Just when I'm finally starting to get a bit of a handle on what's going on in particle physics (or at least map out the areas of my ignorance), along comes Howard Georgi with "Unparticle Physics":
I discuss some simple aspects of the low-energy physics of a nontrivial scale invariant sector of an effective field theory--physics that cannot be described in terms of particles. I argue that it is important to take seriously the possibility that the unparticle stuff described by such a theory might actually exist in our world. I suggest a scenario in which some details of the production of unparticle stuff can be calculated. I find that in the appropriate low-energy limit, unparticle stuff with scale dimension d[script U] looks like a nonintegral number d[script U] of invisible particles. Thus dramatic evidence for a nontrivial scale invariant sector could show up experimentally in missing energy distributions.
Were it anywhere other than Physical Review Letters, I would suspect that this was a put-on, just to mess with low-energy experimentalists. It's an editor's recommendation at PRL, though, so if it is a gag, it goes all the way to the top of the profession.
I'm aware that this thing has been on the ArXiV for months, and has generated a number of papers in response. Which hopefully means that somebody out there understands what it's about, and can explain it to me as if I were a rather stupid child of four, because I've got nothing, here.
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That's a nice challenge, I'll take a crack at it. The basic point is that particles are derived entities in quantum field theory, the basic entities are fields, which are continuous media. In the familiar case where the fields are weakly interacting one can choose a useful basis of states which can be interpreted as having localized excitations interacting weakly (a.k.a. particles, or sometimes quasi-particles). This is a useful set of states because it approximates the low energy states of the system pretty accurately.
However, there are many more possibilities for the low energy states of quantum fields, especially when they are not weakly interacting (e.g. QCD where the fundamental fields -quarks and gluons- do not correspond to particles, similarly in many strongly coupled electron systems). In many cases the observable (low energy) physics does not correspond to a bunch of quasi-particles interacting freely.
So, the state mentioned in the abstract is one such state, almost the opposite of the familiar many-particle state. It is a state that has scale invariance, and the number of particles in it is indefinite- therefore the particle basis is not very useful. Or in the theorist jargon, it has no particle interpretation.
(Not sure how this is used for low energy phenomenology, what problems it is useful for, that is another story...)
Unparticle is a godawful name, however.
Off-topic, but the aspect of particles being not only derived entities, but observer-dependent is a difficult one for me.
Unparticle is a godawful name, however.
chad orzel 6.2.07 blog reporting doubleplusungood refs unparticles rewrite fullwise
I first saw it on a general interest science site and I was wondering if it was some kind of post modernism new-age type thing that got past the editors. I am not a physicist but I can often tell if something is the product of a fringe theorist. I was half and half on this one but if it was in Physical Review Letters it must at least be worthy of rebuttal. I hope that if there is something to it that someone learns how to explain it. I can understand the term "scale invariant" but other than that it might as well be in hieroglyphs.