W boson’s surprise mass threatens to upend the standard model

Physicists analysing data from a long-defunct experiment have found that the W boson — a fundamental particle that carries the weak nuclear force — is notably heavier than theory predicts. Although the difference between the theoretical prediction and experimental value is only 0.09%, it is significantly larger than the result’s error margins, which are less than 0.01%. The finding also disagrees with some other measurements of the mass. If it is confirmed by other experiments, it could be the first major breach in the standard model of particle physics, a theory that has been spectacularly successful since it was introduced in the 1970s.