CarlB said:
(1) +++
(3) ++-, +-+, -++
(3) +--,-+-, --+
(1) ---
This is also related to the use of spinors to build representations of SO(2n).
I thought a little about it, the passing from a - to a + being equivalent to the application of a fermionic creation operator. If this operator has some charge, then we see how the charge progresses. For instance assume we use two fermionic operators of electric charge +1 and we start from a particle of charge q. We have
-- : one particle of charge q
+-, -+: two particles of charge q+1
++: one particle of charge q+2
and if we ask the final particle to be opposite of the starting one, then -q=q+2, and q=-1. Obviusly, with three operators, -q=q+3, and q=-3/2 and so on.
If we ask the final particle to have the same charge than the starting one, then there is no way to solve q=q+2; we need more quanta of charge in the SU(n) way: colours. So for 2 creation operators
--: neutral
-+: North, +-: South
++: NorthSouth, ie neutral again
For 3:
--- neutral
+-- Red -+- Blue --+ Green
++- RB, +-+ RG, -++BG
+++ RGB
and so on. But then what is funny is to consider both
----- neutral
+--- R, -+--- G, --+-- B, ---+- N, ----+ S
.
.
.
+++++ RGBNS
For the last one to be neutral, you may to consider that NS cancel as well as RGB. Got 32 particles
12 coloured: (RGB)0, (RGB)N, (RGB)S, (RGB)NS
12 anticoloured: [RGB]0, [RGB]N, [RGB]S, [RGB]NS
8 uncoloured: 0, N, S, NS, RGB, RGBN, RGBS, RGBNS
No surprise as we are building a kind of SU(5) or SO(10), the GUT groups.
Looking to the uncoloured sector, aiming to get leptons, we can first try to set the electric charge of N to be +1, the one of S to be -1, and then... the quarks do not fit.
We need to relax the N,S sector. For instance, take N to be of electric charge +1, S to be 0, and RGB of charge -1/3. Then RGBNS is still neutral (but neither RGB or NS are). You can check that with this assignation we get the right electric charge:
(RGB)0 = -1/3
(RGB)N = +2/3
(RGB)S= -1/3
(RGB)NS= +2/3
etc.
I think this is one of the traditional explanations of how to fit the standard model into SO(10).
A worry happening here is that if we consider that we are jumping from ----- to +---- and then to +--+- and so on by using fermionic creation operators, then half of the fermions have a sort of wrong grading. The answer could be to use a sixth generator, neutral, only to duplicate the number of elements. Then for each combination of charges we have one element with even grade and another one with odd grade.
Superpartners, if you wish. But with a minor objection: if every application of the fermionic creation is expected to increase the spin by +1/2, we need to start with a neutral particle of spin -3/2 to get 15 particles of spin -1/2, another 15 of +1/2 and another one of +3/2. Note here "a particle" means "a degree of freedom".