Cassions

Cassions are a class of hypothetical particles proposed in speculative extensions of the Standard Model to mediate a new fundamental interaction, sometimes described as a fifth force. The term is used for both vector (spin-1) and scalar (spin-0) variants, which couple weakly to ordinary matter and are characterized by their mass and the symmetry structure of the underlying theory.

In many models, cassions arise from new gauge symmetries or hidden sectors that communicate with the Standard Model through portals such as kinetic mixing or scalar portals. Depending on the model, cassions may be massless or acquire mass via symmetry breaking. A light cassion could produce a long-range force; a heavier one would yield a short-range interaction.

Experimentally, cassions have not been observed. Constraints come from precision tests of gravity and electromagnetism, beam-dump and collider searches, and astrophysical observations. While much of the parameter space is constrained, ranges of mass and coupling remain plausibly allowed and are active areas of experimental and theoretical study.

Variants include multiple cassions with different masses. The concept overlaps with other dark-sector mediators such as axions and dark photons but differs in assumed couplings and symmetry structure. Unlike typical dark matter candidates, cassions are described as force carriers rather than stable relics.

History and usage vary across literature; the term appears in speculative works since the early 2000s. As a hypothetical construct, definitions depend on the chosen framework, and ongoing research seeks to clarify viability and experimental signatures.