The origin of the word 'strangeness' is purely historical. The short answer is that
strangeness refers to the amount of strange quark content in a given baryon.
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 (more info 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'
For more on these particles and the history of the subject, I recommend 'The Experimental
Foundations of Particle Physics,' by Robert Cahn and Gerson Goldhaber.
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley
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