A recent news report suggests that time–reversal violation may have been observed in neutrino reactions. (You can read the paper on which the report is based here.) Why is this important? Because it may shed light on a long-standing puzzle and that is why it is that in the universe we inhabit, matter is vastly more abundant than anti-matter.

Why is this a puzzle? Because when matter is created out of pure energy, it seems to be always the case that the amount of matter and anti-matter are identical. So in the Big Bang when energy was transformed into all the matter and anti-matter now in the universe, there should have been equal numbers of both. But since we now see so little anti-matter, it has been argued that this is because of the violation of what is known as ‘time-reversal symmetry’, that causes anti-matter to decay at a different (and faster) rate than matter, leading to its current depleted quantities. [UPDATE: See a correction to this in the comments.]

In the world of physics, conservation laws provide strict constraints on what kinds of reactions can and cannot happen. A conservation law says that during any process the total value of some quantity (say the sum of the energy of all the entities involved) has to remain unchanged even as the value of that quantity for each entity changes. Most of the conservation laws we are familiar with such as for energy, momentum, and angular momentum can be observed on a macroscopic level, but there are others, such as baryon and lepton number conservation, that require observations on the level of sub-nuclear particles. Particularly interesting conservation laws are those involving ‘charge conjugation’, ‘parity’, and ‘time reversal’. I briefly discuss this in my book The Great Paradox of Science (p. 289-90).

There is also a conservation law involving CPT, where C stands for ‘charge conjugation’, P stands for ‘parity’, and T for ‘time reversal. Conservation under charge conjugation means that if we replace every particle involved in a reaction by its corresponding anti-particle, the reaction would proceed identically. Conservation of parity means that we would not be able to tell if an elementary particle reaction we observe was seen by us directly or was observed reflected in a mirror, where left and right-handedness would be switched. Conservation under time reversal means that we could not tell if the film of a reaction that was being shown to us was running forward or backward. What CPT conservation tells us is that in any reaction, we would get the same result when all three operations C, P, and T are applied to the particles involved in the reaction. CPT conservation is thought to be strictly true under all circumstances. This is not the case for C, P, and T separately or in any other combination. At various times, each one was thought to be conserved too but over time violations of those conservation laws were found.
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[I]t used to be thought that in addition to CPT conservation, just CP alone (i.e., a combination of just charge conjugation and parity) was also conserved in every reaction involving elementary particles. Why did we believe this universal claim? Because no reaction violating it had ever been seen. Postulating the violation of CP-conservation required finding a reaction that violated it, thus constituting a new existence claim. Some scientists suspected that it might be violated under certain conditions and one such rare reaction was detected in 1964, which was confirmed in subsequent experiments. It was only then that the universal claim that CP was always conserved was accepted as not being always true.

We now have the possibility that neutrino oscillations may also violate time-reversal symmetry. This article explains why this result is significant.

That’s where the T2K experiment comes in. T2K is based at the Super-Kamiokande neutrino observatory, based underground in the Kamioka area of Hida, Japan.

Researchers used the facility’s detector to observe neutrinos and their antimatter counterparts, antineutrinos, generated 295km away at the Japanese Proton Accelerator Research Complex (J-Parc) in Tokai. T2K stands for Tokai to Kamioka.

As they travel through the Earth, the particles and antiparticles oscillate between different physical properties known as flavours.

Physicists think that finding a difference – or asymmetry – in the physical properties of neutrinos and antineutrinos might help us understand why matter is so prevalent compared with antimatter. This asymmetry is known as charge-conjugation and parity reversal (CP) violation.

It is one of three necessary conditions, proposed by the Russian physicist Andrei Sakharov in 1967, that must be satisfied to produce matter and antimatter at different rates.

After analysing nine years’ worth of data, the researchers found a mismatch in the way neutrinos and antineutrinos oscillate by recording the numbers that reached Super Kamiokande with a flavour different from the one they had been created with.

The result has also reached a level of statistical significance – called three-sigma – that’s high enough to indicate that CP violation occurs in these particles.

The results have been published in the journal Nature.

“While CP violation involving quarks is experimentally well established, CP violation has never been observed for neutrinos,” said Stefan Söldner-Rembold.

“The violation of CP symmetry is one of the (Sakharov) conditions for a matter-dominated Universe to exist, but the quark-driven effect is unfortunately much too small to explain why our Universe is mainly filled with matter.

“Discovering CP violation with neutrinos would be a great leap forward in understanding how the Universe was formed.”

He said a theory called leptogenesis links the dominance of matter to CP violation involving neutrinos. “These leptogenesis models predict that the matter domination is actually due to the neutrino sector. If you were to observe neutrino CP violation, that would give us a strong indication that the leptogenesis model is the way forward,” said Prof Söldner-Rembold.

The results from T2K “give strong hints” that the CP violation effect could be large for neutrinos.

Symmetry laws and their violations are powerful tools in scientific investigations.