Gómez-Ceballos, G

Matorras, F

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Search for Doubly Charged Higgs Bosons at LEP2

DELPHI Collaboration DELPHI 2002-010 CONF 551March 11, 2002Search for Doubly Charged Higgs Bosons at LEP2G. Go´mez-Ceballos, F. MatorrasInstituto de F´ısica de Cantabria,Universidad de CantabriaSantander, SpainAbstractA search for pair-produced doubly charged Higgs bosons has been performed usingthe data collected by the DELPHI detector at LEP at centre-of-mass energiesbetween 189 and 209 GeV. No excess is observed in the data with respect to theStandard Model background. A lower limit for the mass of 99.1 GeV/c2 at the 95%confidence level has been set for doubly charged Higgs bosons in left-right symmetricmodels.PRELIMINARYContributed paper to the Spring 2002 conferences1 IntroductionDoubly charged Higgs bosons (H±±) appear in several extensions to the StandardModel [1], such as left-right symmetric models, and can be relatively light. In Super-symmetric left-right models usually the SU(2)R gauge symmetry is broken by two tripletHiggs fields, so-called left and right handed. Pair-production of doubly charged Higgsbosons is expected to occur mainly via s-channel exchange of a photon or a Z boson. Inleft-right symmetric models the cross-section of e+e− → H++L H−−L is different from thatfor e+e− → H++R H−−R , where H±±L and H±±R are the left-handed and right-handed Higgsbosons. The formulae for the decays and the production of these particles can be foundin [2].In these models the doubly charged Higgs couples only to charged lepton pairs, otherHiggs bosons, and gauge bosons, at the tree level. The current limit and the mass rangeof this analysis is restricted to the interval between 45 GeV/c2, the LEP1 limit set byOPAL [3], and the kinematic limit at LEP2, that is around 100 GeV/c2. The dominantdecay mode of the doubly charged Higgs boson is expected to be a same sign chargedlepton pair, the decay proceeding via a lepton number violating coupling. As discussedin [2], due to limits that exist for the couplings of H±± → e±e± from high energy Bhabhascattering, H±± → µ±µ± from the absence of muonium to anti-muonium transitions andH±± → µ±e± from limits on the flavour changing decay µ± → e∓e±e±, electron and muondecays are not likely. In addition, most of the models expect that the coupling to ττ willbe much larger than any of the others. Therefore, only the doubly charged Higgs bosondecay H±± → τ±τ± is considered here.The partial width for the H±± decay into two τ leptons is, at the tree level [2]:Γττ (H±± → τ±τ±) = h2ττ8pimH(1−2m2τm2H)(1− 4m2τm2H)1/2 (1)where mτ is the mass of the τ lepton and hττ is the unknown Hττ Yukawa couplingconstant. In this analysis, we assume that hττ is sufficiently large (hττ ≥ 10−7) to neglectthe decay length of the Higgs boson.Searches in this channel have already been performed at LEP2 by the OPAL experi-ment [4].2 Data AnalysisThe data collected by DELPHI during the LEP runs at centre-of-mass energies from189 GeV to 209 GeV were used. The total integrated luminosity of these data samples is∼ 570 pb−1. The DELPHI detector and its performance have already been described indetail elsewhere [5, 6].Signal samples were simulated using the PYTHIA generator [7]. In this analysissamples with doubly charged Higgs boson with masses between 50 and 100 GeV/c2, in10 GeV/c2 steps, were used at different centre-of-mass energies, both for left-handed andright-handed bosons.The background estimates from the different Standard Model processes were based onthe following event generators, interfaced with the full DELPHI simulation program [6]:the WPHACT [8] generator was used to produce four-fermion events Monte Carlo sim-1ulation. The four-fermion samples were complemented with dedicated two-photon colli-sion samples generated with BDK, BDKRC [9] and PYTHIA [7]. Samples of qq¯(γ) andµ+µ−(γ) events were simulated with the KK2f generator [10]. Finally, KORALZ [11] wasused to simulate τ+τ−(γ) events and the generator BHWIDE [12] was used for e+e−(γ)events.3 SelectionThe final state resulting from e+e− → H++H−− → τ+τ+τ−τ− consists of four narrowand low multiplicity jets coming from the τ decays. An initial set of cuts was applied toselect events with four jets of low multiplicity. A charged particle multiplicity between4 and 8 was required. Events were clustered into jets, requiring each jet to be separatedfrom the others by at least 15 degrees, and only events with four reconstructed jetswere accepted. To improve the reconstruction of the tau energy, the tau momenta wererescaled, imposing energy and momentum conservation and keeping the tau directions attheir measured values. If any jet momentum was negative, the event was rejected, as theyare commonly not genuine four-jet events.The two-photon background was reduced by the following energy and momentumrequirements: the visible energy outside a cone of 25◦ around the beam had to be greaterthan 0.15√s, the momenta of the jets were required to be larger than 0.01√s and thetotal neutral energy had to be less than 0.35√s.The four lepton background was rejected by requiring that the momentum of the mostenergetic lepton identified (electron or muon) was less than 0.25√s and the momentumof the second most energetic lepton identified was less than 0.15√s. Algorithms used inthe lepton identification were the same as those used in the selection of fully-leptonic Wpairs [13].The calculated tau momenta, defined above, were used to reconstruct the Higgs mass.The charge of the tau jet was calculated as the sum of the charges of the charged particles.If this value was not ±1, then the charge of the most energetic charged particle wasassumed as to be the charge of the tau jet. For events with two positive τ lepton candidatesand two negative τ lepton candidates the charge was used to assign the pairing of bothdoubly charged Higgs bosons. If the total charge was not equal to 0, the pairing was chosento minimise the difference between the two reconstructed masses of the Higgs bosons. Theratio|MH++−MH−−|0.5·(MH+++MH−−)was required to be less than 0.7. Finally the reconstructed eventmass, defined as the average of the mass of the two pairings, had to be greater than 40GeV/c2. The reconstructed event mass was used as an additional discriminant variablein the computation of the confidence levels.The effects of the selection cuts are shown in Table 1 for the combined 189-209 GeVsample. After all cuts were applied only one event was observed in the data with a mass of69±3 GeV/c2, while 0.9 events were expected from background processes. The candidatewas collected at√s=206.7 GeV and is compatible with the assignment ZZ → τ+τ−τ+τ−.The most probable reconstructed masses with different sign leptons are indeed compatiblewith a 90-90 GeV/c2 mass hypothesis at the one sigma level. The signal efficiency wasaround 40% for a wide range of masses between 70 and 100 GeV/c2 for both left-handedand right-handed doubly charged Higgs, as shown in Table 2. The final reconstructedmass plot and the expected mass distribution in simulated signal events are shown in2Figure 1. The good level of agreement between data and simulation observed at differentstages of the analysis is demonstrated in Figure 2.cut data total bkg. llll other εH++LH−−LFour jets preselection 60 67.4 44.0 23.4 59.2%anti γγ cuts 26 31.0 28.9 2.1 52.3%anti 4 lepton cuts 1 1.9 1.7 0.2 48.7%Mass requirements 1 0.9 0.8 0.1 44.2%Table 1: The total number of events observed and the expected background after thedifferent cuts used in the analysis for the combined 189-209 GeV sample. The last columnshows the efficiency for a left-handed doubly charged Higgs boson signal with mH±±L=100 GeV/c2 at√s=206.7 GeV.channel MH±± (GeV/c2)50 60 70 80 90 100left-handed 32.7 36.6 40.5 44.8 43.4 44.2right-handed 31.8 37.0 40.0 44.0 44.8 45.2Table 2: Selection efficiencies (in %) for left-handed and right-handed H++H−− →τ+τ+τ−τ− for several H±± masses at√s=206.7 GeV. The statistical error is around1.5% in all cases.4 Systematic uncertaintiesSeveral sources of systematic uncertainties were investigated for their effect on the signalefficiency and the background level. The particle identification was checked on di-leptonsamples both at the Z peak and at high energy. The discrepancy on the efficiencies be-tween the data and the simulation were found to be lower than 2% in all cases. Thetrack selection and the track reconstruction efficiency was also studied with these sam-ples. These effects were studied by the comparison between data and simulation at theboundaries of sub-detectors. The systematic error of these effects was about 1.5%.The errors on the background and signal rates from the modelling of the preselectionvariables and the detector response were a few percent. Different variables at preselectionlevel have been studied, with good agreement between data and simulation observed. Thedistributions in relevant variables before the anti γγ cuts and the anti four lepton cutsare shown in Figure 2. The masses reconstructed from both same sign and different signlepton pairs, before the anti four lepton cuts were applied, are shown in Figure 3. Forthe opposite sign lepton pairs only the mass of the combination closest to the Z mass hasbeen given and the Z peak is clearly visible.The dominant part of the background uncertainty (∼ 12%) is due to the limitedsimulation statistics available. The total systematic error on the background is about13%. The total systematic error on the efficiency is about 5%.35 Determination of the mass limitNo evidence for H++H−− production was observed. A likelihood ratio technique [14]has been used to compute the cross-section and mass limits. The reconstructed eventmass was used as discriminant variable in the computation of the confidence levels. Allcentre-of-mass energies was treated as separate experiments in the likelihood function.A very similar behaviour, both in terms of efficiency and of mass distributions, wasobserved for the left-handed and the right-handed doubly charged Higgs bosons. Hence,the average of both contributions were used to calculate the confidence levels. The ex-pected left-handed and right-handed cross-sections were calculated using the PYTHIAgenerator [7].Previous searches for H±± pair production have already excluded MH±± < 45.6GeV/c2 [3]. Therefore, we have limited this search to masses greater than this value.Figure 4 shows the 95% confidence level upper limit on the cross-section at√s = 206.7GeV for the production of H++H−− → τ+τ+τ−τ−. The comparison of this limit with theexpected cross-section for left-handed H±±L and right-handed H±±R pair production yields95% confidence level lower limits on the mass of the H±±L and H±±R bosons of 99.6 and99.1 GeV/c2 respectively.6 ConclusionA search for pair-produced doubly charged Higgs bosons was performed using the datacollected by DELPHI at LEP at centre-of-mass energies from 189 GeV to 209 GeV inR-parity conserving supersymmetric left-right symmetric models. No significant excesswas observed and a lower limit on the doubly charged Higgs mass of 99.1 GeV/c2 is setat 95% confidence level.AcknowledgementsWe are greatly indebted to our technical collaborators, to the members of the CERN-SLDivision for the excellent performance of the LEP collider, and to the funding agenciesfor their support in building and operating the DELPHI detector.We acknowledge in particular the support ofAustrian Federal Ministry of Science and Traffics, GZ 616.364/2-III/2a/98,FNRS–FWO and IWT, Belgium,FINEP, CNPQ, CAPES, FUJB and FAPERJ, Brazil,Czech Ministry of Industry and Trade, GA CR 202/96/0450 and GA AVCR A1010521,Danish Natural Research Council,Commission of the European Communities (DG XII),Direction des Sciences de la Matie`re, CEA, France,Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie, Germany,General Secretariat for Research and Technology, Greece,National Science Foundation (NWO) and Foundation for Research on Matter (FOM),The Netherlands,Norwegian Research Council,State Committee for Scientific Research, Poland, 2P03B06015, 2P03B1116 and4SPUB/P03/178/98,JNICT–Junta Nacional de Investigac¸a˜o Cient´ıfica e Tecnolo´gica, Portugal,Vedecka grantova agentura MS SR, Slovakia, Nr. 95/5195/134,Ministry of Science and Technology of the Republic of Slovenia,CICYT, Spain, AEN96–1661 and AEN96-1681,The Swedish Natural Science Research Council,Particle Physics and Astronomy Research Council, UK,Department of Energy, USA, DE–FG02–94ER40817.References[1] J.F. Gunion, H.E. Haber, G.L. Kane and D. Dawson, The Higgs Hunter’s Guide,Frontiers in Physics, Lecture Notes Series, Addison Wesley, 1990;C. S. Aulakh, A. Melfo and G. Senjanovic, Phys. Rev. D57 (1998) 4174;Z. Chacko and R. N. Mohapatra, Phys. Rev. D58 (1998) 15003.[2] M. L. Swartz, Phys. Rev. D40 (1989) 1521;K. Huitu et al., Nucl. Phys., B487 (1997) 27.[3] OPAL Collaboration, P. D. Acton et al., Phys. Lett. B295 (1992) 347.[4] OPAL Collaboration, G. Abbiendi et al., Search for doubly charged Higgs bosonswith the OPAL detector at LEP, CERN-EP-2001-082.[5] DELPHI Collaboration, P. Aarnio et al., Nucl. Instr. and Meth. A 303 (1991) 233.[6] DELPHI Collaboration, P. Abreu et al., Nucl. Instr. and Meth. A 378 (1996) 57.[7] T. Sjo¨strand, PYTHIA 5.719 / JETSET 7.4, Physics at LEP2, eds. G. Altarelli,T. Sjo¨strand and F. Zwirner, CERN 96-01 (1996) Vol 2, 41.[8] E. Accomando and A. Ballestrero, Comp. Phys. Comm. 99 (1997) 270.[9] F.A. Berends, P.H. Daverveldt, R. Kleiss, Comp. Phys. Comm. 40 (1986) 271, 285and 309.[10] S. Jadach, B.F.L. Ward, Z. Was, Phys. Lett. B449 (1999) 97;S. Jadach, B.F.L. Ward, Z. Was, Comp. Phys. Comm. 130 (2000) 260.[11] S. Jadach, B.F.L. Ward, Z. Was, Phys. Lett. 390 (1997) 298.[12] S. Jadach, W. Placzek, B.F.L. Ward, Comp. Phys. Comm. 79 (1994) 503.[13] DELPHI Collaboration, P. Abreu et al., W pair production cross-section and Wbranching fractions in e+e− interactions at 189 GeV, Phys. Lett. B479 (2000) 89.[14] A.L. Read, in CERN Yellow Report 2000-005, p. 81.5DELPHI10-111010 240 50 60 70 80 90 100Reconstructed mass (GeV/c2)Events / 4 GeV/c2Figure 1: The reconstructed mass distribution after all cuts. The hatched histogramcorresponds to the expected background and the dot with the error bar shows the oneremaining candidate event. The dashed line corresponds to mH±±L= 70 GeV/c2 and thedotted line corresponds to mH±±L= 100 GeV/c2.6DELPHI05101520250 0.05 0.1 0.15 0.2P min jet / EcmEvents / 0.0050246810120 0.25 0.5 0.75 1Evis | cos(Θ) | ⊆ 0.9  / EcmEvents / 0.050123456780 0.2 0.4 0.6 0.8Pmax lepton / EcmEvents / 0.0501234567890 0.2 0.4 0.6P 2nd lepton / EcmEvents / 0.05Figure 2: Event selection variable distributions at different stages of the analysis. The topplots show the minimum momentum of the jets and the visible energy outside 25◦ aroundthe beam pipe scaled by√s after the four jet preselection cuts. The bottom plots showthe momentum of the most energetic identified lepton and the momentum of the secondmost energetic identified lepton scaled by√s after the anti γγ cuts. The solid lines showthe expected background, the dots the observed data and the dashed lines correspond tomH±±L= 100 GeV/c2 in arbitrary units.7DELPHI01234560 20 40 60 80 100Same sign Mass (GeV/c2)Events / 4 GeV/c2012345678920 40 60 80 100 120 140Opposite sign Mass (GeV/c2)Events / 4 GeV/c2Figure 3: Reconstructed mass with same sign lepton pairs (top) and opposite sign leptoncombination nearest to the Z mass (bottom). These distributions are shown before theanti four lepton cuts.8DELPHI10-210-1150 60 70 80 90 100limit on σσlσrMH (GeV/c2)σ(H++H--) (pb)Figure 4: The solid line shows the 95% confidence level upper limit on the H±± pairproduction cross-section at√s=206.7 GeV assuming 100% branching ratio for the decayof H±± into τ±τ±. The dashed and dotted lines show the expected production cross-section of H±±L and H±±R pairs in left-right symmetric models.9

Search for Doubly Charged Higgs Bosons at LEP2

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