for(int j=0;j<aD.GetLength();j++)
{
// Mass of the anti-D0 Candidate
double mass=aD[j].M();

TLorentzVector d1_p4=aD[j].Daughter(0)->P4();   
TLorentzVector d2_p4=aD[j].Daughter(1)->P4(); 
TLorentzVector moth_p4=d1_p4+d2_p4;

// Mass of the anti-D0 by the four-vectors of the two daughters
double mass2=moth_p4.Mag();
}