The BaBar collaboration has made the first direct observation of time-reversal (T) violation. The results are in agreement with the basic tenets of quantum field theory and reveal differences in the rates at which the quantum states of the B0 meson transform into one another. The researchers say that this measured lack of symmetry is statistically significant and consistent with indirect observations.

The BaBar detector at the PEP-II facility at SLAC in California was designed to study the collisions of electrons and positrons and to determine the differences between matter and antimatter. In particular, physicists working on the experiment are interested in the violation of the charge–parity symmetry (or CP violation). Although the detector was decommissioned in the spring of 2008, data collected during the period of operation continue to be analysed.

Symmetries of the universe

Our current understanding of the universe suggests that it is governed by certain fundamental symmetries. One of these symmetries looks at the relation between charge (C), parity or "handedness" (P), and time (T) – meaning that if you apply a CPT transformation to a system, it shows no difference from the original system. However, physicists are constantly searching for any possible signs of CPT-violation, which could indicate the presence of new physics. In the realm of the weak force, however, instances of the breaking of individual symmetries have been observed in cases of parity inversion or a combination of parity inversion and charge conjugation (CP). Therefore, it was expected that these systems would also show asymmetries when time was reversed. That is, transformation from one state to another would occur at different rates when the process is reversed in time, thus showing a T-violation.

"While CP violation in the B sector is well established by both BaBar and Belle, all CPT-violation tests have always been consistent with zero," says Patrick Koppenburg, a physicist from the Dutch National Institute for Subatomic Physics (Nikhef), and a member of the LHCb collaboration at CERN. "So, the observation of T violation is not a surprise, but it still needed to be tested." Indeed, physicists have waited for nearly 50 years to make this direct observation since the discovery of CP violations in 1964. The discovery also comes 14 years after another experiment – the CPLEAR experiment – claimed to have the first experimental proof of the violation in 1998 but this claim proved controversial.

Probing the arrow of time

Electron–positron collisions inside BaBar are tuned to just the right energy for producing Υ(4S) mesons, which are composed of a bottom quark and its antiquark. These Υ particles swiftly decay into B mesons, such as the neutral B0 mesons used in this study.

In 10 years, BaBar detected almost half a billion pairs of B and anti-B mesons. Since these pairs are created from the same Υ, they inherit their quantum numbers from the parent Υ. This "entanglement" of the two simultaneously produced B0 mesons is crucial to observing T violations. "Since the global quantum numbers of the B0-antiB0 system are fixed by the Υ(4S) decay, the state of the first B0 meson to decay – whatever it may be – dictates the state of the other B0 meson at that time, which itself decays after some time into another state," explains Fernando Martinez-Vidal, who is at the Institute for Particle Physics at the University of Valencia and Spain's National Research Council (CSIC), and is one of the physicists who worked on this study. "By appropriately choosing the states into which the first and second B0 mesons decay, we can prepare the processes to be studied and compared."

Forwards and backwards

In the world of quantum physics, the individual mesons can be expressed as superpositions, in terms of linear combinations of both B0 and anti-B0 flavour states. The transformations studied are the change of a B0 meson from a "flavour" state to a "linear-combination" state, and the time-reversed change from a "linear-combination" state to a "flavour" state. To begin with, the BaBar physicists identified the flavour of the first meson in the pair to decay (B0 or anti-B0) and used this information to "tag" the flavour of the second meson. Taking the instant this decay occurred as the starting time, they measured the time it took the second meson to transform into a linear-combination state. They then performed the measurement in reverse: if the first meson transforms into a linear-combination state, this information can be used to determine the linear-combination state of the second meson and measure the time taken for it to decay into a "flavour" state.

Thus, by exchanging the initial and final states of the transformation, the physicists could see if there were any differences in the rates of each of these transformations. Unsurprisingly, they found the difference they were looking for, with a significance of 14σ – in particle-physics experiments, a significance of 5σ and above is considered a definite discovery.

While BaBar may have gone silent nearly half a decade ago, hopefully more new results will emerge from the collected data.

The work is published in Physical Review Letters.