question about fundamental particles

[gib65]

How do we know when we have a truly fundamental particle? For example, how do we know that an electron can't be broken down into smaller particles?

[swansont]

Some of the ideas-

 

Can you break it down, and what would it break down into are two questions you'd have to ask. i.e. does it decay? If so, it can't be fundamental. If not, then it may just be the lowest energy state of some constituents, so you proceed.

 

Then, is there the possibility of an internal structure? i.e. what happens when you slam some of them into each other? With baryons, you get other baryons: the proton is made up of three quarks, uud, with a spin of 1/2. You can get a uud combination with spin 3/2 and a larger mass ([math]\Delta^+[/math] Sort of a proton in an excited state or resonance) You'd see these resonance in scattering experiments, which would suggest (or not) an internal structure.

 

Of course, you have to approach this from the other way: you observe these particles and measure their properties, and see the pattern that leads you to the model that they are made up of something else. Which predicts new particles, and you find evidence of them.

 

The evidence for an electron is that it's vanishingly small on its own, with no hint of structure. It's light, so there's nothing for it to decay into.

[gib65]

So I take it there is no formal proof. Seems a good guess anyhow.

[iNow]

So I take it there is no formal proof. Seems a good guess anyhow.

 

Here's a nifty little link across which I just stumbled:

 

 

http://filer.case.edu/sjr16/advanced/extras_particlephys.html

Particle Physics is a constantly changing science. It seeks to understand the fundamental building blocks of everything - the particles that cannot be broken down into anything else. Over the decades, it has spawned many new sciences based upon what it once thought was fundamental.

 

Here you will find a relatively complete overview of particle physics. This page will not delve too much into the mathematics nor physics of the subject - there are many college and graduate school classes that teach this - but you will be able to learn about the basics, and you will be able to find enough information here in order to understand the terminology in the rest of the site.

 

This page starts from the ground up, starting with the fundamental forces and then building up with Bosons and Fermions. Then, the page talks about heavier particles that are made of parts of quarks, continuing with a brief discussion of atomic physics, and culminating with a discussion of antimatter.

 

 

Be sure to click the link to see the real meat of the page.

 

 

.

[swansont]

So I take it there is no formal proof. Seems a good guess anyhow.

 

I'm not a particle physics guy, so I'm not about to start discussing the details of the standard model. At one point people thought atoms were the smallest building block, but then other particles kept showing up. So you keep looking smaller, or higher energy, figuring out the pattern, and deciding whether what you see are the constituents or the result of some other process (e.g. an electron is not part of the nucleus, though it is emitted in beta decay). At some point along the way, you come up with your model. The Higgs, AFAIK, is the last piece of the standard model. But you keep looking, in case that's not the complete picture (and there are things not predicted in the standard model that have to be measured instead).

[BenTheMan]

So I take it there is no formal proof. Seems a good guess anyhow.

 

Basically you hit things really hard and see what happens. If it breaks, then it's not fundamental, and if it doesn't break, then either you didn't hit it hard enough, or the thing is actually fundamental.

 

You are correct in that there's no formal proof, but the experiments do place a lower bound on ``compositeness'', as it were. I'll try to remember to come back and post something about how you can use dimensional analysis to look at all of these things when I have more time.