Instabilities in neutrino systems induced by interactions with scalars

TL;DR

This paper investigates how weakly coupled scalar particles can induce rapid helicity reversals in neutrino-anti-neutrino clouds, leading to instabilities that could impact early universe neutrino physics.

Contribution

It introduces a novel mechanism for neutrino helicity instabilities driven by scalar interactions, with implications for cosmology and neutrino physics.

Findings

Neutrino-anti-neutrino clouds can experience rapid helicity reversals due to scalar interactions.

The transition time depends logarithmically on seed strength, making it relatively insensitive to seed size.

A limit on the scalar coupling constant is established to avoid excessive effective neutrino numbers in the early universe.

Abstract

If there are scalar particles of small or moderate mass coupled very weakly to Dirac neutrinos, in a minimal way, then neutrino-anti-neutrino clouds of sufficient number density can experience an instability in which helicities are suddenly reversed. The predicted collective evolution is many orders of magnitude faster than given by cross-section calculations. The instabilities are the analogue of the ``flavor-angle" instabilities (enabled by the Z exchange force) that may drive very rapid flavor exchange among the neutrinos that emerge from a supernova. These exchanges do require a tiny seed in addition to the scalar couplings, but the transition time is proportional to the negative of the logarithm of the seed strength, so that the size of this parameter is comparatively unimportant. For our actual estimates we use a tiny non-conservation of leptons; an alternative would be a neutrino magnetic moment in a small magnetic field. The possibility of a quantum fluctuation as a seed is also discussed. Operating in the mode of putting limits on the coupling constant of the scalar field, for the most minimal coupling scheme, with independent couplings to all three $\nu$, we find a rough limit on the dimensionless coupling constant for a neutrino-flavor independent coupling of $G<10^{-10}$, to avoid the effective number of light neutrinos in the early universe being essentially six. If, on the other hand, we wish to fine-tune the model to give a more modest excess (over three) in the effective neutrino number, as may be needed according to recent WMAP analyses, it is easy to do so. \pacs{13.15.+g}