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Searches for Physics Beyond the Standard Model at Lep * SEARCHES FOR PHYSICS BEYOND THE STANDARD MODEL AT LEP * ANDRE TILQUIN Centre de Physique Des Particules de Marseille 163 Avenue de Lummy, Case 907, F-13288 Marseille Cedex E-mail: [email protected] An overview of experimental results in searches for physics beyond the Standard Model at LEP is presented. The selected topics include searches 2for SUSY, extended Higgs sector, new fermions, compositness, and low scale gravity in extra dimensions. No evidence for physics beyond the Standard Model has been observed. 1 Introduction Many theoritical problems remain unexplained in the Standard Model (SM): the particle generation structure, quantum numbers, the large number of free parameters and the hierarchy of energy scales between the electroweak (EW) and gravitational interactions. The Large Electron Positron collider (LEP) ran from 1989-2000 with four experiments (ALEPH, DELPHI, L3 and OPAL) at centre-of-mass energies ranging from 89 GeV up to 210 GeV and a total collected luminosity of about 900 pb^1 for each experiment. All following limits are given at 95% confidence level. 2 Searches for Supersymmetry The Minimal Supersymmetric extension of the Standard Model (MSSM) pre­ dicts SUSY partners for fermions (sfermions) and for SM gauge bosons (gaugi- nos). Gauginos will mix with Higgsinos bosons to form neutral state (neutrali- nos) and charged states (charginos). The lepton and baryon number conserva­ tion translated into the "R-parity" conservation. The Lightest Supersymmetric *This work is supported by the High-Energy Physics Foundation 412 WGB: Electroweak Sz physics beyond SM 413 Particle (LSP) is stable, neutral and weakly interacting to fit the cosmological observation. A widely accepted SUSY breaking model is the constrained MSSM. The unification of masses amnd couplings at the GUT scale allow the EW scale phenomenology to be set by few parameters: tan(/?), M2, ra0, and the trilinear coupling Af. 2.1 Searches for sleptons Scalar leptons can be produced in pair in e+e~~ collisions. They decay for example, to the LSP Xi and a lepton of the same flavor. The sensitivity of observation depends on AM = Mf — M^o since the Xi *s undetectable. The combined LEP lower limits for a neutralino mass hypothesis of 40 GeV are: 2.2 Searches for squark Due to the large top mass, mixing in the stop sector can be large and the lightest stop can become the lightest squark. The mass of the sbottom could also be low if tan(/3) takes large value. Sbottom squark will mainly decay intyo &x?> and the stop squark will decay into ex?, or blv if the sneutrino is light enough. The best combined lower limits for a stop is 97 GeV (AM = - M^o = 40 GeV) and 65 GeV independently of AM. A lower limit of 95 GeV on the sbottom mass is set for AM = ~ M^o = 20 GeV. 2.3 Searches for charginos and neutralinos Charginos (x^) can be produced in pairs in e+e~ collisions. They typically can decay as The lower limit for chargino set by the four LEP collaborations is nearly the kinematical limit. For a sneutrino mass greater than 300 GeV, the lower limit is M~+ > 103.5 GeV. Limit on the Xi LSP mass are obtained by combining limit on charginos and MSSM higgs bosons. The lower limit on LSP mass is 45.6 GeV for constrained MSSM and a top quark mass of 175 GeV. 414 A. Tilquin DIS 2003 3 Searches for Higgs bosons beyond SM Two Higgs Doublet Models (2HDM) are the simplest extensions of the SM, with two complex scalar fields doublets, which give rise to five physical states: h°, H°, A, and H±. Two types of 2HDM are possible: in type I, one doublet couples to fermions, the other to bosons, while in type II, one doublet couples to "up" fermions and the other to "down" fermions. 3.1 Searches for MSSM Higgs MSSM is a particular case of a type II2HDM. At LEP energies, the h° and A are produced via two complementary processes: ete~ —> h°Z and e+e~ —> h°A. The results of the SM Higgs searches are used for the e+e~ —> h°Z channel. The second process leads to two different channels: hA —> bbbb and hA —• T+T~&&. Figure 1 shows the MSSSM exclusion regions for the ra^o-max benchmark scenario which is designed to yield the maximum value of m^o in the model. The lower limit set by the combination of the four LEP experiments are: ra^o > 91 GeV and mA > 91.9 GeV. 3.2 Searches for exotic Higgses In many extension of the SM, the h —> bb is not dominant. All LEP collabora­ tions have searched for hadronic Higgs decay without specific flavour tagging. Under the assumption of a SM production and a purely hadronic decay of the Higgs boson, the LEP combined limit is rrih > 112.9 GeV. In SUSY model, the Higgs boson might decay into invisible neutralinos. Searching for acoplanar jets or leptons, the four LEP collaborations exclude this hypothesis for a Higgs boson mass up to 114.4 GeV for an invisible branching ratio of 1. Type I 2HDM have been constructed that allow only Higgs boson coupling to the vector bosons and no coupling to the SM fermions. For masses less then 100 GeV, the decay h —> 77 via a W loop has the largest branching ratio. For the branching ratio expected in fermiophobic Higgs model, masses less than 109.7 GeV are excluded (Figure 1). Charged Higgs are predicted by all 2HDM. At LEP they are produced by pairs and the irreducible background is the W pair production. Assuming WGB: Electroweak & physics beyond SM 415 Figure 1: Left hand side: The MSSM exclusion for the 777^0-max benchmark scenario. Right hand side: Fermiophobic Higgs limit. the LEP combined limit is mH± > 78.6 GeV for any branching ratio. Doubly charged Higgs appear naturally in left-right symmetric models with triplets Higgs fields. Searches have been conducted by DELPHI and OPAL yeilding mass limit of about 100 GeV. 4 Compositness A consequence of fermion compositness is the existence of excited states (#*,/*)• Assuming a gauge group coupling of unity, the scale of compositness should be above 1 TeV. In addition a direct constraint on the e* mass has been obtained by DELPHI and OPAL: me* > 103 GeV, Lepto quark are resonant states carrying both baryon and lepton number. At LEP they can be produced by pair or could be observed indirectly by their effect, via a t channel exchange, on SM processes. Typically LEP experiments 416 A. Tilquin DIS 2003 exclude such lepto quark for masses below 99 GeV. 5 Extra dimensions There are several classes of models with large extra dimensions. The gravity has a scale Mo and propagates in N extra dimensions with size R. Indirect effect from Kaluza-Klein excitations of the graviton GKK exchange in e+e~" —> e+e~~ process are calculated by using a UV cutoff called Ms of the order of MD- Searches for direct production e+e~~ —> JGKK has been carried out by DELPHI. Depending on the number of extra dimensions (6,4 and 2), lower limit on M& has been set at 0.59, 0.84 and 1.36 TeV. Result of a fit to the differencial Bhabha scaterring cross-section from the four LEP experiments strongly excludes Ms < 1 TeV. 6 Conclusions No evidence for physics beyond the Standard Model have been observed. Limits from LEP exclude new particles for masses below 100 GeV and scale of new physics is above 1 TeV. Prom an experimantal point of view there is no indication that new physics is needed. The hope for next decade is that new physics scale is not to far from few TeV. Acknowledgements I would like to thank all LEP collaborations for their nice results. I apologize to those whose research I did not have time to cover. References [1] All references can be found on http://lephiggs.web.cern.ch/. .
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