if((k_prime.px())*(pair_4p.px())+(k_prime.py())*(pair_4p.py())+(k_prime.pz())*(pair_4p.pz())>0.||ptmp>=10.)continue;//angle between jpsi and scattered electron < pi/2, 3-momentum mag < 10.
if(dp>10.-ptmp){//if there are more than one candidate of scattered electron, choose the one with highest 3-momentum mag