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      Simulating the weak death of the neutron in a femtoscale universe with near-Exascale computing

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          Abstract

          The fundamental particle theory called Quantum Chromodynamics (QCD) dictates everything about protons and neutrons, from their intrinsic properties to interactions that bind them into atomic nuclei. Quantities that cannot be fully resolved through experiment, such as the neutron lifetime (whose precise value is important for the existence of light-atomic elements that make the sun shine and life possible), may be understood through numerical solutions to QCD. We directly solve QCD using Lattice Gauge Theory and calculate nuclear observables such as neutron lifetime. We have developed an improved algorithm that exponentially decreases the time-to solution and applied it on the new CORAL supercomputers, Sierra and Summit. We use run-time autotuning to distribute GPU resources, achieving 20% performance at low node count. We also developed optimal application mapping through a job manager, which allows CPU and GPU jobs to be interleaved, yielding 15% of peak performance when deployed across large fractions of CORAL.

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          Solving Lattice QCD systems of equations using mixed precision solvers on GPUs

          Modern graphics hardware is designed for highly parallel numerical tasks and promises significant cost and performance benefits for many scientific applications. One such application is lattice quantum chromodyamics (lattice QCD), where the main computational challenge is to efficiently solve the discretized Dirac equation in the presence of an SU(3) gauge field. Using NVIDIA's CUDA platform we have implemented a Wilson-Dirac sparse matrix-vector product that performs at up to 40 Gflops, 135 Gflops and 212 Gflops for double, single and half precision respectively on NVIDIA's GeForce GTX 280 GPU. We have developed a new mixed precision approach for Krylov solvers using reliable updates which allows for full double precision accuracy while using only single or half precision arithmetic for the bulk of the computation. The resulting BiCGstab and CG solvers run in excess of 100 Gflops and, in terms of iterations until convergence, perform better than the usual defect-correction approach for mixed precision.
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            A percent-level determination of the nucleon axial coupling from Quantum Chromodynamics

            The \(\textit{axial coupling of the nucleon}\), \(g_A\), is the strength of its coupling to the \(\textit{weak}\) axial current of the Standard Model of particle physics, in much the same way as the electric charge is the strength of the coupling to the electromagnetic current. This axial coupling dictates the rate at which neutrons decay to protons, the strength of the attractive long-range force between nucleons and other features of nuclear physics. Precision tests of the Standard Model in nuclear environments require a quantitative understanding of nuclear physics rooted in Quantum Chromodynamics, a pillar of the Standard Model. The prominence of \(g_A\) makes it a benchmark quantity to determine theoretically - a difficult task because quantum chromodynamics is non-perturbative, precluding known analytical methods. Lattice Quantum Chromodynamics provides a rigorous, non-perturbative definition of quantum chromodynamics that can be implemented numerically. It has been estimated that a precision of two percent would be possible by 2020 if two challenges are overcome: contamination of \(g_A\) from excited states must be controlled in the calculations and statistical precision must be improved markedly. Here we report a calculation of \(g_A^{QCD} = 1.271\pm0.013\), using an unconventional method inspired by the Feynman-Hellmann theorem that overcomes these challenges.
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              Author and article information

              Journal
              03 October 2018
              Article
              1810.01609
              e04869a0-f521-4a64-ba2c-9c385ded55fe

              http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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              Custom metadata
              LLNL-JRNL-749850
              2018 Gordon Bell Finalist: 9 pages, 9 figures
              hep-lat cs.DC nucl-th physics.comp-ph

              High energy & Particle physics,Mathematical & Computational physics,Networking & Internet architecture,Nuclear physics

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