where and , so that the mechanism of spontaneous symmetry breaking can be used. There is a parameter here, at first hidden within the shape of the potential, that is very important. In a unitarity gauge one can set formula_65 and make formula_66 real. Then formula_67 is the non-vanishing vacuum expectation value of the Higgs field. formula_68 has units of mass, and it is the only parameter in the Standard Model which is not dimensionless. It is also much smaller than the Planck scale; it is approximately equal to the Higgs mass, and sets the scale for the mass of everything else. This is the only real fine-tuning to a small nonzero value in the Standard Model, and it is called the hierarchy problem. Quadratic terms in and arise, which give masses to the W and Z bosons: