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Photoproduction of the f 1 ( 1285 ) meson
The f 1 (1285) meson with mass 1281.0 ± 0.8 MeV/c 2 and width 18.4 ± 1.4 MeV (full width at half maximum) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the ηπ + π - , K + K 0 π - , and K - K 0 π + decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the ηπ + π - final-state Dalitz distribution are consistent with the axial-vector J P = 1 + f 1 (1285) identity, rather than the pseudoscalar 0-η (1295). The production mechanism is more consistent with s -channel decay of a high-mass N * state and not with t -channel meson exchange. Decays to η π π go dominantly via the intermediate a$^±_0$ (980) π ∓ states, with the branching ratio Γ [a 0 π (no $\overline{K}K$) ] / Γ [ηππ (all) ] = 0.74 ± 0.09 . The branching ratios Γ ($\overline{K}K$ π )/ Γ (ηππ) = 0.216 ± 0.033 and Γ (γρ 0 ) / Γ (ηππ) = 0.047 ± 0.018 were also obtained. The first is in agreement with previous data for the f 1 (1285), while the latter is lower than the world average.
Coherent photoproduction of {pi}{sup +} from {sup 3}He
We have measured the differential cross section for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction. This reaction was studied using the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.50 to 1.55 GeV were incident on a cryogenic liquid {sup 3}He target. The differential cross sections for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model.
K*{sup +} Photoproduction at CLAS
The study of hyperon resonances has entered a new era of precision with advent of high-statistics photoproduction data from the CLAS detector at Jefferson Lab and elsewhere. These data have multi-particle final states, allowing clean identification of exclusive reactions associated with strange mesons and baryons. In this paper, cross sections for K* photoproduction are presented that will provide a future test of the existence of the purported K{sub 0}(800) meson via comparison with theoretical models.
Upper limits for the photoproduction cross section for the Φ⁻⁻(1860) pentaquark state off the deuteron
We searched for the Φ⁻⁻(1860) pentaquark in the photoproduction process off the deuteron in the Ξ⁻π⁻-decay channel using CLAS. The invariant-mass spectrum of the Ξ⁻π⁻ system does not indicate any statistically significant enhancement near the reported mass M=1.860 GeV. The statistical analysis of the sideband-subtracted mass spectrum yields a 90%-confidence-level upper limit of 0.7 nb for the photoproduction cross section of Φ⁻⁻(1860) with a consecutive decay intoΞ⁻π⁻ in the photon-energy range 4.5GeV γ <5.5GeV.
Photoproduction of K{Sigma}(1385) from the nucleon
The reactions of K{Sigma}(1385) photoproduction, i.e., {gamma}p {yields} K{sup +}{Sigma}{sup 0}(1385) and {gamma} {yields} K{sup +}{Sigma}{sup -}(1385), are investigated in the resonance energy region for studying the role of the nucleon and {Delta} resonances of masses around 2 GeV. The Lagrangians for describing the decays of these resonances into the K{Sigma}(1385) channel are constructed and the decay amplitudes are obtained, which allows us to determine the coupling constants using the predictions of quark models or the data listed by the Particle Data Group. The resulting cross sections are compared to the data from the Thomas Jefferson National Accelerator Facility and the SPring-8, which indicates nontrivial contributions from the two-star-rated resonances in the Particle Data Group as well as from some missing resonances predicted by a quark model.
Determining pseudoscalar meson photoproduction amplitudes from complete experiments
In preparation for a new generation of complete experiments with the goal of performing a high precision extraction of pseudoscalar meson photo-production amplitudes, we present expressions that allow the direct numerical calculation of matrix elements with arbitrary spin projections from Chew-Goldberger-Low-Nambu (CGLN) amplitudes. We use this numerical tool to verify the most general analytic form of the cross section, dependent upon the three polarization vectors of the beam, target and recoil baryon, including all single, double and triple-polarization terms involving 16 spin-dependent observables. Analytic expressions that determine the recoil baryon polarization are presented, together with examples of their potential use with quasi-4? detectors to deduce observables. We assemble the analytic equations relating the 16 experimental observables and the CGLN amplitudes and use our independent method of numerical evaluation to resolve sign differences that exist in the literature.
Neutral Pion Photoproduction on Neutron
The reaction γn → π 0 n is investigated both theoretically and experimentally as an important step toward determining the electromagnetic coupling constants of the N* and Δ* resonances. We analyze the data on the collisions of γ quanta with energies between 200 and 800 MeV with a deuterium target collected by the A2 experiment in Mainz, Germany. Furthermore, these complement the data for neutral-pion photoproduction on protons obtained by the same experiment.
Transverse polarization of Σ + ( 1189 ) in photoproduction on a hydrogen target in CLAS
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Polarization Transfer in Wide-Angle Compton Scattering and Single-Pion Photoproduction from the Proton
Wide-angle exclusive Compton scattering and single-pion photoproduction from the proton have been investigated via measurement of the polarization transfer from a circularly polarized photon beam to the recoil proton. The wide-angle Compton scattering polarization transfer was analyzed at an incident photon energy of 3.7 GeV at a proton scattering angle of theta(p)(cm) cm = 70 degrees. The longitudinal transfer K-LL, measured to be 0.645 +/- 0.059 +/- 0.048, where the first error is statistical and the second is systematic, has the same sign as predicted for the reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton. However, the observed value is similar to 3 times larger than predicted by the generalized-parton-distribution-based calculations, which indicates a significant unknown contribution to the scattering amplitude.
Photoproduction of the f 2 ( 1270 ) Meson Using the CLAS Detector
The quark structure of the f 2 ( 1270 ) meson has, for many years, been assumed to be a pure quark-antiquark (q$\bar{q}$) resonance with quantum numbers J PC = 2 ++ . Recently, it was proposed that the f 2 ( 1270 ) is a molecular state made from the attractive interaction of two ρ mesons. Such a state would be expected to decay strongly to final states with charged pions due to the dominant decay ρ → π + π - , whereas decay to two neutral pions would likely be suppressed. Here, we measure for the first time the reaction γp → π 0 π 0 p , using the CEBAF Large Acceptance Spectrometer detector at Jefferson Lab for incident beam energies between 3.6 and 5.4 GeV. Differential cross sections, dσ/dt , for f 2 ( 1270 ) photoproduction are extracted with good precision due to low backgrounds and are compared to theoretical calculations.
First measurement of the helicity asymmetry E in photoproduction on the proton
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Single-energy amplitudes for pion photoproduction in the first resonance region
We consider multipole amplitudes for low-energy pion photoproduction, constructed with minimal model dependence, at single energies. A previous fit making minimal use of Watson's theorem has been reexamined in light of more recent measurements. Problems associated with the choice of a unique solution are discussed. Comparisons with more recent fits to the full resonance region are made. Explanations are suggested for the discrepancies and more precise measurements of the recoil polarization, P, are motivated.
Extracting the photoproduction cross sections off the neutron, via the {gamma}n{yields}{pi}{sup -}p reaction, from deuteron data with final-state interaction effects
The incoherent pion photoproduction reaction {gamma}d {yields} {pi}{sup -}pp is considered theoretically in a wide energy region E{sub th}{<=}E{sub {gamma}}{<=}2700 MeV. The model applied contains the impulse approximation as well as the NN and {pi}N final-state-interaction (FSI) amplitudes. The aim of the paper is to study a reliable way for getting the information on elementary {gamma}n {yields} {pi}{sup -}p reaction cross sections beyond the impulse approximation for {gamma}d {yields} {pi}{sup -}pp. For the elementary {gamma}N {yields} {pi}N, N N {yields} N N, and {pi}N {yields} {pi}N amplitudes, the results of The George Washington University (GW) Data Analysis Center (DAC) are used. There are no additional theoretical constraints. The calculated cross sections d{sigma} /d{Omega}({gamma}d {yields} {pi}{sup -}pp) are compared with existing data. The procedure used to extract information on the differential cross section d{sigma} /d{Omega}({gamma}n {yields} {pi}{sup -}p) on the neutron from the deuteron data using the FSI correction factor R is discussed. The calculations for R versus {pi}{sup -}p center-of-mass (CM) angle {theta}{sub 1} of the outgoing pion are performed at different photon-beam energies with kinematic cuts for a ''quasifree'' process {gamma}n {yields} {pi}{sup -}p. The results show a sizable FSI effect R{ne}1 from the S-wave part of pp-FSI at small angles close to {theta}{sub 1}{approx}0: this region narrows as the photon energy increases. At larger angles, the effect is small (|R - 1|<<1) and agrees with estimations of FSI in the Glauber approach.
Differential photoproduction cross sections of the Σ 0 ( 1385 ) , Λ ( 1405 ) , and Λ ( 1520 )
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Recent N* results from photoproduction experiments at CLAS
The recent breakthroughs in the technology of polarised targets have enabled a new generation of meson photo-production experiments to be carried out. A measurement of a full set of polarisation observables off both polarised proton and neutron targets and in a large number of meson-production channels has come within sight. Such a measurement would very significantly reduce model-dependence in the analysis of the data and thus has the potential to resolve long-standing issues, such as the 'missing resonance' problem, and shed new light on the nucleon excitation spectrum. This has formed the motivation for the recent N* experimental programme of CLAS.
Coherent photoproduction of {pi}{sup +} from {sup 3}He with CLAS at Jefferson Lab
We have measured the differential cross section for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction. This reaction was studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.5 to 1.55 GeV were incident on a cryogenic liquid {sup 3}He target. The differential cross sections for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model.
The Determination of Pseudoscalar Meson Photoproduction Amplitudes from Complete Experiments
A new generation of complete experiments is currently underway with the goal of performing a high precision extraction of pseudoscalar meson photo-production amplitudes. Here we review the most general analytic form of the cross section, dependent upon the three polarization vectors of the beam, target and recoil baryon, including all single, double and triple-polarization terms involving 16 spin-dependent observables. Analytic expressions that determine the recoil baryon polarization are also presented. Different conventions are in use in the literature and we have used a numerical calculation of cross sections from Chew-Goldberger-Low-Nambu amplitudes with arbitrary spin projections to clarify apparent sign differences. As an illustration of the use of this machinery, we carry out a multipole analysis of the {gamma}p {yields} K{sup +}{Lambda} reaction and examine the impact of recently published polarization measurements. In fitting multipoles, we use a combined Monte Carlo sampling of the amplitude space, with gradient minimization, and find a shallow {chi}{sup 2} valley pitted with a very large number of local minima, despite the inclusion of recent data on 8 different observables. We conclude that, while a mathematical solution to the problem of determining an amplitude free of ambiguities may require 8 observables, as has been pointed out in the literature, experiments with realistically achievable uncertainties will require a significantly larger number.