Nucleon isovector couplings fromNf=2lattice QCD

Abstract

We compute the axial, scalar, tensor and pseudoscalar isovector couplings of the nucleon as well as the induced tensor and pseudoscalar charges in lattice simulations with N f = 2 massdegenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions. The simulations are carried out down to a pion mass of 150 MeV and linear spatial lattice extents of up to 4.6 fm at three different lattice spacings ranging from approximately 0.08 fm to 0.06 fm. Possible excited state contamination is carefully investigated and finite volume effects are studied. The couplings, determined at these lattice spacings, are extrapolated to the physical pion mass. In this limit we find agreement with experimental results, where these exist, with the exception of the magnetic moment. A proper continuum limit could not be performed, due to our limited range of lattice constants, but no significant lattice spacing dependence is detected. Upper limits on discretization effects are estimated and these dominate the error budget.

Baryons as relativistic three-quark bound states

“…4.12 and Ref. [464] for a compilation of results, although agreement may be within reach [401,465,584]. It is interesting to note that also in chiral perturbation theory one finds strong cancellations at leading and next-to-leading order in the chiral expansion for g A [447,585].…”

Stochastic method with low mode substitution for nucleon isovector matrix elements

“…Most attempts have resulted in values ∼10% below the experimental number for the axial-vector coupling [1][2][3][4][5][6][7][8], while a few claim that their results could be consistent with experiment [9][10][11][12]. For the quark momentum fraction x u−d , overestimation by ∼20 -30% is common in most of the calculations [3,7,[13][14][15] except [8].…”

“…Recently, attention has been paid to lattice QCD calculation of the isovector scalar matrix element g 3 S in the proton [2,11,16,17] due to its role in constraining possible scalar interactions at the TeV scale [18].…”

New Physics Searches from Nucleon Matrix Elements in Lattice QCD

“…The results plotted in Fig. 2 correspond to: N f =2+1 Clover by LHPC/BMW [28]; N f =2+1 Domain Wall fermions (DWF) by LHPC [28]; N f =2+1 DWF/staggered by LHPC [28]; N f =2+1+1 Twisted Mass fermions (TMF) by ETMC [29]; N f =2 Clover fermions by RQCD [30]; N f =2 TMF & Clover by ETMC [5]; N f =2+1+1 Highly Improved Staggered Quarks (HISQ) by PNDME [7]. As mentioned above, the scalar charge suffers from excited state contamination, so it is important to perform different analysis to address this systematics.…”

“…Moreover, the scalar current couples to the vacuum, and thus, a vacuum subtraction is required. π , corresponding to: N f =2+1 Clover fermions by LHPC/BMW [28] (down green triangles); N f =2+1 DWF by LHPC [28] (orange diamonds); N f =2+1 DWF/staggered by LHPC [28] (red circles); N f =2+1+1 TMF by ETMC [29] (open blue square); N f =2 Clover fermions by RQCD [30] (green x); N f =2 TMF & Clover by ETMC [5] (filled blue square); N f =2+1+1 HISQ by PNDME [7] (turquoise right triangles).…”