Experimental limits on the free parameters of higher-derivative gravity
  Fourth-derivative gravity has two free parameters, $\alpha$ and $\beta$, which couple the curvature-squared terms $R^2$ and $R_{\mu\nu}^2$. Relativistic effects and short-range laboratory experiments can be used to provide upper limits to these constants. In this work we briefly review both types of experimental results in the context of higher-derivative gravity. The strictest limit follows from the second kind of test. Interestingly enough, the bound on $\beta$ due to semiclassical light deflection at the solar limb is only one order of magnitude larger. 
Breno L. Giacchini
gr-qc