This text was initially printed at The Conversation. The publication contributed the article to House.com’s Knowledgeable Voices: Op-Ed & Insights.
Zoltan Fodor, Professor of Physics, Penn State
When the outcomes of an experiment don’t match predictions made by the very best principle of the day, one thing is off.
Fifteen years in the past, physicists at Brookhaven National Laboratory found one thing perplexing. Muons – a sort of subatomic particle – have been transferring in surprising ways in which didn’t match theoretical predictions. Was the speculation unsuitable? Was the experiment off? Or, tantalizingly, was this proof of latest physics?
Physicists have been attempting to unravel this thriller each since.
I am a theoretical physicist and the spokesperson and one among two coordinators of the Budapest-Marseille-Wuppertal collaboration. This can be a giant–scale collaboration of physicists who’ve been attempting to see if the older theoretical prediction was incorrect. We used a new method to calculate how muons work together with magnetic fields.
My staff’s theoretical prediction is totally different from the unique principle and matches each the outdated experimental proof and the brand new Fermilab knowledge. If our calculation is appropriate, it resolves the discrepancy between principle and experiment and would recommend that there’s not an undiscovered drive of nature.
Our result was published in the journal Nature on April 7, 2021, the identical day as the brand new experimental outcomes.
Associated: A tiny, wobbling muon simply shook particle physics to its core
The muon and the Normal Mannequin
The muon is a heavier, unstable sister of the electron. Muons are throughout us and are, for instance, created when cosmic rays collide with particles in the Earth’s atmosphere. They can move by way of matter, and researchers have used them to probe the inaccessible interiors of buildings from giant volcanoes to the Egyptian pyramids.
Muons, like electrons, have an electrical cost and generate tiny magnetic fields. The power and orientation of this magnetic subject is known as the magnetic second.
Nearly every part within the universe – from how atoms are constructed to how your cellphone works to how galaxies transfer – might be described by 4 interactions. You might be most likely conversant in the primary two: gravity and electromagnetism. The third is the weak interaction, which is answerable for radioactive decay. Final is the strong interaction, the drive that holds the protons and neutrons in an atom’s nucleus collectively. Physicists name this framework – minus gravity – the Normal Mannequin of particle physics.
All interactions of the Normal Mannequin contribute to the muon’s magnetic second and every achieve this in a number of other ways. Physicists very exactly know the way electromagnetism and the weak interaction achieve this, however figuring out how the robust interplay contributes to the muon’s magnetic subject has confirmed to be extremely onerous to do.
A magnetic thriller
Of the entire results that the robust interplay has on the muon’s magnetic second, the biggest and in addition hardest to calculate with the required precision is known as the Main Order Hadronic Vacuum Polarization.
Up to now, to calculate this impact, physicists used a combined theoretical–experimental method. They’d accumulate knowledge from collisions between electrons and positrons – the other of electrons – and use it to calculate the robust interplay’s contribution to the muon’s magnetic second. Physicists have been utilizing this method to further refine the estimate for decades. The most recent outcomes are from 2020 and produced a very precise estimate.
This calculation of the magnetic second is what experimental physicists have been testing for many years. Till April 7, 2021, probably the most exact experimental outcome was 15 years outdated. For this measurement, at Brookhaven Nationwide Laboratory, researchers created muons in a particle accelerator after which watched how they moved by way of a magnetic subject utilizing an enormous, 50-foot-wide (15-meter) electromagnet. By measuring how muons moved and decayed, they have been in a position to instantly measure the muon’s magnetic second. It got here as fairly the shock when Broohaven’s 2006 direct measurement of the muon’s magnetic moment was bigger than it ought to have been in keeping with principle.
Confronted with this discrepancy, there have been three choices: Both the theoretical prediction was incorrect, the experiment was incorrect or, as many physicists believed, this was an indication of an unknown drive of nature.
So which was it?
My colleagues and I selected to pursue the primary choice: The speculation may be off not directly. So, we determined to attempt to discover a higher technique to calculate the prediction. Our staff of physicists took probably the most fundamental underlying equations of the robust interplay, put the equations on an space-time grid and solved as lots of them as doable directly.
The approach is sort of like making a climate forecast. As business aircrafts fly their routes, they measure stress, temperature and the pace of wind at given factors on Earth. Equally, we positioned the robust interplay equation on a space-time grid. The climate knowledge at particular person factors are then put right into a supercomputer that mixes the entire knowledge to foretell the evolution of the climate. Our staff put the robust interplay forces on a grid and regarded for the evolution of those fields. The extra planes gathering knowledge, the higher the prediction. On this metaphor, we used billions of airplanes to calculate probably the most exact magnetic second we may utilizing thousands and thousands of laptop processing hours at a number of supercomputer facilities in Europe.
Our new method produces an estimate of the power of the muon’s magnetic subject that carefully matches the experimental worth measured by the Brookhaven scientists. It basically closes the hole between principle and experimental measurements and, if true, confirms the Normal Mannequin that has guided particle physics for many years.
However my colleagues and I’ve not been the one ones pursuing this thriller. Different scientists, like the ones at Fermilab, a particle accelerator near Chicago, have chosen to check the second choice: that the experiment was off.
At Fermilab, physicists have been persevering with the experiment that was finished at Brookhaven to get a extra exact experimental measurement of the muon’s magnetic second. They used a extra intense muon supply that gave them a extra exact outcome. It matched the old measurement almost perfectly.
The Fermilab outcomes strongly recommend that the experimental measurements are appropriate. The brand new theoretical prediction made by my colleagues and me matches with these experimental outcomes. Whereas it could have been thrilling to find hints of latest physics, our new principle appears to say that this time, the Normal Mannequin is holding up.
One thriller stays although: the hole between the unique prediction and our new theoretical outcome. My staff and I imagine that ours is appropriate, however our result’s the very first of its kind. As at all times in science, different calculations should be finished to substantiate or refute it.
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