First results from Fermilab muon g-2 experiment: more confusion

Particle physicists have been waiting for the results from the Fermilab muon g-2 experiment with great anticipation. Results from the first run of the experiment were announced on April 7, 2021. I congratulate the E989 collaboration for performing this extraordinarily difficult experiment and equally difficult data analysis.

Following the news there was great hype about the first results in the media. Hundreds of newspaper articles, blogposts, and YouTube videos appeared suddenly announcing that experimental proof was found for a new force of nature or new particle. This is not true (not yet). We were hoping for more clarity but the party was spoiled by a group of theorists (known as the BMW collaboration) who published a new theoretical prediction which is very close to the Fermilab result. All the fun was gone 😦

The rationale for the Fermilab experiment was to check the measurement made by the BNL (Brookhaven National Laboratory) 20 years ago and improve the accuracy 4 times. If the difference between the measured value and the predicted value (using the Standard Model of particle physics) was statistically significant (larger than 5 sigma) then we would know that there is physics beyond the Standard Model. Now, the predicted value is in doubt and we have no idea whether the difference is statistically significant or not.

What is the solution? Better experimental accuracy, of course. We need a smaller error bar from the Fermilab experiment.

Accuracy of the Fermilab measurement (1st run) is not an improvement over the Brookhaven measurement yet. Compare the error bars in the figure shown above. The Fermilab error bar will be 4 times smaller when all data from runs 1-5 are combined. By that time the difference between the two theoretical predictions should be resolved as well.

My expectation is that the difference between the measured value and the predicted value will grow to 5 sigma and more.

My previous posts on muon g-2

Theoretical value of muon g-2 updated

Muon g-2 mystery

Science journalists

‘Last Hope’ Experiment Finds Evidence for Unknown Particles (read this first)

Blog posts by physicists

Why is it when something happens it is ALWAYS you, muons?

New Muon G-2 Results!

Muon g-2 Result

Muon’s Escalating Challenge to the Standard Model

Fermilab strengthens muon g-2 anomaly

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