 Solution

The unique dimension-five operator built solely from Standard Model fields is the Weinberg operatorwhich gives a Majorana mass term $m_{\nu }\sim c_5{v}^2/\Lambda$ after the Higgs condenses. A dimension-six example that mediates proton decay iswith Lorentz and weak indices contracted appropriately; $u^c{u}^c{d}^c{e}^c/{\Lambda }^2$ is another. Their dimensions explain why neutrino masses are tiny and proton decay is rare: both probe a high scale, while the stronger proton-lifetime bound places especially severe constraints on baryon-number violation.