Judith A. McGovern

Proton polarisability contribution to the Lamb shift in muonic hydrogen at fourth order in chiral perturbation theory

We calculate the amplitude T1 for forward doubly virtual Compton scattering in heavy-baryon chiral perturbation theory, to fourth order in the chiral expansion and with the leading contribution of the

Spin polarisabilities of the nucleon at NLO in the chiral expansion

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Compton scattering from the proton in an effective field theory with explicit Delta degrees of freedom

N

Compton scattering on the proton, neutron, and deuteron in chiral perturbation theory to O (Q4)

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ABSENCE OF FIFTH-ORDER CONTRIBUTIONS TO THE NUCLEON MASS IN HEAVY-BARYON CHIRAL PERTURBATION THEORY

form factor. This provides a model-independent expression for the amplitude in the low-momentum region, which is the dominant one for its contribution to the Lamb shift. It allows us to significantly reduce the theoretical uncertainty in the proton polarisability contributions to the Lamb shift in muonic hydrogen. We also stress the importance of consistency between the definitions of the Born and structure parts of the amplitude. Our result leaves no room for any effect large enough to explain the discrepancy between proton charge radii as determined from muonic and normal hydrogen.

Pairing in many-fermion systems: an exact renormalisation group treatment

Abstract We present a calculation of the fourth-order (NLO) contribution to spin-dependent Compton scattering in heavy-baryon chiral perturbation theory, and we give results for the four spin polarisabilities. No low-energy constants, except for the anomalous magnetic moments of the nucleon, enter at this order. For forward scattering the fourth-order piece of the spin polarisability of the proton turns out to be almost twice the size of the leading piece, with the opposite sign. This leads to the conclusion that no prediction can currently be made for this quantity. For backward scattering the fourth-order contribution is much smaller than the third-order piece which is dominated by the anomalous scattering, and so cannot explain the discrepancy between the CPT result and the current best experimental determination.

Convergence of the chiral expansion for the nucleon mass

We analyse the proton Compton-scattering differential cross section for photon energies up to 325 MeV using Chiral Effective Field Theory (χEFT) and extract new values for the electric and magnetic polarisabilities of the proton. Our approach builds in the key physics in two different regimes: photon energies ω ≲ mπ (“low energy”), and the higher energies where the Δ(1232) resonance plays a key role. The Compton amplitude is complete at N4LO,

Colour superconductivity in finite systems

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