First Measurement of the Tensor Structure Function $b_1$ of the Deuteron

The \Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6 GeV/c positron beam of \Hera. The use of a tensor polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry $\At$ and the tensor structure function $\bd$ for average values of the Bj{\o}rken variable $0.01<< x><0.45$ and of the squared four-momentum transfer $0.5 {\rm GeV^2}<< Q^2 > <5 {\rm GeV^2}$. The quantities $\At$ and $\bd$ are found to be non-zero. The rise of $\bd$ for decreasing values of $x$ can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.