Thursday, January 25, 2007

Strange Fruit

Taking time away from work isn't easy here. The institute is extremely conducive to steady concentration which is wonderful but it does mean I probably haven't been taking it as easy as I should have while I'm on the road to full recovery from the flu. Still, with long hours in the office followed by a five minute stroll to the guest house, there's not much to do there except read and continue with the work. No TV, currently no other guests and no internet makes for complete peace!

About the only distraction is the stunning view from my office window with the green mountains, the red trees and the almost constantly changing sky.

Photos don't seem to be uploading to blogger at the moment so they will have to wait.

I've seen more talks in the first four days here than I normally would in a month at the ITP. The Yukawa institute is unusual in its international feel, with many foreign visitors and postdocs.

On Wednesday we had a great talk from Frank Wilczek. He spoke about the high pressure, low temperature phase of QCD, which is the condition found at the centre of neutron stars (low temperature here is not in the astrophysical sense, but on the scale of the QCD phase diagram). In this region the phase is said to be colour superconducting and one can perform a weak coupling expansion using BCS theory and find the spectrum of states.

In particular in this phase there is a diquark condensate formed which induces colour-flavour locking and the SU(3) colour symmetry along with the left and right flavour symmetries are broken to the diagonal subgroup of the colour and flavour symmetry. The condensate which is formed has non-zero electromagnetic charge but only a particular combination of the electromagnetic U(1) with the SU(3) colour symmetry remains unbroken, meaning that the asymptotic states of the theory are integrally charged under the new U(1).

The asymptotic states of the theory are found to be single quarks, gluon bilinear pairs and goldstone modes. Though this looks rather different from the non-extreme phase of QCD it turns out that the quantum numbers of these states in the basis chosen by the particular symmetry breaking pattern are exactly the same as in normal QCD: an octet of baryons, an octet of mesons and an octet of pseudo goldstone bosons.

It is found by studying the fermi surface from the point of view of colour superconductivity that the numbers of u,d and s quarks are equal and because of the equality of the number of quarks in this phase, the whole is electrically neutral, meaning that there is no sea of electrons to counter the positive charge that one would have in ordinary quark matter (just u and d quarks with a quark bilinear condensate). Photons (of the unbroken U(1)) are neither absorbed nor reflected, meaning that though the particle density is very high, the substance is also transparent. This has been compared to a very high pressure diamond and it's thought that in the centre of neutron stars may reside a huge diamond like phase (though the edge of this region will merge with the low density region on the outside of the neutron star complicating astrophysical comparison with the weak coupling expansion from the theory).

(See here for a review of the subject, though this doesn't contain the more recent diquark studies of baryon spectra)

It's fascinating to see that there are places in the universe where not only such strange matter may reside but that we can make predictions about it from our knowledge of QCD. Frank mentioned that once we get measurements from gravitational wave detectors (and here) to high enough precision we will be able to make measurements of coalescing neutron stars which we can test against the predictions from the above theory. Exciting stuff indeed. There have been some nice articles detailing quite how unbelievable the gravitational wave detectors are. As an example of how difficult these machines are to construct and use, in order to make a measurement of a gravitational wave it is necessary to measure the changes in the length of a beam several kilometers long which will be on the scale of one hundred millionth the diameter of a hydrogen atom. It's been said before but this is simply preposterous!

(It's very likely that I've misunderstood some of the statements made in the talk so if anybody knows that I've made mistakes, please do correct me. Unfortunately the current valuable research time means that I haven't had a proper chance to read up in detail on this subject yet).

In addition to this lecture there have been a couple of highly theoretical string theory lectures and, as I remembered from being here last year, the questioning during the lectures can be rather intense. Whenever I give my lecture here, which will be on a topic I haven't discussed in detail before, I'll have to aim to a pretty high level. There are many all-rounders here (lattice QCD experts who know their way around an M2 brane for instance) so you never know where the questions may spring from.

Anyway, I gotta get back to work. Projects are progressing reasonably well but as always there's much more to do

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