  We study the beam single spin asymmetry in semi-inclusive hadron production
in deep inelastic scattering in at order 1/Q. There are two competing
contributions: the leading order transverse momentum dependent parton
distribution $h_1^\perp(x,k_\perp)$ convoluted with chiral-odd fragmentation
function $\hat e(z)$, and the chiral-odd distribution function $e(x)$
convoluted with Collins fragmentation function $H_1^\perp(z,k'_\perp)$. We
estimate this asymmetry and compare with the experimental measurements from
CLAS and HERMES collaborations.
