Search for solar axions emitted in the M1-transition of ⁷Li^* with Borexino CTF

Abstract

Results of background measurements with a prototype of the Borexino detector were used to search for 478 keV solar axions emitted in the M1-transitions of ⁷Li^*. The Compton conversion of axion to a photon A+e → e+γ, axioelectric effect A+e+Z → e+Z, decay of axion in two photons A → 2γ and Primakoff conversion on nuclei A+Z → γ+Z are considered. The upper limit on constants of interaction of axion with electrons, photons and nucleons - g_Aeg_AN ≤ (1.0-2.4)×10^-10 at m_A ≤ 450 keV and g_Aγg_AN ≤ 5×10^-9 GeV^-1 at m_A ≤ 10 keV are obtained (90%c.l.). For heavy axions with mass at 100 < m_A < 400 keV the limits g_Ae < (0.7-2.0)×10^-8 and g_Aγ < 10^-9-10^-8 are obtained in assumption that g_AN depends on m_A as for KSVZ axion model. These limits are stronger than obtained in previous laboratory-based experiments using nuclear reactor and artificial radioactive sources.