 in quantum field theory, the mass gap is the difference in energy between the vacuum and the next lowest energy state. the energy of the vacuum is zero by definition, and assuming that all energy states can be thought of as particles in plane-waves, the mass gap is the mass of the lightest particle. since exact energy eigenstates are infinitely spread out and are therefore usually excluded from a formal mathematical description, a stronger definition is that the mass gap is the greatest lower bound of the energy of any state which is orthogonal to the vacuum. ==mathematical definitions== for a given real field \phi(x), we can say that the theory has a mass gap if the two-point function has the property :\langle\phi(0,t)\phi(0,0)\rangle\sim \sum_na_n\exp\left(-\delta_nt\right) with \delta_0>0 being the lowest energy value in the spectrum of the hamiltonian and thus the mass gap. this quantity, easy to generalize to other fields, is what is generally measured in lattice computations. it was proved in this way that yangâmills theory develops a mass gap on a lattice... the corresponding time-ordered value, the propagator, will have the property :\lim_{p\rightarrow 0}\delta(p)=\mathrm{constant} with the constant being finite. a typical example is offered by a free massive particle and, in this case, the constant has the value 1/m2. in the same limit, the propagator for a massless particle is singular. ==examples from classical theories== an example of mass gap arising for massless theories, already at the classical level, can be seen in spontaneous breaking of symmetry or higgs mechanism. in the former case, one has to cope with the appearance of massless excitations, goldstone bosons, that are removed in the latter case due to gauge freedom. quantization preserves this property. a quartic massless scalar field theory develops a m
Particle_physics Quantum_field_theory