I don’t know if there is a quick and dirty answer to this, but it struck me just now that of all the subatomic particles I know about, only quarks come in so many varieties. How do we know that up and down quarks aren’t, for example, really quarks and squarks? Especially since the theory that discusses quarks details the very point that they cannot exist in isolation?
Are there other particles besides quarks which come in so many darned varieties?
It depends on what you consider two different particles, and what you consider different types of the same particle. So, there are six kinds of particle (plus their six antiparticles) which all share some similar properties. For the sake of keeping things organized, we call all of those things “quarks”. Similarly, there are six different kinds of leptons (plus six antileptons), but the only one we deal with on a regular basis is the electron. And there are five or six different vector bosons (counting particles and antiparticles), one of which is the photon.
But even at that, that’s the simplest way we can categorize them. Take a look at all of the baryons (particles composed of three quarks, such as protons) or mesons (composed of a quark and an antiquark) sometime. Or all of the hadrons, which includes the baryons and mesons. There are dozens which have been discovered, and probably many more not yet discovered. It was only with Gell-Mann’s theory of quarks that the vast “particle zoo” started to make some sense.
IANAHighEnergyPhysicist, but I know that many people are uncomfortable with the seemingly complex nature of the quarks, with colors and spin and charge and ‘flavor’ and believe that this complexity is evidence that there remains an underlying explanation we haven’t yet uncovered. So in some sense, there might be many types of quarks because the quarks themselves are just made up of other, even more fundamental particles.
Also, don’t forget that science generally doesn’t concern itself with the “why.” It can explain what stuff (apparently) is and how it (apparently) works, but the “why” is left to the philosophers.
I can perosnally recomned this book “The Particle Explosion” by
Frank Close, Michael Marten, Christine Sutton.
I have read it through a couple of times, although that was a while ago now. One detail that lodged in my mind was that there are more sub-atomic particles known than there are atomic elements!
It was by tabulating the sub-atomic particles, in much the same way as the elements were tabulated in the periodic table, that allowed as yet undiscovered particles to be predicted and first provided evidence of an underlying structure.
I am sure that I remember reading somewhere that scientists thought there were good reasons for thinking that quarks were themselves fundemental particles. I am curious to know why that should be - scientists have said that sort of thing before.
A squark, being the supersymmetric partner of the quark, by definition would have a different spin (zero, IIRC, as opposed to 1/2 for quarks). All varieties of quarks have spin 1/2, so none of them can be squarks.
Some “preon” theories have suggested that quarks are made up of even more fundamental particles. These have not been successful - they predict particles that have not been seen, don’t answer important questions about quark properties, etc.
Leptons come in a similar array as quarks. There are six quark flavors (up, down, strange, charm, top, and bottom), and six lepton flavors (electron, muon, tau, e neutrino, mu neutrino, tau neutrino). The big difference is that each quark flavor comes in three colors, but the leptons have no color. (This is not their actual visible color but a mathematical property which is called “color”.) When you line them all up it makes a very pretty table (like the periodic table of the elements!) but as yet there is no good explanation for the patterns we see.
It struck me this morning to wonder if “quark” was more like “lepton” than it was “electron”. I see my fellow dopers have done me well here. Chronos, your post makes me want to ask a thousand more questions, lol, but I’ll try to stay on topic here.
Cervaise, it wasn’t meant to be an ontological question per se, but we have a theory of quarks that someone created and in this theory we have particles (I thought!) that come in so many darned varieties that no other particles did. So why did this situation happen? You see?
FriendRob, :o I didn’t mean to wonder about supersymmetric partners at all. Squark just rhymed with quark in this case. Sorry 'bout that.
But… leptons have no color. As a rule? Are there particles we can (in principle, according to the current theories) produce or “see” that will have an overall color?
Most theorists believe in “color confinement”. Roughly, it would take an infinite amount of energy to produce a particle with overall color. This has never been rigorously proven, AFAIK. Maybe we just need a bigger collider. Thus you can only “see” color by going inside a proton or neutron, or going back to the early Big Bang when the universe was so dense that it was effectively a single huge nucleon.
In some unified theories, “leptonness” is a fourth color, denoted “lilac” (since it starts with “l”). But in the Standard Model leptons have no color, ever. Color is the “charge” that the Strong force sees, just as electric charge is what the electromagnetic force sees. (Technically, the Strong Force is an SU(3) Yang-Mills gauge theory. Sorry, I just wanted to type that.) Leptons don’t participate in Strong interactions.
“Color is the ‘charge’ that the Strong Force sees…” That helps clarify things quite a bit for me, actually. How damn interesting.
Thanks…
Does anyone have a link to a picture of this nifty table of subatomic particles?
I can’t seem to find it on Google.
A bigger collider wouldn’t help. Color confinement comes from the properties of the color force. When you pull a negatively charged particle away from a positively charged one, the force between them falls off with distance. This means that you only need a finite amount of energy to move the charged particles essentially infinitely far from each other.
With color charges, the carriers of the color force themselves have color, and the force between the colored particles remains constant, and the amount of energy required to separate two of them is proportional to the distance of separation. The important feature of this is that as you pull them apart, you eventually put in enough energy to create a quark - antiquark pair, so the quark you were trying to isolate acquires a new companion which cancels out its color.
This leads to predictions of particle ‘jets’, where a quark has enough (kinetic) energy to make several quark-antiquark pairs, and those additional particles all head off in the same direction. These jets have been observed (see figure at bottom of link).
I seem to remember something about quarks possibly having an internal structure…made up of particles called Rishons .
I googled, but only found references to Judaism.
Anecdote time:
Up and down were the first quarks discovered. Then they discovered a third quark. This quark didn’t appear in protons and neutrons and so they though it strange. That was a bit harsh on the poor quark, so not making the same mistake twice, the next quark in the standard model was given a more charming name, “charm”. Well, from strange and charm it was on to the “b” quark and the “t” quark which were given the overly fanciful “beauty” and “truth” names before the particle theorists got their heads out of the clouds and settled for “bottom” and “top”.
I think this is basically a mixture of reality and myth, but a particle physicist told me the story and I thought the best venue for sharing it would be here. Hope you all enjoyed it.
