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 exclusive photodisintegration of He-3 with polarized photons is reported. Cross sections and asymmetries for He-3(gamma over arrow pointing right, pn)p and He-3(gamma over arrow pointing right, pp)n at 235 less-than-or-equal-to E(gamma) less-than-or-equal-to 305 MeV 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. The pp data require the inclusion of three-nucleon absorption to describe the cross section and beam asymmetry over all momenta.

A feasibility study has been carried out for high-precision measurements of two-photon reactions leading to hadrons at the DAΦNE electron-positron collider. This new facility will operate at a c.m. energy of 1.02 GeV and up to a maximum of 1.5 GeV. It will have a luminosity ofL⋍5·1032 cm−2s−1, 100 times larger than the present available e+e− colliders. DAΦNE offers an excellent opportunity to study γγ reactions at low energy with high statistics, using the KLOE detector equipped with electron tagging facilities as described in this report. Among the various topics that one may address in these studies, we concentrate on the following: 1) Precision measurements of the polarizabilities of charged and neutral pions via the two-photon reactions γγ→ππ. These will allow tests of chiral theories and chiral perturbation techniques, as well as other theoretical approaches. 2) Measurements for the first time of the azimuthal correlations of the γγ→ππ and other two-photon reactions. These provide new tests of the validity of chiral and other theories. 3) Formation of theC=+1 light pseudoscalar mesons, π0, η and η′. This allows the study of the\(q\bar q\) and possibly gluonic inner structure of these mesons. For the π0 its measured radiative width will also improve the currently known lifetime. 4) The possibility to realize a double tagging at DAΦNE will allow reliable measurements of the currently little known total γγ→hadrons cross-section below 1 GeV, to be compared with several models. The physics program described abovecannot be realized without tagging the two-photon reactions to suppress the background from e+e− annihilation channels and other sources.

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.

A procedure is described for determining, for any point within the volume of a scintillating detector, the relative amount of light collected as the result of the deposition of a known amount of energy at that point. The response to muons traversing many well defined paths through the scintillator is combined with Fourier reconstruction techniques of computed tomography to produce three-dimensional images of the nonuniformities in light generation and collection. This information can be combined with a Monte Carlo simulation of energy deposition in electromagnetic showers to accurately predict the resolution for the detection of γ-rays of any energy. This has been applied to two large NaI(Tl) detectors and the calculated resolutions agree with measured spectra over the full range for which data are available, E γ = 6−45 MeV.

The Baldin sum rule for the nucleon has been recalculated at the light of the most recent photoabsorption cross section measurements. The proton value (alpha + beta)_p =13.69 +/- 0.14 is smaller but consistent with the one usually quoted in literature. However, the value for the neutron (alpha + \beta)_n =14.40 +/- 0.66 turns out to be three standard deviations away from the previously calculated one., Comment: 6 pages, LaTex2e, 2 figures, 2 tables. Phys. Rev. C to appear

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.

The Laser Electron Gamma Source (LEGS)[1] has recently begun operations at the National Synchrotron Light Source (NSLS) of Brookhaven National Laboratory. At LEGS, laser light is Compton backscattered from the 2.5 GeV electrons circulating in the X-Ray storage ring of the NSLS. The backscattered photons have gamma ray energies, k < 320 MeV. The corresponding scattered electrons are detected in a tagging spectrometer which is incorporated into the storage ring. This tag provides a determination of the photon energy to ~ 5 MeV [2]. Since the laser light is polarized, and since the spin-flip amplitude in Compton backscattering is very small, the resulting polarization of the gamma-ray beam is very high.

Threshold π+ electroproduction has been measured at momentum transfers ‖k2‖ of 0.45, 0.58 and 0.88 (GeV/c)2, extending the range of earlier measurements. Using PCAC and current algebra-based models we have deduced the axial vector form factor of the nucleon GA(k2), and find that the dipole parametrisation is favoured over the monopole. The value of MA in the dipole parametrisation is, in a weak PCAC model, 0.96 ± 0.03 GeV, in excellent agreement with neutrino measurements. The threshold cross section is also in excellent agreement with the predictions of a fixed-t dispersion relation model.

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.

Worldwide data on deuteron photodisintegration between 10 and 120 MeV have been analyzed and compared with the standard theory including MEC+IC contributions. No real effect can be envisaged for the breakdown of the current theory at intermediate energy.

The electromagnetic properties of charmed hadrons are discussed in the framework ofSU6 ⊗Uc. Magnetic moments and mass differences between variousI-spin multiplets have been calculated both for baryons and mesons. Our results are consistent with those obtained by other authors with different methods. It is shown indeed that electromagnetic mass differences of uncharmed baryons are in very good agreement with the theory. As to the mesons we find satisfactory results for the D+-D0 mass difference, while the calculated D*+-D*0 mass difference is far from the experimental one.

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.

Preliminary results of an experiment to determine the rate of the capture process μ − + p → n + v μ are given.

An experiment is described which will measure the d(..gamma..,p) reaction with polarized photon beams from LEGS ranging in energy from 50 MeV to 530 MeV. The data will yield a new high-resolution high-statistics determination of the total unpolarized cross section, will isolate for the first time the polarization dependent part of the reaction, and will measure the photon-beam asymmetry with much higher resolution and accuracy than available from existing data. These will provide one of the most stringent tests for models of the nuclear force. In addition, high-resolution data will be taken with both linear and circularly-polarized ..gamma.. rays to search for the effects of bibaryons.

The total cross section for deuteron photodisintegration has been measured in the $\ensuremath{\gamma}$-ray energy range between 15 and 75 MeV, by use of the monochromatic LADON photon beam of the Frascati National Laboratories and detection of the proton. The results are in substantial agreement with the standard theory and do not provide evidence for contributions of quark degrees of freedom.

We describe the calibration of a modular neutron counter for neutrons and protons of between 100 and 500 MeV kinetic energy. We present results for the detection efficiency, time and spatial resolution, and multifiring. We emphasise the necessity of calibrating the crosstalk between neutron and photon identification.

In electroproduction on hydrogen at momentum transfers −0.3, −0.6, −0.9 (GeV/ c ) 2 and W = 2.3 GeV we have measured, over a range of backward centre-of-mass angles, the yield of neutrons with momenta between 0.4 and 1.2 GeV/ c , and in addition, protons over a limited range with momenta greater than 0.8 GeV/ c . We compare the neutron yields with those of the protons and also with previous experiments. We find that the yield of neutrons is significantly greater than that of protons.

We have measured pπ0 electroproduction at −k2 = 0.88 (GeV/c)2 near threshold. Comparison with models shows that a ‘weak PCAC’ model, a fixed-t dispersion relation model and a pseudo-vector-Born approximation all adequately describe the small threshold cross section. Away from threshold the cross section dependence suggests that the pseudo-vector-Born approximation is not adequate.

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.

When completed, the Laser Electron Gamma Source (LEGS) is expected to provide intense beams of monochromatic and polarized (circular or linear) gamma rays with energies up to 500 MeV. The gamma-ray beams will be produced by Compton backscattering uv laser light from the electrons circulating in a storage ring. Progress with installation of the facility is described, particularly the Ar-ion laser and tagging spectrometer. Tests of the tagging spectrometer coponents is reported, and a second laser is described for higher energy operation. Estimates are given of expected beam parameters. Experimental equipment for the planned research projects to be carried out at the LEGS facility is discussed. (LEW)

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.

New preliminary results on the deuteron photodisintegration at LEGS are presented. The experimental activity at LADON is rapidly reviewed and some indication is given for the possible activity at GRAAL.