Wednesday, April 17, 2013

Neutrinos (Part 2)

If you have not read the first part of this post, I highly recommend you read it first. I'll even provide you a convenient link, because I'm nice like that. Neutrinos (part 1)

We shall pick up the neutrino story from the perspective of understanding the physics of the Sun. In 1957, Margaret and Geoffrey Burbidge, William Fowler, and Fred Hoyle published the groundbreaking paper "Synthesis of the Elements in Stars." Neil DeGrasse Tyson routinely cites this paper as one of the most important works in all of astrophysics. "Synthesis of the Elements in Stars" (often abbreviated B2FH, after the authors' last initials) proposed the existence of the nuclear fusion reactions necessary to create all of the elements we see in the universe apart from those initially created in the Big Bang (which I discuss briefly in my History of the Universe post).

To make a rather long story short, the Sun exists, and continues to exist, because it is fusing hydrogen into helium in its very hot, very dense core. The net reaction (ignoring the intermediate steps) is graphically shown below. This process releases energy and keeps the Sun hot, which is incidentally what prevents the Sun from collapsing on itself from its own weight. There are a lot of nuclear reactions that contribute to the production of helium in the Sun (occupies multiple pages of An Introduction to Modern Astrophysics), the most important point is that a number of these reactions result in beta-plus decays where protons are forced to change into neutrons to keep newly formed nuclei stable. This is the opposite of the reaction that was shown in Part 1, so rather than getting electrons and anti-electron neutrinos, we get positrons and electron neutrinos. (Recall an electron neutrino is just the type of neutrino that is specifically associated with electrons.)
Image slightly modified to include neutrinos. Image credit: Addison Wesley Publishing
If you crunch the numbers, you find that the Sun should be producing something like 1034 neutrinos every second, and that's just the ones that are a high enough energy for us to detect. It's pretty natural that astrophysicists would want to confirm the existence of these neutrinos, because that would tell us whether or not we were right about the physics going on deep inside the Sun. Remember, neutrinos don't like interacting with anything, so they can fly right out of the center of the Sun no problem. Unfortunately, the same generally holds true for our detectors.

Fortunately, we know what reactions need to take place for us to detect a neutrino. Therefore, we can do experiments to measure the probability that a neutrino interacts with your detector before you actually put it to use.

The first dedicated solar neutrino detector was the Raymond Davis and John Bacall's Homestake detector, which came online in 1970. Like most neutrino detectors, the Homestake detector was placed in an old mine in North Dakota. This shielded the detector from cosmic rays entering Earth's atmosphere that can produce neutrinos. The detector itself was quite literally a giant tank of dry cleaning fluid, a molecule made up of 2 carbon and 4 chlorine atoms. The chlorine atoms are the part that matters. When an energetic neutrino interacts with chlorine, the chlorine atom can be changed into a radioactive isotope of argon. Every few weeks, Davis would extract the argon and count how much was created, which would tell him about how many neutrinos had interacted with his detector.

While Davis was in charge of the experimental measurements, Bacall calculated the number of argon atoms that the detector was expected to see. When the data were compared to the calculations, it turns out that Davis' measurements came up with roughly one third of the neutrinos that Bacall predicted.

At first, everyone thought that Bacall's calculations were just wrong, or that he hadn't accounted for something properly. Eventually, more neutrino detectors were built: Kamiokande in 1983 which became Super-Kamiokande in 1996, GALLEX in 1991, SNO in 1999. While I won't discuss the finer points of these detectors, each of them works in a slightly different way, but saw the same result; the Sun was producing only one third of the electron neutrinos it was expected to. The distinction of electron neutrino is important because that is both the type of neutrino the Sun creates and the type that our detectors are primarily sensitive to. In textbooks, this is routinely referred to as the "solar neutrino problem".

Not all neutrino experiments use solar neutrinos though. As I mentioned above, cosmic rays entering Earth's atmosphere can produce neutrinos that can either be annoying background noise or a useful signal! For obvious reasons, we call these "atmospheric neutrinos" and we've been observing these since 1965. Neutrinos are also created in nuclear reactors from beta decays that occur in the reactors. Because these "reactor neutrinos" tend to be pretty high energy, they can be used for neutrino physics experiments. We can also use particle accelerators to produce "neutrino beams". Basically, this involves creating a bunch of particles travelling in the same direction that will eventually break apart and create neutrinos (generally muon neutrinos, actually) that are all travelling in the same direction as one another. Then you stick a detector in their way (or better yet, aim the beam at a detector that already exists) and you have your very own neutrino experiment!

"Well that's cool and all, but what does this have to do with the solar neutrino problem?" you may be asking. This goes back to work done (and promptly ignored) by Italian physicist Bruno Pontecorvo in 1957. Pontecorvo suggested that neutrinos could spontaneously change flavors as they traveled through space. But for this to be the case, neutrinos had to have mass, which, according to the Standard Model of particle physics, they did not. Because the Standard Model had been so bloody successful, there was no reason to believe the theory was wrong.

Until 1998. In 1998, Super-Kamiokande began to see hints of neutrinos changing their flavor in its atmospheric and solar neutrino data. When SNO came online a year later, it also saw evidence that neutrinos were changing flavors. This marks the first, and so far only, time that the Standard Model was shown to be wrong, which required an extension of the theory to allow for neutrinos with mass whose flavors could change more or less spontaneously. Now we even have long baseline (hundreds of miles) neutrino beam experiments such as MINOS and T2K that are designed to detect these flavor changes (and they do). This phenomenon is now referred to as "neutrino oscillations".

This may seem like a strange term for "changing flavors", but it actually makes sense, in an odd sort of way. In particle physics jargon, neutrinos can change their flavor because neutrino mass eigenstates don't line up with neutrino flavor eigenstates. This means that when a neutrino of a certain flavor, let's say an electron neutrino, is created, it is a pure electron neutrino in flavor, but its mass is some combination of electron, muon, and tau neutrino masses. As the neutrino moves through space, as shown in the diagram below for a simple two-flavor case, the mass combination changes a bit (for the life of me, I couldn't tell you why). The change in the mass combination will change the flavor of the neutrino, so rather than being a pure electron neutrino, it has some probability of being a muon or tau neutrino as well.

The oscillation comes in because (in a simple two-flavor picture), as the neutrino travels through space, it will flip back and forth between the two flavor states. Not shown in the diagram, the rate of oscillation depends largely on the energy of the neutrino.
Figure adapted from Scientific American, March 19, 2013 "Neutrino Experiments Light the Way to New Physics"
Oscillations can also occur as neutrinos travel through not-empty space. In fact, they happen much quicker. This is called the Mikheyev-Smirnov-Wolfenstein (yeah, I just wanted to write out those names) or MSW effect. It turns out that the MSW effect is mostly responsible for the oscillation of solar neutrinos rather than their trip through empty space to Earth. This explains the solar neutrino problem because we expect that the neutrinos will become roughly evenly divided amongst all three flavors during their trip from the center of the Sun to our detectors on Earth.

Phew. That was probably the hardest three paragraphs I've ever tried to write. These last paragraphs alone were why this post took so damn long to write. On the bright side, you now all know why neutrinos are so weird and part of why I love them so much. I hope this post was understandable to most of you, and there will be a Part 3 in the works. But before that, I expect you'll be getting some exoplanet news some time tomorrow. Stay tuned!

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