New calculations look to person put a 25-year-old particle physics puzzle to rest. But they’ve besides created a conflict pinch different experimental results.
Introduction
For 25 years, physicists person been puzzled by an evident one-part-in-a-million problem. Their expectations of the measurement that definite particles should wobble successful a magnetic section were clashing pinch what they saw successful experiments. The discrepancy was an electrifying hint that they mightiness beryllium seeing grounds of chartless particles.
Then successful 2021, that hint seemed to evaporate. When researchers updated the measurement they did their theoretical calculations, they recovered that their predictions matched the experimental results overmuch much precisely than before, to 1 portion successful 100 billion.
But that, successful turn, has created different puzzle: The aged calculations look perfectly valid. So why don’t they lucifer the caller calculations? Those older predictions were not purely based connected theory; they were besides inferred from different experiments. If the older calculations conflicted pinch newer results, and the older calculations were based connected experimental data, was thing unusual going connected successful those aged experiments?
One promising hint comes from a particle collider successful Siberia, which has precocious started seeing its experiments dramatically diverge from what it and different colliders saw successful the past. Its results person sparked a flurry of activity arsenic physicists effort to find whether the conflicting measurements are a broadside effect of different experimental procedures, aliases a motion that caller particles are popping up aft all.
Weird Wobbles
The particle astatine the halfway of the enigma is the muon, a heavier relative of the electron. A muon behaves a spot for illustration a mini barroom magnet. Spin 1 successful a circle wrong a magnetic section and the magnetism will make it wobble, tracing retired its own, smaller circles. The sizes of these circles are wished by a number called a “g-factor.”
If the muon sat isolated from different particles, its g-factor would beryllium precisely 2. But quantum mentation requires that each different particles power the g-factor. As the muon wobbles, it releases particles specified arsenic photons, which are excessively short-lived to show up successful detectors. These tin merchandise different particles, which tin merchandise still much particles. The muon quickly reabsorbs each these fleeting particles, and the only trace they time off down is that the muon wobbles a small spot more. Through these intricate chains of emanation and reabsorption, each particle successful beingness has immoderate mini effect connected the activity of the muon.
That makes the precise size of the excess wobble, the muon’s “g–2,” invaluable arsenic a model into the quantum world. “The measurement of muon g–2 is simply a proxy for saying really galore particles beryllium successful the universe,” said Alex Keshavarzi, a elder investigation chap astatine University College London.
So erstwhile an research astatine Brookhaven National Laboratory connected Long Island measured the muon’s g-factor successful 2001, physicists were thrilled that it came retired larger than expected. To some, it hinted that caller particles — possibly moreover particles that could relationship for acheronian matter — were astatine work.
Physicists group retired to cheque the consequence pinch an moreover much precise measurement. In 2013, Brookhaven’s 50-foot-wide magnetic ringing was moved via an elaborate bid of barges and trucks to Fermi National Accelerator Laboratory (Fermilab) successful Illinois, wherever an upgraded type of the research would return moreover much data.
To hole for that caller experiment, physicists besides made a immense effort to understand the theoretical prediction that disagreed pinch the data. Their situation was to understand the muon’s chains of emanation and reabsorption successful utmost detail. In particular, really overmuch do the particles associated pinch each of nature’s 4 basal forces participate successful these chains?
The calculation is straightforward for 3 of nature’s 4 forces. Gravity is truthful anemic that physicists tin disregard it outright. And some the electromagnetic unit and the anemic atomic unit tin beryllium deduced utilizing a modular technique.
The beardown force, however, is not truthful easy to woody with. That unit tightly binds particles known arsenic quarks into composite particles specified arsenic protons and neutrons. Standard theoretical techniques don’t activity connected the beardown force. So physicists person to get creative.
In his doctoral thesis successful 2018, Keshavarzi helped hone an replacement measurement of knowing the beardown force, called the data-driven method. In this method, physicists don’t effort to foretell really often muons will emit and sorb groups of quarks. They spell retired and measurement it.
The main measurement that happens is by colliding electrons and their antimatter partners, positrons. The matter and antimatter annihilate each other, creating different particles, including bundles of quarks. If tons of quarks appear, physicists cognize they person a tight quantum nexus to particles specified arsenic electrons and positrons. In short, the much quarks look successful electron-positron collisions, the much powerfully they will impact the muon.
Using the data-driven method, physicists group retired to cipher the expected size of the muon’s magnetic wobble. That prediction, which was released successful June 2020, sharply differed from Fermilab’s precise experimental measurement, which came retired successful April 2021. The discrepancy was truthful beardown that it astir crossed the stringent period required for physicists to declare they had discovered caller particles.
But a different theoretical calculation would show a different story.
Wrangling Lattices
Not each physicists pursued the data-driven method to cipher the muon’s wobble. Some were moving connected a much purely theoretical method to make their prediction.
The attack resembles what happens successful upwind forecasting. While it is possible, successful principle, to understand the upwind by keeping way of the precise contour of each breeze successful the atmosphere, successful believe that task is absurd. Instead, meteorologists disagreement the ambiance into large boxes — a 3D grid — and cipher really each container changes connected mean complete time.
Likewise, it’s excessively difficult for physicists to support way of each strong-force relationship betwixt each brace of quarks. So physicists usage a method called lattice QCD (short for quantum chromodynamics, the mentation of the beardown force), to usage a large grid to simulate the wide behaviour of quarks.
In 2014, a collaboration among researchers successful Budapest, Hungary; Marseille, France; and Wuppertal, Germany — the BMW group — started connected a task to usage lattice QCD to cipher the muon g-factor.
At first, their predictions were 10 times fuzzier than data-driven inferences. Low-energy particles thin to dispersed out, truthful capturing their imaginable positions requires utilizing a immense lattice. High-energy particles request a comparatively smaller grid, but 1 pinch an highly good mesh. “Back then, it was unimaginable that 1 time lattice would scope the aforesaid precision” arsenic the data-driven method, said Kalman Szabo, a professor astatine Wuppertal who was progressive successful the effort.
It took a decade of processing clever computational techniques — and waiting for accrued computing powerfulness — for the BMW group to wrangle grids that were some sufficiently large and sufficiently detailed. But wrangle them they did. In 2021, connected the aforesaid time that Fermilab released its updated muon g–2 measurement, the BMW group’s result appeared successful the diary Nature.
According to the BMW group’s lattice calculation, Fermilab’s muons were wobbling precisely arsenic they should. Since then, independent lattice groups person published matching calculations.
Today, galore physicists judge the muon enigma is nary more: According to the lattice simulations, the muon’s other wobble tin beryllium explained wholly by the emanation and reabsorption of known particles obeying the known laws of the known forces.
So why does the data-driven method bespeak otherwise?
Inconsistent Experiments
To fig retired what’s going on, physicists are drilling into the electron-positron collisions driving the data-driven method. These collisions are expected to beryllium a nonstop model into quark behavior, but calculations based connected this information disagree pinch some the latest experimental results and BMW’s prediction. So what’s really going connected successful the aftermath of those collisions?
In the metropolis of Novosibirsk successful confederate Siberia, the VEPP-2000 collider has been crashing electrons into positrons connected and disconnected since the move of the millennium. It’s a comparatively gentle collider, operating astatine 6,000 times little power than CERN’s Large Hadron Collider, adjacent Geneva.
The VEPP-2000 features 2 detectors that precisely count really often definite bundles of quarks, known arsenic pions, popular retired of the electron-positron crashes — information that physicists person been utilizing to infer really overmuch the beardown unit was messing pinch muons.
In 2010, physicists installed a wholly caller detector. They past utilized it to much precisely measurement this pion accumulation rate, which they published successful 2023. After the refresh, they recovered that the complaint changed significantly.
Fedor Ignatov, a physicist astatine the University of Liverpool successful the UK, was a personnel of the squad that measured a mysterious caller complaint of pion accumulation astatine the VEPP-2000 collider.
Courtesy of Fedor Ignatov
“It was a surprise. No 1 expected it to beryllium for illustration that,” said Fedor Ignatov, a physicist astatine the University of Liverpool successful the UK and personnel of the team.
Physicists had seen faint hints that thing unusual was going connected pinch the pion rate. They noticed that measurements of it from experiments successful Italy and the United States were starting to drift apart. But the melodramatic divergence of the caller measurement from the detector’s ain past results, on pinch the collaboration’s declare of precocious precision, made the business difficult to ignore.
Physicists person pored complete the result. “No measurement has been scrutinized more,” Keshavarzi said. So far, nary problems person been found.
Members of the BMW group past utilized a lattice simulation to predict the pion complaint successful 2024. Their calculation mostly agrees pinch the astir caller results. And preliminary information from the different detector astatine the VEPP-2000 collider besides look to lucifer the caller rate. Meanwhile, a 2023 analysis of information collected earlier astatine yet different research astatine a collider, BABAR successful California, sits successful striking statement pinch the older rate.
All of this leaves physicists wondering what’s really going connected successful each these collider experiments. The discrepancies constituent either to signs of unknown particles meddling pinch the quarks, aliases to overlooked specifications generating the mistaken belief that quarks are misbehaving. Either way, particle physicists can’t remainder until they person solved the caller electron-positron mystery, and figured retired whether the aged pion rate, aliases the caller pion rate, is the correct one.
“There are 4 decades of measurements that preceded that, that were each done successful different ways, that were each done by different people, that were each done by different experiments, that each overgarment a wholly different picture,” Keshavarzi said. “There is truthful overmuch still near to do.”
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