Regge-plus-resonance approach to strangeness production from the deuteron

Electromagnetic production of strangeness plays a prominent role in our quest to chart the excitation spectrum of the nucleon. Despite the large body of high-quality p(g*,K)Y data, phenomenological analyses of these reactions have not led to an unequivocal outcome. The persistent lack of consensus on the role of different nucleon resonances in the reaction dynamics can be attributed in part to the important role played by non-resonant dynamics. In this dissertation, photon- and electron-induced kaon production off nucleons and deuterium is studied using an efficient reaction model developed at Ghent University, coined Regge-plus-resonance, that successfully reconciles an isobaric approach in the resonance region with Regge phenomenology at higher energies. The dominant non-resonant amplitudes are modelled in terms of K+(494) and K*+(892) t-channel Regge-trajectory exchange and are fixed by high-energy data. In a next step, this amplitude is extrapolated into the resonance region and refined by adding individual N* and Delta* terms in the s-channel. We show how the Regge-plus-resonance model, which is optimised to data obtained off proton targets, can be transformed to reliably describe all reactions with a kaon-hyperon pair in the final state. In addition to the study of strangeness production off the proton, it pays to consider the same reaction on more complex targets, such as the deuteron. In this dissertation quasi-free kaon production from the deuteron is investigated using the Regge-plus-resonance elementary operator within the relativistic plane-wave impulse approximation. In addition, the effects of the hyperon-nucleon final-state interaction are analysed using the non-relativistic one-boson-exchange model developed at Jülich as input. Numerical results for (semi-)inclusive and exclusive differential cross sections are presented and we investigate the formalism’s sensitivity to its different ingredients. The characteristic shape of the cross section is studied and distinct kinematic regions are identified where the 2H(g,K)YN reaction can be exploited to learn about elementary kaon production or hyperon-nucleon rescattering. Finally, model calculations are confronted with the first published 2H(g,K)YN data

Hyperon resonances in radiative kaon capture

We use crossing symmetry to extend our Regge model for electromagnetic kaon production from the proton to the case of radiative kaon capture. Our model is based on the exchange of the K(494) and K*(892) Regge trajectories in the t-channel. We use the parameters fitted to describe the p(gamma,K+)Y reaction to make predictions for the p(K-,gamma)Y process. The differential cross sections of the latter for kaon momenta of 520 and 750 MeV/c show a satisfactory agreement with data from the Crystal Ball Collaboration

Regge-plus-resonance predictions for neutral-kaon photoproduction from the deuteron

We present a Regge-inspired effective-Lagrangian framework for neutral-kaon photoproduction from the deuteron. Quasi-free kaon production is investigated using the Regge-plus-resonance elementary operator within the relativistic plane-wave impulse approximation. The Regge-plus-resonance model was developed to describe photoinduced and electroinduced charged-kaon production off protons. We show how this elementary operator can be transformed to account for the production of neutral kaons from both protons and neutrons. Our results compare favourably to the sole 2H(gamma,K0)YN dataset published to date.Comment: 4 pages, 1 figure; Proceedings 12th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Williamsburg VA, May 31-June 4, 201

Consistent interactions for high-spin fermion fields

We address the issue of consistent interactions for off-shell fermion fields of arbitrary spin. These interactions play a crucial role in the quantum hadrodynamical description of high-spin baryon resonances in hadronic processes. The Rarita-Schwinger description of high-spin fermion fields involves unphysical degrees of freedom, associated with their lower-spin content. These enter the interaction if not eliminated outright. The invariance condition of the interaction under the unconstrained Rarita-Schwinger gauge removes the lower-spin content of the fermion propagator and leads to a consistent description of the interaction. We develop the most general, consistent interaction structure for high-spin fermions. We find that the power of the momentum dependence of a consistent interaction rises with the spin of the fermion field. This leads to unphysical structures in the energy dependence of the computed cross sections when the short-distance physics is cut off with standard hadronic form factors. A novel, spin-dependent hadronic form factor is proposed that suppresses the unphysical artifacts.Comment: 17 pages, 10 figure

The incompleteness of complete pseudoscalar-meson photoproduction

[Background] A complete set is a minimum set of observables which allows one to determine the underlying reaction amplitudes unambiguously. Pseudoscalar-meson photoproduction from the nucleon is characterized by four such amplitudes and complete sets involve single- and double-polarization observables. [Purpose] Identify complete sets of observables, and study how measurements with finite error bars impact their potential to determine the reaction amplitudes unambiguously. [Method] The authors provide arguments to employ the transversity representation in order to determine the amplitudes in pseudoscalar-meson photoproduction. It is studied whether the amplitudes in the transversity basis for the $\gamma p \to K^+\Lambda$ reaction can be estimated without ambiguity. To this end, data from the GRAAL collaboration and mock data from a realistic model are analyzed. [Results] It is illustrated that the moduli of normalized transversity amplitudes can be determined from precise single-polarization data. Starting from mock data with achievable experimental resolution, it is quite likely to obtain imaginary solutions for the relative phases of the amplitudes. Also the real solutions face a discrete phase ambiguity which makes it impossible to obtain a statistically significant solution for the relative phases at realistic experimental conditions. [Conclusions] Single polarization observables are effective in determining the moduli of the amplitudes in a transversity basis. Determining the relative phases of the amplitudes from double-polarization observables is far less evident. The availability of a complete set of observables does not allow one to unambiguously determine the reaction amplitudes with statistical significance.Comment: 15 pages, 6 figures, 8 table

The p(γ,K+)Λp(\gamma,K^+)\Lambda reaction: consistent high-spin interactions and Bayesian inference of its resonance content

A Bayesian analysis of the world's $p(\gamma,K^+)\Lambda$ data is presented. We adopt a Regge-plus-resonance framework featuring consistent interactions for nucleon resonances up to spin $J = 5/2$. The power of the momentum dependence of the consistent interaction structure rises with the spin of the resonance. This leads to unphysical structures in the energy dependence of the computed cross sections when the short-distance physics is cut off with standard hadronic form factors. A plausible, spin-dependent modification of the hadronic form factor is proposed which suppresses the unphysical artifacts. Next, we evaluate all possible combinations of 11 candidate resonances. The best model is selected from the 2048 model variants by calculating the Bayesian evidence values against the world's $p(\gamma,K^+)\Lambda$ data. From the proposed selection of 11 resonances, we find that the following nucleon resonances have the highest probability of contributing to the reaction: $S_{11}(1535)$, $S_{11}(1650)$, $F_{15}(1680)$, $P_{13}(1720)$, $D_{13}(1900)$, $P_{13}(1900)$, $P_{11}(1900)$, and $F_{15}(2000)$.Comment: 5 pages, 4 figure

The Regge-plus-resonance model for kaon production on the proton and the neutron

The Regge-plus-resonance (RPR) framework for kaon photoproduction on the proton and the neutron is an economical single-channel model with very few parameters. Not only does the RPR model allow one to extract resonance information from the data, it has predictive power. As an example we show that the RPR model makes fair predictions for the $p(e,e'K^{+})\Lambda$ and the $n(\gamma,K^{+})\Sigma ^{-}$ observables starting from amplitudes optimized for the reaction $p(\gamma, K ^{+})\Lambda$ and $p(\gamma,K^{+})\Sigma ^{0}$ respectively

Regge-plus-resonance predictions for charged-kaon photoproduction from the deuteron

We present a Regge-inspired effective-Lagrangian framework for charged-kaon photoproduction from the deuteron. Quasi-free kaon production is investigated using the Regge-plus-resonance elementary operator within the non-relativistic plane-wave impulse approximation. The Regge-plus-resonance model was developed to describe photoinduced and electroinduced kaon production off protons and can be extended to strangeness production off neutrons. The non-resonant contributions to the amplitude are modelled in terms of K+(494) and K*+(892) Regge-trajectory exchange in the t-channel. This amplitude is supplemented with a selection of s-channel resonance-exchange diagrams. We investigate several sources of theoretical uncertainties on the semi-inclusive charged-kaon production cross section. The experimental error bars on the photocoupling helicity amplitudes turn out to put severe limits on the predictive power when considering quasi-free kaon production on a bound neutron.Comment: 5 pages, 2 figures; Proceedings 19th International IUPAP Conference on Few-Body Problems in Physics, Bonn, 200

Regge-plus-resonance predictions for kaon photoproduction from the neutron

We present predictions for n(gamma,K+)Sigma- differential cross sections and photon-beam asymmetries and compare them to recent LEPS data. We adapt a Regge-plus-resonance (RPR) model developed to describe photoinduced and electroinduced kaon production off protons. The non-resonant contributions to the amplitude are modelled in terms of K+(494) and K*+(892) Regge-trajectory exchange. This amplitude is supplemented with a selection of s-channel resonance diagrams. The three Regge-model parameters of the n(gamma,K+)Sigma- amplitude are derived from the ones fitted to proton data through SU(2) isospin considerations. A fair description of the n(gamma,K+)Sigma- data is realized, which demonstrates the Regge model's robustness and predictive power. Conversion of the resonances' couplings from the proton to the neutron is more challenging, as it requires knowledge of the photocoupling helicity amplitudes. We illustrate how the uncertainties of the helicity amplitudes propagate and heavily restrain the predictive power of the RPR and isobar models for kaon production off neutron targets.Comment: 14 pages, 4 figures; Minor revisions; Published in Physics Letters