Polarized Photon Beam Research Articles

Measurement of spin-density matrix elements forϕ-meson photoproduction from protons and deuterons near threshold

The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for $\gamma p \to \phi p$, $\gamma d \to \phi p n$ and $\gamma d \to \phi d$ at forward production angles. A linearly polarized photon beam at $E_{\gamma}$=1.6-2.4 GeV was used for the production of $\phi$ mesons. The natural-parity Pomeron exchange processes remains dominant nearthreshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from nucleons but is greatly reduced in the coherent production from deuterons. There is no strong $E_{\gamma}$-dependence, but some dependence on momentum-transfer. A small but finite value of the spin-density matrix elements reflecting helicity-nonconserving amplitudes in the $t$-channel is observed.

POLARIZED HD TARGET FOR FUTURE LEPS EXPERIMENTS AT SPRING-8 IN JAPAN

We are carrying out hadron photoproduction experiments by using polarized photon beams with energies of 1.5-2.9 GeV at SPring-8 in Japan. In 2005, we started developing a polarized HD target for future LEPS experiments using both the polarized photon beams and the polarized target. To introduce a polarized target plays an important role in upgrading the LEPS experiments to the next higher step. The measurement of double polarization asymmetries is expected to give important informations to investigate the nucleon hidden structure, hadron photoproduction dynamics, and exotic hadron property. We report on the present status of the development of the polarized HD target at RCNP. We will carry out a test experiment to confirm that the polarized HD target can be transported from Osaka university to SPring-8 without any problems in 2010. If we succeed in the test experiment, we will carry out a long experiment to take physics data in 2011-2012.

POLARISED PHOTON BEAMS AT ELSA AND RECENT RESULTS OF DOUBLE POLARISATION MEASURMENTS

The puzzle of the excitation spectrum of the nucleon, composed of broad and overlapping resonances, is still unresolved. For a complete experiment at least 8 well chosen single and double polarisation observables are needed. At the electron accelerator ELSA of the University of Bonn single- and double polarisation experiments are currently performed with the Crystal Barrel/TAPS experiment. The experimental setup includes a ɸ-symmetric calorimeter system covering almost the complete solid angle and it is ideally suited to detect single and multiple neutral meson final states. Circularly or linearly polarised photon beams impinge on a longitudinal polarised butanol target in the center of the setup. The linearly polarised photon beams provide the basis for the measurement of azimuthal beam asymmetries, such as Σ and, in combination with the polarised target, G . Circularly polarised photon beams allow the extraction of the beam-target asymmetry E . Preliminary results for the polarisation observables Σ, G and E will be presented.

HYPERON PRODUCTION AND POLARIZATION WITH CLAS AND CLAS12 AT JLAB

Many baryon resonances are predicted studying the channels with π, but very few established. This is the so-called "missing resonance" problem. It is important to provide data to investigate the spectrum of baryon ( N * and Δ) resonances, with the decay in KY. A comprehensive study of the electromagnetic strangeness production has been undertaken at Jefferson Lab using circularly and linearly polarized photon beam and polarized hydrogen and deuterium target. Recent results in strangeness photoproduction on protons, including cross sections and spin observables, are discussed in this paper, as well preliminary results on photoproduction on bound neutron. Also an overview of the future perspectives for the 12 GeV upgrade.

PossibleΣ(12−)under theΣ*(1385)peak inKΣ*photoproduction

The LEPS Collaboration has recently reported a measurement of the $\ensuremath{\gamma}n\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\Sigma}}^{*\ensuremath{-}}(1385)$ reaction using a linearly polarized photon beam around the resonance region. The observed beam asymmetry is sizably negative at ${E}_{\ensuremath{\gamma}}=1.8$--$2.4$ GeV, in contrast to the presented theoretical prediction. In this paper, we calculate this process using the framework of the effective Lagrangian approach. By including a newly proposed $\ensuremath{\Sigma}({J}^{P}={\frac{1}{2}}^{\ensuremath{-}})$ state with a mass around 1380 MeV, the experimental data for both $\ensuremath{\gamma}n$ and $\ensuremath{\gamma}p$ experiments can be well reproduced. It is found that the $\ensuremath{\Sigma}({\frac{1}{2}}^{\ensuremath{-}})$ and/or the contact term may play an important role and may deserve further investigation.

Near-ThresholdΛ(1520)Production by theγ→p→K+Λ(1520)Reaction at ForwardK+Angles

Differential cross sections and photon-beam asymmetries for the gamma(p)-->K{+}Lambda(1520) reaction have been measured with linearly polarized photon beams at energies from the threshold to 2.4 GeV at 0.6<cos(theta){c.m.}{K}<1. A new bump structure was found at W approximately 2.11 GeV in the cross sections. The bump is not well reproduced by theoretical calculations introducing a nucleon resonance with J<or=3/2. This result suggests that the bump might be produced by a nucleon resonance possibly with J>or=5/2 or by a new reaction process, for example, an interference effect with the phi photoproduction having a similar bump structure in the cross sections.

A high-pressure polarized 3He gas target for nuclear-physics experiments using a polarized photon beam

Following the first experiment on three-body photodisintegration of polarized $^3$He utilizing circularly polarized photons from High Intensity Gamma Source (HI$\gamma$S) at Duke Free Electron Laser Laboratory (DFELL), a new high-pressure polarized $^3$He target cell made of pyrex glass coated with a thin layer of sol-gel doped with aluminum nitrate nonahydrate has been built in order to reduce the photon beam induced background. The target is based on the technique of spin-exchange optical pumping of hybrid rubidium and potassium and the highest polarization achieved is $\sim$62% determined from both NMR-AFP and EPR polarimetry. The $X$ parameter is estimated to be $\sim0.06$ and the performance of the target is in good agreement with theoretical predictions. We also present beam test results from this new target cell and the comparison with the GE180 $^3$He target cell used previously at HI$\gamma$S. This is the first time that sol-gel coating technique has been used in a polarized $^3$He target for nuclear physics experiments.

Spectral Structure of the Pygmy Dipole Resonance

High-sensitivity studies of E1 and M1 transitions observed in the reaction 138Ba(gamma,gamma{'}) at energies below the one-neutron separation energy have been performed using the nearly monoenergetic and 100% linearly polarized photon beams of the HIgammaS facility. The electric dipole character of the so-called "pygmy" dipole resonance was experimentally verified for excitations from 4.0 to 8.6 MeV. The fine structure of the M1 "spin-flip" mode was observed for the first time in N=82 nuclei.

Spin and parity assignments to dipole excitations of the odd-mass nucleus 207Pb from nuclear resonance fluorescence experiments with linearly-polarized γ-ray beams

Pb(γ⃗,γ′) photon scattering reactions were studied [1] with the nearly monochromatic, linearly polarized photon beams at the High Intensity γ-ray Source (HIγS) at the DFELL. Azimuthal scattering intensity asymmetries measured with respect to the polarization plane of the beam have been used for the first time to assign both the spin and parity quantum numbers of dipole excited states of 206,207,208Pb at excitation energies in the vicinity of 5.5 MeV. Evidence for dominant particle-core coupling is deduced from these results along with information on excitation energies and electromagnetic transition matrix elements.

Photoproduction of multi-meson final states — recent results from the Crystal-Barrel/TAPS experiment at ELSA-

Multi-meson final states such as γp → pπ0π0 and γp → p π0η have been investigated with the Crystal-Barrel/TAPS experiment at ELSA. Baryon cascades via Δπ and Δη and also via higher mass baryon resonances are clearly observed. Based on this data and on data from other reactions a partial wave analysis has been performed from which partial decay widths of various N* and Δ* states into the different pπ0π0 and pπ0η decay channels have been determined, leading partly to unexpected results. In addition, polarisation observables have been investigated. The beam asymmetry ∑ has been determined for both reactions and double polarisation data using a longitudinally polarised target and a circularly or linearly polarised photon beam has been taken. Given the angular coverage of the Crystal-Barrel/TAPS experiment this data covers almost the full angular range and phase space. This data will provide new and important information for the partial wave analyses and bring us one step closer towards a complete experiment.

A new beam asymmetry measurement from pion photoproduction on the neutron using CLAS

We present a preliminary analysis of the photon beam asymmetry observable (Σ) from the photoproduction reaction channel γn → pπ− in the invariant mass range 1.6–2.3 GeV. The measurement was obtained using the near-4π CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory, USA, employing a linearly polarised photon beam with an energy range 1.1–2.3 GeV, incident on a liquid deuterium target. The measurement will provide new data to address the poorly established neutron excitation spectrum and will greatly expand the sparse world data-set both in energy and angle.

Photoproduction of ω-Mesons at ELSA and the role of azimuthal asymmetries

Measurements of ω photoproduction of the Crystal-Barrel/TAPS experiment at the ELSA accelerator of Bonn University are presented which used linearly polarized tagged photon beam from threshold to Eγ = 1700 MeV. The azimuthal asymmetries ∑ and ∑π indicate s-channel resonance contributions on top of the established t-channel exchange processes. These findings are further enhanced by a very first measurement of the G-asymmetry which, in addition to the polarized photon beam, also requires a longitudinally polarized proton target. An intuitive interpretation of the specific sensitivity of the azimuthal asymmetries to the reaction mechanisms involved is given.

Experimental confirmation of left-right asymmetry in photoionization

Single-photon single ionization of noble gas atoms by linearly polarized synchrotron radiation has been studied by employing angle- and energy-resolved photoelectron spectroscopy. Left-right asymmetry parameters were measured for photoelectrons ejected from the outer s-shells of noble gas atoms He, Ne, Ar and Xe using a linearly polarized photon beam. A non-zero left-right asymmetry was observed relative to the photon propagation direction.

A method to polarise antiprotons in storage rings and create polarised antineutrons

An intense circularely polarised photon beam interacts with a cooled antiproton beam in a storage ring. Due to spin dependent absorption cross sections for the reaction gamma+antiproton > pi- + antineutron a built-up of polarisation of the stored antiprotons takes place. Figures-of-merit around 0.1 can be reached in principle over a wide range of antiproton energies. In this process antineutrons with Polarisation > 70% emerge. The method is presented for the case of 300 MeV/c cooled antiproton beam.

Competition between excited core states and [formula omitted] single-particle excitations at comparable energies in 207Pb from photon scattering

The Pb(γ→,γ′) photon scattering reaction has been studied with the nearly monochromatic, linearly polarized photon beams at the High Intensity γ-ray Source (HIγS) at the DFELL. Azimuthal scattering intensity asymmetries measured with respect to the polarization plane of the beam have been used for the first time to assign both the spin and parity quantum numbers of dipole excited states of 206,207,208Pb at excitation energies in the vicinity of 5.5 MeV. Evidence for dominant particle–core coupling is deduced from these results along with information on excitation energies and electromagnetic transition matrix elements. Implications of the existence of weakly coupled states built on highly excited core states in competition with 1ℏω single particle (hole) excitations at comparable energies are discussed.

Charged particle detection at GRAAL

Experimental results on proton and charged pion detection obtained from a study of the γ+n→p+π− reaction are reported in detail. Data have been collected using the tagged and linearly polarized photon beam, impinging on a deuterium target, and the large solid angle apparatus of the GRAAL facility in Grenoble (France). The energy of the charged particles was measured using a BGO calorimeter. A comparison of the experimental data with a GEANT3-based simulation is also presented.

Measurements ofH→D→(γ→,π)and Implications for the Convergence of the Gerasimov-Drell-Hern Integral

We report new measurements of inclusive pi production from frozen-spin HD for polarized photon beams covering the Delta(1232) resonance. These provide data simultaneously on both H and D with nearly complete angular distributions of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH) sum rule. Recent results from Mainz and Bonn exceed the GDH prediction for the proton by 22 microb, suggesting as yet unmeasured high-energy components. Our pi0 data reveal a different angular dependence than assumed in Mainz analyses and integrate to a value that is 18 microb lower, suggesting a more rapid convergence. Our results for deuterium are somewhat lower than published data, considerably more precise, and generally lower than available calculations.

Multipole mixing ratios of transitions inB11

The mixing ratios of $M1$ and $E2$ radiation for transitions in $^{11}\mathrm{B}$ have been determined by measuring the azimuthal asymmetry of the radiation emitted from levels populated by resonant absorption of polarized photons. The photon-scattering experiments were carried out at the Free-Electron Laser Laboratory at Duke University using nearly monoenergetic and linearly polarized photon beams. The mixing ratios were deduced from a comparison of the measured azimuthal asymmetries with calculations for the angular distributions of mixed transitions.

The Crystal-Barrel/TAPS experiment at ELSA Current status of the CsI(Tl) calorimeters

The Crystal Barrel experiment at ELSA, as a medium-energy hadron physics experiment, is focused on baryon-spectroscopy and meson-photoproduction. The experimental setup includes three different, ψ-symmetric calorimeters covering almost the complete solid angle. The detector system, together with a polarised target and a linearly or circularly polarised photon beam, allows in addition to the measurement of cross sections also the measurement of single- and double-polarisation observables. Baryon resonances up to masses of 2.5 GeV can be investigated.In the course of a major upgrade of the CsI(Tl) calorimeters, two different techniques to detect the light generated in the crystals are compared with the known performance of the existing photodiode readout. The aim of the new techniques is to enable the calorimeter to be a first level trigger source while keeping a very good energy resolution comparable to the present pho-todiode readout.The first solution is a complete redesign of the crystal readout, featuring two 10mm x 10mm APDs mounted directly on the crystals. With an integrated, fast preamplifier, this readout will allow both an energy measurement and the generation of a fast timing-/trigger-signal. The second solution is a modification of the existing setup. The photodiode will continue to provide the energy information while two additional SiPMs will create the timing-/trigger-signal.

On von Neumann and Bell Theorems Applied to Quantumness Tests

The issues, raised in arXiv:0809.011, concerning the relevance of the von Neumann theorem for the single-system's quantumness test proposed in arXiv:0704.1962 and performed for the case of single photon polarization in arXiv:0804.1646, and the usefulness of Bell's inequality for testing the idea of macroscopic quantum systems are discussed in some details. Finally, the proper quantum mechanical description of the experiment with polarized photon beams is presented.