Isolated Lepton Production at Colliders

The production of isolated leptons with high transverse momentum in high energy e-e, p-p or e-p collisions is reviewed. The leptons are produced either through boson splitting or by boson-boson collision and yield experimentally simple and spectacular topologies which can be exploited to validate the Standard Model or to search for new phenomena.Comment: 50 page

Events with Isolated Charged Leptons and Large Missing Transverse Momentum at HERA

Striking events with isolated charged leptons, large missing transverse momentum and large transverse momentum of the hadronic final state were observed at the electron proton collider HERA in a data sample corresponding to a luminosity of about 130 pb-1. The H1 collaboration observed 11 events with isolated electrons or muons and with transverse momentum above 25 GeV. Only 3.4+-0.6 events were expected from Standard Model (SM) processes. Six of these events have a transverse momentum of greater than 40 GeV, while 1.3+-0.3 events were expected. The ZEUS collaboration observed good agreement with the SM. However, ZEUS found two events with a similar event topology, but tau leptons instead of electrons or muons in the final state. Only 0.2+-0.05 events were expected from SM processes. For various hypotheses the compatibility of the experimental results was investigated with respect to the SM and with respect to possible explanations beyond the SM. Prospects for the high-luminosity HERA-II data taking period are given

Observation of Events with Isolated Charged Leptons and Large Missing Tra nsverse Momentum and of Events with Multi-Electrons at HERA

Striking events with isolated charged leptons, large missing transverse momentum and large transverse momentum of the hadronic final state (PTX) have been observed at the electron proton collider HERA. In the full HERA-I data sample corresponding to an integrated luminosity of about 130 invpb, the H1 experiment observes 10 events with isolated electrons or muons and with PTX >25 GeV. Only 2.9 pm 0.4 events are expected from Standard Model (SM) processes. Six of these events have PTX >40 GeV, while 1.1 pm 0.2 events are expected. The ZEUS experiment observes good agreement with the SM. However, in a preliminary search ZEUS has found two events with a similar event topology, but tau-leptons instead of electrons or muons in the final state . Only 0.12 pm 0.02 events are expected from SM processes. Moreover, six events with two or more electrons forming an invariant mass bigger than 100 GeV have been observed by the H1 experiment. Three events have two electrons and three events have three electrons, while only 0.25 events are expected in each case. The ZEUS measurement is in agreement with the SM expectation.Comment: talk given at 38th Recontres de Moriond Electroweak Interactions and Unified Theories, Les Arc (France) 200

Searches for New Particles

The status of searches for new particles and new physics during the past year at the Fermilab Tevatron, at HERA and at LEP is summarized. A discussion of the hints for the Standard Model Higgs boson from LEP2 data is presented. Searches for non-Standard Model Higgs bosons are also described. Many searches have been carried out for the particles predicted by supersymmetry theories, and a sampling of these is given. There have also been searches for flavor changing neutral currents in the interactions of the top quark. In addition, searches for excited leptons, leptoquarks and technicolor are summarized.Comment: LaTeX2e, ws-p10x7.cls, 16 pages, 25 figures, Proceedings of the XX International Symposium on Lepton and Photon Interactions at High Energies, Rome, Italy, 23-28 July, 200

Study of Rare Exclusive Electroweak Processes at HERA

Results on multi-lepton events at high transverse momenta, isolated lepton events with large missing transverse energy and single W production reported to this conference are based for the first time on the full data samples taken by two colliding experiments, H1 and ZEUS, at HERA. The data correspond to an integrated luminosity of about 1fb-1 from both experiments.Comment: Parallel talk at ICHEP08, Philadelphia, USA, July 2008. 4 pages, LaTeX, 4 ps figure

Isolated tau leptons in events with large missing transverse momentum at HERA

A search for events containing isolated tau leptons and large missing transverse momentum, not originating from the tau decay, has been performed with the ZEUS detector at the electron-proton collider HERA, using 130 pb^-1 of integrated luminosity. A search was made for isolated tracks coming from hadronic tau decays. Observables based on the internal jet structure were exploited to discriminate between tau decays and quark- or gluon-induced jets. Three tau candidates were found, while 0.40 +0.12 -0.13 were expected from Standard Model processes, such as charged current deep inelastic scattering and single W-boson production. To search for heavy-particle decays, a more restrictive selection was applied to isolate tau leptons produced together with a hadronic final state with high transverse momentum. Two candidate events survive, while 0.20 +-0.05 events are expected from Standard Model processes.Comment: 28 pages, 4 figures, 3 tables, accepted by Phys. Lett. B. Updated with minor changes to the text requested by the journal refere

Search for Particles and Forces Beyond the Standard Model at HERA ep and Tevatron p\bar{p} Colliders

A review of searches for physics beyond the Standard Model carried out at high energy lepton-hadron and hadron-hadron facilities is presented, with emphasis on topics of interest for future data taking at the upgraded Tevatron $p{\bar p}$ and HERA $ep$ colliders. The status and discovery prospects are discussed for leptoquarks, Technicolour and supersymmetry, forbidden lepton and quark flavour-changing processes, extra gauge bosons, excited states of composite fermions, generic contact interactions and extra compactified dimensions.Comment: 64 pages, 34 figures, submitted for publication in "Progress in Particle and Nuclear Physics

Searches for New Phenomena at the Tevatron and at HERA

Recent results on searches for new physics at Run II of the Tevatron and highlights from HERA are reported. The searches cover many different final states and a wide range of models. All analyses have at this point led to negative results, but some interesting anomalies have been found.Comment: Invited talk at the XXVI Physics in Collisions Conference (PIC06), Buzios, Brasil, July 2006, 20 pages. PSN THUPL0

Report of the GDR working group on the R-parity violation

This report summarizes the work of the "R-parity violation group" of the French Research Network (GDR) in Supersymmetry, concerning the physics of supersymmetric models without conservation of R-parity at HERA, LEP, Tevatron and LHC and limits on R-parity violating couplings from various processes. The report includes a discussion of the recent searches at the HERA experiment, prospects for new experiments, a review of the existing limits, and also theoretically motivated alternatives to R-parity and a brief discussion on the implications of R-parity violation on the neutrino masses.Comment: 60 pages, LaTeX, 22 figures, 2 table