Aron M. Bernstein

Neutral pion photoproduction from the proton near threshold

Abstract Differential and total cross sections for the photoproduction of neutral pions from the proton have been measured for incident photon energies from 140–270 MeV, using the photon spectrometer TAPS at the tagged photon beam of the 855 MeV Mainz Microtron. The energy dependence of the s- and p-wave multipoles close to threshold was deduced from a multipole fit and a multipole analysis. The extracted s-wave amplitude E 0+ at threshold is found to be significantly smaller than the prediction of the classical low energy theorems, but is in reasonable agreement with the chiral perturbation theory.

The 42, 44Ca(p, d)41, 43Ca reactions at 26.5 MeV

Abstract The 42Ca(p, d)41Ca and 44Ca(p, d)43Ca reactions at 26.5 MeV have been examined with 100 keV resolution to obtain nuclear structure information on the levels observed in 41Ca and 43Ca up to an excitation energy of approximately 8 MeV and on the ground state wave functions of 42Ca and 44Ca. The strong 2 s 1 2 and 1 d 3 2 hole states were verified at 2.68 MeV and 2.02 MeV, respectively, in 41Ca and at 1.96 MeV and 0.99 MeV, respectively, in 43Ca. Isobaric analog states (T = Tz+1) were located at 5.84±0.05 MeV (J π = 3 2 + ), 6.82±0.05 MeV ( 1 2 + ) and 7.13±0.05 MeV ( 7 2 − ) in 41 Ca and 7.97±0.05 MeV ( 3 2 + ) in 43 Ca . The presence of additional levels in both nuclei with ln = 0 and ln = 2 transitions implies that the one-hole state strength is distributed over several levels. Evidence for the splitting of the 1 f 7 2 single-particle strength has been found with the excitation of states with ln = 3 transitions at 2.98 MeV and 3.28 MeV in 41Ca and 43Ca, respectively, whose spectroscopic strengths are approximately 11% and 7.5%, respectively, of the corresponding ground state transitions. Excitation of the 7.13 MeV 7 2 − , T = 3 2 ) level in 41Ca indicates the presence of four-particle-two-hole components in the 42Ca ground state. Using sum rules, we find the average number of 1 f 7 2 protons in the 42Ca ground state to be approximately 0.6. The different relative spectroscopic strengths for the (p, d) and (d, p) reactions leading to 3 2 − states in 43Ca at 0.59 MeV and 2.05 MeV demonstrates that both [(1 f 7 2 2 (2 p 3 2 ) 2 ] and [(1 f 7 2 3 (2 p 3 2 )] admixtures are present in the 44Ca ground state. Indication of an [(1 f 7 2 3 (1 f 5 2 )] admixtures found from the excitation of the 5 2 − state at 0.373 MeV in 43Ca. A non-pick-up angular distribution to the 1.68 MeV level in 43Ca was observed. If the previously made tentative assignment of 11 2 − for this state is correct, this transition proceeds by higher-order reaction mechanisms. The spectroscopic strengths for the levels excited were derived from DWBA calculations using form factors modified by finite-range and non-local corrections in the local-energy approximation. The resulting shapes of the angular distributions were in better agreement with the experimental results than those for zero-range local calculations; the derived spectroscopic strengths were also less sensitive to the choice of optical potential parameters and in better agreement with sum-rule predictions. Comparisons between the experimental spectroscopic strengths and those predicted by theoretical wave functions are made where such wave functions are known.

Neutron and proton transition matrix elements and inelastic hadron scattering

Abstract The ratios of the neutron and proton transition matrix elements ( M n / M p ) have been studied for 0 + → 2 + transitions in single-closed-shell (SCS) nuclei by comparing inelastic hadron scattering and electromagnetic transition rates. The results show a general consistency from hadron scattering to an accuracy ≈15% and show the systematic trend that M n / M p > N / Z (the collective-model result) for SCS neutron-valence nuclei and N / Z for SCS proton-valence nuclei, in agreement with schematic-model calculations of core-polarization effects.

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.

LIGHT QUARK MASS DIFFERENCE AND ISOSPIN BREAKING IN ELECTROMAGNETIC PION PRODUCTION

Abstract It is demonstrated that there is a dynamic isospin breaking effect in the near threshold γ ∗ N→πN reaction due to the mass difference of the up and down quarks, which also causes isospin breaking in the πN system. The photopion reaction is affected through final state πN interactions (formally implemented by unitarity and time reversal invariance). It is also demonstrated that the near threshold γ N →πN reaction is a practical reaction to measure isospin breaking in the πN system, which was first predicted by Weinberg about 20 years ago but has never been experimentally tested.It is demonstrated that there is a dynamic isospin breaking effect in the near threshold

The (α, p) reaction on 40, 42, 44, 48Ca and 48Ti at 31 MeV

\gamma^{*} N\to \pi N

New Measurement of the π0 Radiative Decay Width

reaction due to the mass difference of the up and down quarks, which also causes isospin breaking in the

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

\pi N

A qualitative description of the spectra 17O and 41Ca

system. The photopion reaction is affected through final state

Test of the shell model for 206Pb by the 208Pb(p, t) reaction

\pi N