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

Abstract

Abstract 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 → 2 γ ) , 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 ( g A γ ), axion–electron ( g Ae ), and isovector axion–nucleon ( g 3 A N ) couplings are obtained: | g A γ × g 3 A N | ≤ 2.3 × 10 - 11 GeV - 1 and | g Ae × g 3 A N | ≤ 1.9 × 10 - 13 at m A < 1 MeV (90% c.l.). The Borexino results exclude new large regions of g A γ , and g Ae coupling constants and axion masses m A , and leads to constraints on the products | g A γ × m A | and | g Ae × m A | for the KSVZ- and the DFSZ-axion models.