Study of the form factor in semileptonic $\Lambda_b$ decays

We measure the $\Lambda_b$ baryon form factor using the exclusive decay ${\Lambda^0_b \rightarrow \Lambda^+_c \mu^- \bar{\nu}}$ followed by $\Lambda^+_c \rightarrow p^+ K^0_S$ and $K^0_S \rightarrow \pi^+ \pi^-$. The data sample corresponds to an integrated luminosity of approximately 1~fb$^{-1}$, recorded with the D\O\ detector in $p\bar{p}$ collisions at $\sqrt{s} =1.96$~TeV. We present a preliminary result on the Isgur-Wise slope.