![]() In March 2021 scientists from CERN-Europe’s particle-physics laboratory-reported evidence that the bottom quark decays into electrons and muons in uneven numbers, contradicting the Standard Model. For particle physicists, that is an exciting prospect. If the W boson is more massive than the Standard Model predicts, it implies that something else is tugging on it too-an as-yet-undiscovered particle or force. That is how they predicted the mass of the top quark (discovered in 1995) and the mass of the Higgs boson (discovered in 2012), before either particle had been detected. That allows scientists to use the W boson to calculate the mass of those other particles. ![]() These protean powers mean that the mass of the W boson is linked to the mass of several other elementary particles. It can also flip quarks from one type to another-up to down, top to bottom, and the whimsically named “strange” quark to a “charm” one. For example, it can transform the electron (and two of its cousins, the muon and tau) into neutrinos. What really distinguishes the W boson, however, is its ability to change the type-or “flavour”-of other elementary particles it comes across. The W boson is 90 times heavier than a hydrogen atom. Unlike other force-carrying particles, however, the W and Z bosons have mass-and a lot of it. Together with its sibling the Z boson, it mediates the weak nuclear force that governs radioactive decay. The W boson is a force-carrying particle. Using detailed recordings of the scattering trajectories of the menagerie of particles present in such collisions, the scientists could calculate the mass of the W boson with unprecedented accuracy. Between 20 (when it ran for the last time), the Tevatron produced approximately 4m W bosons in collisions between particles called quarks and their antimatter counterparts, antiquarks. The scientists at Fermilab analysed historical data from the Tevatron, a circular particle collider which was the most powerful in the world until the Large Hadron Collider ( LHC) came online in 2009. It places the odds that the result is spurious at only one in a trillion (“seven sigma”, in the statistical lingo), well above the one in 3.5m (five sigma) that physicists require to consider a finding robust. The difference is small-only a hundredth of a percent-but the measurement’s precision exceeds that of all previous experiments combined. In a paper published last week in Science, a team of researchers from the Fermi National Accelerator Laboratory (Fermilab) in America announced that the mass of an elementary particle called the W boson appears to be greater than the Standard Model predicts. But the Standard Model has fought back, stubbornly predicting the results of every experiment physicists have thrown its way.īut that may perhaps be changing. Physicists have spent much time, effort and money performing ever-more elaborate experiments in an effort to see where the Standard Model fails, in the hopes of finding a clue to the theory that will replace it. It cannot explain gravity, dark matter (mysterious stuff detectable only by its gravitational pull), or where all the antimatter in the early universe went.
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