MiniBooNE results at 6.1 sigma: Potential evidence for sterile neutrinos

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  • #2
Orodruin
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I assume that by "IceCube" you mean "MiniBooNE".

You have to take these results with a (big) grain of salt. The best-fit they present is already ruled out by other experiments. Even more so if you consider experiments that measure different channels. In addition, the oscillation fit is still a pretty bad one. Although they have a ##\chi^2/##dof of 35.2/28, many of the bins are not in the region where they have a signal and I imagine that if you would focus on that region the ##\chi^2/##dof would be rather nasty. They also cover their bases on this in the last sentence of the abstract: "Although the data are fit with a standard oscillation model, other models may provide better fits to the data."

Edit: Let me also point out that Sabine has misinterpreted some of the paper in her blog post. In particular with respect to what other experiments are shown in the "money plot". Those shown are other experiments trying to measure the exact same oscillation channel. There is no way to reconcile the best-fit with OPERA even assuming sterile neutrino oscillations. Something is going on, but it is probably not sterile neutrinos.
 
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  • #3
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Also, MiniBooNE was against sterile neutrinos before they were for them. :olduhh:
 
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  • #4
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Based on figure 4, they rule out ##\Delta m^2 < 0.01 eV^2## with more than 3 sigma. This means at least one neutrino mass eigenstate has to be at least 100 meV, right? 140 meV if we go by the 95% CL. And this is for the optimal case of ##\theta = 45^\circ##. This could come in conflict with cosmological measurements in the not so distant future - in addition to the existing conflict with the other measurements of the same parameter.

Edit: Oops, wrong sign
 
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Is this interpretation correct?
LSND experimet observed and excess number of electron neutrinos,so by considering a sterIle neutrino we can explain that the muon neutrinos oscillate 5o sterile neutrinos and then theses sterile neutrinos oscillate to electron neutrinos in the short baseline experiment?
 
  • #6
Orodruin
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Based on figure 4, they rule out ##\Delta m^2 > 0.01 eV^2## with more than 3 sigma. This means at least one neutrino mass eigenstate has to be at least 100 meV, right? 140 meV if we go by the 95% CL. And this is for the optimal case of ##\theta = 45^\circ##. This could come in conflict with cosmological measurements in the not so distant future - in addition to the existing conflict with the other measurements of the same parameter.
The best fit is ruled out by so many things (including the OPERA results that they show in the figure!) that I think cosmology would be one of the weaker ... It is completely incompatible with essentially everything else we know about neutrinos. Also, a state with maximal mixing is not very sterile ...

I also believe the best fit would actually already be ruled out by Planck. A "sterile" neutrino with that kind of interactions would easily thermalise and send ##\Delta N_{\rm eff}## to at least 1. And then we have not even started to talk about atmospheric and reactor neutrino experiments ...

Is this interpretation correct?
LSND experimet observed and excess number of electron neutrinos,so by considering a sterIle neutrino we can explain that the muon neutrinos oscillate 5o sterile neutrinos and then theses sterile neutrinos oscillate to electron neutrinos in the short baseline experiment?
Many would put it like this as a kind of a mental picture. However, it is a quantum process and you are never measuring the neutrino state in between production and detection so you can not say it was a sterile neutrino at some point. A more accurate way of putting it is that it would change the interference pattern among the neutrino mass eigenstates.
 
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  • #7
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The best fit is ruled out by so many things (including the OPERA results that they show in the figure!) that I think cosmology would be one of the weaker ... It is completely incompatible with essentially everything else we know about neutrinos. Also, a state with maximal mixing is not very sterile ...

I also believe the best fit would actually already be ruled out by Planck. A "sterile" neutrino with that kind of interactions would easily thermalise and send ##\Delta N_{\rm eff}## to at least 1. And then we have not even started to talk about atmospheric and reactor neutrino experiments ...


Many would put it like this as a kind of a mental picture. However, it is a quantum process and you are never measuring the neutrino state in between production and detection so you can not say it was a sterile neutrino at some point. A more accurate way of putting it is that it would change the interference pattern among the neutrino mass eigenstates.
It is a very naive and basic question,but I just get confused, that all the neutrinos are like that, that they do not exist at some points, or just for sterile neutrinos?
if for others also then how we can say we have a muon neutrino beams or electron neutrinos observed by their interactions?
 
  • #8
Orodruin
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It is a very naive and basic question,but I just get confused, that all the neutrinos are like that, that they do not exist at some points, or just for sterile neutrinos?
I did not say they did not exist. I said you do not measure their flavour state.
 
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I did not say they did not exist. I said you do not measure their flavour state.
I understand,and the reason that long baseline experiments could not see any result about sterile neutrinos, is because in their baseline and their energy ranges the feature of the sterile neutrinos could not affect the probabilities of neutrino oscillation?
 
  • #10
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At longer baselines the oscillations of eV range sterile neutrinos would be completely averaged out. That does not necessarily mean you would have no information, but it would be more difficult to extract it.
 
  • #12
jim mcnamara
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@Ygggdrasil - that is primarily why I posted this in the first place. I was confused - the original title of the post reflected a popular science name 'Ice Cube', @mfb corrected me. Thanks for that.

What also confused me is why I saw nothing on PF about it except Bee H's blog, and the kinds of articles you cited.
 
  • #13
Orodruin
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The results are being picked up by the popular press:
A major physics experiment just detected a particle that shouldn't exist. https://www.nbcnews.com/mach/scienc...-detected-particle-shouldn-t-exist-ncna879616

Evidence Found for a New Fundamental Particle. https://www.quantamagazine.org/evidence-found-for-a-new-fundamental-particle-20180601/
A clear example of why one should not always trust the popular press. The NBC article does not mention that the interpretation as sterile neutrinos is in direct conflict with other experiments until pretty far down and you certainly do not get that impression from the first part of the article. The Quantamagazine article is a bit more forthcoming with this information.

I was confused - the original title of the post reflected a popular science name 'Ice Cube', @mfb corrected me. Thanks for that.
You are welcome. :-p
Also I am not sure IceCube is more "popular science" than MiniBooNE. Both are actual names of particle physics experiments. One just happens to be more known than the other.
 
  • #14
Ygggdrasil
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clear example of why one should not always trust the popular press. The NBC article does not mention that the interpretation as sterile neutrinos is in direct conflict with other experiments until pretty far down and you certainly do not get that impression from the first part of the article. The Quantamagazine article is a bit more forthcoming with this information.
Exactly why after seeing and skimming through the NBC article (mainly to see if the claims had any basis in a published article), I came here to see if you all had anything to say about the paper.
 
  • #15
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John Baez offers some interesting comments here: https://johncarlosbaez.wordpress.com/2018/06/02/miniboone/. The fact is MiniBooNE detected an excess of electron neutrinos over their 15 years of data collection. It isn't a huge number, but, given the confidence we have in the expected number of detections is very high, it is enough to establish a very high confidence [4.8 sigma] that something very curious is going on that is not explained by the standard model. That does not mean it is proof of sterile neutrinos, but, that is probably as good a guess as anyone has offered thus far. It should be interesting to see if MiniBooNE can nudge up the signal they currently have to up over the magic 5 sigma level with more data.
 
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  • #17
Orodruin
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John Baez offers some interesting comments here: https://johncarlosbaez.wordpress.com/2018/06/02/miniboone/. The fact is MiniBooNE detected an excess of electron neutrinos over their 15 years of data collection. It isn't a huge number, but, given the confidence we have in the expected number of detections is very high, it is enough to establish a very high confidence [4.8 sigma] that something very curious is going on that is not explained by the standard model. That does not mean it is proof of sterile neutrinos, but, that is probably as good a guess as anyone has offered thus far. It should be interesting to see if MiniBooNE can nudge up the signal they currently have to up over the magic 5 sigma level with more data.
First of all, it is a 4.8 sigma difference with the no oscillation scenario for the best fit. This best fit happens to already be strongly excluded by other experiments, which puts the interpretation as sterile neutrinos in serious doubt. I would certainly agree that it is curious and worthy of scrutiny, but I would be very surprised if the signal is due to sterile neutrinos (see the comments I made on Backreaction).

Edit: Also, it is as good a guess as someone has offered thus far and has therefore also been much more scrutinised. It has been scrutinised to the point that it seems unlikely to be able to explain the MiniBooNE low-energy excess as oscillations just don't give a good fit of the data.
 
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  • #18
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My interest in sterile neutrinos dates back to this paper: https://arxiv.org/abs/1402.2301, Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters, which sparked an enduring controversy over the plausibility of sterile neutrinos as a component of the dark matter budget. Particle physics is not really my thing, although I have tried to keep a finger on the sterile neutrino pulse since then.
 
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  • #21
ohwilleke
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The MiniBooNE anomaly, if it was a sterile neutrino would be a best fit to one of about 0.5 eV/c^2 in mass and a mixing fraction of about 0.01.

But, a sterile neutrino interpretation of this anomaly is not going to hold up, and doesn't deserve the exaggerated significance claimed by the MiniBooNe collaboration, whose own paper is at best ambivalent about the interpretation of its own results. It reports the data without purposing an explanation in the body text.

The Excess Observed Is A Poor Fit To A Sterile Neutrino Hypothesis

The paper even notes that the MiniBooNE data itself isn't a great fit to an oscillation model with additional neutrino types. If there was a fourth neutrino type into which ordinary neutrinos were oscillating the excess appearances shouldn't bunch up in the low energy part of the spectrum along with the background where they appear; they should instead be concentrated around mid-range energies.

One of the authors of the MiniBooNE paper states in the comments at Bee's blog (Backreaction):

Hi, my name is Teppei and I am one of authors of this and let me give my comment. We checked everything and there is excess, but somehow not quite same from what I expected. I don't think we made mistakes in data taking..., identifying isolated electromagnetic shower is very easy for a spherical Cherenkov detector. So the data is data. Question is what it is exactly. It always bothers me that the shape of data distribution is more similar with gamma rays (background, sharp peak at low energy), than electrons (from electron neutrino and oscillation candidates, mild peak at middle energy). Indeed, new BSM mechanism to produce a single gamma ray (such as dark photon oscillation, new neutrino NC gamma production process) could be a solution and we might make a big discovery! well, but since I am experimentalist, I would suspicious any potential source of gamma rays we are missing from our simulation. Let's assume the excess is not oscillation signal (electrons) but background (gamma rays). Then it's unlikely these high energy gamma rays are produced in the detector..., for this we need to be 100% wrong to predict pi0 (pi0-->gamma+gamma) production rate or Delta resonance (Delta-->N+gamma) rate, this is unlikely since we can calibrate these processes internally (I am also bit suspicious about this process). Then the only source is unknown process to produce single gamma (either SM or BSM) or gamma rays sneaking into the detector in the way we couldn't simulate by the current simulation. We are continuously working on this. Meantime, follow up experiment called MicroBooNE will release data soon. That is located right next to MiniBooNE (so they receive same neutrino beam) and it has an ability to distinguish gamma rays and electrons, so they may clarify the issue before MiniBooNE.
The main comment by Mattias at Bee's blog, is quite eloquent and also on point:

I believe there are several things that need to be pointed out regarding the MiniBooNE results, which are all reasons for neutrino phenomenologists not being nearly as excited about this as you might expect. First and foremost of these is the OPERA exclusion from the money plot, which measures the exact same channel as MiniBooNE with the same L/E in an accelerator experiment. The exclusion limits in the plot are actually all from other experiments examining the same channel and so the oscillation interpretation of the best-fit is directly excluded by other experiments. On top of this, something that is not shown in the figure are the exclusion limits that would come from other oscillation channels just from the assumption of probabilities adding up to one. This is where atmospheric and reactor neutrinos come in and this essentially rules out the entire fit region.

The second thing to note is that the fit is a rather bad one. Although the chi2/dof is seemingly fine, the signal is concentrated in a few bins at low energy where the background is the largest and likely most poorly understood (the first MiniBooNE papers did not include this region as a signal region exactly because of this). The rise with energy is simply put just too steep to be due to oscillations.

Third, the people doing global fits with sterile neutrinos typically disregard this region of MiniBooNE data because of what I just mentioned. Regardless of what they do with steriles, they end up with a fit that does not explain the MiniBooNE excess at low energies. Some find a good fit with steriles and LSND results, but that result would typically be compatible with a MiniBooNE excess at higher energies, as was initially expected. MiniBooNE itself does not provide much statistical weight to those fits.

With the global picture in mind, it seems very unlikely to me that the appropriate resolution of this discrepancy is sterile neutrinos. It is very possible that something is afoot, be it new physics or a background that is poorly understood, but that something is probably not sterile neutrinos. The best way of testing this in the future is using experiments with multiple detectors to decrease the reliance on Monte Carlo simulations of backgrounds and fluxes. The Fermilab short-baseline program should do pretty well in this regard.

Also, regarding the right-handed neutrinos in the canonical seesaw, they are typically not Dirac. We introduce an additional right-handed SM singlet to build Dirac mass terms via the Yukawa couplings to the left-handed lepton doublet in the same way we give up-type quarks masses. The big difference is that the right-handed neutrinos are SM singlets and therefore allow for a heavy Majorana mass term between them. The resulting neutrinos (both light and heavy) are Majorana particles. Of course, you have no a priori handle on the scale of the right-right Majorana mass and if it is very very small you end up just with light pseudo-Dirac neutrinos.
Theorists Rush In Where Angels Fear To Tread

As much as I have a high opinion of Sabine Hossenfelder, I think her conclusion that it is time for the theorists to chime in is premature. The claims made are extraordinary and have been previously ruled out in the LSDN case, and the experimentalists need to kick the tires more before accepting these results at face value.

Likewise, John Baez is a great guy, but I think he's jumping in with two feet to provide a beyond the Standard Model explanation a bit too prematurely, while giving too short shrift to skepticism about the experimental result.

We saw similar rushes by theorists to explain anomalies like the OPERA superluminal neutrino result, the primordial gravitational wave claim, and the 750 GeV scalar boson that later turned out to be experimental flukes or systemic errors.

The Reactor Data Taken As A Whole Disfavors Sterile Neutrinos


There is a reason that you have to start a PhD dissertation with a review of the literature. Cherry picking the LSDN and MiniBooNE anomalies without also including data from other reactor experiments is statistically unsound, so the 6.1 sigma anomaly quoted isn't meaningful. Data from at least nine other experiments (OPERA, KARMEN, MINOS, MINOS+, Ice Cube, STEREO, Daya Bay, Juno and Reno) flatly contradict the sterile neutrino conclusion that MiniBooNE seeks to support with these anomalies, and so does the cosmology data.

KARMEN and OPERA combined directly contradict all of the MiniBooNE parameter space and almost all of the LSDN parameter space.

nu2.png


If you include all available evidence, pro- and con- that is known and available, as any valid statistical probability estimate should, the case for a fourth neutrino type that oscillates with the three Standard Model neutrinos is greatly diminished. Of course, it is also true that even one or two omitted or underestimated sources of significant systemic error can also greatly reduce the statistical significance of the anomaly, which matters when at least one of those source of error (differences in fuel mixture) is indeed omitted.

For example, the MiniBooNE collaboration notes OPERA data in their paper that flatly contradicts their conclusion, while still selling the effect as 6.1 sigma based on only a subset of the data that excludes the data from OPERA that they consider. This verges on dishonest. It is a bit like saying that a coin appears to be biased if you exclude all instances when it came up heads twice in a row.

The LSDN anomaly that preceded the MiniBooNe anomaly was contradicted contemporaneously by the KARMEN data using comparable methods. As the article cited by Jim McNamara explains:

The excess events pile up in the low end of the detector’s energy range, notes Tommaso Dorigo, a particle physicist with Italy's National Institute for Nuclear Physics in Padua, on his blog. That’s also where the "backgrounds" from various other types of particles pile up. So if MiniBooNE researchers underestimated their background, Dorigo explains, then they may have essentially mistaken background events for a signal.
The fact that new results from MiniBooNE differ from previous results there also flag this as a likely case of systemic error.

The MINOS and MINOS+ reactor experiments rule out a light sterile neutrino. The abstract of a pre-print on their results states that:

A simultaneous fit to the charged-current muon neutrino and neutral-current neutrino energy spectra in the two detectors yields no evidence for sterile neutrino mixing using a 3+1 model. The most stringent limit to date is set on the mixing parameter sin2θ24 for most values of the sterile neutrino mass-splitting Δm241>10−4 eV2.
The MINOS data explores a range of values for Δm41 between the lightest mass state and the sterile neutrino mass state of 10 meV to 32,000 meV, where the bounds on the sum of the three neutrino masses from cosmology in the currently experimentally preferred normal hierarchy is 60 meV to 110 meV. For example, the MINOS data shows that:

At fixed values of ∆m241 the data provide limits on the mixing angles θ24 and θ34. At ∆m241 = 0.5 eV2, we find sin2θ24 less than [0.0050 (90% C.L.), 0.0069 (95% C.L.)] and sin2θ34 less than [0.16 (90% C.L.), 0.21 (95% C.L.)].
In each of those cases, a lower bound of zero is consistent with the data and is the modal possibility.

A Daya Bay and Juno paper abstract from 2014 states that:

In this work, we show that the high-precision data of the Daya Bay experiment constrain the 3+1 neutrino scenario imposing upper bounds on the relevant active-sterile mixing angle sin 2 2 θ14 . 0 .06 at 3 σ confidence level for the mass-squared difference ∆ m 2 41 in the range (10 − 3 , 10 − 1 ) eV 2 . The latter bound can be improved by six years of running of the JUNO experiment, sin2 2θ14 . 0.016, although in the smaller mass range ∆m2 41 ∈ (10 − 4 , 10 − 3 ) eV 2 . We have also investigated the impact of sterile neutrinos on precision measurements of the standard neutrino oscillation parameters θ13 and ∆ m 2 31 (at Daya Bay and JUNO), θ12 and ∆ m 2 21 (at JUNO), and most importantly, the neutrino mass hierarchy (at JUNO). We find that, except for the obvious situation where ∆ m 2 41 ∼ ∆ m 2 31, sterile states do not affect these measurements substantially.
Fuel Differences May Explain The Anomaly

Further data from Daya Bay in 2017 further disfavors this hypothesis and may explain the anomaly. The abstract of this paper notes that:

The Daya Bay experiment has observed correlations between reactor core fuel evolution and changes in the reactor antineutrino flux and energy spectrum. Four antineutrino detectors in two experimental halls were used to identify 2.2 million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cycles for each of six 2.9 GWth reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective 239Pu fission fractions, F239, from 0.25 to 0.35, Daya Bay measures an average IBD yield, σ¯f, of (5.90±0.13)×10−43 cm2/fission and a fuel-dependent variation in the IBD yield, dσf/dF239, of (−1.86±0.18)×10−43 cm2/fission. This observation rejects the hypothesis of a constant antineutrino flux as a function of the 239Pu fission fraction at 10 standard deviations. The variation in IBD yield was found to be energy-dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the evolution in the IBD spectrum show general agreement with predictions from recent reactor models, the measured evolution in total IBD yield disagrees with recent predictions at 3.1σ. This discrepancy indicates that an overall deficit in measured flux with respect to predictions does not result from equal fractional deficits from the primary fission isotopes 235U, 239Pu, 238U, and 241Pu. Based on measured IBD yield variations, yields of (6.17±0.17) and (4.27±0.26)×10−43 cm2/fission have been determined for the two dominant fission parent isotopes 235U and 239Pu. A 7.8% discrepancy between the observed and predicted 235U yield suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.
The Reno Collaboration also points to fuel discrepancies as the source of the reactor anomaly. It's abstract to a June 2, 2018 preprint states:

We report a fuel-dependent reactor antineutrino yield using six 2.8\,GWth reactors in the Hanbit nuclear power plant complex, Yonggwang, Korea. This analysis uses an event sample acquired through inverse beta decay (IBD) interactions in identically designed near and far detectors for 1807.9 live days from August 2011 to February 2018. Based on multiple fuel cycles, we observe a fuel dependent variation in the IBD yield of (6.03±0.21)×10−43~cm2/fission for 235U and (4.17±0.29)×10−43~cm2/fission for 239Pu while a total average IBD yield per fission (y⎯⎯⎯f) is (5.79±0.11)×10−43~cm2/fission. The hypothesis of no fuel dependent IBD yield or identical spectra of fuel isotopes is ruled out at more than 6\,σ. The measured IBD yield per 235U fission shows the largest deficit relative to a reactor model prediction. Reevaluation of the 235U IBD yield per fission may mostly solve reactor antineutrino anomaly. We also report a hint of correlation between the 5\,MeV excess in observed IBD spectrum and the reactor fuel isotope fraction of 235U.
Despite these hints at an explanation for the anomaly, the MiniBooNE paper doesn't consider or analyze this possibility or even acknowledge fuel discrepancies as a potential source of systemic error, particularly in cases where the results differ over time at the same experiment which they have lumped into a single temporal bin.

Cosmology Data Disfavors Sterile Neutrinos That Oscillate Together With Active Neutrinos


As of 2015, the constraint with Planck data and other data sets was 3.04 ± 0.18 (even in 2014 cosmology ruled out sterile neutrinos). Neff equal to 3.046 in a case with the three Standard Model neutrinos and neutrinos with masses of 10 eV or more not counting in the calculation. So, the four neutrino case is ruled out at a more than 5.3 sigma level already, which is a threshold for a scientific discovery that there are indeed only three neutrinos with masses of 10 eV or less, ruling out the sterile neutrino hypothesis for a stable sterile neutrino of under 10 eV (when a best fit of potential anomalies from reactors predicts a sterile neutrino mass of about 1 eV also here).

A 2015 pre-print notes that:

The 95% allowed region in parameter space is Neff < 3.7, meff s < 0.52 eV from PlanckTT + lowP + lensing + BAO. This result has important consequences for the sterile neutrino interpretation of short-baseline anomalies. It has been shown that a sterile neutrino with the large mixing angles required to explain reactor anomalies would thermalize rapidly in the early Universe, yielding ∆Neff = 1. The preferred short-baseline solution then corresponds to ms of about 1 eV and ∆Neff = 1 and is strongly excluded (more than 99% confidence) by the above combination of Planck and BAO data.
There Are Only Three Active Neutrinos.

Weak boson decays have long ago ruled out the possibility of a number of weakly interacting neutrinos different than three. The number of weakly interacting neutrinos of less than 45 GeV upon Z boson decay is 2.992 ± 0.007 (with a mean value 1.14 sigma from 3) which is consistent with the Standard Model, in a quantity that must have an integer value. The two neutrino and four neutrino hypotheses are ruled out at the 140+ sigma level, when a mere 5 sigma result is considered scientifically definitive.
 

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  • #23
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In the below linked APS article on the LSND excess electron neutrino/antineutrino anomaly experiment, it's stated on page 2 that the production of kaons, (and heavier mesons), is "negligible". In the same paragraph it's indicated that the proton beam impinging on the fixed target has an energy of 800 MeV. Kaons are more than three times the mass of pions (the main source of neutrinos in the experiment), the former being 497 MeV versus 139 MeV for the pions. So, I assume the much greater mass of the kaons is why the 800 MeV beam produces them in much less abundance than the pions. But what formula is used to calculate the production rate of kaons. Since their production is "negligible" I gather it's not a simple inverse linear relationship of mass versus production rate, and also taking into account the beam energy.
 
  • #24
Vanadium 50
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At KE = 800 MeV there is not enough energy to produce a kaon.
 

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