# Neutrino oscilation

1. Mar 24, 2006

### michealsmith

cann anyone tell me about tghe neutrino oscilation experimtent

2. Mar 24, 2006

### Staff: Mentor

3. Mar 27, 2006

### michealsmith

ok what do u know about the T2k

4. Apr 11, 2006

### Gluonium

The outcome was that Neutrino's oscilate, which means that they must have mass. This shook the standard model a bit because it predicted that they did not have mass.

If i recall correctly, they experiment was just a beam of neutrinos and on a few occaisions some of the neutrinos oscillated.

5. Apr 12, 2006

### arivero

Short history: Georgi told the proton was to decay at a measurable probability, Japanese did big inversion money to build the detectors but no proton was detected to disintegrate. Then they turned the detectors into neutrino detectors and oscillation of neutrinos was detected! Big success because nobody had taken upon his shoulders the work of neutrino detection. After this, laboratory experiments are done not to confirm it but to refine the measurements so we can get some hints on the value of the mass. The recent experiment if one of these.

If the USA had filled with water the SuperCollider it had been better inversion than just to bury it.

6. Apr 20, 2006

### SpaceTiger

Staff Emeritus
On the contrary, I think Ray Davis and John Bahcall had taken quite a lot on their shoulders.

7. Apr 20, 2006

### Staff: Mentor

Lots of accelerator experiments involving neutrinos had tried to detect neutrino oscillations as a sideline to their main work, but none had found any. I did a search like that as my PhD dissertation project.

The experiments that finally observed neutrino oscillations explored different energy and distance ranges, where the effects are easier to detect.

8. Apr 20, 2006

### ZapperZ

Staff Emeritus
Er.. the MINOS collaboration had just had their first results reported here just last week! In fact, if Fermilab doesn't get funding beyond 2009 for the Tevatron, it WILL become predominantly a neutrino factory for MINOS. So I don't think it is a side project any longer.

Zz.

9. Apr 20, 2006

### Staff: Mentor

Right, MINOS is very much an accelerator-based experiment. I was referring to earlier generations of accelerator-based neutrino experiments, say from the 1970s up to the early to mid 1990s. A lot of them had their data analyzed in various ways to set limits on neutrino oscillation parameters, long before the first positive evidence came from Super-K etc.

There was a colloquium at the University of South Carolina last week about the recent results, but we're at the end of the semester here so things are crazy enough with tests and exams that I couldn't take the afternoon off to drive down to Columbia.

10. May 4, 2006

### Michael Mozina

I have a couple of layman's questions that perhaps someone could explain.

I can't quite grasp why the absense of atmospheric muon nutrinos from the far side of the earth automatically equates into neutrino flavor changes. In other words, why couldn't the muon neutrinos simply be more apt to be absorbed or deflected by dense mass compared to the electron neutrino?

If the fact that there are three types of neutrinos means that at least some of them must have mass, aren't we assuming different masses for different kinds of neutrinos? If they are different masses to begin with, how does a neutrino "gain or lose mass" to change to a different type? Wouldn't a change of mass violate conservation of energy laws?

Pardon my naivate' on this issue, I just don't understand how evidence of a deficit of recieved neutrinos of one type automatically equates into evidence of a flavor change.

11. May 4, 2006

### Staff: Mentor

I haven't seen the detailed analysis of these experiments, but I'm sure it must include estimating what fraction of neutrinos of each type are absorbed while traveling through the earth, using neutrino interaction cross-sections predicted by the standard model (and studied experimentally) and current models of the earth's structure.

Neutrino oscillations are not "mass oscillations." They are "flavor oscillations." The basic idea is that neutrinos of a particular flavor (e, mu or tau) do not have a single definite mass, but rather have certain probabilities of being three different masses. For each flavor, the possible masses are the same, but the probabilities are different.

According to a quantum-mechanical treatment of this system, if you create a neutrino of a particular flavor in a way that does not give you knowledge of which mass it has, its wavefunction is a superposition of the wavefunctions for all three masses. As the neutrino travels, the three wavefunctions in the superposition interfere with each other, giving oscillating probabilities for each of the three flavors. So when we detect it, it might be any one of the three flavors. However, it has the same mass at production and at detection, chosen at random from one of the three possible values.

12. May 4, 2006

### Michael Mozina

The fact that neutrinos actually posess mass seems pretty "new" from a particle physics standpoint. I guess I'm a bit skeptical that we can already rule out some sort of scattering/absortion affect this early in the process.

I guess this concept is just hard for me to "wrap my head around". In the realm of photons, the mass of a photon does not vary, but it's wavelength changes, thus we have "high" and "low" energy photons. A photon however does not typically change energy states unless there is some kind of interaction with something else. If the 'mass' of a neutrino particle varies, how does one determine the energy state of the three types of neutrinos?

I suppose that is as comprehensive an answer as I'm likely to get based on my own lack of understanding of the mechanical models that are being used to describe a triple wave function for a single particle.

I suppose I'd feel a lot better if we could aim neutrinos at a detector and measure (detect) the fact that some of the neutrinos actually changed into another form of neutrino. As it stands, it seems like a lack of a "detection" of a single kind of neutrino is simply being "interpreted" as a change from one state to another without actually seeing/detecting such the actual transition into another form. At the moment we only detect a miss, rather than detecting a hit of a different kind of neutrino. Detecting a missing neutrino is not identical to detecting a hit of a different kind of neutrino, but that seems to be the way the data is "interpreted" at the moment.

Thank you for your clear explanation of the triple wave function that is currently attributed to a neutrino. I admit I remain skeptical, but that explanation does seem to help. Thanks. :)

13. May 4, 2006

### Michael Mozina

A couple more layman type questions came to me at lunch.

Why does current theory favor an "intrinsic triple wavelength" rather than some kind of transition occuring along the way due to say a interaction at the nuclear level? In other words, at first I originally assumed that neutrinos might change "wavelength" since a photon is a close neighbor from a mass standpoint. I could grasp how the neutrino wavelength might be affected along the way, based upon a physical process inside an atom, but I don't really "grok" the whole three wavelengths at once concept.

Wouldn't it make more sence to believe the neutrino's wavelength was altertered by an interaction with an atomic nucleus, rather than believing it has three separate wavelengths at once?

14. May 4, 2006

### Staff: Mentor

Remember, the mass of a particular single neutrino doesn't vary with time. Some neutrinos turn out to have one mass, some have another mass, and the rest have a third mass. The mass of any particular individual neutrino is the same at production and at detection; but the flavor at detection may be different from the flavor at production.

There are experiments in progress or in the works that are going to test this. They produce neutrinos of a specific type at an accelerator, then detect them far enough away so that oscillation effects should be significant.

In the meantime, there are results from the Sudbury Neutrino Observatory in Canada that detects electron, muon and tau neutrinos from the sun. The sum of the three flavors agrees (within experimental statistical uncertainty) with predictions of the number of electron-neutrinos produced by the sun according to standard solar models.

Earlier solar-neutrino detectors detected only electron-neutrinos, and they found fewer neutrinos than the solar models predict. This was the long-standing "solar neutrino puzzle" which has now apparently been resolved.

15. May 4, 2006

### SpaceTiger

Staff Emeritus
Aren't the neutrinos generally in flavor eigenstates at production -- that is, not in a particular mass eigenstate?

16. May 4, 2006

### Staff: Mentor

Yes. And when the neutrino interacts (is detected) it does so as one of the flavor eigenstates, which may or may not be the one that it was created in. But energy and momentum are conserved, so because $E^2 = (pc)^2 + (mc^2)^2$, the mass of a particular individual neutrino, whichever mass it turns out to be, must be conserved. We can't know which mass it is, without making extremely precise measurements of the energies and momenta of the other particles involved in the production and decay processes, which is impossible in practice. At best we can state the probablilites that the neutrino has each of those masses.

The mass eigenstates and the flavor eigenstates are related by a matrix of coefficients, something like this:

$$|\nu_e> = a_{11} |\nu_1> + a_{12} |\nu_2> + a_{13} |\nu_3>$$

$$|\nu_\mu> = a_{21} |\nu_1> + a_{22} |\nu_2> + a_{23} |\nu_3>$$

$$|\nu_\tau> = a_{31} |\nu_1> + a_{32} |\nu_2> + a_{33} |\nu_3>$$

One of the major goals of neutrino oscillation research is to narrow down the values of the coefficients.

Last edited: May 4, 2006
17. May 5, 2006

### SpaceTiger

Staff Emeritus
But isn't that just reducing to another Schrodinger's cat question -- i.e. whether the particle had a "real" mass and we just didn't know what it was or whether it was in a superposition of states. I'm not particularly opinionated on the issue, but it seems to be a debatable point, at the least.

18. May 5, 2006

### Staff: Mentor

You can make some interesting parallels between neutrinos and other QM situations. For example, suppose we produce a muon-neutrino from pion decay:

$$\pi^+ \rightarrow \mu^+ + \nu_\mu$$

Suppose (hypothetically) that the pion has a very precisely known energy and momentum. They get divided up among the muon and the neutrino, so at the moment of production, the neutrino's energy, momentum and mass are uncertain. But now suppose (hypothetically again) that we measure the energy and momentum of the outgoing muon very precisely. Together with our knowledge of the pion's energy and momentum, this determines the neutrino's energy and momentum. If we do this precisely enough, we can determine which mass the neutrino has. In this case there are no flavor oscillations! When the neutrino interacts, it can still do so as any of the three flavors, but the probabilities of the different flavors are constant. They don't oscillate with time or distance traveled.

Disclaimer: I haven't actually seen this written up anywhere. It's based on my understanding of the QM of neutrino oscillations. Nobody who knows the subject well has contradicted me on this yet, but I'm definitely open to corrections.

This is very much like the classic two-slit interference setup for photons or electrons or whatever. If you make measurements that allow you to determine which slit the particle went through, you destroy the two-slit interference pattern.

19. May 5, 2006

### SpaceTiger

Staff Emeritus
Isn't that why the existence of neutrino oscillations suggests flavor violation in the charged sector? If the flavor eigenstates of the charged leptons weren't exactly equal to their mass eigenstates, then the energy-momentum states of the muon would be tangled with those of the neutrino. Then the experiment you're describing would be analogous to EPR -- precise measurements of the energy and momentum of the muon would cause the neutrino mass wave function to "collapse".

20. May 5, 2006