Muon G-2 Experiment

The famous 50-foot-diameter superconducting muon storage ring.

The famous 50-foot-diameter superconducting muon storage ring.

 

Muon G-2 experiment is a very recent experiment that will be conducted using the accelerator in Fermilab. The experiment is a continuation of G-2 experiment in Brookhaven National Laboratory (BNL) in order to unveil the mystery of the discrepancy between the theoretical and the measured value of “g.”

 

G value is the ratio between the magnetic moment and the spin of a specific particle. It was predicted in Paul Dirac’s theory that g value is exactly two for muon. The anomalous, however, exists due to interactions of muon involving virtual particles. Thus, a more precise measurement of g value would reveal information about virtual particles. The sea of virtual particles is believed to exist even in the vacuum. These particles, abiding the law of quantum physics, have rather ephemeral period of existence and thus explains why vacuum appears to be empty to us. This understanding is widely accepted by the science community. Many hope the Muon G-2 experiment will unveil unexpected yet crucial evidences of the unknown subatomic world beyond the border of the Standard Model.

 

Eager to explore the unknown, scientists at BNL initiated the E821 G-2 experiment. The measured and predicted values differ with a significance of about 3.5σ, which is not enough(5σ) to confirm a real discrepancy. The E-989 muon g-2 experiment at Fermilab aims to measure the anomalous magnetic moment of the muon to unprecedented accuracy.  Sensitive detectors installed along the storage ring will measure the orbits and the precessions of these positrons to 140 parts per billion (ppb.) To put in a perspective, 140 ppb, or 0.14 ppm, is a fourfold improvement over the previous experiment at BNL. According to Ph.D. Lee Roberts, one of the co-spokespersons, the experiment will use the Fermilab Main Injector to inject the muon beam into the relocated Brookhaven muon storage ring. The ultimate goal for the experiment is a factor-of-20 increase in statistics and a significant reduction in systematic uncertainties compared to the BNL experiment.

The accuracy Fermilab aim to achieve

The accuracy Fermilab aim to achieve

 

The following paragraph is an excerpt from the Fermilab Muon g-2 page:

“A muon has an internal magnet, sort of like a miniature bar magnet. It also has an angular momentum, called spin, much like a spinning top. The strength of the magnet and the rate of the magnet’s gyration determine the muon’s gyromagnetic ratio ‘g’. When placed in a magnetic field, the muon’s internal magnet wants to rotate itself to align along the magnetic field axis like a compass that aligns with the Earth’s magnetic field. However, the muon’s angular momentum prevents this from happening. Instead, the muon’s spin axis rotates, or precesses, about the magnetic field axis. This is similar to a spinning top whose spin axis is not exactly vertical — angular momentum prevents the top from tipping over due to gravity. Physicists can predict precisely the precession rate of the muon’s spin axis about the magnetic field axis.”

Depiction of How the g value is measured (illustrated by BNL)

Depiction of How the g value is measured (illustrated by BNL)

The famous storage ring is considered by many the key to the project. To save both time and budget, physicists decide to transport the already built magnetic storage ring from BNL, New York to FNAL, Illinois. Due to the delicacy of the magnet, the transportation has to take a detour and thus the magnet has spent more than a month on its way to the new home. The so called “Big Move” has drawn much publicity and attention to this unique experiment. According to the plan, the magnet will be powered in 2015 and the beam will be sent as early as the end of 2016.

The route of the Big Move

The route of the Big Move

http://www.liv.ac.uk/particle-physics/experiments/gminus2/

http://www.hep.ucl.ac.uk/muons/g-2/

http://muon-g-2.fnal.gov/2-the-physics-of-g-2.shtml

http://www.g-2.bnl.gov/

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