I Open Questions about Neutrinos Today

[Vanadium 50]

I'm not sure I would consider KATRIN "new and important". The limit has moved down from 1.1 eV to 0.9 eV, or about 20%. But it's still an order of magnitude below the cosmological limit - that's that the sum of the neutrino masses is below 0.26 eV. (Which means a 0.09 eV upper limit on any of them)

 

[exponent137]

I'm not sure I would consider KATRIN "new and important". The limit has moved down from 1.1 eV to 0.9 eV, or about 20%. But it's still an order of magnitude below the cosmological limit - that's that the sum of the neutrino masses is below 0.26 eV. (Which means a 0.09 eV upper limit on any of them)

I admit that the analyses of DESI, EUCLID, and of other cosmological measurements will be really important because they will give the lower bound of the neutrino masses. (The results of DESI will even happen in 5 years.) Project 8 and Holmes will give better results than KATRIN, and conditionally some beta decays will give the mass of the neutrino.

But, when we wait, KATRIN will give an independent confirmation and every few percents are important. This measurement is from the first principle, where cosmological measurements have some additional unknown parameters.

I read that the cosmological upper bound for the sum of neutrino masses is 0.12 eV. So, do you think that 0.26 eV is a more conservative bound?

 

[ohwilleke]

I'm not sure I would consider KATRIN "new and important". The limit has moved down from 1.1 eV to 0.9 eV, or about 20%. But it's still an order of magnitude below the cosmological limit - that's that the sum of the neutrino masses is below 0.26 eV. (Which means a 0.09 eV upper limit on any of them)

The credible cosmology based limits are as low as 110 meV (i.e. 0.11 eV) for the sum of the three masses, and assuming that the differences between the three neutrino masses from oscillation data are correct to within their margins of error, absolute neutrino mass boundaries can be quite tightly bounded, with the best fit value being less than that.

The difference between the first and second neutrino mass eigenstate is roughly 8.66 +/- 0.12 meV, and the difference between the second and third neutrino mass eigenstate is roughly 49.5 +/- 0.5 meV, See, e.g., the Particle Data Group global averages.

This implies that the sum of the three neutrino mass eigenstates cannot be less than about 65.34 meV with 95% confidence, in addition to being not more than 110 meV.

Assuming the 0.11 eV sum of neutrino mass constraints, the neutrino mass differences from oscillation data, and a normal hierarchy (which almost all observational data favors, although not necessarily decisively), implies the following bounds on absolute neutrino mass, most of the uncertainty in which is driven by the uncertainty in the lightest neutrino mass which is shared in all three absolute mass estimates:

v1: 0 meV to 12 meV
v2: 8.42 meV to 21.9 meV
v3: 56.92 meV to 72.4 meV

By comparison, Katrin bounds v1 to less than 900 meV, which is 75 times the bound derived from cosmology and neutrino oscillations and an assumption of a normal mass hierarchy.

In an inverted hierarchy of neutrino masses, the minimum sum of the three neutrino masses given current neutrino oscillation data is around 98 +/- 1 meV (which leaves only about 4 meV of shared uncertainty in each of the three neutrino masses if indeed the sum of the three is not more than 110 meV), which is again, still far less than Katrin bound.

 

[mfb]

Mixing only gives you differences between squared masses (excluding smaller effects they are not sensitive to yet). If you take the square root of these you get a maximal mass difference, not a mass difference. Mixing alone doesn't set relevant upper limits on the neutrino masses.
The 0.11 eV cosmological constraint is not without criticism, direct measurements are useful for verification.

 

[exponent137]

At the end of this article, I saw two additional future projects of the sky surveys, CSST and PFS, beside of DESI, EUCLID, and SDSS-V.
https://phys.org/news/2021-04-scientists-dark-energy.html
Are there any other future surveys in progress?

I suppose that all of these surveys give also the upper bound of the neutrino masses? And, it is predicted, that they will give also the lower bound.

The most probably disagreement of two sorts of measurements of the Hubble constant does not influence the neutrino masses?

 

[exponent137]

exponent137 said:

We have been waiting for this since Sep. 2019 https://arxiv.org/abs/1909.06048
Or when we first hear this old result?

$m^2$ of the old result was

View attachment 281719
And the upper bound was $1.1$ eV.
Now $m^2$ is positive.

A combination of the Runs 1 and 2 is additionally a little better, the upper bound is 0.8 eV.
https://www.sciencenews.org/article/neutrino-max-possible-mass-tiny-new-estimate-particle-physics

Some interesting headings of presentations are here. Can someone view them?
http://meetings.aps.org/Meeting/APR21/Session/Q14

 

[exponent137]

By comparison, Katrin bounds v1 to less than 900 meV, which is 75 times the bound derived from cosmology and neutrino oscillations and an assumption of a normal mass hierarchy.

In an inverted hierarchy of neutrino masses, the minimum sum of the three neutrino masses given current neutrino oscillation data is around 98 +/- 1 meV (which leaves only about 4 meV of shared uncertainty in each of the three neutrino masses if indeed the sum of the three is not more than 110 meV), which is again, still far less than Katrin bound.

I am adding here another aspect: The goal of KATRIN is "<200 meV". This new result "<800 meV" gives a promise that this goal will be achieved. But this wished bound will be so 17 times the bound derived from cosmology, what will not be too big, and what will help to confirm the results from cosmology.According to posts #52 and #55, I am adding here the projects WFIRST and LSST as another two future cosmological projects for neutrino masses. And the following analysis will help at all of them:
https://phys.org/news/2021-05-supernovae-twins-possibilities-precision-cosmology.html

 

[exponent137]

And here is the KATRIN preprint of "<800 (900) meV":
https://arxiv.org/abs/2105.08533

Besides, the main phase of DESI is beginning.
https://newscenter.lbl.gov/2021/05/17/start-of-dark-energy-survey/
Maybe it will be the first one that will give the lower bound of the neutrino masses.

 

[ohwilleke]

And here is the KATRIN preprint of "<800 (900) meV":
https://arxiv.org/abs/2105.08533

Besides, the main phase of DESI is beginning.
https://newscenter.lbl.gov/2021/05/17/start-of-dark-energy-survey/
Maybe it will be the first one that will give the lower bound of the neutrino masses.

Thanks for the links. Mixing gives a pretty meaningful low bound on the sum of the neutrino masses, but a lower bound on the lightest neutrino mass, or even a confirmation that it was non-zero, would be serious progress.

 

[exponent137]

Thanks for the links. Mixing gives a pretty meaningful low bound on the sum of the neutrino masses, but a lower bound on the lightest neutrino mass, or even a confirmation that it was non-zero, would be serious progress.

Yes, I was not precise enough, they try to measure the sum of masses of the three neutrinos, now they measured only their upper bound.
https://arxiv.org/abs/2006.09395
It will not be perfect, but it will be the next level.

 

[mfb]

If the neutrino masses follow the patterns of quarks or charged leptons then upper bounds will improve until it's clear that we have one "heavy" state and two light states - maybe in parallel with establishing the mass ordering from mixing, so we have two independent measurements of that. But measuring the mass of the lightest state won't work with current or near-future experiments.

 

[Vanadium 50]

If the neutrino masses follow the patterns of quarks or charged leptons then upper bounds will improve until it's clear that we have one "heavy" state and two light states

If that's the case, the heaviest neutrino weighs 0.050 eV. In that case, we get the mass ordering from mixing long, long before we see the 0.050 eV directly.

 

[mfb]

Do you have projections for how the cosmology constraints are expected to improve (assuming normal ordering) in the next years?
Mixing is clearly the more robust approach here, there I have seen projections and it shouldn't be that far away now. But an independent cross check would still be great.

 

[Vanadium 50]

Do you have projections for how the cosmology constraints are expected to improve (assuming normal ordering) in the next years?

No. When I ask my colleagues, they say "much better", in the same tone as "top men" in Raiders of the Lost Ark.

 

[exponent137]

Do you have projections for how the cosmology constraints are expected to improve (assuming normal ordering) in the next years?

Does this help at this dilemma:
https://arxiv.org/abs/2006.09395

 

[exponent137]

Another fresh result for the sum of neutrino masses is "<0.13 eV".
It agrees with the above-mentioned values "<0.11 eV" and "<0.12 eV".
Probably before 2026 there will be no big improvements.

 

[mfb]

Does this help at this dilemma:
https://arxiv.org/abs/2006.09395

Upcoming data from the CMB Stage-4 (CMB-S4) experiment, as well as the Dark Energy Spectroscopic Instrument (DESI) and Euclid galaxy surveys, will reduce these error bars dramatically, and it is expected that these experiments will measure the sum of neutrino masses at least at the 3σ level

Sounds promising.
A 3 sigma measurement with a central value of ~0.06 eV would also rule out inverted ordering with ~3 sigma (very roughly).

 

[exponent137]

exponent137 said:

A combination of the Runs 1 and 2 is additionally a little better, the upper bound is 0.8 eV.
https://www.sciencenews.org/article/neutrino-max-possible-mass-tiny-new-estimate-particle-physics

Some interesting headings of presentations are here. Can someone view them?
http://meetings.aps.org/Meeting/APR21/Session/Q14

Can anyone explain to me what is the difference now:
https://www.nature.com/articles/s41567-021-01463-1
The upper bound is the same, 800meV. Is data upgraded in any way, or this is the same data but officially published?

 

[ohwilleke]

Can anyone explain to me what is the difference now:
https://www.nature.com/articles/s41567-021-01463-1
The upper bound is the same, 800meV. Is data upgraded in any way, or this is the same data but officially published?

I think this is just the published version of the results previously released as preprints and conference presentations.

 

[valenumr]

Interesting... Does that appear by squaring the sum with the PMNS coefficients?
I would like to see a way to describe chirality to a layman without making them scratch their heads, given that it's a very mathematical construct (in contrast to helicity).
Wouldn't it be enough to ask questions about them in terms of participating in interactions (for example with the W^\pm-bosons)? E.g. we know that (anti)neutrinos don't interact with W^{-(+)} (results from not having a right-handed singlet representation, as for example quarks do).

This a confusing point for me. If neutrinos have mass, can they have chirality? And another point... Right handed neutrinos "don't exist" because they don't interact in EW theory. At least that is my understanding.

 

[ohwilleke]

This a confusing point for me. If neutrinos have mass, can they have chirality? And another point... Right handed neutrinos "don't exist" because they don't interact in EW theory. At least that is my understanding.

All SM fundamental fermions have chirtality.

 

[exponent137]

By end of year 2023 KATRIN will release their results (page 26): https://indico.in2p3.fr/event/29681.../attachments/76424/110930/04-TLasserre-v1.pdf
And then in 2025.

"next data release by end 2023 (<0.5 eV sensitivity)
target sensitivity: m < 0.2-0.3 eV by 2025"

 

[Vanadium 50]

I think KATRIN is a great experiment. But one must be careful to regard projections as estimates. They will release their results when they are ready. Sometimes it takes longer.

 

[exponent137]

News about KATRIN:
https://www.katrin.kit.edu/130.php#Anker0
https://pos.sissa.it/431/011/pdf

"Currently the combined analysis for measurement campaigns one to five is ongoing with an expected sensitivity of ∼ 0.5 eV."

But it seems that value 0,75 eV is a new one?
"The KATRIN collaboration has determined a neutrino mass limit in the sub-eV range. This is the current world-best limit from direct single β-decay measurements at 𝑚_𝜈 < 0.75 eV (90 % CL)"

 

[Vanadium 50]

So, we have two possibilities.

(A) The KATRIN experiment has chosen to update its results by presenting them at an obscure conference, with absolutely no explanation of what has changed. Indeed, the citation is to the older number.

(B) Some who gave the talk rounded a number differently in their presentation.

And you are asking us to go with Option A.

 

[exponent137]

So, we have two possibilities.

(A) The KATRIN experiment has chosen to update its results by presenting them at an obscure conference, with absolutely no explanation of what has changed. Indeed, the citation is to the older number.

(B) Some who gave the talk rounded a number differently in their presentation.

And you are asking us to go with Option A.

Yes, in principle, we are waiting for ∼ 0.5 eV. It was predicted at the end of the year 2023. But here, this prediction was repeated; maybe the update is close.

About 0,75 eV: I do not understand, but it is not as important as 0.5 eV. But I hope that this is more than rounding...

 

[Vanadium 50]

And you are asking us to go with Option A.

Yes,

Wow. Just wow.

 

[ohwilleke]

FWIW, "The KATRIN experiment has chosen to update its results by presenting them at an obscure conference"

I have no idea about this particular case, but this is often how new results are first spread, with publication following due to more rigorous publication than conference paper standards.

 

[Vanadium 50]

No they do not. They may announce preliminary results, but they are subject to change until submission to a journal, refereeing and publication.

They never, despite the claims of some non-physicists, release a result by putting a new number in the proceedings of an obscure conference, do not explain what has changed, and then cite the old published results.

Once again, wow.

 

[mfb]

* The finished analysis sets an upper limit of 0.75 eV.
* The ongoing analysis expects to set an upper limit of around 0.5 eV (assuming no signal), but the precise number will only be known once the analysis is done.

What's unclear?

The result of the ongoing analysis will be shown at a major conference and with a corresponding paper draft.

 

[Vanadium 50]

What's unclear?

The actual published paper rounds to 0.8 eV.

(It's more complicated than this, as the paper publishes multiple statistical procedures to indicate that no matter which one you pick, you get a similar number - which is useful information)

 

[exponent137]

By the end of year 2023 KATRIN will release their results (page 26): https://indico.in2p3.fr/event/29681.../attachments/76424/110930/04-TLasserre-v1.pdf
And then in 2025.

"next data release by end 2023 (<0.5 eV sensitivity)
target sensitivity: m < 0.2-0.3 eV by 2025"

And the next data release happened, half a year later after it was promised:
https://arxiv.org/pdf/2406.13516
The new upper bound is mν<0.45 eV.
Was this paper the first announcement?

 

[Vanadium 50]

And the next data release happened, half a year later after it was promised:

The fiends! Imagine the chutzpah of waiting until the analysis was complete before going public with it! The nerve! What kind of scientists are they?

I wonder if they can be criminally charged with waiting until they were sure of a result rather than publishing something fishy on your preferred schedule. Hang 'em high! Stake them to anthills!

 

[exponent137]

I wonder if they can be criminally charged with waiting until they were sure of a result rather than publishing something fish on your preferred schedule. Hang 'em high! Stake them to anthills!

I thought nothing negative with this sentence. Only for evidence, I gave 1/2 year. It is a natural law, it seems.

It was one post one year ago when someone explicitly criticized such delay. I did not.

 

[ohwilleke]

And the next data release happened, half a year later after it was promised:
https://arxiv.org/pdf/2406.13516
The new upper bound is mν<0.45 eV.
Was this paper the first announcement?

This pushes the limit on the sum of the three neutrino masses to 1.41 eV in a normal hierarchy and 1.46 eV in an inverted hierarchy, when considered together with neutrino oscillation observations.

After a full run of data collection, KATRIN is expected to lower that bound to 0.2 eV. This would push the limit on the sum of the three neutrino masses to 0.66 eV in a normal hierarchy and 0.71 eV in an inverted hierarchy. This still isn't nearly as tight as the cosmology based bounds, but is much less model dependent and is still of the same order of magnitude.

The best take on the cosmology bound in light of DESI is that:

Cosmological neutrino mass bounds are becoming increasingly stringent. The latest limit within ΛCDM from Planck 2018+ACT lensing+DESI is ∑mν < 0.072eV at 95% CL, very close to the minimum possible sum of neutrino masses (∑mν > 0.06eV), hinting at vanishing or even ''negative'' cosmological neutrino masses.

In this context, it is urgent to carefully evaluate the origin of these cosmological constraints. In this paper, we investigate the robustness of these results in three ways: i) we check the role of potential anomalies in Planck CMB and DESI BAO data; ii) we compare the results for frequentist and Bayesian techniques, as very close to physical boundaries subtleties in the derivation and interpretation of constraints can arise; iii) we investigate how deviations from ΛCDM, potentially alleviating these anomalies, can alter the constraints.

From a profile likelihood analysis, we derive constraints in agreement at the ∼10% level with Bayesian posteriors. We find that the weak preference for negative neutrino masses is mostly present for Planck 18 data, affected by the well-known "lensing anomaly". It disappears when the new Planck 2020 HiLLiPoP is used, leading to significantly weaker constraints. Additionally, the pull towards negative masses in DESI data stems from the z=0.7 bin, which is in ∼3σ tension with Planck expectations. Without these outliers, and in combination with HiLLiPoP, the bound relaxes to ∑mν<0.11eV at 95% CL. The recent preference for dynamical dark energy alleviates this tension and further weakens the bound. As we are at the dawn of a neutrino mass discovery from cosmology, it will be very exciting to see if this trend is confirmed by future data.

Daniel Naredo-Tuero, et al., "Living at the Edge: A Critical Look at the Cosmological Neutrino Mass Bound" arXiv:2407.13831 (July 18, 2024).

 

[Mordred]

Thanks for the links will prove useful at several Breit Wigner cross sections I'm currently looking into

 

[exponent137]

The best take on the cosmology bound in light of DESI is that:



Daniel Naredo-Tuero, et al., "Living at the Edge: A Critical Look at the Cosmological Neutrino Mass Bound" arXiv:2407.13831 (July 18, 2024).

There is a new release of data. The upper limit of the sum of neutrino masses is still ever 0,07 eV? Namely, according to the Naredo paper, there should be some outliers in data.

https://arxiv.org/pdf/2411.12022

 

[ohwilleke]

There is a new release of data. The upper limit of the sum of neutrino masses is still ever 0,07 eV? Namely, according to the Naredo paper, there should be some outliers in data.

https://arxiv.org/pdf/2411.12022

I've seen the paper and blogged it yesterday. In that post, I discussed some assumptions from the body text that this new cosmology based neutrino mass limitation is based upon, among other things. I stated that:

The DESI collaboration has found that the sum of the three neutrino masses should be less than 0.071 eV at 95% confidence (assuming as a prior only that the sum of the neutrino masses is greater than zero). This disfavors an inverted neutrino hierarchy the demands roughly a minimum of a 0.100 eV sum of neutrino masses, while a normal neutrino hierarchy requires a minimum sum of neutrino masses of only about 0.059 eV. The preference for a normal hierarchy is only about two sigma, however. This estimate is heavily dependent upon the assumed dark energy model, however, and assumes a fixed cosmological constant.

Before consideration of the full-shape data from DESI and without the limitation of an assumed zero mass bound, the DESI experiment had exhibited a weak preference for negative neutrino masses, and there was a fair amount of discussion in the wake of that about why it did so, which I discussed in this blog post about Daniel Naredo-Tuero, et al., "Living at the Edge: A Critical Look at the Cosmological Neutrino Mass Bound" arXiv:2407.13831 (July 18, 2024) which notes a key outlier in the DESI data that drives some of this result.

The abstract from that paper stated (emphasis mine):

Cosmological neutrino mass bounds are becoming increasingly stringent. The latest limit within ΛCDM from Planck 2018+ACT lensing+DESI is ∑mν < 0.072eV at 95% CL, very close to the minimum possible sum of neutrino masses (∑mν > 0.06eV), hinting at vanishing or even ''negative'' cosmological neutrino masses.

In this context, it is urgent to carefully evaluate the origin of these cosmological constraints. In this paper, we investigate the robustness of these results in three ways: i) we check the role of potential anomalies in Planck CMB and DESI BAO data; ii) we compare the results for frequentist and Bayesian techniques, as very close to physical boundaries subtleties in the derivation and interpretation of constraints can arise; iii) we investigate how deviations from ΛCDM, potentially alleviating these anomalies, can alter the constraints.

From a profile likelihood analysis, we derive constraints in agreement at the ∼10% level with Bayesian posteriors. We find that the weak preference for negative neutrino masses is mostly present for Planck 18 data, affected by the well-known "lensing anomaly". It disappears when the new Planck 2020 HiLLiPoP is used, leading to significantly weaker constraints. Additionally, the pull towards negative masses in DESI data stems from the z=0.7 bin, which is in ∼3σ tension with Planck expectations. Without these outliers, and in combination with HiLLiPoP, the bound relaxes to ∑mν<0.11eV at 95% CL. The recent preference for dynamical dark energy alleviates this tension and further weakens the bound. As we are at the dawn of a neutrino mass discovery from cosmology, it will be very exciting to see if this trend is confirmed by future data.

I didn't see a discussion of the z=0.7 bin outlier in the most recent paper whose preprint was made available on arXiv yesterday, although it is possible that I missed it as I read only the parts of the paper that were most interesting to me, rather than reading it cover to cover, carefully and in depth.