A few people last week were linking to this press release from Fermilab, which probably says more about the state of American particle physics than anything else: it’s about an experiment that they expect to be approved in 2012, to break ground in 2013, and start running in 2016. I guess with the Tevatron shutting down and nothing noteworthy from the LHC yet, this is what you have to talk about.
The experiment in question is an update of an experiment from Brookhaven in 2001, which measured the anomalous magnetic moment of the muon. The value they get differs from the best theoretical value by the traditional 3 standard deviations– enough to look interesting, but not enough to be concrete evidence of anything. Ten years of effort haven’t turned up an error in either experiment or theory that would conclusively close the gap, so it’s a mystery.
One of the nice things about writing up particle physics results is that everything is on the arxiv, so here’s the latest experimental value for the muon g-factor:
g/2=1.001 165 920 80(54)(33)
(The two sets of numbers in parentheses are the statistical and systematic uncertainties in the last two digits.) Meanwhile, the latest theoretical value is:
g/2=1.001 165 918 28(49)
One of the annoying things about writing about particle physics results, however, is that they tend to pretend there’s nothing other than particle physics results. In fact, as impressive as those numbers look, they’ve got a ways to go to catch a measurement from my corner of physics, namely the g-factor of the electron, whose best current value is:
g/2 = 1.001 159 652 180 73 (28)
One of the reasons the muon g-factor is interesting is that the electron g-factor has been measured to such amazing precision, and agrees perfectly with theory. In fact, that measurement is better than the best independent measure of the “fine structure constant” which is a key input to the theory, so the paper giving that result (arxiv link) casts it as a measure of the fine-structure constant. The muon’s magnetic moment is more sensitive to exotic physics (the details are a little subtle, but roughly speaking, it’s because it’s a heavier version of the electron, and couples to more things), but if the electron’s magnetic moment didn’t agree to such high precision, it would be hard to argue that the muon discrepancy was all that meaningful.
The electron measurement really is a tour de force, and the theoretical calculation is quite an achievement in its own right, involving up to eighth order Feynman diagrams. And neither side of that measurement is done– The PI on the experiment, Gerald Gabrielse of Harvard, gave a prize talk at DAMOP where he talked about the next generation experiment, and the theory team is already working on the tenth order calculation to match the expected precision of the experiment (and cut down the error bars on the current numbers). It’s worth looking at that paper just for the pages of crazy Feynman diagrams that they have to evaluate to get their result.
So, that’s an important bit of background to remember when reading about this. When it eventually happens, the measurement will be pretty impressive, but its success will build not only on the Brookhaven work, but also the exceptional work of the people making measurements and calculations of the electron’s properties.