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 →3 He + A (5.5 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 (A → 2γ ), inverse Primakoff conversion on nuclei (A + Z → γ + Z), the Compton conversion of axions to photons (A + e → e + γ ) and the axio-electric effect (A + e + Z → e + Z). Model-independent limits on product of axion–photon (gAγ ), axion–electron (gAe), and isovector axion–nucleon (g3AN) couplings are obtained: |gAγ × g3AN| ≤ 2.3 × 10−11GeV−1 and |gAe × g3AN| ≤ 1.9×10−13 at mA <1MeV (90% c.l.). The Borexino results exclude newlarge regions of gAγ , and gAe coupling constants and axion masses mA, and leads to constraints on the products |gAγ × mA| and |gAe × mA| for the KSVZ- and the DFSZ-axion models.