Polarisation parameters in neutral pion photoproduction

The polarisation parameters Σ, P and T have been measured for the process γp→π⁰p in the photon energy range 1300–2100 MeV and c.m. angles between 30° and 110°, in an experiment with a polarised beam and polarised target. The results are compared with a recent theoretical analysis which fits data from threshold to 16 GeV. The new data are in general agreement with the analysis, but with some significant discrepancies in detail.     

A measurement of the beam asymmetry parameter Σ for neutral pion photoproduction in the energy range 1.2–2.8 GeV

An experiment has been completed at the Daresbury synchrotron to measure the asymmetry in the photoproduction cross section of neutral pions on hydrogen, for photons polarised normal to and in the production plane. The source of polarised photons was coherent bremsstrahlung of electrons traversing the lattice structure of diamond and the polarisation P of the γ beam was calculated from the measured intensity of the coherent spike. The asymmetry parameter Σ, defined as Σ = (σ⊥ ? σ|)/(σ⊥ + σ|) where σ⊥(σ|) are the cross sections for photons polarised perpendicular (parallel) to the production plane, has been measured over a range of photon energies from 1.2 to 2.8 GeV and over a range of ?t (the square of the four-momentum transfer) from 0.13 (GeV/c)² to 1.4 (GeV/c)². A marked energy variation in the value of Σ is found over the energy region 1.6–1.8 GeV.     

Differential cross sections of neutral pion photoproduction from hydrogen in the energy region from 390 MeV to 975 MeV

The differential cross sections of the reaction γp→π⁰p have been measured at photon energies between 390 MeV and 975 MeV with an energy step of 20 or 25 MeV. The angular region covers 11 angles between 15° and 130° in the c.m.s. A combination of a magnetic spectrometer and photon detector was used for the measurement in the angular region from 50° to 130°. At forward angles of 15°, 35° and 50°, π⁰ mesons were detected using a π⁰ detector consisting of a pair of photon detectors. The data obtained with two different detection systems overlap at the angle of 50° and coincide with each other within the experimental error. The results are compared with those of phenomenological partial-wave analyses.     

Messung der zirkularen Polarisation der inneren und äußeren Bremsstrahlung

The polarisation of the internal bremsstrahlung γ's was detected by the Compton-forward-scattering with polarised electrons for three differentβ-decays: 1. for the allowed transition (P³²), 2. for the unique forbidden transition (Sr⁹⁰+Y⁹⁰), and 3. for the forbidden transition with forbidden spectrum (Bi²¹⁰=RaE). The detected polarisation values of P³² and (Sr⁹⁰+Y⁹⁰) agree with the theoretical values for the allowed transitions. The detected polarisation values for Bi²¹⁰ are smaller than the theoretical values of the allowedβ-transition by a factor of 0.62±0.08. The polarisation of the external bremsstrahlung was measured as a function of the atomic number (Z) using thick absorbers. The polarisation of the external bremsstrahlung decreases slowly with the increase of the atomic number. The polarisation of bremsstrahlung, which was extrapolated to an absorber with atomic numberZ=0, is in well agreement with the theoretical values of the infinitively thin absorbers.     

Spin dependent parton densities

We propose a set of polarised-beam polarisedtarget experiments to determine the complete set of spin dependent parton densities. The polarised valence quark densities are determined through semi-inclusive hadroproduction in polarised muon-proton collisions. This, together with present data then fixes the polarised sea quark densities. Polarised gluon densities are measurable through asymmetries in large,p _T direct photon production in polarisedp-p collisions. An independent measurement of polarised sea quark densities comes from direct double photon production in polarisedp-p collisions at rapidities close to zero. We illustrate these results with numerical estimates.     

Conventional and collision photon echo in ytterbium vapor

The polarization properties of the photon echo generated by two linearly polarized pulses of resonant radiation at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition of ¹⁷⁴Yb are investigated. A complicated polarization behavior of the photon echo versus an angle between the polarization vectors of the excitation pulses is revealed in a mixture of ytterbium vapor with inert gas. For the angles ranging from 0° to 75°, a conventional echo with its linear polarization coinciding with the second excitation pulse dominates and the echo amplitude decreases with an increasing angle. For the angles ranging from 75° to 89°, the photon echo is elliptically polarized. Finally, for an angle of 90°, the conventional echo disappears and the collision echo becomes linearly polarized along the first excitation pulse.     

Differential cross sections for photoemission of deuterons from the nitrogen nucleus

Momentum distributions for inclusive photoemission of deuterons from nitrogen have been measured at laboratory angles of 43° and 50°, and for maximum energies of the bremsstrahlung photon beam of 725, 775 and 825 MeV. The double differential cross sections of deuterons emitted from nitrogen are obtained for photon energies of 750 and 800 MeV and for laboratory angles of 43° and 50°. The photon energy is determined by the photon subtraction method. Results are examined by a π⁰ photoproduction process off a quasi-deuteron in nitrogen (γ″d″→π⁰d) and by a two-step process. The two-step process means the following: first, a nucleon is produced by a photo-pion production process from a quasi-free nucleon in the nitrogen nucleus (γ″N″→πN); second, the nucleon produced at the first step picks up another nucleon in the nucleus and then a deuteron is emitted (N+A→d+anything).     

Charged-pion photoproduction measurement on deuterium in the Δ(1236) region using a quasi-monochromatic photon beam

Charged-pion photoproduction spectra on hydrogen and deuterium have been measured using an in-flight positron annihilation photon beam. Pions were detected in magnetic spectrometers. The experiment consisted of π^± angular distributions at 300 MeV, and fixed-angle excitation curves at 20° (π^±) and 55° (π^±) in the photon energy range 300–460 MeV. Experimental data are compared to a calculation by Laget which includes final-state nucleon-nucleon interaction and Pauli correlations. The overall agreement is satisfactory; however, the model slightly overestimates the nucleon-nucleon rescattering contribution.     

A surprising method for polarising antiprotons

We propose a method for polarising antiprotons in a storage ring by means of a polarised positron beam moving parallel to the antiprotons. If the relative velocity is adjusted to v/c ≈ 0.002 the cross-section for spin-flip is as large as about 2 ^. 10¹³ barn as shown by new QED calculations of the triple spin cross-sections. Two possibilities for providing a positron source with sufficient flux density are presented. A polarised positron beam with a polarisation of 0.70 and a flux density of approximately 1.5 ^. 10¹⁰ /(mm² s) appears to be feasible by means of a radioactive ¹¹C dc-source. A more involved proposal is the production of polarised positrons by pair production with circularly polarised photons. It yields a polarisation of 0.76 and requires the injection into a small storage ring. Such polariser sources can be used at low (100MeV) as well as at high (1GeV) energy storage rings providing a time of about one hour for polarisation build-up of about 10¹⁰ antiprotons to a polarisation of about 0.18. A comparison with other proposals show a gain in the figure of merit by a factor of about ten.     

Laser with simultaneous Gaussian and conical refraction outputs

We report a double-sided two-mirror neodymium-doped KGd(WO₄)₂/Nd:KGW conical refraction laser which produces a linearly polarised output from the flat output coupler and a cone-refracted output from the curved output coupler. The linearly polarised output had a polarisation extinction ratio of 20:1 with a Gaussian beam profile and a measured M ²=1. The unpolarised output had the intensity and polarisation distribution corresponding to a cone-refracted Gaussian beam with extinction ratio of 2.7:1.     

Spin-rotation formalism for Meson-Baryon scattering

A spin-rotation formalism is developed for the extraction of physical quantities from the reaction π^- p↑→K°Λ, using the Λ-decay as polarisation analyser. The method described here uses the parameters polarisation,P, and spin-rotation angle, β. Effects due to apparatus acceptance are explicitly included. The method has advantages in minimising the sensitivity to various systematic errors.     

Photon structure functions: Are they measurable?

It is shown that 3 of the 8 independent photon structure functions are simply calculable using leading logarithm techniques in QCD. We then discuss which of these are most readily measurable in the processese ⁺e^?→e⁺e^?+X,γp→μ ⁺ μ ^? X andμp→μμ ⁺ μμ ^? X. Detailed predictions are given for the latter processes including the case where beam and target are longitudinally polarised. Given sufficient beam intensities, the contributions from the anomalous (as opposed to vector meson-like) component of the photon structure function would be easily identifiable above the Bethe-Heitler pair production background. However, currently available beam intensities make such measurements impractical at present. We discuss simple algorithms for estimating the polarised and unpolarised quark and gluon distributions inside the proton and the hadronic (?-like) piece of the photon.     

Forward photoproduction of neutral pions on protons in the third resonance region

Four angular distributions of the differential cross section of the reaction γ + p → π⁰ + p have been measured at the Bonn 2.5 GeV Synchrotron at pion c.m. angles between 3° and 63° in the photon energy range from 0.9 GeV to 1.2 GeV. The π⁰ mesons were detected by their two decay photons. The data are compared with data of other laboratories and predictions of two recent partial-wave analyses.     

Measurement of polarized target asymmetry in γp → π+n below 1.02 GeV

The polarized target asymmetry for the process γp → π⁺n has been measured for incident photon energies below 1.02 GeV over a range of c.m. angles from 40° to 160°. π⁺ mesons from a polarized butanol target were detected by a magnetic spectrometer. The results are compared with predictions given by existing analyses. A tentative interpretation of the data is performed, and a larger contribution of S-wave resonances is suggested. The photocouplings of dominant resonances were hardly changed by the inclusion of new data and they seem to be almost uniquely determined.     

In situ SEOP polarised He neutron spin filter for incident beam polarisation and polarisation analysis on neutron scattering instruments

We discuss the development and characterisation of a new in situ spin exchange optical pumping (SEOP) based ³He neutron spin filter polarisation device. We present results from a recent test of the prototype system developed with the Institut Laue-Langevin. The polariser was installed on the polarised reflectometer CRISP at ISIS in the analyser position. The ³He was pumped continuously in situ on the beamline. The system also integrated a ³He adiabatic fast passage spin flipper that allowed reversal of the ³He and therefore neutron polarisation state, allowing for measurement of all four polarisation cross-sections. The system was run for a number of days reaching a ³He polarisation of 63%.     

Classical theory of the molecule alignment in a laser field

In the classical mechanics framework the solution is given for the problem of the two-atomic molecule alignment in a field of linearly polarised laser pulse. The numerical results are presented for the Cl₂ molecule, together with the analytical results in the limit of a very short and strong laser pulse. The width of the angular distribution for molecular axes in field is demonstrated to depend on the radiation intensity as S ^?1/4, the alignment along the polarisation direction is possible as well as in the transversal direction. The analysis of some experimental data is given.     

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.     

Two-body photodisintegration of 3He and 4He in the Δ(1236) region

Differential cross sections for the reactions ³He(γ, p)d and ⁴He(γ, p)t were measured at proton c.m. angles of 60° and 90° for photon energies ranging from 150–450 MeV with an average resolution of 8 MeV. Bremsstrahlung was used as the photon source; deuterons/tritons were analyzed in a magnetic spectrometer whereas coincident protons were detected in a plastic scintillator telescope. The experimental method includes a calibration by means of ¹H(γ, π⁰)p differential cross section measurement at 90° c.m. in the same photon energy range. The ³He and ⁴He two-body photodisintegration differential cross sections show a monotonically decreasing variation with photon energy. In addition, partial data on the differential cross section of the reaction ⁴He(γ, n)τ at 90° and 120° neutron c.m. angle are given.     

Angular dependence of the polarization of the recoil proton from the reaction γp→π0 p in the region of the second resonance

The polarization of the recoil proton from the reactionγp→π ⁰ p has been measured for photon energies between 600 and 1,200 MeV and pion c.m. angles between 90° and 150°.     

Double internal bremsstrahlung in electron capture

Double internal bremsstrahlung has been observed in the electron capture decay of ³⁷Ar. The energy distribution of the two photons has been investigated, using a dual parameter pulse-height analysis system, at an angle of θ = 90° between two photon propagation vectors. For sum photon energies in the interval of 210–810 keV and at θ = 90° the ratio of the transition probabilities of double to single bremsstrahlung emissions was found to be (4.8±0.4)×10^?5.