BRST Cohomology in Quantum Affine Algebra U_q(widehat{sl₂})

Using free field representation of quantum affine algebra $U_q(\widehat{sl_2})$, we investigate the structure of the Fock modules over $U_q(\widehat{sl_2})$. The analisys is based on a $q$-analog of the BRST formalism given by Bernard and Felder in the affine Kac-Moody algebra $\widehat {sl_2}$. We give an explicit construction of the singular vectors using the BRST charge. By the same cohomology analysis as the classical case ($q=1$), we obtain the irreducible highest weight representation space as a nontrivial cohomology group. This enables us to calculate a trace of the $q$-vertex operators over this space.