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.
The Compton double-polarization observable Σ 2z has been measured for the first time in the Δ (1232) resonance region using a circularly polarized photon beam incident on a longitudinally polarized ...proton target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using either a dispersion relation calculation or a baryon chiral perturbation theory calculations with the weighted average of these two fits resulting in γE1E1=–2.87±0.52, γM1M1=2.70±0.43, γE1M2=–0.85±0.72 , and γM1E2=2.04±0.43 , in units of 10–4 fm4 .
Precise helicity-dependent cross sections and the double-polarization observable E were measured for η photoproduction from quasifree protons and neutrons bound in the deuteron. The η → 2γ and η → ...3π0 → 6γ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of η photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin (σ1/2). It is absent for reactions with parallel spin orientation (σ3/2) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of P11 and S11 partial waves to explain the narrow structure.
The SPASCHARM experiment is aimed at a systematic study of the nucleon spin structure and the spin dependence of the strong interaction of antimatter and matter with matter at energies up to 45 GeV. ...As part of the first stage of the experiment, the study of the spin properties of hadrons will take place in a beam of negatively charged hadrons on existing beamline 14 at the operating SPASCHARM setup at the U70 facility. At the second stage, the production of polarized beams of protons and antiprotons is envisaged in beamline 24A of the U-70 accelerator facility. A polarized antiproton beam will certainly become a unique beam in the world. It is planned to measure single-spin asymmetries in dozens of reactions, both on hydrogen and on various nuclei. At the SPASCHARM facility, it is also possible to measure the transverse polarization of hyperons and elements of the spin density matrix of vector mesons. The spin structure of the nucleon will be investigated in the study of quarkonium production to determine the contribution of gluons to the proton spin. The presence of two types of polarized beams and eight types of nonpolarized beams (π
±
,
K
±
,
p
,
,
d
,
C
), in combination with a polarized target, expands the range of studies of polarization phenomena and enhances the uniqueness of the project.
The helicity-dependent cross sections for the photoproduction of π0η pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI ...accelerator with the combined Crystal Ball – TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the σ1/2 (photon and target spin antiparallel) and σ3/2 (parallel spins) components to be measured for quasi-free production of π0η pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, A1/2n,p and A3/2n,p. Identical couplings for protons and neutrons are typical for Δ resonances and identical A1/2 and A3/2 components are only possible for J≥3/2 states, which constrains possible contributions of nucleon resonances.
The new SPASCHARM experiment for systematic studies of polarization phenomena in strong interactions is under construction now at IHEP, Protvino. The technical beam runs for the experiment first ...stage are planned for the Fall 2016 and Spring 2017. At this stage, the polarization measurements will be carried out with unpolarized hadronic beams of various compositions (π±, K±, p, antiprotons), using the polarized target. The universal large acceptance experimental set-up is capable detecting and identification most charged and neutral particles and reconstructing a large number of resonances produced in beam interactions at polarized proton target, and later on at the second stage, in collisions of polarized proton and antiproton beams with fixed targets of various materials. The large acceptance and wide data acquisition bandwidth would provide the capabilities for simultaneous data accumulation for a number of physics analyses from the measurements of single-spin asymmetries in inclusive and exclusive reactions to reconstructions of final state hyperon polarizations and spin density matrix elements for vector mesons in a wide range of kinematic variables (pT, xF).
Abstract
The target asymmetry
T
, recoil asymmetry
P
, and beam-target double polarization observable
H
were determined in exclusive
$$\pi ^0$$
π
0
and
$$\eta $$
η
photoproduction off quasi-free ...protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the
$$\gamma p \rightarrow \pi ^0 p$$
γ
p
→
π
0
p
and
$$\gamma p \rightarrow \eta p$$
γ
p
→
η
p
reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the
$$\eta n$$
η
n
system at
$$W = 1.68\ \textrm{GeV}$$
W
=
1.68
GeV
. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the
$$S_{11}(1535)$$
S
11
(
1535
)
and
$$S_{11}(1650)$$
S
11
(
1650
)
resonances within the
$$S_{11}$$
S
11
-partial wave.
A new experiment SPASCHARM for systematic study of polarization phenomena in inclusive and exclusive hadronic reactions is currently under commissioning at IHEP. The universal experimental setup will ...detect dozens of various resonances and stable particles produced in collisions of unpolarized beams with the polarized target, and at the next stage, using polarized beams. At the first stage with polarized target, the final states composed of light quarks (u, d, s) will be reconstructed. Hyperon polarization and spin density matrix elements of the vector mesons will be measured along with the single-spin asymmetries. The 2π-acceptance in azimuth, which is extremely useful for reduction of systematic errors in measurements of spin observables, will be implemented in the experiment. The solid angle acceptance of the setup, Δθ≈250 mrad vertically and 350 mrad horizontally in the beam fragmentation region, covers a wide range of kinematic variables pT and xF. This provides the opportunity for separating dependences on these two variables which is usually not possible in the setups with a small solid angle acceptance. Unlike some previous polarization experiments, the SPASCHARM will be able to simultaneously accumulate and record data on the both, charged and neutral particle production.
The target asymmetry
T
, recoil asymmetry
P
, and beam-target double polarization observable
H
were determined in exclusive
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\begin{document}$$\pi ^0$$\end{document}
π
0
and
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\begin{document}$$\eta $$\end{document}
η
photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the
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\begin{document}$$\gamma p \rightarrow \pi ^0 p$$\end{document}
γ
p
→
π
0
p
and
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\begin{document}$$\gamma p \rightarrow \eta p$$\end{document}
γ
p
→
η
p
reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the
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\begin{document}$$\eta n$$\end{document}
η
n
system at
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\begin{document}$$W = 1.68\ \textrm{GeV}$$\end{document}
W
=
1.68
GeV
. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the
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\begin{document}$$S_{11}(1535)$$\end{document}
S
11
(
1535
)
and
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\begin{document}$$S_{11}(1650)$$\end{document}
S
11
(
1650
)
resonances within the
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\begin{document}$$S_{11}$$\end{document}
S
11
-partial wave.
The double-polarization observable Eand the helicity-dependent cross sections σ1/2and σ3/2have been measured for the first time for single π0photoproduction from protons and neutrons bound in the ...deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the π0 meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the π0N final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry E since the σ1/2 and σ3/2 components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.
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