The latest in Ph-word: April 2020 part #1

T2K, Tokai-to-Kamioka, is an experiment studying neutrinos, the mysterious particles that pass by the dozens (of trillions) through earth each second without stopping. In some way its name reflects this behaviour of neutrinos: Tokai and Kamioka are two japanese villages separated by 300 kilometers, but when an accelerator at one of them creates and sends underground streams of neutrinos to the other, they pass through the 300km of solid rock undisturbed. Then they reach T2K’s vast tank of water, where very very few happen to fall heads-on on the nuclei of atoms, giving off a signal that the experiment records.

T2K has been working for decades with those tiny weirdos, steadily finding out more about them. Now it announced that things are progressing in its current and quite important mission: to see whether both neutrinos and antineutrinos, their antimatter equivalents, behave in exactly the same way or if they obey CP-violation.

In brief (but you can read more in a previous post if you are so inclined), in brief then, CP-violation is the very enigmatic fact that some nuclear decays don’t happen in the same way in matter and antimatter. It has been observed in quarks but seeing whether it holds for neutrinos hadn’t been easy, for reasons that should be obvious.

T2K, however, takes advantage of one peculiar feature of neutrinos: there are three types of them and as time passes each neutrino changes its type. The Tokai accelerator generates only one type of neutrinos (or antineutrinos), so the experiment can measure this change, even if in handfuls.

When all’s said and done, T2K can tell if there is any difference in this behaviour between neutrinos and antineutrinos. Actually it’s been at it for a few years now, and last month it announced its results so far. It has not seen any undoubtable CP-violation yet but it’s also seen some hints of it. The important part is that it seems to be halfway done with the measurements it must do before concluding the matter. So it made everybody hopeful that a couple more years and we’ll know at last what’s really going on, at least in this aspect of life.

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