The new hybrid BBN model with the photon cooling, X particle, and the primordial magnetic field

The Big Bang Nucleosynthesis theory accurately reproduces the abundances of light elements in the universes, except for the 7Li abundance. The calculated 7Li abundance with the baryon-to-photon ratio fixed by the observations of the cosmic microwave background (CMB) is inconsistent with the observed lithium abundances on the surface of metal-poor halo stars, and this problem is called “7Li problem”. Previous studies proposed to resolve this 7Li problem include photon cooling (possibly via the Bose–Einstein condensation of a scalar particle), the decay of a long-lived X particle (possibly the next-to-lightest supersymmetric particle), or an energy density of a primordial magnetic field (PMF). We review and analyze the results of these solutions both separately and in concert, and the constraint on the X particles and the PMF parameters from observed light-element abundances with a likelihood analysis. We can discover parameter ranges of the X particles which can solve the 7Li problem and constrain the energy density of the PMF.