The golden modesB0→J/ψKS,Lin the era of precision flavor physics

$CP$ violation is a major challenge of contemporary particle physics. It has been discovered in kaon decays and appears also in $B$ decays, where the $B^0\to J/\psi K_{\mathrm{S},\mathrm{L}}$ channels are considered to be clean probes of this phenomenon. Recent $B$-factory data challenge the description of $CP$ violation in the standard model of particle physics, showing some “tension” with theoretical predictions. We take a detailed look at certain standard-model contributions, which are usually neglected, and point out that they can be included unambiguously through measurements of the $B^0\to J/\psi {\pi }^0$ observables. Using the most recent data, we show that the tension with the standard model is softened, and we constrain a possible new-physics phase in $B^0-{\overline{B}}^0$ mixing. Our strategy is crucial to fully exploit the accuracy of the search for this kind of new physics at the LHC and future super-flavor factories.