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Question: What are the advantages and disadvantages of flipped $SU(5)$ model?

We know the fermions (same form for three families), each consisting of the representations

  • $\overline{5}_{−3}$ for the lepton doublet, L, and the up quarks $u_c$;

  • $10_1$ for the quark doublet, Q, the down quark,$d_c$ and the right-handed neutrino, $N$;

  • $1_5$ for the charged leptons, $e_c$.

Some claims from Wiki: Flipped $SU(5)$ is not a fully unified model, because the $U(1)Y$ factor of the Standard Model gauge group is within the $U(1)$ factor of the GUT group. The addition of states below Mx in this model, while solving certain threshold correction issues in string theory, makes the model merely descriptive, rather than predictive.

So my understanding is that

  1. $U(1)Y$ factor of the Standard Model is not in the $SU(5)$ but in the $U(5)$ gauge group?

  2. Why Flipped $SU(5)$ model is merely descriptive, rather than predictive? Are there precise disadvantages?

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