where α α is the fine-structure constant, e i is the charge of quark with flavor i , and q i A , B ( x 1 , 2 ) denote the parton distribution function of in hadron A and hadron B , with momentum x 1 and x 2 respectively. Similarly q ¯ ¯ i A , B ( x 1 , 2 ) denotes the antiquark distributions.