Unlike our daily experience, the world of elementary particle physics is mostly symmetrical in time. Run the clock backward on your day and it won't work; run the clock backward on a process in particle physics and things are just fine. However, to preserve certain fundamental aspects of space-time the Standard Model predicts that certain reversible events nevertheless have different probabilities, depending on which way they go. This time-reversal asymmetry is remarkably hard to observe in practice since it involves measurements of highly unstable particles.

New results from the BaBar detector at the Stanford Linear Accelerator Center (SLAC) have uncovered this asymmetry in time. Researchers measured transformations of entangled pairs of particles, including the rates at which these transformations occurred. Through analyzing over 10 years of data, they found clear time-reversal asymmetry with an error of only one part in 1043, a clear discovery by any standard. These results are a strong confirmation of predictions of the Standard Model, filling in one of the final missing details of that theory.

A direct consequence of relativity in particle physics is the presence of three related symmetries, known as CPT: charge, parity, and time. Charge symmetry (C) involves operations wherein particles are swapped with their antiparticles; parity (P) deals with interactions that may depend on which direction in space they take place. Time reversal (T) is perhaps the most subtle of the three: some processes are predicted to occur differently depending on their order in time. While our everyday lives demonstrate that time has a definite direction (we get older and more decrepit, a tragically dropped pizza will not spontaneously reform itself and become edible again, etc.), fundamental processes involving particles are almost all reversible.

Bottom physics

Mesons are particles consisting of a quark and an antiquark. The B mesons of various sorts all contain a bottom antiquark. Bottom quarks are the second most massive quark, and are highly unstable. As a result, the B mesons quickly transform into other particles. The different types of B particles are determined by the second quark they contain: B0 has a down quark, for example. The corresponding antiparticles are denoted B , and contain bottom quarks. Generally speaking, I won't distinguish between the mesons and anti-mesons, referring to both as B mesons for simplicity. Mesons are particles consisting of a quark and an antiquark. The B mesons of various sorts all contain a bottom antiquark. Bottom quarks are the second most massive quark, and are highly unstable. As a result, the B mesons quickly transform into other particles. The different types of B particles are determined by the second quark they contain: Bhas a down quark, for example. The corresponding antiparticles are denoted, and contain bottom quarks. Generally speaking, I won't distinguish between the mesons and anti-mesons, referring to both as B mesons for simplicity. Additionally, some types of B mesons can oscillate into different types, a process similar to what we see with neutrinos. This oscillation was the key to measuring time-reversal asymmetry in the new BaBar results.



Together, CPT appears to be a true symmetry of nature, to the best of our ability to test it. However, some interactions involving the weak force (one of the four fundamental forces, along with the strong force, electromagnetism, and gravity) violate the CP symmetries. That means that to preserve CPT, these interactions must also violate T—but that has proven remarkably difficult to demonstrate experimentally.

The key to finding T-violating processes is finding particles that exchange identities through oscillations that can go either way. (This is in contrast to the decay of B mesons, which is irreversible.) The new BaBar results examined transitions of different types of B mesons that can transform into other types, and measured the probability of each process happening. To cite one example: the particle B - can change into B0, and vice versa. If T symmetry is violated, then the probability of a B - changing into a B0 will be different from the reverse process.

The BaBar team performed measurements on 468 million pairs of B and B mesons, produced in the decay of Υ(4S) (upsilon) mesons. They observed T-violation in four different processes and their reverses:

B - → B0 B 0 → B - B + → B0 B 0 → B +

(The mesons denoted with a subscript "+" or "-" refer not to the quark makeup, but to the particular way they decay.) These processes were already known to violate CP, so they were prime candidates for looking for T-violation as well. Additionally, each decay of a Υ(4S) produced a B B pair that, by virtue of their common origin, were entangled: measurement of the spin state of one meson revealed the outcome of measurement of its partner.

By identifying the particular B type (its "flavor" in particle physics parlance) and determining the decay process for each pair produced, the researchers measured the rate of transition of one B meson type to another. Since each pair was entangled upon production, the second meson was necessarily in a complementary state at all times, enabling precision measurement of the different transition routes enumerated above. Entanglement is usually the domain of quantum information and measurement theory, making its application to the T-reversal problem exciting and unique.

Thanks to over 10 years of data, the BaBar team measured T-violation to the 14 sigma level, meaning there is only 1 chance in 1043 that this effect is not real. (For comparison, a positive detection of the Higgs boson last summer was announced at the 5 sigma level.) These results are a strong verification of the predictions of the Standard Model of particles, demonstrating that—at least for some elementary particle processes—the direction of time matters.

Physical Review Letters, 2012. DOI: 10.1103/PhysRevLett.109.211801 (About DOIs).