Minimal SU(5) theory on the edge: The importance of being effective

It is well known that the minimal renormalizable <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>S</a:mi><a:mi>U</a:mi><a:mo stretchy="false">(</a:mo><a:mn>5</a:mn><a:mo stretchy="false">)</a:mo></a:math> grand unified theory is ruled out: it predicts the same masses of down quarks and charged leptons, the gauge couplings do not unify, and neutrinos are massless. We show here that all this can be cured simultaneously by the addition of higher-dimensional effective operators. However, the theory lives on the edge since the unification scale turns out as low as roughly <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msup><e:mn>10</e:mn><e:mn>14</e:mn></e:msup><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>GeV</e:mi></e:math>, threatening proton longevity. If the lower bound on the proton lifetime was to be increased by an order of magnitude, the usual desert in energies between the weak and unification scales would be populated. We also revisit two minimal extensions of this theory that offer a dynamical seesaw origin of neutrino mass and discuss the resulting consequences.