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Hierarchical clustering in chameleon $f(R)$ gravity

We use a suite of high resolution state-of-the-art N-body Dark Matter simulations of chameleon $f(R)$ gravity to study the higher order volume averaged correlation functions $\bar{ξ_n}$ together with the hierarchical $n$-th order correlation amplitudes $S_n=\barξ_n/\barξ_2^{n-1}$ and density distribution functions (PDF). We show that under the non-linear modifications of gravity the hierarchical scaling of the reduced cumulants is preserved. This is however characterised by significant changes of both the $\bar{ξ_n}$ and $S_n$'s values and their scale dependence with respect to General Relativity gravity (GR). In addition, we measure a significant increase of the non linear $σ_8$ parameter reaching $14, 5$ and 0.5% in excess of the GR value for the three flavours of our $f(R)$ models. We further note that the values of the reduced cumulants up to order $n=9$ are significantly increased in $f(R)$ gravity for all our models at small scales $R\simlt 30\hmpc$. In contrast the values of the hierarchical amplitudes, $S_n$'s, are smaller in $f(R)$ indicating that the modified gravity density distribution functions are deviating from the GR case. Furthermore we find that the redshift evolution of relative deviations of the $f(R)$ hierarchical correlation amplitudes is fastest at high and moderate redshifts $1\leq z \leq4$. The growth of these deviations significantly slows down in the low redshift universe. We also compute the PDFs and show that for scales below $\sim 20\hmpc$ they are significantly shifted in $f(R)$ gravity towards the low densities. Finally we discuss the implications of our theoretical predictions for measurements of the hierarchical clustering in galaxy redshifts surveys, including the important problems of the galaxy biasing and redshifts space distortions.