| 1. |
- Kliger, Robert, 1950, et al.
(författare)
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Vibration response of long cable-stayed timber footbridge – case study
- 2013
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Ingår i: Proceedings of International Conference on Timber Bridges 2013- Las Vegas, Nevada USA.
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Konferensbidrag (refereegranskat)abstract
- Timber footbridges with a span larger than 30 meters are sensitive to vibrations and dynamic factors must therefore be taken into considerations in the design. A case study of a cable-stayed footbridge, Älvsbacka Bridge, with a free span of 130 meters, is presented. The dynamic design of the bridge is compared, based on the old Swedish code, BRO 2004 and the Eurocode 5. Simplified force models presented in BRO 2004 and the ISO 10137 international standard are compared. Vertical and lateral accelerations are measured on the bridge with accelerometers attached to the bridge deck. According to the simplified force model, the vertical acceleration limit was met for a group of twenty pedestrians, which was regarded as a reasonable design situation. From the measured accelerations, the damping factor was calculated as 1.2% of critical damping, which is twice the value used in design.
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| 2. |
- Capel, Pierre, et al.
(författare)
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Effective field theory analysis of the Coulomb breakup of the one-neutron halo nucleus 19 C
- 2023
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Ingår i: European Physical Journal A. - 1434-601X .- 1434-6001. ; 59:11
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Tidskriftsartikel (refereegranskat)abstract
- We analyse the Coulomb breakup of 19 C measured at 67A MeV at RIKEN. We use the Coulomb-Corrected Eikonal (CCE) approximation to model the reaction and describe the one-neutron halo nucleus 19 C within Halo Effective Field Theory (Halo EFT). At leading order we obtain a fair reproduction of the measured cross section as a function of energy and angle. The description is insensitive to the choice of optical potential, as long as it accurately represents the size of 18 C. It is also insensitive to the interior of the 19 C wave function. Comparison between theory and experiment thus enables us to infer asymptotic properties of the ground state of 19 C: these data put constraints on the one-neutron separation energy of this nucleus and, for a given binding energy, can be used to extract an asymptotic normalisation coefficient (ANC). These results are confirmed by CCE calculations employing next-to-leading order Halo EFT descriptions of 19 C: at this order the results for the Coulomb breakup cross section are completely insensitive to the choice of the regulator. Accordingly, this reaction can be used to constrain the one-neutron separation energy and ANC of 19 C.
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| 3. |
- Svensson, Isak, 1988, et al.
(författare)
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Bayesian estimation of the low-energy constants up to fourth order in the nucleon-nucleon sector of chiral effective field theory
- 2023
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 107:1
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Tidskriftsartikel (refereegranskat)abstract
- We use Bayesian methods and Hamiltonian Monte Carlo (HMC) sampling to infer the posterior probability density function (PDF) for the low-energy constants (LECs) up to next-to-next-to-next-to-leading order (N3LO) in a chiral effective field theory (χEFT) description of the nucleon-nucleon interaction. In a first step, we condition the inference on neutron-proton and proton-proton scattering data and account for uncorrelated χEFT truncation errors. We demonstrate how to successfully sample the 31-dimensional space of LECs at N3LO using a revised HMC inference protocol. In a second step we extend the analysis by means of importance sampling and an empirical determination of the neutron-neutron scattering length to infer the posterior PDF for the leading charge-dependent contact LEC in the S01 neutron-neutron interaction channel. While doing so we account for the χEFT truncation error via a conjugate prior. We use the resulting posterior PDF to sample the posterior predictive distributions for the effective range parameters in the S01 wave as well as the strengths of charge-symmetry breaking and charge-independence breaking. We conclude that empirical point-estimate results of isospin breaking in the S01 channel are consistent with the PDFs obtained in our Bayesian analysis and that, when accounting for χEFT truncation errors, one must go to next-to-next-to-leading order to confidently detect isospin breaking effects.
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| 4. |
- Svensson, Isak, 1988, et al.
(författare)
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Bayesian parameter estimation in chiral effective field theory using the Hamiltonian Monte Carlo method
- 2022
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 105:1
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Tidskriftsartikel (refereegranskat)abstract
- The number of low-energy constants (LECs) in chiral effective field theory (chi EFT) grows rapidly with increasing chiral order, necessitating the use of Markov chain Monte Carlo techniques for sampling their posterior probability density function. For this we introduce a Hamiltonian Monte Carlo (HMC) algorithm and sample the LEC posterior up to next-to-next-to-leading order (NNLO) in the two-nucleon sector of chi EFT. We find that the sampling efficiency of HMC is three to six times higher compared to an affine-invariant sampling algorithm. We analyze the empirical coverage probability and validate that the NNLO model yields predictions for two-nucleon scattering data with largely reliable credible intervals, provided that one ignores the leading-order EFT expansion parameter when inferring the variance of the truncation error. We also find that the NNLO truncation error dominates the error budget.
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| 5. |
- Svensson, Isak, 1988
(författare)
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Efficient sampling of Bayesian posteriors and predictive distributions in χEFT
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 6. |
- Svensson, Isak, 1988
(författare)
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Extending the reach of uncertainty quantification in nuclear theory
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The theory of the strong interaction—quantum chromodynamics (QCD)—is unsuited to practical calculations of nuclear observables and approximate models for nuclear interaction potentials are required. In contrast to phenomenological models, chiral effective field theories (χEFTs) of QCD grant a handle on the theoretical uncertainty arising from the truncation of the chiral expansion. Uncertainties in χEFT are preferably quantified using Bayesian inference, but quantifying reliable posterior predictive distributions for nuclear observables presents several challenges. First, χEFT is parametrized by unknown low-energy constants (LECs) whose values must be inferred from low-energy data of nuclear structure and reaction observables. There are 31 LECs at fourth order in Weinberg power counting, leading to a high-dimensional inference problem which I approach by developing an advanced sampling protocol using Hamiltonian Monte Carlo (HMC). This allows me to quantify LEC posteriors up to and including fourth chiral order. Second, the χEFT truncation error is correlated across independent variables such as scattering energies and angles; I model correlations using a Gaussian process. Third, the computational cost of computing few- and many-nucleon observables typically precludes their direct use in Bayesian parameter estimation as each observable must be computed in excess of 100,000 times during HMC sampling. The one exception is nucleon-nucleon scattering observables, but even these incur a substantial computational cost in the present applications. I sidestep such issues using eigenvector-continuation emulators, which accurately mimic exact calculations while dramatically reducing the computational cost. Equipped with Bayesian posteriors for the LECs, and a model for the truncation error, I explore the predictive ability of χEFT, presenting the results as the probability distributions they always were.
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| 7. |
- Svensson, Isak, 1988, et al.
(författare)
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Inference of the low-energy constants in Δ -full chiral effective field theory including a correlated truncation error
- 2024
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 109:6
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Tidskriftsartikel (refereegranskat)abstract
- We sample the posterior probability distributions of the low-energy constants (LECs) in Δ-full chiral effective field theory (χEFT) up to third order. We use eigenvector continuation for fast and accurate emulation of the likelihood and Hamiltonian Monte Carlo to draw effectively independent samples from the posteriors. Our Bayesian inference is conditioned on the Granada database of neutron-proton (np) cross sections and polarizations. We use priors grounded in χEFT assumptions and a Roy-Steiner analysis of pion-nucleon scattering data. We model correlated EFT truncation errors using a two-feature Gaussian process, and find correlation lengths for np scattering energies and angles in the ranges 45-83 MeV and 24-39 degrees, respectively. These correlations yield a nondiagonal covariance matrix and reduce the number of independent scattering data with factors of 8 and 4 at the second and third chiral orders, respectively. The relatively small difference between the second- and third-order predictions in Δ-full χEFT suppresses the marginal variance of the truncation error and the effects of its correlation structure. Our results are particularly important for analyzing the predictive capabilities in ab initio nuclear theory.
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| 8. |
- Wesolowski, S., et al.
(författare)
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Rigorous constraints on three-nucleon forces in chiral effective field theory from fast and accurate calculations of few-body observables
- 2021
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 104:6
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Tidskriftsartikel (refereegranskat)abstract
- We explore the constraints on the three-nucleon force (3NF) of chiral effective field theory (χEFT) that are provided by bound-state observables in the A=3 and A=4 sectors. Our statistically rigorous analysis incorporates experimental error, computational method uncertainty, and the uncertainty due to truncation of the χEFT expansion at next-to-next-to-leading order. A consistent solution for the H3 binding energy, the He4 binding energy and radius, and the H3β-decay rate can only be obtained if χEFT truncation errors are included in the analysis. The β-decay rate is the only one of these that yields a nondegenerate constraint on the 3NF low-energy constants, which makes it crucial for the parameter estimation. We use eigenvector continuation for fast and accurate emulation of no-core shell model calculations of the few-nucleon observables. This facilitates sampling of the posterior probability distribution, allowing us to also determine the distributions of the parameters that quantify the truncation error. We find a χEFT expansion parameter of Q=0.33±0.06 for these observables.
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