There are six types of quarks, known as flavors.
- Why where these types called flavors?
- Why do the flavors have such odd names (up, down, charm, strange, top, and bottom)?
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There are six types of quarks, known as flavors.
It is important to remember that words like this get used in spit-balling sessions and then stick. You have to think of a couple of guy sitting by a black board, coffee in hand saying something like
OK, OK! SO that doesn't work. But what if we assume these things come in three flavors and ...
It's just a word made up on the spot.
That said, I think that "up" and "down" came from an analogy with spin. "Strange" interaction were so called because they violated rules of thumb for other known interactions...
After that things got out of hand.
Side note: "flavor" is also used to distinguish the generation of leptons (and will no doubt be applied to any sterile neutrinos that make an appearance).
Side note$^2$: For a long time there were competing names for the third generation flavors. I'm a little sad that "top" and "bottom" won out against "truth" and "beauty".
There are six flavors of quarks. "Flavors" just means different kinds. The two lightest are called up and down.
The third quark is called strange. It was named after the "strangely" long lifetime of the K particle, the first composite particle found to contain this quark.
The fourth quark type, the charm quark, was named on a whim. It was discovered in 1974 almost simultaneously at both the Stanford Linear Accelerator Center (SLAC) and at Brookhaven National Laboratory.
The fifth and sixth quarks were sometimes called truth and beauty in the past, but even physicists thought that was too cute.
The bottom quark was first discovered at Fermi National Lab (Fermilab) in 1977, in a composite particle called Upsilon ($\Upsilon$).
The top quark was discovered last, also at Fermilab, in 1995. It is the most massive quark. It had been predicted for a long time but had never been observed successfully until then.
Copied shamelessly from How did quarks get their silly names?
when physicists played with the strong interactions, the linguistic creativity was at the top, partially due to the influence of people such as Murray Gell-Mann who is a keen linguist (and borrowed the word quark itself).
Flavor is something that distinguishes the quark species $u,c,t; d,s,b$ - especially in between the generations - and similarly for the charged leptons and neutrinos. They needed a comprehensible word that defines a certain "property" of the particles - but it's a completely new kind of property.
So the 3 possible QCD-charged versions of the quarks were named colors - red, green, blue are 3 independent colors we can see, so the analogy works so perfectly that sometimes people misunderstand that the 3 quark colors have nothing to do with the real colors haha.
And the different generations were called flavors. While colors are kind of interchangeable - you can permute R,G,B to get a similarly meaningful picture, and then transform it back, and correspondingly, the $SU(3)$ symmetry is unbroken - flavors are qualitatively different. Bitter is not the same thing as sweet or sorrow or salty. The physicists clearly enumerated all senses we use to perceive "properties of the world" - vision, hearing, tasting etc. - and the vision (colors) was used for the confined QCD charges while the flavors (tasting) was used to distinguish the particles across the families.
Up and down quarks were named in this way because they form the upper and lower component of the isospin doublet. Isospin $SU(2)$ is mathematically fully analogous to the $SU(2)$ group of spin that rotates the real space. A double - the fundamental representation of the spin $SU(2)$ - is conventionally described by a basis in which one state is spinning with spin up and the other has spin pointing down. So they used the same names - written as words - for the isospin as well.
That was the first two quarks. Hadrons containing the third lightest quark, the strange quark, were behaving a bit strangely, so Gell-Mann gave the strangely (partly) conserved quantum number the name strangeness, and the quark that carried it had to be strange as a consequence.
It became clear that it had to have an $SU(2)$ partner as well. They looked at the first three quarks and realized that the lower-charged quarks, bottom and strange, were named by some kind of inferior names. So they had to choose a random description for the new superior quark - they chose charm.
While inventing names for the third family (or generation - none of those names has been eliminated yet), it was already a naming industry. So they chose truth for the superior species and beauty for the inferior species - because beauty is less important than the truth - apologies to all the hotties who are reading this text. However, it was too playful so the new up-like and down-like names which were more modest and had the same initial letter - top and bottom - were adopted instead.
There are many other properties one should use for the new internal quantum numbers. For example, I am convinced that R-parity in supersymmetry should be described as sex. Everything we observe is R-parity even and should be "female" - they have an even number of X-chromosomes. R-parity odd superpartners - to be seen - should be called "male" because they have an odd number of X chromosomes. ;-) If you collide two "male" particles, you may create a female one. But it is not a real sex (like when you emit an LSP): instead, it is a surgery when you pick the X-chromosomes from both fathers haha. Unfortunately, physicists are much less creative than in the 1960s so my terminology won't be adopted.
Best wishes Lubos