Speakers
Description
The exotic, two-neutron halo nucleus $^{11}$Li has proven to be an extraordinarily fruitful laboratory for the study of the nuclear many-body system under very low density conditions. The halo, consisting of the two valence neutrons outside the $^{9}$Li core, form a thin mist of nuclear matter, held together by a subtle mix of the bare NN-interaction and the induced interaction arising from the exchange of collective dipole and quadrupole vibrations. The resulting intertwining of pairing (gauge space) and surface (3D-space) correlations manifests itself not only in the structure of the ground state of $^{11}$Li, but also in the low-lying resonant dipole strength observed at an excitation energy of ~1 MeV.
Recent studies suggest that this symbiotic collaboration between pairing and dipole surface modes would result in an enhanced two-neutron transfer cross section for the population of the dipole mode in the reaction $^{9}$Li(t,p)$^{11}$Li*. Such an experiment constitutes a novel probe of the low-lying dipole strength (pygmy dipole resonance, PDR), and would highlight the role of multipole (in this case, dipole) pairing vibrations in their structure. Such a probe would complement the much-studied characterization of the PDR in terms of its isovector decomposition. It would also confirm the predicted vorticoid character of such a mode, by populating its most elementary manifestation: a single quantum vortex.
The new ISOLDE Solenoidal Spectrometer is one of the most promising tools to perform this challenging experiment, beyond past, conventional approaches. The spectrometer will enable the exclusive measurement of this reaction with unprecedented resolution and sensitivity, which are mandatory to disentangle the $^{11}$Li structure and shed light on the character of the low-lying dipole strength. The expected $^{9}$Li beam yield of around 10$^{6}$ pps makes ISOLDE together with the ISS unique for this type of low-energy measurements not possible in other facilities.
