Lepton and quark generations.

[GutZ]

From what I've read it seem to say that Lepton; Electron, Muon, Tau, with their respective neutrinos, and quarks: Up, Down, Top, Bottom, Charmed, Strange....*takes breath* all are split up into different generations. So my question, does that mean that they actually were created in different generations or is that just a way to express them? I know higher generations decay into smaller ones, so is that a discrete value at which they start to decay?

 

Maybe just an example of how one changes to another and why, would help me out now.

[Severian]

The three generations are identical copies of one another except for their masses. No-one knows why there are 3, although certain effects (like CP violation) would be absent with only one.

 

The heavier generations just decay into lighter ones because they are heavier, e.g. [math]\mu^- \to W^- \nu_\mu[/math] (where the W^- is virtual) followed by [math]W^- \to e^- \bar \nu_e[/math].

[5614]

I made you a generation table:

Where:

Ve = electron neutrino

Vu = muon neutrino

Vt = tau neutrino

u = muon

and all the others are the normal meanings. I am aware that it should not just be a normal 'v' to represent neutrinos, but this is Paint! It'll do for now! (Same with mu and u for a muon.)

 

OK, so the 4 fundemental particles in the middle red box are first generation. The 4 particles in the 2nd blue box (Vu, u, c, s) are second generation and the outter 4 in the green box (appologies, when I saved as .jpg the blue and green now look similar) are the third generation.

 

This brings us back to the question; what's the difference?

 

There are only two differences.

1) energy (or mass, same thing)

2) flavour

As labelled in the diagram, oh, I wrote mass in the diagram and not energy, nevermind, same thing! The lower generations have a lower energy, this means that they are more stable. So if you look around you will see, or the things you see will be made up of, first generation particles. This is because particles prefer the lowest energy state they can reach, and attain it through decaying. Atoms are made from up and down quarks and orbitting electrons. Whilst the electron neutrino does not easily interact with anything, they are almost everywhere.

 

In particle accelerators where there are a lot of high energy particles it is common place to find charm and strange quarks (as well as 3rd generation particles), although remember that quarks are never isolated due to confinement, they are always in groups of quarks. There are some theories which say that at even higher energies there may be a 4th generation, however the Standard Model and some other experiments suggest that it is theoretically impossible to have a 4th generation.

 

So my question, does that mean that they actually were created in different generations or is that just a way to express them?

I'm not quite sure what you mean by that. They are grouped that way according to their energies. Whilst I would think it more likely that higher energy particles were only discovered more recently, as high energy particle accelerators were only made more recently, I do not know the precise order in which the particles were discovered, nor does it have anything to do with which generation they are in.

 

Maybe just an example of how one changes to another and why, would help me out now

As for an example of one changing, I know one example off the top of my head:

top quark --> bottom quark + W boson

I know that one because that decay happens so quickly that the top quark does not even have time to form a hadron, ie. it effectively bypasses the confinement rule, which says that no quark is ever found by itself, because it decays so quickly.

 

Long post, hope it helps, although I suppose you know some(/most?) of it already.

[GutZ]

I see, I knew most of that, but some I didn't so thanks.

 

I guess I mean more like; at a specific time in the past (very early) did...say the tau only exists, then later on it lost energy and the muon was created, and so forth? or were they all created at the same time and you only see them at high energy collisions?(If thats the right terminology...) or shorter is the term generation used literially?

 

I guess I should think more in terms of different energy levels and spins, instead of names making me think they have more then just that, that makes them unique.

[YT2095]

5614, Nice Post!

[5614]

Thank you very much YT! Always good to hear

 

at a specific time in the past (very early) did...say the tau only exists, then later on it lost energy and the muon was created, and so forth? were they all created at the same time and you only see them at high energy collisions?(If thats the right terminology..

Well, the only time you could be referring to is the big bang. I don't know, but using logic I can say that at the time of the big bang, when the energies were massive there most certainly would be a lot of high energy particles, so there would be a majority of quarks leptons being tau.

 

Tau quarks particles will quickly decay into other lower energy particles, however if we go to a moment after the big bang but before tau quarks particles had time to decay... I'm not sure, we're getting into the region of what exact particles were created at the instant of the big bang - no one knows. Were there only tau and no muons created? Was there a bit of all particles? Was there some even heavier undiscovered particle which decayed into a tau? The last is unlikely, but as far as I know we don't know which, if either, of the first two it was.

 

You do see high energy particles during high energy collisions. So if you have an e- and an e+ with a high kinetic energy, then when they annihilate the photon produced will have sufficient energy to split into other particles. Or you could get no photons and some other particle instead, such as a W+ and W- pair or a single Z boson.

 

As for how to think of it; when someone names a particle I think of it as just that. But when I think of them in relation to their generation I see electrons and e.neutrinos as, well, 'normal' particles. Then the 2nd generation as big things zipping around quickly with the view of decay, then as 3rd generation particles as rare, massive particles with a lot of energy that will decay, most probably, very quickly.

[GutZ]

Oh there are tau quarks? I thought they were elementry particles. It hard rememebering all the combinations.

 

Thank-you very much 5614, all the generation stuff makes sense now. and of course thanks to severian for the inital reply, and YT for the compliment on 5614 post, you modivate people well.

[5614]

Ooh dear gawd, tau quarks, doh! No, there's no such thing!

 

A tau is a fundemental particle and is a lepton, not a quark! There are only 6 quarks, and they are the 6 quarks (u, d, c, s, t, b) which can be found on the right side of the generation diagram. There's also another 6 corresponding anti-quarks, although generally you would say there are 6 quarks, it almost goes without saying that they have anti-quark partners which you're not including in the 6.

[GutZ]

Ooh dear gawd, tau quarks, doh! No, there's no such thing!

 

haha, I would of believed you!

[5614]

Lucky you asked then. Always check if you're not sure, everyone makes mistakes.

[timo]

Since 5614 replaced the term quark with particle: Of course taus are particles but the term you were probably looking for is lepton.

 

About tracing back to high energies: I see no initial reason why taus should be produced more than electrons. At most, I´d see a point in claiming that the energies involved are so high that one can neglect the masses of the particles and that therefore electrons behave exactly as taus or myons (because they are equal except for their mass). A little catch there is that I doubt that you can even speak of particles in the normal sense in a scenario such as early stages of the universe (normally particles are defined as free, non-interacting particles).

 

The term generation is not related to an order of production in the universe.

[GutZ]

hmmm, ok I think I understand that.

 

I guess I was thinking as well that maybe decay had some relationship with the universe (in a sense, I don't know how to properly word it) so during an earlier time the "heavier" particles would not decay, then over time they would, given that the conditions were there to allow it.

 

But like you said, it has nothing to do with the production. I don't know how you guys do it, so much information, I'd go loony.

[Royston]

This is the second subject today that I've recently written essays about...Gutz, when I get back tonight, I'll give you a short basic order of events if that helps, starting from the planck era, and what forces became distinct as the universe evolved as well as the times quarks and leptons and then hadrons were created. Then you might see why certain masses of particles don't coincide with certain epochs.

[GutZ]

Alrighty, thanks.

[Royston]

Hold that thought, but I'll promise I'll post you an order of events tomorrow sometime...just had a beer, and my essay is strewn about my bedroom somewhere. I won't be able to do it from memory without making the odd mistake or three.

[Royston]

Right, around 5 x 10-44 seconds after the big bang, the universe reached the planck epoch, very hot, very dense...light could travel a mere 10-35 m at this time (planck length) travelling at 3 x 10^8 m s-1, and the mean energy per particle was 10^19 GeV...so time, size and energy are all proportional with each other. To put things in perspective, an atomic nucleus is roughly 10-14 m, so the planck length compared to an atomic nucleus, is the equivalent of an atomic nucleus compared to the earth !

 

Around this time gravity became distinct from the other 3 forces (the strong and weak nuclear forces, and electromagnetism.) So a theory of quantum gravity is needed to understand gravity at this time. It's thought matter and anti-matter were spontaneously being created from energy (photons) so...

 

photons<--->particle + anti-particle. Where two photons are produced when matter and anti matter annihilate.

 

A theoretical particle (X Boson) was responsible for converting quarks to leptons and vice versa at this time. So the X Boson, is the quanta of what is know as the grand unified theory (unification of the strong and weak interactions, and electromagnetism) though unified really means that the forces are not distinct from one another. All flavours of quark/anti-quark and lepton/anti-lepton were created at this time, where the matter or anti-matter X boson could decay into either matter or anti-matter particles.

 

Roughly 10-36s after the planck time, the strong and electroweak forces became distinct (electroweak being the weak nuclear force and electromagnetism being combined or really non-distinct...the Higgs boson is thought to be particle that unifies (for want of a better word) these two processes) The universe had expanded and so cooled down to 10^28 Kelvins...think of it as a gas, when it's condensed more collisions, more energy, as it expands the mean energy drops. In fact between this time and 10-32 s, the so-called inflation period caused rapid expansion, which explains the size of the universe today.

 

The 'desert' period came after this, where no new processes arose until about 10-11s...Gutz I've just realised I have to be somewhere, but I'll carry on late this evening.

[Severian]

Not to be picky, but you do realise that everything so far, up to the 'desert' has been complete speculation?

[Royston]

Yeah, but thanks for clarifying anyway, I was only half way through the post...if Gutz is interested in the rest, I'll carry on.

[GutZ]

Very much so, I'd like all the information I can get, I get so frustrated not know how the universe works. I have to look up some of this planck units (probably the smallest measurement possible from what I read) to understand better, but I'll get it all eventually.

[Guv]

The higgs boson would be interesting to look up, still speculation but may have the largest effect on the mass of particals. (generational effect)