Results on the cross section measurement of Z boson pair-production at LEP
are presented. The more general case of neutral-current four-fermion production
and the particular case of ZZ events enriched in b quarks are also discussed.
All the results agree with the Standard Model predictions.Comment: To appear in the proceedings of ICHEP 2000, Osaka, Japan, July
  27th-August 2nd, 200

Mele, Salvatore

English

arXiv

Results on the cross section measurement of Z boson pair-production at LEP are presented. The more general case of neutral-current four-fermion production and the particular case of ZZ events enriched in b quarks are also discussed. All the results agree with the Standard Model predictions.Results on the cross section measurement of Z boson pair-production at LEP are presented. The more general case of neutral-current four-fermion production and the particular case of ZZ events enriched in b quarks are also discussed. All the results agree with the Standard Model predictions

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arXiv:hep-ex/0009004v1  1 Sep 2000To appear in the proceedings of ICHEP 2000, Osaka, Japan, July 27th – August 2nd, 2000ZZ CROSS SECTION MEASUREMENTSSALVATORE MELEEP Division, CERN, CH-1211, Gene`ve 23, Switzerland∗E-mail: Salvatore.Mele@cern.chResults on the cross section measurement of Z boson pair–production at LEP1,2 are presented. Themore general case of neutral–current four–fermion production and the particular case of ZZ eventsenriched in b quarks are also discussed. All the results agree with the Standard Model predictions.1 IntroductionLEP was successfully operated in the yearsfrom 1997 through 2000 at centre–of–mass(√s) energies from 183GeV up to 208GeV.This allowed each of its four experiments tocollect more than 540 pb−1 of data above theZ boson pair–production threshold, as sum-marised in Table 1.This process tests the Standard Modelof electroweak interactions in the neutral–current sector and is sensitive to New Physicsscenarios such as couplings between neutralgauge bosons4 or extra space dimensions5.The results presented here refer to a par-ticular subset of all the possible diagrams forneutral–current four–fermion production, de-noted as NC02 and depicted in Figure 1. TheFigure also shows the ZZ production crosssection as calculated with the YFSZZ andZZTO programs3. These diagrams define theZ pair production signal, also defined by someexperiments with a wider part of the fullfour–fermion phase space compatible with Zpair–production.The related topics of general neutral–current four–fermion production and b quarkcontent in Z–pair events are also investigated,and discussed in the following.Detailed accounts of the data sets, analy-sis techniques and results of each experimentcan be found elsewhere1,2. All the results at√s = 183 and 189GeV are published whilethe others are preliminary.∗On leave of absence from INFN Sezione di Napoli,I-80126, Napoli, Italy.2 Data analysisThe four experiments devised analysis strate-gies that rely on the identification of the sig-natures of the pair–production of two par-ticles with equal masses, compatible with Zbosons.Multivariate techniques are used for thelargest statistic fully hadronic final state(49% of the Z pair decays), where a largebackground is expected from the QCD andW pair–production processes. Event shapevariables are used to reject the first, focus-ing on the signal four–jet topology, also com-mon to hadronic decays of W pairs. Thisbackground can be discriminated thanks tothe different boson mass that reflects into thetopology of the decay products. Moreover,W decays lack b quarks, present in 39% ofthe fully hadronic decays of the Z pairs. Theuse of the b tag techniques developed for theHiggs search hence increases the signal pu-rity.The final state with a Z decaying intohadrons and the other into neutrinos is thesecond most populated (28%) and is investi-gated with hadronic events with large missingenergy. The hermeticity of the detectors al-lows to reconstruct the four–momentum ofthe Z decaying into neutrinos. The mainbackgrounds are the production of a Z inassociation with an undetected high energyand low polar angle initial state radiationphoton, semileptonic decay of W pairs withthe charged lepton escaping detection or themore general case of a W decaying into100.511.5170 180 190 200Ecm [GeV]sZZ NC02 [pb] LEP Summer 00 - Preliminary20/07/2000±2.0% uncertaintyZZTOYFSZZe+e- e-e+ZZ ZZFigure 1. Measured and predicted cross sections forthe Z boson pair–production process via the two di-agrams shown in the lower right corner.hadrons produced together with a non reso-nant system of a low polar angle electron anda neutrino. Sequential cuts or multivariatetechniques that enforce the signal topology ofan undetected Z suppress these backgrounds.High signal purity is achieved in the lowerstatistic (14%) final state with hadrons andcharged leptons, which benefits of the highresolution measurements of the lepton mo-menta. Kinematic fits in the hypothesis of anequal mass of the lepton–lepton and hadronsystems are performed, requiring then thismass to be compatible with the Z mass.Topological variables such as the angles of theleptons and the hadronic jets are also used.The final states with two charged lep-tons and two neutrinos (4%) and four chargedleptons (1%) are penalised by low statis-tics, even though identified with good pu-rity thanks to the high lepton resolutions.The background from lepton pair–productionand four–fermion processes is mainly rejectedby requiring the lepton invariant and recoilmasses to be compatible with the Z mass.The undetectable final state with fourTable 1. LEP√s and sum of the integrated luminosi-ties (L) collected by the four experiments togetherwith the measured (σZZ ExpNC02) and expected (σZZ ThNC02)ZZ cross sections. Data collected in the year 2000are grouped into the two last energy bins. The firstuncertainty is statistical, the second systematic.√s L σZZ ExpNC02 σZZ ThNC02GeV pb−1 pb pb182.7 221 0.23± 0.08± 0.02 0.25188.7 686 0.70± 0.07± 0.03 0.65191.5 114 0.60± 0.18± 0.04 0.77195.6 310 1.04± 0.12± 0.04 0.90199.6 326 0.98± 0.12± 0.04 0.98201.7 152 0.88± 0.18± 0.04 1.01205.0 241 1.00± 0.15± 0.05 1.04206.8 123 0.70± 0.21± 0.05 1.06neutrinos accounts only for 4% of the Z–pairdecays.3 ResultsThe four experiments measured the Z pair-production cross section at all the√s abovethreshold1,2. Combined results are presentedin Table 1 and Figure 1. This average takesinto account sources of common and corre-lated systematic uncertainties, mainly due touncertainties on the background cross sec-tions and modelling, as well as uncorrelatedones, dominated by Monte Carlo statisticsand detector related effects, in particular forthe b tag procedure.All the measurements agree with theStandard Model predictions. The ratio be-tween the measured and predicted cross sec-tions is formed at each√s, and its averageover all the√s yields 0.99 ± 0.06, what re-veals an overall agreement within the com-bined accuracy of 6%, mainly statistical.4 Four–fermion productionThe DELPHI Collaboration extends its Zpair–production analysis to the case of a Zboson produced in association with a pair of2fermions from a virtual photon. Final stateswith two quarks and either a muon– or aneutrino–pair are analysed. In the latter casethe mass of the hadronic system must be be-low 60GeV. The cross sections over the fulldata sample are respectively expected to be0.19− 0.25pb and 0.13− 0.16pb, decreasingwith√s. The measured cross sections read:σ183GeV−208GeVe+e−→Zγ⋆→µ+µ−qq¯= 0.22± 0.05± 0.02 pbσ183GeV−208GeVe+e−→Zγ⋆→νν¯qq¯= 0.19± 0.06± 0.02 pb ,where the first uncertainty is statistical andthe second systematic.5 B quark contentThe experimental investigation of Z–pair fi-nal states containing b quarks validates thecapability of the LEP experiments to detectthe production and decay of the Higgs boson.These two process have a similar topol-ogy, as the Higgs boson production wouldpreferentially manifests as a pair of b quarksfrom the decay of a heavy object that recoilsagainst a Z. The expected cross sections arealso similar, as illustrated in Figure 2. TheFigure also shows the results of the measure-ment by the L3 Collaboration, in agreementwith the Standard Model predictions. In thismeasurement Z decays into hadrons, neutri-nos and charged leptons are considered in as-sociation with the b quark pair.The OPAL Collaboration measures thebranching ratio of the Z into b quarks in theselected Z–pair events, als finding agreementwith the value measured at LEP at the Z res-onance.In conclusion, if such rare processes withthese topologies can be observed, a Higgs bo-son light enough to be produced at LEP willnot escape detection.AcknowledgementsI wish to thank my L3 and LEP colleaguesworking on these subjects for all the construc-√s (GeV)Cross Section (pb)ZZ→bbX Datae+e- →ZZ→bbXMH = 95 GeVMH = 100 GeVMH = 105 GeVL3Preliminary00.10.20.30.40.5170 180 190 200 210Figure 2. Measured and predicted cross sections forneutral–current four–fermion events compatible withZ pairs with at least a b quark pair. Data are pre-sented for the average√s of the years 1998, 1999 and2000. The computed cross section for the productionof the Standard Model Higgs boson is also presentedfor different mass hypotheses.tive discussions we had throughout the lastyears and for sharing with me their prelimi-nary results.References1. ALEPH Collab., R. Barate et al., Phys.Lett. B 469, 287 (1999), Contr. #289a;Contr. #287; Contr. #188; DELPHICollab., P. Abreu et al., Preprint CERN-EP/2000-89, Contr #659; Contr. #459;Contr. #460; L3 Collab., Contr. #502;Contr. #411; OPAL Collab., G. Ab-biendi et al., Phys. Lett. B 476,256 (2000), Contr. #153; Contr #154;Contr. #168.2. L3 Collab., M. Acciarri et al. Phys. Lett.B 450, 281 (1999); Phys. Lett. B 465,363 (1999).3. G. Passarino, these proceedings4. C. Matteuzzi, these proceedings.5. G. Landsberg, these proceedings.a Notation for contributed papers to this conference3

ZZ Cross Section Measurements

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ZZ Cross Section Measurements

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