Wednesday, 6 March 2013

What can we infer about nuclear matter from giant quadrupole resonances?

The title of this post is the subject of a recent paper, published in Physical Review C by Xavier Roca-Maza and co-workers.  It is an attempt to link what can be observed in actual atomic nuclei with a model fictitious substance called infinite nuclear matter.  The reason anyone might want to do this is twofold.  Firstly, this stuff - infinite nuclear matter - which doesn't quite exist in nature - is easy to calculate.  That might sound a bit weak, but the ability to calculate things at all in nuclear physics is a great boon, and one can characterise nuclear matter in various useful ways - such as how compressible it is.  But still - why calculate things that don't exist at all?  That leads to the other reason for calculating nuclear matter.

Infinite nuclear matter is an approximation to a few different things.  It's like the central part of atomic nuclei - the tiny combinations of protons and neutrons at the centre of atoms, and its also the same stuff that makes up neutron stars.  They interact according to the same rules of physics, as far as we know.  If we can understand the ways that the model infinite matter corresponds to both actual nuclei and actual neutron stars then we might learn something about the way protons and neutrons interact.

One difficulty in doing this is that real nuclei are so small, and are so dominated by the fact that they have surfaces, rather than being bulk matter, means that it's hard to make this link.  Fortunately, there are some things that actual nuclei do that don't rely too much on surface effects.  Prime amongst these are giant resonances.  These are ways in which nuclei vibrate when excited in experiments.  The recent paper discusses so-called isovector giant quadrupole resonances, in which protons and neutrons vibrate in opposite ways to each other with a particular geometry.

In their paper, the researchers systematically tweaked the properties of a model nuclear force and found a marked connection between the vibrational frequency of the giant resonance in lead nuclei, the radii of the proton and neutron matter distribution in the same nucleus, and a nuclear matter quantity called the symmetry energy, which measures the extent to which protons and neutrons like to exist in equal numbers.  They take care to rule out, as far as possible, dependence on the particular model of the nuclear force, and have added a little bit more to the story of understanding what makes protons and neutrons stick together.

Roca-Maza, X., Brenna, M., Agrawal, B., Bortignon, P., Colò, G., Cao, L., Paar, N., & Vretenar, D. (2013). Giant quadrupole resonances in ^{208}Pb, the nuclear symmetry energy, and the neutron skin thickness Physical Review C, 87 (3) DOI: 10.1103/PhysRevC.87.034301