The stuff we deal with every day is made of matter. It is inescapable. However, it wasn’t always this way. When the universe was vey young and very hot, there were large amounts of both matter and antimatter present. Particles of antimatter have the same mass as their matter counterparts, but the charges are reverses. So, for example, the electron encountered in ordinary matter has a negative charge, while the anti-electron (or positron) has a positive charge. When such counterparts meet, they annihilate each other releasing high-energy radiation in the form of gamma-rays. In experiments, matter and antimatter particles are always made in equal amounts, satisfying sensible rules such as charge balance. (if you start off with no net electrical charge before making any particles, after you’ve made some new particles, their charges must still add up to zero.)
Given these rules and these observations, you would think there would be the same amount of matter and antimatter in the universe, but in fact, that’s not correct. With rare and fleeting exception, everything we observe in the universe is made of matter. There is no net antimatter left from the early days of the universe. That means that unequal amounts of matter and antimatter was made in the early universe, and only the dominant matter survived to the present day. This suggests that, despite the apparent identical behavior of matter and antimatter particles (other than charge), there must be some kind of deep, fundamental difference between the two. For this kind of imbalance to have occurred, physicists believe that there must have been a violation in two of what they consider to be the three fundamental symmetries of the universe.
What constitutes a fundamental symmetry? These are conditions whereby you obtain the same experimental results when you reverse conditions, and most of these are kind of obvious. The three consist of time (T), charge(C), and parity (P). Taking them in reverse order, parity symmetry asks, if you set up your experiment as the exact mirror image of what you did, would you get the same result? That should be a clear yes. Charge symmetry asks, if you reverse the charges of the particles in an experiment, would you get the same result? Coulomb’s law governing electrical forces gives the same result if you reverse charges, so again, we expect the answer to be yes. Finally, what would happen if we reverse time, and run the experiment backward? It turns out that the kinematic laws of physics run backwards the same way they run forward, so the answer again would be that you would get the same result.
But in order to explain the imbalance of matter and antimatter in the early universe—resulting in a present-day universe consisting of matter only, two of these symmetries, C and P, must be violated. So, there has been a decades-long search for experimental evidence of CP symmetry violation. In the analysis of a decade’s-worth of data from experiments observing neutrinos, those ghostly near-massless particles that rarely interact with anything, scientists in Japan believe they have observed a hint of just such a violation strong enough to explain why the universe is made up of just matter.
For more details, and tonight’s comments, make the jump. But first, a word from our sponsor:
Here at Top Comments we strive to nourish community by rounding up some of the site's best, funniest, most mojo'd & most informative commentary, and we depend on your help!! If you see a comment by another Kossack that deserves wider recognition, please send it either to topcomments at gmail or to the Top Comments group mailbox by 9:30pm Eastern. Please please please include a few words about why you sent it in as well as your user name (even if you think we know it already :-)), so we can credit you with the find!
So, what are these neutrinos doing that have piqued the interest of physicists. First, a little bit about neutrinos. Neutrinos interact so weakly with other particles that they pass through matter most of the time as though it wasn’t there. There are three flavors of neutrinos (yes, that’s what the property is called), each associated with a particle in a class of particles that respond to the weak nuclear force, but not to the strong nuclear force. These particles are, in order of increasing mass, the electron, the muon, and the tau particle. It was discovered back in the 1980s that the neutrinos associated with each of these particles, as they travel through space, oscillate among the three possible flavors. So, for example, an electron neutrino after a while will transform into a tau particle neutrino for a while before transforming again into a muon neutrino.Such oscillation is important to establish before considering the experiment and its results.
The experiment was called T2K, which is short for Tokai to Kamioka. There is a particle accelerator in Tokai which, among other things, generates lots of neutrinos. The detector for these neutrinos is in Kamioka, 295 kilometers away. The particle accelerator in Tokai produces muon neutrinos and antineutrinos, which are then detected in Kamioka at a detector called SuperKamiokande which contains nearly 50,000 tons of pure water located deep underground, and 13,000 phototubes trained to observe any flashes due to an interaction with a neutrino. The detector can discern whether a light flash is due to an electron neutrino (or antineutrino), or a muon neutrino (or antineutrino). In their journey from Tokai, some of the muon neutrinos are bound to transform into electron neutrinos, and if CP symmetry held true, there would be, on average, equal numbers of muon antineutrinos that transform into electron antineutrinos. In this experiment’s decade of operation, it has detected flashes from 90 electron neutrinos, but just 15 electron antineutrinos. This suggests that muon neutrinos and their antiparticles have different rates of transformation, pointing to a significant difference between the behavior of matter and antimatter in this instance. The implication is CP violation.
Maybe. The result has 95 % confidence (2 sigma). The standard in physics for identification of a new phenomenon is 5 sigma, and this result is nowhere near that standard. Other, bigger experiments are in the process of being built which will confirm or refute the SuperKamiokande result, but it will take many years of collecting data for that to happen. So, we’ll have to wait a while to see.
Top Comments (April 18, 2020):
No submissions or highlights tonight.
Top Mojo (April 17, 2020):
Top Mojo is courtesy of mik! Click here for more on how Top Mojo works.
Top Photos (April 17, 2020):
Tonight’s picture quilt is courtesy of jotter!