Strange particles, what are they and why are they strange

What is a strange particle

I’ll start with a few statements which describe strange particles and then explain each one:

  • All strange particles contain a strange quark or anti-strange quark. 
  • All strange particles have surprisingly long lives, as much as 100,000 times as long as similar particles. This is why they were called strange in the first place.
  • Strange particles are always produced in pairs, which leads us to reason that they have a quality (which we now call strangeness) which is conserved.

Table of fermions to show that kaons are mesons and hadrons

Strange quarks

First of all, what is a strange quark. Quarks and leptons are the fundamental particles which are fermions, the building blocks of our known universe. Of the quarks, the up and down quarks make up protons and neutrons and, together with electrons, they make up all the atoms and materials we know. The other quarks are unstable with brief lives and are produced in high energy collisions. Particles made up from quarks are called hadrons. If the particle contains three quarks it is a baryon and if it contains two it is a meson. A strange particle (either meson or hadron) will contain at least one strange quark.

This explanation will be in terms of the strange particles most frequently studied, which are K-mesons or kaons. Since kaons are mesons they must contain two quarks. A kaon contains either an up or a down quark and a strange quark. One of the pair is an antiquark.  There are four possibilities (the colours are not significant):


A description of the four possible kaons / k-mesons

An up antiquark and a strange quark, the up antiquark has a charge of -2/3 and the strange quark -1/3 a total for the K- meson of -1

An up quark and a strange antiquark total charge is +2/3 and 1/3 equals +1

A down quark and a strange antiquark total of -1/3 and +1/3 gives zero charge

A down antiquark and a strange quark, total of +1/3 and -1/3 gives zero charge

The charge on an up quark is 2/3, on a down quark -1/3 and the same -1/3 on a strange quark. The anti quarks have the opposite charges.

Strangeness is defined as:

S = (no. of anti-strange quarks) – (no. of strange quarks)

Where do kaons and other strange particles come from

Kaon production via the strong force in high energy collision

Kaons are usually produced by strong interactions in collisions between high energy particles (eg cosmic rays) and atomic nuclei. Pairs of particles are produced and these balance to ensure that strangeness is conserved. A strange and anti-strange quarks are created at the same time. In the example illustrated here, the total strangeness of the proton and pion is zero since neither contain a strange quark. The total after is also zero, with a balance between strange and anti-strange quarks.

Kaon decay

Kaon decay is relatively slow and is by the weak force or weak interaction. In a weak interaction, strangeness may not be conserved – in other words there are likely to be no strange quarks in the decay products. Below are two examples of Feynman diagrams illustrating K+ and K quarks decaying via the weak force. In both cases the products are a pair of leptons, which of course, have zero strangeness.

Charged kaons decaying via the weak interaction to a pair of leptons