Restricted cross sections

Multiparticle processes with only neutral final state particles are shown to be useful to study multiperipheral dynamics without the presence of recently introduced corrections due to diffraction, clusterings, multi-Pomeranchukon contribution, etc.     

On the calculation of scattering cross sections from absorption cross sections

A dispersion-type relation between photon scattering and absorption cross sections is derived. It is shown that the relation gives the correct scattering cross section in the low and high frequency limits, and is consistent with the Kramers-Heisenberg formula in the vicinity of a resonance. The practical application of the relation is illustrated by calculations of the scattering cross section and refractive index of He and Ne at STP and dry air at 1200°K from known absorption cross section data. The method is useful for obtaining scattering cross sections, polarizabilities, and refractive indices at elevated temperatures.     

Heavy-ion fusion cross sections

The absolute cross sections for heavy-ion induced reactions in the rare earth region are studied systematically as functions of projectile energy, charge and mass. Targets and projec-tiles are chosen so that the same final states are fed in erbium, holmium and dysprosium nueleides with mass numbers A = 159–162. As the low lying states in these final nucleides are well known, the cross sections for the various reaction channels can be determined from singles γ-ray spectra. By summing these cross sections, the heavy-ion fusion cross sections are determined, and compared to theoretical calculations based on the Bass model as well as to total reaction cross sections deduced from optical model fits to elastic scattering data.     

Integrated pion-nucleus cross sections

We propose an improved analysis for transmission experiments which determines uniquely, up to statistical error, the imaginary part of the forward nuclear amplitude. The analysis we propose takes the Coulomb interaction into account and is especially useful when the Coulomb-nuclear interference effects are large. In some cases the real part of the forward nuclear amplitude may also be extracted.     

Rising total cross sections

As two hadrons pass one another the vacuum between them becomes polarised. It is shown that quark-loop contributions to the vacuum polarisation account for most, if not all, of the rise of the total cross section with energy.     

Quasi-elastic reaction cross sections

Starting from the idea that a critical phenomenon separates deep-inelastic from quasi-elastic reactions, we introduce a simple procedure to calculate quasi-elastic cross sections. The results are compared with experimental data.     

Singular cross sections in muon colliders

We address the problem that the cross section for the collisions of unstable particles diverges, if calculated by standard methods. This problem is considered for beams much smaller than the decay length of the unstable particle, much larger than the decay length and finally also for pancake- shaped beams. We find that in all cases this problem can be solved by taking into account the production/propagation of the unstable particle and/or the width of the incoming wave packets in momentum space. Received: 4 March 2003 / Revised version: 8 April 2003 / Published online: 23 May 2003 RID="a" ID="a" e-mail: chrisd@sci.kun.nl RID="b" ID="b" e-mail: kleiss@sci.kun.nl     

Partial cross sections in a plasma

The collision integral for a plasma obtained in the ring approximation with a preliminary ladder renormalization is used to determine the first two partial cross sections. Numerical calculations show that the partial cross sections display a very nonlinear behavior (in logarithmic scale) and do not diverge at small values of the plasma parameter.     

Compound-nucleus effects in spin-spin cross sections

By comparison with recent data, it is shown that spin-spin cross sections for low-energy neutrons may be dominated by a simple compound-elastic level-density effect, independent of spin-spin terms in the nucleon-nucleus optical-model potential.     

Nonperturbative corrections in resummed cross sections

We show that the resummation of large perturbative corrections in QCD leads to ambiguities in high energy cross sections that are suppressed by powers of large momentum scales. These ambiguities are caused by infrared renormalons, which are a general feature of resummed hardscattering functions in perturbative QCD, even though these functions are infrared safe order-by-order in perturbation theory. As in the case of the operator product expansion, the contributions of infrared renormalons to coefficient functions may be absorbed into the definition of higher-dimensional operators, which induce nonperturbative corrections that are power-suppressed at high energies. The strength of the suppression is determined by the location of the dominant infrared renormalon, which may be identified explicitly in the resummed series. In contrast to the operator product expansion, however, the relevant operators in factorized hadron-hadron scattering and jet cross sections are generally nonlocal in QCD, although they may be expressed as local operators in an effective theory for eikonalized quarks. In this context, we verify and interpret the presence of 1 /Q corrections to the inclusive Drell-Yan cross section with Q the pair mass. In a similar manner, we find exp (-b² In Q) corrections in the impact parameter space of the transverse momentum distributions of the Drell-Yan process and e⁺6^- annihilation. We also show that the dominant nonperturbative corrections to cone-based jet cross sections behave as 1 /(Qδ), with δ the opening angle of the jet and Q the center of mass energy.     

Energy integrated photonuclear cross sections

Energy integrated photonuclear cross sections are calculated in the electric dipole approximation for the nuclei ²H, ⁴He, ¹²C, ¹⁶O, ²⁸Si, ³²S and ⁴⁰Ca. The ground-state wave functions are calculated by the Brueckner-Hartree-Fock procedure. The dependence of the cross sections on the different types of realistic nucleon-nucleon potentials is discussed as well as the dependence on the underlying harmonic oscillator states. The enhancement factors κ are also given.     

Heavy-ion total reaction cross sections

A one-parameter expression has been found which predicts with a smaller χ² than previous expressions, the total reaction cross sections for a large number of heavy-ion systems covering a wide energy range. The expression uses an interaction radius which is a sum of the equivalent uniform matter radii of the interacting nuclei plus and energy-dependent term.     

Double pomeron jet cross sections

We treat hadron-hadron collisions where the final state is kinematically of the kind associated with double-pomeron-exchange (DPE) and has large transverse momentum jets. We show that in addition to the conventional factorized (FDPE) contribution, there is a nonfactorized (NDPE) contribution which has no pomeron beam jet. Within a simple model we compute DPE-two-jet total and differential cross sections at Tevatron energy scales, and show that the NDPE contribution is dominant.     

Cesium halide photodissociation cross sections

The cross sections for the photodissociation of CsCl, CsBr and CsI have been measured using a cesium halide seeded shock tube. These measurements were made by using the ratio of two absorption signals: one in the incident and one in the reflected shock region. The cesium halide was the absorption species in the incident shock region and a halogen negative ion species absorbed in the reflected shock region. Previously measured and calculated halogen negative ion photodetachment cross sections were used in calculating the cesium halide photodissociation cross section. Measurements were made between 4000 and 2000 Å at 1900°K.     

Pionization effects in proton-nucleus total cross sections

A simple unitary model for meson production effects in proton-nucleus scattering was used earlier to estimate the pionization corrections to the proton-deuteron total cross section. This model is here used to study the corresponding effects in heavy nuclei using a generalization of the optical limit. An increase in the total cross sections at the upper limit of NAL energies of roughly 20% is found.