HIGGS and SUSY searches performed by the ALEPH Experiment at LEP are
interpreted in the framework of two constrained R-parity conserving models:
Minimal Supergravity and minimal Gauge Mediated Supersymmetry Breaking.Comment: 8 page

de VIVIE, Jean-Baptiste

English

arXiv

Higgs and SUSY searches performed by the ALEPH experiment at LEP are interpreted in the framework of two constrained R-parity conserving models: minimal supergravity and minimal gauge mediated supersymmetry breaking. (4 refs).HIGGS and SUSY searches performed by the ALEPH Experiment at LEP are interpreted in the framework of two constrained R-parity conserving models: Minimal Supergravity and minimal Gauge Mediated Supersymmetry Breaking

de Vivie, Jean Baptiste

CERN Document Server

LAL 99-58October 1999Interpretation of Higgs and Susysearches in mSugra and Gmsb modelsJean Baptiste de VivieE-mail: devivie@lal.in2p3.frLaboratoire de l’Acce´le´rateur Line´aireIN2P3-CNRS et Universite de Paris-Sud, BP 34, F-91898 Orsay CedexAbstractHiggs and Susy searches performed by the ALEPH Experiment at LEP are interpretedin the framework of two constrained R-parity conserving models: Minimal Supergravity andminimal Gauge Mediated Supersymmetry Breaking.1 IntroductionSearches for supersymmetric and Higgs particles have been performed using the data collectedwith the ALEPH detector at LEP, at centre-of-mass energies up to 189 GeV. All have givennegative results. This can be interpreted as constraints on the parameter space of a given model.The results are here presented in the framework of two dierent R-parity conserving constrainedmodels: Minimal Supergravity (mSugra) [1] and Gauge Mediated Supersymmetry Breaking(Gmsb) [2] models. Scans of their parameter space are carried out [3, 4] in order to set a lowerlimit on the mass of the lightest neutralino in mSugra and on the universal Susy mass scale inGmsb. The detailed interpretation of searches in such constrained frameworks is of particularrelevance to understand the interplay of the various experimental constraints. Moreover, itallows a direct comparison with TEVATRON limits.2 Interpretation in the Minimal Supergravity model2.1 Model parameters and constraintsThe mSugra model allows to reduce the parameter space of the Minimal SupersymmetricStandard Model (Mssm) down to four parameters and one sign. In this framework, severalconstraints are assumed at the Grand Unication (GUT) scale: all scalar soft masses (includingthose pertaining to the Higgs sector) are equal to m0, all gaugino masses are equal to m1/2,and the trilinear couplings are unied in a single parameter A0. These three parameters areused as inputs to the renormalization group equations, to determine the low energy parameters.The bilinear coupling between the two Higgs elds is traded for the ratio of their vacuumexpectation values tan β. Finally, µ is determined up to a sign ambiguity by requiring thatElectroweak Symmetry Breaking be dynamically triggered by radiative corrections due to thelarge top Yukawa coupling.1From a given (m0,m1/2,A0,tan β,sign(µ)) set, all couplings and masses of Susy particlesare computed. Several constraints are applied to dene the allowed parameter sets. First,theoretical constraints are imposed to determine whether a set is physically acceptable: noparticle should be tachyonic, a proper Electroweak Symmetry Breaking is required and the topYukawa coupling should not develop a Landau pole up to the GUT scale. The top quark massis set to 175 GeV/c2. Experimental constraints are then applied. LEP1 limits on non-standardcontributions to the total or invisible Z width are used. Nonetheless, the main ro^le is played bydirect searches for new particles at LEP2. Low m1/2 values are excluded by chargino searchesand further constrained by slepton searches at low m0. Higgs boson searches allow for a wideexclusion in the low tan β regime. Finally searches for heavy stable charged particles are alsoused to deal with quasi-stable staus.2.2 Results in the (m0,m1/2) planeThe results of the scan are rst presented for A0 = 0. Exclusion domains in the (m0,m1/2) planefor tan β = 3 (top) and 10 (bottom) are shown in Fig. 1. Chargino searches lead to a lowerlimit on m1/2 approximately independent of m0, except for low m0, where the experimentalsensitivity is reduced. In this region, slepton searches can be used. For tanβ = 3 and negativeµ where stop mixing is small, Higgs searches exclude a large fraction of the parameter space.The impacts of the further mSugra constraints are studied with a special attention payedto the corridor region, corresponding to small sneutrino masses. In this region, the charginoproduction cross section is reduced due to the negative interference between s and t-channelprocesses and two body decays of charginos to lepton-sneutrino dominate, leading to almostinvisible nal states when 0 < mχ± −mν˜ < 3 GeV/c2. An example of such a region is shownin Fig. 2(a), for tan β = 4.2 and µ < 0. The combination of the Z width and the charginoconstraints yields a lower limit on m1/2 of 63 GeV/c2; adding slepton searches increases thislimit to 94 GeV/c2. Finally Higgs searches allow to set a lower limit on m1/2 of 107 GeV/c2,corresponding to a χ mass of 45.4 GeV/c2.2.3 Mass limit for the lightest neutralinoThe mass limit for the lightest neutralino is presented in Fig. 2(b), as a function of tan β. Forpositive µ, it is approximately independent of tan β. For negative µ, a structure is observed.At small tan β, Higgs searches are used to cover the corridor region. As tan β increases, Higgssearches become less ecient and slepton searches become constraining. However, they cannotcompletely cover the corridor for intermediate tan β, and therefore a gap appears in the mχlimit. Altogether, for A0 = 0, the lower limit on mχ is 41.5 GeV/c2. However, as can be seen inFig. 2(b), the impact of non zero A0 value cannot be neglected. Scanning over A0 leads to thenal mass lower limit for the lightest neutralino within mSugra, for tan β  10:mχ > 35.8 GeV/c2This result improves by  3 GeV/c2 on the one obtained in the less constrained minimalmodel (MSSM) considered in Ref. [3]. The limitation to relatively low tan β values comes fromthe fact that the combined analyses are optimized for the case of negligible stau mixing. Inthe MSSM interpretation, the latter condition is met by ne-tuning the trilinear coupling Aτ .However, in mSugra such an adjustment is not any longer possible; the regions in the parameterspace where the stau is the NLSP and nearly degenerate with the lightest neutralino are lesscovered by direct selectron searches. For these parameter sets, charginos predominantly decayinto ~τντ with very soft tracks from subsequent stau decays which are dicult to detect. In suchcases, the lower limit on m1/2 is set by the Z width measurement leading to a degradation of the2  ALEPH PRELIMINARY01002000 200 400 600 800 1000m1/2 (GeV)tanβ = 3, µ < 0A0 = 01 2346mχ+= 94.3 GeV/c2meR= 94.3 GeV/c201002000 50 100 150 200tanβ = 3, µ > 0A0 = 00501001502000 50 100 150 200m0 (GeV)m1/2 (GeV)tanβ = 10, µ < 0A0 = 050501001502000 50 100 150 200m0 (GeV)tanβ = 10, µ > 0A0 = 0Figure 1: Regions of the (m0,m1/2) plane excluded for A0 = 0 and tan β = 3 (upper plots) and tanβ = 10(lower plots). Region 1 is theoretically forbidden. The other regions are excluded by LEP1 constraints(2), chargino (3), slepton (4), heavy stable charged particle (5) and Higgs (6) searches. The thin dashedlines represent the kinematic limit for direct chargino and selectron searches.6070809010011012055 60 65 70 75 80 85m0 (GeV)m1/2 (GeV)AA'A''mχ+= 94.3 GeV/c2tanβ = 4.2µ < 0A0 = 0ALEPH404550Mχ (GeV/c2)3absolute lower limit on the LSP mass. To recover sensitivity in these cases, specic neutralinoanalyses should be included.3 New topologies in Gauge Mediated Supersymmetry Breakingmodels3.1 Model parametersThe other generic class of models studied by the LEP collaborations are the Gmsb models. Inthis framework, Supersymmetry breaking is mediated to the observable sector by a number ofmessenger pairs charged under the Standard Model (SM) gauge groups. In minimal models, veparameters and one sign are needed to determine masses and couplings: F, the Susy breakingscale which xes the coupling of superparticles to the gravitino, tan β, sign(µ), N, the eectivenumber of messenger pairs, , the universal mass scale of Susy particles and Mmess, the meanmessenger mass, used as starting point for the RGEs. In these models, the soft masses arecomputed from N,  and Mmess, as well as the trilinear couplings which however are expectedto be small.3.2 Relevant topologiesIn Gmsb, the Lightest Supersymmetric Particle (LSP) is the gravitino, which can be consideredas massless and not interacting from an experimental point of view. The experimental topologiesdepend on the nature of the Next to Lightest Supersymmetric Particle (NLSP) and its lifetime,determined by the gravitino mass: τ / m2G˜/ (F/Mpl)2. The lightest neutralino and the sleptonsare in general the only superparticles relevant for LEP2 searches, in addition to the gravitino.The lightest neutralino NLSP caseIn this case the relevant process is the pair-production of neutralinos, decaying into a photon anda gravitino. For short neutralino lifetimes, the experimental topology consists of two acoplanarphotons and missing energy. No excess of candidates has been observed in the data.In the minimal model, and for very short lifetimes, this search excludes χ masses smallerthan 90 GeV/c2. Since the pair production of bino neutralinos can only occur via a t-channelselectron exchange, this limit depends slightly on the selectron mass as shown in Fig. 3. Thissearch allows to almost completely rule out the Gmsb interpretation of the CDF event in theselectron pair-production scenario.For moderate lifetimes, the previous search is complemented by a search for a single photonwith large \impact parameter" (non pointing photon), which takes advantage of the high gran-ularity of the ALEPH electromagnetic calorimeter. For very long lifetimes, neutralinos decayoutside the detector, giving rise to an invisible nal state.The slepton NLSP caseDepending on tan β, two distinct scenarii can be considered: the slepton co-NLSP scenarioat low tan β, where all right handed sleptons are mass-degenerate, and the stau NLSP scenario,at large tan β where the stau becomes lighter due to a large mixing. Again, topologies dependon lifetime. For very short lifetimes, the standard Mssm slepton searches for very high m(the mass dierence between the slepton and the LSP) can be used. The topology consists ofa pair of acoplanar leptons and missing energy. For intermediate lifetimes, sleptons can fly anddecay in the tracking volume and therefore give rise to detectable kinks. If they decay insidethe beam pipe, the associated track has a large impact parameter. Finally, for long lifetimes,sleptons decay outside the detector. The nal state therefore consists of a pair of heavy stablecharged particles, with two main characteristics: the kinematics of the pair-production and the4high specic ionization in the Time Projection Chamber. The number of candidate events ob-served in the data and their properties are compatible with the SM expectation. The negativeresults of these searches are translated into a lower limit for the mass of the slepton NLSP,independent of its lifetime: in the co-NLSP scenario, m˜`R> 85 GeV/c2 and for the stau NLSP,mτ˜R > 68 GeV/c2.To extend the sensitivity of the search in the case of short lifetimes, a search for slep-tons produced in cascade decays of neutralinos has been performed. The process of interestis e+e− !χχ ! ` ~`` ~`! `+`− ~G`+`− ~G. Here advantage is taken of the multi lepton topology.Two leptons may be soft (from the neutralino decays) and two are hard (from the slepton de-cays). Due to the majorana nature of the neutralino, the charge (and flavour) of the two hardleptons are uncorrelated, leading to spectacular nal states, e.g. with two energetic equallycharged muons. Moreover, the neutralino production (via t-channel selectron exchange) crosssection is large, since the neutralino is mainly bino and the right handed selectron is light. Formχ < 87 GeV/c2 and mχ − mτ˜ > mτ , this search is used to improve the stau mass limit to84 GeV/c2.4 Scan of the Gmsb parameter space4.1 Ranges for the parameters and constraintsIn the absence of any excess with respect to the SM, all these searches are used to put constraintson the parameter space. The ve parameters presented in 3.1 are varied in order to cover alarge fraction of the parameter space, relevant for LEP2 studies and compatible with the mainmotivations for Gmsb models: ve values of N are probed (from 1 to 5),pF is varied from 10to 104 TeV/c2, Mmess from 10 to 109 TeV/c2,  from 1 TeV/c2 to min(pF,Mmess) and tan β isrequired to be above 1.3. Several constraints are applied on each parameter set. Direct searchesfor acoplanar taus and heavy stable charged particles performed at LEP1 and constraints fromthe Z width are used to exclude very low NLSP masses. Standard Susy searches for charginosand sleptons at LEP2 are also used to cover long neutralino lifetimes. Finally all searchespresented above contribute to exclude a large fraction of the parameter space.4.2 Interplay between the various searches and mass limit for the NLSPAn example of the complementarity of the standard Mssm and Gmsb searches is shown in Fig 4,where N, Mmess and tan β have been chosen in order for the lightest neutralino to be the NLSP.For long lifetimes, the neutralino escapes the detector and therefore only indirect limits can beset, as in the usual Mssm. For short lifetimes, the acoplanar photons complemented by thenon pointing photon searches determine the limit. Finally in this case, the lower limit on mχ is45 GeV/c2.The mass limit for the NLSP is obtained by varying all the parameters in the consideredrange. The results of this scan, presented as excluded regions in the (mχ,mτ˜ ) plane, are illus-trated in Fig. 5. It can be emphasized that all searches contribute, in particular that the multilepton topology signicantly extends the exclusion domain for sleptons nearly degenerate withthe lightest neutralino. Altogether, the mass limit is 45 GeV/c2 for the neutralino NLSP, and67 GeV/c2 for the stau NLSP.4.3 Lower limit on ΛIn Gmsb models, the parameter which sets the overall scale for Susy masses is . The lowerlimit on  as a function of tan β obtained in this study is shown in Fig. 6. It is essentially derivedfrom the mass limit for the NLSP. Within the considered parameter ranges, the absolute lower53040506070809010011012080 100 120 140 160 180 200 eR mass  (GeV/c2) χ 10  mass limit (GeV/c2)CDF  RegionExcluded at 95% C.L.ALEPH  PRELIMINARYFigure 3: Excluded region in the (me˜R ,mχ) plane for a bino neutralino.20304050607080900 1 2 3 4 5 6 7 8log10(cτχ/cm)Mχ GeV/c2ALEPHFigure 4: Mass limit for the lightest neutralino as a function of its lifetime.6limit on  is 9 TeV/c2, found at low tan β for a quasi stable χ NLSP. This can be translatedinto an indirect mass lower limit for the gravitino: mG˜ > 2 10−2 eV/c2.5 ConclusionThe search for Supersymmetry is one of the paramount physics goal of LEP2. A detailedinterpretation of searches has been performed by the ALEPH Collaboration in the constrainedframework of mSugra. For tan β  10, this results in a lower limit on the mass of the lightestneutralino: mχ > 35.8 GeV/c2. Nevertheless, there are regions in the parameter space atlarge tan β where the constraints are weak and this limit is degraded. New specic topologiespertaining to Gmsb models have been extensively searched for. In the absence of any hints fora signal, limits are set on masses and model parameters. In particular, the overall mass scaleof Susy particles has to be larger than 9 TeV/c2, almost independently of all the other Gmsbparameters.AcknowledgementsI would like to thank J.-F. Grivaz and M. Kado for their help in preparing this talk. It is apleasure to thank L. Duflot, G. Ganis and M. Kado for careful reading and comments on thismanuscript.References[1] H. P. Nilles, “Supersymmetry, Supergravity and Particle Physics”, Phys. Rep. 110 (1984)1.[2] S. Ambrosanio et al., “Signals for gauge-mediated supersymmetry breaking models the atCERN LEP2 collider”, Phys. Rev. D56 (1997) 1761.[3] The ALEPH Collaboration, “Search for Charginos and Neutralinos in e+e− Collisions atCentre-of-Mass Energies near 183 GeV and Constraints on the MSSM Parameter Space”,CERN-EP/99-014, submitted to European Physics Journal.The ALEPH Collaboration, “Search for Charginos and Neutralinos in e+e− Collisions atps = 188.6 GeV and Mass Limit for the Lightest Neutralino”, ALEPH 99-011, CONF99-006.[4] The ALEPH Collaboration, “Search for Gauge Mediated Susy Breaking topologies atps 189 GeV”, ALEPH 99-045,CONF 99-024.70204060801001201400 20 40 60 80 100 120 140Mχ (GeV/c2)Mτ (GeV/c2)∼ALEPHLEP1n. e.4 l∼χ → γG∼ ∼l → lGNot allowedby theoryNot allowedby theory0204060801001201400 20 40 60 80 100 120 140Mχ (GeV/c2)Mτ (GeV/c2)∼ALEPHLEP1χ±∼stable ln.e.∼l → lχNot allowedby theoryNot allowedby theoryFigure 5: Excluded regions in the (mχ,mτ˜ ) plane for short (left) and long (right) NLSP lifetimes. Then.e. region is not excluded and the not allowed by theory regions correspond to regions unaccessiblewithin the parameter ranges considered.010203040506010 20 30tanβΛ TeV/c2ALEPHN5=1N5=3N5=5Figure 6: Lower limit on  as a function of tanβ for three dierent values of the eective number ofmessenger pairs.8

Interpretation of HIGGS and SUSY searches in MSUGRA and GMSB models

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