Testing modified gravity using a marked correlation function
  In theories of modified gravity with the chameleon screening mechanism, the
strength of the fifth force depends on environment. This induces an environment
dependence of structure formation, which differs from $\Lambda$CDM. We show
that these differences can be captured by the marked correlation function. With
the galaxy correlation functions and number densities calibrated to match
between $f(R)$ and $\Lambda$CDM models in simulations, we show that the marked
correlation functions from using either the local density or halo mass as the
marks encode extra information, which can be used to test these theories. We
discuss possible applications of these statistics in observations.
