Search for solar axions produced in the p(d,He3)A reaction with Borexino detector

A search for 5.5-MeV solar axions produced in the p+d→3He+A(5.5MeV) reaction was performed using the Borexino detector. The Compton conversion of axions to photons, A+e→e+γ; the axio-electric effect, A+e+Z→e+Z; the decay of axions into two photons, A→2γ; and inverse Primakoff conversion on nuclei, A+Z→γ+Z, are considered. Model-independent limits on axion-electron (gAe), axion-photon (gAγ), and isovector axion-nucleon (g3AN) couplings are obtained: |gAe×g3AN|≤5.5×10−13 and |gAγ×g3AN|≤4.6×10−11GeV−1 at mA< 1 MeV (90% c.l.). These limits are 2-4 orders of magnitude stronger than those obtained in previous laboratory-based experiments using nuclear reactors and accelerators.