Every neutrino ever observed has been left-handed—its spin and linear momentum point in opposite directions—and every antineutrino has been right-handed. That’s because the weak interaction, the basis for all neutrino detection, violates the symmetries of both charge conjugation C (the replacement of particles by their antiparticles) and parity P (spatial inversion). It treats particles and their antiparticles not identically, but as mirror images of one another.

But the combination symmetry, CP, isn’t quite exact. Weak interactions that transform the flavors of quarks can differ from their antiparticle counterparts not just in their spatial arrangements but in their products and rates of production. CP violation is important because of its relevance to a fundamental question: Why is there anything in the universe at all?

The Big Bang should have yielded equal amounts of matter and antimatter, which should promptly have annihilated each other, leaving nothing but photons. Somehow, though, one...

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