A UV Completion Candidate: WHD as a Brane–Antibrane String Vacuum

Read this first. This paper makes two claims that are easy to conflate, and they are not the same one. Realizability — that WHD’s low-energy physics sits at a genuine, consistent point in the string landscape — is shown, within the framework, three independent ways: the Higgs is forced to be the open-string tachyon (its mass fixed, m² = −1/2α′), the tachyon kink is the brane, and the Z2 mirror that defines it is an exact string orbifold whose residual K-theory class evaluates to zero, with consistent tadpoles across the whole RR-flux family. That is what “a place in the landscape” means here — a construction that exists and is consistent, not merely a plausible story [C] in-framework; the one open residual is a bounded closure computation [W], not a doubt about whether the vacuum exists. Selection is the different, weaker claim: that this is the vacuum nature chose. That we do not make — it sits at genuine strong coupling with no weak-coupling corner, so selection is capped at [M], irreducibly. Realizable is not selected. And none of it is load-bearing: every required numerical gate elsewhere in this series stands whether or not a sentence here survives future scrutiny. Marks — [C] proven within the framework, [F] a forced fingerprint, [M] motivated but not demonstrated, [W] weak/unquantified.

The question this paper asks

Everywhere else in this series, four numbers — a mass parameter, a self-coupling, a gauge coupling, a cutoff scale — are simply declared: chosen for naturalness and unit convenience, the way any effective field theory declares its inputs. This paper asks the next question down: could those four numbers be outputs of something, instead of choices?

The one idea: the Higgs is the tachyon

The candidate answer is that WHD is a specific kind of string vacuum: a brane sitting next to its own anti-brane, which are pulled together by their mutual attraction. Stretched between them is an open-string field that is unstable — a “tachyon” in the technical sense, meaning it sits at an unstable maximum of its own energy, exactly like a pencil balanced on its tip. The candidate’s central claim [F] is that this tachyon is the same field this whole program calls the Higgs — one field, one scale, appearing twice under two different names depending on which part of the story you’re looking at. If that identification holds, the field’s mass isn’t a free dial at all: it’s fixed by the string tension itself (m² = −1/2α′), the sharpest, most falsifiable piece of the whole candidate.

Why the brane doesn’t just annihilate to nothing

A brane and an anti-brane, left alone, generically annihilate completely — that’s the textbook outcome, and it’s the obvious objection to this whole idea: why is there a universe left over, rather than empty string vacuum? The resolution [C, conditional] turns on the same mirror symmetry the rest of this program relies on: the tachyon is forced to be odd under the identification, and an odd field cannot sit at any single uniform value except zero — the least stable point there is. The only way to be both odd and rolling downhill is a two-sided roll, our side one way and the mirror side the opposite way, crossing zero in between. That forced, lopsided roll is a kink, not a uniform collapse — and a kink does not have “annihilate to nothing” available to it. This is conditional on exactly one open premise [W]: that the mirror symmetry is exact in the full string construction, not just an effective rule adopted at low energy.

A dark sector as a consequence, not a patch

The construction is required to satisfy a strict bookkeeping rule (charges threading the compact extra dimensions must cancel). Satisfying the leading version of that rule [C] is exactly what forces a specific structural fact: an even, minimum-of-two count of chiral matter families (forbidding an odd count is a clean consequence of an anomaly in the gauge group, not a choice), and — the genuine payoff — that same family count forces a decoupled, off-wall dark sector to exist. Dark matter here is not assumed to patch a gap; it’s a structural consequence of having the number of matter families we already observe.

The one sharp, near-term test

This dark sector is not a weak-scale WIMP, and underground detectors built to find one [C] cannot test it — there’s no light portal particle, only gravity and string-suppressed couplings, both far below current direct-detection sensitivity. The one real, quantitative, near-term handle [C for existence and sign; M for size] is dark radiation: because this program’s vents (real black holes that make our ordinary matter) are brane-directed, any leak into the hidden sector heats it less than it heats us — a small, positive excess in the effective number of relativistic species (ΔN_eff), never the large “democratic” value a symmetric leak would give (which is already excluded). Upcoming CMB surveys are precise enough to catch this or rule it out.

What this candidate does not claim

It does not claim to be the completion, or the one nature necessarily selects — realizable is not the same as selected. It does not claim the construction is fully checked (a deeper bookkeeping condition tied to the string coupling itself does not cancel by the usual static argument, and the proposed dynamical resolution sits in a regime where the approximations that would verify it are least trustworthy [C, for the gap; M, for the proposed resolution]). And it explicitly withdraws an earlier, over-stated claim that a safe outcome is generic [W]: the physics plausibly favors the dangerous configuration at least as much as the safe one, and this paper does not paper over that.

If you want the full derivation — the exact fingerprint formulas, the tadpole and dilaton bookkeeping, the ΔN_eff calculation — the PDF is the authoritative version, once it exists; this page is a faithful restatement, not a substitute, and it is exactly as conditional as the work it restates.