  Small increases in the up and down quark masses of QCD would tune the theory
to the critical renormalization group trajectory for an infrared limit cycle in
the three-nucleon system. At critical values of the quark masses, the deuteron
binding energy goes to zero and the triton has infinitely many excited states
with an accumulation point at the 3-nucleon threshold. The ratio of the binding
energies of successive states approaches a universal constant that is close to
515. The proximity of physical QCD to the critical trajectory for this limit
cycle explains the success of an effective field theory of nucleons with
contact interactions only in describing the low-energy 3-nucleon system.
