Measurement of the spin-dependent structure-functions of the proton and the deuteron

% NA47 %title \\ \\The physics motivation of the experiments of the Spin Muon Collaboration is to better understand how the nucleon spin is built-up by its partons and to test the fundamental Bjorken sum rule. \\ \\The spin-dependent structure functions $g _{1}(x)$ of the proton and the deuteron are determined from the measured cross section asymmetries for deep inelastic scattering of longitudinally polarized muons from longitudinally polarized nucleons. The experiment is similar to the NA2 one of the European Muon Collaboration in which the violation of the Ellis-Jaffe sum rule for the proton was found. \\ \\The apparatus is the upgraded forward spectrometer which was used originally by the European and New Muon Collaborations. To minimize the systematic uncertainties the target contains two oppositely polarized cells, which were exposed to the muon beam simultaneously. For the experiments in 1991 and 1992 the original EMC polarized target was reinstalled. In 1993 a new polarized target was put into operation. It has higher cooling power, higher field homogeneity and longer target cells. Deuteron target material, in the form of deuterated butanol beads, was used in 1994 and 1995. Proton target material was used in 1993 and 1996, but in 1996 butanol was replaced by ammonia which provides a 30 \% better dilution factor. Measurements of the transverse spin-dependent asymmetry were performed during the 1993 proton run and during the 1995 deuteron run. The beam polarization is either determined from the energy spectrum of the positrons from muon decay in flight or from the asymmetry in muon scattering from the polarized electrons in a parallel or anti-parallel magnetized iron foil.\\ \\The structure function $g_1(x)$ is measured in the range $0.003<x<0.7$, which gives contributions to the first moments $\int_{0.003}^{0.7}g_1^p(x,Q^2_0)dx =0.139 \pm0.006$(stat.) $\pm0.010$(syst.) and $\int_{0.003}^{0.7}g_1^d(x,Q^2_0)dx =0.041 \pm0.007$(stat.) $\pm0.005$(syst.) for the proton and the deuteron respectively. The results for the full moments from 0 to 1 depend on how the measurements are extrapolated down to $x=0$. However, independently whether a Regge or a QCD like approach is used, the Ellis-Jaffe sum rule is violated and the Bjorken sum rule is confirmed.\\ \\The work will continue in 1998 with a more detailed QCD analysis, studies of less inclusive channels, and the final publications.\\ \\