Abstract
Final results are presented from the inclusive measurement of deep-inelastic polarised-muon scattering on longitudinally polarised deuterons using a 6LiD target. The data were taken at 160 GeV beam energy and the results are shown for the kinematic range 1(GeV/c)2<Q2<100(GeV/c)2 in photon virtuality, 0.004<x<0.7 in the Bjorken scaling variable and W>4GeV/c2 in the mass of the hadronic final state. The deuteron double-spin asymmetry A1d and the deuteron longitudinal-spin structure function g1d are presented in bins of x and Q2. Towards lowest accessible values of x, g1d decreases and becomes consistent with zero within uncertainties. The presented final g1d values together with the recently published final g1p values of COMPASS are used to again evaluate the Bjorken sum rule and perform the QCD fit to the g1 world data at next-to-leading order of the strong coupling constant. In both cases, changes in central values of the resulting numbers are well within statistical uncertainties. The flavour-singlet axial charge a0, which is identified in the MS‾ renormalisation scheme with the total contribution of quark helicities to the nucleon spin, is extracted at next-to-leading order accuracy from only the COMPASS deuteron data: a0(Q2=3(GeV/c)2)=0.32±0.02stat±0.04syst±0.05evol. Together with the recent results on the proton spin structure function g1p, the results on g1d constitute the COMPASS legacy on the measurements of g1 through inclusive spin-dependent deep inelastic scattering.
Highlights
About a quarter of a century ago, measurements of the spindependent structure function g1p by EMC [1] led to the very surprising result that the quark spin contribution to the nucleon spin of 1/2 might be very small or even vanishing, albeit with large experimental uncertainties
In this Letter, results are presented on the longitudinal doublespin asymmetry Ad1 and the longitudinal spin structure function g1d of the deuteron
We have presented new results on the longitudinal spin structure function g1d from data taken in 2006 and we have combined this data with our previous measurements
Summary
About a quarter of a century ago, measurements of the spindependent structure function g1p by EMC [1] led to the very surprising result that the quark spin contribution to the nucleon spin of 1/2 might be very small or even vanishing, albeit with large experimental uncertainties. In the last two decades, several new experiments were set up at various laboratories to study the longitudinal spin structure of the nucleon in even more detail These experiments included COMPASS at CERN using the CERN SPS muon beam line at energies 160 GeV and 200 GeV, HERMES at DESY using the 27.5 GeV electron beam of HERA, many experiments at the 6 GeV electron beam of Jefferson Laboratory, as well as PHENIX and STAR at the protonproton collider RHIC with a centre of mass energy of 270 GeV. In this Letter, results are presented on the longitudinal doublespin asymmetry Ad1 and the longitudinal spin structure function g1d of the deuteron They are obtained from data taken in 2006 with the CERN 160 GeV longitudinally polarised muon beam and a longitudinally polarised 6LiD target. The reader is referred to Ref. [10] for a detailed description of the analysis
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