Higher Twist, Polarised Structure Functions and the Bjorken System of   Equations
  We discuss the present situation with regard to polarised nucleon structure function measurements. In particular we examine the status of the Bjorken sum rule in the light of the recent data on the spin structure functions of (i) the deuteron obtained by the SMC group at CERN and (ii) the neutron by the E142 group at SLAC. In order to fully exploit all the available data it is necessary to study the complete Bjorken system of equations, which may be done in any of several equivalent ways. Combining the new data with that already obtained for the proton by the EMC group and earlier SLAC/YALE collaborations, together with bounds obtained on the strange quark polarisation, we show that the Bjorken system of equations is violated at about the 2 sigma level. We also show, using unpolarised data, that arguments based on possible higher-twist contributions are unable to account for this discrepancy. In conclusion, a simple explanation in terms of a non-perturbative renormalisation of the relevant Wilson coefficients is presented. 
Philip G. Ratcliffe