 In video one of this module, we saw how cosmic rays provided the earliest test bed for discovering new particles. In those days, one or two scientists could make a discovery, and the cloud chambers that they required for their research could sit on a standard bench top. These days, we're searching for much heavier and more exotic particles, and the people and resources we need to make these new discoveries as fast. In the next two videos, we're going to talk about this much grander scale of discovery by exploring a recent particle physics experiment. The search for the Higgs boson. The search began way back in 1964. Two papers came out at around the same time, one by Peter Higgs and one by Robert Brout and Francois Engelbert. Both explored a question that had been bothering scientists for a long time. How do particles get their mass? You see, there was a bit of a problem with a theory that united the weak and electromagnetic forces. It only worked if all the exchange particles, the photon, the W and Z bosons, were massless. In fact, any quantum theory that required fermions to communicate via exchange bosons required that those exchange bosons be massless. This is fine for the photon, as it's massless, but the W and Z bosons aren't massless, as you've already learned. So to make a very long story short, this meant there needed to be some new clever way to generate W and Z exchange bosons with mass. This is where Higgs Brout and Engelbert's theory comes in. They postulated that something we now call the Higgs field exists everywhere in the universe. This field interacts with particles differently, and this different interaction leads to particles of different mass. Massless particles, like the photon, don't interact with the field, while massive particles, like the W and Z bosons, interact quite strongly. Glacier, Salam, and Weinberg used Higgs's ideas to generate massive W and Z bosons in their electroweak theory, which made everyone reasonably happy. Except for one thing. There was no experimental proof of the Higgs field's existence. Without it, the theory could have been simply a mask for underlying problems in the electroweak theory, which, aside from the mass problem, worked really well. As you can imagine, this was deeply troubling to physicists. Theories are only as good as the experiments we used to test them. This is where the Higgs boson comes in. You see, the Higgs field has to have a way to communicate with particles that interact with it, meaning it needs an exchange boson as well. The Higgs boson is this exchange boson, and it has a few specific predicted properties. It's a fundamental particle with zero spin, no charge, and non-zero mass.