Muon Neutrino To Electron Neutrino Oscillation Conclusively Shown

New submitter Chris Greenley writes "The T2K long baseline neutrino experiment in Japan has just announced conclusive evidence for electron to muon neutrino oscillation at the 7.5 sigma level. (The level needed for discovery is 5 sigma.) This experiment generates a focused beam of electron neutrinos using an accelerator in the J-PARC facility north of Tokyo which is aimed at the massive Super-Kamiokande detector 295 km (185 miles) away, near the west coast of Japan. 'This T2K observation is the first of its kind in that an explicit appearance of a unique flavor of neutrino at a detection point is unequivocally observed from a different flavor of neutrino at its production point.' This result clears the way for CP-violation neutrino studies which could show that 'regular' neutrinos act differently than their antimatter counterparts, a phenomenon that so far has only been observed in quarks. If neutrino CP-violation is found, it could explain why there is such a large predominance of matter over antimatter in the universe."

Comments

General implications

[JoshuaZ]

They detected 28 electron neutrino interactions, where they would have expected 5 such events without the oscillation in question. This helps underscore how incredibly hard is it to get neutrinos to show up with anything: even when one is manufacturing millions of them, one is lucky to get a tiny set to then show up in your detector. This is connected to how most neutrino detectors are basically large vats of water or some other liquid, because the most we can generally hope for is that if we put enough mass in the way, some neutrinos will by sheer chance run into things.

This is also relevant to what we expect for stellar neutrino observation. Understanding neutrino oscillation gets us a better idea of what sort of neutrino ratios to expect (as a function of energy levels) in other circumstances. Right now, we can observe a lot of natural neutrinos from the sun. But the only neutrinos we've observed from an identified extra solar location, the 1987A http://en.wikipedia.org/wiki/SN_1987A, which was a very close supernova (so close it could be seen with the naked eye). In fact, in that case, the neutrinos arrived before we saw the light. That's not at all connected to the erroneous claim from a few years ago that neutrinos were going faster than light speed. What is happening here is that most of the light in a supernova is formed in the core, and the core of a star is very dense. So it takes a long time for the light to reach the surface of the star. But from the standpoint of neutrinos even very dense star isn't that much of an issue so they can get to the surface much faster. It is possible that this sort of work will give us better understanding both such neutrinos and what to expect when we do observe them from other close supernova.

Neutrinos are still a major area where there's a lot we don't understand, and this research is going to possibly have major implications for our understanding of these elusive particles.

Re:General implications

[rgbatduke]

Um, how? Using Dark Matter detectors? Look, neutrinos couple to existing, known particles -- leptons, although via other more complex processes e.g. inverse beta decay they can couple to protons in nuclei as well. The response we can detect is in proportion to the density of those particles in a medium that can function as a detector, which in turn is proportional to straight up mass density. The interaction probability is phenomenally low for all of the known particles, so one requires large volumes of material to make ANY kind of detector. There are strict limits on the density of ordinary matter, and even more stringent limits on the density of matter that can conceivably used as a detector

So I'm curious -- given that the ratio between the mass density of water and the mass density of e.g. Tungsten, Uranium, Plutonium is only a single order of magnitude, and a reaction cross section that AFAIK depends solely on the mass density, where exactly are the other two orders of magnitude going to come from from any possible variation in materials?

"Systems", well, perhaps. If we use underground cavities filled with water to look for Cerenkov radiation, or chlorine detectors to look for outgassing Argon, we can always make them 10x bigger and hence increase the detector volume 1000 fold. But this is morally equivalent to building 1000 detectors like we have today and combining the data, and it still leaves us with the same issues if one wishes to determine flavor information and not just raw e.g. neutral current flux. Indeed, to get flavor information we will very likely be limited to building 1000 detectors to get 1000 fold increase in data simply because there are size constraints on detectors that are going to detect and resolve e.g. a muon produced in a charged current interaction.

So sure, we can always scale up our existing technology or minor improvements of it to improve detection rates. But I don't think that materials or systems that improve the scaling of the detection technologies we already have are particularly plausible, based on what I now know. So what did you have in mind (as in, do you know something I don't)? Are there other materials that are likely to have orders of magnitude higher cross section for inverse beta decay, or are you just thinking of building bigger/more detectors?

Not flames, BTW -- an honest question.

rgb