# How do scientists know the structure of the nucleus

1. Mar 25, 2009

### dangerbird

was it something that was figured out solely through atom smashers?

Last edited: Mar 25, 2009
2. Mar 25, 2009

### malawi_glenn

3. Mar 25, 2009

### Bob S

The first indication of the nuclear size was scattering alpha particles (from radioactive decay) off various nuclei including thin gold foils by Rutherford in about 1909, which showed that the nucleus was not like plum pudding but a very small concentrated positive charge.

4. Mar 25, 2009

### dangerbird

interesting stuff thanks

5. Mar 26, 2009

### Lok

The latest experiment i've read about was deep inelastic scattering, using high energy electrons to study the nucleons. The results were as predicted: 3 particles(quarks), signs of internal structure among the particles. and the fractional charge predicted by the quark theories.

6. Mar 26, 2009

### Staff: Mentor

Those are not exactly new. Deep inelastic scattering experiments have been done as far back as the late 1960s. They've been done with electrons, neutrinos and muons, at least.

7. Mar 26, 2009

### malawi_glenn

well, one can not find that there are 3 quarks in a nucleon, since they have more than just 3.. There is a difference between the naive quark model of hadrons and what the hadrons really are composed of. In the hadrons there is also alot of gluon content and quark-antiquark soup, which one discover in deep inelastic scattering.

8. Mar 26, 2009

### humanino

It's called "Gross-Llewellyn-Smith rum rule". You get it from the integral of the sum of the structure functions $F_3$ for neutrino and antineutrino.
$$\frac{1}{2}\int dx \left(F_3^{\nu p}(x,Q^2)+F_3^{\bar{\nu} p}(x,Q^2)\right)=3\left(1-\frac{\alpha_s}{Q^2}-3.58\left(\frac{\alpha_s}{Q^2}\right)^2\cdots\right)$$
If that's what you mean, the result is not exactly 3, it depends on $Q^2$, but quite in agreement with what we expected, there are 3 valence quarks in the proton.

9. Mar 26, 2009

### Lok

Wikipedia: In principle, some baryons could be composed of further quark-antiquark pairs in addition to the three quarks (or antiquarks) that make up basic baryons. Baryons containing a single additional quark-antiquark pair are called pentaquarks. Evidence for these states was claimed by several experiments in the early 2000s, though this has since been refuted. No evidence of baryon states with even more quark-antiquark pairs has been found.

I never heard of more than 3 quarks in a baryon, even though some theories predict them at relatively low energy levels, observable in nowadays experiments. I'm willing to wait for results in this direction, but i am skeptical about it.

10. Mar 26, 2009

### malawi_glenn

again, you are only refering to the naive quark model, which treats the so called "valence quarks". You can read about the sea-quarks in any book on particle physics.

http://en.wikipedia.org/wiki/Quark

wiki:
http://arxiv.org/abs/nucl-th/0003061

Books:
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-3527402977.html
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470032944.html [Broken]
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-3527406018.html

Last edited by a moderator: May 4, 2017
11. Mar 26, 2009

### humanino

The putative pentaquarks, as their name suggest, have 5 valence quarks (or 4 quarks, plus one antiquark). The concept of "valence quark is not limited to "naive quark models". The structure function I was referring to above have certainly little to do with "naive quark models". There is nothing wrong in principle with describing a hadron as a tower of Fock states, the valence part of which determines its quantum numbers. The reason people think of this picture in terms of "naive quark models" is only because in is difficult to calculate in practice.

12. Mar 26, 2009

### malawi_glenn

That is true humanino, I just wanted to point out that there is a difference on talking about the sea-quarks and the valence quarks. It seemed to me that Lok did not know of this

13. Mar 26, 2009

### Lok

We do still talk about theory here. If not Thanks for the input.

14. Mar 26, 2009

### malawi_glenn

theory? The sea-quarks are found experimentally as well.

15. Mar 26, 2009

### dangerbird

k so the way its known is by colliding particles into each other.
what source did they use to make muons for the scattering experiments? where do those come from? did they come from the moon

Last edited: Mar 26, 2009
16. Mar 27, 2009

### malawi_glenn

why should they come from the moon? :-S

One simply produced muons by colliding particles at a target, and then filtered what was produced. One can use magnetic fields etc to pick up those particles that are interesting for ones purpose. http://www.isis.rl.ac.uk/MUONS/index.htm?content_area=/MUONS/muonsIntro/whatMuons/whatMuons.htm&side_nav=/MUONS/muonsSideNav.htm& [Broken]

Last edited by a moderator: May 4, 2017
17. Mar 27, 2009

### dangerbird

gooooooood

Last edited by a moderator: May 4, 2017