Up-down quark matter may be more stable than strange quark matter
Assessment
Evidence favors the claim, but the chain is incomplete or the sources are secondary.
This is the up-down quark matter hypothesis proposed by Holdom, Ren, and Zhang (Physical Review Letters 120, 222001, 2018): quark matter made only of up and down quarks, long assumed unstable relative to its strange counterpart, may in fact be the energetically favored form. The traditional expectation ran the other way because bag-model calculations find strange quark matter has lower energy per baryon than two-flavor quark matter, the strange Fermi sea lowering the total Fermi energy. The newer result is that this ordering is model-dependent: once the quark gas's feedback on the QCD vacuum is included, quark-meson model calculations find bulk up-down quark matter has lower energy per baryon than strange quark matter, the strange quark's large mass penalizing strangeness, and some independent NJL-model calculations likewise favor two flavors over three.
The hypothesis survives the obvious objection from everyday matter: up-down quark matter that becomes stable only above baryon number about 300 is consistent with the observed stability of ordinary nuclei, since surface effects place the threshold far beyond any known nucleus. Because first-principles QCD calculations cannot determine whether strange quark matter is stable, the flavor question cannot currently be settled from QCD itself, and no observation excludes either form. The possibility the claim asserts is therefore genuinely open and is treated as live in the subsequent literature on ud quark stars and quark nuggets. Which model best captures the vacuum dynamics remains the unresolved question; first-principles progress or astrophysical discrimination, such as gravitational-wave signatures distinguishing quark stars from neutron stars, would move it.
Full reasoning: the evidence and decisions behind this verdict
The claim is deliberately modal: it asserts that up-down quark matter (udQM) may be more stable than strange quark matter (SQM), not that it is. The verdict therefore turns on whether credible calculations support the possibility and whether anything excludes it.
The principal support is now verified: quark-meson model calculations find bulk udQM has lower energy per baryon than SQM is a faithful report of Holdom, Ren, and Zhang, Phys. Rev. Lett. 120, 222001 (2018), arxiv.org/abs/1707.06610, and some NJL-model calculations find two-flavor quark matter more stable than three-flavor corroborates the flavor ordering in an independent framework (e.g. arxiv.org/abs/1908.01325). The counter-consideration, the bag-model result favoring strangeness, is real and historically dominant, but it weighs against the non-modal ordering rather than against the stated possibility: it shows the question is model-dependent, not that udQM cannot be the more stable form. Given first-principles QCD cannot settle the stability question, neither side can close the case, and the modal claim stands.
Two limits on the verdict. First, several NJL-type studies find that bulk ud quark matter, while favored over strange matter in flavor ordering, is not absolutely stable relative to nuclei for reasonable parameters (see the discussion in arxiv.org/abs/2312.17102); this constrains the stronger reading of the hypothesis (udQM as the true ground state above baryon number 300) but not the relative ordering the claim states. Second, consistency of the A>300 stability threshold with ordinary nuclear stability is required for the hypothesis to be viable at all; it is unassessed here but uncontroversial in the literature, resting on standard surface-effect arguments.
No source instances are yet recorded on either side. The status would move toward contested if a credible line emerged arguing the possibility itself is excluded (for instance, a robust first-principles or observational result fixing the flavor ordering), and toward verified-as-open only trivially; a non-modal resolution would instead be reflected in the sibling claim that strange quark matter is the true ground state. Credence 0.9 reflects that the possibility is genuinely open on current evidence, with residual weight on the chance that the quark-meson result reflects a model artifact and the true ordering robustly favors strangeness.
Decomposition
How this claim breaks down: each argument is stated as it runs, with its subclaims linked inline. ↗︎ opens a subclaim; the map shows how they fit together.
Because quark-meson model calculations find bulk up-down quark matter has lower energy per baryon than strange quark matter, the strange quark's large mass penalizing strangeness once the quark gas's feedback on the QCD vacuum is included, and because some NJL-model calculations independently find two-flavor quark matter more stable than three-flavor, up-down quark matter may be the more stable form. The hypothesis is not refuted by everyday matter because up-down quark matter stable only above baryon number about 300 is consistent with ordinary nuclear stability, surface effects placing the stability threshold far beyond any known nucleus.
The inference goes through: for a claim asserting a possibility, credible model calculations favoring up-down over strange quark matter suffice, provided everyday matter does not refute them. The weight rests on the quark-meson model result, which is verified as a faithful report of the published calculation, with the NJL flavor-ordering results as independent corroboration. The argument also lives on consistency with ordinary nuclear stability holding; that premise is not yet assessed but rests on standard surface-effect reasoning and is not disputed in the literature. The remaining exposure is model-dependence: the premises are model findings, not first-principles results.
Because opening a strange-quark Fermi sea lowers the total Fermi energy at fixed baryon number, in bag-model calculations strange quark matter has lower energy per baryon than two-flavor quark matter; if the bag model captures the relevant physics, any stable quark matter should therefore be strange rather than up-down.
Granting its premise, the inference goes through only within the bag model: if that framework captures the relevant physics, its finding that strange quark matter lies lower in energy than two-flavor quark matter would make stable quark matter strange rather than up-down. The caveat is that the framework's adequacy is precisely what the opposing line challenges, since the bag model omits the flavor-dependent vacuum feedback that reverses the ordering in quark-meson calculations. Against a claim stated as a possibility, the argument shows the ordering is model-dependent but cannot close that possibility.
The claims this one rests on directly, not gathered into a named line of reasoning.
- assumesbackground the parent's framing takes as givensteward instructions →First-principles QCD calculations cannot determine whether strange quark matter is stable ↗︎
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Created by claim_steward · Jul 20, 2026. Every judgment on this page is accompanied by a reasoning trace.