I've read that because Kaons are produced by the strong interaction and decay by the weak interaction that this makes them strange. However protons and neutrons decay by the weak interaction as well i.e B+ or B- decay. So why aren't protons and neutrons also strange?

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    $\begingroup$ They are strange because they have an s-quark, protons and neutrons don't. $\endgroup$ Commented Mar 23, 2017 at 14:18
  • $\begingroup$ Ok, that makes sense but I've read that strangeness is a property allocated to them to explain their behaviour, i.e taking longer than expected to decay. I don't understand the significance of them decaying by weak interaction, when non strange particles do as well. $\endgroup$ Commented Mar 23, 2017 at 14:24

1 Answer 1


Here is the historical origin of the term strangeness,

Very early in the study of high-energy physics the only source of high energy particles was in cosmic rays (the first accelerators were not yet in existence). People exposed emulsions or set up bubble chambers to record the passage of particles raining down on the surface of the earth from the atmosphere. Most of these particles were identified as electrons and protons. Others came to be identified as anti-protons and pions. At that time we did not have the quark theory to describe these particles (see this link for more on quarks), we simply had a list of particles and their masses and charges.

In the late 40s and early 50s people noticed particles that left very unusual tracks in their emulsions. We now call these particles 'kaons' and 'lambda hyperons,' but at the time they were simple 'strange particles.' They decayed into charged particles, so the tracks they left had kinks in them or formed a 'v.' They were generally heavier than the pions and protons that people were familiar with.

We now know that protons and pions are made up of up and down quarks, while these 'strange particles' have at least one of a new type of quark that is much like the down quark, only heavier. But in the early 50s we didn't have the quark theory. However, in the early 50s Murray Gell-Mann suggested that these new particles had a conserved quantum number which he called 'strangeness.' This was originally just a bookkeeping device that helped explain the decay patterns of the new particles into the old familiar particles. There was no underlying physical meaning to 'strangeness.' Later, when the quark theory was introduced, this new property was associated with a new particle, the unimaginatively-named 'strange' quark.

  • $\begingroup$ Just put the link as a comment. $\endgroup$
    – ProfRob
    Commented Mar 23, 2017 at 20:02
  • $\begingroup$ @RobJeffries that is what I had done at the beginning, but then I thought to give search engines some field on history of strangeness. It took me some time to find this write up, which I had lived through starting in 1965 (my thesis was from data of K-proton at 8.25 GeV/c in the CERN two meter bubble chamber), and comments have a way of disappearing and I do not think are scanned by search engines. $\endgroup$
    – anna v
    Commented Mar 23, 2017 at 20:19

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