119267197
One- and Two-Neutron Halos in Effective Field Theory
I discuss recent work our group has undertaken on effective-field-theory analyses of experimental data pertaining to one- and two-neutron halo nuclei.
653219
Restoration of rotational invariance of bound states on the light front
We study bound states in a model with scalar nucleons interacting via an exchanged scalar meson using the Hamiltonian formalism on the light front. In this approach manifest rotational invariance is broken when the Fock space is truncated. By considering an effective Hamiltonian that takes into account two meson exchanges, we find that this breaking of rotational invariance is decreased from that which occurs when only one meson exchange is included. The best improvement occurs when the states are weakly bound. (c) 2000 The American Physical Society.
119329980
Compton Scattering on He-3
We present first calculations for Compton scattering on $^3$He. The objective of the calculation is an extraction of the neutron polarizabilities.
17094763
Explicit Delta(1232) Degrees of Freedom in Compton Scattering off the Deuteron
We examine elastic Compton scattering off the deuteron for photon energies between 50 MeV and 100 MeV in the framework of chiral effective field theories to nextto-leading order. We compare one theoretical scheme with only pions and nucleons as explicit degrees of freedom to another in which the ∆(1232) resonance is treated as an explicit degree of freedom. Whereas pion degrees of freedom suffice to describe the experimental data measured at about 70 MeV, the explicit ∆(1232) gives important contributions that help to reproduce the angular dependence at higher energies. The static isoscalar dipole polarizabilities αE and β s M are fitted to the available data, giving results for the neutron polarizabilities αE = (14.2 ± 2.0 (stat) ± 1.9 (syst)) · 10−4 fm, βn M = (1.8±2.2 (stat)±0.3 (syst)) ·10−4 fm. These values are in good agreement with previous experimental analyses. Comparing them to the well-known proton values we conclude that there is currently no evidence for significant differences between the proton and neutron electromagnetic dipole polarizabilities. Suggested PACS numbers: 13.40.-f, 13.60.Fz, 25.20.-x, 21.45.+v Suggested
33324518
Short-range interactions in an effective field theory approach for nucleon-nucleon scattering
We investigate in detail the effect of making the range of the {open_quotes}contact{close_quotes} interaction used in effective field theory (EFT) calculations of NN scattering finite. This is done in both an effective field theory with explicit pions, and one where the pions have been integrated out. In both cases we calculate NN scattering in the {sup 1}S{sub 0} channel using potentials which are second order in the EFT expansion. The contact interactions present in the EFT Lagrangian are made finite by use of a square-well regulator. We find that there is an optimal radius for this regulator, at which second-order corrections to the EFT are identically zero; for radii near optimal these second-order corrections are small. The cutoff EFT{close_quote}s which result from this procedure appear to be valid for momenta up to about 100{endash}150MeV/c. We also find that the radius of the square well cannot be reduced to zero if the theory is to reproduce both the experimental scattering length and effective range. Indeed, we show that, if the NN potential is the sum of a one-pion-exchange piece and a short-range interaction, then the short-range piece must extend out beyond 1.05 fm, regardless of its particular form. {copyright} {ital 1997}more » {ital The American Physical Society}« less
118801994
Perturbation schemes for systems of nucleon and pions: The Relationship of covariant perturbation theory, the convolution integral and time ordered perturbation theory
This paper is the first in a series of three which attempt to resolve the difficulties that have plagued the $NN-\pi NN$ problem for the past ten years. Various theoretical inconsistencies in the current formulation have been pointed out and this work aims to eliminate these inconsistencies and so, we hope, produce agreement with experiment. This is to be done by using covariant perturbation theory, in which these inconsistencies are not present. The covariant perturbation theory is developed starting from a model Lagrangian, in order to fix notation and phases. It is shown that both old-fashioned "time-ordered" perturbation theory and the convolution integral of Kvinikhidze and Blankleider may be recovered from the covariant perturbation theory when certain approximations are made. The connection of these results with the work of Klein, L\'evy, Macke and Kadyshevsky is discussed. Two forthcoming papers will pursue this covariant calculation in the $NN-\pi NN$ system using the model and perturbation scheme developed in this paper and derive fully covariant $NN-\pi NN$ equations without the double counting problems present in previous covariant equations.
119183013
Curvature Constraints from the Causal Entropic Principle
Current cosmological observations indicate a preference for a cosmological constant that is drastically smaller than what can be explained by conventional particle physics. The causal entropic principle (Bousso et al.) provides an alternative approach to anthropic attempts to predict our observed value of the cosmological constant by calculating the entropy created within a causal diamond. We have extended this work to use the causal entropic principle to predict the preferred curvature within the 'multiverse'. We have found that values larger than {rho}{sub k}=40{rho}{sub m} are disfavored by more than 99.99% peak value at {rho}{sub {lambda}}=7.9x10{sup -123} and {rho}{sub k}=4.3{rho}{sub m} for open universes. For universes that allow only positive curvature or both positive and negative curvature, we find a correlation between curvature and dark energy that leads to an extended region of preferred values. Our universe is found to be disfavored to an extent depending on the priors on curvature. We also provide a comparison to previous anthropic constraints on open universes and discuss future directions for this work.