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Recent results on searches for flavour changing neutral currents in the Top sector and for heavy and excited leptons at LEP are reported. Searches for different possible extensions of the fermion sector of the Standard Model (SM) are summarised in this report. The detailed descriptions of the analyses can be found in several contributions to this conference made by the four LEP collaborations ,ALEPH [1], DELPHI [2],L3 [3], Opal [4], and by the Exotica LEP Working Group[5]. The analysed data were collected at centre-of-mass energies ranging from 189 GeV to 208 GeV and with luminosities between 350 pb−1 and 650 pb−1. All results are preliminary and all limits set are at 95 % Confidence Level (CL). 1. Search for Heavy and Excited Leptons New leptons couple to gauge bosons Decay Mode Model Dirac Majorana Sequential 101.3 89.5 L0 → e W Vector 102.6 —Mirror 100.8 89.5 Sequential 101.5 90.7 L0 → μ W Vector 102.7 —Mirror 101.0 90.7 Sequential 90.3 80.5 L0 → τ W Vector 99.3 —Mirror 90.3 80.5 Table 1: L3 lower masses limits in GeV/c for pairproduced neutral heavy leptons. through their SU(2) × U(1) charges and can be be pair produced at LEP. The pair production cross section is proportional to the pair production cross section of the corresponding SM fermions, but affected by a factor g which depends on the type of the lepton (for instances g=2 for vector leptons) and by a space phase factor which is equal to zero at the kinematic limit. Single production of new leptons is also possible at LEP through their mixing with the SM leptons. In the case of excited leptons the SU(2) and U(1) couplings are given as a function of the constants f and f ′ respectively, and several scenarios are possible. ∗Speaker. P r H E P h e p 2 0 0 1 International Europhysics Conference on HEP Mário Pimenta New leptons can decay into SM leptons by radiating a γ, Z or W , according to the type of lepton and the scenario considered. In the case of the charged heavy lepton the possibilities of decaying into its neutral partner or even not decaying inside the detector were also considered. The combination of the several production and Decay Mode Model Sequential 100.8 L± → νW Vector 101.2 Mirror 100.5 Sequential 101.9 L± → L0 W Vector 102.1 Mirror 101.9 Sequential 102.6 STABLE Vector 102.6 Mirror 102.6 Table 2: L3 lower masses limits in GeV/c for pair-produced charged heavy leptons. decay modes gives rises to many and very different topologies, from final states with only γs to isolated leptons or jets. In most of the topologies there is an important missing energy. The main backgrounds are WW , qq(γ) and ZZ. No signal for new heavy or excited leptons was observed. The lower limits on the masses of heavy leptons obtained by L3 on the pair production of neutral and charged leptons are shown in Tables 1 and 2 respectively. The lower limits on the masses of pair produced excited leptons obtained by DELPHI and OPAL are shown in Table 3. Two scenarios were considered: f = f ′ and f = − f ′ . e∗ μ∗ τ∗ ν∗ e ν∗ μ ν∗ τ f = f ′ 103.0 103.1 102.2 102.0 102.4 95.3 f = −f ′ 98.0 98.0 98.0 102.7 102.8 102.8 f = f ′ 102.9 102.9 102.8 — — — Table 3: Lower limits (in GeV/c) at 95% CL on the excited lepton masses from the pair production modes. The first two lines corresponds to results obtained by DELPHI while the last line reports those obtained by OPAl. For the single production of excited leptons DELPHI and OPAL derived limits on the ratio of the coupling (f) to the Compositness scale (Λ) as a function of the excited lepton mass. In the case of charged excited leptons (electrons, muons and taus) the result obtained by the LEP Exotica Working Group combining the DELPHI and OPAL results (with f = f ′ ) are shown in figure 1. The stronger limit in the case of electrons is due to the contribution of the t-channel diagram. The limit on the excited electron production for f = f ′ can be extended above the LEP kinematic threshold searching for deviations in the e+e− → γγ(γ) differential cross section [6]. The result obtained by DELPHI is shown in figure 2. For the single production of isosinglet heavy neutrinos L3 extended the limits on the mixing angle to a mass range above the W mass as is shown in figure 3. 2. Search for Single Top production The most prominent signature for the direct observation of Flavour Changing Neutral Currents (FCNC) processes is the single Top quark production in the reaction e+e− → tc

Search for single top (FCNC) and heavy and excited fermions at LEP