In theoretical physics, the doublet-triplet (splitting) problem is a potential problem of some Grand Unified Theories, such as those associated with the Lie groups SU(5) and SO(10).

A theory of this kind predicts Higgs bosons that form representations of the large, grand unified group . When this representation is decomposed into irreducible representations of the smaller Standard Model group, we find not only color-neutral, two-dimensional representations of the electroweak SU(2) group (the Higgs doublets ), but also colored representations that transform as three-dimensional representations of SU(3) (QCD) (the Higgs triplets).

The latter are not seen and the correct explanation, assuming the validity of SU(5) or SO(10), must be that these triplet states are very massive, at the order of the GUT scale. Not only that, light triplets would mediate extremely rapid proton decay and interfere with the gauge coupling unification. The doublet-triplet problem is the question 'what keeps the doublets light while the triplets are heavy?'.

In SU(5):

In SO(10):

Most models which solve the doublet-triplet splitting problem are considered very contrived.

Non-supersymmetric theories suffer from quadratric radiative corrections to the mass squared of the electroweak Higgs boson (see hierarchy problem). In the presence of supersymmetry, the triplet Higgsino needs to be more massive than the GUT scale to prevent proton decay; there it is not enough simply to require the triplet to have a GUT scale mass.