# I How do we know what a neutron is made of?

1. Jul 4, 2018

### momoichi8382

we have physical evidence for the composition of a proton because it decays from a neutron

however all experiments such as https://en.wikipedia.org/wiki/Deep_inelastic_scattering etc, always involve a proton and possibly a neutron, in fact you can't accelerate a neutron without a proton, deuterium etc also must have protons, targets etc. have protons, since all these experiments about neutrons have protons how do we know we're not getting readings from protons instead of neutrons

what i'm asking is, a neutron is said to be 2 down quarks and 1 up quark, well how do we know a neutron isn't actually 3 up quarks which then decays into 2 up and and 1 down when it turns into a proton, i'm not saying i believe a neutron is actually 3 up quarks, i'm just saying how do we know it isn't?

do we know it isn't from purely mathematical reasons or can we experimentally prove it as well? do we simply know because we smash atoms together and count the ratios of up and down quarks?

Last edited: Jul 4, 2018
2. Jul 4, 2018

### mathman

Up quarks have a charge of +2/3, while down quarks have a charge of -1/3. Since the neutron has a charge of 0, it must consist of 1 up quark and 2 down quarks. For a proton to get a charge of +1 it needs 2 up quarks and 1 down quark. The experimental evidence is for the existence of quarks inside these particles.

3. Jul 4, 2018

### momoichi8382

see but that seems like more of a mathematical proof, for all we know a quark could attract itself, without a positive or a negative charge (i don't believe such in the slightest), but i think if it's only mathematical we're grasping at straws when we say a neutron is 2 down and 1 up

i don't believe it is purely mathematical because my knowledge about quarks is very poor, so i'm hoping responds with experimental observations

4. Jul 5, 2018

### Staff: Mentor

The total electric charge is always the sum of the components. This is independent of any attraction or not. This alone shows that the neutron has to have two down one up already.

We have found the particle made out of three up quarks. It is called $\Delta^{++}$. Due to symmetries it has to have a mass significantly above the mass of proton and neutron (1232 MeV instead of 940 MeV). This has been predicted, and it has been measured.

Experiments with deuterons or heavier nuclei show the total amount of quarks in these nuclei. You can subtract the contributions from protons and get the contributions from neutrons - twice as much down as up.

And so on. There are literally hundreds of measurements that would all be in horrible disagreement with predictions if the neutron would be made out of three up quarks. The non-conservation of charge would be the most obvious one.

5. Jul 5, 2018

### Droidriven

I'm no expert, but this makes no sense from my simple perspective. It attracts itself with no charge? Wouldn't that go against symmetries and charge=attraction/opposition? Without charge, what do you propose would cause it to "attract itself"?

How about gauge symmetry or the usage of arbitrary "names", such as up/down or +/-, and whether our picture or description "matters" to nature and what is actually going on. One of the non-descript "it is that way because it is, however we choose to label it"

Last edited: Jul 5, 2018
6. Jul 5, 2018

### Orodruin

Staff Emeritus
The nucleons are held together by the strong interactions, not the electromagnetic one.

7. Jul 6, 2018

### Droidriven

I was replying more to his comment about "attracts itself" than what does or doesn't actually bind them.

8. Jul 13, 2018

### DarMM

I've never been sure if talk about nucleons being "held together" by the Strong Force or them being "made of quarks" actually make sense. When you look at QCD nonperturbatively protons and neutrons are simply eigenstates of the Hamiltonian and quark and gluon states aren't elements of the Hilbert space.

Sometimes it seems that quark and gluon fields simplify the field algebra, at the cost of unphysical states.