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 $p+d\to {}^3\mathrm{H}\mathrm{e}+A(5.5\mathrm{M}\mathrm{e}\mathrm{V})$ 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 $(\mathrm{A}\to 2\gamma )$, inverse Primakoff conversion on nuclei $(\mathrm{A}+Z\to \gamma +Z$), the Compton conversion of axions to photons $(\mathrm{A}+e\to e+\gamma )$ and the axio-electric effect $(\mathrm{A}+e+Z\to e+Z$). Model-independent limits on axion-photon ($g_{A\gamma }$), axion-electron ($g_{Ae}$), and isovector axion-nucleon ($g_{3AN}$) couplings are obtained: $\mathrm{\mid }{g}_{A\gamma }\times g_{3AN}\mathrm{\mid }\le 2.3\times {10}^{-11}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$ and $\mathrm{\mid }{g}_{Ae}\times g_{3AN}\mathrm{\mid }\le 1.9\times {10}^{-13}$ 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 $\mathrm{\mid }{g}_{A\gamma }\times m_A\mathrm{\mid }$ and $\mathrm{\mid }{g}_{Ae}\times m_A\mathrm{\mid }$ for the KSVZ- and the DFSZ-axion models.