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Search for New Particles at LEP

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Search for New Particles at LEP Search for New Particles at LEP S. Rosier-Lees LAPP (INPPS-CNRS) Annecy-Le-Vieux - France Abstract The LEP energy upgrade up to fi =189 GeV has allowed us to extend substantially the potential of searches for new physics. Results on searches for Higgs bosoms and supersymmetric particles obtained by the ALEPH, DELPHI, L3, and OPAL exper- iments are reported. No evidence of any signal is observed. Therefore, new limits on the Higgs boson masses as well as on the masses of the various supersymmetric particles are derived. They significantly improve those obtained either at LEPl or LEP1.5. The LEPBOO discovery potential for the neutral Higgs bosons is also shown. @ 1998 by S. Rosier-Lees. -409- . be discovered at LEP200 when running at &=200 GeV and assuming an integrated luminosity of 200 pb-’ collected by each experiment. LEP PRELIMINARY Individual Limit LEP Combined 11 Table 1: Individual and LEP combined observed and expected mass limits for the Stan- dard Model Higgs boson [3], up to fi = 183 GeV. 4 _?I: 80 82 84 86 88 90 92 94 10 80 82 84 86 88 90 92 mH(GeV/ z5 k --- HZ-Signal(mn=85GeV) 2 1.5 1 0.5 0 0 20 40 60 80 loo 0 mF(GeV) Figure 1: Mass distribution for the candidate events selected by the OPAL experiment in the searches for e+e- + HZ at center-of-mass energies up to 183 GeV [3]. I,,,I,,,,,,/,I lo 80 82 84 86 88 90 92 94. 80 82 84 86 88 90 92 94 mH(GeV/c’) mH(Ge V/c2) 2.2 The MSSM Higgs Bosons Figure 2: Average expected (dashed lines) and observed (solid lines) confidence levels, CL,, obtained In the MSSM, all SUSY particle masses, their couplings, and their production cross sec- from combining the results of the four LEP Collaborations using the four statistical methods. The tions and decay widths are predicted in terms of only six free independent parameters: intersections of the curves with the horizontal lines at 0.05 define the 95% confidence level lower rnilz (the common gaugino mass parameter at the GUT scale), me (the common mass for bounds, ezpected and observed, for the mass of the 5X4 Higgs boson. scalar fermions at the GUT scale), p (the higgsino mixing parameter), tan/3 (the ratio of the vacuum expectation values of the two Higgs doublets), mA (the mass of the neutral CP-odd Higgs boson A), and A (the trilinear coupling in the Higgs sector at the GUT scale); the renormalization group equations are used to determine the parameters at low -411- h ] Mh.a Lower Limit (GeV/c2) ] PRELIMINARY Range of ~ Lower ’ Individual Limit LEP Combined (ADLO) y#p#p h (Obs.) 70.7-74.4 78.8 (Exp.) 67.4-70.3 76.3 A (Obs.) 71.0-76.1 79.1 (ExP.) 68.4-72.0 76.3 Table 4: Individual and LEP combined mass limits for the neutral h and A bosons [3]. background contamination. The 95% C.L. individual limits on the charged Higgs boson Mu*=60 GeV/c’ r-o,r+u, T+V,SC cssi: Efficiency 24% 42% 40% Expected events 9.2 30.1 99.4 Observed events ] 6 1 28 1 93 0 100 200 300 400 500 mA (GeV/c2) Table 5: Typical efficiencies and number of expected and observed events obtained by the L3 experiment in the charged Higgs boson searches at a c.m. energy of 183 GeV (141. Individual Limit (ADLO) LEP Combmed 10 11 Table 6: Individual and LEP combined mass limits for the charged Higgs bosons. mass is shown as a function of the branching ratio Br(H* + r*vT) in Fig. 4. Combining all LEP results, a lower limit on the charged Higgs boson mass of 68 GeV is established at the 95% C.L., assuming that the sum BR(H+ + r’+uT) + Br(H+ + cS) is equal to one. It represents a gain of almost 10 GeV with respect to the individual limits, Table 6. 0 20 40 60 80 loo 120 140 3 Other SUSY Particles mh (GeV/c2) The supersymmetric particles searched for at LEP2, not counting the Higgs bosons, are Figure 3: LEP combined exclusion domains (method D) in the plane (MA, tan,@ (top) or sleptons, stops, sbottoms, charginos, and neutralinos as detailed in Refs. [17] and [18]. (Mh, tan,@ (bottom) by the LEP direct searches for e+e- + hZ and hA at center-of-mass The most popular SUSY breaking mechanisms are the gravity and the gauge mediated energies up to 183 GeV assuming either a maximal mixing or no mixing [3] and [IS]. SUSY breaking models (GMSB). The results of SUSY searches at LEP are interpreted in the framework of these two models. 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