The day the gap closed

For a quarter of a century the muon's magnetic wobble sat about one part in a million away from theoretical prediction, and that deviation was taken as a hint of undiscovered particles. In 2021 the Budapest-Marseille-Wuppertal collaboration's lattice QCD calculation showed the muon wobbling exactly as known physics requires, and the result was released the same day as Fermilab's updated measurement.[1]

The same number can be reached by a second route. The data-driven prediction published by Alex Keshavarzi and colleagues in June 2020 inferred quark behaviour from electron-positron collisions, and it clashed with Fermilab's April 2021 measurement so sharply that it nearly reached the stringent threshold required to claim a new particle. The lattice calculation did not dissolve that clash; it moved where the clash sits.[1]

The measurement left carrying the weight

The data-driven route depends on measurements of pion production rates in electron-positron collisions. When a new detector was installed at the VEPP-2000 collider in Novosibirsk in 2023 and the rates were measured again, the values changed significantly. Fedor Ignatov of the University of Liverpool describes it this way: "It was a surprise. No one expected it to be like that". The BABAR collider's 2023 analysis, meanwhile, sits in striking agreement with the older rate.[1]

A theoretical calculation meeting the experiment carried the disagreement out from between theory and measurement and set it down among the measurements themselves: what now diverges is one method's own inputs rather than two rival methods. An unknown particle counts as only one account of this. Overlooked procedural differences between experiments spread across decades could produce the same divergence, and in that case nothing in the Standard Model needs touching.[1]

What to watch

Keshavarzi says no measurement has been scrutinised more than VEPP-2000's and that so far no problems have been found; his own summary bears it out: "There are four decades of measurements...all done in different ways...that all paint a completely different picture." What to watch is whether an independent measurement of the pion rate lands on VEPP-2000's new value or in the cluster formed by BABAR and the older results. The muon's own number may hold steady for a while yet; until the two ends of the road leading to it converge, no one can say which side holds new physics.[1]