The lower panel shows the ratio of measured data at Daya Bay (China) to the theoretical prediction. The spectrum overall shows a 6 percent deficit when compared with the data. But in the 5- 7 MeV range the bump in the spectrum shows up as an excess compared with model predictions. The total flux of reactor neutrinos detected at Daya Bay has also fallen short of predictions.
Let's cut to the chase: DO neutrinos exist or not? The inherent problem, based on the results of collective experiments, may be in the question itself. That is, instead we ought to be asking: Does the neutrino exist as a stable, permanent subatomic particle or identity? And the answer so far appears to be that it does not.
To fix ideas, and for reference, two Nobel -winning scientists showed that neutrinos - which are found in three “flavors,” - can oscillate from one flavor to another. In other words, they can change identities like a spy on the run, and hence there is no such entity as "the neutrino" - i.e. which is relatively permanent in its properties (namely its flavor).
Logically then, it makes more sense to refer to "flavor states" than such and such a neutrino. These states are: electron, muon and tau - which are in fact superpositions of mass eigenstates.
Because of the oscillations and quantum interference we need to reckon in a "misalignment" between flavor and the basic neutrino masses. This is done by reference to three independent "mixing angles": Θ 12 , Θ 23 and Θ 13. To a good approximation, oscillation in any one regime is characterized by just one Θ ij and a corresponding mass difference, defined:
D m ij2 = [m j2 - m i2]
As an example, the probability that a muon neutrino of energy E acquires a different flavor after traversing distance L is:
P = sin2 Θ 23 sin2 (l23)
where l23 is the energy -dependent oscillation length, given by:
4ħ E c / (D m 322)
How well do we know the parameters? Atmospheric neutrino observations yield:
Θ 23 ~ 45 degrees, while D m 322 = 0.0024 eV2.
Meanwhile, solar neutrino data yield roughly 33 degrees for Θ12 and D m 212 = 0.00008 eV2. (Note: ħ is the Planck constant of action divided by 2 π) If then:
D m 312 = [D m 212 + D m 322 ] = 0.00008 eV2 + 0.0024 eV2
We know, D m 312 = 0.00248
which is close to D m 322
The experiments reported in Physics Today (May, p. 16) were initiated to track the apparent "disappearance" of electron antineutrinos from nuclear power plants proximate to the Daya Bay reactor. But the measurements have shown a puzzling divergence between those applicable to models for antineutrino production in reactors. This has led to the unsavory acceptance of one of two conclusions: 1) the emergence of a new physics superseding the "Standard Model" of particle physics, or 2) there exist major deficiencies in our understanding of nuclear reactor physics.
Hence, the need for more detailed neutrino experiments. These are now about to commence with the construction of a combined $1 billion experiment funded by the U.S. Dept. of Energy, CERN and institutions from 30 countries.
One component of the experiment is based at the Deep Underground Neutrino Experiment (DUNE) at Fermilab, a DOE national laboratory in Batavia, IL. It will fire an intense beam of neutrinos at near light speed (c = 300,000 km/sec) through the Earth's mantle toward detectors at the Sanford Underground Research Facility in Lead, SD. The total time for the trip is estimated at 0.004 s or 4 milliseconds.
Construction began last Friday on the specialized neutrino detectors at the Sanford lab site. These will each be about a mile deep and filled with 70,000 tons of liquid argon. The construction of the Fermilab, beamline, meanwhile, is scheduled to begin in 2021. Its first neutrino beam will fire in about ten years.
The project, make no mistake, is immense and dwarfs the Daya Bay experiments. More than one thousand physicists from around the world will observe and record the changes the neutrinos undergo in their 4 millisecond journey from Illinois to South Dakota. They will be expected to analyze the interactions that then result when the neutrinos strike the extremely cold liquid and ascertain the extent to which 'flavor states' change in transit.
At the end of the experiments might we expect all neutrino results (e.g. from nuclear reactors, solar flux, atmospheric) can finally be reconciled? We can't say for certain yet, but at the least neutrino physicists hope that the divergences between the theoretical model predictions and actual data will be significantly reduced. Certainly below the 6 percent threshold noted at Daya Bay.