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The FermiLabs G-2 experiment and what it actually means

  • Writer: Sriyashas Kalluri
    Sriyashas Kalluri
  • Apr 14, 2021
  • 3 min read

Updated: May 19, 2021


Over the past few days, a single announcement has taken the scientific world by storm and news about it seems to be popping up everywhere. This announcement, made by FermiLab was about the results of their G-2 experiment regarding the ‘anomalous magnetic dipole moment of the muon’.


There have been quite a few articles published about this discovery in various popular newspapers, talking about what it's results imply(most of which are right and I’m not going to question that), but let’s be real, most people don’t know what the standard model or a muon is and most of this just passes over their heads.


Now, this is intended towards the guy who doesn't know what any of this means and wants to know why people are making a big fuss over this. So, if you came here looking to find a large technical essay, I’m sure the original announcement which I have put a link for at the end will interest you.


The standard model


To put it simply the standard model is a framework that defines and segregates all the particles and forces we know of till now(except gravity but that’s a separate problem). It has successfully explained and weathered through most experiments and has proven to be incredibly useful.





Picture of the standard model (taken from google)



What Anomalous Magnetic Dipole Moment of the Muon means


Those six words may seem overwhelming, so let’s break it down into Anomalous, Dipole Moment and Muon (Explained in reverse order).


Muon - To put it simply, a Muon is the heavier cousin of the electron with stability issues. It has the same charge properties, etc except it’s about 207 times heavier and lasts for about 2.2 microseconds before decaying away


Magnetic Dipole Moment - When an object with 2 distinct magnetic poles is placed in another magnetic field, it turns to align itself with the field(sort of like a compass). The strength of interaction with the magnetic field (and by extension torque) to turn and align itself is called the Magnetic Dipole Moment of an object. The same goes for particles like electrons and muons


Anomalous - The anomalous part of the name is a little complicated. When the Dipole Moment of particles like electrons is to be calculated, since they don’t actually have angular momentum but have a property called spin, the answer for conventional bodies is multiplied by something called the G-factor. This G- factor was worked out to be exactly 2, but further calculations showed its value was actually 2.00159…. This 0.00159 is called the Anomalous Magnetic Dipole Moment (Hence the name G-2)



The Experiment itself


The experiment involved using a particle accelerator to produce muons and then analyze their Dipole Moment. But, when the experiment concluded, their results were slightly offset from the calculated value. There is a massive caveat here though.


The degree of sureness that the result wasn’t just an anomaly was only 4.2𝜎 while the significance required was 5𝞂. Although this means that it is not official yet, it still is exciting. Partly because this was a repeat of another experiment at Brookhaven National Laboratory in 2001 where they got the same result but only to 3.7𝞂.



Diagram of a Particle Accelerator(taken from symmetry magazine)


What the Results Mean for Us/Conclusion


You probably are thinking why people are making a big fuss just because the results of an experiment didn’t match with theory.


The reason people are so excited is because the theory accounted for all known particles and still didn’t match the experiment. This means that there is something else we don’t know that is causing a shift in interactions. Possibly new particles, forces or even a new branch of physics


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