Neutrino effective potential in a fermion and scalar background
We consider the neutrino self-energy in a background composed of a scalar particle and a fermion using a simple model for the coupling of the form $\lambda\bar f_R\nu_L\phi$. The results are useful in the context of Dark Matter-neutrino interaction models in which the scalar and/or fermion constitute the dark-matter. The corresponding formulas for models in which the scalar particle couples to two neutrinos via a coupling of the form $\lambda^{(\nu\nu\phi)}\bar\nu^c_R\nu_L\phi$ are then obtained as a special case. In the presence of these interactions there can be new contributions to the neutrino effective potential and index of refraction in the context of neutrino collective oscillations in a supernova and in the Early Universe hot plasma before neutrino decoupling. The formulas obtained here can be used to estimate those effects and/or put limits on the model parameters based on the contribution that such interactions can have in those contexts. A particular feature of the results is that the contribution to the neutrino self-energy or effective potential in a neutrino background due to the $\nu\nu\phi$ coupling is proportional to the antineutrino-neutrino asymmetry $(n_{\bar\nu} - n_{\nu})$, in contrast to the standard $Z$ contribution which is proportional to $(n_{\nu} - n_{\bar\nu})$. Therefore the two contributions tend to cancel. If the cancellation is significant, it is conceivable that the $O(1/m^4_\phi)$ terms give the dominant contribution. Alternatively, a limit can be set by requiring that the contribution of the $\nu\nu\phi$ interaction to the neutrino effective potential does not cancel the standard contribution in an appreciable way.