Electroweak Theory and the Origin of the Fundamental Forces

**SPEAKER_1** (0:00)
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**Matt O'Dowd** (1:00)
Our universe seems pretty complicated. We have this weird zoo of elementary particles that interact through very different fundamental forces. But some extremely subtle clues in nature have led us to believe that the forces of nature were once unified, ruled by a single grand symmetry. But how does one force separate into multiple? And how do the forces of nature arise from mathematical symmetries in the first place?
The best way to understand how the universe went from a simpler, more symmetric state to its current complicated condition is to look at the unification that we understand the best. Today, we're gonna begin an exploration of how the electromagnetic and weak forces were once a single thing. And this dive into electroweak unification will lead us inevitably to the Higgs field and to an understanding of how particles gain mass. Let's start with the mysterious and often misunderstood weak interaction. You may have heard that the weak force is responsible for some types of radioactive decay. That's not very satisfying, but it's how the weak force was first identified. Beta decay is when a neutron turns into a proton by emitting an electron in a neutrino. The electron was called a beta particle by Ernest Rutherford back in 1899, before we even knew that these things were electrons. It's one of the main ways radioactive nuclei decay, the other being alpha decay, where the emitted alpha particle is really a helium-4 nucleus.
Fast forward to the early 30s. While the brand-new field of quantum mechanics could describe the behavior of electrons, nuclear processes remained mysterious.
Enrico Fermi made the first attempt at a full quantum description of beta decay with his four-fermion interaction. Basically, he tried to model this as a direct interaction in which all four fermion particles literally touch. So an in-going neutron is directly converted into an outgoing proton, electron and neutrino with all the conservation laws satisfied. Fermi was motivated by the apparently extremely short range of that interaction. And that short range is what earned it the name the weak interaction. But Fermi's model only worked at low energies, and neither it nor its successors explain why the weak interaction violates charge parity symmetry.
Meanwhile, there was a very different effort to explain electromagnetism that was enjoying much more success. That effort was quantum electrodynamics in which charge particles interact not by touching, but via a mediating particle that transmits the force. The midi-chlorian. I mean, the photon.
By the way, force-mediating particles are bosons, as opposed to the fermions that make up matter. QED is what we call a gauge theory. It's force-

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**Matt O'Dowd** (5:00)
Fearing fields and particles arise from the symmetries of the quantum equations of motion. We'll come back to that. In 1957, Julian Schwinger proposed a set of force-mediating gauge bosons for the weak interaction. Given that the weak interaction could change a neutral particle into a pair of charged particles, this mediating particle must itself be charged. This was an early hint that somehow the electromagnetic force, which acts on charged particles, was playing a role here, although the involvement of the neutron and neutrino meant it couldn't be entirely electromagnetism. Not only that, but experiments at the time indicated that if they existed, these new weak bosons, W bosons, had to have mass due to the short-range nature of the interaction, and quite a lot of mass, in fact. The short-range nature of those forces that have massive force carriers is usually attributed to the energy time uncertainty of the relation, and we've presented it that way previously, though as we'll discuss in a future episode, that is not the full story. Anyway, this new gauge theory of the weak interaction seemed to be okay with parity violation, and it wasn't only accurate at low energies. So, problem solved. Easy peasy, right? Well, not even close.