Magnetic Polarisabilities from lattice QCD
Background field methods provide an important nonperturbative formalism for the determination of hadronic properties which are complementary to matrix-element calculations. The magnetic polarisability of the proton, neutron and the pion are calculated using the background field method. I will discuss how a careful consideration of nonperturbative improvement can be used to remove the spurious background field-dependent additive mass renormalisation arising from the Wilson term. Free of this artefact, the magnetic polarisabilities of the charged and neutral pion are calculated using a relativistic formalism. Finally, the magnetic polarisability of the proton and neutron are calculated using a quark-level projection technique which provides enhanced overlap with the energy-eigenstates of these hadrons in a background magnetic field. A chiral effective field theory analysis is used to connect the lattice QCD results for the proton and neutron to the physical regime, obtaining magnetic polarisabilities in good agreement with experimental measurements.
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