Jannes Nys

Finite-Energy Sum Rules in Eta Photoproduction off a Nucleon

The reaction gamma N -> eta N is studied in the high-energy regime (with photon lab energies E gamma(lab) > 4 GeV) using information from the resonance region through the use of finite-energy sum rules. We illustrate how analyticity allows one to map the t dependence of the unknown Regge residue functions. We provide predictions for the energy dependence of the beam asymmetry at high energies.

Amplitude analysis and the nature of the Zc(3900)

Abstract The microscopic nature of the XYZ states remains an unsettled topic. We show how a thorough amplitude analysis of the data can help constraining models of these states. Specifically, we consider the case of the Z c ( 3900 ) peak and discuss possible scenarios of a QCD state, virtual state, or a kinematical enhancement. We conclude that current data are not precise enough to distinguish between these hypotheses, however, the method we propose, when applied to the forthcoming high-statistics measurements should shed light on the nature of these exotic enhancements.

Electroweak single-pion production off the nucleon: From threshold to high invariant masses

Neutrino-induced single-pion production (SPP) provides an important contribution to neutrino-nucleus interactions, ranging from intermediate to high energies. There exists a good number of low-energy models in the literature to describe the neutrino production of pions in the region around the Delta resonance. Those models consider only lowest-order interaction terms and, therefore, fail in the high-energy region (pion-nucleon invariant masses, W greater than or similar to 2 GeV). Our goal is to develop a model for electroweak SPP off the nucleon, which is applicable to the entire energy range of interest for present and future accelerator-based neutrino-oscillation experiments. We start with the low-energy model of [E. Hernandez, J. Nieves, and M. Valverde, Phys. Rev. D 76, 033005 (2007).], which includes resonant contributions and background terms derived from the pion-nucleon Lagrangian of chiral-perturbation theory [S. Scherer and M. R. Schindler, A Primer for Chiral Perturbation Theory (Springer, Berlin, 2012), p. 1.]. Then, from the background contributions, we build a high-energy model using a Regge approach. The low-and high-energy models are combined, in a phenomenological way, into a hybrid model. The hybrid model is identical to the low-energy model in the low-W region, but, for W > 2 GeV, it implements the desired high-energy behavior dictated by Regge theory. We have tested the high-energy model by comparing with one-pion production data from electron and neutrino reactions. The hybrid model is compared with electron-proton scattering data, with neutrino SPP data and with the predictions of the NuWro Monte Carlo event generator. Our model is able to provide satisfactory predictions of the electroweak one-pion production cross section from pion threshold to high W. Further investigation and more data are needed to better understand the mechanisms playing a role in the electroweak SPP process in the high-W region, in particular, those involving the axial current contributions.

Amplitude extraction in pseudoscalar- meson photoproduction: towards a situation of complete information

A complete set for pseudoscalar-meson photoproduction is a minimum set of observables from which one can determine the underlying reaction amplitudes unambiguously. The complete sets considered in this work involve single- and double-polarization observables. It is argued that for extracting amplitudes from data, the transversity representation of the reaction amplitudes offers advantages over alternate representations. It is shown that with the available single-polarization data for the Λ reaction, the energy and angular dependence of the moduli of the normalized transversity amplitudes in the resonance region can be determined to a fair accuracy. Determining the relative phases of the amplitudes from double-polarization observables is far less evident.

Model discrimination in pseudoscalar-meson photoproduction

To learn about a physical system of interest, experimental results must be able to discriminate among models. We introduce a geometrical measure to quantify the distance between models for pseudoscalar-meson photoproduction in amplitude space. Experimental observables, with finite precision, map to probability distributions in amplitude space, and the characteristic width scale of such distributions needs to be smaller than the distance between models if the observable data are going to be useful. We therefore also introduce a method for evaluating probability distributions in amplitude space that arise as a result of one or more measurements, and show how one can use this to determine what further measurements are going to be necessary to be able to discriminate among models.

On the η and η′ Photoproduction Beam Asymmetry at High Energies

Abstract We show that, in the Regge limit, beam asymmetries in η and η ′ photoproduction are sensitive to hidden strangeness components. Under reasonable assumptions about the couplings we estimate the contribution of the ϕ Regge pole, which is expected to be the dominant hidden strangeness contribution. The ratio of the asymmetries in η ′ and η production is estimated to be close to unity in the forward region 0 − t / GeV 2 ≤ 1 at the photon energy E lab = 9 GeV , relevant for the upcoming measurements at Jefferson Lab.

Analyticity constraints for hadron amplitudes: Going high to heal low-energy issues

Analyticity constitutes a rigid constraint on hadron scattering amplitudes. This property is used to relate models in different energy regimes. Using meson photoproduction as a benchmark, we show how to test contemporary low energy models directly against high energy data. This method pinpoints deficiencies of the models and treads a path to further improvement. The implementation of this technique enables one to produce more stable and reliable partial waves for future use in hadron spectroscopy and new physics searches.

K+ Λ electroproduction above the resonance region

Background: In π+ n and π− p electroproduction, conventional models cannot satisfactory explain the data above the resonance region, in particular the transverse cross section. Although no high-energy L-T-separated cross-section data is available to date, a similar scenario can be inferred for K+ Λ electroproduction. Purpose: Develop a phenomenological model for the p(γ*,K+)Λ reaction at forward angles and high-energies. Propose a universal framework for interpreting charged-kaon and charged-pion electroproduction above the resonance region. Method: Guided by the recent model for charged-pion electroproduction, developed by the authors, a framework for K+ Λ electroproduction at high energies and forward angles is constructed. To this end, a Reggeized background model for K+ Λ photoproduction is first developed. This model is used as a starting base to set up an electroproduction framework. Results: The few available data of the unseparated p(γ*,K+)Λ cross section are well explained by the model. Predictions for the L-T-separation experiment planned with the 12 GeV upgrade at Jefferson Lab are given. The newly-proposed framework predicts an increased magnitude for the transverse structure function, similar to the situation in charged-pion electroproduction. Conclusions: Within a hadronic framework featuring Reggeized background amplitudes, s-channel resonance-parton effects can explain the observed magnitude of the unseparated p(γ*,K+)Λ cross section at high energies and forward angles. Thereby, no hardening of the kaon electromagnetic form factor is required.

Modeling neutrino-nucleus interaction at intermediate energies

We present the current status of the research activities of the Ghent group on neutrino-nucleus interactions. These consist in the modeling of some of the relevant neutrino-nucleus reaction channels at intermediate energies: low-energy nuclear excitations, quasielastic scattering, two-nucleon knockout processes and single-pion production. The low-energy nuclear excitations and the quasielastic peak are described using a Hartree-Fock-CRPA (continuum random phase approximation) model that takes into account nuclear long-range correlations as well as the distortion of the outgoing nucleon wave function. We include two-body current mechanisms through short-range correlations and meson-exchange currents. Their influence on one- and two-nucleon knockout responses is computed. Bound and outgoing nucleons are treated within the same mean-field framework. Finally, for modeling of the neutrino-induced single-pion production, we use a low-energy model that contains resonances and the background contributions required by chiral symmetry. This low-energy model is combined with a Regge approach into a Hybrid model, which allows us to make predictions beyond the resonance region.

What is the right formalism to search for resonances? II. The pentaquark chain

We discuss the differences between several partial-wave analysis formalisms used in the construction of three-body decay amplitudes involving fermions. Specifically, we consider the decay