Narrow Results

A Gamma-ray spectroscopic study of excited states of isobaric multiplets has been performed in recent years, with a view to gaining a quantitative understanding of energy differences between excited states in terms of a range of Coulomb and other isospin breaking phenomena. Recently, the experimental programme has been augmented by a study of isobaric analogue states of mirror nuclei populated in mirrored fragmentation reactions. In this presentation, recent results on the analogue states in the mirror pair ⁵³Ni/⁵³Mn will be summarised. In this work, further strong evidence is found for the need to include an anomalous isospin-breaking two-body matrix element for angular-momentum couplings of J = 2, in addition the expected Coulomb contribution, in order to account for the experimental data.

Excited states in the T_z = -1 nucleus ⁴⁴V have been observed for the first time. The states have been identified through recoil-γ-γ coincidences and comparison with analogue states in the mirror nucleus ⁴⁴Sc. Mirror energy differences have been extracted and compared to state-of-the-art fp shell-model calculations which include charge symmetry breaking forces.

The COSY accelerator in Jülich is presented together with its internal and external detectors. The physics programme performed recently is discussed with emphasis on strangeness physics.

We investigate by global and local fits whether existing data on pp → π⁺d and np → π⁰d have indications on charge dependent forces other than the Coulomb force and the effect of differing thresholds. We find evidence in the energy (or pion momentum) dependence of the total cross sections close to threshold.

Charge symmetry breaking effects can be used as an experimental tool to study quark mass effects. Using recent high precision data from TRIUMF and IUCF an international theory collaboration is working on a consistent description within the framework of chiral pertubation theory. The WASA-at-COSY collaboration has started an experimental program extending the studies on the reaction dd → ⁴Heπ⁰ towards higher excess energies in order to provide necessary information on the contribution of p-waves. First preliminary results on the total cross section at Q = 60 MeV are presented.

One of the key experiments of the physics program of WASA-at-COSY is the determination of the p- wave contributions to the Charge Symmetry breaking amplitude in the reaction dd → ⁴Heπ⁰ at 1.2 GeV/c beam momentum. As a first step towards realisation of this goal the Charge Symmetry conserving reaction dd → ³Henπ⁰ was measured. For an overall theoretical analysis of CSB reactions in terms of Chiral Perturbation Theory this reaction channel will be used to gain information on the deuteron-deuteron initial state interaction for s- and p- wave pion production.

The comparison of analogue E1 transitions of mirror nuclei offers a valuable tool in the investigation of isospin symmetry breaking. Differences in the measured B(E1) reduced strengths in fact can be interpreted as the signature of isospin mixing. In this work, the isospin T_z = 1/2 nuclei ⁶⁷As and ⁶⁷Se were investigated. The nuclei were produced via the fusion evaporation reaction of ³²S on ⁴⁰Ca, at the ATLAS accelerator at Argonne National Laboratory. Gammasphere, Microball and the neutron wall arrays were employed for output channel selection.

The lifetime of the mirror 9/2⁺ states has been measured using the centroid shift method. A confirmation of the validity of this technique is given by the excellent reproduction of the known lifetime of the 9/2⁺ state in ⁶⁹As, previously determined via a recoil distance experiment. From a preliminary comparison of the extracted B(E1), our results indicate that isospin symmetry in these nuclei is not significantly broken. Further comparison with large scale shell model calculations is however still required.