25 Days of Particles: Day 18

W Boson

Classification: boson
Fundamental: yes
Mass: 80400 MeV
Interactions: electromagnetic (+1 or -1) and weak
Spin: 1
Lifetime: 3e-25s

Having met all the known matter particles, let's turn to how they talk to each other, or the four forces. The force of gravity had been identified as such by Sir Isaac Newton in the seventeen century, and electromagnetism was the cutting edge of physics during the nineteenth. The discovery of radioactive decays required a new force, which was called the weak force because it happened so infrequently, and the discovery of all the positively charged protons in the atomic nucleus required a strong force.

During the 1950s and 1960s, many theoretical physicists focused on developing quantum theories of these forces. When they were done, they had managed not only to recast three of the forces into quantum terms, but they had also managed to unify the electromagnetic and weak forces into one mathematical element. These new quantum theories make up what we now have as the Standard Model, and predicted two new heavy gauge bosons that carried the weak part of the electroweak force. In particular, the decays were ascribed to the activity of the charged W boson.

Take a beta decay, the one that helped all this stuff get discovered. In beta decay, a neutron decays into a proton and emits an electron and a neutrino. Taking a closer look, what really happens is that a down quark emits a negatively charged W boson and an up quark. The up quark stays behind in the baryon, which is now a proton. The W decays into any pair of particles, which could be an electron and an electron anti-neutrino. It could also decay into another down quark and an anti-up quark, which could form a pion, or into a muon and muon anti-neutrino.

The W particles also give us an explanation for why the weak force is weak. The W boson is heavy, far heavier than the energy carried by most particles. So the average particle sitting in the nucleus doesn't have the energy to emit the weak force carriers. It only happens rarely by a quantum mechanically fluke available to particles. So the W bosons get created only rarely, so the weak force doesn't happen all the time. It happens once in a while with a predictable probability, that gives us the half-life of radioactive substances.

The W boson is also the only boson that links up particles between families according to how the particles mix, allowing up-type quarks to decay into down-type quarks even if they aren't in the same family. In my mind, this makes the W bosons the work horses of the weak force. They help particles get down to the lowest energy state, and they definitely make life interesting for particle physicists.