[en] The diffractive photoproduction of J/ψ mesons is measured with the H1 detector at the ep collider HERA using an integrated luminosity of 78 pb^-1. The differential cross section dσ(γp→J/ψY)/dt is studied in the range 2< vertical bar t vertical bar <30 GeV², where t is the square of the four-momentum transferred at the proton vertex. The cross section is also presented as a function of the photon-proton centre-of-mass energy W_γp in three t intervals, spanning the range 50< W_γp<200 GeV. A fast rise of the cross section with W_γp is observed for each t range and the slope for the effective linear Pomeron trajectory is measured to be α'=-0.0135±0.0074(stat.)±0.0051(syst.) GeV^-2. The measurements are compared with perturbative QCD models based on BFKL and DGLAP evolution. The data are found to be compatible with s-channel helicity conservation

[en] A search for events with a high-energy isolated electron or muon and missing transverse momentum has been performed at the electron-proton collider HERA using an integrated luminosity of 13.6 pb^-1 in e^-p scattering and 104.7 pb^-1 in e⁺p scattering. Within the Standard Model such events are expected to be mainly due to W boson production with subsequent leptonic decay. In e^-p interactions one event is observed in the electron channel and none in the muon channel, consistent with the expectation of the Standard Model. In the e⁺p data a total of 18 events are seen in the electron and muon channels compared to an expectation of 12.4±1.7 dominated by W production (9.4±1.6). Whilst the overall observed number of events is broadly in agreement with the number predicted by the Standard Model, there is an excess of events with transverse momentum of the hadronic system greater than 25 GeV with 10 events found compared to 2.9±0.5 expected. The results are used to determine the cross-section for events with an isolated electron or muon and missing transverse momentum

[en] A narrow resonance in D*^-p and D*⁺p-bar invariant mass combinations is observed in inelastic electron-proton collisions at centre-of-mass energies of 300 GeV and 320 GeV at HERA. The resonance has a mass of 3099±3(stat.)±5(syst.) MeV and a measured Gaussian width of 12±3(stat.) MeV, compatible with the experimental resolution. The resonance is interpreted as an anti-charmed baryon with a minimal constituent quark composition of uuddc-bar, together with the charge conjugate

[en] Deep-inelastic positron-proton interactions at low values of Bjorken-x down to x ∼ 4 . 10^-5 which give rise to high transverse momentum π -mesons are studied with the H1 experiment at HERA. The inclusive cross section for π -mesons produced at small angles with respect to the proton remnant (the forward region) is presented as a function of the transverse momentum and energy of the π and of the four-momentum transfer Q² and Bjorken-x. Measurements are also presented of the transverse energy flow in events containing a forward π -meson. Hadronic final state calculations based on QCD models implementing different parton evolution schemes are confronted with the data. (orig.)

[en] A search for squarks in R-parity violating supersymmetry is performed in e^±p collisions at HERA using the H1 detector. The data were taken at a centre-of-mass energy of 319 GeV and correspond to an integrated luminosity of 64.3 pb^-1 for e⁺p collisions and 13.5 pb^-1 for e^-p collisions. The resonant production of squarks via a Yukawa coupling λ' is considered, taking into account direct and indirect R-parity violating decay modes. No evidence for squark production is found in the multi-lepton and multi-jet final state topologies investigated. Mass dependent limits on λ' are obtained in the framework of the Minimal Supersymmetric Standard Model. In addition, the results are interpreted in terms of constraints on the parameters of the minimal Supergravity model. At the 95% confidence level squarks of all flavours with masses up to 275 GeV are excluded in a large part of the parameter space for a Yukawa coupling of electromagnetic strength. For a coupling strength 100 times smaller, masses up to 220 GeV can be ruled out. (orig.)

[en] A narrow resonance in D^*-p and D^*+p invariant mass combinations is observed in inelastic electron-proton collisions at centre-of-mass energies of 300 GeV and 320 GeV at HERA. The resonance has a mass of 3099±3(stat.)±5 (syst.) MeV and a measured Gaussian width of 12±3(stat.) MeV, compatible with the experimental resolution. The resonance is interpreted as an anti-charmed baryon with a minimal constituent quark composition of uuddc, together with the charge conjugate. (orig.)

[en] Cross sections for the production of two isolated muons up to high di-muon masses are measured in ep collisions at HERA with the H1 detector in a data sample corresponding to an integrated luminosity of 71 pb^-1 at a centre of mass energy of √s=319 GeV. The results are in good agreement with Standard Model predictions, the dominant process being photon-photon interactions. Additional muons or electrons are searched for in events with two high transverse momentum muons using the full data sample corresponding to 114 pb^-1, where data at √s=301 GeV and √s=319 GeV are combined. Both the di-lepton sample and the tri-lepton sample agree well with the predictions

[en] A Very Forward Proton Spectrometer (VFPS) has been installed in the proton beam line of the H1 experiment. This device, located at 220 m downstream of the interaction point, is based on the Roman Pot technique and consists of two stations in the cold section of the proton beam line. We discuss the major physics issues which can be addressed with these forward proton taggers. (author)

[en] Results are presented on the photoproduction of isolated prompt photons, inclusively and associated with jets, in the γp center of mass energy range 142 < W < 266 GeV. The cross sections are measured for the transverse momentum range of the photons 5 < E_T^γ < 10 GeV and for associated jets with E_T^jet > 4.5 GeV. They are measured differentially as a function of E_T^γ, E_T^jet, the pseudorapidities η^γ and η^jet and estimators of the momentum fractions x_r and x_p of the incident photon and proton carried by the constituents participating in the hard process. In order to further investigate the underlying dynamics, the angular correlation between the prompt photon and the jet in the transverse plane is studied. Predictions by perturbative QCD calculations in next to leading order are about 30% below the inclusive prompt photon data after corrections for hadronisation and multiple interactions, but are in reasonable agreement with the results for prompt photons associated with jets. Comparisons with the predictions of the event generators PYTHIA and HERWIG are also presented. (orig.)

[en] The cross section for anti-deuteron photoproduction is measured at HERA at a mean centre-of-mass energy of W_γp = 200 GeV in the range 0.2 < p_T/M < 0.7 and vertical stroke y vertical stroke < 0.4, where M, p_T and y are the mass, transverse momentum and rapidity in the laboratory frame of the anti-deuteron, respectively. The numbers of anti-deuterons per event are found to be similar in photoproduction to those in central proton-proton collisions at the CERN ISR but much lower than those in central Au-Au collisions at RHIC. The coalescence parameter B₂, which characterizes the likelihood of anti-deuteron production, is measured in photoproduction to be 0.010 ± 0.002 ± 0.001, which is much higher than in Au-Au collisions at a similar nucleon-nucleon centre-of-mass energy. No significant production of particles heavier than deuterons is observed and upper limits are set on the photoproduction cross sections for such particles. (orig.)