25 Days of Particles: Day 6

Pion

Classification: boson, meson
Fundamental: no
Mass: 139.6 MeV (pi+ and pi-), 135.0 (pi_0)
Interactions: Electromagnetic (for pi+ and pi-; not for pi_0), Strong, Weak, Gravity
Spin: 0
Lifetime: 2.6e-8 s (pi+ and pi-), 8.4e-17 s (pi_0)

We now have a nice picture of atomic structure, with protons and neutrons clumped together in the nucleus at the center of the atom and the electrons zipping around the outer edge, and this nice picture had issues from its inception. We'll skip over the question of how the negatively charged electron doesn't just crash straight into the positively charged nucleus; this is the question that quantum mechanics was invented to answer. Instead we'll turn to a similar problem with the nucleus itself.

How do a bunch of positively charged protons stay packed tightly into the nucleus? All that positive charge should repel so strongly that the nucleus couldn't hold itself together. The answer that isn't really an answer is that there must be some other, nuclear force strong enough to over-power the electromagnetic repulsion. This is a statement of fact, not an explanation, and physicists immediately began working to figure out what that force was. In 1935, Hideki Yukawa published his theory that there was an intermediate mass particle, called a meson, that carried the strong nuclear force between protons and neutrons.

Wait, hold on, particles carrying forces?

Yup. In particle physics, we think of some particles as being what makes up matter. These are the fermions with half-integer spins like protons and electrons and neutrons. Other particles are force carriers; they are emitted or absorbed by particles, and the emission or absorption is where the interaction happens. These guys are the bosons with integer spins. So Yukawa looked at how big atomic nuclei could get before they became unstable, concluded that was the distance a force carrier could travel, and used that to predict the mass of this meson. It, or rather they, were discovered in 1947 and got the final name of pions.

There are three pions, as they come in positively charged, negatively charged, and neutral varieties. Yukawa wasn't entirely right about the pions being the strong force carriers, either, though this was a step in the right direction and pions play an important role. Their discovery marks the beginning of the particle zoo, the ever-expanding family of particles that continues to grow to this day, and led to a much more complete picture of all the strongly interacting particles pull it off.