Scientists have studied and detected the different types of quark matter over the last decades, from up to strange. While we obviously know how matter composed solely of up and down matter looks like, what would matter composed of at least one strange quark and/or one charm quark and/or one top/bottom look like?
[Physics] What would strange and charm matter look like
matterparticle-physicsquarksstandard-model
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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".
Yes, the 6 antiquarks are antiparticles of the 6 quarks – in other words, they're particles of "antimatter". The word "antimatter" sometimes represents just a relative label – antimatter of something (antimatter of antimatter is matter again), sometimes it means the antimatter of the particles we routinely see in the world around us.
Because the 6 antiquark flavors – anti-up, anti-down etc. – have the same properties as the quarks (up to the opposite signs), they're not counted as "independent types of elementary particles". Quite generally, we don't consider antiparticle species to be "independent species" because it's a completely general fact that every particle type has an antiparticle (although, in some cases such as the photon, Z-boson, or Higgs boson, they coincide with the original particle).
No one would ever say that there are "12 types of quarks" because of the antiquarks. We either consider antiquarks "not to be quarks" when we talk about "quarks" in a strict sense, or we do include antiquarks among quarks but the antiparticles are considered to be pretty much the same thing as the original quarks (despite the sign flip in all quantum numbers) which is why we still have just 6 quark flavors (the types are called flavors; each of them also has 3 colors and 2 spin polarizations).
Leptons are not composed of quarks. Leptons and quarks are two equally large but mutually disjoint sets of elementary particles – leptons plus quarks are known as "elementary fermions".
The four forces are mediated by the photons (electromagnetic), W-bosons and Z-bosons (weak nuclear force), gluons (strong force), and gravitons (the gravitational force). Physics is pretty much equally sure about all four or five of them. The only way in which gravitons differ is that gravity is such an extremely weak force that individual gravitons are pretty much undetectable. But they're detectable if they're coming in sufficiently strong beams or packages – gravitational waves – and the 1993 physics Nobel prize was given out for the evidence that gravitational waves existed exactly as predicted by Einstein's general theory of relativity.
The Higgs boson is a boson (i.e. not fermion) but it's the only boson in the list that doesn't mediate a fundamental force. It's still very important in the scheme of the Universe because it guarantees that W-bosons, Z-bosons, (charged) leptons, and quarks are massive – via the Higgs/BEH mechanism. The Higgs boson was discovered last July.
Quarks differ by their carrying a color - interacting via the strong force (one mediated by gluons and described by QCD). Leptons don't carry any color so they don't interact by the strong force – which is the reason why their name, "leptons", is related to words like "skinny" in Greek.
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What it would look like I can't say (that would be a heck of a challenge to work out from first principles for stuff made of protons and neutrons and electrons never mind other baryons).
The properties of those composite baryons is better know.
You'll find a list of baryons here together with their known properties (not all known !).
The key number is mean lifetime. Of these particles only the proton is thought to be stable (we've never detected a decay and we've been looking). The neutron has a lifetime that's short in human terms, but long enough for it's role in the nucleus (where things work slightly differently anyway).
The rest are either unobserved or have extremely short lifetimes.
These can't (as far as we know) form stable atoms. There is no baryon that could "replace" a proton as a nucleus for a stable Hydrogen-like atom. That pretty much rules out stars as we know them and heavier nuclei like Helium and beyond.
So no people, or planets as we know them.
In fact the universe had a chance to "go in this direction" for want of a better expression. When hadrons were first formed after the Big Bang, the fact is that only the proton and neutron were stable enough to form atoms. The rest just can't exist long enough to do that.
So in a sense the universe has already tested this idea and "decided" it was unworkable : it's survival of the fittest out there !