  A good understanding of conformal field theory (CFT) at c=0 is vital to the
physics of disordered systems, as well as geometrical problems such as polymers
and percolation. Steady progress has shown that these CFTs should be
logarithmic, with indecomposable operator product expansions, and
indecomposable representations of the Virasoro algebra. In one of the earliest
papers on the subject, V. Gurarie introduced a single parameter b to quantify
this indecomposability in terms of the logarithmic partner t of the stress
energy tensor T. He and A. Ludwig conjectured further that b=-5/8 for polymers
and b=5/6 for percolation. While a lot of physics may be hidden behind this
parameter - which has also given rise to a lot of discussions - it had remained
very elusive up to now, due to the lack of available methods to measure it
experimentally or numerically, in contrast say with the central charge. We show
in this paper how to overcome the many difficulties in trying to measure b.
This requires control of a lattice scalar product, lattice Jordan cells,
together with a precise construction of the state L_{-2}|0>. The final result
is that b=5/6 for polymers. For percolation, we find that b=-5/8 within an XXZ
or supersymmetric representation. In the geometrical representation, we do not
find a Jordan cell for L_0 at level two (finite-size Hamiltonian and transfer
matrices are fully diagonalizable), so there is no b in this case.
