Determination of αs and the nucleon spin decomposition using recent polarized structure function data

Abstract

New data on polarized µ−p and e−p scattering permit a first determination of α s using the Bjorken sum rule, as well as higher precision in determining the nucleon spin decomposition. Using perturbative QCD calculations to O( α s /π) 4 for the non-singlet combination of structure functions, we find α s (2.5 GeV 2 ) = 0.375 +0.062 −0.081 , corresponding to α s (M 2 Z ) = 0.122 +0.005 −0.009 , and using calculations to O( α s /π) 3 for the singlet combination we find ∆u = 0.83 ± 0.03, ∆d = −0.43 ± 0.03, ∆s = −0.10 ± 0.03, ∆Σ ≡ ∆u + ∆d + ∆s = 0.31 ± 0.07, at a renormalization scale Q 2 = 10 GeV 2 . Perturbative QCD corrections play an essential role in reconciling the interpretations of data taken using different targets. We discuss higher-twist uncertainties in these determinations. The ∆q determinations are used to update predictions for the couplings of massive Cold Dark Matter particles and axions to nucleons.

Nucleon matrix elements of higher-twist operators from the instanton vacuum

“…The nucleon isovector axial coupling constant in the large-N c quark model is g A = 0.3 obtained from an analysis of polarized nucleon structure functions of [32], this would mean positive values for f (2) : f…”

Flavor decomposition of the polarized quark distributions in the nucleon from inclusive and semi-inclusive deep-inelastic scattering

“…The comparison is performed by using the ∆q 0 value and the QCD corrections from Ref. [14]. We obtain values for the up, down and strange contributions which differ from our semi-inclusive result.…”

“…The understanding of the spin structure of the nucleon in terms of quarks and gluons remains a challenge since it was demonstrated by EMC [1] and later experiments [2][3][4][5][6][7][8][9][10][11][12][13] using inclusive deep-inelastic scattering (DIS) that only a fraction of the nucleon spin can be attributed to the quark spins and that the strange quark sea seems to be negatively polarized [14]. These conclusions follow from the extraction of the first moments of up, down and strange quark spin distributions from the inclusive data by assuming SU(3) f flavor symmetry.…”

“…[14,51,52]. Higher twist corrections are expected to be small [53][54][55][56] and have been neglected.…”

The Strong Coupling Constant

“…Measurements of the strong coupling constant so far only exist based on the spindependent structure functions of the nucleon. A first analysis [29] had comparable experimental and theoretical errors, a recent update [30] finds α s (µ) = 0.320 +0.031 −0.053 (exp) ± 0.016(theo) for a scale µ = 1.73 GeV. Evolved to the Z-mass one obtains α s (M Z ) = 0.118 +0.005 −0.007 with an error dominated by the experimental uncertainties.…”