Pion Photo Research Articles

Sensitivity of multipoles to phase variations in pion photo- and electro-production analyses

We use the Athens Model Independent Analysis Scheme (AMIAS) to examine the validity of using the Fermi–Watson theorem in the multipole analyses of pion photoproduction and electroproduction data. A standard practice in this field is to fix the multipoles’ phases from $$\pi N$$ scattering data, making use of the Fermi–Watson theorem. However, these phases are known with limited accuracy and the effect of this uncertainty on the obtained multipole extraction has not been fully explored yet. Using AMIAS we constrain the phases within their experimentally determined uncertainty. We first analyze sets of photoproduction pseudodata of increasing statistical precision and subsequently we apply the methodology for a re-analysis of the Bates/Mainz electroproduction data. It is found that the uncertainty induced by the $$\pi N$$ phases uncertainty to the extracted solutions would be significant only in the analysis of data with much higher precision than the current available experimental data.

Two Pion Photo- and Electroproduction with CLAS

Exclusiveπ+π-pphoto- and electroproduction data from CLAS have considerably extended the information on the spectrum and structure of nucleon resonances. The data from theπ+π-pandNπchannels have provided results on the electrocouplings of most resonances in the mass region up to 1.8 GeV and at photon virtualities up to 5.0 GeV2. The recent CLAS data onπ+π-pphotoproduction have improved knowledge on the photocouplings of nucleon resonances in the mass range of 1.6 GeV <MN*<2.0 GeV and on their decays to theπΔ andρpfinal hadron states. Analyses of the combinedπ+π-pphoto- and electroproduction data have revealed evidence for the candidate-stateN’(1720)3/2+. The new results on the nucleon resonance spectrum, electroexcitation amplitudes from analysis of the CLASπ+π-pphoto- and electroproduction data, and their impact on the exploration of strong QCD are presented.

Recent results on the nucleon resonance spectrum and structure from the CLAS detector

The CLAS detector at Jefferson Lab has provided the dominant part of all available worldwide data on exclusive meson electroproduction off protons in the resonance region. New results on the $\gamma_{v}pN^*$ transition amplitudes (electrocouplings) are available from analyses of the CLAS data and will be presented. Their impact on understanding of hadron structure will be discussed emphasizing the credible access to the dressed quark mass function that has been achieved for the first time by a combined analysis of the experimental results on the electromagnetic nucleon elastic and $N \rightarrow N^*$ transition form factors. We will also discuss further convincing evidences for a new baryon state $N^{\, '}(1720)3/2^+$ found in a combined analysis of charged double pion photo- and electroproduction cross sections off the protons.

Resonance matrix elements on the lattice

The notion of resonance matrix elements is discussed. The analogue of the Lellouch-Luscher formula for the pion photo(electro)production amplitude in the Δ(1232) resonance region is given. A prescription for the pole extraction of the ΔNγ * transition form factors is provided. Theoretical prospects on the lattice calculation of B → K *(892)l + l − form factors are briefly summarized.

Pion photo- and electroproduction in relativistic baryon ChPT

We present a calculation of pion photo- and electroproduction in manifestly Lorentz-invariant baryon chiral perturbation theory up to and including order q^4. We fix the low-energy constants by fitting experimental data in all available reaction channels. Our results can be accessed via a web interface, the so-called chiral MAID.

Lorentz Invariance Violation on UHECR propagation

In the present work possible consequences of the Lorentz symmetry breaking are presented. Those consequences are encoded through the modification of dispersion relations which are applied to the processes of pair and pion photo production generated at UHECR scale. The produced physics from such modifications contains scenarios where the expected photo production thresholds under Lorentz symmetry are shifted; the other scenarios predict that the cosmic ray flux will be recover at higher energies due to the suppression of the production processes. This phenomenon is repeated for different components of the diffuse photon background and generates opacity bands in distinct parts of the energy spectrum; we expose scenarios that have not been considered previously by other authors. Moreover, in contrast with previous studies, in this work the scenarios generated by the Lorentz symmetry breaking are measured by a generic parameter, unique for each process, independent of the correction origin (E2 or p2) in the dispersion relations, as well as the nature of the particle involved and possible corrections to the relations of conservation energy-momentum laws. Additionally, the Pierre Auger result for the pion photo production is used to estimate the energy scale parameter of the Lorentz symmetry breaking.

Pion photo- and electroproduction in relativistic baryon chiral perturbation theory and the chiral MAID interface

We present a calculation of pion photo- and electroproduction in manifestly Lorentz-invariant baryon chiral perturbation theory up to and including order ${q}^{4}$. We fix the low-energy constants by fitting experimental data in all available reaction channels. Our results can be accessed via a Web interface, the so-called chiral MAID. We explain how our program works and how it can be used for further analysis.

Unified description of hadron-photon and hadron-meson scattering in the Dyson-Schwinger approach

We derive the expression for the nonperturbative coupling of a hadron to two external currents from its underlying structure in QCD. Microscopically, the action of each current is resolved to a coupling with dressed quarks. The Lorentz structure of the currents is arbitrary and thereby allows to describe the hadron's interaction with photons as well as mesons in the same framework. We analyze the ingredients of the resulting four-point functions and explore their potential to describe a variety of processes relevant in experiments. Possible applications include Compton scattering, the study of two-photon effects in hadron form factors, pion photo- and electroproduction on a nucleon, nucleon-pion or pion-pion scattering.

Cosmogenic gamma rays and the composition of cosmic rays

We discuss the prospects of detecting the sources of ultrahigh energy (UHE) cosmic ray (CR) nuclei via their emission of cosmogenic $\ensuremath{\gamma}$ rays in the GeV to TeV energy range. These $\ensuremath{\gamma}$ rays result from electromagnetic cascades initiated by high energy photons, electrons, and positrons that are emitted by CRs during their propagation in the cosmic radiation background and are independent of the simultaneous emission of $\ensuremath{\gamma}$ rays in the vicinity of the source. The corresponding production power by UHE CR nuclei (with mass number $A$ and charge $Z$) is dominated by pion photo production ($\ensuremath{\propto}A$) and Bethe-Heitler pair production ($\ensuremath{\propto}{Z}^{2}$). We show that the cosmogenic $\ensuremath{\gamma}$-ray signal from a single steady UHE CR source is typically more robust with respect to variations of the source composition and injection spectrum than the accompanying signal of cosmogenic neutrinos. We study the diffuse emission from the sum of extragalactic CR sources as well as the point-source emission of the closest sources.

Electromagnetic excitation of nucleon resonances

Recent progress on the extraction of electromagnetic properties of nucleon resonance excitation through pion photo- and electroproduction is reviewed. Cross section data measured at MAMI, ELSA, and CEBAF are analyzed and compared to the analysis of other groups. On this basis, we derive longitudinal and transverse transition form factors for most of the four-star nucleon resonances. Furthermore, we discuss how the transition form factors can be used to obtain empirical transverse charge densities. Contour plots of the thus derived densities are shown for the Delta, Roper, S 11, and D 13 nucleon resonances.

Baryon resonance analysis from SAID

We discuss the analysis of data from πN elastic scattering and single pion photo- and electroproduction. The main focus is a study of low-lying non-strange baryon resonances. Here we concentrate on some difficulties associated with resonance identification, in particular the Roper and higher P11 states.

Color transparency and short-range correlations in exclusive pion photo- and electroproduction from nuclei

A relativistic and quantum mechanical framework to compute nuclear transparencies for pion photo- and electroproduction reactions is presented. Final-state interactions for the ejected pions and nucleons are implemented in a relativistic eikonal approach. At sufficiently large ejectile energies, a relativistic Glauber model can be adopted. At lower energies, the framework possesses the flexibility to use relativistic optical potentials. The proposed model can account for the color-transparency (CT) phenomenon and short-range correlations (SRC) in the nucleus. Results are presented for kinematics corresponding to completed and planned experiments at Jefferson Lab. The influence of CT and SRC on the nuclear transparency is studied. Both the SRC and CT mechanisms increase the nuclear transparency. The two mechanisms can be clearly separated, though, as they exhibit a completely different dependence on the hard-scale parameter. The nucleon and pion transparencies as computed in the relativistic Glauber approach are compared with optical-potential and semiclassical calculations. The similarities in the trends and magnitudes of the computed nuclear transparencies indicate that they are not subject to strong model dependences.

A dispersive approach to pion photo- and electroproduction

The relativistic amplitudes of pion photo- and electroproduction are calculated by dispersion relations at constant t. Several sum rules and low-energy theorems for the threshold amplitudes are investigated within this technique. The continuation of the amplitudes to sub-threshold kinematics is shown to provide a unique framework to derive the low-energy constants of chiral perturbation theory by global properties of the excitation spectrum.

Unitary isobar model --MAID2007

The unitary isobar model MAID2007 has been developed to analyze the world data of pion photo- and electroproduction. The model contains both a common background and several resonance terms. The background is unitarized according to the K-matrix prescription, and the 13 four-star resonances with masses below 2 GeV are described by appropriately unitarized Breit-Wigner forms. The data have been analyzed by both single-energy and global fits, and the transverse and longitudinal helicity amplitudes have been extracted for the four-star resonances below 2 GeV. Because of its inherent simplicity, MAID2007 is well adopted for predictions and analysis of the observables in pion photo- and electroproduction.

Chiral symmetry and diffractive neutral pion photo- and electroproduction

We show that diffractive production of a single neutral pion in photon-induced reactions at high energy is dynamically suppressed due to the approximate chiral symmetry of QCD. These reactions have been proposed as a test of the odderon exchange mechanism. We show that the odderon contribution to the amplitude for such reactions vanishes exactly in the chiral limit. This result is obtained in a nonperturbative framework and by using PCAC relations between the amplitudes for neutral pion and axial vector current production.

Electroproduction of nucleon resonances

The unitary isobar model MAID has been extended and used for a partial wave analysis of pion photo- and electroproduction in the resonance region W < 2 GeV. Older data from the world data base and more recent experimental results from Mainz, Bates, Bonn and JLab for Q^2 up to 4.0 (GeV/c)^2 have been analyzed and the Q^2 dependence of the helicity amplitudes have been extracted for a series of four star resonances. We compare single-Q^2 analyses with a superglobal fit in a new parametrization of Maid2003 together with predictions of the hypercentral constituent quark model. As a result we find that the helicity amplitudes and transition form factors of constituent quark models should be compared with the analysis of bare resonances, where the pion cloud contributions have been subtracted.

Dispersion relation analysis of neutral pion photoproduction and electroproduction at threshold

Neutral pion photo- and electroproduction at threshold is analyzed in the framework of dispersion relations. For this purpose, we evaluate the real threshold amplitudes in terms of Born contributions and dispersion integrals determined by the imaginary parts of the MAID and SAID multipoles. The results show considerable cancellations between Born terms and resonance contributions. Good agreement with the data is found for photoproduction. While our dispersion analysis suggests considerable discrepancies for electroproduction, the present state of the experimental multipole analysis at finite Q^2 does not permit drawing conclusions at this time.

Pion photo- and electroproduction and the partially conserved axial current

The relevance of the axial current for pion production processes off the nucleon with real or virtual photons is revisited. Employing the hypothesis of a partially conserved axial current (PCAC), it is shown that, when all of the relevant contributions are taken into account, PCAC does not provide any additional constraint for threshold production processes that goes beyond the Goldberger-Treiman relation. In particular, it is shown that pion electroproduction processes at threshold cannot be used to extract any information regarding the weak axial form factor. The relationships found in previous investigations are seen to be an accident of the approximations usually made in this context.

Dispersion relation formalism for virtual Compton scattering and the generalized polarizabilities of the nucleon

A dispersion relation formalism for the virtual Compton scattering (VCS) reaction on the proton is presented, which for the first time allows a dispersive evaluation of four generalized polarizabilities at a four-momentum transfer ${Q}^{2}&lt;~0.5 {\mathrm{GeV}}^{2}.$ The dispersive integrals are calculated using a state-of-the-art pion photo- and electroproduction analysis. The dispersion formalism provides a new tool to analyze VCS experiments above pion threshold, thus increasing the sensitivity to the generalized polarizabilities of the nucleon.

A phenomenological description of [formula omitted] photo- and electroproduction in nucleon resonance region

The π −Δ ++ production on the nucleon by real and virtual photons is discussed as initial step in a simple model approach for the two pion photo- and electroproduction on the nucleon, with emphasis on nucleon resonance excitation which is of interest for new facilities like TJNAF. A calculation for π −Δ ++ channel in resonance excitation region is presented and compared to existing experimental data along with a discussion of physical effects that we find to be of relevance. The calculation is proposed as a starting basis for the investigation of N ∗ electromagnetic form factors using experimental data about two pion production by real and virtual photons.