Researchers at Super-Kamiokande report solar neutrino signal is slightly stronger at night (Update)

Mar 14, 2014 by Bob Yirka report

(Phys.org) —Researchers working at the Super-Kamiokande Collaboration in Kamioka, Japan are reporting slightly stronger neutrino detection occurring at night, due they say to changes that occur in flavor as the neutrinos pass through the Earth. In their paper published in Physical Review Letters, the researchers describe the results they found when analyzing a year's worth of data from their detector, which showed a flux of solar neutrinos during nighttime that was approximately 3.2 percent greater than what was measured during the day.

Scientists have known for a while that neutrino's change "flavor" as they journey through space—such as when they travel from the sun to our planet. The term "flavor" refers to its characteristics, which can be one of three: electron, muon, and tau. But until now, it wasn't clear if traveling through an object could also cause them to change flavor. In this new effort, the researchers have found that they likely do indeed, which means that sometime in the future, could be used to learn more about the interior of our planet.

Scientists have found that approximately half of the low energy electron neutrinos emitted by the sun change to a tau or muon flavor before they reach us (which reduces the chances of electron neutrino detectors detecting them). An even smaller number of high energy electron neutrinos find their way here. Now the researches in Tokai are reporting that they've found evidence that because fewer electron neutrinos are detected at night, it's reasonable to conclude that that some of them have changed back to muon or tau flavors as they pass through the Earth (it's not likely the sun produces fewer neutrinos when the detector is on the dark side of the planet) which means that passing through the Earth has caused the neutrinos to revert back to the flavor they started out as.

Perhaps just as exciting is that the measured results agree with theories made by Russian physicists Stanislav Mikheyev and Alexei Smirnov in 1986 who were basing their research on work done previously by Lincoln Wolfenstein back in 1978—they describe what has come to be known as the MSW effect.

Because the work done falls below the 5σ needed for classification as a new discovery, however, more work will have to be done to find additional evidence of changes to neutrino flavors as they pass through a material before what the team has found will be considered as generally accepted by the physics community.

Explore further: What's next for the Large Hadron Collider?

More information: 1. www-sk.icrr.u-tokyo.ac.jp/sk/index-e.html

2. First Indication of Terrestrial Matter Effects on Solar Neutrino Oscillation, Phys. Rev. Lett. 112, 091805 – Published 7 March 2014. dx.doi.org/10.1103/PhysRevLett.112.091805 . On Arxiv: arxiv.org/abs/1312.5176

ABSTRACT
We report an indication that the elastic scattering rate of solar B8 neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through Earth during nighttime. We determine the day-night asymmetry, defined as the difference of the average day rate and average night rate divided by the average of those two rates, to be [−3.2±1.1(stat)±0.5(syst)]%, which deviates from zero by 2.7σ. Since the elastic scattering process is mostly sensitive to electron-flavored solar neutrinos, a nonzero day-night asymmetry implies that the flavor oscillations of solar neutrinos are affected by the presence of matter within the neutrinos' flight path. Super-Kamiokande's day-night asymmetry is consistent with neutrino oscillations for 4×10−5  eV2≤Δm221≤7×10−5  eV2 and large mixing values of θ12, at the 68% C.L.

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billpress11
2.5 / 5 (2) Mar 14, 2014
Quote from article: "Now the researches in Tokai are reporting that they've found evidence that because fewer electron neutrinos are detected at night, it's reasonable to conclude that that some of them have changed back to muon or tau flavors as they pass through the Earth (it's not likely the sun produces fewer neutrinos when the detector is on the dark side of the planet) which means that passing through the Earth has caused the neutrinos to revert back to the flavor they started out as."

True, "it's not likely the sun produces fewer neutrinos when the detector is on the dark side of the planet" But it is also reasonable to assume that a few more of the electron neutrinos were absorbed traveling through the earth than the present theory allows for.

marklade
Mar 14, 2014
This comment has been removed by a moderator.
CrossMan
4 / 5 (1) Mar 14, 2014
@billpress11, a few more, yes, but 3% more? No.
arenshaw
5 / 5 (4) Mar 14, 2014
"Researchers working on the J-PARC project in Tokai, Japan…"

This statement is completely incorrect. The research was done by the Super-Kamiokande collaboration with the Super-K detector located in Kamioka, Japan. Also it should be pointed out that the event display which is being shown is an atmospheric muon neutrino event from 1996, observed during the first phase of Super-K. The current research being described was done using solar neutrinos.

"Now the researches in Tokai are reporting that they've found evidence that because fewer electron neutrinos are detected at night…which means that passing through the Earth has caused the neutrinos to revert back to the flavor they started out as."

The first part of this statement is also incorrect and contradicts a statement in the first paragraph. We report that the flux during the nighttime is 3.2%, which means we observe more electron neutrinos during the nighttime, which is what the solar neutrinos are created as.
arenshaw
5 / 5 (1) Mar 14, 2014
**3.2% larger during the nighttime"
billpress11
not rated yet Mar 14, 2014
@billpress11, a few more, yes, but 3% more? No.

Since there is little agreement about the actual shortage or the percentage of electron neutrinos missing from the sun 3% is well with the margin of error. I have read it is as high as 40% fewer than theory calls for. Therefor one cannot state with any degree of certainty why there are fewer electron neutrinos detect on the night side.
arenshaw
5 / 5 (1) Mar 14, 2014
@billpress11 "Therefor one cannot state with any degree of certainty why there are fewer electron neutrinos detect on the night side."

This statement is incorrect, there are more electron neutrinos observed during the nighttime, please see the PRL paper because this article has many incorrect statements. In fact the observed day-night flux asymmetry is quite in line with what is predicted based on currently observed neutrino oscillation parameters, ~3.0%.
GSwift7
5 / 5 (1) Mar 14, 2014
The first part of this statement is also incorrect and contradicts a statement in the first paragraph


I was wondering about that one myself. Maybe something got flipped in translation to english. I've seen some fairly comical errors in science story translation from time to time.
billpress11
not rated yet Mar 14, 2014
Quote from article: (it's not likely the sun produces fewer neutrinos when the detector is on the dark side of the planet) which means that passing through the Earth has caused the neutrinos to revert back to the flavor they started out as."

Arenshaw, I'm not sure which one of us is reading the quote above incorrectly. We seem to have come to an opposite conclusion on this part of the article.

I take that to mean they have detected fewer electron neutrinos on the dark side of the earth or during the night time AFTER the electron neutrinos have passed through the earth.

Some parts of this article are a little confusing.
arenshaw
5 / 5 (1) Mar 14, 2014
billpress11, the article above makes contradictory statements; in the first paragraph it states "which showed a flux of solar neutrinos during nighttime that was approximately 3.2 percent greater than what was measured during the day." Then in the third paragraph it states "Now the researches in Tokai are reporting that they've found evidence that because fewer electron neutrinos are detected at night." Both cannot be correct, and in fact it is the first statement which is correct. If you read the PRL paper it is very clear which is the correct statement.
billpress11
not rated yet Mar 14, 2014
Arenshaw, the PRL paper does state it much more clearly, thanks.

Quote from article abstract: "We report an indication that the elastic scattering rate of solar B8 neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through Earth during nighttime."

I assume by elastic scattering rate they are referring to the energy of the neutrinos detected. I wonder how this compares to the actual number of neutrinos detected on the day vs night side? I have read in the past fewer solar neutrinos were detected on the night side.
billpress11
not rated yet Mar 14, 2014
This article does state that more work is needed to confirm these results.

I would like to know the actual number of electron neutrinos detected on the day vs night side of the earth. Just by detecting 3.2% more electron neutrinos with a larger scattering rate on the night side does not in itself prove neutrino oscillation has occurred. Exactly what are they comparing here, the number with the day side?

To determine if neutrino oscillation has actually occurred one would have to determine the actual number AND scattering rates of electron neutrinos detected on the day vs night side. And the number of the ones with a larger scattering rate on the night should be larger than the number detected on the day side.
arenshaw
5 / 5 (4) Mar 14, 2014
billpress11, elastic scattering refers to the type of interaction the neutrinos are undergoing inside the Super-K detector, and it just so happens that electron flavor neutrinos have a ~6 times larger cross section than the other two flavors. Because the oscillations in vacuum between the Earth and the Sun are average out (the distance is much larger than the oscillation length), any difference between a day and night measurement must come from the interaction of the matter with the neutrinos. Further more, because the neutrinos are created as electron flavor in the core of the Sun and oscillate to muon and tau on the way to Earth, the fact that we see more electron flavor neutrinos at night tells us that the neutrinos must be oscillating back to the electron flavor. I can't imagine a more direct proof that matter affects solar neutrino oscillation than comparing the daytime and nighttime number of events (as is done in this work).
billpress11
not rated yet Mar 14, 2014
Arenshaw I don't think the abstract states quite the same thing you did. Here is a quote: "We determine the day-night asymmetry, defined as the difference of the average day rate and average night rate divided by the average of those two rates, to be [−3.2±1.1(stat)±0.5(syst)]%"

I don't see how one can assume that any neutrinos have change flavor just from that statement without knowing the TOTAL number of electron neutrinos detected in a direct day-night comparison. The reason, the total number of neutrinos with a larger elastic scattering rate (LSR) on the day side could be larger than on the night side while the total number electron neutrinos with a LSR detected on the night side may increase by 3.2% of the total on the night side. I certainly don't know if that is the case but without that information one cannot assume any neutrinos have oscillated. Because the neutrinos with a smaller elastic scattering rate may be more readily absorbed by matter.
Q-Star
5 / 5 (3) Mar 14, 2014
Further more, because the neutrinos are created as electron flavor in the core of the Sun and oscillate to muon and tau on the way to Earth, the fact that we see more electron flavor neutrinos at night tells us that the neutrinos must be oscillating back to the electron flavor. I can't imagine a more direct proof that matter affects solar neutrino oscillation than comparing the daytime and nighttime number of events .


The oscillation between types takes place as they move from the core to the surface of the Sun. In the vacuum of space they don't generally change flavor because there are few free electrons for them to interact with. The relative percentages leaving the Sun are the same as arrive at the earth. This finding shows that some of the neutrinos are changing flavor as they pass through the earth. That was the surprise. They must be interacting with leptons to some degree while passing through the earth.
arenshaw
5 / 5 (3) Mar 14, 2014
billpress11, The average rate is extracted using an unbinned extended maximum likelihood fit to the solar zenith angle distribution of events. This extracts not only the total number of events, but the number of events as a function of the solar zenith angle, hence giving us the day-night asymmetry. The fit is also done as a function of energy, so this should account for the changing elastic scattering rate which I believe you talk about. I suggest reading the paper, since it is relatively short, and not relying on this exceptionally bad article with many erroneous statements.
arenshaw
5 / 5 (4) Mar 14, 2014
Q-Star, "The oscillation between types takes place as they move from the core to the surface of the Sun. In the vacuum of space they don't generally change flavor because there are few free electrons for them to interact with.""

Yes you are correct that the MSW effect in the Sun maximally converts the electron neutrinos to muon and tau neutrinos, but this only effects solar neutrinos above ~3 MeV, reducing the survival probability (the probability of the neutrino being electron in the detector) to ~30%. But I disagree with you saying that in the vacuum of space they don't generally change flavor because there are few free electrons, they do! Neutrinos oscillations dictate that they oscillate even in vacuum, by how much all depends on the oscillation length and the distance between the source and the detector.
billpress11
1 / 5 (1) Mar 14, 2014
Q-star the missing neutrinos from the sun is not proof of neutrino oscillation. We know less than what we think about the nature of neutrinos. For example, what if neutrinos are a type of neutral radiation similar to electromagnetic radiation only they do not alternate electromagnetically? If they are they would drop off by the inverse square rule just like EMR explaining the missing neutrino from the sun.
billpress11
not rated yet Mar 14, 2014
Arenshaw, where are the numbers? One cannot draw a conclusion without seeing the raw data. Why don't they give us these numbers? And if these raw numbers show fewer large elastic scattering rate neutrinos after they pass through the earth there is not proof of any neutrino oscillation. Until they do give us these raw numbers and have them verified by other sources I will NOT be convinced that neutrinos oscillate.
Q-Star
5 / 5 (4) Mar 14, 2014
Q-Star, "The oscillation between types takes place as they move from the core to the surface of the Sun. In the vacuum of space they don't generally change flavor because there are few free electrons for them to interact with.""

Yes you are correct that the MSW effect in the Sun maximally converts the electron neutrinos to muon and tau neutrinos, but this only effects solar neutrinos above ~3 MeV, reducing the survival probability (the probability of the neutrino being electron in the detector) to ~30%. But I disagree with you saying that in the vacuum of space they don't generally change flavor because there are few free electrons, they do! Neutrinos oscillations dictate that they oscillate even in vacuum, by how much all depends on the oscillation length and the distance between the source and the detector.


You might correct, it's not my field. I'll yield to your take on it.
Q-Star
5 / 5 (6) Mar 14, 2014
Q-star the missing neutrinos from the sun is not proof of neutrino oscillation. We know less than what we think about the nature of neutrinos. For example, what if neutrinos are a type of neutral radiation similar to electromagnetic radiation only they do not alternate electromagnetically? If they are they would drop off by the inverse square rule just like EMR explaining the missing neutrino from the sun.


There are no "missing" neutrinos" from the sun. They are all accounted for just as predicted. The only thing that is completely unresolved is what changes they go through between their initial creation and when they are detected. I will grant you that there is much unknown. But they aren't "missing".
Q-Star
5 / 5 (3) Mar 14, 2014
Arenshaw, where are the numbers? One cannot draw a conclusion without seeing the raw data. Why don't they give us these numbers?


http://arxiv.org/abs/1312.5176

I don't think the PDF is behind a pay-wall. I just downloaded it.
billpress11
not rated yet Mar 14, 2014
Q-star, I agree, in a way, there is no missing energy and momentum in the undetected neutrinos from the sun. I checked out you link provided but I still cannot find the data i want and need to convince me that neutrinos do in fact oscillate.
Whydening Gyre
not rated yet Mar 15, 2014
...in the vacuum of space they don't generally change flavor because there are few free electrons, they do! Neutrinos oscillations dictate that they oscillate even in vacuum, by how much all depends on the oscillation length and the distance between the source and the detector.

What is the determining factor in oscillation length?
And - are you saying the 8000 mile diameter of the Earth (approx. distance between night n day) can also have an effect?
Torbjorn_Larsson_OM
5 / 5 (4) Mar 15, 2014
Neat, but FWIW neutrino detectors have already been used to learn more about the interior; they confirmed that ~ 45 % of the net heat outflow stems from radioactivity.

arenshaw, thanks for the corrections! I was wondering about some of that. Yes, Yirka often manage to write the most horrendously mangled articles...

@billpress: The observation is nearly 5 sigma, yet initially you claim uncertainties!? Methinks you protest too much. (I.e. there's likely an agenda.)
MRBlizzard
not rated yet Mar 15, 2014
Could someone point me to a discussion of how both momentum and energy are conserved during neutrino oscillation in empty space. The only thing I can think of is that energy is conserved because some energy is drawn from an associated field, when the more massive tau neutrino changes into a lighter electron neutrino and speeds up thus conserving momentum.

With respect to the actual counts, let's look at the arxiv paper.
billpress11
not rated yet Mar 16, 2014
MRBlizzard, you bring up a very good point. I cannot answer that question but I assume their answer would go something like this, the energy and momentum are conserved only the type or flavor has changed. Of course one might then ask what is the actual difference? It is my understanding that all three types of neutrinos can have a multitude of different energy and momentum levels. I have looked at the arxiv and many other papers and have not been able to find any of the raw numbers used.

TL, I am not questioning the 3.2% difference on the dark vs day side of the earth. What I am questioning is the conclusion they have drawn from the increased percentage. What I want to know is the actual number of the larger elastic scattering rates neutrinos on dark and daylight sides. If the actual number of the larger ones on the dark side is the same or smaller I do not see how they can make a claim of oscillation.
Arrowstone
not rated yet Mar 16, 2014
And how do we know the tau neutrino is more massive than that of the electron? Not just because of the differing masses of the respective leptons.(?)
arenshaw
5 / 5 (2) Mar 17, 2014
The tau neutrino is not more massive than the electron neutrino. The flavor states are made up of a superposition of the mass eigenstates, therefore the flavor states do not have definite mass, but a mixture of the three mass states. The electron neutrino is mostly made up of the first mass eigenstate and the tau neutrino the third. However, it is currently unknown which mass state is heavier, the third mass eigenstate or the first mass eigenstate, this know as the mass hierarchy problem and future experiments are looking to determine this.

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