Understanding of the roles of three-nucleon forces (3NFs) in nuclear few- and many-body systems is one of the fundamental subjects in nuclear physics. Recently, 3NFs are constructed with chiral effective field theory in which two-, three-, and many-nucleon forces are treated consistently and systematically. The chiral 3NF effects have been analyzed in few-body systems and nuclear matter, and the binding energies of light nuclei and the saturation property in symmetric nuclear matter were well reproduced. Furthermore, it was found that the chiral 3NF effects improve the agreement between theoretical and measured cross sections for nucleus-nucleus elastic scattering.
In this talk, we propose to use proton knockout reactions (p,2p) at intermediats and high energies, which can be regarded as a two-proton quasielastic scattering, as a new probe into chiral 3NF effects on reaction observables. In a many-body system, 3NF effects can be represented by the density-dependence of nucleon-nucleon effective interaction. Proton knockout reactions from a deeply bound orbit should be suitable for probing 3NF effects since such reactions occur mainly in the internal region of the target nucleus in which the density is high. We clarify the roles of chiral 3NF for knockout reactions based on the distorted-wave impulse approximation with a nucleon-nucleon g-matrix interaction including the 3NF effects. The chiral 3NF effects significantly change the peak height of the triple differential cross section of (p,2p) reactions.