First observation of quantum interference in the process ϕ→KSKL→π+π−π+π−: A test of quantum mechanics and CPT symmetry

We present the first observation of quantum interference in the process ϕ→KSKL→π+π−π+π−, using the KLOE detector at the Frascati e+e− collider DAΦNE. From about 5×104 neutral kaon pairs both decaying to π+π− pairs we obtain the distribution of Δt, the difference between the two kaon decay times, which allows testing the validity of quantum mechanics and CPT invariance: no violation of either is observed. New or improved limits on coherence loss and CPT violation are presented.