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 $27.6\mathrm{GeV}/c$ positron beam of DESY 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 $A_{zz}^d$ and the tensor structure function $b_1^d$ for average values of the Bjorken variable $0.01<⟨x⟩<0.45$ and of the negative of the squared four-momentum transfer $0.5{\mathrm{GeV}}^2<⟨Q^2⟩<5{\mathrm{GeV}}^2$. The quantities $A_{zz}^d$ and $b_1^d$ are found to be nonzero. The rise of $b_1^d$ for decreasing values of $x$ can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.

Figure 1

The tensor asymmetry $A_{zz}^d(x)$. The error bars are statistical and the shaded band shows the systematic uncertainty.

Figure 2

The tensor structure function presented as (top) $b_1^d(x)$ and (middle) $x{b}_1^d(x)$. The error bars are statistical and the shaded bands show the systematic uncertainty. The bottom panel shows the average value of $Q^2$ in each $x$ bin.