðXþ!udÞ are always better than the upper limits onBðHþ!csÞ.
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 TABLE I. Expected and observed 95% C.L. upper limits onBðt!HþbÞforHþmasses of 60
to150 GeV=c2.
mHþðGeV=c2Þ 60 70 90 100 110 120 130 140 150
Expected 0.13 0.19 0.22 0.15 0.13 0.12 0.10 0.10 0.09
Observed 0.09 0.12 0.32 0.21 0.15 0.12 0.08 0.10 0.13
2] ) [GeV/c M(H+
60 80 100 120 140 160
) = 1.0s c→+ b) with B(H+ H→B(t
0 0.1 0.2 0.3 0.4 0.5
0.6 Observed @ 95% C.L.
Expected @ 95% C.L.
68% of SM @ 95% C.L.
95% of SM @ 95% C.L.
FIG. 2 (color online). The upper limits onBðt!HþbÞat 95%
C.L for charged Higgs masses of 60 to 150 GeV=c2 except a region formHþ mW. The observed limits (points) in2:2 fb1 CDF II data are compared to the expected limits (solid line) with 68% and 95% uncertainty band.
101803-6
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, United Kingdom; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; and the Academy of Finland.
aDeceased.
bVisitors from Universiteit Antwerpen, B-2610 Antwerp, Belgium.
cVisitors from the University of Bristol, Bristol BS8 1TL, United Kingdom.
dVisitors from the Chinese Academy of Sciences, Beijing 100864, China.
eVisitors from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.
fVisitors from the University of California Irvine, Irvine, CA 92697, USA.
gVisitors from the University of California Santa Cruz, Santa Cruz, CA 95064, USA.
hVisitors from Cornell University, Ithaca, NY 14853, USA.
iVisitors from the University of Cyprus, Nicosia CY-1678, Cyprus.
jVisitors from University College Dublin, Dublin 4, Ireland.
kVisitors from the University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
lVisitors from Universidad Iberoamericana, Mexico D.F., Mexico.
mVisitors from Queen Mary, University of London, London, E1 4NS, England.
nVisitors from the University of Manchester, Manchester M13 9PL, England.
oVisitors from Nagasaki Institute of Applied Science, Nagasaki, Japan.
pVisitors from the University of Notre Dame, Notre Dame, IN 46556, USA.
qVisitors from University de Oviedo, E-33007 Oviedo, Spain.
rVisitors from Texas Tech University, Lubbock, TX 79409, USA.
sVisitors from IFIC(CSIC-Universitat de Valencia), 46071 Valencia, Spain.
tVisitors from the University of Virginia, Charlottesville, VA 22904, USA.
uVisitors from Bergische Universita¨t Wuppertal, 42097 Wuppertal, Germany.
vOn leave from J. Stefan Institute, Ljubljana, Slovenia.
wVisitors from the University of Massachusetts Amherst, Amherst, MA 01003, USA.
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