0000000000358684
AUTHOR
S. S. Kamalov
Two-body Mechanisms in Pion Scattering and Pion Photoproduction on the Trinucleon
A breakdown of the Impulse Approximation is studied in pion photoproduction and pion charge exchange on 3He at high momentum transfers. The usual DWIA formalism with Faddeev wave functions which works well for small momentum transfers deviates from experimental measurements by up to two orders of magnitude for Q 2 > 6 fm−2. It is found that the explicit inclusion of two-body mechanisms, where the photon or pion is absorbed on one nucleon and the pion is emitted from another nucleon removes most of the disagreement with the data.
Measurement of the Recoil Polarization in thep(e→,e′p→)π0Reaction at theΔ(1232)Resonance
The recoil proton polarization has been measured in the $p(\stackrel{\ensuremath{\rightarrow}}{e}{,e}^{\ensuremath{'}}\stackrel{\ensuremath{\rightarrow}}{p}){\ensuremath{\pi}}^{0}$ reaction in parallel kinematics around $W\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1232\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$, ${Q}^{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.121\phantom{\rule{0ex}{0ex}}(\mathrm{GeV}/c{)}^{2}$, and $\ensuremath{\epsilon}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.718$ using the polarized cw electron beam of the Mainz Microtron. All three proton polarization components, ${P}_{x}/{P}_{e}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{-…
2p-shell admixtures in pion single charge exchange on 13C
Abstract The pion charge exchange process 13 C (π + , π 0 ) 13 N(gs) is calculated with an extended nuclear wave function for the A=13 system that includes configurations outside the 1p-shell. A small amount of 2p-shell admixtures leads to an improved description of the experimental excitation function. We also find a strong sensitivity of the differential cross sections as well as asymmetry to contributions from higher shells.
Asymmetries in elastic scattering of 100 MeVπ+from a polarizedHe3target
We report the observation of a large asymmetry in the elastic scattering of 100 MeV {pi}{sup +} from a polarized {sup 3}He target. Measurements have been made at laboratory angles of 60{degree}, 80{degree}, and 100{degree}, with the largest value {ital A}{sub {ital y}}=0.89{plus minus}0.12 occurring at 80{degree}, near a cross-section minimum. This asymmetry is the largest observed to date in pion scattering from a spin-1/2 nucleus. The {ital A}{sub {ital y}} data are qualitatively reproduced by a schematic model; however, agreement with the data is significantly improved when realistic three-body Faddeev wave functions and a full nonlocal distorted-wave impulse approximation reaction model…
Pion photoproduction on3He including final-state interaction
Coherent π0 and π+ photoproduction and elastic and charge-exchange pion scattering on3He has ben calculated in a consistent model. Realistic three-body Faddeev wave functions have been used and full nonlocal DWIA results for pion photoproduction are obtained. Furthermore, two-step processes such as3He(γ, π0)3He(π0, π+)3H are included giving rise to unexpected large results. The comparison with experimental data is very good over a wide range of photon energies and nuclear momentum transfers. A long-time discrepancy between previous impulse-approximation calculations and experimental data has been removed.
Recent results from the MAID and Dubna-Mainz-Taipei PWA analysis
The unitary isobar model MAID2007 has been developed to analyze the world data of pion photo- and electroproduction. The model contains both a common background and several resonance terms. The background is unitarized according to the K-matrix prescription, and the 13 four-star resonances with masses below 2 GeV are described by appropriately unitarized Breit-Wigner forms. The data have been analyzed by both single-energy and global fits, and the transverse and longitudinal helicity amplitudes have been extracted for the four-star resonances below 2 GeV. Because of its inherent simplicity, MAID2007 is well adopted for predictions and analysis of the observables in pion photo- and electropr…
Photo- and Electroproduction of Eta Mesons on Nucleons and Nuclei
Eta photo- and electroproduction off the nucleon is investigated in an approach that contains Born terms, vector meson and nucleon resonance contributions. In a comparison with the new Mainz data we find a large sensitivity on the elementary ηN N coupling. Our analysis results in a pseudoscalar ηN N coupling with a coupling constant of g ηN N 2 /4π=0.4. Furthermore, we also study coincidence cross sections for eta electroproduction and present calculations for structure functions and kinematical conditions that are most sensitive to the S 11(1535) and the D 13(1520) resonances. Finally, we show results on the inclusive eta photoproduction off complex nuclei with a very good agreement with r…
0+-0+transition in charged photopion reactions
In the near-threshold region for the (\ensuremath{\gamma},${\mathrm{\ensuremath{\pi}}}^{\ifmmode\pm\else\textpm\fi{}}$) reaction the spin-flip (Kroll-Ruderman) term dominates most nuclear reactions. The smaller non-spin-flip term is of considerable interest because it is very sensitive to the reaction dynamics and in particular to the \ensuremath{\Delta} channel, even in the near-threshold region. We propose to study the non-spin-flip term via the ${0}^{+}$\ensuremath{\rightarrow}${0}^{+}$ transition in the $^{14}\mathrm{C}$(\ensuremath{\gamma},${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${)}^{14}$N reaction; the cross section is shown to be insensitive to the details of nuclear …
Excitation of S11 resonances in pion scattering and pion photoproduction on the proton
A self-consistent analysis of pion scattering and pion photoproduction within a coupled channels dynamical model is presented. In the case of pion photoproduction, we obtain background contributions to the imaginary part of the S-wave multipole which differ considerably from the result based on the K-matrix approximation. Within the dynamical model these background contributions become large and negative in the region of the S_{11}(1535) resonance. Due to this fact much larger resonance contributions are required in order to explain the results of the recent multipole analyses. For the first S_{11}(1535) resonance we obtain as a value of the dressed electromagnetic helicity amplitude: A_{1/…
Meson exchange currents in theHe3(γ,π+)3H reaction
We generate meson exchange currents mechanisms for the (\ensuremath{\gamma},${\mathrm{\ensuremath{\pi}}}^{+}$) reaction in nuclei starting from the \ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}N amplitude on one nucleon and allowing one of the pions to be produced off shell and be absorbed by a second nucleon. Detailed calculations are presented for the \ensuremath{\gamma} $^{3}\mathrm{\ensuremath{\pi}}^{+3}$H reaction, where we show that the cross section at large momentum transfers is dominated by these mechanisms, helping improve the agreement with experimental data. It is also shown that the meson exchange currents produce important effects in the photon a…
Pion Scattering and Pion Photoproduction on 3He
Using realistic 3-body Faddeev wave functions π± scattering and π+ photoproduction on 3He have been calculated in a consistent model. We demonstrate that at T π ≤ 180MeV polarization observables in pion scattering contain very little information about nuclear structure and can be understood in terms of free πN amplitudes. At higher energies the sign-flip of the asymmetry A y offers a new possibility to study higher-order effects of pion nuclear interaction. In pion photoproduction we find that two-step processes such as 3 He(γ, π°)3 He(π°, π+)3 H give rise to unexpected large contributions to the differential crosssection as well as to γ-asymmetry and with this improved a long time discrepa…
Meson exchange currents in kaon scattering on the lightest nuclei
The K^+ scattering on the lightest nuclei, d, 3He and 4He is studied in the framework of multiple scattering theory. Effects from MEC tied to the K^+N-->KNpi reaction are evaluated. We found that at momentum transfers Q^2<0.5 (GeV/c)^2 contributions from MEC are much smaller than kaon rescattering corrections. This makes the conventional multiple scattering picture a reliable tool to study these reactions in this kinematical domain and to extract the K^+n scattering amplitude from the K^+d data. At larger transferred momentum MEC can become more relevant.
Pion Photo- and Electroproduction on the Proton
Using a unitary isobar model a new operator for pion photo- and electroproduction has been developed for nuclear applications at photon equivalent energies up to 1 GeV. The model contains Born terms, vector mesons and nucleon resonances (P 33(1232), P 11(1440), D 13(1520), S 11(1535), F 15(1680), and D 33(1700)). Within this model we have obtained good agreement with the experimental data for pion photo- and electroproduction on the nucleon for both differential cross sections and polarization observables.
T- and P-asymmetries in the (γ, π−) reaction on 13C and 15N
Abstract A simultaneous theoretical analysis of the polarization observables for negative pion photoproduction off the 13 C and 15 N nuclei is performed. A sensitivity of the target and recoil asymmetries to different ingredients of the theory (the Fermi motion, the final state interaction and the nuclear model input) is investigated. The importance of the high (2 hω ) configurations in the nuclea ground states is shown.