Spectroscopy
Spectroscopy is the benchmark calculation of lattice QCD – the
masses of the lowest-lying light hadrons (containing u, d, s quarks) are well
known experimentally. But spectroscopy
is also a valuable tool for gleaning information about the nature of the QCD
interaction, and for investigating various models of QCD, such as the quark
model. Finally, lattice spectroscopy
is a vital complement to the experimental studies of the spectrum being undertaken
at Jefferson Laboratory and elsewhere.
Studies performed by members of the LHPC collaboration include:
Excited Nucleon Spectrum
The observed excited nucleon spectrum poses many important questions, such as the nature of the Roper resonances, and whether the Lambda (1405) is indeed a true three-quark state rather than a molecular state. We can also make contact with the quark model by searching for states which have been computed in that model, but which have not been observed experimentally.
There has been considerable interest by LHPC collaborators in the spectrum of negative-parity baryons; the mass splitting within the parity doublets is a direct consequence of spontaneously broken chiral symmetry, and it is only recently that attempts have been made to compute this splitting.
One of the first calculations was performed by Frank Lee and Derek Leinweber, using a highly improved fermion action. This calculation showed a non-zero splitting between the N(1535), the lowest-mass negative-parity nucleon, and the N(938). Using a highly-improved fermion action on an anisotropic lattice, Frank Lee was even able to extract a signal for the first excited radial excitations.
The non-perturbatively improved clover fermion action removes all O(a) discretisation errors from masses; it has been extensively studied in the quenched approximation to QCD, and estimates made of the discretisation and finite-volume uncertainties. Recently, a determination of the mass of the lightest negative parity states has been made by LHPC, in collaboration with the UKQCD and QCDSF collaborations. By working at a variety of lattices, finite-volume dependence has been estimated and an extrapolation to the continuum limit performed.