Thursday, May 27, 2021

New Data Confirms Low Sum Of Neutrino Masses

New Cosmology Bounds On Neutrino Mass

The Dark Energy Survey (DES) has confirmed that the sum of the three neutrino masses in a lambdaCDM model should be less than 130 meV/c^2 with 95% confidence, using the combined results of DES Year 3, Planck, and other low redshift datasets, all of which are consistent with each other. 

There will probably be no major adjustments in these bounds before the year 2026.

A January 2021 article explains the methodology involved to set this bound. The pre-DES bound from "the latest 2018 Planck data, in conjunction with measurements of the baryon acoustic oscillations (BAO) from the Baryon Oscillation Spectroscopic Survey (BOSS)" was 120 meV/c^2 at 95% C.L. 

Direct Measurements Of Neutrino Mass

The tightest directly measured bound on the mass of the electron anti-neutrino (which should be the same as the electron neutrino) is that it is less than 800 meV/c^2 with 90% confidence. This limit from the KATRIN experiment is expected to ultimately be reduced to 200 meV/c^2 with 90% confidence when the experiment has run its course and collected all of the data it plans to collect.

Limits on Neutrino Masses From Neutrino Mixing

The differences between the three neutrino mass eigenstates is known with much greater precision from neutrino mixing data, although the observational data only mildly favor the "normal" ordering of neutrino masses seen in the charged leptons and quarks, over an "inverse" ordering of neutrino masses.

The square of the difference between the first and second neutrino mass is 75.3 ± 1.8 meV^2/c^4 (i.e. about 8.7 meV/c^2, and at least 8.5 meV/c^2 at two sigma).

The square of the difference between the second and third neutrino mass is 2453 ± 34.2 meV^2/c^4 if there is a a normal ordering (i.e. about 49.5 meV/c^2 and at least 48.8 meV/c^2 at two sigma), or -2546 + 34 - 40 meV^2/c^4 in an inverse ordering.

So, if the lightest neutrino mass is negligibly greater than zero, then the sum of the three neutrino masses is at two sigma (roughly 95% confidence interval) not less than 57.3 meV/c^2.

Limits on Neutrino Majorana Mass From Neutrinoless Double Beta Decay

Neutrinoless double beta decay has not been observed. If neutrinos have Dirac mass, it never occurs. If neutrinos have Majorana mass, the non-detection of neutrinoless double beta decay places an upper bound on the Majorana mass of neutrinos at 180 meV/c^2 as a 90% confidence interval.

Like the direct mass detection bound from KATRIN, this bound is not competitive with the upper bounds on neutrino mass from cosmology observations and neutrino mixing.

The Combined Limits On Neutrino Masses

The neutrino mixing and cosmology bounds constrain the sum of the three neutrino masses in a normal ordering to be between 57.3 meV/c^2 and 130 meV/c^2, with about 90% of the possible variation within that range being a function of the lightest neutrino mass eigenstate, which must be less than 24.3 meV/c^2, with a value at the low end of this range favored. Uncertainty in mass differences from neutrino mixing accounts for about only 2.2 meV/c^2 of the possible variation within this range.

There are good theoretical arguments, none of which are established definitively, however, for the lightest neutrino mass to be non-zero, even if it is arbitrarily small. This is because particles with zero mass have qualitatively different properties than particles with non-zero mass, because all other fermions have non-zero rest mass, and because all other particles that interact via the weak force have non-zero rest mass. Also, all other particles which experience CP violation have non-zero rest mass, and there is strong evidence that neutrino oscillation has a non-zero CP violating phase. CP violation (which is equivalent to time reversal asymmetry) makes less theoretical sense in the case of a particle with zero rest mass, because particles with zero rest mass do not experience the passage of time in their own reference frame.

In an inverse neutrino mass hierarchy the sum of the three neutrino masses must be at least 96 meV/c^2 at two sigma, which is quite close to the 130 meV/c^2 upper bound from the combined cosmology data.

The direct measurement of electron anti-neutrino mass is not competitive with these boundaries but is also not inconsistent with them. This is also true of the bound on the Majorana mass of neutrinos from neutrinoless double beta decay detection experiments. 

The cosmology constraint on the lightest neutrino mass is 33 times stronger than the direct measurement constraint, and could be as little as 8 times stronger when the KATRIN experiment is complete. It is about 7.5 times stronger than the bound imposed by the non-detection of neutrinoless double beta decay in a scenario in which neutrinos have exclusively Majorana mass.

The Number of Neutrino Types

Data from W and Z boson decays likewise tightly constrain the number of active neutrinos with masses of less than 45,000,000,000,000 meV/c^2 to exactly three.

Cosmology data also strongly supports the hypothesis that there are exactly three generations of neutrinos (with no sterile neutrinos having a mass of 1,000,000,000 meV/c^2 or less) (also here). A far heavier sterile neutrino, however, would not be discernible as a neutrino from cosmology data and instead would look like a type of dark matter particle.

Neutrino mixing data less definitively rule out the possibility of one or two additional neutrinos that oscillate, but do not interact via the electromagnetic, weak or strong forces, although the neutrino mixing data still favors the default three neutrino hypothesis over the one and two sterile neutrino alternatives (see also here).


neo said...

The most detailed dark-matter map of our universe is weirdly smooth

But some results were surprising. “We found hints that the universe is smoother than expected,” says Jeffrey. “These hints are also seen in other gravitational-lensing experiments.”

This is not what is predicted by general relativity, which suggests that dark matter should be more clumpy and less uniformly distributed. The authors write in one of the 30 papers being released that “though the evidence is by no means definitive, we are perhaps beginning to see hints of new physics.” For cosmologists, “this would correspond to possibly changing the laws of gravity as described by Einstein,” says Jeffrey.

mond + dark matter

andrew said...

Thanks for the link. I'm working my way through the 30 preprints from DES but it is always nice to have some perspective. The core observation is consistent with what has been observed at the galaxy scale for decades.

andrew said...

The paper referenced has a really "hide the lede" title: L. F. Secco, "Dark Energy Survey Year 3 Results: Cosmology from Cosmic Shear and Robustness to Modeling Uncertainty" (2021). The link to the paper is:

"This work and its companion paper, Amon et al. (2021), present cosmic shear measurements and cosmological constraints from over 100 million source galaxies in the Dark Energy Survey (DES) Year 3 data. We constrain the lensing amplitude parameter S8 ≡ σ8 p Ωm/0.3 at the 3% level in ΛCDM: S8 = 0.759+0.025−0.023 (68% CL). Our constraint is at the 2% level when using angular scale cuts that are optimized for the ΛCDM analysis: S8 = 0.772+0.018−0.017 (68% CL). With cosmic shear alone, we find no statistically significant constraint on the dark energy equation-of-state parameter at our present statistical power. We carry out our analysis blind, and compare our measurement with constraints from two other contemporary weak-lensing experiments: the Kilo-Degree Survey (KiDS) and Hyper-Suprime Camera Subaru Strategic Program (HSC).

"We additionally quantify the agreement between our data and external constraints from the Cosmic Microwave Background (CMB). Our DES Y3 result under the assumption of ΛCDM is found to be in statistical agreement with Planck 2018, although favors a lower S8 than the CMB-inferred value by 2.3σ (a p-value of 0.02). This paper explores the robustness of these cosmic shear results to modeling of intrinsic alignments, the matter power spectrum and baryonic physics.

"We additionally explore the statistical preference of our data for intrinsic alignment models of different complexity. The fiducial cosmic shear model is tested using synthetic data, and we report no biases greater than 0.3σ in the plane of S8 × Ωm caused by uncertainties in the theoretical models."

andrew said...

The quote you pull from the MIT tech review is captured more technically in the final paragraph of the linked paper which states:

"while the assumption of ΛCDM as the ultimate end-to-end model connecting the early- and late- Universe has withstood another test, our result should be understood within a broader context. It is still an open puzzle that modern weak lensing surveys, independently and in blind analyses, find a lower lensing amplitude than predicted by the CMB, and the difference between DES cosmic shear itself with respect to Planck has increased from 1.0σ in DES Y1 to 2.3σ in Y3."

The first three paragraphs of the introduction state:

"Discoveries and advances in modern cosmology have resulted in a remarkably simple standard cosmological model, known as ΛCDM. The model is specified by a spatially flat universe, governed by the general theory of relativity, which contains baryonic matter, dark matter, and a dark energy component that causes the expansion of the Universe to accelerate.

"Although remarkably simple, it appears to be sufficient to describe a great many observations, including the stability of cold disk galaxies, flat galaxy rotation curves, observations of strong gravitational lensing in clusters, the acceleration of the expansion of the Universe as inferred by type Ia supernovae (SNe Ia), and the pattern of temperature fluctuations in the Cosmic Microwave Background (CMB). Yet despite all this, ΛCDM is fundamentally mysterious in the sense that the physical nature of its two main components, dark matter and dark energy, is still completely unknown.

"The success of ΛCDM has, however, been shaken in recent years by new experimental results. We have seen tentative hints that the model might fail to simultaneously describe the late- (low redshift) and early-time (high redshift) Universe. To take one prominent example, constraints on the local expansion parameter H0 obtained from the local distance ladder and SNe Ia appear to be in tension with those inferred by the CMB at a statistically significant level, with varying levels of significance being reported by different probes. In a separate but analogous tension, the value of the S8 ≡ σ8(Ωm/0.3) 1/2 parameter — the amplitude of mass fluctuations σ8 scaled by the square root of matter density Ωm — differs when inferred via cosmological lensing from the value obtained using Planck (assuming ΛCDM) at the level of 2 − 3σ. Other probes of the late Universe, in particular spectroscopic galaxy clustering, redshift-space distortions and the abundance of galaxy clusters, also all tend to prefer relatively low values of S8. Although the evidence is by no means definitive, we are perhaps beginning to see hints of new physics, and so stress-testing ΛCDM with new measurements is extremely important."