Neutrino Mass from a Higher-Dimensional Operator

We calculate the neutrino mass induced at one loop in the cascade seesaw mechanism. The ratio between the two neutrino masses, which are respectively generated from an operator of mass dimension (3+4n) occurring at one loop and an operator of dimension (5+4n) at tree level, is also given. Detailed studies show that a relatively low new physics scale could accommodate the tiny neutrino mass of desired order without demanding small couplings. We also find that the ratio will reach order one when the new physics scale is higher than several TeV and n is small. In this case, the contribution to neutrino mass from higher order quantum effects becomes important and cannot be ignored.