Problems with fundamental particles and quarks

[Owen-]

Problems with fundamental particles and quarks :(

Hi, I have no idea where to post this so I hope its ok...

I'm studying A level physics. We have a topic on fundamental particles.

Yea couple of questions...

1. Whats the difference between a (insert lepton here)-neutrino, and its antiparticle, or any particle that has no charge and its antiparticle

I know they have negative lepton/baryon numbers, but in reality what effect does that have...? (also the difference between the pion pi0 and its antiparticle...?)

2. How does one determint the quark composition of... say a proton. its supposed to be up up down. why could it not be top top bottom, or charm charm strange, when all these have the same baryon number and charge respectively...

3. How is a "fundamental particle" fundamental if its made up of quarks

4. I don't get what generations mean either...

Sorry about noobish questions but I am totally confused :s

Thanks,
Owen.

 

[diazona]

1. A particle can annihilate with its antiparticle to produce something else, typically a photon. So there is definitely a physical difference between the particle and its antiparticle, even if it's not something so obvious as charge. The lepton/baryon number is one way we have of quantifying that difference.

2. Different quarks have different masses. Also, the heavier quarks (charm/bottom/top mostly) are unstable, i.e. they will spontaneously decay into lighter particles in a short time, so if they were in the proton, we would expect protons to decay quickly. That doesn't happen.

3. It's really not. People just say that sometimes, because once you get down to protons and neutrons, it's all really really small

4. Generations are kind of like rows in the periodic table. The first generation/row contains the up and down quarks, and the electron and its neutrino. The second generation contains a set of particles that are pretty much identical except for having higher masses (except that we're not sure about the neutrino). Same for the third generation.

 

[Owen-]

Ah thanks a lot - cleared pretty much everything up for me - your a hero :)

 

[Parlyne]

Let me add to the explanation for number 1 that it is, in fact, possible that neutrinos are their own anti-particles. (In the literature, this is discussed under the name "Majorana neutrinos.") In fact, every model I've seen that has a natural way of making the neutrino masses small without unreasonably tiny Higgs couplings uses Majorana neutrinos.

This, of course, violates lepton number. However, even in the standard model, lepton number alone is not always conserved. There are non-perturbative electroweak configurations called "sphaelerons" which break the individual conservations of baryon and lepton numbers, but conserve their difference. Making neutrinos Majorana particles would violate this conservation as well.

 

[tom.stoer]

Regarding 2: there are a lot more possible classifications of a particle than only via its electric charge; two examples are
- the different types of quarks are classified acdcording to their flavor (u, d, ...);
- the strong interaction couples not to electric charge but to "color" (not the color we can see, of course :-)

Mathematically flavour, color etc. are something like "charge"; they share some properties with the electric charge.

 

[Prathyush]

1. A particle can annihilate with its antiparticle to produce something else, typically a photon. So there is definitely a physical difference between the particle and its antiparticle, even if it's not something so obvious as charge. The lepton/baryon number is one way we have of quantifying that difference.

Can a neutrino and anti neutrino annihilate into a photon? There is no vertex "neutrino photon neutrino" vertex in the standard model.

 

[tom.stoer]

Neutrinos interact only via Z-bosons; so they would annihilate into Z's

 

[Parlyne]

Can a neutrino and anti neutrino annihilate into a photon? There is no vertex "neutrino photon neutrino" vertex in the standard model.

This would have to be a virtual photon; but, the answer is yes. There's a 1-loop diagram that creates this coupling. Essentially, the neutrino emits a virtual W^+ and becomes a charged lepton of any flavor, the antineutrino absorbs the W^+ and becomes the corresponding antilepton, and the charged leptons annihilate into a (virtual) photon.

 

[tom.stoer]

OK, via higher loops it's possible. But is there a way to have only photons in the final state?

 

[Parlyne]

OK, via higher loops it's possible. But is there a way to have only photons in the final state?

Sure. There are box diagrams that give \nu \overline{\nu} \rightarrow \gamma \gamma. They're highly suppressed, but definitely there.