What was the first discovery of the delta baryon $\Delta^{++}$?

Question

The delta baryons (also called delta resonances) are a family of subatomic hadron particles which have the symbols $\Delta^{++}$, $\Delta^{+}$, $\Delta^{0}$, and $\Delta^{−}$ and electric charges +2, +1, 0 and −1 elementary charge respectively.

1. i want first reference claims delta baryon discovery

2. why the mass of all delta particles are the same!?

3. proton and neutron quarks content are the same with $\Delta^{+}$, $\Delta^{0}$, So why they have so different mass!?

Answer 1

You may find useful this paper from the particle data group listings:N AND Delta RESONANCES . It gives a concise explanation and references of what the resonances are. You may chase the original references there.

There are many Delta resonances as you can see in the table, with different masses. It is only within each isospin multiplet of mass that the mass is the same. The are named N resonances if the isospin is 1/2 ( nucleon like) and delta, if it is 3/2.