On the chirality of the SM and the fermion content of GUTs

Understand

The Standard Model (SM) is a chiral theory, where right- and left-handed fermion fields transform differently under the gauge group.

• Extra fermions, if they do exist, need to be heavy otherwise they would have already been observed.
• With no complex mechanisms at work, such as confining interactions or extra-dimensions, this can only be achieved if every extra right-handed fermion comes paired with a left-handed one transforming in the same way under the Standard Model gauge group, otherwise the new states would only get a mass after electroweak symmetry breaking, which would necessarily be small ($\sim100\textrm{ GeV}$).
• Such a simple requirement severely constrains the fermion content of Grand Unified Theories (GUTs).