The use of polarization modulation and amplitude-sensitive optical heterodyne interferometry for linear birefringence parameters measurement

A novel common-path polarization modulation and amplitude-sensitive optical heterodyne polarimeter is setup in order to characterize a phase retardation plate (PRP) in real time. The phase retardation ΔΦ and fast-axis angle β of the linear birefringence parameters (LBP) of a PRP are measured simultaneously. Meanwhile, the dynamic ranges of 0° < ΔΦ < 180° and 0° < β < 180° are demonstrated experimentally. In order to measure LBP in real time, a polarization modulation is introduced by continuously rotating the tested PRP such that ΔΦ and β are able to be obtained in terms of the ratio of the amplitudes of S polarization and the ratio of P polarization of the heterodyne signals, respectively. Consequently, this novel method, which combines optical heterodyne interferometry with a polarization modulation technique, not only improves the detection sensitivity, but also provides a real time capability to measure LBP. In addition, the error in the LBP measurement is derived and analyzed.     

Rigorous pion-pion scattering lengths from threshold πN → ππN data

A new evaluation of the universal ππ scattering length relation is used to extract the ππ s-wave scattering lengths from threshold pion production data. Previous work has shown that the chiral perturbation series relating threshold pion production to ππ scattering lengths appears to converge well only for the isospin-2 case, giving a₂ = −0.031 ± 0.007m_π⁻¹. A model-independent and data-insensitive universal curve then implies a₀ = 0.235 ± 0.03m_π⁻¹ for the isospin-0 scattering length.     

The role of meson retardation in the NN interaction above pion threshold

A model is developed for the hadronic interaction in the two-nucleon system above pion threshold which is based on meson, nucleon and Δ degrees of freedom and which includes full meson retardation in the exchange operators. For technical reasons, the model allows maximal one meson to be present explicitly. Thus the Hilbert space contains besides NN and NΔ also configurations consisting of two nucleons and one meson. For this reason, only two- and three-body unitarity is obeyed, and the model is suited for reactions in the two-nucleon sector, where only one pion is produced or absorbed. Starting from a realistic pure nucleonic retarded potential, which had to be renormalized because of the additional π and Δ degrees of freedom, a reasonable fit to experimental NN-scattering data could be achieved.     

A phase-shift analysis of elastic pion-deuteron scattering

Recent data on differential and total cross sections in the region 82 < T_π (MeV) < 292 from SIN and CERN and an isolated polarization measurement formed the material for a πd phase-shift analysis. For virtually all cases χ² < 1 could be reached, but extracted phase parameters are not unique. We emphasize the strong constraints polarization data would exert on presently ambiguous phase parameters. We present our results in Argand plots and briefly discuss their behaviour.     

Note on finite temperature sum rules for vector and axial-vector spectral functions

An updated analysis of vector and axial-vector spectral functions is presented. The resonant contributions to the spectral integrals are shown to be expressible as multiples of 4π²f_π², encoding the scale of spontaneous chiral symmetry breaking in QCD. Up to order T² this behavior carries over to the case of finite temperature.     

Measurement of the reaction γp → K0Σat photon energies up to 2.6 GeV⋆

The reaction p K⁰⁺ was measured in the photon energy range from threshold up to 2.6 GeV with the SAPHIR detector at the electron stretcher facility, ELSA, in Bonn. Results are presented on the reaction cross-section and the polarization of the ⁺ as a function of the kaon production angle in the centre-of-mass system, cos^c.m._K, and the photon energy. The cross-section is lower and varies less with photon energy and kaon production angle than that of p K⁺⁰. The ⁺ is polarized predominantly at cos^c.m._K 0. The data presented here are more precise than previous ones obtained with SAPHIR and extend the photon energy range to higher values. They are compared to isobar model calculations.     

Simultaneous measurements of the p+d→(A=3)+π reactions

A stack of annular detectors made of high-purity germanium was used to measure simultaneously pd → ³H π⁺ and pd → ³He π⁰ differential cross sections at beam momenta of 750 MeV/c, 800 MeV/c, and 850 MeV/c over a large angular range. The extracted total cross sections for the pd → ³He π⁰ reactions bridge a gap between near threshold data and those in the resonance region. The ratio of the cross sections for the two reaction channels taken at the same η=p^cm_π/m_π yields 2.11±0.08 indicating that a deviation from isospin symmetry is very small.     

Exploring the reaction mechanism of eta prime photoproduction

We examine theoretical implications of studying the differential cross-section and some key polarization observables in the η′ photoproduction reaction off protons, from its threshold to about 2 GeV of cm energy, the region of the so-called “missing baryon resonances”. We compare implications of different models all of which generate the observed peak in the total η′ photproduction cross section, but have very different differential cross section and polarization profields. Their measurements are thus crucial in distinguishing these reaction mechanisms, of interest to understanding QCD in its nonperturbative domain.     

Pion–nucleon amplitude near threshold: the sigma-term and scattering lengths beyond few loops

The pion–nucleon amplitude is considered in the vicinity of the elastic scattering threshold within a relativistic dynamical model dressing the πNN and πNΔ vertices self-consistently with an infinite number of meson loops. The dressing is formulated as solution of a system of coupled integral equations incorporating unitarity, crossing symmetry and analyticity constraints. The calculated scattering lengths and the sigma-term agree with recent data analyses. The dressing is important in this model both below and at threshold. The contribution of the Δ resonance is discussed, including effects of the consistent dressing of the πNΔ vertex. A comparison with the approaches of chiral perturbation theory and the Bethe–Salpeter equation is outlined.     

π-electroproduction on hydrogen near threshold at four-momentum transfers of 0.2, 0.4 and 0.6 GeV

The reaction e + p → e + π⁰ + p was measured near the one pion threshold, detecting the final electron and proton in coincidence for values of q² = 0.2, 0.4 and 0.6 GeV². The slope of the cross section at threshold is determined. The data are compared with those of the e + p → e + π⁺ + n reaction, measured simultaneously, and with the results of pseudovector Born approximation and with dispersion theoretical calculations.     

On the pp → ppη(η′) reactions near threshold

The production rate for η′ in pp → ppη′ at rest is calculated in a covariant one boson exchange model, previously applied to study π⁰ and η production in NN collisions. The transition amplitudes for the elementary BN → η′N processes with B being the meson exchanged (B = π, σ, η, , ω and a₀) are taken to be the sum of s- and u-channels with a nucleon in the intermediate states, and an a₀ meson pole in a t-channel. The couplings of the η′ to hadrons are a factor 0.4 weaker than the respective η-hadron couplings, as suggested by a quark model and a singlet-octet mixing angle θ = −23°. The model reproduces near threshold cross sections for the quasielastic processes π⁻p → nη(η′) and pp → ppη(η′) reactions.     

Regge-pole description of γN→πN backward-scattering data

Baryon Regge-pole contributions to u-channel helicity amplitudes for the γN→πN processes are derived, with attention to kinematic singularities and threshold conditions. An N, Nγ and Δδ Regge-exchange model is proposed that describes the backward scattering data on γp→π^op and γp→π⁺n at high energy. The N and Nγ trajectories are found to be nearly degenerate, with residues in the ratio β(Nγ)/β(N)≈0.6. Structure in the differential cross sections is explained as dominance at small u giving way to dominance at large u. An isoscaar-isovector admixture for the γ-coupling to is required by the fits. The solution extrapolates through the mean γp→π^op 180^o differential cross section at intermediate energies, as required by duality.     

Search for up-down quark mass differences via isospin symmetry breaking

We performed a series of experiments employing the two pion production reactions p + d → ³He + π⁰ and p + d → ³H + π⁺. The two isopin related reactions differ by a Clebsch Gordan coefficient of 2. We searched for deviations in this ratio in complete angular distributions in the range of the Δ excitation as well as in the vicinity of the η production threshold.     

Optimization of pulse shaping using nonlinear polarization rotation

Pulse shaping using nonlinear polarization rotation has been analyzed in detail. It is found that there is an optimal, power-dependent polarization ellipticity for the maximum nonlinear transmission. It is also found that the polarization ellipticity should be kept smaller than 0.414 so that higher nonlinear transmission can be obtained. Compared to the well-known scheme of Stolen et al., the peak transmission can be increased more than three times, from 7% to 24%, for an incident power that generates self-phase modulation (φ_spm) of π.     

Measurement of the average polarization of B produced by polarized muons in the μ + C → νμ + B reaction

The average polarization of ¹²B produced by the capture of polarized muons in ¹²C has been measured using recoil implantation techniques. From the result we deduce the average polarization of the ¹²B ground state ¹²B(0) corresponding to the ¹²C → ¹²B(0) Gamow-Teller reaction: J_μ(0) = 0.43 ± 0.10. The sizeable deviation of this polarization from the value of 2/3, characteristic of a “bare” 0⁺ → 1⁺ process, is a fair evidence for induced axial-vector interaction(s) in muon capture. The ratio of the induced pseudoscalar and the axial-vector coupling constants is deduced to be: g^μ_P/g^μ_A = 12 ± 5.