In this paper we will present the measurement of the Z0Z0 production cross section through the four-charged-lepton decay channel

Bauce, M.

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DOI 10.1393/ncc/i2011-10796-xColloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010First observation of Z0Z0 in the four-lepton decay channel at CDFM. Bauce on behalf of the CDF CollaborationINFN and Universita` di Padova - via Marzolo 8, 35131 Padova, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online l’8 Febbraio 2011)Summary. — In this paper we will present the measurement of the Z0Z0 produc-tion cross section through the four-charged-lepton decay channel.PACS 14.70.Hp – Z bosons.1. – IntroductionZ0Z0 events are produced at the Tevatron, in pp collisions at√s = 1.96TeV, withan expected cross section of 1.4 ± 0.1 pb at NLO calculation in the Standard Model(SM). The study of this process is an important test of the SM and can give hints ofnew physics beyond that. The measurement of the Z0Z0 production cross section isnecessary to evaluate one of the main backgrounds to the search of the Higgs boson,which decays significantly to a Z0 pair for mH ≥ 140GeV. Finally the Z0Z0 productioncross section is sensitive to new physics through anomalous trilinear gauge couplings [1]and large extra-dimensions [2].2. – Analysis techniquesData used in this analysis has been collected by the CDF II experiment which uses amulti-purpose detector whose description can be found in [3].The Z0Z0 → lll′l′ candidates are selected using a high momentum muon and highenergy electron triggers corresponding to an integrated luminosity of 4.8 fb−1. The eval-uation of the background contamination was performed using real data, collected withhigh energy jet triggers. A Monte Carlo (MC) simulation based on Pythia as generatorand CDFSim as emulator of the detector response is used to determine signal efficiencyand acceptance.The Z0Z0 events selection is based on the reconstruction of both Z0 bosons. Inthe final state, exactely four reconstructed leptons (e, μ) are required. One of themmust be the lepton that triggered the event, with pT ≥ 20GeV/c, while the others arerequired to have pT ≥ 10GeV/c. We combine the four leptons to form two pairs ofsame flavour and opposite charge leptons, requiring one of the two pairs to have anc© Societa` Italiana di Fisica 235236 M. BAUCE on behalf of the CDF COLLABORATION]2Z [GeV/cT leading PllM20 40 60 80 100 120 140]2Z [GeV/cT subleading PllM20406080100120140 ZZ )+jetsγZ(Data CDF Run II Preliminary-1 L dt = 4.8 fb∫Fig. 1. – Invariant mass distribution of the two reconstructed Z0.invariant mass in the range 76–106GeV/c2. To increase the acceptance of the analysis,the second pair is allowed to have an invariant mass in a wider range, 40–140GeV/c2.Given a signal selection efficiency of ∼ 10%, the number of expected Z0Z0 events is4.68± 0.02(stat.)± 0.76(syst.).The background contribution to this analysis is low because there are no physicalprocesses leading to a four-lepton final state. The main contribution is instead due to]24-leptons mass [GeV/c0 50 100 150 200 250 300 350 400 450 500Events00.20.40.60.811.21.41.61.82CDF Run II Preliminary-1 L dt = 4.8 fb∫MC SignalBackgroundDataFig. 2. – Four-lepton invariant mass distribution.FIRST OBSERVATION OF Z0Z0 IN THE FOUR-LEPTON DECAY CHANNEL AT CDF 237fake leptons, Z0 + γ events in which one or more jets are mis-identified as leptons. Adata sample composed mainly of jets is used to evaluated the probability for a jet tobe reconstructed as a lepton. This probability is used to weigh events containing oneor two jets in the final state, that replace the real leptons. The background predictionobtained is 0.04 ± 0.02(stat.) ± 0.03(syst.) events, where the systematic error is due tothe uncertainties in the fake rates calculation and is the main systematics uncertainty inthe calculation of the signal significance.3. – ResultsIn 4.8 fb−1 of collected data we observe 5 events with a statistical significance of 5.7σ.This is the first observation of Z0Z0 production at CDF. Figure 1 shows the scatterplotof the two Z0 invariant masses for the MC simulation, background expectations anddata (stars). Solid lines represent the cuts applied. The four lepton invariant mass isdisplayed in fig. 2.With the observed events we evaluate the production cross section σ(pp→ Z0Z0) =1.56+0.80−0.63(stat.) ± 0.25(syst.) pb, that is in agreement with the previous measurementsdone by CDF [4] and DØ [5] and with the theoretical expectation at NLO, σZZ =1.4± 0.1 pb.REFERENCES[1] Hagiwara K., Peccei R. D. et al., Nucl. Phys. B, 282 (1987) 253.[2] Kober M., Koch B. and Bleicher M., Phys. Rev. D, 76 (2007) 125001.[3] Abe F. et al. (CDF Collaboration), Nucl. Instrum. Methods A, 271 (1988) 387.[4] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. Lett., 100 (2008) 201801.[5] Abazov V. M. et al. (DØ Collaboration), Phys. Rev. Lett., 101 (2008) 171803.

First observation of Z0Z0 in the four-lepton decay channel at CDF

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First observation of Z0Z0 in the four-lepton decay channel at CDF

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