Andringa, S

Gonçalves, P

Onofre, A

Peralta, L

Pimenta, M

Tomé, B

CERN Document Server

DELPHI Collaboration DELPHI 2002-086 PHYS 9218 June, 2002Search for a fermiophobic Higgs at LEP 2 (update)S. Andringa, P. Gonc¸alves, A. Onofre,L. Peralta, M. Pimenta and B. Tome´LIP-ISR-FCUL, Av. Elias Garcia, 14, 1, P-1000 Lisboa, PortugalAbstractHiggs bosons decaying into two photons were searched for in the data collectedwith the DELPHI detector at centre-of-mass energies ranging from 183 to 209 GeV,corresponding to a total integrated luminosity of nearly 650 pb−1. Upper limits onthe cross-section of the process e+e− → h0Z0 with h0 decaying to photons werederived as a function of the Higgs boson mass. Assuming SM couplings of the Higgsto gauge bosons but no coupling to fermions, a lower mass limit of 104.1 GeV/c2 isobtained at 95% CL.1 IntroductionThis note is an update of the analysis reported in [1]. It presents results of the searchfor a fermiophobic Higgs boson, produced in association with a Z0 and decaying into twophotons.The data collected with the DELPHI detector at centre-of-mass energies ranging from183 to 209 GeV, and corresponding to a total integrated luminosity of about 650 pb−1,were analysed. All the possible decays of the Z0 boson were considered, leading to qq¯γγ,νν¯γγ and l+l−γγ final states.The final analyses of qq¯γγ and l+l−γγ were unchanged with the respect to the onesdescribed in [1], but new data samples were included. For qq¯γγ, the main change in theresults is due to the change of the qq¯(γ) background generator from Pythia [2] to KK2f[3] – with a more detailed description of initial state radiation. Two more events werefound in the 189 GeV data, due to the change to the final processing.The νν¯γγ analysis is very similar to that of [1], but some of the criteria have undergoneslight changes. The most relevant one was the constraint on the quality of the kinematicfit imposing the missing mass to be equal to MZ0 , the χ2/ndof was required to have amaximum value of 5, instead of 10. Signal selection efficiency and number of expectedbackground events are stable with respect to the previous version. Concerning the data,one more event was found (in the 202 GeV data set) and two events were lost (from the189 GeV and 200 GeV data sets).2 ResultsA good agreement between data and SM expectations was found, with a total of 54 eventsselected and 51±1 expected. The detailed numbers per channel and centre-of-mass energyare shown in table 1.√s L qq¯γγ νν¯γγ l+l−γγ(GeV) (pb−1) DATA MC DATA MC DATA MC183 49 4 2.64 ±0.12 2 1.59 ±0.65 – –189 153 8 6.85 ±0.33 5 4.82 ±0.55 1 1.70 ±0.44192 26 0 1.17 ±0.05 1 0.93 ±0.13 0 0.27 ±0.07196 77 4 3.24 ±0.15 2 2.29 ±0.26 2 0.62 ±0.15200 85 4 3.15 ±0.15 2 2.31 ±0.24 0 0.60 ±0.15202 42 3 1.66 ±0.08 1 0.99 ±0.15 0 0.25 ±0.07205 78 3 3.13 ±0.10 2 2.46 ±0.16 0 0.61 ±0.16206.5 55 3 2.20 ±0.10 4 1.58 ±0.53 0 0.29 ±0.07207 84 3 3.11 ±0.14 0 2.49 ±0.16 0 0.69 ±0.17TOT 647 32 27.2 ±0.5 19 19 ±1 3 5.0 ±0.6Table 1: Selected data and background expectations for the three topologies and all datasamples considered. The errors on the background are from MC statistics.Limits on the production cross-section of h0Z0 with h0 → γγ were obtained combiningall channels. Figure 1 shows the 95% CL upper limit on the ratio of the h0Z0 production1cross-section to the Standard Model (SM) one, multiplied by the branching ratio BR(h0 →γγ), as a function of the Higgs boson mass. The cross-section ratio is constant (equalto sin2 (α− β)) in the fermiophobic 2HDM model [4]. For this model and the range ofmasses studied, the width of the higgs boson is always below the mass resolution of theanalysis.In a model where the Higgs couplings to bosons have SM values, but the couplings tofermions vanish, a 95% CL limit on the Higgs boson mass (given by the intersection ofthe cross-section limit with the BR curve, also shown in figure 1) is set at 104.1 GeV/c2,while the expected limit is of 104.6 GeV/c2.3 ConclusionsAround 650 pb−1 of LEP2 data, corresponding to centre-of-mass energies ranging from183 to 209 GeV, were analysed in the search for h0Z0 production with subsequent decayof the Higgs boson to two photons. All possible Z0 decays were analysed.A good agreement with the SM expectations was found and limits were set on theproduction cross-section and BR(h0 → γγ) for masses between 10 and 115 GeV/c2.References[1] S. Andringa et al, ”Search for a fermiophobic Higgs at LEP 2”, DELPHI-2001-073CONF 501 (2001)[2] T. Sjo¨strand, Comp. Phys. Comm. 135 (2001) 238[3] S. Jadach and B.F.L. Ward and Z.Wa¸s, Comp. Phys. Comm. 130 (2000) 260[4] A. Barroso, L. Bru¨cher, R. Santos, Phys. Rev. D60 (1999) 035005210-210-1120 40 60 80 100Mh(GeV/c2)σhZ/σSM .BR(h→γγ)BRexpected limitobserved limitDELPHIpreliminaryFigure 1: 95% CL limit on the h0Z0 production cross-section normalized to the SM value× BR(h → γγ). The observed limit (full line) and the expected limit (dashed line) areshown. Also shown is the fermiophobic BR (dotted line), obtained by keeping the SMcouplings of the Higgs to boson pairs and setting the h0f f¯ couplings to 0.3

Search for a fermiophobic Higgs at LEP 2 (update)

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