On the algebra generated by $\mathbf C$, $\mathbf{P}$ and $\mathbf T$: $\mathbf {I^2 = T^2 = P^2 = I P T = -1}$, with applications to pseudo-scalar mesons.

The representation of the discrete Lorentz symmetry operations of parity $\mathbf P$ and time reversal $\mathbf T$ involve complex phases when acting on fermions. If the phase of $\mathbf P$ is a rational multiple of $\pi$ then $\mathbf P^{2 n}=1$ for some positive integer $n$ and it is shown that, when this is the case, $\mathbf P$ and $\mathbf T$ generate a discrete group, a dicyclic group (dicyclic groups are generalisations of the dihedral groups familiar from crystallography). Charge conjugation $\mathbf C$ introduces another complex phase and, again assuming rational multiples of $\pi$, $\mathbf T \mathbf C$ generates a cyclic group of order $2 m$ for some positive integer $m$. There is thus a doubly infinite series of possible finite groups labelled by $n$ and $m$. Demanding that $\mathbf C$ commutes with $\mathbf P$ and $\mathbf T$ forces $n=m=2$ and the group generated by $\mathbf P$ and $\mathbf T$ is then the quaternion group. We propose an experiment to check this by measuring the phase of $\mathbf P$. Neutral pseudo-scalar mesons can be simultaneous $\mathbf C$ and $\mathbf P$ eigenstates. $\mathbf T$ commutes with $\mathbf P$ and $\mathbf C$ when acting on fermion bi-linears so neutral pseudo-scalar mesons can also be $\mathbf T$ eigenstates. The $\mathbf T$-parity should therefore be experimentally observable and the $\mathbf{CPT}$ theorem dictates that $T= C P$.