  Sufficiently dimerized quantum antiferromagnets display elementary S=1
excitations, triplon quasiparticles, protected by a gap at low energies. At
higher energies, the triplons may decay into two or more triplons. A strong
enough magnetic field induces Bose-Einstein condensation of triplons. For both
phenomena the compound IPA-CuCl3 is an excellent model system. Nevertheless no
quantitative model was determined so far despite numerous studies. Recent
theoretical progress allows us to analyse data of inelastic neutron scattering
(INS) and of magnetic susceptibility to determine the four magnetic couplings
J1=-2.3meV, J2=1.2meV, J3=2.9meV and J4=-0.3meV. These couplings determine
IPA-CuCl3 as system of coupled asymmetric S=1/2 Heisenberg ladders
quantitatively. The magnetic field dependence of the lowest modes in the
condensed phase as well as the temperature dependence of the gap without
magnetic field corroborate this microscopic model.
