Accidental massless scalar
= Accidental massless scalar
{title2=$M^2\xi=0$}
An <accidental massless scalar> has zero quadratic mass without being required by the stated broken continuous symmetry. A singlet with potential $\kappa z^4$ around $z=0$ is an example: its <Hessian matrix> vanishes although no continuous symmetry of that single real field protects a mass term. Such a zero mass can require tuning once quantum corrections are included.