What does the term 'strangeness' refer to?

Asked by: Bill Wehner Jr.

Answer

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' quark.

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