There’s an interesting article on the Physicsworld.com website that discusses a proposal by Maxim Pospelov, a physicist at the Perimeter Institute in Waterloo, Canada, for a new particle to explain the ‘lithium problem’ — the fact that there is not as much lithium in the universe as there should be, according to predictions based on the theory of big bang nucleosynthesis
Pospelov points out that, for several years, physicists thought that neutrons produced by the decay of unstable “supersymmetric” particles might have converted lithium-7 into lighter nuclei such as helium-4. However, those neutrons would eventually have fused with spare protons to create more deuterium, making the theoretical abundance of that isotope too high. Any extra helium-4, in contrast, would have been almost unnoticeable, given its abundance. “The amount of deuterium has been measured very accurately in the last few years,” he says, “so supersymmetric scenarios have been completely disfavoured.”
To overcome this problem, Pospelov and colleagues propose a previously unknown “X” particle that is electrically neutral and fairly stable, which interacts fairly strongly with both protons and neutrons, and has a mass lying somewhere between 1.6–20 MeV.