HERA Energies Research Articles

Photoproduction within the two-component Dual Parton Model: Amplitudes and cross sections

In the framework of the Dual Parton Model an approximation scheme to describe high energy photoproduction processes is presented. Based on the distinction between direct, resolved soft, and resolved hard interaction processes we construct effective impact parameter amplitudes. In order to treat low mass diffraction within the eikonal formalism in a consistent way a phenomenological ansatz is proposed. The free parameters of the model are determined by fits to high energy hadro- and photoproduction cross sections. We calculate the partial photoproduction cross sections and discuss predictions of the model at HERA energies. Using hadro- and photoproduction data together, the uncertainties of the model predictions are strongly reduced.

Photoproduction of high- pT jets in next-to-leading order QCD

The photoproduction of high p T jets at HERA energies is considered in next-to-leading order perturbative QCD. The cancellation of O( α em α S 2) dependence of the cross section on the factorisation scale M γ of the collinear singularities of the photon is shown explicitly to occur when the direct and resolved photon terms are both considered. A detailed numerical analysis is presented and compared with previous studies, showing that the theoretical predictions are affected by large uncertainties related to the poorly known photon structure functions. Sizeable numerical differences in the analyses published so far are also emphasised.

The role of energy conservation in the BFKL equation

We study a modification to the BFKL equation at zero momentum transfer due to the imposition of energy conservation. The significance of our modification, which enters in the form of an ultraviolet cutoff, is illustrated directly and is discussed within the context of the gluon diffusion in kT. We conclude that, in the kinematic range accessible to HERA energy conservation leads to a suppression of the semi-hard cross sections (relative to the BFKL predictions) but that it does not appreciably alter the value of the leading eigenvalue of the BFKL kernel.

Formula omitted] lepton production at HERA

The ep collider HERA provides an alternative to search for physics beyond the standard model. In this context, we study the phenomenology of a heavy spin- 3 2 lepton at HERA energies. Estimates for production cross sections and decay rates are given, as well as Monte Carlo event reconstruction of several relevant distributions. We contrast our results with those obtained in the case of a heavy spin- 3 2 lepton, and point out thet a clear distinction between these two signals can be made.

Associate J/ ψ+ γ production as a clean probe of heavy quark production models

Associated J/ ψ+ γ production in deep inelastic e( μ) p collisions is suggested as a clean probe for distinguishing between heavy quark production models. At HERA energies, the event rates, possibly accompanied by spectator-jets of the incoming photon, and/or a measurement in particular of the γ γ distribution unambiguously allow to discriminate between the semi-local duality model and the color-singlet model. At a fixed target μp experiment ( E μ =280 GeV, say), a mere confirmation of the existence of several J/ ψ+ γ events would already rule out the color-singlet model.

Total photoproduction cross section measurement at HERA energies

We present first results on the total photoproduction cross section measurement with the H1 detector at HERA. The data were extracted from low Q 2 collisions of 26.7 GeV electrons with 820 GeV protons. The γp total cross section has been measured by two independent methods in the γp center of mass energy range from 90 to 290 GeV. For an average center of mass energy of 195 GeV a value of σ tot ( γp) = 159 ± 7 (stat.) ± 20 (syst.) μb was obtained.

Leptoproduction of heavy flavour at high energies

A k ⊥-dependent form of high energy factorization has been recently proposed in order to find the small- x behaviour of heavy mass production in QCD. Here we review the method and discuss some applications to heavy flavour lepto-production at asymptotic and Hera energies.

Higher order QCD corrections to the photoproduction of a direct photon at HERA

The contribution of resolved photons to the photoproduction cross section of direct photons is reexamined. Higher order,O(α2αs), QCD corrections to this contribution are calculated and turn out to be important at HERA energies. The observation of direct photons in the medium-p⊥ range (p⊥∼5 GeV/c) should provide the opportunity to measure the gluon content of the photon.

Model-independent treatment of QED corrections for deep inelastic neutral current scattering

We discuss complete model-independent, semi-analytical QED corrections connected with the lepton legs to neutral-current deep inelastic scattering at HERA energies in four different kinds of variables: leptonic, hadronic, mixed, and Jaquet-Blondel variables. For the first three kinds, details of the approach and numerical results are presented.

Final states in heavy quark leptoproduction at small x

The process of heavy quark production in ep collisions at very high energies is investigated in detail using a new Monte Carlo simulation of QCD which includes leading contributions in the small- x region. The simulation generates a gluon structure function which satisfies the Lipatov evolution equation, together with the associated initial-state gluon radiation, and combines this with the off-mass-shell gluon-photon fusion cross section. The resulting heavy quark and associated final-state hadron distributions are asymptotically different from those of the conventional treatment based on Altarelli-Parisi evolution and on-shell gluon-photon fusion, but the differences are not large at HERA energies.

The Monte Carlo program LESKO-F for deep inelastic e ±p → e ± X scattering at HERA including QED bremsstrahlung from the lepton line

A Monte Carlo event generator for deep inelastic electron (position) proton neutral current scattering at HERA energies is described. It includes O (α) QED bremsstrahlung from the lepton line and allows for longitudinal polarization of the lepton beam. Final particles are produced on the parton level, i.e. the final state is described by the four-momenta of lepton, quark, photon and proton remnant. Information on each generated event is stored in the standardized HEPEVT event record. The program is constructed in such a way that it may be easily connected with other event generators and/or detector simulation programs. Some examples of numerical results of this program are presented.

Order- αs2 QCD corrections to the deep inelastic proton structure functions F2 and FL

We present the calculation of the order- α s 2 correction to the deep inelastic coefficient functions C i ( x, Q 2) ( i=2, L). Its effect on the behaviour of the proton structure functions F i ( x, Q 2) for fixed target and HERA energies is studied. We investigate the dependence of the O( α s 2) QCD corrected structure functions on the mass factorization scale and the specific parametrization chosen for the parton densities. Also discussed are the implications of the higher-order QCD corrections for the extraction of the parton densities, in particular the gluon distribution function, from the data. Our analysis reveals that the O( α s 2) contributions to F 2( x, Q 2) are appreciable and can amount from +10% (large- x region) to −20% (small- x region), depending on the chosen value for Q 2 and the parton density set. In the case of F L( x, Q 2) the corrections in the small- x region can be even larger (about 45%). The large corrections in the region 0.5< x<1, which can be attributed to soft-gluon radiation, may explain the difference observed between the combined SLAC-BCDMS data and the fit based on the next-to-leading-log (NLL) approximation for the proton structure function F 2( x, Q 2).

Heavy flavour production at high energies

A form of high energy factorization has been recently proposed in order to find the small-¢x behaviour of heavy mass production in QCD. Here we review the method and discuss its application to heavy flavour photo- and lepto-production at asymptotic and Hera energies.

The total and jet photoproduction cross sections at HERA and Fermilab energies

We show that the hadronic structure of the photon can lead to a significant rise in the total and jet photoproduction cross sections at HERA and Fermilab energies. We present our predictions for the total and jet photoproduction cross sections for the forthcoming Fermilab E683 and ZEUS and H-1 experiments at the HERA collider. We discuss the relevance of the hadronic character to the anomalous muon content recently observed in cosmic ray showers associated with point sources such as Cygnus X-3, Hercules X-1 and the Crab Nebula.

The total and jet photoproduction cross sections at HERA and Fermilab energies

We show that the hadronic structure of the photon can lead to a significant rise in the total and jet photoproduction cross sections at HERA and Fermilab energies. We present our predictions for the total and jet photoproduction cross sections for the forthcoming Fermilab E683 and ZEUS and H-1 experiments at the HERA collider. We discuss the relevance of the hadronic character to the anomalous muon content recently observed in cosmic ray showers associated with point sources such as Cygnus X-3, Hercules X-1 and the Crab Nebula.

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.

Exploring the Lipatov equation

Properties of the Lipatov equation for the small x behaviour in deep inelastic scattering are explored. In particular, the effect of imposing upper and lower cutoffs on the transverse momentum integral in the equation is analysed. The lower cutoff, needed to avoid integration over momenta where the gluon propagators are nonperturbative, does not change the output power. Imposing both a lower and an upper cutoff, so as to take account of kinematic limitations at subasymptotic values of Q 2, has an important effect even at HERA energies. This will make it difficult to separate the perturbative pomeron from the nonperturbative one seen already in soft hadronic collisions.

HERACLES: an event generator for ep interactions at HERA energies including radiative processes

A Monte Carlo event generator simulating neutral and charged current ep interactions at HERA energies and beyond is described. The present version 4.0 of the generator optionally treats the ep scattering either by means of structure-function parametrizations or on the basis of parton distribution functions in the framework of the quark-parton model. Single-photon emission from the lepton line as self-energy corrections and the complete set of one loop weak corrections can be included. These corrections are sufficient to describe the cross-section with an accuracy of a new percent or, if Q 2 ≤2 x 10 3 GeV 2, of less than 1%.

Electron-state bremsstrahlung processesep→γep(X) at HERA

The analytic formulae for the bremsstrahlung photon spectra in the processeslN→γlN andlN→γlX (taking into account γ-andZ-boson exchange diagrams) are derived. The energy and angular distributions of photons are computed at HERA energies. The very large sensitivity of spectra for the deep inelastic channel to the choice of the structure functions is revealed in the region of small angles and large energies of the emitted photons.

Leading log radiative corrections to ep scattering including jet measurement

>The leading log radiative corrections to deep inelastic neutral and charged current ep scattering are calculated both for the case of jet measurement and when Q2 is measured from the leptonic and y from the hadronic vertex. They are found to be smaller compared to the case of lepton measurement. The corrections to heavy flavour production amount to −3.5%for cc, −4.4% for bb and − 12% (−16%) for top production with mt = 90 (120) GeV at HERA energies.