Abstract
The gluon polarisation in the nucleon has been determined by detecting charm production via D0 meson decay to charged K and π in polarised muon scattering off a longitudinally polarised deuteron target. The data were taken by the COMPASS Collaboration at CERN between 2002 and 2006 and correspond to an integrated luminosity of 2.8 fb−1. The dominant underlying process of charm production is the photon–gluon fusion to a cc¯ pair. A leading order QCD approach gives an average gluon polarisation of 〈Δg/g〉x=−0.49±0.27(stat)±0.11(syst) at a scale μ2≈13 (GeV/c)2 and at an average gluon momentum fraction 〈x〉≈0.11. The longitudinal cross-section asymmetry for D0 production is presented in bins of the transverse momentum and the energy of the D0 meson.
Highlights
Pioneering experiments on the spin structure of the nucleon performed in the seventies at SLAC [1] were followed by the EMC experiment at CERN which obtained a surprisingly small quark contribution to the proton spin [2], in contrast to the naive expectation that the spin of the nucleon is built mainly from valence quark spins [3]
Due to the limited range in the four-momentum transfer squared, Q2, covered by the experiments, the QCD analyses (e.g. [5]) show limited sensitivity to the gluon helicity distribution as a function of the gluon momentum fraction x, ∆g(x), and to its first moment, ∆G. (The perturbative scale, μ2, in these QCD analyses is set to Q2.) The determination of ∆g(x) from QCD evolution has to be complemented by direct measurements in dedicated experiments
Assuming that ∆g/g(x) is approximately linearly dependent on x in the range covered, ∆g/g x gives a measurement of ∆g/g( x ), where x is calculated using the signal weights. This assumption is supported by the results of the COMPASS QCD analysis [5]
Summary
Pioneering experiments on the spin structure of the nucleon performed in the seventies at SLAC [1] were followed by the EMC experiment at CERN which obtained a surprisingly small quark contribution to the proton spin [2], in contrast to the naive expectation that the spin of the nucleon is built mainly from valence quark spins [3] This result triggered extensive studies of the spin structure of the nucleon in polarised lepton nucleon scattering experiments at CERN by the SMC [4] and COMPASS [5], at SLAC [6], at DESY [7] and at JLAB [8] as well as in polarised proton–proton collisions at RHIC [9, 10]. Due to the limited range in the four-momentum transfer squared, Q2, covered by the experiments, the QCD analyses (e.g. [5]) show limited sensitivity to the gluon helicity distribution as a function of the gluon momentum fraction x, ∆g(x), and to its first moment, ∆G. (The perturbative scale, μ2, in these QCD analyses is set to Q2.) The determination of ∆g(x) from QCD evolution has to be complemented by direct measurements in dedicated experiments
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