 In 1993 William A. Bardeen and Christopher T. Hill studied the properties of these systems implementing both the heavy quark symmetry and the
chiral symmetries of light quarks in a Nambu-Jona-Lasinio model approximation.This described the phenomenon and gave a prediction of the mass gap of formula_13 MeV, which would be zero if the chiral symmetry breaking was turned off.The excited states of non-strange, heavy-light mesons are usually short-lived resonances due to the principal strong decay mode formula_14, and are therefore hard to observe.