180 research outputs found
Models, measurements, and effective field theory: proton capture on Beryllium-7 at next-to-leading order
We employ an effective field theory (EFT) that exploits the separation of
scales in the p-wave halo nucleus to describe the process
up to a center-of-mass energy of 500 keV.
The calculation, for which we develop the lagrangian and power counting, is
carried out up to next-to-leading order (NLO) in the EFT expansion. The power
counting we adopt implies that Coulomb interactions must be included to all
orders in . We do this via EFT Feynman diagrams computed in
time-ordered perturbation theory, and so recover existing quantum-mechanical
technology such as the two-potential formalism for the treatment of the
Coulomb-nuclear interference. Meanwhile the strong interactions and the E1
operator are dealt with via EFT expansions in powers of momenta, with a
breakdown scale set by the size of the Be core, MeV.
Up to NLO the relevant physics in the different channels that enter the
radiative capture reaction is encoded in ten different EFT couplings. The
result is a model-independent parametrization for the reaction amplitude in the
energy regime of interest. To show the connection to previous results we fix
the EFT couplings using results from a number of potential model and
microscopic calculations in the literature. Each of these models corresponds to
a particular point in the space of EFTs. The EFT structure therefore provides a
very general way to quantify the model uncertainty in calculations of
. We also demonstrate that the only
NLO corrections in come from an
inelasticity that is practically of NLO size in the energy range of
interest, and so the truncation error in our calculation is effectively
NLO. We also discuss the relation of our extrapolated to the
previous standard evaluation.Comment: 68 pages, 10 figures, and 4 table
Halo effective field theory constrains the solar Beryllium-7 + proton -> Boron-8 + photon rate
We report an improved low-energy extrapolation of the cross section for the
process Beryllium-7+proton -> Boron-8+photon, which determines the Boron-8
neutrino flux from the Sun. Our extrapolant is derived from Halo Effective
Field Theory (EFT) at next-to-leading order. We apply Bayesian methods to
determine the EFT parameters and the low-energy S-factor, using measured cross
sections and scattering lengths as inputs. Asymptotic normalization
coefficients of Boron-8 are tightly constrained by existing radiative capture
data, and contributions to the cross section beyond external direct capture are
detected in the data at E < 0.5 MeV. Most importantly, the S-factor at zero
energy is constrained to be S(0)= 21.3 + - 0.7 eV b, which is an uncertainty
smaller by a factor of two than previously recommended. That recommendation was
based on the full range for S(0) obtained among a discrete set of models judged
to be reasonable. In contrast, Halo EFT subsumes all models into a controlled
low-energy approximant, where they are characterized by nine parameters at
next-to-leading order. These are fit to data, and marginalized over via Monte
Carlo integration to produce the improved prediction for S(E).Comment: 7 pages, 3 figures, 2 tables, and 1 supplemental materia
- …