![]() ![]() Poking at the places where the Standard Model breaks or otherwise deviates from observations, physicists say, is one of the best ways to search for “new physics,” their catch-all term for finding additional, possibly more fundamental building blocks of the universe. More recently, physicists making such measurements have focused less on refining the Standard Model’s core competencies and more on probing its failures-it does not, for instance, incorporate gravity, dark matter, neutrino masses or a number of other troublesome phenomena. Then, using the W boson mass and top quark mass, researchers made a similar prediction for the Higgs boson-which bore out spectacularly in 2012. It was a rough measurement of the W boson mass that allowed physicists in 1990 to predict the mass of the top quark with reasonable accuracy five years before that particle was first observed. You almost feel betrayed because suddenly they’re sawing off one of the legs that really support the whole structure of particle physics.” Questing for Quarks “Nobody was waiting for this discrepancy,” says Martijn Mulders, an experimental physicist at CERN near Geneva, who was not involved with the new research but co-wrote an accompanying commentary in Science. ![]() Taken by surprise, particle physicists are only beginning to grapple with the implications. The well-studied W boson, it seems, still holds plenty of secrets about the workings of the subatomic world-or at least about how we investigate it. Although these numbers differ by only about one part in 1,000, the uncertainties for each are so minuscule that even this small divergence is of enormous statistical significance-it is exceedingly unlikely to be an illusion produced through sheer chance. The new measurement, which is more precise than all previous measurements combined, is nearly 77 MeV higher than the Standard Model’s prediction. (One MeV is about twice the mass-energy contained within a single electron.) But in a new analysis published on Thursday in Science, physicists on the CDF collaboration have instead found the W boson mass to be 80,433.5 ± 9.4 MeV. Plugging data into the Standard Model framework, however, predicts that the so-called W mass should be 80,357 mega-electron-volts (MeV), plus or minus 6 MeV. Because its mass is constrained by (and itself constrains) many other particles and parameters within the Standard Model-particle physicists’ theory of fundamental particles and how they behave-the W boson has become a target for researchers seeking to understand where and how their best theories fail.Īlthough physicists have long known the W boson’s approximate mass, they still do not know it exactly. Crucially, the W boson is responsible for certain forms of radioactive decay, allowing neutrons to convert into protons. The W boson is massive, some 80 times heavier than a proton. Now a fresh analysis of old CDF data has unearthed a stunning discrepancy in the mass of an elementary particle, the W boson, that could point the way to new, as yet undiscovered particles and interactions. ![]() A decade ago the 4,100-metric-ton Collider Detector at Fermilab (CDF) reached the end of its life and was shut down, stripped of its parts for use in other experiments. In particle physics, data long outlives the detectors that generate it. ![]()
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