25 Days of Particles: Day 10

Strange Quark

Classification: quark, fermion
Fundamental: yes
Family: second
Mass: ~101 MeV
Interactions: Electromagnetic (charge -1/3), Strong, Weak, Gravity
Spin: 1/2
Lifetime: unstable, but lifetime depends on baryon or meson

If you remember back to our discussion of the discovery of the kaon, you remember that one of the odd things about kaons is that they needed a new quantum number to describe them. This quantum number was called strangeness, and when the quark description of hadrons was developed, physicists figured out that strangeness meant the particle contained a specific quark. That quark inherited the name of the strange quark and particles that contain strange quarks are called strange particles. To put the kaons into context this way:

K+ = up quark + anti-strange quark
K- = strange quark + anti-down quark
K_0 = a linear combination of strange + anti-down and down + anti-strange quarks

Now, it may seem that the strange quark's properties seem awfully familiar. We've met a quark with electric charge -1/3 and spin of 1/2 before; those are the down quark's properties as well. Down quarks and strange quarks have all the same quantum numbers and can interact in all the same ways, so in the mathematical theories one could almost wonder if they were truly different particles.

But only almost wonder, because the strange quark is about twenty times more massive than the down quark. That is a lot of extra mass.

The strange quark is the first particle we've met from the second family or generation. The first family contains the electron, electron neutrino, up quark, and down quark. The second family contains four particles as well, each a more massive version of one of the particles in the first family. There is a third family of particles as well. The Standard Model, the big mathematical theory physicists have developed to describe all the known particles and their interactions, contains twelve fundamental particles.

The three families also present an interesting dilemma, since the Standard Model as originally created predicted all the particles to be massless. The particles clearly are not massless and mass is the determining feature between the three families. The Standard Model can be mathematically extended to include mass in many different ways, and this was first done in the 1960s. Fifty years later, experimentalists are still trying to figure out which method is right.