Sunday, April 7, 2013

Neutrinos (Part 1)

Because I've been pretty busy lately, I haven't really had time to post too much, which also means I missed out on the opportunity to write an April Fools post. As a way of making up for that, I'm going to start my series of articles on neutrinos, which are easily my favorite type of particle in the universe. These guys are so weird that they may as well be nature's April Fools joke on all of us.

Before I dive into this, let me point out that we physicists love our conservation laws. Energy (or mass-energy if you're talking about something relativistic), momentum (normal and angular), charge, lepton number, baryon number (and many more) had better be conserved in nature (I'll explain what leptons and baryons are later on). Violating some kind of conservation law is the easiest way for a physicist to determine that a certain theory or model is wrong.

To start our discussion, let's jump back in time to 1930. Quantum mechanics and relativity were both coming of age, and with them, our understanding of the physics that governs how atoms work. One of the more interesting new results was the understanding of radioactivity as changes that took place within the nucleus of an atom.

There are three different types of radiation that physicists know about: alpha, beta, and gamma (yeah, we're really creative), and each consists of a different sub-atomic particle. An alpha particle is the combination of two protons and two neutrons (like the nucleus of helium-4). Gamma radiation is just very high energy photons, or light particles. Beta radiation comes in two different types: negatively charged and positively charged. Turns out that these are just electrons and their anti-matter counterpart positrons.

Alpha and beta radiation from an atom will change one type of atom into another. Alpha radiation makes a nucleus smaller by two protons and two neutrons, and is commonly observed in heavy, unstable nuclei like uranium. Beta radiation can either change a proton into a neutron (by ejecting a positron) or a neutron into a proton (by ejecting an electron). This also makes nuclei more stable by bringing it closer to a combination of protons and neutrons that makes the nucleus more happy. Gamma radiation just brings nuclei into a lower energy state, which makes it more stable (a lot like taking your energetic kid to the park until he or she tires him/herself out).

While studying beta decay, Austrian physicist Wolfgang Pauli realized that the reaction disobeyed particular conservation laws (energy, momentum, angular momentum). Of course, this is bad, so Pauli predicted the existence of another particle that is involved in beta decays. He knew that it had to be very light (massless), neutrally charged, and weakly interacting (or it would have been detected already). In 1933, Italian physicist Enrico Fermi coined the name "neutrino" for this hypothetical particle, which means "little neutral one". Pauli is later said to have lamented "I have a done a terrible thing. I have postulated a particle that cannot be detected."

Fortunately for us, Pauli was wrong in one major respect: neutrinos can be detected. It's just not easy.

To explain how this was first done, I need to spend some time talking about how beta decays actually work. So to help out, I'll show you a Feynman diagram of a beta decay reaction. A Feynman diagram is a great way to visualize what occurs in particle physics reactions, but they also represent some incredibly complicated mathematics regarding interaction probabilities and such (check out the Wikipedia page if you want to read something that is utterly incomprehensible to non-particle physicists). But for our purposes, Feynman diagrams are almost brilliantly simple graphical representations of these type of reactions.
Feynman diagram of beta decay reaction that changes a neutron into a proton by kicking out an electron showing the quark structure of both the neutron (bottom) and proton (top). Time progresses forward along the y axis.
The figure above shows what we call a beta-minus decay, which just means that the ejected particle (an electron) has a negative charge. The particle in the middle of the reaction is a W-minus boson, which is one of the particles that is responsible for one of our four fundamental forces. Specifically, the W-minus boson is one of three particles that is involved in the "weak force", which you pretty much only see in beta decay type reactions (with some exceptions). In this reaction, the W-minus is not actually an observable particle because it is just a force carrier. What we can observe are the electron and the neutrino. In this case, the neutrino is specifically an anti-neutrino (as indicated by the vertical bar above the particle's symbol).

Why does it have to be an anti-neutrino? This comes down to some of those conservation laws I mentioned earlier. Electrons and neutrinos both belong to a family of particle known as leptons. Because lepton number is one of those things that has to be conserved, if we have 0 leptons at the beginning of the reaction, we had better get 0 net leptons out. When we count leptons (or any type of conserved particle, for that matter), leptons like electrons and neutrinos have a lepton number of +1, and the anti-leptons (positrons, anti-neutrinos) have a lepton number of -1. Therefore, with an electron and an anti-neutrino, we still have a lepton number of 0 afterwards.

We also need to conserve baryon number in particle reactions. Baryons are particles that are made up of three quarks, like neutrons and protons. It's easier to see that baryon number is also conserved in this reaction, because you get one baryon (neutron) in and one baryon (proton) out. We also conserve charge. Neutrons (as indicated by their name) and neutrinos are neutrally charged, whereas electrons and protons have opposite charges (negative and positive, respectively). Because this reaction is allowed to occur by the laws of nature, this makes particle physicists happy.

Interestingly, you can also run this reaction backwards. If an energetic anti-neutrino interacts with a proton, it can cause that proton to change into neutron and kick out a positron in the process. (Reality check: does this reaction conserve all of the quantities I mentioned above?) The positron emitted will quickly encounter an electron (because our universe is chock full of electrons) and annihilate, producing two gamma ray-energy photons.

This was successfully detected by Cowan et al. in 1956, which eventually got them the Nobel Prize in Physics in 1995. Amusingly enough, the discovery of a the muon neutrino won a Nobel Prize before the discovery of neutrinos themselves did (1988 compared to 1995).

Muon neutrinos are a type, or "flavor" (as we like to call different varieties in particle physics for some reason), of neutrino that is specifically associated with muons, which are very similar to electrons, but more massive. Muons are also leptons, but because they're more massive than electrons (about 200 times more massive), they're unstable in nature and tend to decay into electrons. Muons are typically created in high-energy particle collisions like one would create at the Large Hadron Collider or from energetic cosmic rays plowing into Earth's atmosphere. So up until now, we've strictly been discussing electron neutrinos, which is why the neutrino in the diagram above has a subscript e.

Turns out, as shown by Perl et al. in 1975 with data from the Stanford Linear Accelerator Center (SLAC), electrons have yet another lepton cousin, the tau (we don't call these guys tauons because that just sounds dumb). Tau particles are, themselves, about 15 times more massive than muons, and have their own neutrino, the (creatively named) tau neutrino. This discovery shared the 1995 Nobel Prize with the discovery of neutrinos in the first place.

So, now you know the basics of how neutrinos work and what kinds of neutrinos are out there. Next time, I'll be talking about the things that make neutrinos truly weird and unique particles, and how they tie in to astrophysics.

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