Determination of quark distributions in ep collisions

A method is presented for extracting various interesting quark distributions from measurements of differential cross sections for neutral and charged current processes at an ep collider such as HERA. The complete structure of ep interactions as described by the parton model in leading-order QCD and lowest-order electroweak theory is taken into account. Using Monte Carlo data we illustrate the experimental feasibility of the proposed unfolding procedure.     

Open charm production in diffractive ep collisions

The cross section for the diffractive reactionγ + p → cc + p with a real or virtual photon is calculated in the nonperturbative two-gluon exchange model of Landshoff and Nachtmann. Numerical predictions are given for cross sections and spectra at typical HERA values of c.m. energy and photon virtuality. The contribution of charm to the diffractive structure function is evaluated and found to be rather small in the model, and the ratio between the production rates for bb and cc is tiny.     

Search for lepton flavour violation in ep collisions at HERA

A search for the lepton flavour violating processes ep→μX and ep→τX is performed with the H1 experiment at HERA. Final states with a muon or tau and a hadronic jet are searched for in a data sample corresponding to an integrated luminosity of 66.5 pb^-1 for e⁺p collisions and 13.7 pb^-1 for e^-p collisions at a centre-of-mass energy of 319 GeV. No evidence for lepton flavour violation is found. Limits are derived on the mass and the couplings of leptoquarks inducing lepton flavour violation in an extension of the Buchmüller–Rückl–Wyler effective model. Leptoquarks produced in ep collisions with a coupling strength of λ=0.3 and decaying with the same coupling strength to a muon–quark pair or a tau–quark pair are excluded at 95% confidence level up to masses of 459 GeV and 379 GeV, respectively.     

Gaugino-higgsino mixing and slepton-squark production in ep collisions

We consider gaugino-higgsino mixing in the minimal supersymmetric extension of the standard model and determine the mass eigenstates and eigenvalues taking the physical masses of the lightest chargino and neutralino as input in the diagonalization procedure. Using the results we calculate cross sections and charge asymmetries for the associated production of sleptons and squarks in ep collisions and investigate the dependence on the chargino and neutralino properties and on the slepton and squark spectrum. The numerical analysis is performed within the minimal supergravity model as well as in a more general framework. Although predictions are made also for higher energies, the main focus is on the HERA energy range. We indicate the values of sparticle masses accessible to SUSY searches at HERA and illustrate the expected reach beyond the present bounds.     

Tau lepton production in ep collisions at HERA

The production of tau leptons in ep collisions is investigated using data recorded by the H1 detector at HERA in the period 1994–2000. Tau leptons are identified by detecting their decay products, using leptonic and hadronic decay modes. The cross section for the production of tau lepton pairs is measured for the first time at HERA. Furthermore, a search for events with an energetic isolated tau lepton and with large missing transverse momentum is performed. The results are found to be in agreement with the Standard Model predictions.     

Diffractive photoproduction of dijets in ep collisions at HERA

Diffractive photoproduction of dijets was measured with the ZEUS detector at the ep collider HERA using an integrated luminosity of 77.2 pb^-1. The measurements were made in the kinematic range Q² < 1 GeV², 0.20<y<0.85 and x_IP<0.025, where Q² is the photon virtuality, y is the inelasticity and x_IP is the fraction of the proton momentum taken by the diffractive exchange. The two jets with the highest transverse energy, E_T ^jet, were required to satisfy E_T ^jet>7.5 and 6.5 GeV, respectively, and to lie in the pseudorapidity range -1.5<η^jet<1.5. Differential cross sections were compared to perturbative QCD calculations using available parameterisations of diffractive parton distributions of the proton.     

Search for single top quark production in ep collisions at HERA

The European Physical Journal C - A search for single top quark production is performed in $e^{\pm}p$ collisions at HERA. The search exploits data corresponding to an integrated luminosity of 118.3...     

Production of exotic E6 leptons in ep collisions

We consider the possibility of producing single, charged E₆ exotic leptons in ep collisions at high energies via flavor-changing couplings to neutral gauge bosons. We find that at HERA energies the production rate for these particles is quite small due to small mixing effects. At higher energies, as may be possible at a LEP ep collider, the production rates become reasonably large: ⋍100–1000 events per year.     

t-dependences of vector meson diffractive production in ep collisions

We calculate p_T-dependences of diffractive vector meson photo- and electroproduction in electron-proton collisions with and without proton dissociation at the small momenta transfers. The calculated slopes are in good agreement with the data.     

Non-linear QCD dynamics and exclusive production in ep collisions

The exclusive processes in electron–proton (ep) interactions are an important tool to investigate the QCD dynamics at high energies as they are in general driven by the gluon content of proton which is strongly subject to parton saturation effects. In this paper we compute the cross sections for the exclusive vector meson production as well as the deeply virtual Compton scattering (DVCS) relying on the color dipole approach and considering the numerical solution of the Balitsky–Kovchegov equation including running coupling corrections. We show that the small-x evolution given by this evolution equation is able to describe the DESY-HERA data and is relevant for the physics of the exclusive observables in future electron–proton colliders and in photoproduction processes to be measured in coherent interactions at the LHC.     

Gluon helicity distributions from hyperon production in proton-proton collisions

We point out a sensitive way of analyzing the gluon polarization inside a polarized proton using hyperon production at largep _T inPP collisions, by using initial and final state polarizations.     

Formation and decay of scalar Leptoquarks/Squarks in ep collisions

The cross sections for the formation of scalar resonances, leptoquarks or squarks, in electron/positron-proton collisions at HERA are presented, including next-to-leading order QCD corrections. Depending mildly on the mass of the resonances, the K-factors increase the production cross sections by up to 30% if the target quarks are valence quarks. The QCD corrections to the partial decay widths of leptoquarks/ squarks to leptons and quarks are small. The electron spectrum in the decays is nevertheless softened by perturbative gluon radiation at a level of 2 GeV for a leptoquark/ squark mass of 200 GeV.     

Photon fragmentation in large-Q^2 ep collisions at next-to-leading order QCD

We study the production of photons accompanied by jets in large- deep inelastic scattering. Numerical results for the cross section differential with respect to the fraction of momentum carried by a photon inside a jet at large , up to in perturbative QCD, are presented. The sensitivity to the fragmentation contribution allows one to study the quark-to-photon fragmentation function. Our results can be confronted with future experimental data from HERA. Received: 9 August 1999 / Published online: 28 September 1999     

Polarized prompt photon production in high energy\vec ep collisions

It is shown that high-p _T polarized prompt photons produced by high energy longitudinally polarized electrons scattered off unpolarized protons may be utilized to determine the spin-dependent parton distributions of the photon.