High Energy Ep Collisions Research Articles

Heavy flavour production in high-energy ep collisions

A selection of recent results on heavy quark production at the HERA collider by the H1 and ZEUS collaborations are presented with a focus on charmonium production in DIS, charm fragmentation and beauty production.

Diffraction in high energy ep collisions

The H1 and ZEUS experiments at HERA both measure diffraction in both inclusive and exclusive processes. Presented here is an overview of those measurements which allow a consistent QCD picture of diffraction to be made.

Spectroscopy in ep collisions at HERA

Recent spectroscopy studies performed in high energy ep collisions by the H1 and ZEUS Collaborations are described. The status of the searches for the strange pentaquark, the θ + , is presented as well as results on searches for further exotic states such as the Σ − − . The evidence for the observation of a pentaquark state containing the charmed quark, the θ c , is discussed and the result is given on searches for glueball candidates at HERA.

Hard diffractive scattering in high energy ep collisions and the Monte Carlo Generator RAPGAP

The program RAPGAP generates events in ep collisions where the electron is scattered on a pomeron P coupled to the proton p. A gap in rapidity between the fast moving proton and the remaining hadronic system is observed. The program is applicable to photoproduction ( Q 2 ⋍ 0 ) as well as deep inelastic scattering ( Q 2 > 0). Different types of hard interaction processes such as γ∗ q → q′ and γrg → q q ̄ for light and heavy quarks are included. Several options for the parametrisation of the pomeron and its parton density are available within the program. The hadronisation is performed using the LUND string fragmentation model. The Monte Carlo implementation is described and some phenomenological consequences are discussed.

Electroweak vector boson production in high-energy ep collisions

We present the results of a comprehensive study of W and Z production in high-energy ep collisions. The processes ep → eW ± X, ep → νW su− X, ep → eZ X and ep → νZ X are investigated. The region of small momentum transfer in eW and eZ production, with a fermion exchanged in the u-channel, is treated using the photon structure function approach, and carefully matched to the deep inelastic region. Low momentum photon exchange contributions to νW and eZ production, where the proton either stays intact or is transformed into a nucleon resonance, are calculated using form factors functions fitted directly to experimental data. These processes will be the source of rare but spectacular events. Total cross sections for W and Z production at HERA and at an ep collider in the LEP tunnel (LEP/LHC), and detailed results for the characteristics of eW, νW, eZ and νZ events are presented. In ep→eW X, eZ X the uncertainties of the quark densities in the proton and photon allow the total cross section predictions to vary by ±40% at HERA and ±15% at LEP/LHC.

Neutral vector boson production in high energyep collisions for minimal extensions of the standard model

We study the direct production of neutral vector bosons in the processep→eZX in the standard model and two of its minimal extensions with differentZ-fermion couplings. We find that a measurement of the integrated lepton asymmetry with longitudinally polarized electron beams seems promising to discriminate between the standard model and these minimal extensions, even at HERA energies.

Transverse-polarization effects at ep colliders.

Possible effects of the transverse polarization of electrons in high-energy ep collisions is studied. It is shown that (1) the azimuthally integrated cross section never depends on the polarization and that (2) the possible azimuthal asymmetry which is proportional to the polarization vanishes in the standard model. The condition for vanishing polarization effects is stated and two examples showing nonzero polarization dependence are analyzed.

E 6 Exotic quark production in ep collisions

We examine the possibility of producing exotic quarks from E 6 theories via flavor changing couplings in high energy ep collisions at HERA and the proposed LEP × LHC. We find that the rate is rather small and very mixing angle dependent. Assuming maximal mixing, the production rates are ≌ 10−30 events per year at HERA (for masses up to 100 GeV) and ≌200 events per year at LEP × LHC (for masses up to 300 GeV).