In the nuclear transmutation represented by 168o(p, \alpha) 137n, the emitted particle is

Nuclear reaction: ¹⁶O + p⁺ → ¹³N + α (alpha particle).
Alpha decay is radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and transforms into an atom with an atomic number that is reduced by two and mass number that is reduced by four.
When oxygen-16 gain one proton, atomic mass is 17, but when lose alpha particle atomic mass reduces by four to 13.