Efficient sampling of Bayesian posteriors and predictive distributions in χEFT
Licentiate thesis, 2021

In this thesis I employ Bayesian statistics to quantify parametric and epistemic uncertainties in chiral effective field theories (χEFT) and propagate these forward to predictions of observables in low-energy nuclear physics. Two primary sources of uncertainty---experimental errors and the theoretical error induced by the truncation of the EFT at up to next-to-next-to-leading-order---are modelled and accounted for in the posterior distributions of the unknown low-energy constants (LECs) that govern interaction strengths in χEFT. These posteriors are computationally challenging to extract and I therefore introduce an advanced Markov chain Monte Carlo (MCMC) algorithm, known as Hamiltonian Monte Carlo, and investigate its performance. I compare its sampling efficiency to standard MCMC algorithms and find reductions in computation time by factors around 3-6 in the present work. I exploit the extracted posteriors to produce predictive distributions for neutron-proton and proton-proton scattering cross sections below and above the pion production threshold and check the consistency of the model predictions against empirical data and higher-order point estimates. I find that the predictive distributions provide reliable credibility intervals as long as the size of the truncation error is estimated from expansion coefficients at next-to-leading-order and above. The LEC posteriors are also central to uncertainty quantification in few- and manybody systems, and as part of a larger collaboration I explore constraints on three-nucleon forces imposed by light-nuclei observables.

two-nucleon scattering

Markov chain Monte Carlo

nuclear physics

Bayesian parameter estimation

chiral effective field theory

Opponent: Arnau Rios Huguet, University of Surrey, Storbritannien


Isak Svensson

Subatomic, High Energy and Plasma Physics PP

S. Wesolowski, I. Svensson, A. Ekström, C. Forssén, R.J. Furnstahl, J. A. Melendez, and D. R. Phillips. Fast and rigorous constraints on chiral three-nucleon forces from few-body observables

I. Svensson, A. Ekström, and C. Forssén. Bayesian parameter estimation in χEFT using Hamiltonian Monte Carlo

Strong interactions for precision nuclear physics (PrecisionNuclei)

European Commission (EC) (758027), 2018-02-01 -- 2023-01-31.

Subject Categories

Subatomic Physics


Basic sciences


C3SE (Chalmers Centre for Computational Science and Engineering)


Chalmers University of Technology



Opponent: Arnau Rios Huguet, University of Surrey, Storbritannien

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