Linearly polarised photon beams at ELSA and measurement of the beam asymmetry in \pi^{0}_{} photoproduction off the proton

At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on the basis of the experimental conditions is obtained. Polarisation degrees of % are typically achieved at half of the primary electron energy. The determination of is confirmed by measuring the beam asymmetry, , in photoproduction and by a comparison of the results to independent measurements using laser backscattering.     

A System for Registering Shadow Images of Rapid Processes in the UV Spectral Range under the Conditions of Intense Background Illumination

A system is presented for registering shadow images obtained from dense pulsed plasma with a high background illumination level, wherein an LGI-21 molecular nitrogen laser is used as an illuminator (laser pulse power 12 µW, duration 20 ns), and a modified Canon EOS 1000D camera is used as a registering unit. A micropinch discharge such as a high-current low-inductance vacuum spark (HCLIVS) has been used as the plasma object under investigation.     

Quark-gluonium content of the scalar-isoscalar states f0(980), f0(1300), f0(1500), f0(1750), and f0(1420 ?70+150 ) from hadronic decays

On the basis of the decay couplings f ₀ → ππ, K , ηη, ηη′ found earlier in the study of analytical (IJ ^PC =00⁺⁺) amplitude in the mass range 450–1900 MeV, we analyze the quark-gluonium content of the resonances f ₀(980), f ₀(1300), f ₀(1500), and f ₀(1750) and the broad state f ₀(1420 ₋₇₀ ⁺¹⁵⁰ ). The K-matrix technique used in the analysis makes it possible to evaluate the quark-gluonium content both for the states with switched-off decay channels (bare states, f ₀ ^bare ) and for the real resonances. We observe a significant change in the quark-gluonium composition in the evolution from bare states to real resonances, which is due to the mixing of states in the transitions f ₀(m ₁) → real mesons → f ₀ (m ₂) responsible for the decay processes as well. For f ₀(980), the analysis confirmed the dominance of q component, thus proving the n /s composition found in the study of the radiative decays. For the mesons f ₀(1300), f ₀(1500), and f ₀(1750), the hadronic decays do not allow one to determine uniquely the n , s , and gluonium components, providing relative percentage only. The analysis shows that the broad state f ₀(1420 ₋₇₀ ⁺¹⁵⁰ ) can mix with the flavor singlet q component only, which is consistent with gluonium origin of the broad resonance. From Yadernaya Fizika, Vol. 66, No. 4, 2003, pp. 772–785. Original English Text Copyright ? 2003 by Anisovich, Nikonov, Sarantsev. This article was submitted by the authors in English.     

Photoproduction of πmesons off protons from the Δ(1232) region to Eγ = 3 GeV

Photoproduction of π⁰-mesons was studied with the Crystal-Barrel detector at ELSA for incident energies from 300MeV to 3GeV. Differential cross-sections dσ/dΩ, dσ/dt, and the total cross-section are presented. For E _γ < 3GeV, the angular distributions agree well with the SAID parametrization. At photon energies above 1.5GeV, a strong forward peaking indicates t-channel exchange to be the dominant process. The rapid variations of the cross-section with energy and angle indicate production of resonances. An interpretation of the data within the Bonn-Gatchina partial-wave analysis is briefly discussed.     

Quark-diquark systematics of baryons: Spectral integral equations for systems composed by light quarks

For baryons composed by the light quarks (q = u, d) we write spectral integral equation using the notion of two diquarks: (i) axial-vector state,D ₁¹, with the spin S _D = 1 and isospin I _D = 1 and (ii) scalar one, D ₀⁰, with the spin S _D = 0 and isospin I _D = 0. We present spectral integral equations for the qD ₀⁰ and qD ₁¹ states taking into account quark-diquark confinement interaction.     

Process πp → ππN at high energies and moderate momenta transferred to the nucleon and the determination of parameters of the f0(980) and f0(1300)

We present the results of simultaneous analysis of the S-wave ππ spectra in the reactions π ⁻ p → (ππ)_S n at p _lab=38 GeV/c (GAMS) and π ⁻ p → (ππ)_S n at p _lab=18 GeV/c (E852 Collaboration) at moderate momenta transferred to the nucleon, |t| ≲ 1.5 (GeV/c)². The t distributions are described by the Reggeized π and a ₁ exchanges provided by the leading and daughter trajectories, while the M _ππ spectra are determined by a set of scalar-isoscalar resonances. With M _ππ distributions averaged over different t intervals, we have found several solutions given by different t-channel-exchange mechanisms at |t| ∼ (0.5–1.5) (GeV/c)², with resonance parameters close to each other. We conclude that, despite a poor knowledge of the structure of the t exchange, the characteristics of resonances such as masses and widths can be reliably determined using the processes under discussion. As to pole positions, we have found (1031±10)−i(35±6) MeV for f ₀(980) and (1315±20)−i(150±30) MeV for f ₀(1300). From Yadernaya Fizika, Vol. 66, No. 5, 2003, pp. 960–972. Original English Text Copyright ? 2003 by Anisovich, Sarantsev. This article was submitted by the authors in English.