Excited states in lattice QCD using the stochastic LapH method

A new method for computing the mass spectrum of excited baryons and mesons from the temporal correlations of quantum-field operators in quantum chromodynamics is described. The correlations are determined using Markovchain Monte Carlo estimates of QCD path integrals formulated on an anisotropic space-time lattice. Access to the excited states of interest requires determinations of lower-lying multi-hadron state energies, necessitating the use of multi-hadron operators. Evaluating the correlations of such multi-hadron operators is difficult with standard methods. A new stochastic method of treating the low-lying modes of quark propagation which exploits a new procedure for spatially-smearing quark fields, known as Laplacian Heavi-side smearing, makes such calculations possible for the first time.