The recent publication in Phys.Rev.Lett. 126, 141801 Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm says that

A fast pulsed-kicker magnet deflects the muon bunch into a 9-cm-diameter storage aperture, resulting in $\approx 5000$ stored muons per fill.

Considering that those muons are moving with almost the speed of light ($\gamma\approx30)$ and at rest they would only last about $2.2\ \mu s$ on average before decaying, catching one would be no mean feat.

What am I missing?

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    $\begingroup$ Very briefly... $\endgroup$
    – tparker
    Apr 17, 2021 at 17:06

2 Answers 2


As the paper says, the muons are deflected with a magnet into a storage ring, where they are stored for a fraction of a second. The muons are still moving with nearly the speed of light, which extends their average lifetime by a factor of $\gamma$.

A small fraction of a second later, the muons have all decayed and the storage ring is ready to receive another fill of muons.

It sounds like you might be under the impression that the muons are brought to rest or stored for a long period of time. Neither is true- they are stored at high speed and for a small fraction of a second. Granted, hundreds of microseconds is an eternity by the standards of many particle physics experiments.

  • $\begingroup$ Is it possible to store them for longer? Has anybody thought about what techniques it might require? $\endgroup$ Apr 18, 2021 at 7:02
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    $\begingroup$ @TomLisankie All that is needed is more speed. Of course, that is a very fluffy usage of the word "all". $\endgroup$
    – PcMan
    Apr 18, 2021 at 7:16

With a Lorentz boost of gamma=29.3, muons lifetime is about 64.4 microseconds, and most of them decay before 700 microseconds after the injection in the ring. This is actually enough time for them to be detected by their decay positrons, considering the fast electronics in use.


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