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My Research

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Jan. 23rd, 2009 | 10:12 am

When I created this blog, the intention was for it to cover anything related to my work - from teaching ideas and methods, to interesting news in physics, to my research area in particular, to University life in general. Some recent modest success related to a couple of my research publications leads me to finally put finger to keyboard about the subject of my research:

Researchers in Nuclear and Particle Physics will be aware of a long-standing online database of publications called SPIRES, on which you can search for publications using keywords and author names. Every now and then I look myself up on it because as well as showing me a (slightly incomplete) list of my publications, it gives an approximate citation count for each one, and it's interesting to see what of my work has been noticed by the physics community and found useful, and what hasn't. The last time I looked, I noticed that one of my articles had and extra label attached to it in the database. The label reads "TOPCITE = 50+" which means that it has now attracted at least 50 citations by other articles that the database knows about. To the uninitiated, 50 might not sound like very much. Indeed, it depends on the field of science to a great extent on what is considered a lot of citations, but the fact that SPIRES marks this out as special gives at least some indication that 50 is pretty decent in my field. In actual fact, I've heard, and I can believe, that the modal number, and probably the median number of citations of scholarly journal articles is zero.

So, slightly as an excuse to point this achievement out, and hence to show off a little, I thought I'd blog about what is in that paper. It grew out of the work I did for my D.Phil thesis. I had developed a new model for the nuclear interaction - the force that keeps protons and neutrons bound together in the nucleus. To test out my model, I made various calculations along the lines of "okay, so if this is my model of the nuclear force, what does it predict the mass of a certain lead isotope will be. What about its radius, its shape? What about isotopes of tin? calcium?" Then, if my calculations agree with the observed values, I can hope that there is something right about my model and use it to make predictions. One set of predictions I used my new force for was various properties of neutron stars. Neutron stars are (as their name suggests) stars made of neutrons. Well, that's not strictly true - there's other stuff there besides neutrons, but basically they are the remnants of stars who have burned all their nuclear fuel and have collapsed (following a supernova) in to what is basically a massive nucleus, consisting largely of neutrons, but with protons and other things in there too. Since neutron stars are made of the same stuff as nuclei, nuclear force models should be able to predict their observed properties - their mass and size and so on.

So, I wrote a paper based on my thesis about these calculations for neutron stars, and well, it's been of quite modest interest to the rest of the world (8 citations according to SPIRES!) but my D.Phil supervisor followed up that work, with me and some others, to apply the same techniques to a different and more established model of the nuclear interaction. That paper has turned out to be the surprisingly popular one. In it, we compared many different variants of this interaction, and classified them according to their ability to produce reasonable properties of neutron stars. Some don't predict the existence of neutron stars at all, some do well, and others are somewhere inbetween. We worked out that this was linked to how the variants dealt with very asymmetric matter - in which the ratio of protons to neutrons became very small - much smaller than is found in everyday nuclei, and this, along with the rather extensive testing of the variants, has turned out to be of interest to a lot of researchers. It's nice when that happens :-)

The other thing relates to a much more recent paper, written by a current PhD student and me, on his thesis work. This work has to do with what happens when nuclei collide with each other. When they do so, they sometimes combine (fuse), sometimes not, but always the resulting system is highly excited - the protons and neutrons in the resulting nucleus or nuclei are not in their normal stable configuration. This means that the nuclei de-excite, often by emitting some of the protons or neutrons. The upshot of this is that if you collide two nuclei together, you can end up with a whole lot of different possible final residual nuclei depending on how the deexciting goes. One of the leading theories (called time-dependent Hartree-Fock) does a pretty good job at some aspects of describing these collisions, but it doesn't do well at describing just how diverse the spread of possible final residues can be. My student implemented an extension to the theory which makes a great improvement of these predictions. The article hasn't been cited much yet (it's just a few months old) but it's been selected by the editors of the journal as a highlight article - of particular interest of importance. They only do that to a few papers per year, and he and I are pretty pleased!

If you actually want to read either article they are available online. The neutron star one is here and the collisions one is here.

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