Kota Mitsui

Absence of $\mathrm{O} (2)$ symmetry in the Vicsek model

The phase transition in the Vicsek model is widely believed to be associated with spontaneous symmetry breaking of the two-dimensional rotational symmetry $\mathrm{O} (2)$. In this paper, we revisit the original Vicsek model introduced by Vicsek \textit{et al.} [Phys.~Rev.~Lett.~75,~1226~(1995)] and demonstrate that the original angle-based update rule is not invariant under global phase shifts at the level of angular increments when it is implemented using the principal-value arctangent, as in the original definition. As a consequence, we numerically demonstrate that the phase transition reported in the original paper vanishes when the global phase is adaptively chosen.