Muons on the Move

Thanks, G, for the excellent questions. I'll do another post later on about the physics behind g-2 (both past and future), but in the mean time one of our experts weighed in on your comment. Physicist Bill Morse worked on the experiment here at Brookhaven, and he'll stay involved with it at Fermi. He gets technical, but I'll flesh out some of the finer points in a future post.

On the anomaly seen when the experiment ran at Brookhaven Lab:

A charged spin ½ particle creates a tiny magnetic field. That magnetic field is given by the magnetic moment = egS/2m. All the physics is in g, the rest just makes the units come out right, well except for the 2. The Dirac equation predicted g=2 for spin ½ point particles. The electron has g = 2.002…, where the 0.002… is the anomalous magnetic moment (g-2) due to virtual particles. However, the proton g value was measured in the 1930s to be 5.6! This was finally explained in the 1960s by the quark model. So maybe the muon is made up of other stuff also (predicted by some models called prions). There is another muon anomaly found at the PSI Lab. Muonic hydrogen (bound state of a muon and a proton) gives different spectroscopy from what you would expect from hydrogen (bound state of an electron and a proton). There are models about a new force that couples to muons, but not electrons. Other models go by the names Super-symmetry, and Dark Light.

Does whatever-it-is extend the Standard Model, or does it call some of it into question?

Great question! Some theorists say the former, and some the latter.

On the possibility of new particles:

The LHC discovered the Higgs, the last particle predicted by the Standard Model, but didn’t see Super-symmetry. However, the g-2 interpretation would predict that they wouldn’t have seen Super-symmetry, until they double the LHC energy (specifically for large tan (beta) models). The LHC is now off for two years in order to double their beam energy.

The articles at Fermilab describe muons as decay products of pions, each composed of an electron and two neutrinos. But elsewhere, they say that muons break down into neutrinos and positrons. In either case, is there any theoretical or practical reversibility to the breakdown of muons into neutrinos and positrons or electrons?

Great question! Some theories say yes, and some say no. So far none have been discovered. I actually did one of those experiments, before I joined g-2. That’s a very interesting, but different, story.

As I understand it, the vacuum is basically a froth of virtual particles that pop into and out of existence, and average to a net value of zero. Is there any theoretical lower limit to the longevity or other values of those particles?

This is due to Quantum Mechanics. Bohr said that anyone who thinks they understand Quantum Mechanics, and is not deeply disturbed by it, doesn’t understand Quantum Mechanics. Anyway, the quantum fluctuations in the vacuum go as (dE) (dt) = Planck’s constant. dE is the energy fluctuation. dt is the time it does it. The average value is close to zero; however, the muon’s spin is precessing due to the magnetic moment egS/2m. Suppose the muon briefly turns into a W particle (500 times more massive than the muon with S=1, and a neutrino). The mass and spin are very different, for this very brief time. The TOTAL angle the muon’s spin has precessed will be different, i.e., we don’t have to “catch it in the act” for this extremely short dt, we just know it did it. It’s like you are driving from NY to LA on a paved highway at 65mph, but briefly go over a rubble strip at 64mph. The time it takes to get to LA will be different!