New Discovery from the T2K Experiment (7.5 sigma for Electron Neutrino Appearance)!

It has been pointed out to me that this has become more of a historical record than a blog....

If I were to blame it partly on a former student who has been promising to write an extensive article about his finishing his PhD on T2K and then going on to work on the LUX dark matter experiment in the States, that would be very poor form indeed, so I would not do such a thing!

Anyway, today is a big day for us on the T2K experiment (the photo above is the T2K Imperial group from a couple of years ago), and in particle physics as a whole. The early T2K observation from a couple of years back that muon neutrinos actually oscillate into electron neutrinos has become a fully-significant discovery (and is being presented to the European Physical Society meeting in Stockholm today by our friend and colleague Mike Wilking):

Imperial College Press Release on the new discovery by T2K

While the most obvious change is that the number of electron-neutrino candidate events has gone from 6 to 28, which is because of the longer period of time T2K has been running, and the higher intensity of the beam (plus of course the size of the underlying physics which causes the process), it is just as important that a lot of work has gone into understanding of the beam and the detectors and the ways neutrinos interact, to increase our certainty on the inputs that go into the physics statement that we can make.

That is to say that the likelihood that random fluctuations (which is to say just bad luck) could make us see what we see is less than one in a trillion. We are certain at that level that what we are seeing is the appearance of electron neutrinos in a muon neutrino beam, once they have travelled the 295 km from J-PARC to the Super-K detector! In the plot above, the green shows the electron neutrino candidates we'd expect to see if this new physics didn't happen. So much work goes into working out the size of the green histogram, so that we know that the actual electron neutrino candidates we see (in the black dots with uncertainties shown in the vertical bars) must mostly be caused by new physics, shown in pink.

We will still be able to make this measurement better, providing a high-precision value of the parameters that describe this in our current model of neutrino oscillations. This, combined with measurements made by other experiments of related but different processes, and other measurements by T2K itself, will continue to help us learn if our models do indeed describe the universe well, and what their implications are. That could be the topic of a future blog post—are there any finishing PhD students' who'd like to contribute an extensive article on this?

PS. This is the work of many many people, and here I quote the bit in our press release today that describes this:

The T2K experiment was constructed and is operated by an international collaboration. The current T2K collaboration consists of over 400 physicists from 59 institutions in 11 countries [Canada, France, Germany, Italy, Japan, Poland, Russia, Switzerland, Spain, UK and US]. The experiment is primarily supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). Additional support is provided by the following funding agencies from participating countries: NSERC, NRC and CFI, Canada; CEA and CNRS/IN2P3, France; DFG, Germany; INFN, Italy; Ministry of Science and Higher Education, Poland; RAS, RFBR and the Ministry of Education and Science of the Russian Federation; MICINN and CPAN, Spain; SNSF and SER, Switzerland; STFC, U.K.; DOE, U.S.A.

T2K’s First Ever Physics Publication!

A week after our first paper, describing the setup of the T2K Experiment, I am happy to say that we have just submitted our first physics paper, and announced the results in seminars at the host laboratories.

Physics results are what our experiments are all about, and after many years of toil (as mentioned many a time here), it is a wonderful feeling, as always, to present to the world something about our Universe that no one ever seen before.

In the T2K Experiment, we create a beam of muon neutrinos at the J-PARC laboratory at Tokai Village on the eastern coast of Japan, and send them to the Super-Kamiokande neutrino detector 295 km away in the mountains of the north-western part of the country.
The mathematics that seem to describe well the results of other experiments—including Super-K looking at neutrinos in the atmosphere, KamLAND (my previous experiment) and many earlier experiments looking at neutrinos from nuclear reactors, SNO and others looking at neutrinos from the Sun, and MINOS and K2K with neutrinos made in a similar way to T2K but with different optimisations—suggest that with the specific energy and distance that the T2K neutrino beam has, we might be able to see a small fraction of the muon neutrinos turn into electron neutrinos.

This effect would be the third type of neutrino oscillation that has been seen.
The fact that it is the third type may make it sound boring and unimportant, but actually it is quite the opposite—the aforementioned maths tells us that if we see three, we have seen them all and that means that various other phenomena can be explained when you plug the numbers, that are given by the experiments, into the maths.

One of these possible phenomena we may be able to explain is the existence of matter in the Universe today, as opposed to all the matter and anti-matter produced in the Big Bang just annihilating into almost nothing, which is one reason we think it is rather interesting to measure these things.

Neutrinos, on the rare occasions when they indicate their existence by colliding with the atoms that make up matter instead of just passing through it, tend to create the particles that they are labelled with in their names—muon neutrinos create muons, and electron neutrinos create electrons—Super-Kamiokande is very good at distinguishing muons from electrons, so we basically point the beam at Super-K and count the number of times we see electrons created by neutrinos.

Of course, it isn’t quite that simple—the beam is pretty messy to start with and hard to understand (just like almost anything that has to do with neutrinos), and lots of other things can mimic electrons created by neutrinos, and it is the job of we experimental physicists to do our best to sort these issues out, and most importantly, understand them enough that we can estimate what their effects are.

Once we do all that, we get the plot that is shown at the top of this blog entry. This is what we have worked so hard for so many years for!

We previously calculated that if this third type of neutrino oscillation doesn’t exist, we would have seen about 1.5 electron neutrinos (on average) in the data we took over the year or so since the T2K beam started. That is the shown in the plot above by the yellow, green and blue hatched areas.

One and a half.

But in fact, when we looked at the actual data collected, we saw 6, as shown by the black points in the plot above.

SIX!

This is consistent with this new type of neutrino oscillation occurring!

If we put in this new neutrino oscillation at quite a large level, it looks like the red region in this plot:

which shows that the data does look a lot like neutrino oscillations!

But....

Here I have to make it clear that what we see now amounts to what we refer to in physics as an “indication” or a “tantalising hint” to employ a common cliche.

We set up experiments to learn about the Universe, but it often doesn’t just respond with simple “yes” or “no” answers, but it gradually gives us a picture that becomes clearer with time.

In our case, it could easily be that the true average rate of electron neutrinos appearing is much smaller, but we were just lucky and a few came in in quick succession by pure chance.

To make a discovery of the sort that T2K is aiming for is to contribute something new to the current understanding of how the building blocks of the Universe are, and this will affect how we build future experiments, and how we interpret the information that comes from other experiments, and how theoretical models of the Universe are built—so we don’t take it lightly.

What we do know is that if we can send more neutrinos to Super-K, we’ll be able to tell for certain what is going on.

Most importantly of all, the T2K experiment is clearly working well, and it can be seen how well it has been optimised for our measurement, which is why with just a few percent of the beam that it was designed for, we can see anything like this at all.

Here is a press release: http://www.kek.jp/intra-e/press/2011/J-PARC_T2Kneutrino.html and a copy of the paper we have submitted to the journal Physical Review Letters.

Unlike the paper from last week, the paper hasn’t been accepted yet, as it has to go through a lot of peer-review to make sure that the community will accept the results that we have shown.





The data we used for this result is from between early 2010 and the afternoon of the 11th of March 2011, and right now, the beam isn’t running because of the of the earthquake—which hit Tokai village very hard indeed.

Recovering from the earthquake and preparing for future data is what a large fraction of T2K collaborators are working on now, and this will continue for a while. It is very satisfying in the meantime, however, to be able to produce results like this that show the world what an exciting time it is for the experiment.

I’ll finish this post with a photograph of the T2K Collaboration that was taken a month ago during a hectic series of meetings when we were working on finalising this result:


Spot the Imperial group!