Search for high energy 5.5 MeV solar axions with the complete Borexino dataset

Abstract

A search for solar axions and axion-like particles produced in the $p+d\rightarrow\rm{^3He}+A~(5.5\rm{ ~MeV})$ reaction was performed using the complete dataset of the Borexino detector (3995 days of measurement live-time). The following interaction processes have been considered: axion decay into two photons $({\rm A}\rightarrow2γ)$, inverse Primakoff conversion on nuclei $({\rm A}+Z\rightarrowγ+Z$), the Compton conversion of axions to photons $({\rm A}+e\rightarrow e+γ)$ and the axio-electric effect $({\rm A}+e+Z\rightarrow e+Z$). Model-independent limits on axion-photon ($g_{Aγ}$), axion-electron ($g_{Ae}$), and isovector axion-nucleon ($g_{3AN}$) couplings are obtained: $|g_{Aγ}\times g_{3AN}| \leq 2.3\times 10^{-11} \rm{GeV}^{-1}$ and $|g_{Ae}\times g_{3AN}| \leq 1.9\times 10^{-13}$ at $m_A <$ 1 MeV (90\% c.l.). The Borexino results exclude new large regions of $g_{Aγ}$, and $g_{Ae}$ coupling constants and axion masses $m_A$, and leads to constraints on the products $|g_{Aγ}\times m_A|$ and $|g_{Ae}\times m_A|$ for the KSVZ- and the DFSZ-axion models.