Of Particular Significance

The Ghost Story of Neutrino Weirdness

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 10/05/2011

Many of you, learning of the OPERA experiment’s claim (whose plausibility is dropping rapidly) of maybe-faster-than-light neutrinos, may have wondered (a) how do physicists know so much about neutrinos, and (b) what was the OPERA experiment originally designed to do?  [Hint: Only as a side project did the OPERA experiment set out to measure the speed of the neutrinos, and that’s why its design was by no means ideal for that purpose. ]

I’ve written an article that tries to explain the answer.  It describes how there are multiple ways to classify neutrinos that are mutually exclusive, and this leads to a strange, and scientifically crucial, quantum effect called “neutrino oscillations.”  After you’ve read the article, I hope you’ll have the language to understand this description of what the OPERA experiment was originally designed to do:   Its original goal was to study a particular effect of neutrino oscillations, by sending muon-neutrinos traveling through the earth for 730 kilometers, oscillating as they go, and emerging in OPERA as a mixture of mainly muon neutrinos and tau neutrinos; then, any one of them colliding with an atomic nucleus might be converted either into a muon or a tau, an effect that OPERA would observe. [Recall that the three charged leptons in nature are electrons, muons and taus, with taus the heaviest.]  The presence of taus as well as muons would confirm oscillations occur in the way most neutrino physicists currently believe that they do, and the rate of tau production would provide more precise information on neutrinos’ properties.

You’ll also understand something else important and relevant for OPERA: if you mess around with neutrino speeds even a tiny bit, you can easily run into conflicts with a wide variety of neutrino oscillation experiments.  If that sounds cryptic, read the article!

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5 Responses

  1. Thanks for the clarification concerning point 1, silly me :-/… I’ll follow the link at work tomorrow during lunch break using a bigger screen … 🙂

  2. Since you are getting interested in some of the technicalities, I recommend an introductory pedagogical review such as http://arxiv.org/abs/hep-ph/0411274. See also http://www.nu.to.infn.it/Neutrino_Lectures/ for more links.

    In quick but technical answer to your specific questions:
    1. No, there is not [and also, you are making a mistake here; the commutator of x with p_x is “i”, the commutator of y with p_x is 0, and angular momentum involves x p_y – y p_x, which is not a commutator.]

    2. The oscillation frequencies correspond to differences of velocities (which appear in the equations as differences of squares of masses.) This is easy to derive and you can find the derivation in the above-mentioned review.

  3. Dear Prof. Strassler, this nice description of neutrino oscillations makes me curious abaut two things:
    1. Is there a symmetry and conserved quantity that corresponds to the nonzero commutator of the mass and weak neutrino states in analogy to the fact that the commutator of position and momentum leads to angular momentum?
    2. Do the oscillation frequencies
    correspooooond to the oscillation of the elements of a matrix transforming betweenne the mass and weak states? How can they be derived?
    I apologize in advance for typos, my laptop is damaged 🙁 and I’m typing from my smart phone…

  4. Thank you for all of your posts. While I really enjoyed reading about neutrinos in your Neutrino Types post, I am still fascinated by the OPERA experiment. Can you please explain why the plausibility is dropping?

    Also, I am really fascinated by superposition. I watched the TED Talk by Aaron O’Connell called “Making sense of a visible quantum object” and it helped me understand superposition at a very rudimentary level, which was better than my previous understanding, or complete lack of understanding. I would really like to comprehend it on a deeper level and what it means in context to the nature of our universe.

    1. I am posting about the theoretical arguments; the first post is already up http://profmattstrassler.com/2011/10/06/is-the-opera-speedy-neutrino-experiment-self-contradictory/ . About superposition — the subject is a basic one in quantum mechanics, but I don’t know of any way to understand it intuitively, other than to learn in great detail how quantum mechanics works. I am not aware of any quick and easy path to understanding this well. Fortunately most of particle physics is much easier to understand…

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