 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' 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 nonzero mass. The mass of the Higgs boson was an open question. In the early days, scientists started looking for a fairly light particle, somewhere between the mass of an electron and a proton. But later, it became clear it would actually have to have a mass that exceeded the proton mass by at least a factor of 100. Still, they didn't quite know how heavy it was. Luckily, the Higgs was also a very unique fundamental particle because it was predicted to have zero spin. No other fundamental particle has this property. So the search for the Higgs would aim to discover a new, heavy, uncharged particle with zero spin. This long experimental campaign actually spanned multiple decades and successfully more ambitious experimental setups. So for simplicity, we're going to talk about the latest search for the Higgs, which has led to its discovery. This discovery won Peter Higgs and Francois Engelbert the Nobel Prize in 2013, and it was made possible using the Large Hadron Collider, otherwise known as LHC, located at CERN in Europe. Before we talk more about the Higgs boson, we're going to spend some time exploring the main tool we use to discover new particles. Particle accelerators like the LHC have been the key to most of our discoveries in particle physics since the 1950s. The LHC is currently the world's biggest particle accelerator, made for smashing or colliding particles together at very high energies. The basic idea behind experiments using accelerators like LHC is this. If we can smash two very high energy particles together, we might be able to produce new, massive, as yet undiscovered particles. This is because of Einstein's mass energy equivalence, E equals mc squared. If there's a lot of energy available in a reaction, then there's a chance that some of that energy can transform into new particles that have mass. We can then use complex detector systems to see how these new particles behave. This tells us a lot about their properties and helps us see if they fit into the standard model as we know it or if there's something new entirely. A good example of the production of new particles occurs in a phenomenon known as pair production. This is kind of like the inverse of electron-positron annihilation. In this case, you have one gamma ray with an energy of greater than, let's say, 1022 kV. This gamma ray has the possibility of producing an electron-positron pair. Essentially, the energy of the gamma ray or the photon is large enough to allow for some or all of that energy to be transformed into an electron-positron pair. The electron and positron, as you would call, have a mass of 511 kV. Pair production can also refer to the production of other particle-antiparticle pairs in the same way. The Higgs boson search aimed to produce the Higgs boson via a similar reaction mechanism, requiring much higher energies. In the next video, we'll talk about the technology we use to achieve these high energies. Thank you very much for watching. We'll see you in the next video.