- J. Foley, K. J. Juge
- O’Cais, M. Peardon, S. Ryan, J.I. Skullerud…
Experiments show that many excited-state hadrons exist, and there are significant experimental efforts to map out the QCD resonance spectrum, such as Hall B and the proposed Hall D at Jefferson Lab, ELSA associated with the University of Bonn, COMPASS at CERN, PANDA at GSI, and BESIII in Beijing. Hence, there is a great need for ab initio determinations of such states in lattice QCD. To extract excited-state energies in Monte Carlo calculations, correlation matrices are needed and operators with very good overlaps onto the states of interest are crucial. To study a particular state of interest, all states lying below that state must first be extracted, and as the pion gets lighter in lattice QCD simulations, more and more multi-hadron states will lie below the excited resonances. To reliably extract these multi-hadron states, multi-hadron operators made from constituent hadron operators with well-defined relative momenta will most likely be needed, and the computation of temporal correlation functions involving such operators will require the use of all-to-all quark propagators. The evaluation of disconnected diagrams will ultimately be required. Perhaps most worrisome, most excited hadrons are unstable (resonances), so the results obtained for finite-box stationary-state energies in lattice QCD must be interpreted carefully. In this talk, progress by the Hadron Spectrum Collaboration in extracting excited-state baryon masses in lattice QCD using large sets of spatially-extended operators is presented. The use of stochastic estimates of all-to-all quark propagators with variance reduction techniques is described. Such techniques are crucial for incorporating multi-hadron operators into correlation matrices.