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 [Formula: see text] were observed at the electron proton collider HERA in a data sample corresponding to an integrated luminosity of about 130 pb-1. The H1 collaboration observed 11 events with isolated electrons or muons and with [Formula: see text]. Only 3.4±0.6 events were expected from Standard Model (SM) processes. Six of these events have [Formula: see text], 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.

Measurements of Diffractive Vector-Meson Photoproduction at High Momentum Transfer from the ZEUS Experiment at HERA

We discuss recent preliminary results on the diffractive photoproduction of ρ0, ϕ, and J/ψ mesons at high momentum transfer reported by the ZEUS collaboration at HERA. A special-purpose calorimeter served to tag the quasi-real photons (Q2<0.01 GeV 2) in the process γ+p→ VM +Y, where Y represents a dissociated state of the proton. The resulting range in photon-proton center-of-mass energy extends from 80 to 120 GeV. The differential cross sections [Formula: see text] were obtained in the region |t| > 1.2 GeV 2, where t denotes the squared momentum transfer to the proton. The measurements provide good sensitivity to the observed power-law dependence [Formula: see text]. Over the region in momentum transfer covered by the data, the power is found to be approximately n ≃ 3 for ρ0 and ϕ photoproduction, and approximately n ≃ 2 for J/ψ photoproduction.