X5(p,α)X6 This is an equation/part of a question in my nuclear physics unit, where X5 represents the reactant nucleus and X6 is the product nucleus. The alpha symbol is most likely alpha decay, but I have no idea what it means in conjunction with 'p' like that. The rest of the question is: In the carbon cycle that occurs on the Sun, He-4 is built from four protons and C-12. First, C-12 absorbs a proton to form a nucleus, X1. Then X1 decays by positron emission to X2, which then absorbs a proton to become X3, which itself absorbs a proton to become X4. X4 then decays to X5 by positron decay and X5 reacts via: X5(p,α)X6 It's from the Walding Physics textbook if anyone has it.