At first, their predictions have been 10 occasions fuzzier than data-driven inferences. Low-energy particles are likely to unfold out, so capturing their attainable positions requires utilizing an enormous lattice. Excessive-energy particles want a relatively smaller grid, however one with an especially positive mesh. “Again then, it was unimaginable that sooner or later lattice would attain the identical precision” because the data-driven methodology, stated Kalman Szabo, a professor at Wuppertal who was concerned within the effort.
It took a decade of growing intelligent computational strategies — and ready for elevated computing energy — for the BMW group to wrangle grids that have been each sufficiently huge and sufficiently detailed. However wrangle them they did. In 2021, on the identical day that Fermilab launched its up to date muon g–2 measurement, the BMW group’s outcome appeared within the journal Nature.
In keeping with the BMW group’s lattice calculation, Fermilab’s muons have been wobbling precisely as they need to. Since then, unbiased lattice teams have revealed matching calculations.
At this time, many physicists imagine the muon thriller is not any extra: In keeping with the lattice simulations, the muon’s additional wobble may be defined totally by the emission and reabsorption of recognized particles obeying the recognized legal guidelines of the recognized forces.
So why does the data-driven methodology point out in any other case?
Inconsistent Experiments
To determine what’s occurring, physicists are drilling into the electron-positron collisions driving the data-driven methodology. These collisions are alleged to be a direct window into quark habits, however calculations primarily based on this knowledge disagree with each the newest experimental outcomes and BMW’s prediction. So what’s actually occurring within the aftermath of these collisions?
Within the metropolis of Novosibirsk in southern Siberia, the VEPP-2000 collider has been crashing electrons into positrons on and off because the flip of the millennium. It’s a comparatively mild collider, working at 6,000 occasions decrease vitality than CERN’s Giant Hadron Collider, close to Geneva.
The VEPP-2000 options two detectors that exactly rely how usually sure bundles of quarks, often known as pions, come out of the electron-positron crashes — knowledge that physicists have been utilizing to deduce how a lot the robust drive was messing with muons.
In 2010, physicists put in a very new detector. They then used it to extra exactly measure this pion manufacturing charge, which they revealed in 2023. After the refresh, they discovered that the speed modified considerably.
Fedor Ignatov, a physicist on the College of Liverpool within the UK, was a member of the crew that measured a mysterious new charge of pion manufacturing on the VEPP-2000 collider.
Courtesy of Fedor Ignatov
“It was a shock. Nobody anticipated it to be like that,” stated Fedor Ignatov, a physicist on the College of Liverpool within the UK and member of the crew.
Physicists had seen faint hints that one thing unusual was occurring with the pion charge. They seen that measurements of it from experiments in Italy and america have been beginning to drift aside. However the dramatic divergence of the brand new measurement from the detector’s personal previous outcomes, together with the collaboration’s declare of excessive precision, made the scenario exhausting to disregard.
Physicists have pored over the outcome. “No measurement has been scrutinized extra,” Keshavarzi stated. To date, no issues have been discovered.
Current lattice-based simulations align with the newly measured charge. And preliminary knowledge from the opposite detector on the VEPP-2000 collider additionally appear to match. In the meantime, a 2023 evaluation of knowledge collected earlier at yet one more experiment at a collider, BABAR in California, sits in hanging settlement with the older charge.
All of this leaves physicists questioning what’s actually occurring in all these collider experiments. The discrepancies level both to indicators of unknown particles meddling with the quarks, or to neglected particulars producing the mistaken impression that quarks are misbehaving. Both approach, particle physicists can’t relaxation till they’ve solved the brand new electron-positron thriller, and discovered whether or not the outdated pion charge, or the brand new pion charge, is the correct one.
“There are 4 a long time of measurements that preceded that, that have been all finished in numerous methods, that have been all finished by totally different individuals, that have been all finished by totally different experiments, that every one paint a very totally different image,” Keshavarzi stated. “There’s a lot nonetheless left to do.”
Correction: July 30, 2026The authentic model of this text said that the BMW group carried out simulations that immediately predict the brand new pion charge. BMW’s calculations help the brand new charge, however solely not directly. Different lattice teams have extra immediately gone after the pion charge.
