A per-cent-level determination of the nucleon axial coupling from quantum chromodynamics

Abstract

The axial coupling of the nucleon, g, is the strength of its coupling to the 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 that is rooted in quantum chromodynamics, a pillar of the standard model. The importance of g 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 per cent would be possible by 2020 if two challenges are overcome: contamination of g from excited states must be controlled in the calculations and statistical precision must be improved markedly. Here we use an unconventional method inspired by the Feynman-Hellmann theorem that overcomes these challenges. We calculate a g value of 1.271 ± 0.013, which has a precision of about one per cent.

Parton distribution functions from Ioffe time pseudo-distributions

“…In this respect we discussed in detail how our method for the extraction of the matrix element possesses a number of advantages compared to the commonly utilized sequential source technique. As was shown in [67], the matrix element extraction based on the Feynman-Hellman theorem can begin at much earlier times and this is very advantageous for simulations with physical pion mass. Additionally, we have also employed the method of momentum smearing which has proven to substantially improve the overlap of the interpolating fields with the boosted hadron ground state.…”

Lattice QCD and neutrino-nucleus scattering

“…A representative set of recent work can be found in Refs. [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74] Significant improvements have been made to investigate the quark-mass, finite-volume, and finite-lattice-spacing dependence, and the effects of excited-state contamination in the correlation functions. With these technical and algorithmic advances, lattice QCD can calculate not only the isovector contribution but also the computationally more demanding isoscalar and strange-quark contributions, which are needed for neutral-current processes, discussed below.…”

Nucleon axial, scalar, and tensor charges using lattice QCD at the physical pion mass

“…Lattice calculations of these charges were recently reviewed by FLAG [1], and we note some calculations of them in the last few years in Refs. [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The nucleon axial charge is experimentally well determined; the latest PDG value is g A = 1.2724(23) [16].…”