Scientists from the UK have played a crucial role in the world's most precise measurement of the muon magnetic anomaly, according to results released today by the Muon g-2 collaboration at Fermilab hosted by the US Department of Energy's Fermi National Accelerator Laboratory.
UK scientists from the universities of Lancaster, Manchester, Liverpool and University College London made substantial contributions to the experiment, which involved 176 researchers from 34 institutions across seven countries.
The experiment measures the anomalous magnetic moment of the muon, known as g-2, which provides an extremely sensitive test of the Standard Model of particle physics.
The final measurement reaches a precision of 127 parts-per-billion, surpassing the original experimental design goal of 140 parts-per-billion. This represents the culmination of over a decade's work and will likely stand as the definitive measurement for many years to come.
The UK teams, funded by the Science and Technology Facilities Council (STFC), the EU, the Royal Society and the Leverhulme Trust, built one of the two key detector systems for the experiment.
The UK-built straw tracking detectors were crucial in tracing the motion of the muon beam - a central element of the analysis while Lancaster University physicists created novel computer simulations to investigate the motion of the muon.
Dr Ian Bailey, who leads the Lancaster University contribution, said: "This beautifully precise new measurement confirms that the muon can tell us something important about the nature of our Universe. It could be another victory for the Standard Model of Particle Physics, or the tension between measurement and theory could resurge as the theory is understood better. Particles don't give up their secrets easily. We'll need to wait a little longer to fully understand the significance of the result."
STFC's Professor Sinead Farrington, Director of Particle Physics, said: "The UK has played critical roles of which we can be proud, both in leadership and in developing the straw tracking detectors, in this highly international collaboration."
Muons are similar to electrons but about two hundred times more massive; like electrons, muons have a quantum mechanical property called spin that can be interpreted as a tiny internal magnet. In the presence of an external magnetic field, the internal magnet will wobble - or precess - like the axis of a spinning top.
The precession speed in a magnetic field depends on properties of the muon described by a number called the g-factor. Theoretical physicists calculate the g-factor based on the current knowledge of how the universe works at a fundamental level, which is contained in the Standard Model of particle physics.
The experiment continues a century-long scientific quest. Nearly 100 years ago, the g-factor was predicted to be exactly 2, but experimental measurements showed it to be slightly different - hence the name "g-2" for the experiment.
UK researchers contributed significantly to the magnetic field measurement, another critical component of the analysis. The result sets a high benchmark for any future theoretical calculations and will likely remain the world's most definitive measurement until the 2030s.
