Understanding Particle Generations: Organization and Similarities

[Antuanne]

What are generations of particles and how are they organized into the different groups. What similarities do two particles in the same generation share?

 

[mfb]

Wikipedia article
If you don't understand something specific, feel free to ask :)

And see https://www.physicsforums.com/blog.php/blog.php?b=3588 please.

 

[Antuanne]

What I don't understand is why particles are categorized in the generation they are? Why are electron neutrinos in the first generation and why are muon neutrinos in the second? Does it have to do with weight?

 

[mfb]

It has been observed that electrons, muons and taus are very similar - and sorted by mass (and discovery date), they get their own generation. All 3 come with a neutrino, which gets the same generation. The order of their masses is unclear here, but it does not matter.

Similar for down, strange, and bottom-quarks (all with negative charge): Sorted by mass and discovery date.
up<-> down, strange<->charm, bottom<->top are linked, so they get the corresponding generation. This agrees with the masses and discovery dates as well. In addition, the link between strange/charm and the other two generations is stronger than the link between up/down and top/bottom.

 

[Antuanne]

Thank you, but this raises another question. Do the masses of neutrinos increase as they go up generations or are they just organized into their generations by their discovery dates?

 

[Antuanne]

Generation of the Three Neutrinos

Why is it that each the electron, the muon, and the tau each come with a neutrino? Why is the electron neutrino grouped with the electron, and why the muon neutrino with the muon and so on? Do they have similarities between interactions?

 

[mfb]

The neutrinos are put in the corresponding generation of the charged leptons: There is one neutrino which couples to the electron (1st generation), that is called electron-neutrino and it is in the first generation. And similar for the muon- and tau-neutrino.

Do the masses of neutrinos increase as they go up generations

That is unknown at the moment. There is the additional problem that neutrino mixing complicates the definition of "mass of the electron-neutrino".

 

[kurros]

Well, I am not exactly sure how to answer in a straightforward way, without talking about group theory. The charged leptons and their partner neutrinos are intimately connected, indeed before electroweak symmetry breaking they can be well considered to be two pieces of the same particle, or at least identical copies of each other (the charged leptons have no charge or mass before this occurs, and the neutrinos also have no mass, so they could not be distinguished from each other either). You can "mix" the two pieces together in various ways and the physics is unchanged, since this "doublet" is "symmetric under SU(2) transformations".

However, electroweak symmetry breaking occurs in such a way as to give half of the doublet electric charge and mass, but not the other half, thus distinguishing electrons from electron neutrinos, and so on. The "pairing" remains, however, and so during electroweak interactions electrons can "transform" into electron neutrinos by emitting or absorbing W bosons (and similarly for the other generations), and vice versa.

edit: Oh, weird, for some reason none of the other responses were visible to me before I posted this. Oh well.