  Nucleon transfer in symmetric heavy-ion reactions at energies below the
Coulomb barrier is investigated in the framework of a microscopic stochastic
mean-field theory. While mean-field alone is known to significantly
underpredict the dispersion of the fragment mass distribution, a considerable
enhancement of the dispersion is obtained in the stochastic mean-field theory.
The variance of the fragment mass distribution deduced from the stochastic
theory scales with the number of exchanged nucleon. Therefore, the new approach
provides the first fully microscopic theory consistent with the
phenomenological analysis of the experimental data.
