How do scientists know the structure of the nucleus

[dangerbird]

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

 

[malawi_glenn]

what is an atom smasher? :P

But basically, at collider experiments, one performs scattering:

http://hyperphysics.phy-astr.gsu.edu/HBASE/rutsca.html

that site as more good easy read things about nuclear physics

 

[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.

 

[dangerbird]

interesting stuff thanks

 

[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.

 

[jtbell]

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.

 

[malawi_glenn]

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.

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 a lot of gluon content and quark-antiquark soup, which one discover in deep inelastic scattering.

 

[humanino]

one can not find that there are 3 quarks in a nucleon

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.

 

[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.

 

[malawi_glenn]

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.

again, you are only referring 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:

The quarks that contribute to the quantum numbers of the hadrons are called valence quarks. Hadrons also contain virtual quark–antiquark pairs, known as sea quarks, originating from the gluons' strong interaction field.

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
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-3527406018.html

 

[humanino]

again, you are only referring 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.

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.

 

[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

 

[Lok]

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

 

[malawi_glenn]

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

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

 

[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

 

[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&

 

[dangerbird]

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&

gooooooood