Can Oscillations Explain Neutrino Anomalies?

[RobertGC]

TL;DR Summary

Solar neutrino deficit and atmospheric neutrino deficit

The Solar neutrino deficit:

The solar neutrino deficit is a problem where fewer neutrinos from the Suns internal nuclear processes are detected here on Earth than expected.

The atmospheric neutrino deficit:

A similar problem occurs with atmospheric neutrinos where fewer are detected coming up through the Earth compared to down from the sky.

Neutrino oscillation is a proposed solution to these where neutrinos flip into different kinds so the expected kinds aren’t observed.

But how do we know it’s not just that the neutrino cross-section is just larger than we think so more of them just get absorbed or scattered?

Robert Clark

 

[mfb]

We can measure cross sections with known neutrino sources, from reactors or from accelerators, in addition to very robust theory predictions for them.

There is nothing that could scatter MeV neutrinos by any relevant amount in the Solar System. The cross section is orders of magnitude too small for that.

The more modern approach: We simply measure the other neutrino types, too. We have found the "missing" neutrinos.

 

[vanhees71]

What's also convincing for neutrino oscillations as the explanation for the "missing neutrinos" is that one has also found the "reappearance" in long-baseline measurements. See, e.g.,

https://arxiv.org/abs/1502.01550

 

[Vanadium 50]

The solar neutrino deficit is a problem where fewer neutrinos from the Suns internal nuclear processes are detected here on Earth than expected.

Really, not any more. We have known for about 20 years that the number of neutrinos detected is what is expected. The SNOL experiment measured that.

 

[berkeman]

Summary:: Solar neutrino deficit and atmospheric neutrino deficit

The Solar neutrino deficit:

https://www.pbs.org/wgbh/nova/neutrino/missing.html

 

[RobertGC]

We can measure cross sections with known neutrino sources, from reactors or from accelerators, in addition to very robust theory predictions for them.

There is nothing that could scatter MeV neutrinos by any relevant amount in the Solar System. The cross section is orders of magnitude too small for that.

The more modern approach: We simply measure the other neutrino types, too. We have found the "missing" neutrinos.

Can you give me some refs for those reactor experiments that determine the neutrino cross-sections?

The neutrinos continue to offer surprises such as the LSND experiment.

Robert Clark

 

[RobertGC]

What's also convincing for neutrino oscillations as the explanation for the "missing neutrinos" is that one has also found the "reappearance" in long-baseline measurements. See, e.g.,
https://arxiv.org/abs/1502.01550

It is notable that that is described as the muon neutrinos “disappearing”, meaning less detected than expected. So I still want to see references about experimental determination of neutrino scattering or absorbtion.

Robert Clark

 

[Vanadium 50]

The neutrinos continue to offer surprises such as the LSND experiment.

That was a quarter-century ago.

 

[Orodruin]

Always start at PDG for review stuff https://pdg.lbl.gov/2019/reviews/rpp2019-rev-nu-cross-sections.pdf

 

[Orodruin]

There are also several experiments that have measured the appearance of other flavours in single flavour beams/fluxes. Notably nu_e appearance in nu_mu beams (eg T2K and NOvA) and nu_tau appearance (eg OPERA).

 

[RobertGC]

That was a quarter-century ago.

But more recent experiments have shown the anomalous observations to an even stronger extent:



Robert Clark

 

[RobertGC]

Always start at PDG for review stuff https://pdg.lbl.gov/2019/reviews/rpp2019-rev-nu-cross-sections.pdf

Thanks. I’ll give that a read.

Robert Clark

 

[RobertGC]

There are also several experiments that have measured the appearance of other flavours in single flavour beams/fluxes. Notably nu_e appearance in nu_mu beams (eg T2K and NOvA) and nu_tau appearance (eg OPERA).

Thanks. I’ll take a look at those.

Robert Clark

 

[Melbourne Guy]

But more recent experiments have shown the anomalous observations to an even stronger extent

It seems to have been only one more experiment, MiniBooNE, but I felt this presenter unnecessarily adopted vibes of a conspiracy theorist with the claims there hasn't been press coverage of the neutrino problem. There has been, going back years, but such physics news is typically transient and often only the most outlandish events bubble up to the general media outlets (cold fusion, the OPERA 'FTL' results, LHC destroying the world, potential asteroid strikes, Oumuamua being an alien space ship, for example).

Also, given this video is recent, I wonder whether IceCube results wouldn't help with the evidence presented? IceCube has so far failed to find light or heavy sterile neutrinos, but possibly to your question, @RobertGC, there is recent work regarding cross section measurement:

https://journals.aps.org/prd/pdf/10.1103/PhysRevD.104.022001

 

[Vanadium 50]

I'm not going to sit through 12 minutes of listening to Hossenfelder rant about how stupid and dishonest all the other physicists are. If there is a point there worth discussing, please state it. Don't just say "somewhere in this video".

It is false, false, false to claim that more recent experiments confirmed LSND, They had an anomaly, and MiniBoone failed to confirm that anomaly - but did find an anomaly of their own. To make the two consistent requires that they discovered not one but two new particles and to take a theory with 5 parameters and add 9 new parameters to it. (I'm neglecting CP violation which makes this even worse)

There were also contemporary criticisms that the distribution of LSND excess events more closely resembled the background than the expected signal, as well as a particularly contentious debate with the HARP-CPD collaboration (a group that seemed to find itself in a lot of contentious debates) over the background estimation.

Furthermore, the most similar experiment, KARMEN, saw nothing.

 

[Melbourne Guy]

I'm not going to sit through 12 minutes of listening to Hossenfelder rant about how stupid and dishonest all the other physicists are.

I'd never heard of her before, fortunately, she speaks quite clearly...and slowly...so I ran the video at 1.5 speed and saved some time. But there wasn't much bashing going on, perhaps apart of the media. Also, from a PF perspective, her "Science without the goobledygook" approach probably makes her content too vague for these discussions right off the bat.

It is false, false, false to claim that more recent experiments confirmed LSND, They had an anomaly, and MiniBoone failed to confirm that anomaly - but did find an anomaly of their own.

That's interesting. Hossenfelder clearly states at about 8:44 that MiniBooNE does confirm LSND. I'm not sure what her agenda might be, or even her expertise, but if that's wrong, then the entire video should be discounted because that's the foundation of her argument.

 

[ohwilleke]

Sabine Hoffenfelder's latest blog post talks about the sterile neutrino anomaly seen at the Liquid Scintillator Neutrino Detector, LSND for short, which ran from 1993 to 98 and again at the Mini Booster Neutrino Experiment experiment at Fermilab since 2003 seeming to show a six sigma anomaly by 2018. She wonders why it isn't a big deal now.

While it is common to talk about a five sigma threshold for discovery of new physics, there are really two more parts of that test: the result needs to be replicated rather than being contradicted by other experiments, and there has to be a plausible physics based theory to explain the result.

Usually, Sabine is a voice of reason and spot on (I bought her book "Lost in Math" and agree with almost everything that she says in it). But on this score, I don't agree with her. She states that:

15 years ago, I worked on neutrino mixing for a while, and in my impression back then most physicists thought the LSND data was just wrong and it’d not be reproduced.

But, most physicists still think that the LSND/MiniBooNE data is wrong, and it wasn't reproduced by other experiments. Instead, multiple experiments and astronomy observations using different methods that make their results robust contradict the LSND/MiniBooNE result.

For what it is worth, searches for non-standard neutrino interactions (other than CP violation) have also come up empty so far and severely constrained that possibility. See, e.g., a paper from IceCube, a paper from ANTARES, an analysis of data from Daya Bay, and a summary of results from six other experiments.

Furthermore, there are no beyond the Standard Model active neutrinos with masses of under 10 TeV. This is also an important part of the argument that there are also no fourth generation quarks or charged leptons, because, for reasons of theoretical consistency, each generation of Standard Model fundamental fermions must be complete.

Other Experiments Contradict LSND/MiniBooNE And There Are Plausible Sources Of Systemic Error

The big problem with the anomaly is that these two sets of results rather than being replicated, were repeatedly contradicted, and instead a plausible physics based explanation for why it was wrong was established.

Three different recent experiments (STEREO, PROSPECT and DANSS) have contradicted the LSND/MiniBooNE result. See, e.g., Matthieu Licciardi "Results of STEREO and PROSPECT, and status of sterile neutrino searches" (May 28, 2021) (Contribution to the 2021 EW session of the 55th Rencontres de Moriond). See also

Searches for electron antineutrino, muon neutrino, and muon antineutrino disappearance driven by sterile neutrino mixing have been carried out by the Daya Bay and MINOS+ collaborations. This Letter presents the combined results of these searches, along with exclusion results from the Bugey-3 reactor experiment, framed in a minimally extended four-neutrino scenario. Significantly improved constraints on the θμe mixing angle are derived that constitute the most stringent limits to date over five orders of magnitude in the sterile mass-squared splitting Δm241, excluding the 90% C.L. sterile-neutrino parameter space allowed by the LSND and MiniBooNE observations at 90% CLs for Δm241<5eV^2. Furthermore, the LSND and MiniBooNE 99% C.L. allowed regions are excluded at 99% CLs for Δm241 < 1.2 eV^2.

Daya Bay, MINOS+ Collaborations, "Improved Constraints on Sterile Neutrino Mixing from Disappearance Searches in the MINOS, MINOS+, Daya Bay, and Bugey-3 Experiments" arXiv 2002.00301 (February 2, 2020).

The KATRIN experiment also contradicts the sterile neutrino anomaly up to 1000 eV sterile neutrino candidates.

In addition to these issues, an analysis back in 2014 already noticed data contradicting the sterile neutrino hypothesis at the ICARUS and OPERA, and observed that some of the parameters used to make the estimates were off and that using the right ones greatly reduced the statistical significance of the anomaly. See Boris Kayser "Are There Sterile Neutrinos" (February 13, 2014). More recent analysis has likewise downgraded the statistical significance of the anomalies previously reported, although it has not entirely eliminated it.

Cosmology Data Strongly Disfavors Sterile Neutrinos

Cosmology data (measurements of Neff) strongly supports the hypothesis that there are exactly three generations of neutrinos when combined with this result; if sterile neutrinos have a mass of 10,000,000,000 meV/c^2 or less (see, e.g., here and https://www.researchgate.net/publication/283532039_Cosmological_limits_on_neutrino_unknowns_versus_low_redshift_priors and here).

Cosmology measures also place a cap on neutrino mass including the sum of the neutrino masses of about 0.087 eV or less, in a manner indifferent between sterile neutrinos of less than about 10 eV, and active neutrinos, which doesn't leave room for an anomalous sterile neutrino. See Eleonora Di Valentino, Stefano Gariazzo, Olga Mena "On the most constraining cosmological neutrino mass bounds" arXiv:2106.16267 (June 29, 2021).

A far heavier sterile neutrino would not be discernible as a neutrino from cosmology data and instead would look like a type of dark matter particle. But, the LSND/MiniBooNE result was pointing to a sterile neutrino with a mass of under 5 eV, so it would be subject to the cosmology bounds.

Also, there are strict direct detection exclusions on heavier dark matter particles as well, although none of those would bar a truly sterile neutrino with no interactions with ordinary matter other than oscillations with active neutrinos.

The main criticism of reliance on cosmology data is that it is highly model dependent, even though this particular conclusion is quite robust to different cosmology models.

Instead, there is strong evidence that there are no sterile neutrinos that oscillate with ordinary neutrinos that have masses of under 10 eV.

Limits On Active Neutrinos

We can also be comfortable that there are no active neutrinos (e.g. a fourth generation neutrino otherwise identical to the three Standard Model neutrinos) with masses of less than about 10 TeV, when direct measurements paired with oscillation data limit the most massive of the three Standard Model neutrino masses to not more than 0.9 eV, and cosmology data limits the most massive of the three Standard Model neutrino masses to not more than 0.09 eV.

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.

Dark dark matter detection experiments have ruled out particles that make up most of hypothetical dark matter particles having weak force interaction coupling constants equal to Standard Model neutrinos at masses of up to about 10 TeV (i.e. 10,000 GeV). In the chart below, that cross section is the blue dotted line marked "Z portal C(x)=1" by a factor of 1,000,000. So, even if the flux of 45 GeV+ Standard Model neutrinos were a million times smaller than the hypothetical flux of dark matter particles through Earth, they would be ruled out by the direct detection experiments up to about 10 TeV.

Direct measurement of the lightest neutrino mass from the Katrin experiment of about 0.8 eV, which means that all of the active neutrino masses have to be less than about 0.9 eV based upon the oscillation data. This means that the sterile neutrino mass predicted by the LSND/MiniBooNE result relative to the active neutrino masses still couldn't have been so massive that it would have evaded cosmology bounds.

The same experiments will soon be able to confirm or rule out the scenario of sterile neutrinos heavier than 10 eV that cosmology tools cannot constrain.

 

[Orodruin]

Other Experiments Contradict LSND/MiniBooNE And There Are Plausible Sources Of Systemic Error

The big problem with the anomaly is that these two sets of results rather than being replicated, were repeatedly contradicted, and instead a plausible physics based explanation for why it was wrong was established.

A generally well written summary, very good! In this section I think it is also relevant to add a bit of a historical note regarding MiniBooNE replicating LSND. It really didn’t in some sense. The intended signal region is perfectly compatible with standard oscillations and only when looking at the low energy part of the spectrum does something ”fishy” show up. This is where the MiniBooNE signal arises and it is true that a sterile neutrino would be a better fit than just standard oscillations. However, if you look at the spectral shape of the excess it really doesn’t look like an oscillation caused effect at all.

I think a sterile neutrino as an explanation for the LSND result is pretty much out of the window. However, that does not necessarily mean that the experiment in itself was wrong - just that the interpretation of the result as being caused by a sterile neutrino is.

 

[RobertGC]

I'd never heard of her before, fortunately, she speaks quite clearly...and slowly...so I ran the video at 1.5 speed and saved some time. But there wasn't much bashing going on, perhaps apart of the media. Also, from a PF perspective, her "Science without the goobledygook" approach probably makes her content too vague for these discussions right off the bat.That's interesting. Hossenfelder clearly states at about 8:44 that MiniBooNE does confirm LSND. I'm not sure what her agenda might be, or even her expertise, but if that's wrong, then the entire video should be discounted because that's the foundation of her argument.

She has said she started off working with neutrinos early in her career so perhaps she has a great affinity for them and feels this anomaly needs more attention.

By the way, the neutrino is rife with anomalies. Both the IceCube and ANITA neutrino experiments have observed anomalies quite recently for example.

Which brings up my next question.

Robert Clark

 

[berkeman]

Which brings up my next question.

So that was the tease. Did we pause for commercial?

 

[PeroK]

So that was the tease. Did we pause for commercial?

The second part is here:

https://www.physicsforums.com/threads/a-test-for-a-superluminal-neutrino.1007824/

 

[berkeman]

The second part is here:

https://www.physicsforums.com/threads/a-test-for-a-superluminal-neutrino.1007824/

Oh dear...

 

[ohwilleke]

A generally well written summary, very good! In this section I think it is also relevant to add a bit of a historical note regarding MiniBooNE replicating LSND. It really didn’t in some sense. The intended signal region is perfectly compatible with standard oscillations and only when looking at the low energy part of the spectrum does something ”fishy” show up. This is where the MiniBooNE signal arises and it is true that a sterile neutrino would be a better fit than just standard oscillations. However, if you look at the spectral shape of the excess it really doesn’t look like an oscillation caused effect at all.

I think a sterile neutrino as an explanation for the LSND result is pretty much out of the window. However, that does not necessarily mean that the experiment in itself was wrong - just that the interpretation of the result as being caused by a sterile neutrino is.

Good points.

 

[Melbourne Guy]

I think a sterile neutrino as an explanation for the LSND result is pretty much out of the window.

Are sterile neutrinos out the window? IceCube seems to have severely constrained them, but I'll admit I'm not aware of what range might be left for them to inhabit.

 

[ohwilleke]

Are sterile neutrinos out the window? IceCube seems to have severely constrained them, but I'll admit I'm not aware of what range might be left for them to inhabit.

Yes.

The theorists are still hard at it, blithely churning out sterile neutrino papers like Energizer bunnies, but the data to the contrary is increasingly unimpeachable.

You could have a more than 10 eV sterile neutrino with a very small rotation angle, which looks a lot like a dark matter candidate and would fully escape detection from direct detection experiments if it was truly "sterile." But that wouldn't lead to any of the claimed anomalies from experiments to date.

 

[Orodruin]

Are sterile neutrinos out the window? IceCube seems to have severely constrained them, but I'll admit I'm not aware of what range might be left for them to inhabit.

It depends a bit on what you put into the concept of ”sterile neutrino”. Sterile neutrinos with eV scale masses (which is a common interpretation of the term) are severely constrained. However, the heavy right-handed neutrinos of the seesaw are arguably also ”sterile” ...

 

[ohwilleke]

It depends a bit on what you put into the concept of ”sterile neutrino”. Sterile neutrinos with eV scale masses (which is a common interpretation of the term) are severely constrained. However, the heavy right-handed neutrinos of the seesaw are arguably also ”sterile” ...

In my previous post, I am referring to leptons that oscillate to some extent with the Standard Model neutrinos, but otherwise have no interactions via the strong, weak or electromagnetic forces (although they would, of course, interact via gravity).

For what it is worth, I don't think these exist either, but the observational exclusions aren't nearly as strict for this class of sterile neutrinos.

I am also assuming that these neutral leptons have a mass in excess of 10 eV. This cutoff is important (1) because above that mass (give or take) they cease to register as neutrinos for purposes of cosmology measures of the effective number of neutrino species (i.e. N_eff), (2) because they would not be considered in the cosmologically determined sum of neutrino masses, and (3) because a sterile neutrino in this mass range is too massive to fit the main anomalies in neutrino physics alleged to provide an experimental basis for the sterile neutrino hypothesis, which I have explained have been largely discredited for good physics data based reasons.

These would instead primarily serve as dark matter candidates, although they would have to be formed via a means other than thermal freeze out because otherwise they would constitute "hot dark matter" which is inconsistent with the high amount of observed small scale (i.e. galaxy scale) structure in the Universe.

Since they are truly sterile, they would have no cross-section of interaction with protons or neutrons or electrons and hence would evade direct detection experiments.

The main parameter space sweet spot would be sterile neutrinos with a keV order of magnitude "warm dark matter" mass, but warm dark matter models without self-interaction still struggle mightily to produce the observationally inferred shape of dark matter halos in galaxies and galactic clusters.

I am not considering heavy right-handed neutrinos in seesaw models.

 

[Orodruin]

In my previous post, I am referring to leptons that oscillate to some extent with the Standard Model neutrinos, but otherwise have no interactions via the strong, weak or electromagnetic forces (although they would, of course, interact via gravity).

For what it is worth, I don't think these exist either, but the observational exclusions aren't nearly as strict for this class of sterile neutrinos.

I am also assuming that these neutral leptons have a mass in excess of 10 eV. This cutoff is important (1) because above that mass (give or take) they cease to register as neutrinos for purposes of cosmology measures of the effective number of neutrino species (i.e. N_eff), (2) because they would not be considered in the cosmologically determined sum of neutrino masses, and (3) because a sterile neutrino in this mass range is too massive to fit the main anomalies in neutrino physics alleged to provide an experimental basis for the sterile neutrino hypothesis, which I have explained have been largely discredited for good physics data based reasons.

These would instead primarily serve as dark matter candidates, although they would have to be formed via a means other than thermal freeze out because otherwise they would constitute "hot dark matter" which is inconsistent with the high amount of observed small scale (i.e. galaxy scale) structure in the Universe.

Since they are truly sterile, they would have no cross-section of interaction with protons or neutrons or electrons and hence would evade direct detection experiments.

The main parameter space sweet spot would be sterile neutrinos with a keV order of magnitude "warm dark matter" mass, but warm dark matter models without self-interaction still struggle mightily to produce the observationally inferred shape of dark matter halos in galaxies and galactic clusters.

I am not considering heavy right-handed neutrinos in seesaw models.

Sure, I just wanted to make it clear that whether or not steriles are excluded depends on what you put into the concept.

 

[A Majeski]

I like the theory of neutrino oscillations and it explains a lot, however, I am having difficulty with it as a full solution, here is why;
the Solar neutrino deficit is about 2/3 yet There are only two flavors of Solar neutrinos. I would expect the Solar deficit to be closer to 1/2 or something other than 2/3. This value indicates that the dispersion of three neutrino flavors is basically flat regardless of the detectors proximity to a large neutrino source, like the Sun.
also, it seems to me that the neutrino mass change should progress from lightest to heaviest with an accompanied decrease in speed. It should not progress from heaviest to lightest as this would require the speed to increase, I cannot see any example where this occurs in other phenomenon.
either way, this sequence should disrupt the flavor ratio.
just a few thoughts on the matter.

 

[Vanadium 50]

You have numbers in that post. Do you have calculations to go along with them?

 

[A Majeski]

No, I just tried to keep it simple

 

[Vanadium 50]

So, you don't do a calculation but guess at an answer. and when your guess doesn't match reality, it's not that your guess is wrong, it's that neutrino oscillations are a bad theory.

I am unconvinced.

 

[A Majeski]

Not at all, I am just trying to understand the concept, I did not mean to ruffle your feathers

 

[Orodruin]

I would expect the Solar deficit to be closer to 1/2 or something other than 2/3. This value indicates that the dispersion of three neutrino flavors is basically flat regardless of the detectors proximity to a large neutrino source, like the Sun.

This is wrong. The mechanism at work here is the Mikheev-Smirnov-Wolfenstein effect and adiabatic flavor transitions. The MSW matter effect on oscillations imply that neutrinos exit the Sun almost entirely in the second mass eigenstate (provided that they are energetic enough to be produced above resonance). This in turn means that the nu_e survival probability is based almost exclusively on the nu_e content of that mass eigenstate.

It is also wrong that only two flavors are involved. Theory predicts almost equal numbers of mu and tau neutrinos.

It is not the case that oscillations equilibrate the flavors. The oscillation probabilities depend on several things, including the mixing parameters.

also, it seems to me that the neutrino mass change should progress from lightest to heaviest with an accompanied decrease in speed. It should not progress from heaviest to lightest as this would require the speed to increase, I cannot see any example where this occurs in other phenomenon.

This is well known (assuming fixed momentum) and it does not really affect oscillations until wave packets of different mass eigenstates separate which would just average out the oscillations. This does happen for solar neutrinos but as mentioned they effectively only consist of one mass eigenstate anyway so it is not super relevant.

Note: Neutrinos will not change their speed during propagation. They are in a quantum mechanical superposition of their mass states and it is the mass states that move at different velocities.

Not at all, I am just trying to understand the concept, I did not mean to ruffle your feathers

You’re not though. You are making assumptions of how things behave and try to draw conclusions based on those (faulty) assumptions. That will never end well and you will not really learn anything. What you should be doing if you wish to understand is to pick up some reference literature on the subject. For example, section III.G of this review.