Monday, June 22, 2020

Standard Model Neutrino Properties Recapped

There are seven experimentally determined parameters related to neutrinos in the Standard Model: four parameters of the PMNS matrix, and three neutrino mass eigenstates (for background see here). A new preprint has updated the latest state of the measurement of the PMNS matrix parameters and neutrino masses as of 2020.

The results for the parameters whose values aren't expressly recited in the abstract are set forth in Table III of the paper:

Taking the square roots and referring to the normal ordering that is strongly preferred by the data, the m(21) mass difference is 8.66 meV (one sigma error about 2.8%). The m(31) mass difference is 50.60 meV (one sigma error about 1.4%).

For example, the best fit values for a 1 meV first neutrino mass eigenvalue would be:

1 meV
9.66 meV
51.60 meV
Sum of neutrino masses:  62.26 meV.

Cosmology bounds the sum of the three neutrino masses to about 130 meV, which implies a mass of about 0 to 23.58 meV for the lightest neutrino mass eigenstates.

The preference for normal v. inverted mass ordering of the absolute neutrino masses is shown in Table II (in which OSC means oscillation data, and Cosmo refers to cosmology data).

In my view, the balance of the evidence also strongly disfavors the sterile neutrino hypothesis (not considered in this paper) and disfavors less strongly, but still disfavors, the existence of neutrinoless double beta decay.

As of last year, the constants were as follows (per the Particle Data Group):
The data below are in the form in which the actual values are directly measured (sine squared values of real valued mixing angles and squared values of mass differences) rather than the underlying parameter values which are easily derived from them with a scientific calculator.

The full data for the parameters shown as ". . . " in the chart above are as follows:
sin^2(theta23) theta23 could be either side of a 45 degree angle based upon existing measurements and assuming a "normal" mass hierarchy for the neutrino masses. But existing experiments, while capable of determining that theta23 is not 45 degrees, but can't determine if it is greater or smaller than those values, which is why there is both an octant I and an octant II value.

0.5120.022+0.019OUR FIT  Normal ordering, octant I
0.5420.022+0.019OUR FIT  Normal ordering, octant II
0.5360.028+0.023OUR FIT  Inverted ordering

Delta m32^2 with normal ordering is 2.444 ± 0.034 (the number shown in chart is inverse mass hierarchy).

Sum of neutrino masses Σmν < 0.12 eV (95%, CMB + BAO); ≥ 0.06 eV (mixing).

Directly measured neutrino mass limits:

Neutrino Flavors

The number of neutrino flavors in the Standard Model is a theoretically determined, rather than experimentally measured value, but the experimental measurements are consistent at the two sigma level with the Standard Model value of 3:

Effective number of neutrino flavors Neff 2.99 ± 0.17 (cosmology measurements) (the expected value of this measured physical constant with exactly three types of neutrinos is 3.045 rather than zero for technical reasons related to the way that radiation impacts the relevant observables). This measurement includes all light neutrinos (up to the order of roughly 1-10 eV in mass) that oscillate with each other, and is independent of whether or not they interact via the weak force.

Number of light (i.e. less than 45 GeV) neutrino flavors from Z boson decays Nν = 2.984 ± 0.008. The Standard Model theoretical value is 3.
The paper and its abstract are as follows:

[Submitted on 19 Jun 2020]

2020 Global reassessment of the neutrino oscillation picture

We present an updated global fit of neutrino oscillation data in the simplest three-neutrino framework. In the present study we include up-to-date analyses from a number of experiments. Namely, we have included all T2K measurements as of December 2019, the most recent NOνA antineutrino statistics, and data collected by the Daya Bay and RENO reactor experiments. Concerning the atmospheric and solar sectors, we have also updated our analyses of DeepCore and SNO data, respectively. 
All in all, these new analyses result in more accurate measurements of θ13θ12Δm221 and |Δm231|. The best fit value for the atmospheric angle θ23 lies in the second octant, but first octant solutions remain allowed at 2σ. Regarding CP violation measurements, the preferred value of δ we obtain is 1.20π (1.54π) for normal (inverted) neutrino mass ordering. 
These new results should be regarded as extremely robust due to the excellent agreement found between our Bayesian and frequentist approaches. 
Taking into account only oscillation data, there is a preference for the normal neutrino mass ordering at the 2.7σ level. While adding neutrinoless double beta decay from the latest Gerda, CUORE and KamLAND-Zen results barely modifies this picture, cosmological measurements raise the significance to 3.1σ within a conservative approach. A more aggressive data set combination of cosmological observations leads to a stronger preference for normal with respect to inverted mass ordering, at the 3.3σ level.

This cosmological data set provides 2σ upper limits on the total neutrino mass corresponding to mν<0.13 (0.15)~eV in the normal (inverted) neutrino mass ordering scenario.

These bounds are among the most complete ones in the literature, as they include all currently available neutrino physics inputs.
Comments:34 pages, 15 figures, 3 tables
Subjects:High Energy Physics - Phenomenology (hep-ph); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Experiment (hep-ex)
Cite as:arXiv:2006.11237 [hep-ph]
(or arXiv:2006.11237v1 [hep-ph] for this version)


neo said...

if hypothetical boson X17 coupled to neutrinos, could that explain neutrino masses and oscillation?

andrew said...

That would be the most plausible reason for it to be real (although there are lots of other reasons to doubt it).

neo said...

Dynamical Evidence For a Fifth Force Explanation of the ATOMKI Nuclear Anomalies
Jonathan L. Feng, Tim M.P. Tait, Christopher B. Verhaaren

Jun 1, 2020 - 32 pages

e-Print: arXiv:2006.01151 [hep-ph] | PDF

Abstract (arXiv)
Recent anomalies in 8
Be and 4He nuclear decays can be explained by postulating a fifth force mediated by a new boson X. The distributions of both transitions are consistent with the same X mass, 17 MeV, providing kinematic evidence for a single new particle explanation. In this work, we examine whether the new results also provide dynamical evidence for a new particle explanation, that is, whether the observed decay rates of both anomalies can be described by a single hypothesis for the X boson's interactions. We consider the observed 8Be and 4He excited nuclei, as well as a 12C excited nucleus; together these span the possible JP quantum numbers up to spin 1. For each transition, we determine whether scalar, pseudoscalar, vector, or axial vector X particles can mediate the decay, and we construct the leading operators in a nuclear physics effective field theory that describes them. Assuming parity conservation, the scalar case is excluded and the pseudoscalar case is highly disfavored. Remarkably, however, the protophobic vector gauge boson, first proposed to explain only the 8Be anomaly, also explains the 4

He anomaly within experimental uncertainties. We predict signal rates for other closely related nuclear measurements, which, if confirmed, will provide overwhelming evidence that a fifth force has been discovered.
as well as a 12C excited nucleus

andrew said...

I've seen the paper. But, it has serious problems, hasn't been replicated, comes from the lab that has cried wolf before, and isn't terribly well motivated theoretically either.

Matt Strassler explained some of the problems with the hypothesis in November 2019.

See also

The mass is far too low for a proposed tetraquark explanation of the kind made here (even though it wouldn't proposed a new fundamental boson):

A least three papers so far this year have explained the experimental data used as a basis for the X17 hypothesis without new physics:

neo said...

given claims for example by Sabine that particle physics has not progressed, certainly finding X17 would be something new in the field.

of the proposals to explain neutrino masses, i've not seen any papers of it explaining neutrinos couple to a new fundamental force, would this work?

I'm wondering how long definitive answers to X17 can be had.

the paper i cited also suggests looking at carbon 12

regarding your other claims

“From a particle physics perspective, anomalies come and go,” says Daniele Alves, a theoretical physicist at Los Alamos National Laboratory. “We’ve learned over time to not be too biased with one interpretation or the other. The important thing is to get to the bottom of this.”

That means the most likely explanation for the unexplained new signal is that there’s something off with the Hungarian detector’s setup. However, no one is disputing the data. The findings were peer-reviewed and published in the journal Physical Review Letters — the same journal that published the discovery of gravitational waves. Even ideas in prestigious journals can sometimes be explained away as systematic error, but that’s the way science works.

“People are paying attention to see whether this is really a nuclear physics effect or whether it’s something systematic,” Alves says. “It’s important to repeat those experiments ... to be able to test whether this is real or if it’s an artifact of the way they’re doing the experiment.”

Quest to Confirm

And that’s precisely what her group hopes to do. Together with a small team, she’s proposing to repeat the Hungarian experiment using equipment that already exists at Los Alamos. The national lab has been a leader in nuclear physics since the creation of the atomic bomb. And today, thousands of top physicists still work there on problems ranging from safeguarding and studying our nation’s nuclear arsenal, to pioneering quantum computers and observing pulsars.

As it turns out, they also have a detector nearly identical to the one used by the Hungarian team.

When you add all that together, Alves believes Los Alamos has exactly the right combination of facilities and expertise to repeat the experiment. That’s why her group quietly worked on their proposal for the last six months, and recently submitted a funding request for review. To gain approval, it will have to win out in an annual competition alongside other projects at the national lab.

In recent years, several other groups likewise have suggested they’ll look for this force. But at the moment, Alves believes they're the main group in the U.S. working to confirm or refute the finding. If they can’t gain approval, it may be years before a university or other group can secure both the funds and expertise to repeat the experiment with the same sort of parameters the Hungarians used.

As with all extraordinary claims, this potentially paradigm-shifting discovery will require extraordinary evidence before people accept it. So we may have to wait a while before we know whether the X17 particle and its potential fifth force will revolutionize physics, or take its place atop the dustbin of debunked and discarded discoveries.

apparently money is very important to confirm and there's no award of money just yet. but many HEP are taking these claims very seriously

andrew said...


"of the proposals to explain neutrino masses, i've not seen any papers of it explaining neutrinos couple to a new fundamental force, would this work?"

Predominantly my own speculation, by rough analogy to the role played by the W boson in giving rise top the CKM matrix, and the role played by the Higgs boson and Higgs field in giving arise to the masses of all of the Standard Model fundamental particles with rest mass other than the neutrinos.

The W is ca. 80.4 GeV and interacts with all mass SM fermions. The Higgs boson is ca. 125 GeV an imparts mass to everything from the electron at 0.511 MeV to the top quark at 173 GeV. The neutrino masses are on the order of 60 meV or less. So, at the high end a factor of about 3 * 10^13 smaller, and at the low end smaller by a factor of 10^9 or more. A factor of 10^9 smaller Higgs boson would have 125 eV. So, 17 MeV isn't grossly out of the mass range that would seem plausible for a boson analogous to the W and Higgs bosons but for neutrinos. (And, if you've followed my posts long enough, I think that the W and Z bosons may play a larger role in mass generation than commonly appreciated, together with the Higgs boson).

While I don't agree with or endorse this paper, it illustrates something of the gist of how it could work:

Not precisely on point, but another possible source of excess background noise at XENON1T that was not

I'll believe that X17 is plausible when another experiment with a better track record replicates the alleged discovery. The system the X17 researchers are studying is much more complicated with hidden sources of potential systemic error than the XENON1T experiment.

andrew said...

A paper discussing the kind of boson for which an X17 might be a good fit is this one:

neo said...

there are several research groups attempting to validate X17,

the paper arXiv:1608.03591 Particle physics models for the 17 MeV anomaly in beryllium nuclear decays Jonathan L. Feng,

requires X17 U(1)B or U(1)B-L gauge bosons, and requires sterile neutrinos

if verified i wonder what this means for say grand unified theories and SUSY

andrew said...

Feng (2016) is an interpretation of the data, not an attempt to replicate it (perhaps your first sentence was not intended to apply to the second).

Requiring sterile neutrinos is a minus, as multiple lines of evidence disfavor them.

andrew said...

FYI, Feng has a June 1, 2020 preprint on the same topic:

neo said...

2 separate comments.

If a lab confirms Feng's specific X17 proposal and is announced soon, X17 is either U(1)B or U(1)B-L gauge boson, what does this mean for other particle physics theories.
sterline neutrinos may be disfavored but if X17 is U(1)B-L gauge boson, it requires it.

it would seem MOND would die, since either U(1)B or U(1)B-L gauge boson predicts additional fermionic matter to cancel gauge anomolies which the lightest could be dark matter.

i wonder if it is compatible with SUSY, for example. Feng states both U(1)B or U(1)B-L gauge boson there's already an extensive literature that existed in context of various grand unified theories.

if these new particles are added to the standard model, what should the new name be? the extended model?

andrew said...

I'm not holding my breath on confirmation. My Baysean prior for confirmation by a new experiment is on the order of a 2% likelihood or less.

Also, even if X17 was confirmed to be a bona fide new fundamental particle and that it was causing changes in deflection angles in atomic decays, that would be a long, long way from figuring out what it does.

MOND is a phenomenological relationship that is observed. The existence of a new dark matter candidate doesn't mean that it is dark matter. If what is observed is an X17 gauge boson, it decays quickly and should be produced regularly throughout the history of the universe. A dark matter mass of 17 MeV is not a good fit to galaxy dynamics data, which should be in the KeV range or less, and dark matter should have either been produced overwhelmingly in the very early stage of the history of universe (probably well before Big Bang Nucleosynthesis) and then persisted as a thermal relic, which X17 is a poor fit to, or should be distributed throughout the universe like a collisionless sterile particle and yet be produced and decay at equal rates creating a constant total quantity which it is hard to figure out how to do with X17 and its decay products.

I'm sure someone has tried a SUSY mashup. But, the likelihood of SUSY is much lower than the likelihood that I die crossing the street on an ordinary day.

We should refrain from naming them until we have a better idea about what they do and how they fit. Until then, X17 will do.

andrew said...

Another answer to your X17 question from Stacy at Triton Station: