MINOS claims CPT violation for neutrinos

MINOS is an experiment at Fermilab studying neutrino oscillations where one type of neutrino changes into another. Such changes were recognised some time ago as the explanation for missing neutrino fluxes from the sun and can only happen because the neutrinos have a small mass.

MINOS measures these oscillations and deduces a value for the differences between the squared masses of the different neutrino flavours. For the electron and muon neutrino the value they get is 2.35 x 10-3eV2.

Now they have also succeeded in getting a value for the same number using anti-neutrinos and the value is  3.35 x 10-3eV2 This means that for at least one of the neutrino flavours the mass must differ between the particle and its anti-particle. Differences between the interactions of particles and anti-particles are signatures of CP violation and have been well-studied for heavier particles, but this kind of mass difference would signal a much more surprising violation of CPT symmetry.

In relativistic quantum field theory and all other known generalisation of particle physics models, CPT must be conserved, even in string theories. If it is broken then physicists would have to invent a whole new way of building their theories.

Luckily the measurement comes with errors bars and the statistical significance of the result is only 2 sigmas which mean it could be a statistical fluctuation with a probability of 5%. In particle physics this level of certainty is not considered much to dance about. Given the very strong theoretical constraints against CPT violation we should expect this result to fade away as more data is collected.

However, the precision measurement of neutrino mass parameters is itself an outstanding achievement well worth knowing about.

12 Responses to MINOS claims CPT violation for neutrinos

  1. Kea says:

    Ah, now that is a MUCH better post! The future awaits …

  2. Ervin Goldfain says:

    Since any Lorentz invariant field theory must preserve CPT, this claim amounts to a direct violation of Special Relativity. It goes without saying that one ought to be very skeptical about these results.


  3. Kea says:

    … it’s only a SMALL violation … and symmetries should be DERIVED in quantum gravity, right?

  4. philipgibbs says:

    Ervin, I’d say it’s an indirect violation of Soecial Relativity since that conclusion requires theroetical assumptions about the way quantum field theory works. However you would have to have a very radical theory to account for this effect without violation of special relativity, but then violation of special relativity is also quite radical. It’s such a radical experimental result that all bets are off if it holds, but odds on bet is that it is a statistical fluctuation that will go away with more data.

    Kea, I go with the idea that in quantum gravity symmetries such as Lorrentz invariance and CPT are remnants of deeper larger symmetries that include permutation symmetry of space-time events. Perhaps at an even deeper level these symmetries are derived from a principle of universality on the space of all possible theories.

  5. Kea says:

    I agree that this is not a direct violation of Special Relativity, since SR says nothing about antiparticles or quantized masses.

  6. ervin goldfain says:

    Kea and Phil,

    In my opinion, it all depends on the context where Lorentz invariance violation is introduced and discussed. I see two viewpoints here:

    1) Strictly speaking, CPT violation does not directly contradict SR. However, it means a radical departure from relativistic QM and QFT alike.

    2) In a broader sense, CPT violation proves that Lorentz invariance of SR cannot be considered a UNIVERSALLY VALID principle. If free particles and antiparticles are allowed to have different masses, the modulus of their four-momentum p^2 = (E^2/c^2) – p.p = m^2 is no longer the same. This leads immediately to a “c” that is different for particles and antiparticles, in manifest contradiction with SR.

    Regardless of which viewpoint you take, what really matters here is that CPT violation, if proven true, creates a serious theoretical challenge.


  7. Kea says:

    Ervin, the only real issue here is relativistic QM. For a long time it has been clear that QFT requires a reformulation. Now although Dirac managed to PREDICT antiparticles with the same mass, it is not clear from RQM alone that this mass is the entire contribution to the particle mass … since neither RQM nor SR nor QFT say anything about the mass quanta. And eventually, all simple physical rules are broken. In my view, the equality of mass in RQM comes down to the imposition of linearity in QM, and it can only be an approximation from the point of view of QG.

    Your second paragraph is hogwash. SR says NOTHING about particles or antiparticles. However, one may ask, with respect to what are you measuring c? For many years, the Riofrio cosmology has discussed a variation in c on cosmic scales. If you want to discuss a measurement of c from the perspective of particles or antiparticles (rather than us) then we are quite happy for c to vary. But this does not violate SR in the usual sense.

  8. Carl Brannen says:

    To the extent that it violates special relativity where QFT is concerned, this can be papered over by giving up SR and retreating to Lorentz’s version of special relativity. That is, it’s only in the philosophical extension of observation of Lorentz invariance to a full symmetry of space and time that there must be an error (if the MINOS obseravation is correct).

    Assuming a preferred reference frame guts SR without changing any of its predictions and this is the natural way of gettign beyond QFT. Then you can get SR as an accidental symmetry of non relativistic QFT. That is, it can be correct exactly for the particles but still not obeyed as a symmetry principle of the Lagrangian and all that.

  9. ervin goldfain says:

    Kea, you write:

    “Your second paragraph is hogwash”.

    This comment is uncalled for and offensive. It prevents me from continuing the conversation.


  10. philipgibbs says:

    There has been a lot more about this around then blogs and news sites. Anyway, to round things up I would summarise the general classes of possibilities as follows:

    (A) – The observation is a result of low statistics or other experimental error and will go away.

    (B) – The observation is correct but the interpretation that antineutrinos have a different mass is wrong. In this case an alternative particle model, would have to be found, e.g. with a different neutrino showing up due to CP violation only.

    (C) – The interpretation is correct and CPT is violated. To account for this a new theory is required to replace quantum field theory while still keeping to Lorentz invariance.

    (D) – CPT is violated but quantum field theory holds while special relativity does not.

    Can wordpress do polls? I may try it.

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