Quantum effects, brane tension and large hierarchy in the brane-world

Semiclassical Einstein's equation in five-dimension with a negative cosmological constant and conformally invariant bulk matter fields is examined in the brane world scenario with the S^1/Z_2 compactification. When numbers N_{b,f} of bosonic and fermionic fields satisfy I<\kappa^2l^{-3}(N_b-N_f)<32I, we obtain an exact semiclassical solution which has two static branes with positive tension and for which the warp factor can be arbitrarily large. Here, \kappa^2 is the five-dimensional gravitational constant, l is a length scale determined by the negative five-dimensional cosmological constant, and $I$ is a dimensionless positive constant of order unity. However, in order to obtain a large warp factor, fine tuning of brane tensions is required. Hence, in order to solve the hierarchy problem, we need to justify the fine tuning by another new mechanism.