Hierarchy problem

In theoretical physics, a hierarchy problem is a confusing observation that two fundamental quantities with the same units have vastly different values. In other words, it is an apparent paradox: while all the numbers originating from a fundamental theory should naively be comparable to 1, we observe numbers in Nature that are much smaller (or much greater).

More specifically, in particle physics, the hierarchy problem is the big question why the typical energy scale associated with the electroweak symmetry breaking - roughly, the typical size of all masses of elementary particles - is so much ($10^{15}$ times) smaller than the Planck energy. More technically, the question is why the Higgs boson is so much lighter than the Planck mass, although one would expect that the large (quadratically divergent) quantum contributions to the Higgs boson mass would inevitably make the mass huge, comparable to the Planck mass.

The most popular theory - but not the only proposed theory - to solve the hierarchy problem (i.e. to answer the question) is supersymmetry.