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Search for Solar Axions Produced in $p(d,\rm{^3He})A$ Reaction with Borexino Detector

A search for 5.5-MeV solar axions produced in the $p+d\rightarrow\rm{^3He}+A (5.5 \rm{MeV})$ reaction was performed using the Borexino detector. The Compton conversion of axions to photons, ${\rm A}+e\rightarrow e+γ$; the axio-electric effect, ${\rm A}+e+Z\rightarrow e+Z$; the decay of axions into two photons, ${\rm A}\rightarrow2γ$; and inverse Primakoff conversion on nuclei, ${\rm A}+Z\rightarrowγ+Z$, are considered. Model independent limits on axion-electron ($g_{Ae}$), axion-photon ($g_{Aγ}$), and isovector axion-nucleon ($g_{3AN}$) couplings are obtained: $|g_{Ae}\times g_{3AN}| \leq 5.5\times 10^{-13}$ and $|g_{Aγ}\times g_{3AN}| \leq 4.6\times 10^{-11} \rm{GeV}^{-1}$ at $m_A <$ 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.