I've just come back from a trip to the LHC (more popularly known as "the black-hole machine," of course). I arrived quite soon after the official announcement that the collider will re-start in November (or perhaps not, as those who believe that the LHC is being sabotaged by its future self claim), so I was pleased that I got a chance to visit the CMS detector before it's all closed up again. (I was less pleased when I ended up getting locked down there by the overzealous security doors. Getting trapped in a physics experiment never ends well in fiction, as Bruce Banner and Jon Osterman aka Dr Manhattan, pictured, will testify.)
Aside from the usual chat about the LHC repairs, I met with Albert de Roeck who is co-ordinating the hunt for exotic particles with the CMS detector. By now, I thought that everything that could be said about the sort of things they are hunting for (the Higgs, supersymmetry, extra-dimensions, mini-black holes) has been said. But de Roeck was energised because he had just come out of a meeting with some theoretical physicists about the possibility of spotting a new entity, unlike any other that has yet been conceived. Proposed in 2007 by one of the founding fathers of supersymmetry, Harvard's Howard Georgi, this weird type of matter is dubbed the "unparticle."
Unparticles are close to my heart because last year I covered them in a story for New Scientist. "What exactly are unparticles?" you ask. That's not such an easy question to answer. I must have asked about 30 different researchers, including Georgi, when I wrote the article and the answers I got ranged from "matter that's unlike a particle," to "something that can't even be described because it is unlike anything we are used to thinking about." One facet of their weirdness is that they have no fixed identity, but morph between having different masses, depending on how you measure them. So far, they have been invoked to explain dark matter and the asymmetry between matter and anti-matter in the universe, among other things.
Unparticles would only very weakly interact with ordinary matter (explaining why we haven't seen them yet) and their shifting identity naturally makes them tough to pin down in accelerator collisions. If they exist, they won't be seen directly, but like neutrinos, they will show up as missing energy in particle interactions. In the case of unparticles, this missing energy signature will be even more bizarre because it will seem to have been caused by a fractional number of particles. De Roeck is making sure that the CMS exotica team are ready to pounce if such signs are seen after the LHC starts up.
That said, unparticles are highly unlikely to exist. Since they were proposed they have been met with equal helpings of excitement and cynicism. While many (many) theoretical physicists have been feverishly writing papers analysing their possible effects on cosmology and various possible signatures at the LHC, others have rolled their eyes and criticise the whole unparticles program as a cheap way to generate new research papers and citations by jumping on a fantastical bandwagon.
A lot of the cynicism seems to derive from the motivation (or lack of motivation) behind unparticles. Although, as I mentioned, people are now looking at whether unparticles could explain various cosmic mysteries, Georgi didn't originally posit them as a solutions to these problems--or indeed as solutions to any problem at all. Rather, he sat down and began pondering what sort of things could conceivably show up at the LHC that nobody had yet considered. It's an upside-down approach to physics: thinking of things that _might_ be out there, just for the sake of it, without there being any _need_ for such entities to exist. But perhaps criticism of this approach is unfair--why shouldn't profound answers about reality come about just because someone sat down and wondered "what if?" How many times has that happened in the history of physics? (That's not a rhetorical question. I'm interested in the examples that you might have.)
So I found it interesting that despite the scepticism, both the CMS and ATLAS groups are now planning to begin an earnest quest for unparticles that will run alongside their searches for more conventional exotica (if that's not an oxymoron). Both experimental groups have been using the LHC downtime to prepare their experiments for any eventuality--and the possibility of unparticles has added a bit of new excitement to their daily routine.
Right now, de Roeck isn't placing unparticles at the top of his list of "things most likely to be found at the LHC." But he doesn't blame theorists such as Georgi for pondering about them. "It's our fault really. We haven't got the machine running and while they are waiting, theoretical physicists are bound to start thinking up ever more imaginative scenarios and possibilities," he told me. "It's up to us to start the experiment and show that these things aren't there."