NLL stop-antistop production at threshold for unstable squarks

We study the cross section for stop-antistop-squark pair production in the electron-positron collision close to threshold for unstable squarks. Scales appearing in this process are the mass of the stop-squarks m, their 3-momentum ∝mv and their kinetic energy ∝mv2, where v is the relative velocity of the squarks in the c.m. frame. Close to production threshold (v<<1) we need to resum terms ∝(αs/v)n as well as large logarithms of ratios of the physical scales in quantum corrections to the cross section. To achieve this we employ the scalar version of the effective field theory framework vNRQCD. The finite width Γ, which we count as O(mv2), is an additional scale in the problem. Stop quark instability effects generate divergences in the phase space integrals and to deal with these in the effective theory kinematic cuts on the final states are introduced. Finite lifetime and background effects already enter at leading order and are substantially more important than for top quark production.