Observational constraints on secret neutrino interactions from big bang nucleosynthesis

Abstract

We investigate possible interactions between neutrinos and massive scalar bosons via g φ ννφ (or massive vector bosons via g V νγ µ νV µ ) and explore the allowed parameter space of the coupling constant g φ (or g V ) and the scalar (or vector) boson mass m φ (or m V ) by requiring that these secret neutrino interactions (SNIs) should not spoil the success of Big Bang nucleosynthesis (BBN). Incorporating the SNIs into the evolution of the early Universe in the BBN era, we numerically solve the Boltzmann equations and compare the predictions for the abundances of light elements with observations. It turns out that the constraint on g φ and m φ in the scalar-boson case is rather weak, due to a small number of degrees of freedom. However, in the vector-boson case, the most stringent bound on the coupling g V 6 × 10 −10 at 95 % confidence level is obtained for m V 1 MeV, while the bound becomes much weaker g V 8 × 10 −6 for smaller masses m V 10 −4 MeV. Moreover, we discuss in some detail how the SNIs affect the cosmological evolution and the abundances of the lightest elements. * arXiv:1712.04792v2 [hep-ph] 1 Apr 2018 § The BBN theory can also predict the primordial abundances of 3 He and 7 Li. However, the only data on 3 He are available for the solar system and the high-metallicity regions in our Galaxy and it is difficult to infer its primordial abundance [15]. On the other hand, the observed relative abundance of lithium is 7 Li/H| p = (1.6 ± 0.3) × 10 −10 , showing a discrepancy in the baryon density between the BBN and CMB estimates [9,17]. Since the lithium abundance remains an unresolved issue, it will not be used to draw any constraints in this work.

A model for lepton flavor violating non-standard neutrino interactions

“…We have built a SU [37]. After electroweak and U ′ (1) symmetry breaking, the new fermions mix with the ordinary neutrinos of different flavors leading to LFV couplings that we require.…”

“…In the scenario where we gauge a linear combination of lepton and Baryon numbers, if we set a e = 0, a wider mass range for Z ′ will open. Z ′ lighter than ∼ 10 MeV is disfavored by cosmology [37]. For 130 MeV > m Z ′ > 10 MeV we can have g q ∼ g τ ∼ g µ ∼ g ν < 10 −3 without violating any bound [38].…”

Tritium beta decay with additional emission of new light bosons

“…For a keV mass state the most relevant interaction is the inverse decay νν → Z . In such a case the BBN constraint is translated into an upper bound on the coupling of the form [78,79]…”

“…In the case of an eV state one should instead consider the neutrino annihilation processes νν → Z Z which yields the bound [79] g νL 4.6 × 10 −6 .…”

“…While these estimates are already a very good approximation, we have adopted the more refined limits determined in [79] for our analysis. A similar way of reasoning could also be applied for the limits on measurements of N eff at the CMB time.…”