The photoproduction of neutral pions from protons between 0.7 GeV and 1.7 GeV

Measurements of differential cross sections for pi-zero photoproduction from protons have been made at angles between 60° and 140° c.m. in the photon energy range 0.7 GeV to 1.7 GeV. The data are compared with the rits provided by three recent partial-wave analyses of pion photoproduction and some significant discrepancies observed.     

Measurement of the target asymmetry for the reaction γp↑ → π+n at photon energies between 0.7 and 2.2 GeV and a pion CM-angle of 65°

At the Bonn 2.5.GeV electron synchrotron the target asymmetry for the photoproduction of positive pions has been measured. Data were taken at photon energies between 0.7 and 2.2 GeV and a pion CM-angle of 65°.     

πo and π+ production with polarized photons in the energy range 1–2 GeV

The asymmetry of π^o and π⁺ photoproduction from hydrogen has been measured. The π^o-mesons were detected at 130° cms with E_γ ranged from 0.9 to 1.65 GeV, and the π⁺-mesons at 40° cms with E_γ ranged from 0.9 to 1.2 GeV. The results agree with model predictions of single pion photoproduction in the resonance region using fixed-t dispersion relations.     

关于利用库仑光生测量新粒子X(2800)衰变到2γ的宽度ΓX→2γ的建议

对于从J(3095)辐射衰变得出的新粒子X(2800),可以从高能γ光子在核子(原子核)上光生微分截面中紧靠0°的实验数据准确定出X→2γ的宽度。讨论了此过程有关的费曼图,分析和估计了其中的耦合常数,计算了能量为50GeV和100GeV的γ光子在质子上(和100GeV的γ光子在铅核上)从0°—3°的微分截面。结果表明,只要Γ_X→2γ不是远小于1keV,就可以利用这种库仑光生准确定出它的衰变宽度。     

Photoproduction of neutral pions at backward angles in the resonance region and the η-cusp

At the Bonn 2.5 GeV electron synchrotron we have measured the differential cross section of the reaction γp→π⁰ p at a pion CM angle of 170° and at photon energiesK _γ between 0.6 and 1.8 GeV. In comparison to previous measurements the accuracy of the data was improved substantially. For the first time in neutral pion photoproduction a cusp structure at the η-threshold has been confirmed [1].     

Inelastic electron scattering from protons and deuterons at very low Q2

The total cross-section for production of hadrons by inelastic electron scattering at 3.2° from hydrogen and deuterium has been measured in the Q² range 0.015 GeV² to 0.1 GeV², at virtual photon energies ranging from 2 GeV to 8.5 GeV. The transition to photoproduction is observed to be smooth, the ratio σ_D/σ_H being about 1.85 in this range. No evidence is seen for a conjectured rapid Q²-dependence of this ratio at low Q².     

High energy pion photoproduction amplitudes from fixed-t dispersion relations and duality

Contributions to the high energy imaginary parts of the charged and neutral pion photoproduction amplitudes from degenerate ? and A₂ exchanges and ω and B exchanges are parametrized similarly to the dual absorptive model. These contributions together with the imaginary parts of the amplitudes found at low energies from partial-wave analyses are then used to evaluate the high energy real parts of the photoproduction amplitudes from fixed-t dispersion relations. A fit is made to data on pion photoproduction at 3.4 and 16 GeV incident photon energies, for momentum transfers up to ?1(GeV/c)². It is shown that much of the data is reproduced both qualitatively and quantitatively by our model. The high energy imaginary parts of the photoproduction amplitudes given by the fit are shown to be in accord with their average low energy behaviour.     

Electroproduction of neutral pions at energies above the resonance region

The reaction e p→e'pπ⁰ has been measured at W=2.55 GeV a fixed electron scattering angle of 10.3°. Two magnetic spectrometers and a lead glass hodoscope were used to detect all four final state particles. Electroproduction cross sections in the t range ?0.15 to ?1.4 (GeV/c)² at q² = ?0.22, ?0.55 and ?0.85 (GeV/c)² are presented. Above |t|=0.6 (GeV/c)² the cross sections are considerably smaller than those for photoproduction.     

Cross-section measurement of charged-pion photoproduction from hydrogen and deuterium

We have measured the differential cross section for the gamman-->pi(-)p and gammap-->pi(+)n reactions at theta(c.m.)=90 degrees in the photon energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at E(gamma) greater, similar 3.3 GeV exhibit a global scaling behavior for both pi(-) and pi(+) photoproduction, consistent with the constituent counting rule and the existing pi(+) photoproduction data. Possible oscillations around the scaling value are suggested by these new data. The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio of exclusive pi(-) to pi(+) photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.     

Photoproduction of the 1.6 GeV ρ0ρ0 state

We calculate the photoproduction cross section for the 1.6 GeV state recently observed in γγ collisions. The integrated cross section forx _F >0 could be 500nb at 50 GeV, and therefore the state should be detectable in existing photoproduction experiments.     

Study for the ω–N Interaction Through ω Photoproduction Near the Threshold

The total cross section has been measured for ω meson photoproduction on the proton for the center-of-mass energy ${\sqrt{s}}$ ranging from 1.69 to 1.75 GeV. Including the subthreshold region of ω photoproduction, a set of about 40 data points has been obtained with a fine energy step of 1–2 MeV. The data set enables us to deduce the ω-N scattering length.     

Positive pion photoproduction from hydrogen at photon energies between 500 MeV and 1,400 MeV in forward direction

The differential cross section of the reactionγ+p→π ⁺ was measured at pion CM-angles of 20° and 30° for photon energies between 500 MeV and 1,400 MeV. The pions were detected in a magnetic spectrometer. By measuring each pion trajectory and by offline calculation of the initial pion parameters an energy resolution of about 2.5% FWHM was achieved. The results complete a set of data which were measured in recent years at the Bonn 2.5 GeV synchrotron. In comparison to photoproduction analyses two effects were revealed: The η cusp appears in the energy dependence of the cross section as a sharp drop atK _γ=710 MeV. In the region of the third resonance the data show a greater enhancement than predicted by most of the analyses.     

Regge approach to charged pion photoproduction at invariant energies above 2 GeV

A Regge model with absorptive corrections is employed in a global analysis of the world data on positive and negative pion photoproduction for photon energies from 3 to 8GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross-sections and single polarization observables at - t ⩽ 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3GeV. Differences between our predictions and data in this energy region are systematically examined as possible signals for the presence of excited baryons. We find that the data available for the polarized photon asymmetry show promising resonance signatures at invariant energies around 2GeV. With regard to differential cross-sections the analysis of negative pion photoproduction data, obtained recently at JLab, indicates likewise the presence of resonance structures around 2GeV.     

重离子碰撞中的轻矢量介子光生过程(英文)

计算了相对论重离子碰撞中轻矢量介子(ρ,ω和φ)的光生过程。基于窄-宽近似,严格推导了相对论重离子碰撞中ρ,ω和φ半弹性和非弹性光生过程的遍举和单举横截面。从数值结果可以看出对于p-p碰撞,光生过程的贡献是不重要的,但是对于pT2.5 GeV Au-Au碰撞和pT3 GeV Pb-Pb碰撞,轻矢量介子光生过程的贡献是明显的。     

Elastic and proton-dissociative \rho^0 photoproduction at HERA

Elastic and proton-dissociative photoproduction (, , respectively, with ) has been studied in interactions at HERA for photon-proton centre-of-mass energies in the range GeV and for GeV, where is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range . For the elastic process, the invariant mass spectrum has been investigated as a function of . As in fixed target experiments, the resonance shape is asymmetric; this asymmetry decreases with increasing , as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant production. The cross section has been studied as a function of ; a fit to the resonant part with the form gives . The resonant part of the cross section is b at GeV. The dependence of the cross section can be described by a function of the type with (stat.) (syst.) GeV and (stat.) (syst.) GeV. The dependence has also been studied as a function of and a value of the slope of the pomeron trajectory GeV has been deduced. The spin density matrix elements , and have been measured and found to be consistent with expectations based on -channel helicity conservation. For proton-dissociative photoproduction in the mass range, the distributions of the two-pion invariant mass, and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The distribution has been fitted to an exponential function with a slope parameter GeV. The ratio of the elastic to proton-dissociative photoproduction cross section is . Received: 3 December 1997 / Online publication: February 26, 1998     

Resolved Photoproduction of J/ψ at HERA

The J/ψ photoproduction via resolved photon process at HERA is studied. It turns out that the J/ψ production is dominated by the leading order production process in low pt range (p_t < 4 GeV), while in the high pt range (p_t > 4 GeV), it is dominated by the fragmentation processes. The difference of J/ψ cross section caused by using GRV and DO photon structure functions is large, especially in the large negative rapidity range. Hence it is possible to use the J/ψ photoproduction process to probe the parton distribution of photon and proton, as well as to test the gluon and heavy quark fragmentation of J/ψ from the perturbative QCD.     

Polarized target asymmetry for pi-zero photoproduction at 4 GeV

The polarized target asymmetry parameter has been measured for single π^o photoproduction from hydrogen at 4 GeV and values of four-momentum transfer squared between −0.15 (GeV/c)² and −1.8 (GeV/c)².     

Recent Results from LEPS/SPring-8 Experiment

The LEPS experiment generates real photon beam up to 2.4 GeV with high degree of linear polarization by backward-Compton scattering at SPring-8, Japan. The experiment is aimed to understand photoproduction of strangeness in the non-perturbative regime of strong interaction. We measure the photoproduction of ${\phi(1020)}$ , Λ(1520) and K ^*0(892) from protons and deuterons near threshold. Other than the differential cross sections, the photon beam asymmetry is shown as a unique and valuable information for the theoretical modeling of production mechanisms.     

Differential cross sections of proton compton scattering at photon laboratory energies between 700 and 1000 MeV

Differential cross sections of proton Compton scattering have been measured at the Bonn 2.5 GeV synchrotron. 78 data points are presented as angular distributions at photon lab energies of 700, 750, 800, 850, 900, and 950MeV. The c.m. scattering angle ranges from 40°–130°, corresponding to a variation of the four momentum transfer squared betweent=?0.10 tot=?0.96 GeV² at 700 and 950 MeV, respectively. Two additional differential cross sections have been measured at 1000MeV, 35.6° and 47.4°. The angular distributions show forward peaks whose extrapolations to 0° are consistent with calculated forward cross sections taken from literature. The small angle data (|t| ?0.2 GeV²) together with the calculated cross sections at 0° are also consistent with the assumption of a slope parameterB of 5 GeV^?2. For the first time a rerise of the angular distributions towards backward angles has been observed. It becomes less steep with increasing energy. The most interesting feature of the angular distributions is a sharp structure which appears betweent=?0.55 GeV² at 700MeV andt=?0.72 GeV² at 950 MeV. Such a rapid varation of the differential cross section witht has never been ovserved in elastic hadron-hadron scattering or photoproduction processes. It indicates the existence of a dynamical mechanism which could be a peculiarity of Compton scattering.     

Proton-antiproton states in high energy photoproduction

Exclusive and inclusive $\text{p}\text{p}$ photoproduction (with ? 2 accompanying charged particles) have been measured in the photon energy range 44–70 GeV. The exclusive process shows no prominent features; the inclusive process shows evidence for S(1936) production as a resonant-like structure with mass 1930 ± 2 MeV and width 12 ± 7 MeV.