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Volume 580 Issue 7803, 16 April 2020

The mirror crack’d

A fundamental question about the formation of the Universe is how there came to be more matter present than antimatter. It is thought that this disparity is related to a violation of fundamental physics. The laws of physics are essentially symmetrical — what holds true for matter particles in our Universe should also be true for antimatter particles in a mirror universe. But this is not always the case, and it has been shown that quarks do not behave in quite the same way as their mirror-image antiquarks. Such symmetry violation — known as charge-conjugation and parity reversal (CP) violation — has not yet been demonstrated for leptons, the class of fundamental particle that includes electrons, muons and neutrinos. This issue sees a substantial advance in the search for this violation, as the T2K Collaboration reports measurements on discrepancies between neutrinos and antineutrinos with a 99.7% confidence level, leading to an indication of CP violation at the 95% confidence level. Using the Super-Kamiokande detector (pictured) to capture neutrinos produced 295 kilometres away at the J-PARC accelerator complex in Tokai, Japan, the team determined that the probability of a muon neutrino becoming an electron neutrino seems to be different from that of a muon antineutrino becoming an electron antineutrino, thereby providing an indication of CP violation in leptons. If confirmed by more precise measurements in the future, this violation may help point the way to an explanation of how excess matter was formed in our Universe.

Cover image: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo

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