We investigate possibilities of search for the Standard Model Higgs boson in the H → ZZ → 4l ± channel in the upper mass reach in the Compact Muon Solenoid detector (CMS). By using appropriate cuts on the data, it was found that CMS can detect Higgs boson up to the mass of about 750 GeV at ≥ 5σ significance level with the designed integrated luminosity of 105 pb−1 . The mass reach extends to about 850 GeV at 3 · 105 pb−1 .Ispitujemo mogućnosti za traženje Higgsovog bozona prema Standardnom modelu u kanalu H → ZZ → 4l ± u području osjetljivosti za velike mase Kompaktnog mionskog solenoidnog (CMS) detektora. Primjenom pogodnih rezova našli smo da se pomoću CMS detektorskog sustava mogu opaziti Higgsovi bozoni do mase oko 750 GeV na razini pouzdanosti ≥ 5σ, uz pretpostavku integralnog luminoziteta od 105 pb−1 . Za 3 · 105 pb−1 mogli bi se opaziti bozoni do mase 850 GeV

Antunović, Željko

Dželalija, Mile

Kinnunen, Ritva

English

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ISSN1330–0016CODENFIZBE7HEAVY HIGGS BOSON SEARCH IN THE H −→ ZZ −→ 4l± CHANNEL INCMSMILE DŽELALIJA, ŽELJKO ANTUNOVIĆ and RITVA KINNUNEN∗Department of Physics, University of Split, Split, Croatia∗SEFT, University of Helsinki, Helsinki, FinlandReceived 29 March 1996UDC 539.12PACS 14.80.BnWe investigate possibilities of search for the Standard Model Higgs boson in theH −→ ZZ −→ 4l± channel in the upper mass reach in the Compact Muon Solenoiddetector (CMS). By using appropriate cuts on the data, it was found that CMScan detect Higgs boson up to the mass of about 750 GeV at ≥ 5σ significance levelwith the designed integrated luminosity of 105 pb−1. The mass reach extends toabout 850 GeV at 3 · 105 pb−1.1. IntroductionThe most promising channel for the Standard Model (SM) Higgs boson discoveryat Large Hadron Collider (LHC) is H −→ ZZ −→ 4l±. In a previous study thatincluded detailed bremsstrahlung effects for electrons and muons [1,2], it was shownthat CMS will be able to detect Higgs boson with 5σ significance level up to mH ≈400 GeV at 104 pb−1 and up to mH ≈ 650 GeV at 105 pb−1 at the nominal LHCenergy of√s = 14 TeV.The main background to the H −→ ZZ −→ 4l± process is the irreducible ZZcontinuum production coming from qq̄ −→ ZZ and gg −→ ZZ. The tt̄ and Zbb̄backgrounds are small and reducible by a Z-mass cut [3].FIZIKA B 5 (1996) 2, 125–134 125dželalija et al.: heavy higgs boson search . . .Here we investigate the maximal Higgs mass reach in this channel. At highmasses, the main problem stems from the large Higgs natural width which increasesfrom about 110 GeV at mH = 600 GeV to about 250 GeV at mH = 800 GeV,reaching about 0.5 TeV atmH = 1 TeV [4,5]. To observe such a broad signal requiresmaximal luminosity and appropriately chosen cuts to suppress the backgroundwithout affecting the signal too much. Several luminosity options are investigated:104 pb−1; 3 · 104 pb−1; 105 pb−1 and 3 · 105 pb−1 — corresponding to differentworking regimes of LHC.2. Simulation for the signal and backgroundThe events are generated by PYTHIA 5.7 with CTEQ2L structure functions[6] and the top quark mass of 174 GeV according to the CDF results [7]. Theproduction processes are gg fusion and WW/ZZ fusion processes (with gg fusioncontributing about 65% at mH = 800 GeV). The production with heavy flavourscontributes less than 1%. The cross-section times branching ratio forH −→ ZZ −→4l± for Higgs masses from 500 to 950 GeV are given in Table 1. The cross-section isfalling rapidly with increasing Higgs mass. The only important background is theirreducible ZZ continuum production. For this background, only the qq̄ −→ ZZprocess is simulated with PYTHIA. To include the gg −→ ZZ contribution, wemultiply the qq̄ result by 1.33, because of the similarity in topology and kinematicsof the two processes [8]. No higher order corrections are taken into account, neitherfor the signal nor for the background. The internal photon bremsstrahlung is takeninto account by a dedicated photon radiation program PHOTOS 2.0 [9].TABLE 1.Total cross–section for H → ZZ → 4l± and ZZ background times branching ratioand detector acceptance after geometrical and kinematic cuts.mH [GeV] 500 550 600 650 700 750 800 850 900 950 ZZσtotal [fb] 3.18 2.24 1.62 1.18 0.89 0.67 0.51 0.40 0.32 0.25 60.26Accept. [%] 75 78 78 79 79 80 80 79 79 78 44The lepton momentum resolution has been studied by detailed GEANT simu-lations of detector response [10,11]. The resulting muon resolution (∆p/p < 1.4%for pT < 100 GeV/c) is parametrized as a function of p and |η|, where p and |η|are momentum and rapidity of muons, respectively, and these parameters are usedto smear the momenta of muons. The electron momentum measurement is affectedby external bremsstrahlung. Effects of external bremsstrahlung on electron energymeasurements depend on the amount of material traversed and hence on the elec-tron rapidity. The effect is substantial for the present CMS tracker design becauseit has on average 27% of radiation length. Detector response to electrons, ways toreconstruct their energy and the Z mass, the Z reconstruction efficiency and res-olution were studied by detailed GEANT simulations [12] and again parametrizedfor fast simulations.126 FIZIKA B 5 (1996) 2, 125–134dželalija et al.: heavy higgs boson search . . .To account for geometrical inefficiences, a reconstruction efficiency per lepton of0.95 is used for both electrons and muons. It is believed that such a reconstructionefficiency is achievable in the ZZ −→ 4l± final state by inclusion of the trackermeasurements [2].3. Signal selection and significance3.1. Lepton cutsIn the event selection we require: one electron with pT > 20 GeV, one withpT > 15 GeV, and the remaining two electrons with pT > 10 GeV, all within|η| < 2.5. For muons, the corresponding pT cuts are 20, 10 and 5 GeV/c, andthe rapidity coverage is |η| < 2.4. Signal and background acceptances are given inTable 1, together with the total cross-section for the ZZ continuum background.Acceptance slowly varies between 78 and 80%, while for the background, whichis produced less centrally, it is 44%. No lepton isolation cuts are applied, becausethere is no background whose reduction is dependent on isolation. In the wholemass range, ZZ continuum background is the dominant one. To avoid any residualtt̄ background, we introduce a cut on the Z–mass by ml+l− = mZ±4σZ , with σZ =3 GeV.Multi–lepton mass resolution for large Higgs masses is affected mostly by itsnatural width, and to a lesser extent by internal and external bremsstrahlung,lepton reconstruction efficiency of the detector and it’s energy and momentumresolution. In Table 2 we list the number of signal and background events, takinginto account radiation and instrumental effects, using the mass window mZ ± 4σZ .These effects combine to give a Z reconstruction efficiency of about 80% in therange of masses we are interested [2].TABLE 2.The number of signal and background events for an integrated luminosity of 105pb−1 with all instrumental and radiation effects included and signal significance.mH [GeV] 500 550 600 650 700 750 800 850 900 950NS 115.7 79.8 57.4 41.3 30.1 25.0 18.3 14.9 11.1 9.0NB 75.9 55.0 47.9 38.7 31.6 32.4 24.2 25.9 19.1 19.1Signific. [σ] 11.0 9.0 7.1 5.8 4.6 3.9 3.2 2.5 2.2 1.83.2. Signal selection and significanceWe considered different pT cuts both on single Z’s and on their scalar sumpZ1T+pZ2Tdistribution. The best results are obtained if one requires a pair of Z’s withpZ1T+pZ2T> mZZ/1.4. For mH = 700 GeV, the distributions of pZ1T+pZ2Tfor signaland background are shown in Figs. 1a) and b) in the interval mZZ = mH ± 2σH ,where we take σH to be half of the Higgs natural width. To make the intrinsi-cally broad signal stand out more prominently over the background, we tried alsoFIZIKA B 5 (1996) 2, 125–134 127dželalija et al.: heavy higgs boson search . . .to apply cuts based on the angular distribution of leptons in the Z–rest–frame,as suggested previously [8,13,14]. Z’s coming from Higgs bosons are mostly longi-tudinally polarized, whilst Z’s in the ZZ continuum are mostly transversal. Thedifference becomes more pronounced with increasing Higgs mass (since it is pro-portional to m2H/m2Z) [13]. Leptons coming from longitudinally polarized Z’s havean angular distribution in the Z helicity framedNd cosϑ∼ sin2 ϑ, (1)while, transversaly polarized Z’s givedNd cosϑ∼ 1 + cos2 ϑ, (2)where ϑ is the angle of a lepton in the Z-rest-frame measured with respect to theZ direction in the H-rest-frame.Fig. 1. a) Distribution of pZ1T+ PZ2Tfor ZZ background in the mass windowmZZ = mH ± 2σH for mH = 700 GeV. b) Distribution of pZ1T + PZ2T for the Higgssignal at mH = 700 GeV. c) Lepton angular distribution for ZZ background inthe window mZZ = mH ± 2σH for mH = 700 GeV. d) Angular distribution of thesignal for mH = 700 GeV.128 FIZIKA B 5 (1996) 2, 125–134dželalija et al.: heavy higgs boson search . . .We use an asymmetric cut in the two angles ϑ1 and ϑ2, where ϑ1 and ϑ2 arethe decay angles in the respective Z–rest–frames. Figures 1c) and d) show theseangular distributions for signal and background for mH = 700 GeV in the masswindow mZZ = mH ± 2σH . Figure 2 shows the distribution of ϑ1 vs. ϑ2 (ϑ1 > ϑ2)for both signal and background in the range mH ± 2σH , for mH = 700 GeV.Fig. 2. Angular distributions of leptons coming from a pair of Z’s (ϑ1 > ϑ2) in the Zhelicity frame for the ZZ background in the interval mZZ = mH ±2σH and for thesignal, all for mH = 700 GeV. Also shown is an illustration of H −→ ZZ −→ 4l±decay in the Z helicity frame.We calculated the signal significance for a number of combinations of angularand pT cuts. Only at the very high masses (mH ≥ 850 GeV) and highest inte-grated luminosities, the angular cut in combination with a pT cut leads to someimprovement in the significance. This happens because even a slight angular cut(for instance ϑ1 > 450 and ϑ2 > 250), whilst reducing background by about 60%,also reduces the signal by about 30%, which for a broad, non–prominent Higgs sig-nal results in decreased significance. We conclude that a cut on Z’s pT distributionFIZIKA B 5 (1996) 2, 125–134 129dželalija et al.: heavy higgs boson search . . .alone is to be preferred for the Higgs mass region from 500 – 800 GeV.TABLE 3.Number of signal and background events for an integrated luminosity of 105 pb−1and mH = 600 GeV, together with resulting signal significances for different cuts:a) pZ1T+ pZ2T> mZZ/1.4, b) pZ1T+ pZ2T> mZZ/2.1, c) (ϑ1, ϑ2) > (450, 250).Cut type no cut a b c a + c b + cNS 57.4 46.2 54.6 40.0 32.5 38.3NB 47.9 20.4 32.0 23.8 10.5 16.0Significance [σ] 7.1 8.0 7.8 6.6 7.5 7.4Table 3 gives the number of signal and background events and signal significancefor mH = 600 GeV at L = 105 pb−1, and all possible combinations of the followingcutspZ1T + pZ2T >mZZ1.4; pZ1T + pZ2T >mZZ2.1; (ϑ1, ϑ2) > (450, 250).Signal and background are taken in the range mH ± 2σH . The cut that gives thebest significance, pZ1T+ pZ2T> mZZ/1.4, reduces the signal by about 20% while atthe same time it reduces the background by about 60%.Fig. 3. The invariant mass of 4l± for mH = 600 GeV (dotted and dashed lines,respectively) over the continuum background (full line) for an integrated luminosityof 105 pb−1. The circles represent the signal points with their statistical errors.130 FIZIKA B 5 (1996) 2, 125–134dželalija et al.: heavy higgs boson search . . .Figure 3 shows the invariant 4l± mass over the continuum background formH =600 and 800 GeV and L = 105 pb−1. Figure 4 shows the 4l± mass distribution forthe same Higgs masses, superimposed on the ZZ background for different integratedluminosities.TABLE 4.The final number of events for H → ZZ → 4l± and ZZ background for anintegrated luminosity 105 pb−1 after pZ1T+ pZ2T> mZZ/1.4 cut and signalsignificance.mH [GeV] 500 550 600 650 700 750 800 850 900 950NS 91.2 63.8 46.2 33.3 24.3 19.9 14.6 11.9 6.3 6.5NB 30.0 22.6 20.4 16.9 13.8 14.2 10.6 11.3 3.8 6.8Significance [σ] 12.5 10.1 8.0 6.5 5.3 4.4 3.7 3.0 2.5 2.0σH (GeV) 30 40 55 70 85 105 130 155 180 200Table 4 and Fig. 5 give our final results. Table 4 gives the number of signaland background events and significance of the signal in the window mH ± 2σH fordifferent luminosities, using the pZ1T+pZ2T≥ mZZ/1.4 cut. Also shown are values ofσH for different Higgs masses. The plot of signal significance at various luminositiesis shown in Fig. 5.Fig. 4. The invariant mass of 4l± superimposed on the ZZ background for a)mH = 600 GeV and L = 3 · 104 pb−1, b) mH = 800 GeV and L = 105 pb−1, c)mH = 600 GeV and L = 105 pb−1 and d) mH = 800 GeV and L = 3 · 105 pb−1.FIZIKA B 5 (1996) 2, 125–134 131dželalija et al.: heavy higgs boson search . . .Fig. 5. The significance of the signal for different Higgs masses, calculated accordingto Poisson statistics and for various integrated luminosities.We calculate signal significance according to Poisson statistics with signal andbackground events taken in mH ± 2σH .4. ConclusionsWe have studied possibilities of detection of H −→ ZZ −→ 4l± at large masses,mH ≥ 500 GeV. The discovery range extends up tomH ≈ 650 GeV at 3 · 104 pb−1mH ≈ 750 GeV at 105 pb−1mH ≈ 850 GeV at 3 · 105 pb−1.A significant improvement in significance is obtained with a cut on the scalar sumof the pT of the two Z’s. A cut based on the helicity angular distribution of leptonsleads to some improvement of the signal to background ratio, but does not lead toimproved signal significance. A more complete maximum likelihood method couldprobably make better use of the decay angular distributions.132 FIZIKA B 5 (1996) 2, 125–134dželalija et al.: heavy higgs boson search . . .AcknowledgementsWe would like to thank D. Denegri for his guidance and suggestions in this work.The authors are gratefully acknowledge to CMS Collaboration and staff at CERNfor their hospitality and financial support. Finally, the Split group thanks theMinistry of Science and Technology of Croatia for financial support.References1) D. Denegri, R. Kinnunen and G. Roullet, Investigation of H −→ ZZ −→ 4leptons inCMS including radiative effects, CMS TN/93-100, 1993;2) D. Bomeštar, D. Denegri, R. Kinnunen and A. Nikitenko: Study of H −→ ZZ −→ 4l±with a full GEANT simulation of CMS, CMS TN/95-018, 1995;3) A. 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Primjenom pogodnih rezova našli smo dase pomoću CMS detektorskog sustava mogu opaziti Higgsovi bozoni do mase oko750 GeV na razini pouzdanosti ≥ 5σ, uz pretpostavku integralnog luminoziteta od105 pb−1. Za 3 · 105 pb−1 mogli bi se opaziti bozoni do mase 850 GeV.134 FIZIKA B 5 (1996) 2, 125–134

Traženje teškog Higgsovog bozona u kanalu H → ZZ → 4l ± pomoću CMS detektora

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