Recent results on nucleon sigma terms in lattice QCD

TL;DR

This work addresses the long-standing difficulty of precisely determining nucleon sigma terms, with a focus on the strange component $\sigma_s$, whose phenomenology is highly uncertain. It surveys lattice QCD advances across light and strange quark sectors, employing methods such as the Feynman-Hellman relation $\sigma_q = m_q \frac{\partial M_N}{\partial m_q}$ and direct evaluation of disconnected diagrams, in both 2-flavor and 2+1-flavor dynamical ensembles, including refinements to SU(3) chiral extrapolations. A recurring conclusion is that $\sigma_s$ is smaller than earlier phenomenological expectations, with representative results around $\sigma_s \approx 20$ MeV and consistent cross-checks from independent approaches. The practical impact is a substantial reduction in uncertainties for scalar nucleon couplings, leading to tighter predictions for dark-matter direct detection cross sections and enhanced model discrimination in supersymmetric scenarios.

Abstract

It has proven a significant challenge to experiment and phenomenology to extract precise values of the nucleon sigma terms. This difficulty opens the window for lattice QCD simulations to lead the field in resolving this aspect of nucleon structure. Here we report on recent advances in the extraction of nucleon sigma terms in lattice QCD. In particular, the strangeness component is now being resolved to a precision that far surpasses best phenomenological estimates.

Figures (1)

• Figure 1: Summary of lattice QCD calculations of the light-quark sigma term (left panel) and strangeness sigma term (right panel). Triangles, squares and circles denote quenched, 2-flavour and 2+1-flavour lattices, respectively. We caution that different calculations have invested differing degrees of effort into quantifying their error estimate.