Finding things that shouldn’t be there: or new evidence for a fifth fundamental force

A big part of being an experimentalist is looking for things that shouldn’t be there. If every test you do comes out exactly as you planned, you haven’t learned anything. Currently we have a rather robust theory that covers particle physics. It is so robust in fact, that it is dubbed The Standard Model. But there are some problems with it. Mainly that it doesn’t include gravity. There is also hope that the strong nuclear force (QCD) and the electroweak force can be framed in a way that unifies them as the electroweak force is a unification of electromagnetism and the weak nuclear force. But we don’t know how to do that yet.

One way for researchers to find new ways is to find new things that happen in the universe that are unexplained by our current theories. One such unexplained happening is the anomalous magnetic moment of the muon.

It works like this: Many of the fundamental particles have a magnetic moment. One way to think of this is that they act rather like tiny spinning bar magnet. (This is actually totally backward, as the combination of spinning particles are what create the bar magnets you are imagining!) One can calculate these magnetic moments using funny pictures that represent a lot of math. You still need to do the math, but the pictures help. For many particles, this calculation has been done. And for a smaller set of those, they’ve been measured. For the electron, theory and experiment agree to incredibly high precision(1).

But for the muon, there’s a problem. They do NOT agree. Although the difference is tiny, it is significant. Statistically significant at the level of 3.4σ. Just to break that down, that means they calculate the chance that the physics we know is right but the measurement just happens to be off is .07%! Not very likely.

That suggests that there is something going on we don’t know about. But just one clue doesn’t answer everything. In fact, it doesn’t answer anything at all, but it does lead us to ask some new questions. The main question is: What the heck is causing that? A commonly proposed answer is: a new fundamental force, a.k.a the Fifth Force.

A fifth fundamental force would be ground breaking. And it’s been brought up off and on since the late 70s. (I’ll be posting a review of The Rise and Fall of the Fifth Force soon, for more details.) It suffices to say that there’s been a lot of strange things measured just at the edges of our knowledge. It keeps suggesting there is something happening we don’t understand. Theorists have been throwing ideas at the wall for decades to see if anything sticks, but it is hard to come up with a working theory that does some predictions on such little information.

Recently a new paper(2) hit the arXiv (pronounced archive) with a new measurement suggesting that some of these theories are on the right track. What’s the measurement? The element beryllium is pretty rare. With 4 protons and 5 neutrons it is stable. But if you hit it with another neutron it’ll pop another one out making it incredibly unstable. It decays almost instantly from beryllium-8. When measuring this decay they’ve found another anomaly. They suggest that there is a new mediator that would be a gauge boson, but not one that we already know about. They report is anomaly at 6.8σ, which is saying that the chances this would just be a fluke is .000000001046%

The paper suggests that the same mediator here would cause the anomalous magnetic dipole moment of the muon. So what’s this about a ‘gauge boson’? We’ll that’s just a fancy way of talking about a force mediator. Those are things like photons (the mediator of the electromagnetic force) which is also called light, and the gluon and W and Z bosons. This is how particle physics does the good old F = ma thing. And the gauge bosons are the force. So if they found a new type of gauge boson, that means a new fundamental force.

(1) B. Odom, D. Hanneke, B. D’Urso, and G. Gabrielse, New Measurement of the Electron Magnetic Moment Using a One-Electron Quantum Cyclotron, Phys. Rev. Lett. 97, 030801 (2006)

(2) Feng et al. Evidence for a Protophobic Fifth Force from 8Be Nuclear Transitions

Author: The Other St. Louis Arch

My name is Adam J. Archibald. I’m a 7th(?) year graduate student at Washington University in Saint Louis. The bulk of my work has been focused on the development of a torsion balance experiment to investigate gravitation at distances below a centimeter. I've also devoted a great deal of time on an experiment probing for violation of the Weak Equivalence Principle. Outside of the lab I enjoy board games, rock climbing, playing music, reading, and occasionally writing.

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