W boson: Sunshine and stardust

The W boson carries the weak force. It changes the character of particles of matter—allowing the Sun to burn and new elements to form

Discovered in 1983, the W boson is a fundamental particle. Together with the Z boson, it is responsible for the weak force, one of four fundamental forces that govern the behaviour of matter in our universe. Particles of matter interact by exchanging these bosons, but only over short distances.

The W boson, which is electrically charged, changes the very make up of particles. It switches protons into neutrons, and vice versa, through the weak force, triggering nuclear fusion and letting stars burn. This burning also creates heavier elements and, when a star dies, those elements are tossed into space as the building blocks for planets and even people.

The weak force was combined with the electromagnetic force in theories of a unified electroweak force in the 1960s, in an effort to make the basic physics mathematically consistent. But the theory called for the force-carrying particles to be massless, even though scientists knew the theoretical W boson had to be heavy to account for its short range. Theorists accounted for the mass of the W by introducing another unseen mechanism. This became known as the Higgs mechanism, which calls for the existence of a Higgs boson.

As announced in July of 2012 at CERN, scientists have discovered a boson that looks much like the particle predicted by Peter Higgs, among others. While this boson is not yet confirmed as the Higgs boson predicted to make sense of the electroweak force, the W boson had a large part in its discovery.

In March 2012, scientists at Fermilab in the US confirmed the most precise measurement of the W boson’s mass to date, at 80.385 +/- 0.016 GeV/c2. According to the predictions of the Standard Model, which takes into account electroweak theory and the theory of the Higgs mechanism, the W boson at that mass should point to the Higgs boson at a mass of less than 145 GeV. Both the ATLAS and CMS collaborations place the mass of the new Higgs-like boson at about 125 GeV, well within range.

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