Some people naturally assume that atomic nuclei are made of protons and neutrons. That is, they are basicly clumps of protons and neutrons that each maintain its separate existence, like pieces of gravel maintain their existence if you mold them together in a ball with mud for a binding force.
How come neutrons in a nucleus don't decay?
This is a natural assumption. A hydrogen nucleus can have one proton as its nucleus. Nuclei can absorb neutrons to become other isotopes. It's natural to assume that nuclei are clumps of protons and neutrons.
Sometimes if an atomic nucleus gets broken by application of large amounts of energy, typically applied with a fast-moving subatomic particle, they might release a neutron or a proton. So for example, smash an alpha particle into a beryllium nucleus and a neutron comes out. Doesn't that imply that the neutron was in there all along, waiting to get out?
But that reasoning implies that electrons, positrons, muons etc are also inside the nucleus all the time, waiting to get out.
There's an idea that protons and neutrons inside a nucleus swiftly transfer charges. This is analogous to a theory from organic chemistry, where sometimes single and double bonds switch back and forth, increasing stability. We could have quarks getting exchanged rapidly between protons and neutrons, increasing stability. I can see that as increasing stability for the nucleus, but I just don't see it as making the protons and neutrons more stable. If ten Hollywood couples get repeated divorces and marry each other's exes, you wouldn't say that the original marriages are stable.
In the extreme, the quarks might just wander around in a nuclear soup, and the protons and neutrons have no more identity than a bunch of used computers disassembled with the parts on shelves for resale. Maybe you could collect enough parts to take a working computer out of the store with you, but it probably won't be one of the old computers.
So — my question — is there experimental evidence that strongly implies protons and neutrons maintain their separate identities inside atomic nuclei? Or is there data which can be interpreted that way but which can also be easily interpreted another way?
Best Answer
When you ask "do protons and neutrons retain their identity in nuclei" and fail to mention isospin, there is clearly a misunderstanding.
So, a simpler question: Do electrons in a neutral helium atom maintain their spin alignment? Where the electrons are added to the $S$ shell one by one, first spin down, and then spin up to complete the shell without violating Pauli's exclusion principle.
Well that is wrong. The electrons fill the shell with a totally antisymmetric wave function, and all the antisymmetry is in the spins:
$$ \chi = \frac{|\uparrow\rangle|\downarrow\rangle-|\downarrow\rangle|\uparrow\rangle}{\sqrt 2} =|S=0, S_z=0\rangle$$
Each electron is in an indefinite state of $s_z$, but the total state is an eigenvalue of $\hat S^2$ and $\hat S_z$.
Isospin is called isospin ($\vec I$) because it replaces spin ($\vec S$) up/down with the exact mathematical formalism of spin, except with proton/neutron being eigenstates of $\hat I_3$.
So what's a deuteron? It's mostly $S$-wave (with some $D$-wave)...so it's even in spatial coordinates. The spin is one, so it's even in spin. That mean the antisymmetry falls on isospin, where it's clearly $I_3=0$. To be antisymmetric, it must be $I=0$, too:
$$|I=0, I_3=0\rangle = \frac{|p\rangle|n\rangle-|n\rangle|p\rangle}{\sqrt 2}$$
Since the 1st (2nd) bra refers to particle one (two), it is clear that individual nucleons are not in definite states of $I_3$ in a deuteron. In other words, they're 1/2 proton and 1/2 neutron, just as two electron spins aren't definite in the $S_z=0$ states.
There's no need to reference quarks to resolve this, as:
$$ p \rightarrow \pi^+ + n \rightarrow p $$ $$ n \rightarrow \pi^- + p \rightarrow n $$
is perfectly fine (where the intermediate state is a virtual pion binding stuff).
Since a helium atom was mentioned, a good exercise is to workout the spatial, spin, and isospin wave functions: it is very spherical in real space and iso-space...which is why it is so stable.