We show that CERN LHC experiments might well be able to determine ail the p
arameters required for a prediction of the present density of thermal LSP r
elies from the big bang era. If the LSP is an almost pure B-ino we usually
only need to determine its mass and the mass of the SU(2) singlet sleptons.
This information can be obtained by reconstructing the cascade (q) over ti
lde (L)-->(chi )over tilde>(0)(2)q-->(l) over tilde (R)lq--><(<chi>)over ti
lde>(0)(1)l(+l-)q. The only requirement is that m (l) over tildeR<m<(chi )o
ver bar>(0)(2), which is true for most of the cosmologically interesting pa
rameter space. If the LSP has a significant Higgsino component, its predict
ed thermal relic density is smaller than for an equal-mass B-ino, We show t
hat in this case squark decays also produce significant numbers of <(<chi>)
over tilde>(0)(4) and <(<chi>)over tilde>(+/-)(2). Reconstructing the corre
sponding decay cascades then allows us to determine the Higgsino component
of the LSP.