It seems that both $SU(5)$ model and flipped $SU(5)$ model are legal grand unified theories (GUT) for the standard model gauge group. In Wikipedia, it says that:

The flipped $SU(5)$ model has a gauge group breaking down $$(SU(5) × U(1)_ χ )/ Z_5 \to (SU(3) × SU(2) × U(1)_Y )/ Z_6 $$ The flipped $SU(5)$ model representation branchings (very different from $SU(5)$ model): $$\bar{5}_{-3}\to (\bar{3},1)_{-\frac{2}{3}}\oplus (1,2)_{-\frac{1}{2}}$$ ($u_c$ and $l$) $$10_{1}\to (3,2)_{\frac{1}{6}}\oplus (\bar{3},1)_{\frac{1}{3}}\oplus (1,1)_0$$ ($q$, $d_c$ and $ν_c$) $1_{5}\to (1,1)_1$ ($e_c$)

My question is that:

  • do we still have the $B-L$ or $5(B-L)-2Y$ global symmetry for the flipped $SU(5)$ model?

  • Does the proton decay still happens in the flipped $SU(5)$ model?

  • What are the pros and cons of $SU(5)$ model versus flipped $SU(5)$ model?



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