Spinorial Structure of $O(3)$ and Application to Dark Matter
An $O(3)$ spinor, $Φ$, as a doublet denoted by ${\bf 2}_D$ consists of an $SO(3)$ spinor, $ϕ$, and its complex conjugate, $ϕ^\ast$, which form $Φ=\left(ϕ,ϕ^\ast\right)^T$ to be identified with a Majorana-type spinor of $O(4)$. The four gamma matrices $Γ_μ$ ($μ=1\sim 4$) are given by $Γ_i=\text{diag.}\left(τ_i,τ^\ast_i\right)$ ($i=1,2,3$) and $Γ_4=-τ_2\otimesτ_2$, where $τ_i$ denote the Pauli matrices. The rotations and axis-reflections of $O(3)$ are, respectively, generated by $Σ_{ij}$ and $Σ_{i4}$, where $Σ_{μν}=[Γ_μ,Γ_ν]/2i$. While $Φ$ is regarded as a scalar, a fermionic $O(3)$ spinor is constructed out of an $SO(3)$ doublet Dirac spinor and its charge conjugate. These $O(3)$ spinors are restricted to be neutral and cannot carry the standard model quantum numbers because they contain particles and antiparticles. Our $O(3)$ spinors serve as candidates of dark matter. The $O(3)$ symmetry in particle physics is visible when the invariance of interactions is considered by explicitly including their complex conjugates. It is possible to introduce a dark gauge symmetry based on $SO(3)\times\boldsymbol{Z}_2$ equivalent to $O(3)$, where the $\boldsymbol{Z}_2$ parity is described by a $U(1)$ charge giving 1 for a particle and $-1$ for an antiparticle. The $SO(3)$ and $U(1)$ gauge bosons turn out to transform as the axial vector of $O(3)$ and the pseudoscalar of $O(3)$, respectively. This property is related to the consistent definition of the nonabelian field strength tensor of $O(3)$ or of the U(1) charge of the O(3)-transformed spinor. To see the feasibility of our dark matter models, we discuss scalar dark matter phenomenology based on the dark $U(1)$ gauge model.