Girone, M

Gobbo, B

Maggi, M

Musolino, G

Ozel, F

Spagnolo, P

CERN Document Server

EPS97 Ref. eps 617submitted to: Pa-13Pl-09Pl-17Search for invisible decays of theHiggs boson in e+e collisionsThe ALEPH CollaborationAbstractIn a data sample of 21.5 pb 1collected in 1996 by the ALEPH detector at centre-of-mass energies of 161, 170 and 172 GeV, a non-minimal CP-even Higgs boson h has beensearched for in the reaction e+e ! hZ. The decay channels considered involve invisiblenal states for the Higgs boson and both hadronic and leptonic nal states for the Zboson. Limits on the production cross section have been derived as a function of the massof the Higgs boson. These results have been combined with those obtained with LEP 1data. For value of the production cross section equal to that of a minimal standard modelHiggs boson, a mass less than 71.3 GeV/c2is excluded at 95% C.L.(Submitted to the 1997 EPS-HEP conference, Jerusalem)OPEN-99-33519/08/19991 IntroductionIn the Minimal Supersymmetric Standard Model (MSSM), the lightest CP-even Higgsboson h could decay invisibly to the lightest neutralino-neutralino pairh! if R-parity is conserved and the neutralino  is light enough. The production cross sectionof the reaction e+e ! hZ, where the Higgs boson decays to invisible nal states, canbe expressed as 2(e+e ! HZ)SM, where 2represents a model dependent factor thatreduces the prediction for the production of the standard model Higgs boson H.The possibility of invisible decays of Higgs bosons with masses up to the present LEP 2physics reach is mostly excluded by direct chargino searches [1], if the GUT mass relationM1= 5=3 tan2WM2holds. However, once this relation is relaxed, new regions of theparameter space may become accessible. The reaction e+e ! hZ leads to topologiesinvolving acoplanar lepton pairs or pairs of jets, depending on whether the Z decays inleptonic or hadronic nal states.The searches reported in this paper are based on data collected by ALEPH atps of161, 170 and 172 GeV with integrated luminosities of 10.9 pb 1, 1.1 pb 1and 9.5 pb 1. Adetailed description of the detector can be found in Ref. [3]. Results have been combinedwith those already obtained from LEP 1 data [4, 5] and from the search for monojetevents [6], updated with the full LEP 1 statistics.2 Event selectionThe topology of interest consists of a pair of leptons or jets with mass close to the Zmass and large missing energy. Monte Carlo samples describing the background standardprocesses [7] and the signals [8] have been used to optimise the selection procedures andcalculate the relevant eciencies.2.1 The lepton pair nal stateEvents are required to have only two charged particle tracks with an invariant mass greaterthan 5 GeV=c2and acoplanarity angle less than 178:5. The acollinearity, dened as theangle between the two momenta, has to be greater than 125. Events are kept if the totaltransverse momentum (pt) is greater than 12.5(15) GeV/c at 161(170/172) GeV centre-of-mass energy, in order to reduce the background contribution from the e+e ! e+e `+` processes. The ptdistribution is shown in Fig. 1a for data and Monte Carlo.Events from the reaction e+e ! `+` are rejected by requiring that the sum ofthe absolute cosines of the polar angles of the two selected particles is smaller than 1.85and the visible energy is less than 103 GeV. In addition, the transverse missing boost(pt=Mrecoil) is required to be smaller than 0.6.Both charged particles must be identied either as an electron or as a muon pair [9],rejecting 75% of the remaining W pair contamination.1From the measured transverse momenta of the two tracks, pT, and their invariantmass, m, a 2, dened as:2=Xi(1=pT i  1=p0T i)221=pTi  2 logm2Z 2Z(m2 m2Z)2+m2Z 2Z;is minimised. This quantity indicates the compatibility of the lepton pair mass with theZ mass. Events are rejected if the 2is greater than 8(8.5) for the 161(170/172) GeVdata. The 2is shown in Fig. 1b for data and Monte Carlo, after the requirement oftwo charged particles in the events. No candidate events are observed in both 161 and170/172 GeV data samples. The expected background is 0.05 events for data collectedat 161 GeV and 0.35 events for data collected at 170 and 172 GeV, mainly coming frome+e ! `+` and W pair events.2.2 The Jet pair nal stateThe event selection requires hadronic events [10] with a visible energy less than 60% (70%at 170/172 GeV) of the centre-of-mass energy. Events with undetected energetic particlesat low polar angles are rejected by requiring that the direction of the missing momentumpoint away from the beam axis (j cos misj < 0:9). Figure 2a shows a comparison of dataversus Monte Carlo for this variable. Background from two-photon collisions is reduced byrequiring the total transverse momentum to be larger than 5 GeV/c. The ptdistributionis shown in Fig. 2b.The event acoplanarity is dened as the absolute value of the triple product of thenormalised momentum vector of each hemisphere, obtained by the plane perpendicularto the thrust axis, and the unit vector along the beam axis, following the standard modelHiggs analysis for the H nal state [11]. This variable is then required to be larger than0.1 to reduce e+e ! qq background events. Events are kept if the visible mass is greaterthan 70 GeV/c2and less than 100 GeV/c2.To reject the WW background, where one W decays leptonically and the other hadron-ically, no identied lepton [9] with a momentum greater than 15 GeV/c must be found inthe event. The most isolated charged particle with respect to the other charged particletracks cannot be a lepton and its momentum has to be smaller than 5 GeV/c.To further reject We and WW events, the events are rst forced to form two jetsusing the Durham algorithm. The acollinearity, dened as the angle between the two jets,is then required to be greater than 140(130at 170/172 GeV). At 170/172 GeV, thethreshold value of the jet clustering variable Ycutfor which the event topology changesfrom one one jet to two jets is required to be less than 0.4. At 161 GeV, instead, at leastone charged particle is required in all reconstructed jets, once three jets are enforced.At 161 GeV, the sphericity is required to be less than 0.15 in order to remove thesignicant contribution of qq events.At 170/172 GeV the WW background is larger. To reduce this background a b-taggingneural network [11] is employed. If the sum of the two hemisphere neural network outputsis greater than 1, the event contains a Z! bb and is retained. If, on the other hand,the probability of containing b quarks is low, then more stringent kinematic cuts must beapplied to eliminate WW events. For these events the minimum angle between two jets,2once three jets are enforced, is required to be less than 60and the minimum invariantmass among all the possible jet pairs has to be smaller than 21 GeV=c2.No candidates are observed in the data, while 1.13 and 0.48 are expected from back-ground at 170/172 and 161 GeV respectively.The eciency, including all Z decays, for the combined acoplanar lepton and jet pairselections is shown in Fig. 3 as a function of the Higgs boson mass.3 Combined resultsThe results of the search for the Higgs boson in invisible decay modes at 161 and 170/172GeV centre-of-mass energies have been combined with those obtained from LEP 1 data.At the Z mass energies, the production of an invisible Higgs in the reaction e+e ! hZmay appear as a monojet, if the Zmass is small compared to its momentum.An analysis to search for invisible decay modes of the Higgs boson has been alreadypublished by ALEPH using data collected from 1989 to 1992 [4]. A dedicated analysissearching for monojet events has been performed later on a larger sample (data collectedfrom 1989 to 1993) [6]. This analysis has been updated using data collected from 1993 to1995. The cut on the visible mass is increased to 7 GeV/c2. It was designed to reject theexpected background events coming from four fermion processes and + events peakingat low mass values. The analysis is also ecient in selecting leptonic nal states. Thesearch for monojet events with the full LEP 1 data sample has selected one candidate,while 0.9 events are expected from the background.The search for acoplanar jets, optimised for the case of a standard model Higgs bosonproduced in the reaction e+e ! H and decaying into hadrons, is described in Ref. [5]and has been performed using the whole LEP 1 data sample. No candidates were found(1.1 background events expected). The results can be reinterpreted in the present context,with the roles of the Higgs boson and the Zinterchanged.The negative results of the search can be translated in 95% C.L. exclusion region inthe (2, mh) plane (Fig. 4). The dashed line represents the exclusion region obtained bythe analyses performed at 161 and 170/172 GeV. The dotted line represents the resultsobtained in [4]. The solid grey line is the exclusion region obtained with the full LEP 1statistics. Finally, the dark solid line is the exclusion region for the combined results. Asa result, a mass mhless than 71.3 GeV/c2is excluded at 95% C.L. for 2= 1.4 ConclusionsA search has been made for invisible decay modes of the lightest Higgs boson in thereaction e+e ! hZ. Acoplanar jets and acoplanar lepton topologies have been studiedat centre-of-mass energies of 161, 170 and 172 GeV. The existing analysis of acoplanarjets at LEP 1 and monojet searches, updated to include the full LEP 1 statistics, are usedto derive combined exclusion limits. For value of  = 1, a mass mhless than 71.3 GeV/c2is excluded at 95% C.L.3AcknowledgementsWe wish to thank our colleagues from the accelerator divisions for the successful operationof LEP at high energy. We are indebted to the engineers and technicians in all ourinstitutions for their contribution to the excellent performance of ALEPH. Those of usfrom non-member countries thank CERN for its hospitality.References[1] A. Djouadi, P. Janot, J. Kalinowsky and P.M. Zerwas, Phys. Lett. B376 (1996) 220.[2] ALEPH Collaboration, \Search for the Neutral Higgs Bosons of the MSSM in e+e collisions atps from 130 to 172 GeV", CERN-PPE/97-071 (1997). Submitted to Phys.Lett. B.[3] ALEPH Collaboration, Nucl. Inst. Meth. A294 (1990) 121;ALEPH Collaboration, Nucl. Inst. Meth. A360 (1995) 481.[4] ALEPH Collaboration, Phys. Lett. B313 (1993) 312.[5] ALEPH Collaboration, Phys. Lett. B384 (1996) 427.[6] ALEPH Collaboration, Phys. Lett. B334 (1994) 244.[7] ALEPH Collaboration, \Search for Charginos and Neutralinos in e+e Collisions atps = 161 and 172 GeV", EPS-HEP97-614, Jerusalem 19{26 August 1997.[8] P. Janot, \The HZHA generator", in Physics at LEP 2, Eds. G. Altarelli, T. Sjostrandand F. Zwirner, CERN 96-01 (1996) 309.[9] ALEPH Collaboration, Nucl. Inst. Meth. A360 (1995) 481.[10] ALEPH Collaboration, Z. Phys. C53 (1992) 1.[11] ALEPH Collaboration, \Search for the Standard Model Higgs Boson in e+e Colli-sions atps = 161, 170 and 172 GeV", CERN-PPE/97-70 (1997). Submitted to Phys.Lett. B.411010 210 30 10 20 30 40 50 60pt (GeV/c)Number of events a)DataBackground MCSignal MC11010 210 30 2 4 6 8 10 12 14 16 18 20χ2Number of events b)DataBackground MCSignal MCFigure 1: Comparison between data and Monte Carlo for the pt(a) and 2(b) distributionsof the leptonic sample. Full dots are the data and the shaded histograms represent theMonte Carlo events for background processes. Signal Monte Carlo is shown in arbitraryunits (unshaded histogram).511010 210 30 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cos(θmis)|Number of events a)DataBackground MCSignal MC11010 210 30 10 20 30 40 50 60pt (GeV/c)Number of events b)DataBackground MCSignal MCFigure 2: Comparison between data and Monte Carlo for the j cos misj (a) and pt(b)distributions of the hadronic sample. Full dots are the data and the shaded histogramsrepresent the Monte Carlo events for background processes. Signal Monte Carlo is shownin arbitrary units (unshaded histogram).600.050.10.150.20.250.30.350.455 60 65 70 75mh (GeV/c2)Efficiency (All Z decays)161 GeV172 GeVFigure 3: Eciencies, for all Z decays, for the combined analyses at 161 (full circles) and172 GeV (open circles).The error bars are due to the Monte Carlo statistics.700.10.20.30.40.50.60.70.80.9150 55 60 65 70 75 8000.10.20.30.40.50.60.70.80.9150 55 60 65 70 75 8095% C.L.Exclusionmh (GeV/c2)ξ296 datapubl. datafull Z peakall dataALEPHFigure 4: Upper limit on 2excluded at 95% C.L. as a function of the Higgs mass. Thedashed line represents the exclusion region obtained by the analyses performed at 161 and170/172 GeV. The dotted line represents the results obtained in [4]. The solid grey lineis the exclusion region obtained with the full LEP 1 statistics. The dark solid line is theexclusion region for the combined results.8

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