A measurement of the anomalous magnetic moment of the muon to 0.14 ppm using the FNAL g-2 experiment.

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

The electron is the lightest, stable charged particle and its properties are extremely well measured and underpin life through its role in chemical reactions. In 1937 a similar but heavier charged particle, the muon, was discovered in cosmic rays. The muon has been studied for the past 80 years and it seems to behave like a heavier version of the electron with its properties only modified by virtue of it being
approximately 220 times the mass of the electron. It appears, like the electron, to have no structure and is not an excited state of the electron but a distinct fundamental particle.

Its larger mass means it is unstable and decays with a lifetime of 2 x 1/millionth of a second. Like the electron, the muon is charged and has the quantum mechanical property of spin. This in turn means that the muon acts like a subatomic magnet and has a property called a magnetic moment. This microscopic magnetic moment in the case of an electron ultimately determines the macroscopic magnetic properties of a material. The size of this magnetic moment determines the size of the torque that an external magnetic field will exert on the muon. This torque causes the direction of the muon's spin to precess around the direction of the magnetic field with a certain frequency. This frequency is determined by the muon's magnetic moment and it this frequency and hence magnetic moment that we will measure in this project.

We are seeking to measure the magnetic moment of the muon to a precision of 0.14 parts per million which will be over a factor of 4 better than the previous measurement. The reason for making such a precise measurement is that the value of the muon's magnetic moment is very precisely predicted in quantum mechanics and so we can use the measurement to test the predictions of quantum mechanics to a very high level of precision.

We presently know there are 4 types of force or interaction: the strong nuclear force, the electromagnetic force, the weak nuclear force and the gravitational force. The muon is subject to all these forces (interactions) and these in turn affect its magnetic moment. The gravitational contribution is too tiny to be measured but the others are not. Since we know the properties of these forces very well then using quantum theory, we can then predict the magnetic moment of the muon and compare it to experiment. Should the prediction and the measurement differ significantly then that would be evidence that there are new types of interaction or that the muon is not a fundamental particle after all and has some sort of structure. The previous measurement of the muon's magnetic moment from data taken in 2001 was at odds with the prediction such that the probability of them being consistent was only 0.05%. However in science the benchmark for inconsistency is that the chance of them being consistent has to be extremely small (0.0001%). By making a more precise measurement we can better examine this consistency of the measurement and the prediction and determine whether there is indeed evidence of new physics or not.

We will make this measurement by injecting a beam of muons into a circular storage ring (of 7m radius) which is subject to a 1.45 T magnetic field. By examining the direction of the electrons from the muon decay as a function of time (and measuring very precisely i.e. to better than 0.1 parts per million) the magnetic field we can measure the magnetic moment. This will be done in 2016 at Fermilab in the USA.

The UK institutes (Liverpool, UCL, Oxford, QMUL, RAL) will be making key contributions to this measurement. We will build the detectors that measure the muon beam's trajectory, the device to measure the magnetic field and the magnet that injects the beam into the circular storage ring. We hope by 2018 to have completed the measurement and so know whether there is new physics beyond the four known interactions or not.

Planned Impact

See the pathways to impact document submitted as an attachment.

Publications

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Aguillard D (2023) Measurement of the Positive Muon Anomalous Magnetic Moment to 0.20 ppm in Physical Review Letters

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Chislett R (2016) The muon EDM in the g-2 experiment at Fermilab in EPJ Web of Conferences

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Lancaster, M (2014) The New FNAL Muon g-2 Experiment in Proceedings of Science

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Logashenko I (2015) The Measurement of the Anomalous Magnetic Moment of the Muon at Fermilab in Journal of Physical and Chemical Reference Data

 
Description Development of a straw tracking detector and associated electronics capable of measuring the precise trajectory of low energy electrons, positrons and muons with minimal multiple scattering. World's most precise measurement of the muon's anomalous magnetic moment, reaching a global audience of 3B.
Exploitation Route The design of this detector is applicable to other experiments requiring high resolution tracking (at high rate) at low energy.
Sectors Other

 
Description The recently published measurement achieved international publicity with an estimated audience reach of 3B. It was featured in the main evening BBC news and all UK newspapers and radio and in many online resources. A talk was also given to the BEIS.
First Year Of Impact 2021
Impact Types Cultural

 
Description STFC Consolidated Grant
Amount £5,893,418 (GBP)
Funding ID ST/N000285/1 
Organisation Science and Technologies Facilities Council (STFC) 
Sector Public
Country United Kingdom
Start 09/2015 
End 09/2019
 
Description MUSE EU Consortium 
Organisation Helmholtz Association of German Research Centres
Department Helmholtz-Zentrum Dresden-Rossendorf
Country Germany 
Sector Academic/University 
PI Contribution We are sharing best practice on DAQ systems and software development.
Collaborator Contribution Providing access to a beam test facility : ELBE in Dresden.
Impact Outcomes pending.
Start Year 2015
 
Description MUSE EU Consortium 
Organisation National Institute for Nuclear Physics
Country Italy 
Sector Academic/University 
PI Contribution We are sharing best practice on DAQ systems and software development.
Collaborator Contribution Providing access to a beam test facility : ELBE in Dresden.
Impact Outcomes pending.
Start Year 2015
 
Description Interview for BBC News at 6pm and 10pm 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact From market research conducted by Fermilab it was estimated that the 2021 Muon g-2 result reached an audience of 3B through TV, newspapers and online articles. There were articles in all the main UK newspapers and Scientific American, New Scientist etc.
Year(s) Of Engagement Activity 2021
URL https://www.bbc.co.uk/news/56643677