I've made a couple of oblique references to this over the past couple of months, but I have an article in the new issue of Physics World, on experiments using molecules to search for an electric dipole moment of the electron:
When most of us think about searching for physics beyond the Standard Model - the dominant paradigm of particle physics - the first thing that springs to mind is probably a gigantic particle accelerator like CERN's Large Hadron Collider (LHC). Within the collider's 27-km loop, protons slam together at 99.9999991% of the speed of light. Office-building-sized detectors generate terabytes of data for physicists to sift through, seeking elusive traces of new kinds of particles.
But there is another type of search for new physics under way as well, this time in atomic-physics labs. Using apparatus no more than a few metres in size, and energies a trillion times lower than those at the LHC, these experimentalists are trying to detect new particles, too - by measuring the electric dipole moment (EDM) of the electron.
The logic behind their search is that under the basic Standard Model, a detectable electron EDM is forbidden. Hence, finding a tiny-but-finite EDM would indicate that the Standard Model needs revision, thereby opening the door to a new class of "virtual particles". From an experimental standpoint, the task is not easy: how do you measure something that is almost, but not quite, zero? Yet these EDM searches may nevertheless be our best chance of discovering new physics until the LHC reaches its full potential - and perhaps even beyond then.
I believe the article is freely available (registration may be required)-- at least, it didn't prompt me for subscriber information, and I'm pretty sure we don't have an institutional subscription. In which case, you should click the link, and read the whole thing...
This was my first time writing a science article for a magazine-- I've done technical journal articles, and a couple of pieces for Inside Higher Ed, but no short general audience science pieces (the book is much longer, and a very different process). I was impressed by how quickly the whole thing went-- I agreed to do the piece in September, turned in a first draft October 12, went through a couple of rounds of edits, and was looking at page proofs by mid-November. An academic journal would be sending the second reminder to the referees at that point...
I think it turned out pretty well. Dave DeMille at Yale, Ben Sauer at Imperial College, London, and Chris Regan at UCLA were kind enough to answer my questions (some of them pretty dumb), and I hope I've done an adequate job of representing their research accurately. All the inevitable errors are undoubtedly mine.
Physical Science
Comments
Very nicely written article Chad, thanks and I enjoyed it. I was able to access it freely FYI. Prompted me to find the paper by Commins to read.
Posted by: MHC | December 2, 2009 3:48 PM
Nice article.
"The [DeMille] team expects to match or exceed the sensitivity of the Berkeley experiment in early 2010," but it seems like they've had a big improvement just around the corner for a few years now. Much like the LHC, one worries that it will always be a fixed amount of time in the future. But maybe things are changing now....
Your article made me wonder why no one ever quotes a precise value of the Standard Model prediction, just an upper bound. The reason seems to be that it doesn't arise until four-loop order, and would require a real technical feat (or just a huge amount of patience) to calculate. There's a wrong calculation in the literature by Hoogeveen, who thought it arises at three loops. Looks like the right order of magnitude should be 10^-41 or 10^-40 e cm.
Posted by: onymous | December 2, 2009 4:35 PM
The fact that you can do "particle physics" with small scale (if not tabletop) experiments just does not get out there very often, so I'm glad to see you produce such a well-written article on the subject.
The best part of it, by the way, is that the lede makes your point and the tedious detail is down where it doesn't get in the way of the main message.
Posted by: CCPhysicist | December 2, 2009 5:02 PM
Your article made me wonder why no one ever quotes a precise value of the Standard Model prediction, just an upper bound.
It's damnably difficult to get concrete bounds on any of this stuff. That blotches-of-color plot (Figure 2) is from Dave DeMille, who says it took quite a bit of work to assemble. Having dredged up a couple of review articles on the subject to check the numbers, I can totally believe it.
The best part of it, by the way, is that the lede makes your point and the tedious detail is down where it doesn't get in the way of the main message.
That's the one thing I know about journalism-- never bury the lede.
Posted by: Chad Orzel | December 2, 2009 5:18 PM
Very nice article, Chad. I am fondly recalling a class report I did on the electron EDM for an undergraduate physics class many years ago...
I do have a very minor quibble with one sentence: We know that CP violation must occur, because we have observed a pronounced asymmetry between matter and antimatter in the visible universe: for example, we observe far more electrons than positrons, so a world in which their charges were flipped would look quite different.
First of all, asymmetry in the amount of matter and antimatter in the universe is completely consistent with CP conservation. What is *not* consistent with CP conservation is any change in the total baryon number of the universe -- if CP is conserved, we must assume that the matter-antimatter asymmetry is a feature of the universe that must have existed for all time.
No, we know that CP violation must occur because it has been experimentally observed -- first in oscillations in the neutral K-meson system by Fitch and Cronin in 1964 (for which they won the 1980 Nobel Prize in Physics), and later in other K- and B-meson interactions. It wasn't until after the discovery of CP violation that Andrei Sakharov considered it as one of the conditions under which the matter-antimatter asymmetry could be generated in the early universe rather than having to exist for all time.
Posted by: Alex R | December 3, 2009 5:59 PM
Great article. Even though I still have trouble taking a theory called "Extended Technicolor" seriously.
Posted by: Amit | December 7, 2009 4:58 AM
That was fascinating. I am confused about one part:
Another promising contender is David DeMille's experiment at Yale University. The Yale group employs a different technique of holding lead-oxide molecules in a glass cell rather than sending them through the experimental apparatus in a beam. Although the cell needs to be maintained at about 973 K (a significant technical challenge) ...
Why does the cell need to be so hot? And why exactly that temperature?
Posted by: Grep Agni | December 7, 2009 10:36 AM