def dim_dm(self, pars):
        r"""
        :math:`\frac{\partial \hat{\rho''}(\omega)}{\partial m} = - \rho_0 m
        (\omega \tau)^c \frac{sin(\frac{c \pi}{2})}{1 + 2 (\omega \tau)^c
        cos(\frac{c \pi}{2}) + (\omega \tau)^{2 c}}`
        """
        self._set_parameters(pars)
        numerator = -self.otc * np.sin(self.ang)
        result = numerator / self.denom
        result *= self.rho0
        return result