I've searched all over for a down/anti-down (Dd) quark pair but the closest thing I can find is the neutral Pi meson, which is: (Uu + Dd)/Sqrt(2) I have two questions: 1) Can someone explain that equation - neither the numerator nor the denominator makes any sense to me. What does Uu+Dd equal? They are particles not numbers, right? And Sqrt(2)...WTF??? I thought mesons were made up of 2 quarks, not 2 quarks plus 2 others plus a mathematical expression. It makes as much sense as saying a proton is made up of UUD+5 !!! Can someone shed some light on this? 2) If all the other quarks can pair with their respective anti-quarks, then why does the Dd pair not exists alone? Thanks.