S. Stave

Unambiguous identification of the second 2+ state in C12 and the structure of the hoyle state

The second J(π)=2+ state of 12C, predicted over 50 years ago as an excitation of the Hoyle state, has been unambiguously identified using the 12C(γ,α0)(8)Be reaction. The alpha particles produced by the photodisintegration of 12C were detected using an optical time projection chamber. Data were collected at beam energies between 9.1 and 10.7 MeV using the intense nearly monoenergetic gamma-ray beams at the HIγS facility. The measured angular distributions determine the cross section and the E1-E2 relative phases as a function of energy leading to an unambiguous identification of the second 2+ state in 12C at 10.03(11) MeV, with a total width of 800(130) keV and a ground state gamma-decay width of 60(10) meV; B(E2:2(2)+→0(1)+)=0.73(13)e(2)  fm(4) [or 0.45(8) W.u.]. The Hoyle state and its rotational 2+ state that are more extended than the ground state of 12C presents a challenge and constraints for models attempting to reveal the nature of three alpha-particle states in 12C. Specifically, it challenges the ab initio lattice effective field theory calculations that predict similar rms radii for the ground state and the Hoyle state.

Measurement of compton scattering from the deuteron and an improved extraction of the neutron electromagnetic polarizabilities.

The electromagnetic polarizabilities of the nucleon are fundamental properties that describe its response to external electric and magnetic fields. They can be extracted from Compton-scattering data-and have been, with good accuracy, in the case of the proton. In contradistinction, information for the neutron requires the use of Compton scattering from nuclear targets. Here, we report a new measurement of elastic photon scattering from deuterium using quasimonoenergetic tagged photons at the MAX IV Laboratory in Lund, Sweden. These first new data in more than a decade effectively double the world data set. Their energy range overlaps with previous experiments and extends it by 20 MeV to higher energies. An analysis using chiral effective field theory with dynamical Δ(1232) degrees of freedom shows the data are consistent with and within the world data set. After demonstrating that the fit is consistent with the Baldin sum rule, extracting values for the isoscalar nucleon polarizabilities, and combining them with a recent result for the proton, we obtain the neutron polarizabilities as αn=[11.55±1.25(stat)±0.2(BSR)±0.8(th)]×10(-4)  fm(3) and βn=[3.65∓1.25(stat)±0.2(BSR)∓0.8(th)]×10(-4)  fm(3), with χ(2)=45.2 for 44 degrees of freedom.

Chiral Dynamics in Photopion Physics: Theory, Experiment, and Future Studies at the HIγS Facility

We review photopion experiments on the nucleon in the near-threshold region. We compare the results with the predictions of the low-energy theorems of quantum chromodynamics calculated using chiral perturbation theory (ChPT), which is based on the spontaneous breaking of chiral symmetry as well as explicit breaking due to the finite quark masses. As a result of the vanishing of the threshold amplitudes in the chiral limit, the experiments are difficult to perform because the cross sections are small. Nevertheless, the field is now mature in terms of accuracy and sensitivity. We also discuss the accomplishments and limitations of past measurements, as well as future planned experiments at Mainz and HIγS. The technical developments required for the HIγS facility are emphasized. Finally, we show that future experiments will provide even more accurate tests of ChPT and will be sensitive to isospin-breaking dynamics due to the mass difference between the up and down quarks.

Determination of quadrupole strengths in the gamma*p-->Delta(1232) transition at Q2= 0.20 (GeV/c)2

We report new precise p

Energy dependence of the delta resonance: Chiral dynamics in action

(\vec{e},e^\prime p)\pi^0

Compton scattering from the deuteron below pion-production threshold

measurements at the peak of the

Compton scattering from C-12 using tagged photons in the energy range 65-115 MeV

\Delta^{+}(1232)

New results for Compton scattering on deuterium : A better determination of the neutron electromagnetic polarizabilities

resonance at

Using Quasi-Monoenergetic Photon Sources to Probe Photo-Fission Resonances

Q^2=\unit[0.20](GeV/c)^2

Nuclear resonance fluorescence using different photon sources

performed at the Mainz Microtron (MAMI). The new data are sensitive to both the electric quadrupole (