Tuesday, October 18, 2011

MINOS: 3 fertile neutrinos seem to be enough

CERN and Gran Sasso cooperatively investigate the properties of neutrinos by sending muon neutrinos from decaying pions and kaons in the first place and detecting them in the other place.

The OPERA experiment recently became famous or notorious for their conclusion that the neutrinos apparently propagate more quickly than by the speed of light but the original purpose of OPERA was something else than sensational claims "we could punch Albert, baby": it was meant to measure neutrino oscillations and the production of tau neutrinos through these oscillations. And at least one tau neutrino has been seen, indeed. ;-)

Can America compete? The distance between Gran Sasso and CERN is 730 km. However, America has something longer, the MINOS experiment. The Fermilab has been sending muon neutrinos to the Soudan Underground Lab in Minnesota. What's the distance?

At this moment, TRF readers who live in the Obamaland (and one-half of them are) are anxious. They have lost the leadership position in accelerators so how is MINOS doing? Don't worry, the relevant distance in the U.S. is 735 km and beats the European one. ;-)

The MINOS collaboration has carefully measured the oscillations on this journey. They detect mostly muon neutrinos in a huge detector in Illinois, near the source, and they also detect some neutrinos after those 735 km in a smaller detector in Minnesota: in that state of the union, the number of muon and non-muon neutrinos they detect is different than what you would expect if the neutrinos were just freely flying through the country of the free, thus proving the existence of oscillations.

Their newest paper has just been published:
MINOS Search for Sterile Neutrinos
They compare the observations to a model with 3 fertile neutrino flavors, the so-called Standard Model :-), and the agreement is good. But just to be sure, they also try to fit the data to a 3+1-flavor model with 3 fertile and 1 sterile neutrino flavor.

As far as I know, all experimental hints that have ever suggested a possible existence of new neutrino species have been superseded by more accurate experiments that are consistent with the Standard Model.

There may be readers who don't understand the terminology (and physics) here. So neutrinos are almost invisible particles that arise from the beta-decay (or, more microscopically, the decay of the neutron, \(n\to p+e^-+\bar\nu_e\)); the last product is the electron antineutrino. Wolfgang Pauli figured out in 1930 that these neutrinos had to exist because of energy conservation (angular momentum conservation is enough to prove that there has to be another particle, too); within years, the right (effective) 4-fermion interaction involving \(p,n,e^-,\nu\) was written down by Enrico Fermi who also gave them the childish Italian name, in order to distinguish them from the equally neutral neutrons. Feynman and Gell-Mann clarified the vector-axial-vector index structure of this interaction in the early 1960s (despite famous experimenters who incorrectly claimed that the interaction was of the scalar-tensor type) and their FG theory was extended to a more consistent model with W-bosons and Z-bosons, the Standard Model, in the late 1960s.

The neutrinos belong among "leptons" (a Greek term for light particles) so except for the electric charge, they're analogous to electrons, muons, and tau leptons (the latter two are heavier cousins of the electron with the same charge and spin). In fact, at energies much higher than 246 GeV, neutrinos behave in the same way as their charged cousins, as guaranteed by the so-called "electroweak symmetry".

However, at much lower energies, the missing electric charge guarantees that the neutrinos' interactions are negligible (they're only represented by the weak nuclear interaction which is almost invisible in everyday life except for the beta-decay) and these particles penetrate through the whole planet as ghosts. Only a tiny proportion of the neutrinos interact with the atoms that we know.

Because of this association between neutrinos and their charged SU(2) partners, the normal three types of neutrinos are called "fertile neutrinos". Well, this term isn't being used outside this blog but it should be! :-) The idea is that being in an SU(2) doublet is more or less equivalent to sex and production of offspring.

Alternative theories of new physics may assume the existence of new types of elementary particles. Sterile neutrinos – this term is being used outside this blog – are neutrinos that are not associated with any charged partner. Otherwise they should behave much like the fertile ones (even though truly neutral neutrinos shouldn't even interact weakly and should be even more invisible). There is no evidence that such sterile neutrinos exist although there is no universal law of Nature that would prohibit them, either.

So such a new particle may be predicted by some string vacua and even non-stringy model builders who like to add unnecessary stuff sometimes investigate the consequences of a hypothetical sterile neutrino. Of course, chances are that no particle of this sort exists.

Icarus, Glashow, and Cohen

Icarus published another preprint today which brings some more experimental support for Glashow's and Cohen's claim that there was no Čerenkov radiation around the would-be superluminal Opera beam which is what would be expected.

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