Pieter Vancraeyveld

Bayesian analysis of kaon photoproduction with the Regge-plus-resonance model

We address the issue of unbiased model selection and propose a methodology based on Bayesian inference to extract physical information from kaon photoproduction γp→K+Λ data. We use the single-channel Regge-plus-resonance (RPR) framework for γp→K+Λ to illustrate the proposed strategy. The Bayesian evidence Z is a quantitative measure for the models fitness given the worlds data. We present a numerical method for performing the multidimensional integrals in the expression for the Bayesian evidence. We use the γp→K+Λ data with an invariant energy W>2.6 GeV in order to constrain the background contributions in the RPR framework with Bayesian inference. Next, the resonance information is extracted from the analysis of differential cross sections and single- and double-polarization observables. This background and resonance content constitutes the basis of a model, which is coined RPR-2011. It is shown that RPR-2011 yields a comprehensive account of the kaon photoproduction data and provides reasonable predictions for e+p→e′+K++Λ observables.

Electroproduction of kaons from the proton in a Regge-plus-resonance approach

Abstract We present a Regge-plus-resonance (RPR) description of the p ( e , e ′ K + ) Y processes ( Y = Λ , Σ 0 ) in the resonance region. The background contributions to the RPR amplitude are constrained by the high-energy p ( γ , K + ) Y data. As a result, the number of free model parameters in the resonance region is considerably reduced compared to typical effective-Lagrangian approaches. We compare a selection of RPR model variants, originally constructed to describe KY photoproduction, with the world electroproduction database. The electromagnetic form factors of the intermediate N * s and Δ * s are computed in the Bonn constituent-quark model. With this input, we find a reasonable description of the p ( e , e ′ K + ) Y data without adding or readjusting any parameters. It is demonstrated that the electroproduction response functions are extremely useful for fine-tuning both the background and resonant contributions to the reaction dynamics.

Bayesian inference of the resonance content of p(gamma,K^+)Lambda

A Bayesian analysis of the worlds p(g,K+)L data is presented. From the proposed selection of 11 resonances, we find that the following nucleon resonances have the highest probability of contributing to the reaction: S11(1535), S11(1650), F15(1680), P13(1720), D13(1900), P13(1900), P11(1900), and F15(2000). We adopt a Regge-plus-resonance framework featuring consistent couplings for nucleon resonances up to spin J = 5/2. 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 worlds p(g,K+)L data.

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 -> K+ Lambda reaction can be estimated without ambiguity. To this end, data from the GRAAL collaboration and simulations 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 simulations 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.

Kaon photoproduction from the deuteron in a Regge-plus-resonance approach

Abstract We present a Regge-inspired effective-Lagrangian framework for kaon photoproduction from the deuteron. Quasi-free kaon production is investigated using the Regge-plus-resonance (RPR) elementary operator within the relativistic plane-wave impulse approximation. The RPR model was developed to describe photoinduced and electroinduced charged-kaon production off protons. We show how this elementary operator can be transformed in order to account for the production of neutral kaons from both protons and neutrons. The model results for kaon photoproduction from the deuteron compare favourably to the H 2 ( γ , K ) Y N data published to date.

Bayesian model selection for electromagnetic kaon production on the nucleon

Abstract We present the results of a Bayesian analysis of a Regge model to describe the background contribution for K + Λ and K + Σ 0 photoproduction. The model is based on the exchange of K + ( 494 ) and K * + ( 892 ) trajectories in the t-channel. We utilise the Bayesian evidence Z to determine the best model variant for each channel. The Bayesian evidence integrals were calculated using the Nested Sampling algorithm. For different prior widths, we find decisive Bayesian evidence ( Δ ln Z ≈ 24 ) for a K + Λ photoproduction Regge model with a positive vector coupling and a negative tensor coupling constant for the K * + ( 892 ) trajectory, and a rotating phase factor for both trajectories. Using the χ 2 minimisation method, one could not draw this conclusion from the same dataset. For the K + Σ 0 photoproduction Regge model, on the other hand, the difference between the evidence integrals is insufficient to pinpoint one model variant.

Bayesian inference of the resonance content of p(γ, K+)Λ

A bayesian analysis of the worlds p(γ,K^+)Λ data is presented. From the proposed selection of 11 resonances, we find that the following nucleon resonances have the highest probability of contributing to the reaction: S11(1535), S11(1650), F15(1680), P13(1720), D13(1900), P13(1900), P11(1900), and F15(2000). We adopt a Regge-plus-resonance framework featuring consistent couplings for nucleon resonances up to spin J=5/2. 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 worlds p(γ,K+)Λ data.

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

We present a Regge‐inspired effective‐Lagrangian framework for neutral‐kaon photo‐production 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(γ,K0)YN dataset published to date.

BAYESIAN MODEL SELECTION FOR ELECTROMAGNETIC KAON PRODUCTION ON THE NUCLEON

We present the results of a Bayesian analysis of a Regge model for K+Λ photoproduction. The model is based on the exchange of K+(494) and K*+(892) trajectories in the t-channel. For different prior widths, we find decisive Bayesian evidence (Δ ln Ƶ ≈ 24) for a K+Λ photoproduction Regge model with a positive vector coupling and a negative tensor coupling constant for the K*+(892) trajectory, and a rotating phase factor for both trajectories. Using the χ2 minimization method, one could not draw this conclusion from the same dataset.

Strangeness content of the nucleon in quasielastic neutrino-nucleus reactions

We present a systematic study of the sensitivity of quasielastic neutrino-nucleus cross sections at intermediate energies to the strange quark sea of the nucleon. To this end, we investigate the impact of the weak strangeness form factors on the ratio of proton-to-neutron knockout, the ratio of neutral-to-charged current cross sections, on the Paschos-Wolfenstein relation, and on the longitudinal helicity asymmetry. The influence of axial as well as vector strangeness effects is discussed. For the latter, we introduce strangeness parameters from various hadron models and from a recent fit to data from parity-violating electron scattering. In our model, the nuclear target is described in terms of a relativistic mean-field approach. The effects of final-state interactions on the outgoing nucleon are quantified within a relativistic multiple-scattering Glauber approach. Our results are illustrated with cross sections for the scattering of 1-GeV neutrinos and antineutrinos off a {sup 12}C target. Folding with a proposed FINeSSE (anti-)neutrino energy distribution has no qualitative influence on the overall sensitivity of the cross-section ratios to strangeness mechanisms. We show that vector strangeness effects are large and strongly Q{sup 2} dependent.