Friday, April 12, 2019

Sterile Neutrinos Disfavored By New Reactor Experiment Data

Some experiments studying neutrinos created by nuclear reactors have seen hints of a possible fourth sterile neutrino species. The latest data from OPERA disfavors that hypothesis.

Final results on neutrino oscillation parameters from the OPERA experiment in the CNGS beam

OPERA Collaboration
The OPERA experiment has conclusively observed the appearance of tau neutrinos in the muon neutrino CNGS beam. Exploiting the OPERA detector capabilities, it was possible to isolate high purity samples of νeνμ and ντcharged current weak neutrino interactions, as well as neutral current weak interactions. In this Letter, the full dataset is used for the first time to test the three-flavor neutrino oscillation model and to derive constraints on the existence of a light sterile neutrino within the framework of the 3+1 neutrino model. For the first time, tau and electron neutrino appearance channels are jointly used to test the sterile neutrino hypothesis. A significant fraction of the sterile neutrino parameter space allowed by LSND and MiniBooNE experiments is excluded at 90% C.L. In particular, the best-fit values obtained by MiniBooNE combining neutrino and antineutrino data are excluded at 3.3 σ significance.
What does it conclude (note that the angels are reported in radians)?
The data are compatible with the three-flavor neutrino model and constraints on θ23 and θ13 were derived jointly for the first time exploiting tau and electron neutrino appearance channels. A best fit value of θ23 = 0.78+0.32 −0.31 rad at 1 σ C.L. is obtained, while θ13 is constrained to [0, 0.20] rad at 1 σ C.L. 
Additionally, a dedicated sample of OPERA electronic detector data is used to perform a search for the νµ disappearance signal in the CNGS beam. Assuming all other mixing parameters equal to the global fit central values [24], an upper limit ∆m2 32 < 4.1 × 10−3 eV2 at 90% C.L. is obtained. 
This translates in degrees and unsquared mass differences to:

A best fit value of θ23 = 44.7+18.3 −17.8 deg at 1 σ C.L. is obtained, while θ13 is constrained to [0, 11.5] deg at 1 σ C.L.

∆m32 < 64 meV at 90% C.L.

These constraints are consistent with best fit numbers from other data in art because they are much less precise. Previous data has concluded that:

θ23=40.0+2.1/-1/5 degrees or 50.4+1.3/-1.3 degrees, and θ13=8.66+0,44/-0.46 degrees. 

The difference between the second and third neutrino mass states is 49.5 ± 0.5 meV.

Previous sterile neutrino data here


neo said...

if they rule out sterile neutrinos completely, does this automatically imply then that neutrinos are majorana particles?

andrew said...

No. It doesn't.

Indeed, I think that all of the experimental evidence points to neutrinos not being Majorana particles, as does some of the logic behind the need for neutrinos that gave rise to their discovery in the first place.

The current formulation of the mass generation process for the Dirac mass of fundamental fermions via the Higgs mechanism does seem to call for fermions that have both left and right parity, and that approach does work for the other fundamental Standard Model fermions. But, I'm not at all convinced that this nuance of Dirac mass creation is correct, or that it doesn't have an alternative.

I think that all of the see-saw models of neutrino mass generation are wrong. I do not think that they are Majorana particles

And, I think that the false assumption that baryon number and lepton number are zero immediately after the Big Bang has powerfully pulled the best minds in theoretical physics to favor BSM theories that allow baryon number and lepton number non-conserving models that are contrary to reality, systemically tugging them away from approaches that could lead to the right solution.

Indeed, I'm not even confident that the only circumstance in the Standard Model where baryon number and lepton number can be violated, sphaleron interactions, actually every happen physically.

I believe that it is extremely likely that the searches for proton decay and neutrinoless double beta decay and flavor changing neutral currents at the tree level will all come out empty handed.

neo said...

so...where that leave theorists?