Improved big bang nucleosynthesis constraints on heavy neutral leptons

A. Boyarsky,O. Ruchayskiy,V. Syvolap,M. Ovchynnikov

doi:10.1103/physrevd.104.023517

A. Boyarsky, O. Ruchayskiy + Show 2 more

Open Access

https://doi.org/10.1103/physrevd.104.023517

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Abstract

We constrain the lifetime of thermally produced heavy neutral leptons (HNLs) from big bang nucleosynthesis. We show that even a small fraction of mesons present in the primeval plasma leads to the overproduction of primordial helium-4. This constrains the lifetime of HNLs to be ${\ensuremath{\tau}}_{N}<0.02\text{ }\text{ }\mathrm{s}$ for masses above the mass of the pion (as compared to 0.1 s reported previously). In combination with accelerator searches, this allows us to put a new lower bound on the HNL masses and define the ``bottom line'' for HNL searches at the future Intensity Frontier experiments.

Highlights

Heavy neutral leptons (HNLs or right-handed neutrinos) are hypothetical particles capable of explaining neutrino masses and oscillations [1] and resolving other beyond-theStandard-Model phenomena, i.e., the origin of the baryon asymmetry of the Universe and the nature of dark matter [3]
We constrain the lifetime of thermally produced heavy neutral leptons (HNLs) from big bang nucleosynthesis
Once HNLs decay, mesons disappear, and the neutron-to-proton ratio nn=np relaxes solely due to the Standard Model (SM) processes

Summary

INTRODUCTION

Heavy neutral leptons (HNLs or right-handed neutrinos) are hypothetical particles capable of explaining neutrino masses and oscillations [1] and resolving other beyond-theStandard-Model phenomena, i.e., the origin of the baryon asymmetry of the Universe (see, e.g., Ref. [2]) and the nature of dark matter [3]. Once HNLs decay, mesons disappear (instantaneously, as compared to the time scales relevant for BBN), and the neutron-to-proton ratio nn=np relaxes solely due to the SM (weak) processes. If HNLs (and mesons) survive until T ≃ 1.5 MeV and below, there is not enough time to completely relax down to the SBBN value This residual effect leads to a strong upper bound on the HNL lifetime. For temperatures corresponding to such short lifetimes, all Standard Model particles are in thermal equilibrium This makes all other effects of HNLs on BBN irrelevant and allows us to derive the bounds purely analytically, avoiding any computations of complicated Boltzmann equations. Charged pions drive the p ↔ n conversion via [33]

G2F m2p T 2

CONCLUSION

Decays into kaons

Numeric study