Particle physics has become an interesting testing ground for questions of foundations of quantum mechanics. Entangled massive meson-antimeson systems are specially suitable as they offer a unique laboratory to test various aspects of particle physics (CP violation, CPT violation,...) as well to test foundations of quantum mechanics, e.g. Bell inequalities, decoherence effects, quantum marking and erasure concepts, Bohr's complementary relation, possible effects coming from models beyond the Standard Model, ....
In this talk I will elucidate why it is interesting to study foundations of quantum mechanics in systems not consisting of ordinary matter and light. I will as well comment on experiments the KLOE collaboration performed and possible new experiments which can be realized.
Particularly, I show that a certain Bell inequality, testing the realistic and nonlocal feature of quantum mechanics, is surprisingly related to the observed CP violation. Herewith two different powerful concepts in physics, entanglement or its manifestation and symmetry, become related. Then I show a Bell inequality sensitive to the quantum number strangeness which is not violated for the maximally entangled spin singlet state, but surprisingly for a non--maximally entangled state. This opens --- for the first time --- the possibility of a direct experimental test whether local realistic theories can be refuted for the K--meson system.
Last but not least I will review decoherence studies for entangled meson--antimeson systems and show recent bounds obtained by various accelerator experiments (including KLOE) and comment on a quantum marking and erasure experiments with neutral kaons which is interesting, because one can do eraser options which are not available by other quantum systems.
In summary, understanding the nature of entanglement and its manifestation needs also studies of entanglement at different energy scales and as recent works show there are still many open questions concerning entanglement, e.g. about high--dimensional genuine multi--partite entanglement.