A beta minus particle $\left(\beta^{-}\right)$ is an electron. Emission of $\beta^{-}$ involves transformation of a neutron into a proton, an electron and a third particle called antineutrino $(\bar{v})$.
${ }_{0} n^{1}={ }_{1} p^{1}+{ }_{-1} \beta^{0}+\bar{v}$