In one book of Susskind I found the following claim and I wanted to ask for its basis. Susskind says that each kind of boson gives positive contribution to the cosmological constant (the lighter, the better). Each kind of fermion gives negative contribution to the cosmological constant. Thus, if supersymmetry holds, then the cosmological constant is zero. In our universe supersymmetry is slightly broken, so the cosmological constant is small but nonzero. Could you point be to a theory this thesis is based on? I guess this will be the string theory as it involves QFT and gravity.