Pseudoscalar Meson Photoproduction Research Articles

Recoil polarimetry with the Crystal Ball at MAMI

The spectrum and properties of the excited states of the nucleon are still poorly established despite decades of study. These proceedings describe new measurements of pseudo-scalar meson photoproduction at the Crystal Ball at MAMI, employing a new large acceptance nucleon recoil polarimeter. The double- and single-polarisation observables obtained will provide valuable and unique data to be used as part of the world effort to improve our knowledge of this fundamental spectrum.

Photoproduction of scalar and pseudoscalar mesons on a lepton within the local Nambu-Jona-Lasinio model

Using the description of the subprocess γγ* → S(P) in terms of local Nambu-Jona-Lasinio model, we calculate the cross sections of photoproduction of scalar and pseudoscalar mesons in high-energy photonlepton collision processes. The dependence on the transversal momentum and the total cross sections in Weizsäcker-Williams approximation are presented.

Search for new and an investigation of known resonances using experimental data on photoproduction of η and η′ mesons on nucleons

The dynamical model of photoproduction of pseudoscalar mesons on nucleons constructed earlier is used for more accurate determination of the properties of known resonances. The new, “missing,” resonances are sought using modern precise experimental data on photoproduction of η and η′ mesons on nucleons.

Irreducible tensor approach to spin observables in the photoproduction of mesons with arbitrary spin-paritysπ

based on irreducible tensor techniques are i) the number of independent irreducible tensor amplitudes is 4(2s+1), ii) a single compact formula is sufficient to express these amplitudes in terms of allowed electric and magnetic multipole amplitudes and iii) all the spin observables including beam analyzing powers as well as the differential cross section are expressible in terms of bilinear irreducible tensors of rank 0 to 2(s + 1). The relationship between the irreducible tensor amplitudes and the helicity amplitudes is elucidated in general and explicit expressions for the helicity amplitudes are given in terms of the irreducible tensor amplitudes in the particular cases of pseudoscalar and vector meson photo production. The connection between the irreducible tensor amplitudes introduced here and the well known CGLN amplitudes for photo production of pseudoscalar mesons is also established.

Positivity constraints on spin observables in exclusive pseudoscalar meson photoproduction

Positivity constraints have proved to be important for spin observables of exclusive reactions involving polarized initial and final particles. Attention is focused on the photoproduction of pseudoscalar mesons from spin $1/2$ baryons, more specifically $\ensuremath{\gamma}N\ensuremath{\rightarrow}K\ensuremath{\Lambda},\ensuremath{\gamma}N\ensuremath{\rightarrow}K\ensuremath{\Sigma}$, for which new experimental data are becoming available.

Single-channel fits andK-matrix constraints

A K-matrix formalism is used to relate single-channel and multi-channel fits. We show how the single-channel formalism changes as new hadronic channels become accessible. These relations are compared with those commonly used to fit pseudoscalar meson photoproduction data.

Gauge-invariant approach to meson photoproduction including the final-state interaction

A fully gauge-invariant (pseudoscalar) meson photoproduction amplitude off a nucleon including the final-state interaction is derived. The approach based on a comprehensive field-theoretical formalism developed earlier by one of the authors replaces certain dynamical features of the full interaction current by phenomenological auxiliary contact currents. A procedure is outlined that allows for a systematic improvement of this approximation. The feasibility of the approach is illustrated by applying it to both the neutral and charged pion photoproductions.

Recent results from ELSA

One of the foremost challenges in hadron physics is to obtain an understanding of the structure of baryons. The main features of the baryon mass spectrum at low excitation energies are reasonably known and described quite accurately by models, but the high-mass spectrum is still rather obscure. With modern achievements in accelerators, highly polarized nucleon targets and hermetically closed detectors allowing for charged and neutral particle detection high-lying excitations can be investigated through the systematic study of single and multiple meson production. In this contribution the instrumentation and results of experiments from the ELSA accelerator at Bonn as well as their significance for baryon resonance analyses are described. In particular, data on photoproduction of vector and pseudoscalar mesons of different flavors are presented.

Signatures of the handbag mechanism in wide-angle photoproduction of pseudoscalar mesons

Wide-angle photoproduction of pseudoscalar mesons is investigated under the assumption of dominance of the handbag mechanism considering both quark helicity flip and non-flip. The partonic subprocess, meson photoproduction off quarks, is analysed with the help of a covariant decomposition of the subprocess amplitudes which is independent of a specific meson generation mechanism. As examples of subprocess dynamics, however, the twist-2 as well as two-particle twist-3 contributions are explicitly calculated. Characteristic features of the handbag approach are discussed in dependence upon the relative magnitudes of the invariant functions. Differential cross sections and spin correlations are predicted to show a characteristic behaviour which allows to test the underlying assumption of handbag dominance.

Photoproduction of pseudoscalar mesons

Experiments that study the photoproduction of pseudoscalar mesons; pions, etas and kaons, have the potential to increase our knowledge of baryon and hyperon resonance properties. Recent experiments at JLab, Mainz, GRAAL, and Bonn are beginning to produce results in the form of polarization and asymmetry measurements and determinations of the differential and integrated cross sections. These new data are essential to the performance of Partial-Wave Analyses that are less model dependent and coupled-channels calculations that incorporate unitarity dynamically, combining hadronic reaction channels together with electromagnetic processes. This approach is necessary to extract resonance properties and may lead to the identification of missing, but predicted, resonances. Some recent experimental and phenomenological results for single and double pseudoscalar meson photoproduction are discussed.

Associative charm photoproduction with circularly polarized photons

We calculate the asymmetries for the processes γ → + p → →Λ c +(Σ c +)+ D 0 and γ → + p → →Λ c +(Σ c +)+ D ∗ , induced by circularly polarized photons on a polarized proton target in the framework of an effective Lagrangian model. We found large and positive values for the asymmetry induced by the z-component of the proton polarization, in a wide energy region, from threshold up to E γ =70 GeV. In the case of pseudoscalar meson photoproduction, the predictions are qualitatively model-independent. For D ∗ -photoproduction, there is a strong dependence of the asymmetries on NΛ cD(D ∗) -coupling constants.

Quark model approach to the η meson electroproduction on the proton

A recently developed quark model approach to pseudoscalar meson photoproduction is extended to the electroproduction process for the η meson in the kinematics of momentum transfer Q2 ≤ 4 (GeV/c)2 and total centre of mass energy W ≤ 1.6 GeV. Existing data are well reproduced and the roles of the S11 (1535) and D13 (1520) resonances are closely investigated. In the study of the longitudinal excitation of the S11 (1535) resonance, a reliable constraint on the S11 (1535) properties is obtained by cleanly removing the electromagnetic transition from the γ(v)p → S11 (1535) → ηp amplitude. Thus, the fitted quantities can be determined with an uncertainty of about 15%. This could be the first direct constraint on the S11 (1535) properties in theory.

Meson photoproduction at GRAAL: preliminary results

A polarised and tagged \(\vec \gamma \) ray beam is produced at GRAAL with energies ranging from 500 MeV up to 1500 MeV. Results of beam polarisation asymmetries and cross sections for the photoproduction of pseudoscalar mesons on the proton are presented. These very precise measurements cover the angular range 30°–150°, providing stringent constraints to theoretical models.

Effect of form factors in fits to photoproduction data

We compare the effects of several form factor recipes in fits to pseudoscalar meson photoproduction data. The specific examples of pion and kaon photoproduction are used to illustrate how different choices can alter the results of such analyses.

Lessons to be learned from the coherent photoproduction of pseudoscalar mesons

We study the coherent photoproduction of pseudoscalar mesons---particularly of neutral pions---placing special emphasis on the various sources that put into question earlier nonrelativistic-impulse-approximation calculations. These include: final-state interactions, relativistic effects, off-shell ambiguities, and violations to the impulse approximation. We establish that, while distortions play an essential role in the modification of the coherent cross section, the uncertainty in our results due to the various choices of optical-potential models is relatively small (of at most 30%). By far the largest uncertainty emerges from the ambiguity in extending the many on-shell-equivalent representations of the elementary amplitude off the mass shell. Indeed, relativistic impulse-approximation calculations that include the same pionic distortions, the same nuclear-structure model, and two sets of elementary amplitudes that are identical on-shell, lead to variations in the magnitude of the coherent cross section by up to factors of five. Finally, we address qualitatively the assumption of locality implicit in most impulse-approximation treatments, and suggest that the coherent reaction probes---in addition to the nuclear density---the polarization structure of the nucleus.

Polarization in Pseudoscalar Meson Photoproduction

Two problems of current interest in the use of polarization observables for pseudoscalar meson photoproduction to probe the basic dynamics are discussed. The first one concerns the extraction of the ratio of electric quadrupole to magnetic dipole transition strengths in the photo-excitation of the \(\). The predictions of various models are compared with the recent LEGS and MAMI data obtained with polarized photon beams. The second one is a proposal by Saghai and Tabakin to use the nodal structure of spin observables to unveil underlying dynamics in the production process. We demonstrate with the example of \(\) at \(\) photon energy that the assumption of multipole truncation, which enters into their nodal analysis, is questionable.

Unified approach to pseudoscalar meson photoproductions off nucleons in the quark model

An unified approach to the pseudo scalar meson ($\pi, \eta$, and $K$) photoproduction off nucleons are presented. It begins with the low energy QCD Lagrangian, and the resonances in the s- and u- channels are treated in the framework of the quark model The duality hypothesis is imposed to limit the number of the t-channel exchanges. The CGLN amplitudes for each reaction are evaluated, which include both proton and neutron targets. The important role by the S-wave resonances in the second resonance region is discussed, it is particularly important for the $K, \eta$ and $\eta^\prime$ photoproductions.

Pseudoscalar meson photoproduction at high energies: from the Regge regime to the hard scattering regime

A model for high energy pion photoproduction is presented which interpolates between the low and high momentum transfer regions. At low momentum transfer (which is described in terms of Regge trajectory exchanges), the model is able to give an economical description of the numerous high energy unpolarized and polarized γN → πN data. At higher momentum transfer, a description based on saturating Regge trajectories and which satisfies quark counting rules at asymptotically large transfers, is found able to describe the few existing γp → nπ + data. The model can be used as a guidance to interpret forthcoming data at large momentum transfers.

Completeness rules for spin observables in pseudoscalar meson photoproduction

The number and type of measurements needed to ascertain the amplitudes for pseudoscalar meson photoproduction are analyzed in this paper. It is found that eight carefully selected measurements can determine the four transversity amplitudes without discrete ambiguities. That number of measurements is one less than previously believed. We approach this problem in two distinct ways: (1) solving for the amplitude magnitudes and phases directly, and (2) using a bilinear helicity product formulation to map an algebra of measurements over to the well-known algebra of the 4{times}4 gamma matrices. It is shown that the latter method leads to an alternate proof that eight carefully chosen experiments suffice for determining the transversity amplitudes completely. In addition, Fierz transformations of the gamma matrices are used to develop useful linear and nonlinear relationships between the spin observables. These relationships not only help in finding complete sets of experiments, but also yield important constraints between the 16 observables for this reaction. {copyright} {ital 1997} {ital The American Physical Society}

Pseudoscalar meson photoproduction: From known to undiscovered resonances

The role of dynamics in spin observables for pseudoscalar meson photoproduction is investigated using a density matrix approach in a multipole truncated framework. Extraction of novel rules for $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}n, {K}^{+}\ensuremath{\Lambda}$, and $\ensuremath{\eta}p$ reactions based on resonance dominance, and on other broad and reasonable dynamical assumptions, are discussed. Observables that are particularly sensitive to missing nucleonic resonances predicted by quark-based approaches, are singled out.