Observation of the Heavy Baryons Σ_b and Σ_b^*

Article

Reference

Observation of the Heavy Baryons Σ

and Σ

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We report an observation of new bottom baryons produced in pp collisions at the Tevatron.

Using 1.1  fb−1 of data collected by the CDF II detector, we observe four Λ0bπ± resonances in the fully reconstructed decay mode Λ0b→Λ+cπ−, where Λ+c→pK−π+. We interpret these states as the Σ(*)±b baryons and measure the following masses:

mΣ+b=5807.8+2.0−2.2(stat.)±1.7(syst.)  MeV/c2, mΣ−b=5815.2±1.0(stat.)±1.7(syst.)  MeV/c2, and m(Σ∗b)−m(Σb)=21.2+2.0−1.9(stat.)+0.4−0.3(syst.)  MeV/c2.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Observation of the Heavy Baryons Σ

and Σ

. Physical Review Letters , 2007, vol. 99, no. 20, p. 202001

DOI : 10.1103/PhysRevLett.99.202001

Available at:

http://archive-ouverte.unige.ch/unige:38432

Disclaimer: layout of this document may differ from the published version.

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Observation of the Heavy Baryons

and

T. Aaltonen,²³A. Abulencia,²⁴J. Adelman,¹³T. Affolder,¹⁰T. Akimoto,⁵⁵M. G. Albrow,¹⁷S. Amerio,⁴³D. Amidei,³⁵ A. Anastassov,⁵²K. Anikeev,¹⁷A. Annovi,¹⁹J. Antos,¹⁴M. Aoki,⁵⁵G. Apollinari,¹⁷T. Arisawa,⁵⁷A. Artikov,¹⁵ W. Ashmanskas,¹⁷A. Attal,³A. Aurisano,⁵³F. Azfar,⁴²P. Azzi-Bacchetta,⁴³P. Azzurri,⁴⁶N. Bacchetta,⁴³W. Badgett,¹⁷

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K. Yorita,¹³T. Yoshida,⁴¹G. B. Yu,⁴⁹I. Yu,²⁸S. S. Yu,¹⁷J. C. Yun,¹⁷L. Zanello,⁵¹A. Zanetti,⁵⁴I. Zaw,²²X. Zhang,²⁴ J. 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

202001-2

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;

and 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

43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

48Purdue University, West Lafayette, Indiana 47907, USA

49University of Rochester, Rochester, New York 14627, USA

50The Rockefeller University, New York, New York 10021, USA

51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

52Rutgers University, Piscataway, New Jersey 08855, USA

53Texas A&M University, College Station, Texas 77843, USA

54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

55University of Tsukuba, Tsukuba, Ibaraki 305, Japan

56Tufts University, Medford, Massachusetts 02155, USA

57Waseda University, Tokyo 169, Japan

58Wayne State University, Detroit, Michigan 48201, USA

59University of Wisconsin, Madison, Wisconsin 53706, USA

60Yale University, New Haven, Connecticut 06520, USA (Received 26 June 2007; published 12 November 2007)

We report an observation of new bottom baryons produced in pp collisions at the Tevatron. Using 1:1 fb¹ of data collected by the CDF II detector, we observe four ⁰_b resonances in the fully reconstructed decay mode⁰_b!c, wherec !pK. We interpret these states as the_b baryons and measure the following masses: m

b 5807:8^2:0_2:2stat: 1:7syst:MeV=c², m

b

5815:21:0stat: 1:7syst:MeV=c², andm_b m_b 21:2^2:0_1:9stat:^0:4_0:3syst:MeV=c².

DOI:10.1103/PhysRevLett.99.202001 PACS numbers: 14.20.Mr, 13.30.Eg

Recently the CDF II detector at the Fermilab Tevatron has accumulated the world’s largest sample of fully recon- structed ⁰_b baryons, which consist of the u, d, and b quarks, with 318060 (stat.) ⁰_b!_c candidates.

This is made possible by the large bb production cross section inpp collisions at

ps

1:96 TeVand the ability of the CDF II experiment to select fully hadronic decays of bhadrons with a specialized trigger system. In this Letter, we present an observation of four⁰_bresonances, where

⁰_b!_c and_c !pK, using1:1 fb¹of data.

The⁰_bstates are interpreted as the lowest-lying charged _bbaryons and will be labeled_b . The symbol_brefers to_b, while_brefers to_b . Any reference to a specific charge state implies the antiparticle state as well.

The _b baryons contain one b and two u quarks, while the _b baryons contain oneband twodquarks;

these states are expected to exist but have not been ob-

served. Baryons containing one bottom quark and two light quarks can be described by heavy quark effective theory (HQET) [1]. In HQET, a bottom baryon consists of a b quark acting as a static source of the color field surrounded by a diquark system comprised of the two light quarks. In the lowest-lying_b states, the diquark system has strong isospin I1 and J^P1, which couple to the heavy quark spin and result in a doublet of baryons withJ^P1₂ (_b) and J^P3₂ (_b). This doublet is degenerate for infinitebquark mass. As thebquark mass is finite, there is a hyperfine mass splitting between the³₂and¹₂states.

There is also an isospin mass splitting between the_b and_b states.

Predictions for the _b masses come from nonrela- tivistic and relativistic potential quark models [2], 1=N_c expansion [3], quark models in the HQET approxima- tion [4], sum rules [5], and lattice quantum chromodynam- ics calculations [6]. On the basis of [2–6], we expect m_b m⁰_b 180–210 MeV=c², m_b m_b 10–40 MeV=c², and m_b m_b 5–7 MeV=c². The difference between the isospin mass splittings of the _b and _b multiplets is predicted to be m_b m_b m_b m_b 0:400:07 MeV=c² [7].

The natural width of_b baryons is expected to be domi- nated by the P-wave one pion transition _b !⁰_b, whose partial width depends on the available phase space and the pion coupling to a constituent quark. Using an HQET prediction [8], the natural widths for the expected _b masses are _b 7 MeV=c² and _b 13 MeV=c².

The CDF II detector is described in detail elsewhere [9].

Its components and capabilities most relevant to this analy- sis are the tracking system [10] and a displaced track trigger which is employed to select bottom and charmed hadrons [11].

In reconstructing the decays ⁰_b!_c and _c ! pK, the proton from the_c decay and the from the ⁰_b decay both must have p_T>2 GeV=c[12], while theKandcandidates havep_T>0:5 GeV=c. We also require p_Tp> p_T to suppress _c combinatorial background. No particle identification is used in this analy- sis. All particle hypotheses consistent with the candidate decay structure are considered. In a 3-D kinematic fit, the _c daughter tracks are constrained to originate from a single point. The_c candidate is constrained to the known _c mass, and the_c momentum vector is extrapolated to intersect themomentum vector to form the⁰_bvertex.

The probability of the 3-D ⁰_b kinematic vertex fit must exceed 0.1%, and the_c and⁰_bmust havep_Tgreater than 4.5 and 6:0 GeV=c, respectively. To suppress prompt backgrounds from the primary interaction, we make the following decay time requirements: ct⁰_b>250m and its significancect⁰_b=_ct>10. We definect⁰_b

L_xy⁰_bm0

bc=p_T⁰_b as the ⁰_b proper time, where L_xy⁰_b is defined as the projection, ontop_T⁰_b, of the vector connecting the primary vertex to the⁰_bvertex in the transverse plane. We use a primary vertex determined event-by-event when computing this vertex displacement.

To reduce combinatorial backgrounds and partially recon- structed decays, we also requirejd₀⁰_bj<80m, where d₀⁰_bis the impact parameter of the momentum vector of the ⁰_b candidate with respect to the primary vertex. To suppress the contributions from B⁰ !D decays, where D !K, we require mpK to be within 16 MeV=c² of the known _c mass [13], and ct_c 2 70; 200m. We define ct_c L_xycm

cc=p_Tc as the _c proper time, where L_xyc is defined analogously toL_xy⁰_bbut computed with respect to the⁰_bvertex.

The invariant mass distribution of_c candidates is shown in Fig. 1 overlaid with a binned maximum like- lihood fit. A clear ⁰_b!_c signal is observed at the expected ⁰_b mass. The invariant mass distribution is de- scribed by several components: the⁰_b!_csignal, a combinatorial background, partially and fully recon- structedBmesons which pass the_cselection criteria, partially reconstructed⁰_bdecays, and fully reconstructed ⁰_b decays other than _c (e.g., ⁰_b!_cK). The combinatorial background is modeled with an exponen- tially decreasing function. All other components are rep- resented in the fit by fixed shapes derived from Monte Carlo (MC) simulations [14,15]. The normaliza- tions are constrained by Gaussian terms to branching ratios that are either measured (forBmeson decays) or theoreti- cal predictions (for⁰_b decays). In the fit, the⁰_b compo- nents are normalized relative to the ⁰_b!_c signal.

To normalize the B meson components, we explicitly reconstruct a B⁰! K signal in the _c

) GeV/c2 π- + Λc m(

5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 6.1 6.2

2Candidates per 20 MeV/c

0 200 400 600 800 1000 1200 1400

Total Fit Λb Partially-reconstructed Fully-reconstructed B Partially-reconstructed B Combinatorial Data

5.2 5.3 5.4 5.5

FIG. 1 (color online). Fit to the invariant mass of⁰_b!_c candidates. Curves for fully reconstructed ⁰_b decays such as ⁰_b !_c and⁰_b !_cKare not indicated on the figure.

The⁰_bsignal region,m_c 2 5:565;5:670GeV=c², con- sists primarily of⁰_bbaryons, with some contamination fromB mesons and combinatorial events. The discrepancies between the fit and data below the ⁰_b signal region are due to incomplete knowledge of the branching ratios of the decays in this region and are included in the_b background model systematics.

202001-4

sample by replacing the proton mass hypothesis with the pion mass hypothesis. The fit to the invariant_cmass distribution results in 318060 (stat.) ⁰_b!_c candidates.

The reconstruction of _b proceeds by combining ⁰_b candidates in the ⁰_b signal region, m_c 2 5:565;5:670 GeV=c², with all remaining high quality tracks. A pion mass hypothesis is used when computing the invariant mass of the _b candidate. We search for narrow resonances in the mass difference distribution of Qm⁰_b m⁰_b m, wherem⁰_b is the recon- structed_cmass. The_b candidates are divided into two subsamples using the charge of the pion from _b decay, denoted by_b: in the⁰_b subsample, the_b has the same charge as the pion from⁰_bwhile in the⁰_b subsample, the _b has the opposite charge as the pion from⁰_b.

The _b signal region, defined as Q2 30; 100 MeV=c², is motivated by the predictions in [2–6]. The signal is modeled by the PYTHIA [16] event generator where only the decays _b !⁰_b, ⁰_b ! _c, and _c !pK are allowed. We optimize the _b selection criteria by maximizing S_MC=

B p ,

whereS_MCis the efficiency of the_b signal measured in the MC simulation andBis the number of background events in the signal region estimated from the upper and lower sideband regions ofQ2 0;30 MeV=c² andQ2 100; 500MeV=c². These sideband regions are parame- terized by a power law multiplied by an exponential. We combine the⁰_band⁰_bsubsamples to optimize cuts on the p_T of the _b candidate, the impact parameter significance jd₀=_d0j of the _b track, and the cos of the_b track, whereis defined as the angle between the momentum of the _b in the _b rest frame and the direction of the total _b momentum in the lab frame.

The maximum of S_MC= pB

is realized for p_Tb>

9:5 GeV=c,jd₀=_d0j<3:0, andcos>0:35.

In the_b search, the dominant background is from the combination of prompt⁰_b baryons with extra tracks pro- duced in the hadronization of thebquark. The remaining backgrounds are from the combination of hadronization tracks with B mesons reconstructed as ⁰_b baryons, and from combinatorial background events. The percentage of each background component in the⁰_bsignal region, com- puted from the⁰_bmass fit, is89:51:7% ⁰_b baryons, 7:20:6% B mesons, and 3:30:1% combinatorial events. Other backgrounds such as 5-track decays ofB mesons are negligible, as confirmed in inclusive singleb hadron simulations [14,15]. The high mass region above the⁰_b!_csignal in Fig.1determines the combina- torial background shape. ReconstructingB⁰ !Ddata as ⁰_b!_c gives the B hadronization background

shape. The⁰_bhadronization background shape is obtained from a⁰_b !_cPYTHIAsimulation. The events in this simulation are reweighted so that the p_T⁰_b distribution agrees with data. As the simulation has fewer low momen- tum tracks around the⁰_bthan found in data, the simulated events are further reweighted until the p_T spectrum of tracks around the ⁰_b is consistent with data. The back- ground shapes are parameterized by a power law multi- plied by an exponential, and the normalizations are fixed from the percentage of that background component in the ⁰_b signal region. The total background shown in Fig. 2 (inset) is compatible with theQsidebands, and the back- ground shape and normalization are fixed components of the_b fit.

In the Qsignal region, we observe an excess of events over the total background as shown in Fig.2. The excess in the⁰_bsubsample is 118 over 288 expected background candidates. In the⁰_bsubsample, the excess is 91 over 313 expected background candidates.

We perform a simultaneous unbinned maximum like- lihood fit to the⁰_b and⁰_bsubsamples for a signal from each expected_b state plus the background, referred to as the ‘‘four signal hypothesis.’’ Each signal consists of a nonrelativistic Breit-Wigner distribution convoluted with two Gaussian distributions describing the detector resolu- tion, with a dominant narrow core of an1:2 MeV=c²width and a small broad component of a3 MeV=c²width for the tails. The natural width of each Breit-Wigner distribution is computed from the central Q value [8]. The expected difference of the isospin mass splittings within the _b

2 Candidates per 5 MeV/c

0 20 40 60 80

Data Total Fit Background

Σb

* Σb +

Σb ^{0 100 200 300 400 500}

)2Candidates / (10 MeV/c 0

20 40 60 80

2) (MeV/c ) - mπ

0

Λb

) - m(

π

0

Λb

Q = m(

0 50 100 150 200

0 20 40 60 80

-

Σb ^{0 100 200 300 400 500}

)2

0 20 40 60 80

Candidates / (10 MeV/c

FIG. 2 (color online). The _b fit to the ⁰_b and ⁰_b subsamples. The top plot shows the ⁰_b subsample, which contains_b , while the bottom plot shows the⁰_bsubsam- ple, which contains _b . The insets show the expected back- ground plotted on the data forQ2 0; 500MeV=c², while the signal fit is shown on a reduced range ofQ2 0; 200MeV=c².

and_bmultiplets is below our sensitivity with this sample of data. Consequently, we constrain m_b m_b m_b m_b

b. The four _b signal fit to data, which has a fit probability of 76% in the range Q2 0; 200 MeV=c², is shown in Fig.2.

Systematic uncertainties on the mass difference and yield measurements fall into three categories: mass scale, _b background model, and_b signal parameterization.

The systematic uncertainty on the mass scale is determined by the discrepancies of the CDF II measuredQvalues of the D, _c, and _c hadrons from the world average Q values [13]. The Q value dependence of this systematic uncertainty is modeled with a linear function, which is used to extrapolate the mass scale uncertainty for each _b Q value. This is the largest systematic uncertainty for the mass difference measurements, ranging from 0.1 to0:3 MeV=c². The systematic effects related to assump- tions made on the_b background model are: the sample composition of the⁰_bsignal region, the normalization and functional form of the⁰_bhadronization background taken from aPYTHIA simulation, and our limited knowledge of the shape of the⁰_bhadronization background (the largest systematic uncertainty on the yield measurements, ranging from 2 to 15 events). The systematic effects related to assumptions made on the _b signal parameterization are: underestimation of the detector resolution, the uncer- tainty in the natural width prediction from [8], and the constraint thatm_b m_b m_b m_b.

The significance of the signal is evaluated using the likelihood ratio, LRL=L_alt, where L is the likelihood of the four signal hypothesis andL_altis the likelihood of an alternative hypothesis [17]. We study the alternate hypoth- eses of no signal, two _b states (one per ⁰_b charge combination), and three _b states, performed by elimi- nating one of the states in the four signal hypothesis.

Systematic variations are included in the fit as nuisance parameters over which the likelihood is integrated. The resulting likelihood ratios are given in Table I. To assess the significance of the signal, we repeat the four signal

hypothesis fit on samples randomly generated from alter- nate signal hypotheses. In1210⁶ background samples, none had aLRequivalent or greater than the one found in data. We evaluate the probability for background only to produce four signals of this or greater significance to be less than 8:310⁸, corresponding to a significance of greater than 5:2. The probabilities for each of the alter- nate hypotheses to produce the observed signal structure is also given in TableI. The final results for the_bmeasure- ment are quoted in TableII. Using the CDF II measurement of m⁰

b 5619:71:2stat: 1:2syst:MeV=c² [18], we find the absolute masses of the _b states given in TableII. The systematic uncertainties on the absolute_b mass values are dominated by the total ⁰_b mass uncertainty.

In summary, using a sample of318060(stat.)⁰_b! _c candidates reconstructed in 1:1 fb¹ of CDF II data, we search for resonant ⁰_b states. We observe a signal of four states whose masses and widths are consis- tent with those expected for the lowest-lying charged_b baryons. This result represents the first observation of the _b baryons.

We thank T. Becher, A. Falk, D. Pirjol, J. Rosner, and D.

Ebert for useful discussions. We also 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 Foundation; 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 Particle Physics and Astronomy Research Council and the Royal Society, UK; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the TABLE I. Likelihood ratios (LR) in favor of the four signal

hypothesis over alternative hypotheses. Also shown is the proba- bility for each hypothesis to produce the observed data (p-value), calculated using theLRas a test statistic on randomly generated samples. The final column gives the equivalent stan- dard deviations from the normal distribution.

Hypothesis LR p-value Significance ()

No Signal 2:610¹⁸ <8:310⁸ >5:2 Two_b States 4:410⁶ 9:210⁵ 3.7 No_b Signal 1:210⁵ 3:210⁴ 3.4

No_b Signal 49 9:010³ 2.4

No_b Signal 4:910⁴ 6:410⁴ 3.2 No_b Signal 8:110⁴ 6:010⁴ 3.2

TABLE II. Final results for the b measurement. The first uncertainty is statistical, and the second is systematic. The absolutebmass values are calculated using a CDF II measure- ment of the⁰_b mass [18], which contributes to the systematic uncertainty.

State Yield

Qor (MeV=c²b)

Mass (MeV=c²) _b 32¹³⁵₁₂₃ Q

b 48:5^2:00:2_2:20:3 5807:8^2:0_2:21:7 _b 59¹⁵⁹₁₄₄ Q

b 55:91:00:2 5815:21:01:7 _b 77¹⁷¹⁰₁₆₆

b21:2^2:00:4_1:90:3 5829:0^1:61:7_{1:81:8 b} 69¹⁸¹⁶₁₇₅ 5836:42:0^1:8_1:7

202001-6

European Community’s Human Potential Programme under Contract No. HPRN-CT-2002-00292; and the Academy of Finland.

aUniversity of Athens, 15784 Athens, Greece.

bUniversity of Bristol, Bristol BS8 1TL, United Kingdom.

cUniversity Libre de Bruxelles, B-1050 Brussels, Belgium.

dCornell University, Ithaca, NY 14853, USA.

eUniversity of Cyprus, Nicosia CY-1678, Cyprus.

fUniversity College Dublin, Dublin 4, Ireland.

gUniversity of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.

hUniversity of Heidelberg, D-69120 Heidelberg, Germany.

iUniversidad Iberoamericana, Mexico D.F., Mexico.

jUniversity of Manchester, Manchester M13 9PL, England.

kNagasaki Institute of Applied Science, Nagasaki, Japan.

lUniversity de Oviedo, E-33007 Oviedo, Spain.

mUniversity of London, Queen Mary College, London, E1 4NS, England.

nUniversity of CA Santa Cruz, Santa Cruz, CA 95064, USA.

oTX Tech University, Lubbock, TX 79409, USA.

pUniversity of CA, Irvine, Irvine, CA 92697, USA.

qIFIC(CSIC-Universitat de Valencia), 46071 Valencia, Spain.

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