Force correlations in the q-model for general q-distributions
  We study force correlations in the q-model for granular media at infinite depth, for general q-distributions. We show that there are no 2-point force correlations as long as q-values at different sites are uncorrelated. However, higher order correlations can persist, and if they do, they only decay with a power of the distance. Furthermore, we find the entire set of q-distributions for which the force distribution factorizes. It includes distributions ranging from infinitely sharp to almost critical. Finally, we show that 2-point force correlations do appear whenever there are correlations between q-values at different sites in a layer; various cases are evaluated explicitly. 
Jacco H. Snoeijer and J. M. J. van Leeuwen (Instituut-Lorentz, Leiden   University, The Netherlands)