Highly-anticipated Fermilab result strengthens evidence of new physics

A much-anticipated result from an international science experiment provides strong evidence for the existence of an undiscovered subatomic particle or new force.

The result, announced today, from the Muon g-2 experiment at the Fermi National Accelerator Laboratory, provides tantalising evidence that the elementary particles called muons are not behaving in the way they are supposed to according to the leading theory of physics – the Standard Model.

Scientists from the UK, funded by the Science and Technology Facilities Council, have played a vital role in the g-2 experiment. Muon g-2 is an international collaboration between Fermilab and dozens of labs and universities in seven countries, including the UK.

The UK collaboration comprises the universities of Lancaster, Liverpool, Manchester, and UCL and the Cockcroft Institute of Accelerator Science and Technology.

Lancaster University physicists have created novel computer simulations to investigate the motion of the muons.

Dr Ian Bailey, who leads the Lancaster University contribution, said: “Nothing is more exciting to a physicist than finding tensions between theoretical predictions and experimental measurements. High-precision measurements of the properties of particles in dedicated experiments complement the measurements made at high-energy colliders like the LHC, and give us another tool to probe the nature of the Universe. The data collected by the Muon g-2 collaboration continues to be analysed, and we are all excited to see what is revealed next.”

What is Muon g-2?

The Muon g-2 experiment searches for signs of new particles and forces by precisely examining the muon’s interaction with a surrounding magnetic field.

The muon, when placed in the magnetic field, itself acts like a tiny magnetic compass and like a gyroscope, this compass rotates at a certain precise frequency, predicted by the Standard Model.

However, the g-2 collaboration has measured this rotation to be faster than predicted – suggesting that our current understanding of physics is incomplete, and hints at the presence of additional particles or hidden subatomic forces that are manifest in the subatomic quantum fluctuations surrounding the muon.

These high-precision measurements depend on a detailed knowledge of the short-lived muons’ trajectories as they circulate inside the magnetic fields of the experiment.

A decade of waiting

This result has been anticipated for over a decade, since a measurement published in 2006 from an experiment at Brookhaven National Lab stood at odds with the Standard Model.

The highly anticipated result from Fermilab pushes the precision of the experiment into uncharted territory in the quest to confirm or refute that finding.

Professor Mark Lancaster of the University of Manchester is the UK lead for the g-2 experiment and an ex co-spokesperson of the experiment.

He said: “This is a long awaited measurement and provides strong evidence that hints at the existence of new particles and forces.

“We know our current list of fundamental particles and forces is incomplete because they are not sufficient to explain the dark-matter content of the universe or that the universe has very little anti-matter.

“This measurement alone has a one in a thousand chance of being a statistical fluke and one in 40 thousand when combined with the previous measurement from Brookhaven. It falls short of the one in a million gold standard used in particle physics to claim a discovery, but we are already analysing much more data that will significantly improve the precision of the measurement by more than a factor of two.”

This result, along with other recent developments including the recent LHCb result, could suggest that scientists are on the precipice of a new era of physics.

Professor Lancaster added: “When viewed together with the recent measurements from CERN’s LHCb experiment, there seems to be a pattern emerging of muons behaving differently than our theory predicts.

“This measurement is the most precise measurement ever to be made at a particle accelerator: it has an uncertainty of better than 0.5 parts per million.”

STFC has supported the UK’s role in this experiment since 2014 and UK physicists have had a very significant role in the experiment, providing one of the two main detector systems and one of the experiment’s co-spokespersons from 2018-2020.

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