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A recent series of papers (1, 2) presents an $E_8$ GUT in 10 dimensions, where compactification on a $\mathbb T^6/(\mathbb Z_3\times\mathbb Z_3)$ orbifold ($\mathbb T^6/(\mathbb Z_6\times\mathbb Z_2)$ in prework) avoids the usual problem of no complex E8 representations and yields the Standard Model, with natural desirable features like heavily suppressed proton decay, only one light Higgs (which breaks EWS), and a $10^6–10^9\,\mathrm{GeV}$ unification scale.

Heterotic string theory on $\mathbb T^6/(\mathbb Z_3\times\mathbb Z_3)$ is a long-studied, but this is pure string-inspired QFT. E8 symmetry is broken via quartification (SU(4) in prework) to the SM.

While interesting on its own right, what is the content & phenomenological status of this GUT? How viable is it?

§II.A.1 This is enough to create a different mass structure and a viable Cabibbo–Kobayashi–Maskawa (CKM) mixing matrix similarly to the discussion in [20]. The third line contains vector-like SU(3)C triplets masses generated at the compactifi- cation scale, which is desirable. The fourth line contains a large mixing between one of the extra vector-like states and the down-type quarks, meaning that the light down-type quark lies in a linear combination between them, further contributing to the CKM matrix.

What CKM & PMNS matrix values does it permit?

§III The lightest KK mode in this model would be the first KK mode of the SM Higgs doublet, as it gets a radiatively driven negative mass squared to break the EW symmetry. This particle would be a stable LKP. The next-to-lightest ones would be the first KK modes of the gluons and photons which do not receive EW scale mass contributions. The next ones are all SM fermions and extra Higgs bosons. Note that the mass differences between these fields are proportional to the EW scale, but the KK masses must be at compactification scale, which is orders of magnitude larger.

What are the mass bounds of the LKP? What mass ranges does it predict for the LSP, triplets, or flavons?

(Note: This E8 is not off-topic or non-mainstream. It even caught the eye of our esteemed correspondent at The Reference Frame twice.)

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