Why US Fermilab’s findings on ‘muons’ could change the laws of physics as we know them

Fermilab finds that tiny subatomic particle seems to disobey ‘Standard Model’ of physics, but another paper says findings are within theoretical calculations based on this model.

A superconducting electromagnet at Fermilab, Illinois, USA | Representational image: Commons

Bengaluru: Findings from an experiment at the Fermi National Accelerator Laboratory (Fermilab) near Chicago, USA, have thrown up preliminary results that suggest we might have to rethink the basic physics of the universe and what we understand of it.

The experiment was conducted to observe one of the 17 known fundamental particles of the universe, called the muon.

The objective of the experiment was to measure to a precise degree how magnetic muons are, by studying how they wobble in magnetic fields. When the muon was observed in a high-speed collision experiment, called the Muon g-2 experiment, physicists noted that the magnetic field around it was not what our codified laws of physics say it should be.

The latest results come on the heels of similar findings made last month at the Large Hadron Collider in Switzerland. Both experiments show that the muon deviates from the ‘Standard Model’ of physics.

The Fermilab research findings have been published in the Physical Review Letters journal this week. However, another independent paper in the journal Nature states that the muon’s moment is in line with what the Standard Model predicts from theoretical calculations.

The contradictory results make these findings among the most tantalising and fascinating in particle physics in recent times.

Standard Model and subatomic particles

The Standard Model came into being in the 1970s and is the name of a codified set of laws that scientists think govern fundamental subatomic particles and their interactions. It unifies three of the four known fundamental forces — electromagnetic force, strong force, and weak force — but not gravity.

There are 17 named fundamental particles that are grouped into two: The ones that are building blocks of matter (and antimatter) are called fermions, while the ones that govern interactions between fermions are called bosons.

Fermions are further grouped into four: Quarks, leptons, antiquarks, and antileptons. There are 12 known fermions: Up, charm, top, down, strange, bottom, electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino. There are five known bosons: Eight types of gluons, the photon, W, Z, and Higgs.

Back in the 1970s, the Standard Model laid out everything that was known about subatomic particles, and also predicted the existence of newer undiscovered particles.

But the model is not perfect. It offers no explanation for dark matter and dark energy. It predicted the Higgs boson, but was incorrect about the particle’s mass. And it could potentially hold more errors than believed so far.

To evaluate how reliable or accurate the theory is, experiments are conducted to compare results with theoretical predictions. These experiments are typically conducted in particle accelerators or colliders which can simulate extremely high energies at which these elementary particles can be observed.

Experiment and findings

Elementary or fundamental particles rotate or spin in magnetic fields twice as fast as larger objects. This is their ‘gyromagnetic ratio’, and thus, the experiment at the Fermilab was called the ‘Muon g-2’.

Muons are special because understanding their behaviour and properties can give a tremendous boost to understanding the fundamentals of the universe, because it constantly emits or absorbs transient elementary particles that move with it, such as a photon or an electron.

Since muons constantly seem to move with a number of other particles, they could also be moving with particles we are as yet unfamiliar with under the Standard Model. This could potentially offer an explanation to the large chunk of mass missing from the universe, which is called dark matter.

In muon magnetic experiments, high speed muons are shot into a magnetised ring that they zip around in. As they pass through the magnetic field, they rotate. After a muon moves around the ring several hundred times, it decays, emitting an electron that then is detected inside the ring. The entire process takes less than a millionth of a second.

By measuring the energy of the emitted electrons, physicists can calculate and deduce how fast the original muon was spinning, and thus put a definitive number to its g-factor or gyromagnetic ratio.

Since the 1990s, physicists have been attempting experiments to narrow down this number to a precise decimal value, and every finding has deviated from values predicted by the Standard Model.

When a measurement overshoots the predicted margin of error, there is a ‘sigma deviation’. When repeated measurements overshoot the margin by five times the allowed value, it is called a five-sigma deviation, and this is when a definitive new discovery beyond the Standard Model can be claimed.

In the 1990s, the Brookhaven National Laboratory in Long Island announced the first results for the muon’s g-factor with a three sigma deviation. Combined with the newer findings from Fermilab, the results show a 4.2 sigma deviation.

The combined results from Fermilab and Brookhaven show a difference with theory at a significance of 4.2 sigma. The chance that the results are a statistical fluctuation is about 1 in 40,000. #gminus2https://t.co/tUx4oji70S

— Fermilab (@Fermilab) April 7, 2021

This data is still from the first run of the experiment, and researchers are continuing to process data from two other runs. The fourth run is currently ongoing, while a fifth is also planned. The findings, made by a team of over 200 researchers from seven countries, hint at a very exciting new world of undiscovered physics.

The contradictory paper

However, in the paper published in Nature, a team of theoretical physicists known as BMW present new supercomputer-powered calculations that show uncertainties outlined by the Standard Model’s prediction of the muon’s magnetic moment. The team, using a technique known as lattice calculation, computed the muon’s magnetic moment and concluded that the results from Fermilab are very much in keeping with the Standard Model.

This differs significantly from findings made last year by another group called Theory Initiative, who calculated the uncertainty to a much narrower value, honing it down through the years.

But even if the BMW group’s findings are correct, there still remains a huge discrepancy between the observed and predicted values of the muon’s g-factor, which could still potentially awaken a new world of physics.

(Edited by Shreyas Sharma)