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R. Engel

Publications and source records attributed to R. Engel.

360 records · Page 20Linked to original sources
arXiv1998

On photohadronic processes in astrophysical environments

We discuss the first applications of our newly developed Monte Carlo event generator SOPHIA to multiparticle photoproduction of relativistic protons with thermal and power law radiation fields. The measured total cross section is reproduced in terms of excitation and decay of baryon resonances, direct pion production, diffractive scattering, and non-diffractive multiparticle production. Non--diffractive multiparticle production is described using a string fragmentation model. We demonstrate that the widely used `$Δ$--approximation' for the photoproduction cross section is reasonable only for a restricted set of astrophysical applications. The relevance of this result for cosmic ray propagation through the microwave background and hadronic models of active galactic nuclei and gamma-ray bursts is briefly discussed.

astro-ph↗
arXiv1998

Phenomenological description of the gamma* p cross section at low Q2

Low Q2 photon-proton cross sections are analysed using a simple, QCD-motivated parametrisation $σ_{γ^\star p}\propto 1/(Q^2+Q_0^2)$, which gives a good description of the data. The Q2 dependence of the gamma* p cross section is discussed in terms of the partonic transverse momenta of the hadronic state the photon fluctuates into.

hep-ph↗
arXiv1998

Rapidity gaps and the PHOJET Monte Carlo

A model for the production of large rapidity gaps being implemented in the Monte Carlo event generator PHOJET is discussed. In this model, high-mass diffraction dissociation exhibits properties similar to hadron production in non-diffractive hadronic collisions at high energies. Hard diffraction is described using leading-order QCD matrix elements together with a parton distribution function for the pomeron and pomeron-flux factorization. Since this factorization is imposed on Born graph level only, unitarity corrections lead to a non-factorizing flux function. Rapidity gaps between jets are obtained by soft color reconnection. It was previously shown that this model is able to describe data on diffractive hadron production from the CERN-SPS collider and from the HERA lepton-proton collider. In this work we focus on the model predictions for rapidity gap events in p-p collisions at \sqrt{s} = 1800 GeV and compare to TEVATRON data.

hep-ph↗
arXiv1997

Photoproduction Off Nuclei, Part II: Particle and Jet Production

High energy multiparticle photoproduction off nuclear targets is studied. The photon is assumed to interact in direct scattering processes or as a resolved $q\bar{q}$-state according to the Generalized Vector Dominance Model. In the description of resolved interactions multiple soft and hard processes in each $q\bar{q}$--nucleon interaction are taken into consideration. The model, formulated within the framework of the two-component Dual Parton Model, is shown to agree with hadron production data from real and weakly virtual photon-nucleus interactions. Differences between multiparticle production in photon-nucleus and hadron-nucleus interactions and features of jet production in photon-nucleus collisions at HERA-energies are discussed.

hep-ph↗
arXiv1997

Hard diffraction and central diffraction in hadron-hadron and photon-hadron collisions

Hadron production in single and central diffraction dissociation is studied in a model which includes soft hadron interaction as controlled by a supercritical pomeron parametrization and hard diffraction. Within this model, particle production in collisions with pomerons exhibit properties like multiple soft interactions and multiple minijets, quite similar to hadron production in non-diffractive hadronic collisions at high energies. However, important differences occur in transverse momentum jet and hadron distributions. It is shown that the model is able to describe data from the CERN-SPS collider and from the HERA collider. Model predictions are presented for single and central diffraction at TEVATRON.

hep-ph↗
arXiv1997

Diffractive meson production from virtual photons with odd charge-parity exchange

We calculate the cross section of diffractive charge-parity C=+1 neutral meson production in virtual photon proton collision at high energies. Due to the opposite C-parities of photon and meson M (M = eta_C, pi^0, a_2) this process probes the t-channel C=-1 odderon exchange which is described here as noninteracting three-gluon exchange. Estimates for the cross section of the inelastic diffractive process gamma^* p -> eta_C X_p are presented. The total cross section of diffractive η_C meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons (pi^0, a_2) in γ^* p collisions are of the same order if the photon virtuality Q^2 is m^2_C.

hep-ph↗
arXiv1997

Multiparticle production in high-mass diffraction dissociation and the interplay of photons and pomerons

Multiple interaction models satisfying $s$-channel unitarity predict that, in contrast to inelastic processes, factorization is violated in diffractive processes. The size of this effect can be characterized in terms of the rapidity gap survival probability. The possibility of its measurement at HERA is pointed out. Furthermore a method to measure photon diffraction dissociation at LEP2 and planned linear colliders is discussed and cross section predictions are given.

hep-ph↗
arXiv1996

Hadronic photon interactions at high energies

A simple phenomenological introduction to the physics of multi-pomeron exchange amplitudes in connection with the Abramovski-Gribov-Kancheli (AGK) cutting rules is given. The AGK cutting rules are applied to obtain qualitative and quantitative predictions on multiparticle production at high energies. On this basis, particle production in hadron-hadron scattering, photoproduction, and in particular the transition to deep-inelastic scattering is discussed.

hep-ph↗
arXiv1996

Photoproduction off Nuclei and Point-like Photon Interactions Part I: Cross Sections and Nuclear Shadowing

High energy photoproduction off nuclear targets is studied within the Glauber-Gribov approximation. The photon is assumed to interact as a $q\bar{q}$-system according to the Generalized Vector Dominance Model and as a ``bare photon'' in direct scattering processes with target nucleons. We calculate total cross sections for interactions of photons with nuclei taking into account coherence length effects and point-like interactions of the photon. Results are compared to data on photon-nucleus cross sections, nuclear shadowing, and quasi- elastic $ρ$-production. Extrapolations of cross sections and of the shadowing behaviour to high energies are given.

hep-ph↗
arXiv1996

Diffraction dissociation, an important background to photon-photon collisions via heavy ion beams at LHC

The cross sections for the production of hadrons in quasi-real photon-photon collisions in proton-proton and heavy ion interactions are compared with the corresponding cross sections for central diffraction and for photon-pomeron collisions. The signatures, heavy ions or protons with only slightly changed momenta together with two large rapidity gaps and a cluster of produced hadrons in the central region, are nearly identical in all three processes. Therefore, it will be rather difficult to distinguish the reactions experimentally. It is found, that central diffraction is the dominant process in collisions of protons, light and medium-heavy ions. The photon-pomeron and photon-photon processes have quite similar cross sections in collisions of heavy ions like lead.

hep-ph↗
arXiv1996

Quantum effects in radiation on short bunches

The radiation caused by particles of one bunch in the collective electromagnetic field of the short oncoming bunch is studied. Quantum effects are calculated for the spectrum of radiated photons. Using this spectrum, the dependence of the relative energy loss delta on a quantum parameter kappa is dicussed. It is shown that the behaviour of delta changes considerably with the increase of that parameter. In the classical regime (kappa << 1) the energy loss is proportional to the incoming particle energy, while in the extreme quantum regime (kappa >> 1) the energy loss becomes a constant. The coherent e+e- pair production for gamma-e colliders as cross-channel to CBS is considered.

hep-ph↗
arXiv1996

Gamma-Gamma Physics at LEP2

This report is an overview of the gamma-gamma physics capabilities of LEP2, and covers the following topics: structure functions, equivalent photon approximation, tagging conditions etc, soft and semihard physics, large-$p_t$ processes, heavy-quark physics, and exclusive channels.

hep-ph↗
arXiv1995

A new possibility to monitor collisions of relativistic heavy ions at LHC and RHIC

We consider the radiation of particles of one bunch in the collective field of the oncoming bunch, called coherent bremsstrahlung (CBS). The main characteristics of CBS for LHC (in the Pb-Pb mode) and for RHIC are calculated. At LHC about $3.9 10^8 dE_γ/E_γ$ photons per second are expected for photon energies $E_γ\stackrel{<} {\sim} E_c= 93 $ eV. It seems that CBS can be a potential tool for optimizing collisions and for measuring beam parameters. The bunch length can be found from the critical energy of the CBS spectrum; the transverse bunch size is related to the photon rate. A specific dependence of photon rate on the impact parameter between the beams allows for a fast control over the beam displacement.

hep-ph↗
arXiv1995

The equivalent photon approximation for coherent processes at colliders

We consider coherent electromagnetic processes for colliders with short bunches, in particular the coherent bremsstrahlung (CBS). CBS is the radiation of one bunch particles in the collective field of the oncoming bunch. It can be a potential tool for optimizing collisions and for measuring beam parameters. A new simple and transparent method to calculate CBS is presented based on the equivalent photon approximation for this collective field. The results are applied to the $ϕ$--factory $DAΦNE$. For this collider about $ 5 \cdot 10^{14} d E_γ/ E_γ$ photons per second are expected in the photon energy $E_γ$ range from the visible light up to 25 eV.

hep-ph↗
arXiv1995

Hadronic photon-photon interactions at high energies

Photon-photon collisions are investigated in the framework of the two-component Dual Parton Model. The model is shown to agree well to hadron production data from hadron-hadron and photon-hadron collisions. The multiparticle production in hadron-hadron, photon-hadron and photon-photon collisions as predicted by the model is compared. Strong differences are only found as function of the transverse momentum variable. The hadron production in photon-photon collisions at present and future electron-positron colliders is studied using photon spectra according to the equivalent photon approximation, according to beamstrahlung and according to backscattered laser radiation.

hep-ph↗
arXiv1995

Hard diffraction in hadron--hadron interactions and in photoproduction

Hard single diffractive processes are studied within the framework of the triple--Pomeron approximation. Using a Pomeron structure function motivated by Regge--theory we obtain parton distribution functions which do not obey momentum sum rule. Based on Regge-- factorization cross sections for hard diffraction are calculated. Furthermore, the model is applied to hard diffractive particle production in photoproduction and in $p\bar{p}$ interactions.

hep-ph↗