Direct determinations of the nucleon and pion $σ$ terms at nearly physical quark masses

Abstract

We present a high statistics study of the pion and nucleon light and strange quark sigma terms using $N_f=2$ dynamical non-perturbatively improved clover fermions with a range of pion masses down to $m_π\sim 150$ MeV and several volumes, $Lm_π=3.4$ up to $6.7$, and lattice spacings, $a=0.06-0.08$ fm, enabling a study of finite volume and discretisation effects for $m_π\gtrsim 260$ MeV. Systematics are found to be reasonably under control. For the nucleon we obtain $σ_{πN}=35(6)$ MeV and $σ_s=35(12)$ MeV, or equivalently in terms of the quark fractions, $f_{T_u}=0.021(4)$, $f_{T_d}=0.016(4)$ and $f_{T_s}=0.037(13)$, where the errors include estimates of both the systematic and statistical uncertainties. These values, together with perturbative matching in the heavy quark limit, lead to $f_{T_c}=0.075(4)$, $f_{T_b}=0.072(2)$ and $f_{T_t}=0.070(1)$. In addition, through the use of the (inverse) Feynman-Hellmann theorem our results for $σ_{πN}$ are shown to be consistent with the nucleon masses determined in the analysis. For the pion we implement a method which greatly reduces excited state contamination to the scalar matrix elements from states travelling across the temporal boundary. This enables us to demonstrate the Gell-Mann-Oakes-Renner expectation $σ_π=m_π/2$ over our range of pion masses.

31 pages, 18 figures, v2, small changes to text and figures