X(6900) as a hadronic mediator of a gluonic dark-matter portal

Abstract

We investigate ????(6900) as a hadronic realization of a gluonic portal between a dark sector and QCD. The analysis is updated using the 2026 CMS observation of a family of all-charm structures and a recent combined LHCb– ATLAS–CMS analysis of the di-???? ∕???? spectrum. The CMS angular analysis favors ???? ???? ???? = 2++, motivating a spin-two mediator rather than the scalar ansatz used in earlier versions of this study. The combined analysis also demon- strates that the extracted ????(6900) mass and width are significantly model dependent because of interference. We therefore use ???????? = 6.919 GeV and Γ???? = 70.3 MeV as a reference benchmark, corresponding to the narrow-width, weak-interference Model I of the combined analysis, while treating the spread among interference models as a systematic theoretical uncertainty. The interaction is int = − ???????? Λ ???????????? ???? ???????? ???? − ???????? Λ ???????????? ???? ???????? ???? . Using the standard spin-two normalization, Γ(???? → ????????) = ????2 ???? ????3 ???? 10????Λ2 , and the gluonic gravitational form factor ???????? (0), the leading spin-independent nucleon cross section is ????SI ???????? = ????2 ???????? ????2 ???? ????2 ???? ???? ( ???????? ???????? Λ2????2 ???? )2 ???????? (0)2. For ???????? (0) = 0.42, ???????????? = 0.10, and ???????? = 100 GeV, the benchmark reference-scale estimate implies ???????? ∕Λ ≲ 1.87 × 10−8 GeV−1 when the 90% C.L. XENONnT reference scale is imposed. The annihilation channel ????̄???? → ????∗ → ???????? is ????-wave suppressed and resonantly enhanced at ???????? ≃ ???????? ∕2 = 3.46 GeV. We show that the most important phenomenological uncertainty is no longer the existence of ????(6900), which is now highly significant, but the interpretation of its line shape and hence the extraction of its mass, width, and gluonic coupling. We argue that a small set of numerical figures is essential for a publication-quality presentation: the direct-detection exclusion plane, the resonance line shape, and the resonant annihilation line shape.