Violating time-reversal symmetry

The weak interactions of elementary particles have long been known to be asymmetric under CP, the combined operation of parity P and charge conjugation C, the replacement of particles by their antiparticles. But absolute invariance under CPT, the combination of CP with time reversal T, is a bedrock theorem of particle theory’s standard model. So it predicts that the weak interactions must violate T invariance to compensate for the CP violation. But only now has the first clear, direct evidence of T violation been reported. During a decade until its shutdown in 2008, the PEPII electron–positron collider at SLAC produced two hundred million pairs of neutral B mesons in quantum states entangled in such a way that the decay mode of one B instantaneously fixes the state of its partner, perhaps a millimeter away (see Physics Today, May 2001, page 17 ). Carrying the heavy ‘bottom’ quark, the B is about five times as massive as the proton, and it lives for barely a picosecond. The team that ran the collider’s BaBar detector (shown in the photo) has now exploited the entanglement to determine that transition rates between B eigenstates depend on temporal direction in a way that can only be attributed to the violation of T symmetry. The T-violation signal is a robust 14 standard deviations, and its amplitude is consistent with the preservation of CPT symmetry. (J. P. Lees et al., BaBar collaboration, http://arxiv.org/abs/1207.5832 .)—Bertram Schwarzschild