25 Days of Particles: Day 7

Kaon

Classification: boson, meson
Fundamental: no
Mass: 493.7 MeV (K+ and K-), 497.6 MeV (K_short and K_long)
Interactions: Electromagnetic (for pi+ and pi-; not for pi_0), Strong, Weak, Gravity
Spin: 0
Lifetime: 1.23e-8 s (K+ and K-), 0.89e-10 s (K_short), 5.12e-8 (K_long)

During the late 1930s and 1940s, the discovery of new particles was a fairly regular occurrence. The phis and etas and rhos all showed up and were identified, what is now called the particle zoo was starting to get populated, and physicists were running out of Greek letters. In 1947, when the first evidence of the kaons was published, it seems like they could have gotten lost in the shuffle.

They didn't, because kaons have a whole lot to teach physicists about particles. They were important subjects to study in the 40s, and they still are today.

When the kaons were discovered, particle physicists classified particles according to their quantum numbers. Unique particles had to have a unique set of values for their quantum numbers, and this allowed the particles to be grouped according to their properties. But then the kaons came along and complicated things, because they needed a new quantum number to be described. When the weak force was used to produce or decay kaons, they went about it pretty slowly; when strong force interactions (such as those between pions and protons) were invoked, kaons showed up and vanished more rapidly. This was finally explained by introducing a new quantum that the strong force conserved and the weak force didn't. This quantum number was called "strangeness," which gives you a hint of what physicists thought of it at the time.

Strangeness turned out to be important property in sorting out how all the particles in the zoo related to each other, but that is part of tomorrow's story.

One of the biggest contributions from the kaons, and the reason they are still studied so much today, is from the evidence they offer on charge-parity (CP) violation. Parity is another way of saying handedness, or how a particle's spin and momentum vector line up. The two different neutral kaons have different values for CP, and therefore decay into different final states. Except, the neutral kaons can turn into each other; K-longs will turn into K-shorts and vice versa. So, occasionally K-longs decay into particles that only K-shorts can produce, which means the CP numbers had to have changed. Physicists call this CP violation.

What is awesomely, amazingly cool about CP violation is that it can lead to particle decays that produce more matter than anti-matter. See, the mathematical theories all say that matter and anti-matter should be mirror images of each other and should be created in equal amounts. But we live in a world where there's a whole lot more matter than anti-matter. So the pressing question is where did all the anti-matter go? It can't be hiding out in the universe somewhere, because stuff in the universe collides and we would see if a bunch of anti-matter collided with a bunch of matter. CP violation allows for processes that naturally produce more matter than anti-matter, which could explain our nice, stable, matter-dominated universe that we know and love.

Could but hasn't quite yet. Physicists haven't identified enough CP-violating processes to explain all the discrepancy we see. So we keep studying kaons, hoping they have still more to teach us, and anything else that might explain the mystery, too.