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

A search for solar axions and axion-like particles produced in the \(p+d\rightarrow ^3\textrm{He}+A~(5.5 ~\textrm{MeV})\) 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 \((\textrm{A}\rightarrow 2\gamma )\), inverse Primakoff conversion on nuclei \((\textrm{A}+Z\rightarrow \gamma +Z\)), the Compton conversion of axions to photons \((\textrm{A}+e\rightarrow e+\gamma )\) and the axio-electric effect \((\textrm{A}+e+Z\rightarrow e+Z\)).  Model-independent limits on product of axion–photon (\(g_{A\gamma }\)), axion–electron (\(g_{Ae}\)), and isovector axion–nucleon (\(g_{3AN}\)) couplings are obtained:

\(|g_{A\gamma }\times g_{3AN}| \le 2.3\times 10^{-11} \textrm{GeV}^{-1}\)

and

\(|g_{Ae}\times g_{3AN}| \le 1.9\times 10^{-13}\) at \(m_A<\) 1 MeV (90% c.l.)

The Borexino results exclude new large regions of \(g_{A\gamma }\), and \(g_{Ae}\) coupling constants and axion masses \(m_A\), and leads to constraints on the products \(|g_{A\gamma }\times m_A|\) and \(|g_{Ae}\times m_A|\) for the KSVZ- and the DFSZ-axion models.