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Register a new userMeasurement of the virtual-photon asymmetry A2 and the spin-structure function g2 of the proton
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A measurement of the virtual-photon asymmetry A_2(x,Q^2) and of the spin-structure function g_2(x,Q^2) of the proton are presented for the kinematic range 0.004 < x < 0.9 and 0.18 GeV^2 < Q^2 < 20 GeV^2. The data were collected by the HERMES experiment at the HERA storage ring at DESY while studying inclusive deep-inelastic scattering of 27.6 GeV longitudinally polarized leptons off a transversely polarized hydrogen gas target. The results are consistent with previous experimental data from CERN and SLAC. For the x-range covered, the measured integral of g_2(x) converges to the null result of the Burkhardt-Cottingham sum rule. The x^2 moment of the twist-3 contribution to g_2(x) is found to be compatible with zero.
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We will discuss recent results for the spin structure functions, with an emphasis on g2 . High precision g2 data allows for tests of the Burkhardt-Cottingham sum rule, and is needed to consistently evaluate higher twist effects.
Precise measurements of the spin structure functions of the proton $g_1^p(x,Q^2)$ and deuteron $g_1^d(x,Q^2)$ are presented over the kinematic range $0.0041 leq x leq 0.9$ and $0.18 $ GeV$^2$ $leq Q^2 leq 20$ GeV$^2$. The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function $g_1^n$ is extracted by combining proton and deuteron data. The integrals of $g_1^{p,d}$ at $Q^2=5$ GeV$^2$ are evaluated over the measured $x$ range. Neglecting any possible contribution to the $g_1^d$ integral from the region $x leq 0.021$, a value of $0.330 pm 0.011mathrm{(theo.)}pm0.025mathrm{(exp.)}pm 0.028$(evol.) is obtained for the flavor-singlet axial charge $a_0$ in a leading-twist NNLO analysis.
Measuring the spin structure of nucleons (protons and neutrons) extensively tests our understanding of how nucleons arise from quarks and gluons, the fundamental building blocks of nuclear matter. The nucleon spin structure is typically probed in scattering experiments using polarized beams and polarized nucleon targets, and the results are compared with predictions from Quantum Chromodynamics directly or with effective theories that describe the strong nuclear force. Here we report on new proton spin structure measurements with significantly better precision and improved coverage than previous data at low momentum transfer squared between $0.012$ and $1.0$ GeV$^2$. This kinematic range provides unique tests of effective field theory predictions. Our results show that a complete description of the nucleon spin remains elusive. They call for further theoretical works that include the more fundamental lattice gauge method. Finally, our data agree with the Gerasimov-Drell-Hearn sum rule, a fundamental prediction of quantum field theory.
We present a precise measurement of the proton longitudinal double-spin asymmetry $A_1^{rm p}$ and the proton spin-dependent structure function $g_1^{rm p}$ at photon virtualities $0.006~({rm GeV}/c)^2<Q^2 < 1~({rm GeV}/c)^2$ in the Bjorken $x$ range of $4 times 10^{-5} < x < 4 times 10^{-2}$. The results are based on data collected by the COMPASS Collaboration at CERN using muon beam energies of $160~{rm GeV}$ and $200~{rm GeV}$. The statistical precision is more than tenfold better than that of the previous measurement in this region. In the whole range of $x$, the measured values of $A_1^{rm p}$ and $g_1^{rm p}$ are found to be positive. It is for the first time that spin effects are found at such low values of $x$.
New results for the double spin asymmetry $A_1^{rm p}$ and the proton longitudinal spin structure function $g_1^{rm p}$ are presented. They were obtained by the COMPASS collaboration using polarised 200 GeV muons scattered off a longitudinally polarised NH$_3$ target. The data were collected in 2011 and complement those recorded in 2007 at 160,GeV, in particular at lower values of $x$. They improve the statistical precision of $g_1^{rm p}(x)$ by about a factor of two in the region $xlesssim 0.02$. A next-to-leading order QCD fit to the $g_1$ world data is performed. It leads to a new determination of the quark spin contribution to the nucleon spin, $Delta Sigma$ ranging from 0.26 to 0.36, and to a re-evaluation of the first moment of $g_1^{rm p}$. The uncertainty of $Delta Sigma$ is mostly due to the large uncertainty in the present determinations of the gluon helicity distribution. A new evaluation of the Bjorken sum rule based on the COMPASS results for the non-singlet structure function $g_1^{rm NS}(x,Q^2)$ yields as ratio of the axial and vector coupling constants $|g_{rm A}/g_{rm V}| = 1.22 pm 0.05~({rm stat.}) pm 0.10~({rm syst.})$, which validates the sum rule to an accuracy of about 9%.
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