Flipped SO(10)

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Flipped SO(10) is a version of Grand Unified Theory that is related to the usual SO(10) GUT in the same way that flipped SU(5) is related to SU(5). In ordinary SO(10) GUTs, each generation of matter sits in a single 16‑dimensional spinor. When SO(10) is broken to smaller groups, this 16 can be organized so that you realize either Georgi–Glashow SU(5) or flipped SU(5) as a subgroup.

In flipped SO(10), the gauge group is extended by an extra U(1) factor, so the full group is effectively SO(10) × U(1) (up to a discrete identification). The three generations of matter live in three copies of the 16, which contain the Standard Model fermions (including right-handed neutrinos) together with additional heavy vectorlike fermions at the GUT scale.

There is an intermediate breaking path to a subgroup like SU(5) × U(1). In this scenario, the Standard Model fields come from all three 16 representations, and the remaining fermions are vectorlike. By giving a vacuum expectation value to Higgs fields (a 16_H and its conjugate), the GUT group breaks down to the intermediate SU(5) × U(1) stage. Yukawa couplings then pair up some of the extra fermions, making them heavy, while a sterile neutrino can be added to help mass generation. This process removes the unwanted components so that the low-energy theory remains chiral for the Standard Model fermions.

It is not fixed whether the intermediate SU(5) × U(1) is Georgi–Glashow SU(5) or flipped SU(5); both options lead to viable GUT models. Flipped SO(10) is especially interesting because it can arise from an even larger E6 GUT.