Lawrence Livermore National Laboratory

Pavlos Vranas (17-ERD-002)

Executive Summary

Our goal is to perform supercomputer simulations of large-scale, lattice quantum chromodynamics in order to illuminate a great scientific mystery—the lack of antimatter relative to matter in the universe. Filling this gap in the Standard Model of particle physics will inform research in all mission-related areas of nuclear science.

Project Description

The Standard Model of elementary particle physics classifies all the known subatomic particles and incorporates electromagnetism and the weak and strong nuclear forces. However, the Standard Model is incomplete, failing to explain profound observations such as the abundance of matter over antimatter in our universe. A natural candidate to explain this asymmetry is the violation of the lepton number (a lepton being a subatomic particle that does not take part in the strong interaction). All observed Standard Model processes conserve the lepton number (the number of leptons minus the number of anti-leptons in an elementary particle reaction). This asymmetry could be explained if the Standard Model were extended so that new physics does not conserve the lepton number at very high energy. Lepton number violation is also central to our understanding of neutrino mass. The mechanism for generating the masses of neutral fermions (neutrinos) must involve new physics beyond the Standard Model. Large experiments are planned to search for the neutrino-less double-beta-decay process, where two separate neutrons decay to two protons and two electrons by exchanging a particle, but without neutrino emission. A possibility is that this particle is a light Majorana neutrino (lighter than the electron). The smaller mass of the neutrinos suggests a mechanism requiring new Majorana neutrinos, heavier than the neutron, that violate the lepton number. Several possible processes involve the exchange of a heavy particle such as a heavy Majorana neutrino. The resulting two-nucleon processes have to be studied using the fundamental theory of strong nuclear force: quantum chromodynamics. Using Lawrence Livermore's Sierra supercomputer to simulate these processes could shed light on physics beyond the Standard Model. These high-accuracy results will serve as input to the many-body nuclear physics calculations to predict the event rate for the neutrino-less double-beta-decay process.

Calculating the event rate for neutrino-less double-beta-decay processes for candidate theories involving heavy-particle exchange requires large-scale lattice quantum chromodynamics (QCD) simulations. The simulations we are performing will shed light on one of the most significant scientific mysteries of our time: the lack of antimatter relative to matter in the universe. Our method is expected to yield two-nucleon results with controlled statistical and systematic uncertainties. We expect to lay the foundation for a firmer understanding of nuclear processes that can improve capabilities to determine nuclear reaction cross sections for Lawrence Livermore’s nuclear database library with controlled uncertainties. The work will also contribute to Livermore's high-performance computing, help define Sierra's successor, and aid in recruitment.

Mission Relevance

Our project supports DOE's objective of delivering scientific discoveries and major scientific tools that transform our understanding of nature and strengthen the connection between fundamental science advances and technology innovation, as well as the Laboratory's nuclear, chemical, and isotopic science and technology core competency by performing cutting-edge nuclear theory calculations. The stockpile stewardship science research and development challenge also benefits from calculations of the necessary low-energy coefficients for two- and some three-nucleon systems needed by the nuclear data libraries.

FY17 Accomplishments and Results

In FY17, we (1) developed a new mixed lattice quantum chromodynamics action; (2) completed the analytical preparation of the relevant matrix elements; (3) completed the lattice computation of the pion exchange diagram with the operator insertion on the pion line; (4) developed a new method and found lattice source/sinks that provide optimal signal; and (5) started investigating the connection with effective field theories to reach large nuclei.

Publications and Presentations

Berkowitz, E., et al. 2017. "Möbius Domain-Wall Fermions on Gradient-Flowed Dynamical HISQ Ensembles." Physical Review D 96 (5). doi:10.1103/physrevd.96.054513. LLNL-JRNL-719521.

——— (Forthcoming.) "An Accurate Calculation of the Nucleon Axial Charge with Lattice Quantum Chromodynamics." Nature. LLNL-JRNL-719521.