Monday, January 23, 2006

Neutrinos on Ice

In an earlier post, Matthew talked about the AMANDA and IceCube neutrino detectors based at the South Pole, which utilize the antarctic ice shield both as a shielding against cosmic radiation and as the medium for a giant Cherenkov-type detector.

In the most recent issue of Physical Review Letters, members of the AMANDA collaboration report on the limits on neutrino cross sections at ultra-high energies that result from the AMANDA events seen so far. Since the event rate in the detector depends on both the cross section and the flux (both of which are unknown) they can only exclude cross section/flux pairs, but by looking separately at down-going neutrinos (which have crossed only the atmosphere and ice shield) and up-going neutrinos (which have passed through the body of the earth, more than one interaction length at the energies they consider), the authors are able to impose upper limits on the cross section at very high energies and an absolute upper bound on the neutrino flux.

I rarely envy experimentalists, but this Symmetry magazine article about AMANDA, IceCube, ANTARES and NESTOR (two rival neutrino telescopes using the deep sea instead of the Antarctic ice) makes me feel just a little jealous of the opportunities they can get to go to such amazing and distant places.

Thursday, January 12, 2006

Gold, Myrrh and Frankincense

I hope everyone had happy holidays and a great start into 2006! I haven't been posting here for a while, so this is going to be a bit like a rather delayed Santa's bag with something of everything in it.

First, I have been reading in a very interesting book during the last few weeks; I am speaking of G.E. Volovik's The Universe in a Helium Droplet. There are lots of material in there, most of which I am probably not able to appreciate properly, since my knowledge of superfluids is extremely limited. What I find most interesting are the analogies with the electroweak sector of the Standard Model and with General Relativity that occur in the effective theory of quasiparticles in superfluid 3He. While there are a lot of differences with the real universe (most importantly significant violations of the effective Lorentz symmetry), the possibility of having a "relativistic" QFT emerge from a non-relativistic condensed matter system is intriguing, and certainly suggests that the image that Volovik calls "anti-GUT", namely that instead of becoming simpler and more symmetric, nature becomes less symmetric at higher energies. Moreover, there is really no reason to believe that the sequence (effective QFT+GR) << (NRQM system) << (SM+GR) has to stop there, or in fact anywhere. If the Standard Model is an effective theory for something else (say strings, or whatever alternative you prefer), why should this something (just like the Helium atoms) not be reducible to another QFT, which is effective as well? This is a slightly disturbing image, without any truly fundamental physics (except for Quantum Theory, which occurs on, and remains the same at, all scales). But maybe that is how the universe really looks; but then, again, it is unlikely that we will ever know either way.

Second, as if the truly heroic feat of computing the O(α4) contributions to the leptons' anomalous magnetic moment was not enough, T. Kinoshita and collaborators have begun to calculate the O(α5) contributions! This needs some very advanced automation techniques; since my research revolves around automating perturbative calculations, I feel able to appreciate what they are doing, and all I can say is that it is truly impressive: Most people (or at least I) would not want to touch a four-loop diagram with a bargepole, whereas Kinoshita wade right into the midst of the five-loop diagrams. Did I say heroic? I really mean it.

Third, when entering "lattice qcd" into Google, a post on this blog turns up at number 15 of 626,000 -- flattering, but undeserved. It appears that with the current blogging craze, Google are overweighting hits from blogs in their results.

So this post wasn't something of everything, but just three things; so maybe it is more like the Magi's gifts. (Which is which? That's your guess.) And with that, a happy new year again to everyone!