[Show abstract][Hide abstract] ABSTRACT: The exclusive electroproduction of J/psi mesons, ep → epJ/psi, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15 lt Q(2) lt 0.8 GeV2 and 2 lt Q(2) lt 100 GeV2 using integrated luminosities of 69 and 83 pb(-1), respectively. The photonproton centre-of-mass energy was in the range 30 lt W lt 220 GeV and the squared four-momentum transfer at the proton vertex \t\ lt 1 GeV2. The cross sections and decay angular distributions are presented as functions of Q(2), W and t. The effective parameters of the pomeron trajectory are in agreement with those found in J/psi photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, sigma(L)/sigma(T), grows with Q(2), whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.
[Show abstract][Hide abstract] ABSTRACT: 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(-0.13)(+0.12) 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.