Search

New high-precision measurements of p( γ , π) and p( γ , γ) cross sections and beam asymmetries have been combined with other polarization ratios in a simultaneous analysis of both reactions. Compton scattering has provided two important new constraints on the photo-pion amplitude. The E2/M1 mixing ratio for the N → Δ transition extracted from this analysis is EMR = -3.0% ± 0.3 (stat + sys) ± 0.2 (model).

The first measurement of the reaction [sup 3]He([rvec [gamma]],[ital p])[ital X] using linearly polarized photons is reported. Cross sections and beam-polarization asymmetries for [theta][sub [ital p]][sup lab]=60[degree]--100[degree] and [ital E][sub [gamma]]=195--304 MeV are compared with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon absorption alone fails to describe the data at low proton momenta. The inclusion of three-nucleon absorption significantly improves the comparison with the measured cross sections. However, some features of the asymmetry distributions are not explained.

Results from three independent measurements of the {ital p}({rvec {gamma}},{pi}{sup 0}) reaction are presented for incident photon energies between 243 and 314 MeV, and for c.m. angles of 105{degree}, 122{degree}, and 150{degree}. The ratio of cross sections measured with orthogonal states of linear polarization is sensitive to the {ital E}2 excitation of the {Delta} resonace. Comparisons are made to the predictions of various models, all of which fail to reproduce these data.

New precision angular distribution data for the D({gamma},{ital p}){ital n} reaction are presented for incident photon energies of 191 and 222 MeV. The difference of cross sections measured with orthogonal states of linear polarization is sensitive to the short-range behavior of the tensor interaction in models of the nuclear force. Comparisons with the Bonn and Paris potentials illustrate the sensitivity.

Coherent X rays can be produced by Compton scattering of laser light on a relativistic electron beam, whose current is periodically modulated by the same laser radiation in a suitable miniundulator. If electron and laser photon energy are properly matched, the modulated electron beam acts as a moving diffraction grating and the X rays show a high degree of coherence. A practical implementation of this scheme is shown.

The photodisintegration of deuterium with linearly polarized light has been measured in the energy range of 110-315 MeV. The results of five independent measurements using three detector systems, two liquid target geometries, and two photon energy distribution end points are presented. These data are combined into one average data set with a systematic uncertainty of {+-}5%. The statistical precision of this combined data set, coupled with the sensitivity to the strength of the short range tensor interaction observed in the parallel-perpendicular cross section difference, makes this an attractive reaction for investigating aspects of the NN interaction. To do this reliably requires a complete, systematic description of the reaction through the {delta} region. Recent NN-N{delta} coupled-channel calculations which include fully retarded pion potentials are compared with multipole fits to the data. These calculations cannot reproduce the energy and angle dependence of the data. The character of the disagreement suggests that the remaining problems may lie with the treatment of the {delta}. (c) 1999 The American Physical Society.

The properties of the transition from the nucleon to the {Delta}(1232) serve as a benchmark for models of nucleon structure. To first order, N {r_arrow} {Delta} photo-excitation is dominated by a simple M1 quark spin-flip transition. At higher order, small L = 2 components in the N and {Delta} wavefunctions allow this excitation to proceed via an electric quadrupole transition. Since Nucleon models differ greatly on the mechanisms used to generate these L = 2 components,, the ratio of E2/M1 transitions (EMR) provides a sensitive test for structure models. Here, new high-precision measurements of p({rvec {gamma}}, {pi}) and p({rvec {gamma}}, {gamma}) cross sections and beam asymmetries have been combined with other polarization ratios in a simultaneous analysis of both reactions. Compton scattering has provided two important new constraints on the photo-pion amplitude. The E2/M1 mixing ratio for the N {r_arrow} {Delta} transition extracted from this analysis is EMR = {minus}3.0% {+-} 0.3 (stat+sys) {+-} 0.2 (model).

Frozen‐spin HD polarized targets operating between 0.4 and 4K, used with cold‐transfer (4K) techniques, provide great configurational flexibility. Their long depolarization times under target usage conditions assure reasonable match between polarization production and usage times, for weakly ionizing beam fluxes, and the very long relaxation times at fields above 7T (∼1 yr.) provide an economical storage mode and ‘‘off‐the‐shelf’’ availability.

The first measurement of exclusive photodisintegration of 3He with polarized photons is reported. Cross sections and asymmetries for 3He(γ,pn)p and 3He(γ,pp)n at 235≤Eγ≤305 MeV, and fixed proton emission angle θP1=100°, are compared to a theoretical calculation which includes one‐, two‐, and three‐nucleon photon absorption. Two‐nucleon absorption dominates the pn data, but is suppressed in the pp data. Both the pn and pp data require the inclusion of three‐nucleon absorption to describe the cross section and beam asymmetry over all momenta.

In constituent quark models, a tensor interaction, mixing quark spins with their relative motion, is introduced to reproduce the observed baryon spectrum. This has a consequence completely analogous to the nuclear tensor force between the n and p in deuterium. A D state component is mixed into what would otherwise be a purely S-wave object. The D-wave component breaks spherical symmetry, resulting in a non-vanishing matrix element for the nucleon and a static quadrupole moment and deformation for its first excited state, the Δ resonance, at ∽325 MeV. The magnitude and sign of this D-state component are quite sensitive to the internal structure of the proton and have been of great interest in recent years [1].

A magnetic spectrometer is described for tagging a gamma-ray beam produced by Compton backscattering laser light from the electron beam circulating in a storage ring. The spectrometer, as it is now installed, has a design energy resolution of 5.5 MeV, a dispersion of 2.8 MeV/cm, and a dynamic range of 160 MeV. The energy resolution is limited by the energy spread of the stored beam. If dispersion can be introduced into the laser-electron interaction straight section of the storage ring, the resolution can be improved to be as small as 1.5 MeV, depending on the value of this dispersion. Measured properties of the spectrometer are in substantial agreement with the design values.

We have measured at − k 2 = 0.6 (GeV/ c ) 2 and W = 2.3 GeV the inclusive electroproduction cross sections for final-state neutrons and protons from a deuterium target. By comparison with previous data from a proton target, we present cross sections for inclusive nucleon production from neutrons which show great similarity to those from protons. There is evidence that ϱ 0 production is present with essentially the same strength as with a proton target.

The symmetry σ(θ) for the reaction 2H(γ, n) p has been measured at nine angles for Eγ = 19.8 MeV. The source of monochromatic and linearly polarized γ-rays was the Frascati Ladon facility, obtained by Compton scattering of laser light against high-energy electrons. The experimental results are compared with theoretical calculations based on the Reidsoft-core and with the De Tourreil-Sprung (version B) interactions.

Measurements of inelastic electron scattering have been made in the range 2.2 < ν < 3.8 GeV and 0.1 < |Q2| < 0.3 (GeV/c)2, on a selection of nuclei ranging from hydrogen and deuterium to uranium, by measuring the scattered electron only. Detailed calculations have been made of the contribution of radiative tails to the measured yield. The results show a small ‘shadowing’ consistent with other electroproduction experiments, and also with photoproduction experiments in this ν range, but the shadowing decreases rapidly as |Q2| increases.

In the present paper we present the experimental results obtained for the backscattering of laser light against the high-energy electrons stored in Adone with the laser cavity dumping technique. The characteristics of the obtained photon beam (LADON beam) are discussed and compared with the theoretical previsions given by the usual storage ring theory.

We measured the elastic-scattering cross-section of thermalized μp mesoatoms against hydrogen molecules (T=300 °K). The experimental results are in agreement with the predictions obtained by Cohenet al., and by Zel’dovich and Gershtein. We also measured the elastic-scattering cross-section of thermalized μd mesoatoms against deuterium molecules.

A special assembly of high pressure wire proportional counters is described. Their operation defines a volume in a gaseous target, the effective limits of which are set mainly by the electric fields of the counters and not by a continuous material wall. The apparatus, used in a recent experiment — at the CERN 600 MeV synchrocyclotron — while filled with extremely purified hydrogen, is fitted to work with any gas, or mixture of gases, proper to fill gas proportional counters. The device seems suitable to be used in different fields of high-energy physics.

We have observed the rates of the reaction μp+Y→μY+p where negative muons are transferred from a μp mesic atom to a monoatomic element Y mixed to gaseous hydrogen at 26 atmospheres. The elements Y added in known proportions were Xe, Ne, Ar, Kr. The results of our measurements are compared with those obtained by other authors and with theoretical predictions.

Measurement of the asymmetry factor in D(γ, np) is reported and the result is discussed in the framework of Partovi’s theory.

The reaction $^{2}(\ensuremath{\gamma}$,p)n has been studied using a monochromatic and polarized gamma ray beam at energies ${E}_{\ensuremath{\gamma}}$=19.8, 29.0, 38.6, and 60.8 MeV. The beam of an intensity \ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}${10}^{5}$ \ensuremath{\gamma}/sec was obtained by Compton back scattering of mode-locked laser light off electron bunches in the Adone storage ring. Photoneutron yields were measured at nine neutron angles ${\mathrm{theta}}_{\mathrm{n}\mathrm{\ensuremath{\simeq}}}$15, 30, 45, 60, 90, 120, 135, 150, and 165 deg in the center of mass (c.m.) for ${E}_{\ensuremath{\gamma}}$=19.8, 29.0, and 38.6 MeV, and at ${\mathrm{theta}}_{\mathrm{n}\mathrm{\ensuremath{\simeq}}}$30, 60, 90, 120, and 150 deg c.m. for ${E}_{\ensuremath{\gamma}}$=60.8 MeV. The polarization independent component ${I}_{0}$(theta) of the differential cross section and the polarization dependent component ${\mathrm{PI}}_{1}$(theta) were deduced and the angular distribution of the azimuthal asymmetry factor \ensuremath{\Sigma}(theta)=${I}_{1}$(theta)/${I}_{0}$(theta) was obtained. An extensive comparison with theory has been carried out and the inclusion of corrections due to meson exchange currents and to \ensuremath{\Delta}-isobar configurations have been shown to be mandatory at energies ${E}_{\ensuremath{\gamma}}$\ensuremath{\gtrsim}40 MeV. Theoretical and experimental implications of intermediate energy deuteron photo- disintegration studies are discussed in some detail.

The lifetime τ T of the triplet state for (μp) 1S muonic atoms is evaluated as a function of the hydrogen pressure, starting from recent experimental results on the scattering cross section for the process (μp+p→μp+p). It is found that τ T is significantly smaller than expected on the basis of previous calculations.

The asymmetries ..sigma..(theta/sub c.m./ = ..pi../2) for the /sup 2/H(..gamma..,n)p reaction have been measured with use of the monochromatic and linearly polarized ..gamma..-ray beam (LADON), obtained by backward Compton scattering of laser light against high-energy electrons. Contributions from meson exchange currents must be included in the theoretical calculations to reproduce our data.

Elastic electron proton scattering has been used to check the validity of the dipole fit of the proton form factors at momentum transfer between 0.05 and 0.30 (GeV/ c ) 2 . The general behaviour of the cross sections is in agreement with previous measurements and is close to the dipole predictions but there is the suggestion of some small amplitude deviations. It is speculated that these deviations may be related to similar effects in the proton formfactor derived from the ISR pp elastic scattering data via a Chou-Yang model.