Friday, August 26, 2011

MINOS: neutrino-antineutrino mass difference evaporates

A week ago, I linked to a video about the CERN's ASACUSA experiment that showed that the proton and antiproton masses coincide at the precision of 1 part per billion.

The MINOS detector in Minnesota, to be discussed momentarily

The CPT-symmetry which is implied by quantum field theory - and which may be viewed as a consequence of the Lorentz symmetry because it is a kind of "rotating of spacetime by \(\pi\)" - ensures that the masses of particles and antiparticles have to be exactly equal.

These facts are apparently ignored by many experimenters who regularly hype their "result" that various other, harder-to-be-measured particles have completely different masses than their antiparticles, despite the 1 ppb agreement for a generic particle called the proton. Most notoriously, the CDF Collaboration at the Tevatron recently claimed that their "most accurate" measurement of the top and antitop masses indicated a 5% mass difference between the proton and the antiproton. Well, it wasn't too accurate a measurement if the error was 5%! :-)

Needless to say, D0 quickly refuted the claim, showing that the difference is zero within the total error margin - and, if I remember well, reverting the sign.


An almost carbon copy of this top-quark situation appeared in the neutrino sector. In June 2010, MINOS claimed that they found hints of the CPT violation in the neutrino sector. Some antineutrino masses differed from the corresponding neutrino masses.

I said that I didn't believe such a difference - and I never did. Even if there were a bizarre signal, it would almost certainly be due to something else than a CPT-violation - but I didn't believe any signal. However, we had to wait for more than one year for this particular claim to be refuted. Yesterday, MINOS released their new results based on a doubled amount of data:
Fermilab press release

The same press release with a chart

MINOS for scientists: relevant graphs
Of course, after the data was doubled, the deviation dropped to the ballpark of zero: this "signal" is gone, too.

Moral of the story

A lesson from these blunders for the experimenters is: don't mess up with basic principles of physics. The Lorentz symmetry, the CPT symmetry, a few conservation laws, and the basic postulates of quantum mechanics are fundamental, universal, and as far as we can say, inevitably exact properties of Nature.

You surely need at least five-sigma evidence for the invalidity of these pillars of science if you want to claim something about their collapse outside a psychiatric asylum. And even if you will claim something, no sensible person will believe you that you have done your job properly.

If you don't have such evidence, you should maximally exploit your opportunity to shut your mouth. Not only that: you should assume that the error - the source of the deviation - is in your experiment and do everything you can to reduce this error in your future measurements if there's some evidence that your deviation is due to an unaccounted for systematic error.

The CPT-symmetry and other principles are huge empires that won't fall easily; on the other hand, you're just a tiny wee-wee experimenter so adjust your sense of proportion accordingly. Thank you very much.

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