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 to refer to such and such a neutrino. These states are: electron, muon and tau - which are in fact superpositions of mass eigenstates.
Let's review again the
basis for these neutrino flavor states. If there are three such states:
electron, muon and tau, then there must be three different
corresponding neutrino masses which we can call: m1, m2 and
m3. Further, the three "flavors" are really different
superpositions of the 3 basic neutrino mass states. Moreover, and to make
it more complex, we know that quantum interference between mass states means a
neutrino originating in one "flavor" can transmogrify to another over
its transit.
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 » 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 . An ongoing experiment at Daya Bay, near Hong Kong, set amidst no less than six nuclear reactors, has been a first major step in detecting and understanding the neutrino.
The Daya Bay detectors are arrayed in two clusters near two vertices of a "neutrino detection" triangle. The detector triangle itself is comprised of three separate electron anti-neutrino detectors (about 2 km each from the reactors) and which reside in water baths to unmask any cosmic ray interlopers. Each of the detectors measures the electron anti-neutrino (call it -ve ) flux from the reactors by recording any light flashes due to -ve collisions within its 20 tons of liquid.
Daya Bay's results have proceeded basically in two related phases:
I. Small deficits of -ve were
previously recorded at short distances from the reactors, and it was further
reported that Θ13, the last of 3 "mixing
angles" that characterize neutrino oscillation is non-zero.
II. Further measurements of Θ13 disclosed it
was not only non-zero but large enough for the Daya experimenters to begin
investigating neutrinos as factors in the matter-antimatter asymmetry of the
cosmos.
This is big deal stuff, because cosmologists have always been perplexed by the
apparent preponderance of matter in relation to antimatter in our universe. (I
tried to solve this in a high school science fair project by postulating
a separate anti-matter cosmos that operated in the context of
'anti-time' or negative time, i.e. with the time vector negentropic as opposed
to entropic)
The survival of so little antimatter in our cosmos requires a violation of what is called "CP symmetry invariance". We don't know WHY there is this asymmetry, but it may have something to do with Fitch and Cronin's (1963-64) discovery of a violation of CPT invariance. (C for charge conjugation, P for parity (spatial reflection) and T for time reversal.
Up until their 1960s' investigations, it was widely accepted by physicists that nature played no favorites where charge conjugation, parity and time reversal were concerned. The discovery of a fundamental violation (Fitch and Cronin found a tiny fraction: 45 out of 22,700 - K2 mesons, spontaneously disintegrate into 2 pions, e.g. π mesons, (instead of the usual 3) changed all this.
It was suggested by them that this CPT invariance violation
might also - in some way - account for the apparent asymmetry in the
distribution of matter with respect to antimatter. Since then experiments have
disclosed T-invariance can be subsumed by CP symmetry invariance. Trouble is,
the existence of so little antimatter still violates CP invariance. (Weak
quark interactions exhibit some CP violation but too small to explain
cosmological asymmetry between matter and antimatter).
D m 312 = [D m 212 + D m 322 ] = 0.00008 eV2 + 0.0024 eV2
And we already know, D m 312 = 0.00248
This was fine as it went, but a further issue that needed to be resolved was whether the oscillation amplitude, e.g. sin2 (2 Θ 13) (for the disappearance of reactor antineutrinos associated with the D m 312, D m 322 approximations) would still be large enough to detect. This was the core experimental quandary facing the Daya Bay collaborators. They were more or less guided (optimistically!) by an earlier independent results that set an upper limit of 0.16 (the Daya Bay -ve detector array was designed to measure a smallest value of 0.01)
Is the "case" closed? Not necessarily! We always must reckon in necessary and sufficient conditions.
This additional degree of freedom entails an independent phase factor,
exp (iσ)
which dictates the CP -violation. Standard theory can't predict σ so it must be done via experiment. Enter the 'Long Baseline Neutrino Experiment' now in the form of DUNE (Deep Underground Neutrino Experiment). The plan is to direct an intense beam of muon neutrinos from Fermilab at a detector in an underground lab in South Dakota, some hundreds of miles distant. which is close to D m 322
When the neutrinos finally arrive at DUNE, physicists hope they will finally explain how the Big Bang created ever so slightly more matter than its opposite, antimatter — an excess that constitutes everything in the universe today. Oh, and also provide a cross check of the Daya Bay results thus far. (Earlier reported research in Physics Today (May, p. 16, 2017) noted an effort to track the apparent "disappearance" of electron antineutrinos from nuclear power plants proximate to the Daya Bay. But the measurements had shown a puzzling divergence between those applicable to models for antineutrino production in reactors.)
If all goes well, the results from DUNE (and Daya Bay) will turn the elusive neutrino into a known quantity, filling a major gap in scientists’ understanding of the universe and, perhaps, return the United States to its former position at the center of particle physics.
On Feb. 1, after more than a decade of planning and construction, the underground caverns were completed with a last blast of dynamite. The hole is there so now all the physicists — and the universe — must do is fill it.
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.
See Also:
Solar Neutrino Breakthrough: Nuclear Fusion In Sun Now Confirmed With Discovery of Solar Neutrinos
And:
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