Yu. A. Usov

Measurement of the Transverse Target and Beam-Target Asymmetries in η Meson Photoproduction at MAMI

We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the γ p →ηp reaction up to a center-of-mass energy of W=1.9  GeV. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of η meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.

Measurements of double-polarized compton scattering asymmetries and extraction of the proton spin polarizabilities.

The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Δ(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields γ(E1E1)=-3.5±1.2, γ(M1M1)=3.16±0.85, γ(E1M2)=-0.7±1.2, and γ(M1E2)=1.99±0.29, in units of 10(-4)  fm(4).

Observation of significant spin effects in hard collisions at 40 GeV/c

Abstract A single-spin asymmetry in the inclusive π 0 production at small x F was measured. In the experiment 40 GeV/c π − mesons were incident on transversely polarized protons and deutrons. An asymmetry of (40–50)% has been revealed in the hard scattering region.

Measurement of

Differential cross sections for the gamma p -> pi(0)p reaction have been measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C, up to the center-of-mass energy W = 1.9 GeV. The new results, obtained with a fine energy and angular binning, increase the existing quantity of pi(0) photoproduction data by similar to 47%. Owing to the unprecedented statistical accuracy and the full angular coverage, the results are sensitive to high partial-wave amplitudes. This is demonstrated by the decomposition of the differential cross sections in terms of Legendre polynomials and by further comparison to model predictions. A new solution of the SAID partial-wave analysis obtained after adding the new data into the fit is presented.

π^0

A movable polarized target has a volume 140 cm3 (20 cm long and 3 cm diameter). Polarizing magnet was tested to 6.5T, homogeneity is better than 2 × 10−4. The nuclear spin relaxation time in a frozen mode (at a temperature 50 mK and magnetic field 2.5T) is over 1000 h. Maximum values of proton polarization obtained were 80% and 85% for positive and negative polarization, respectively.

photoproduction on the proton at MAMI C

Abstract The paper presents results on measuring the polarization in the reaction π − p→ π 0 n at 40 GeV/ c in the 4-momentum transfer range 0 t |⩽2 (GeV/ c ) 2 . The results based on the statistics of about 2.2 million recorded events made it possible to observe a noticeable value and complex structure in the polarization behaviour up to | t |⩽2 (GeV/ c ) 2 .

A movable polarized target for high energy spin physics experiments

The double polarization observable E and the helicity dependent cross sections σ_{1/2} and σ_{3/2} were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η→2γ and η→3π^{0}. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ_{1/2} and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in η production are only N 1/2^{-} (S_{11}) and N 1/2^{+} (P_{11}). Furthermore, the extracted Legendre coefficients of the angular distributions for σ_{1/2} are in good agreement with recent reaction model predictions assuming a narrow resonance in the P_{11} wave as the origin of this structure.

Polarization in the reaction π−p → π0n at 40 GeV/c

Abstract A SATURNE polarized target has been used for nucleon-nucleon elastic scattering and transmission experiments for 15 years. Continuous improvements resulted in a flexible and reliable facility for spin physics. The polarized proton target was a 70 cm 3 cartridge loaded with pentanol-2, a promising material according to the results obtained. The new acquisition system and data processing was based on the LabView/PC software and increased the accuracy of polarization measurements. For a polarized neutron target, two cartridges loaded with 6 LiD and 6 LiH were set in the refrigerator and could be quickly inserted in the beam. Results from the the first experiments using 6 Li materials in quasielastic pp or pn analyzing power measurements are compared with the same observables measured in free nucleon-nucleon scattering using polarized proton targets. The angular resolution distributions for θ CM determination and azimuthal coplanarity are shown for different targets in nucleon-nucleon scattering. A comparison of analyzing power results for elastic and quasielastic scattering suggests that the contribution of inelastic processes to quasielastic pn scattering may be suppressed by additional constraints.

Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections

Experience with polarized targets [1,2] and achieving of very low temperatures in 1966 at the Laboratory of Nuclear Problems [3] and by another group [4] gave rise to the idea of using a radically new technique based on dissolving 3He in 4He to create a frozen spin polarized target. The short history of the development of such proton and deuteron targets at the LNP is given. The complex Cr(V) compounds in diols are used as target operating material. A characteristic feature of these targets is a long relaxation time (∼ 1000h) in magnetic fields of about 0.5 T. Lately, the Saclay-Argonne frozen spin proton polarized target used initially in E704 experiment at FERMILAB has been upgraded with adding the missing parts and the first physics experiment has been carried out.

Proton and neutron polarized targets for nucleon-nucleon experiments at SATURNE II

Abstract The angular dependece of the pp elastic scattering spin correlation parameter was measured in the angular range from 60° to 97° CM at 14 energies between 1.96 and 2.23 GeV. A rapid decrease of the energy dependence of this observable at 90° CM is observed around 2.11 GeV kinetic energy. This agrees with the behavior of A oonn predicted on the basis of an exotic six-quark structure. The A oonn (90° CM ), together with the known differential cross section, allows the determination of the absolute value of the pure spin-singlet amplitude. The energy dependence of this amplitude shows a shoulder centered at 2.11 GeV, corresponding to a total mass of 2.735 GeV and an estimated width of 17 MeV.