The story of strange quarks starts before the idea of quarks was really invented. It starts with the idea of strangeness. This is the property given to certain particles that took an unusually long time to decay.

In the middle of the 20th century, physicists across the globe were busy creating particles. So many particles were created that the term “zoo of particles” was thrown around, and it was an appropriate name. Particles that differed wildly in mass and in charge, that were rarely stable, and often decayed into the more familiar particles were found. What was strange however, is that there was no particular pattern to how fast or slow these particles might decay.

Some particles with nothing particularly special about their mass or charge, took much longer to decayKaons, for instance, are particularly heavy particles and should decay quickly, but they are surprisingly stable.

. Whilst similarly charged and massive particles decayed rapidly. How could we explain this feature? Well first it was quantified, each particle was described by a certain amount of “strangeness” based off of how long it took to decay.

In 1964, a simple model of just 3 fundamental particles was proposed (along with their anti-particles), which could explain the existence of the zoo of particles. This was the birth of the quark-model of particle physics. A point of view that changed our understanding of nature at the most fundamental level.

The model used three quarks: the up quark, the down quark and the strange quark. And it was sufficient to explain the known particles at that time. With the increasing power of particle colliders, further particles were found which were explained by adding a few more fundamental quarks to the picture.

Up Down Strange
Charge +2/3 e -1/3 e -1/3 e
Mass (MeV/\(\mathbf{c^2}\)) \(2.3 \pm 0.7\) \(4.8 \pm 0.5\) \(95 \pm 5\)

So what makes the strange quark so strange?

To understand this, we need to know about the forces that fundamental particles can experience. These are the strong nuclear force, the weak nuclear force and the electromagnetic force. They also experience gravity but it is too weak to be of concern for the tiny masses that these particles have.

The forces we specifically need to have a look at are the strong and weak nuclear forces. These are the forces that keep nuclei together, but they are responsible for a lot more. Without them, we wouldn’t even have protons and neutrons as particles, never-mind nuclei.

  • The weak nuclear force is felt by all the quarks and leptons - the quarks and the neutrinos and the electron, muon and tau.
  • The strong force however, is only felt by the quarks, it doesn’t affect the leptons at all.

The strong nuclear force binds quarks together. The familiar proton is made of two up quarks and one down quark, and the neutron is made of two down quarks and one up quark.

However, we know that in the nuclei of some elements, a process occurs where a neutron transforms into a proton, or vice-versa. This is only possible by the weak force.