Measurement of the spin-dependent structure function g1( x) of the proton

We have measured the spin-dependent structure function $g_1~p$ of the proton in deep inelastic scattering of polarized muons off polarized protons, in the kinematic range $0.003<x<0.7$ and $1\\,\\mbox{GeV}~2<Q~2<60\\,\\mbox{GeV}~2$. Its first moment, $\\int_0~1 g_1~p(x) dx $, is found to be $0.136 \\pm 0.011\\,(\\mbox{stat.})\\pm 0.011\\,(\\mbox{syst.})$ at $Q~2=10\\,\\mbox{GeV}~2$. This value is smaller than the prediction of the Ellis--Jaffe sum rule by two standard deviations, and is consistent with previous measurements. A combined analysis of all available proton, deuteron and neutron data confirms the Bjorken sum rule to within $10\\%$ of the theoretical value.