In the same underground observatory in Japan where, 18 years ago, neutrinos were first seen oscillating from one “Flavor” to another – a landmark discovery that earned two physicists the 2015 Nobel Prize – a tiny anomaly has begun to surface in the neutrinos’ oscillations that could herald an answer to one of the biggest mysteries in physics: why matter dominates over antimatter in the universe.
More to the point, why does anything – matter or antimatter – exist at all? The reigning laws of particle physics, known as the Standard Model, treat matter and antimatter nearly equivalently, respecting so-called charge-parity, or “CP,” symmetry: For every particle decay that produces, say, a negatively charged electron, the mirror-image decay yielding a positively charged antielectron occurs at the same rate.
Significantly more matter than antimatter must have been created, such that a matter surplus survived the annihilation and now holds sway. The question is, what CP-violating process beyond the Standard Model favored the production of matter over antimatter? Many physicists suspect that the answer lies with neutrinos – ultra-elusive, omnipresent particles that pass unfelt through your body by the trillions each second.
Detecting a difference in the behavior of the neutrinos and antineutrinos would provide an important clue about the preponderance of matter over antimatter, perhaps opening up a route beyond the Standard Model to a more complete theory of nature.
Already, the strange properties of neutrinos provide a possible outline of that fuller story.