26847251
How low-energy weak reactions can constrain three-nucleon forces and the neutron-neutron scattering length.
We show that chiral symmetry and gauge invariance enforce relations between the short-distance physics that occurs in a number of electroweak and pionic reactions on light nuclei. Within chiral perturbation theory, this is manifested via the appearance of the same axial isovector two-body contact term in pi(-)d --> nngamma, p-wave pion production in NN collisions, tritium beta decay, pp fusion, nud scattering, and the hep reaction. Using a Gamow-Teller matrix element obtained from calculations of pp fusion as input, we compute the neutron spectrum obtained in pi(-)d --> nngamma. With the short-distance physics in this process controlled from pp --> de(=)nu(e), the theoretical uncertainty in the nn scattering length extracted from pi(-)d --> nngamma is reduced by a factor larger than 3, to approximately < or = 0.05 fm.
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
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
119337535
Using chiral perturbation theory to extract the neutron-neutron scattering length from pi- d ---> nn gamma
The reaction {pi}{sup -}d{yields}nn{gamma} is calculated in chiral perturbation theory so as to facilitate an extraction of the neutron-neutron scattering length (a{sub nn}). We include all diagrams up to O(Q{sup 3}). This includes loop effects in the elementary {pi}{sup -}p{yields}{gamma}n amplitude and two-body diagrams, both of which were ignored in previous calculations. We find that the chiral expansion for the ratio of the quasi-free (QF) to final-state-interaction (FSI) peaks in the final-state neutron spectrum converges well. Our third-order calculation of the full spectrum is already accurate to better than 5%. Extracting a{sub nn} from the shape of the entire {pi}{sup -}d{yields}nn{gamma} spectrum by using our calculation in its present stage would thus be possible at the {+-}0.8 fm level. A fit to the FSI peak only would allow an extraction of a{sub nn} with a theoretical uncertainty of {+-}0.2 fm. The effects that contribute to these error bars are investigated. The uncertainty in the nn rescattering wave function dominates. This suggests that the quoted theoretical error of {+-}0.3 fm for the most recent {pi}{sup -}d{yields}nn{gamma} measurement may be optimistic. The possibility of constraining the nn rescattering wave function used in our calculation more tightly--and thus reducing the error--is briefly discussed.
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.
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.
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.
55262459
Effects of Inhomogeneity on the Causal Entropic prediction of Lambda
The causal entropic principle aims to predict the unexpectedly small value of the cosmological constant Lambda using a weighting by entropy increase on causal diamonds. The original work assumed a purely isotropic and homogeneous cosmology. But even the level of inhomogeneity observed in our universe forces reconsideration of certain arguments about entropy production. In particular, we must consider an ensemble of causal diamonds associated with one cosmology, and we can no longer immediately discard entropy production in the far future of the universe. Depending on our choices for a probability measure and our treatment of black hole evaporation, the prediction for Lambda may be left intact or dramatically altered.