SLAC National Accelerator Laboratory

Four years after the facility’s closure, researchers have used data from the particle physics experiment BaBar (pictured right, at the SLAC National Accelerator Laboratory in Menlo Park, California) to make the first direct measurement confirming that time does not run the same forwards as backwards — at least for the B mesons that the experiment produced during its heyday.

The application of quantum mechanics to fundamental particles rests on a symmetry known as CPT, for charge-parity-time, which states that fundamental processes remain unchanged when particles are replaced by their antimatter counterparts (C), left and right are reversed (P) and time runs in the reverse direction (T). Violations of C and P alone were first seen in radioactive decays in the 1950s, and BaBar was used to confirm violations of CP in B meson decays in 2001. To keep CPT intact, that implies that time reversal is also violated, but finding ways to compare processes running forwards and backwards in time has proven tricky.

Theoretical physicists at the University of Valencia in Spain worked with researchers on BaBar to exploit the fact that the experiment had generated entangled quantum states of the meson B-zero and its antimatter counterpart B-zero-bar, which then evolved through several different decay chains. By comparing the rates of decay in chains in which one type of decay happened before another with others in which the order was reversed, the researchers were able to compare processes that were effectively time reversed version of each other. They report in Physical Review Letters today that they see a violation of time reversal at an extremely high level of statistical significance.

“It was important to measure time reversal independently of charge-parity violation because there was always the possibility something was wrong with the full picture,” says Fabio Anulli of the National Institute for Nuclear Physics in Rome, who is physics coordinator for BaBar.

Measurements indicating time reversal was probably violated had already been made in kaons at Fermilab in Batavia, Illinois, and at CERN near Geneva, Switzerland, but in those experiments, according to Anulli, the measurements of time reversal were not disentangled from violations of charge-parity that were also present.

In a Viewpoint published today in Physics, physicist Michael Zeller of Yale University concludes that “the long wait for an unequivocal time-reversal violation in particle physics is finally over.”