Andy’s Los Alamos Blog

The titles are almost completely meaningless, it just looks bad without them.

Another interesting day. I got up early and biked around town hoping to avoid the afternoon storms brought daily by the “monsoon season,” enjoyed a nice little ride, then spent a whopping 63 minutes on Los Alamos’ public transportation getting to work. I mean, at least they’re trying, but I could have probably walked to work faster. It’s only 5 miles… For those who have played Sierra’s Half Life computer game, the only disappointing thing about Black Mesa - I mean Los Alamos - National Lab is the lack of the awesome employee rail system. Oh, well.

Today I worked on my final paper for the Los Alamos Summer School for most of the day. I had to keep decreasing my font size to keep it under control… it’s nine pages so far and I’ve hardly even talked about my experiments. When I leave for home, I’ll get it approved by the declassification people and post it up here for your reading pleasure. I should have at least my abstract up soon, but it’s up to the reviewers.

I’ve received a mixed response on that last post. I’ll try to clear things up a bit, I’ll need to think a bit about how to else to approach particle physics. In the mean time, there was a question about neutrino oscillations, which is a pretty neat modern physics topic. I’m afraid this’ll be unavoidably more technical than the last post…

Neutrino Oscillations:

Neutrinos are tricky particles, painfully quantum-mechanical in nature, and free neutrinos absolutely do change flavor (among electron-, muon-, and tau-neutrino). This weirdness was originally observed in the Homestake Experiment, due to a discrepancy between the theorized and observed number of neutrinos coming from the Sun, but not explained until 2001, when the Sudbury Neutrino Observatory (SNO) proved the existence of neutrino flavor charge. Since, a good deal of theoretical work has brought the theory of neutrino oscillation to its current, well-developed state. Technically speaking, free neutrinos exist as superposition of mass eigenstates, meaning that at any time each neutrino is sort of all three at once. If you detect it, however, you only detect one type. This is the “quantumness” of the system; quantum physically, it’s perfectly legit for a particle to be three particles at once, but once you observe it it can of course only be one (there’s a hot and ongoing debate about what constitutes observation). In the words of quantum mechanics, the neutrino has a certain wavefunction, and when you observe it, you collapse the wavefunction to the observed condition. Anyway, this begs the question: if neutrinos are all three at once, how can we define the three separately? It turns out that they are not equal parts all three, but, strangely perhaps, an individual neutrino is defined by the one it has the greatest probability of being observed as. So, an electron neutrino is mostly electron neutrino with a little muon- and tau-neutrino. When you observe it, you’ll probably see an electron neutrino but there’s a slight chance you won’t. The oscillation bit adds another layer of complication: these probabilities change over time. So what is mostly electron neutrino now might be mostly muon neutrino later, for example. Below is a graph from the Wikipedia page on neutrino oscillation, showing these probabilities over time in the case of relatively slow solar electron neutrino oscillation (electron-neutrino-ness=black, muon=blue, tau=red). This should help with visualizing the oscillations.

A few references for more neutrino oscillation information:

• “Neutrino Mass, Mixing, and Flavor Change” http://pdg.lbl.gov/2005/reviews/numixrpp.pdf
• “The Neutrino Oscillation Industry” http://www.neutrinooscillation.org/
• Arxiv.org, search for “neutrino oscillation”