Lattice measurement of $B_{B_s}$ with a chiral light quark action

The computation on the lattice of the bag parameter $B_{B_s}$ associated to the $B_s-\overline{B_s}$ mixing amplitude in the Standard Model is presented. The estimation has been made by combining the static limit of HQET and the Neuberger light quark action which preserves the chiral symmetry on the lattice. We find B^{\bar{MS}}{stat}}_{B_s}(m_b)=0.92(3).