ArgonneLab132 karma2021-04-21 16:16:09 UTC
In particle physics, we have one theory that describes all our understanding of all particles. We call it The Standard Model.
This theory allows the very precise calculation of how the muon interacts with a magnetic field. It turns out that all particles that exist contribute to this.
The (Fermilab Muon g-2) experiment measures this interaction very precisely. Our measurement, as did the predecessor, found that the measurement and the calculation from the theory probably don't agree. This means that the Standard Model is missing some particles or forces.
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ArgonneLab55 karma2021-04-21 16:06:01 UTC
There is an independent experiment in Japan at J-PARC under construction. If everything goes according to their plan, first results are expected ~2025. (link: https://g-2.kek.jp/portal/index.html)
In addition we (Fermilab Muon g-2) took and are still taking more data. This first publication is only roughly 6% of the expected full dataset.
ArgonneLab45 karma2021-04-21 16:47:30 UTC
There is a lot of work to be done on the theory side, too.
The new Lattice QCD calculation done by the BMW collaboration is quite new, and it will take time for the High Energy Physics community to investigate and evaluate their Muon g-2 value and uncertainty.
Just looking at the uncertainties of the experiment, the BMW calculation and those calculations using more conventional methods, no conclusions can be drawn yet.
Stay tuned! The theory community is working hard on it, and we are eager to see what they come up with, too.
ArgonneLab23 karma2021-04-21 16:38:26 UTC
The goal is to reduce the uncertainty of the experimental measurement by roughly a factor of 4.
How many sigmas that will correspond to depends on what the value will exactly be and on future work on the theoretical prediction/calculation. We hope to achieve 5 sigmas. We consider 5 sigmas a discovery.
ArgonneLab21 karma2021-04-21 16:09:35 UTC
The spin is accounted for in the Standard Model. The new force, if it exists, will affect how a muon interacts with a magnetic field, which can be perceived through the motion of the muon's spin.
In this experiment, we are watching how the muon's spin moves. More precisely we actually measure how fast the spin precesses, similar to the precession of a gyroscope.
To be accurate, we have not yet confirmed that this new force exists.
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