Dark matter in the x-channel

In this paper I study a class of scenarios in which a scalar singlet dark matter candidate interacts with the Standard Model in such a way that the Higgs portal is strongly suppressed, even at loop level, through a specific mechanism. The new analysis still awaits peer review, but it already lays out the central claim clearly.

It matters because cosmology operates at the edge of what current instruments can measure, where systematic errors and model assumptions are never trivial. Small discrepancies between independent measurements have historically pointed toward missing physics rather than simple calibration errors, and the ongoing tension in the Hubble constant is a live example of how a persistent disagreement between methods can reshape the theoretical landscape. Each new dataset that approaches this territory with independent systematics adds real information to a problem that has resisted easy resolution for more than a decade. The mediator carries non-trivial gauge or internal quantum numbers that allow only a quartic coupling with the dark matter, and belongs to representations with no components that. I refer to this construction as $x$-channel because no single mediator is exchanged between the dark and visible sectors, to distinguish it from the standard $s$- and $t$-channel.

The suppression is due to a boundary condition on the running Higgs-portal coupling, which makes the tree-level and one-loop Higgs-mediated contributions vanish at zero momentum. I consider here a minimal realisation, in which the mediator is a singly- or doubly-charged colourless $SU(2)_L$-singlet scalar decaying to leptons, and study the complementarity.

Within the scanned ranges and benchmark assumptions, the surviving points are predominantly in the region where the dark matter is heavier than the charged scalar mediator. LHC limits determine the lower edge of this region, part of which can be probed by future colliders through the production of the charged scalars.

Other realisations of the $x$-channel can open different phenomenological directions to explore.

The relevance goes beyond one dataset because even small shifts in measured parameters can matter when the field is testing the limits of the standard cosmological model. The Lambda-CDM framework describes the observable universe with remarkable economy, but its success rests on two components, dark matter and dark energy, whose physical nature remains entirely unknown. Any credible measurement that tightens or loosens the constraints on those components moves the entire theoretical enterprise forward, regardless of whether the immediate result looks dramatic on its own terms.

Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether the effect survives when independent surveys, different calibration strategies and tighter control of systematic uncertainties enter the picture. Programmes such as Euclid, DESI and the Rubin Observatory will deliver datasets over the next several years that cover the same parameter space with largely independent methods. If the current signal persists through those tests, its theoretical implications will become impossible to set aside. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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