Small $x$ behaviour of the chirally-odd parton distribution $h_1(x,Q^2)$
  The small $x$ behaviour of the structure function $h_1(x,Q^2)$ is studied within the leading logarithmic approximation of perturbative QCD. There are two contributions relevant at small $x$. The leading one behaves like $(\frac{1}{x})^0$ i.e. it is just a constant in this limit. The second contribution, suppressed by one power of $x$, includes the terms summed by the GLAP equation. Thus for $h_1(x,Q^2)$ the GLAP asymptotics and Regge asymptotics are completely different, making $h_1(x,Q^2)$ quite an interesting quantity for the study of small $x$ physics. 
R. Kirschner, L. Mankiewicz, A. Sch\"afer, and L. Szymanowski