Isovector and flavor diagonal charges of the nucleon from 2+1+1 flavor QCD

Abstract

We present high-statistics results for the isovector and flavor diagonal charges of the proton using 11 ensembles of 2+1+1 flavor HISQ fermions. In the isospin symmetric limit, results for the neutron are given by the u ↔ d interchange. A chiral-continuum fit with leading order corrections was made to extract the connected and disconnected contributions in the continuum limit and at Mπ = 135 MeV. All results are given in the MS scheme at 2 GeV. The isovector charges, guA−d = 1.218(25)(30), guS−d = 1.022(80)(60) and guT−d = 0.989(32)(10), are used to obtain low-energy constraints on novel scalar and tensor interactions, εS and εT, at the TeV scale. The flavor diagonal axial charges are: guA ≡ ∆u ≡ h1i∆u+ = 0.777(25)(30), gdA ≡ ∆d ≡ h1i∆d+ = −0.438(18)(30), and gsA ≡ ∆s ≡ h1i∆s+ = −0.053(8). Their sum gives the total quark contribution to the proton spin, ∑q=u,d,s(12∆q) = 0.143(31)(36). This result is in good agreement with the recent COMPASS analysis 0.13 <21∆Σ < 0.18. Implications of results for the flavor diagonal tensor charges, guT = 0.784(28)(10), gdT = −0.204(11)(10) and gsT = −0.0027(16) for constraining the quark electric dipole moments and their contributions to the neutron electric dipole moment are discussed. These flavor diagonal charges also give the strength of the interaction of dark matter with nucleons via axial and tensor mediators.