Energy carriers in the Fermi-Pasta-Ulam $\beta$ lattice: Solitons or Phonons?

We investigate anomalous energy transport processes in the Fermi-Pasta-Ulam $\beta$ lattice. They are determined by the maximum sound velocity of the relevant weakly damped energy carriers. That velocity can be numerically resolved by measuring the propagating fronts of the correlation functions of energy/momentum fluctuations at different times. The numerical results are compared with the predictions for solitons and effective (renormalized) phonons, respectively. Excellent agreement has been found for the prediction of effective long wavelength phonons, giving strong evidence that the energy carriers should be effective phonons rather than solitons.