Monday, February 22, 2016

Too many anti-neutrinos: Evidence builds for new anomaly

Bump ahead.
Tl;dr: A third experiment has reported an unexplained bump in the spectrum of reactor-produced anti-neutrinos. Speculations for the cause of the signal so far focus on incomplete nuclear fission models.

Neutrinos are the least understood of the known elementary particles, and they just presented physicists with a new puzzle. While monitoring the neutrino flux from nearby nuclear power plants, three different experiments have measured an unexpected bump around 5 MeV. First reported by the Double Chooz experiment in 2014, the excess was originally not statistically significant
5 MeV bump as seen by Double Chooz. Image source: arXiv:1406.7763
Last year, a second experiment, RENO, reported an excess but did not assign a measure of significance. However, the bump is clearly visible in their data
5 MeV bump as seen by RENO. Image source: arXiv:1511.05849
The newest bump is from the Daya Bay collaboration and was just published in PRL

5 MeV bump as seen by Daya Bay. Image source: arXiv:1508.04233

They give the excess a local significance of 4.1 σ – a probability of less than one in ten thousand for the signal being due to pure chance.

This is a remarkable significance for a particle that interacts so feebly, and an impressive illustration of how much detector technology has improved. Originally, the neutrino’s interaction was thought to be so weak that to measure it at all it seemed necessary placing detectors next to the most potent neutrino source known – a nuclear bomb explosion.

And this is exactly what Frederick Reines and Clyde Cowan set out to do. In 1951, they devised “Project Poltergeist” to detect the neutrino emission from a nuclear bomb: “Anyone untutored in the effects of nuclear explosions would be deterred by the challenge of conducting an experiment so close to the bomb,” wrote Reines, “but we knew otherwise from experience and pressed on.” And their audacious proposal was approved swiftly: “Life was much simpler in those days—no lengthy proposals or complex review committees,” recalls Reines.

Briefly after their proposal was approved, however, the two men found a better experimental design and instead placed a larger detector close by a nuclear power plant. But the controlled splitting of nuclei in a power plant needs much longer to produce the same number of neutrinos as a nuclear bomb blast, and patience was required of Reines and Cowan. Their patience eventually paid off: They were awarded the 1995 Nobel Prize in physics for the first successful detection of neutrinos – a full 65 years after the particles were first predicted.

Another Nobel Prize for neutrinos was handed out just last year, this one commemorating the neutrino’s ability to “oscillate,” that is to change between different neutrino types as they travel. But, as the recent measurements demonstrate, neutrinos still have surprises in stock.

Good news first, the new experiments have confirmed the neutrino oscillations. On short base-lines as that of Daya Bay – a few kilometer – the electron-anti-neutrinos that are emitted during nuclear fission change into to tau-anti-neutrinos and arrive at the detector in reduced numbers. The wavelength of the oscillation between the two particles depends on the energy – higher energy means a longer wavelength. Thus, a detector placed at fixed distance from the emission point will see a different energy-distribution of particles than that at emission.

The emitted energy spectrum can be deduced from the composition of the reactor core – a known mixture of Uranium and Plutonium, each in two different isotopes. After the initial split, these isotopes leave behind a bunch of radioactive nuclei which then decay further. The math is messy, but not hugely complicated. With nuclear fission and decay models as input, the experimentalists can then extract from their data the change in the energy-distribution due to neutrino oscillation. And the parameters of the oscillation that they have observed fit those of other experiments.

Now to the bad news. The fits of the oscillation parameters to the energy spectrum do not take into account the overall number of particles. And when they look at the overall number, the Daya Bay experiment, like other reactor neutrino experiments before, falls about 6% short of expectation. And then there is the other oddity: the energy spectrum has a marked bump that does not agree with the predictions based on nuclear models. There are too many neutrinos in the energy range of 5 MeV.

There are four possible origins for this discrepancy: Detection, travel, production, and misunderstood background. Let us look at them one after the other.

Detection: The three experiments all use the same type of detector, a liquid scintillator with Gadolinium target. Neutrino-nucleus cross-sections are badly understood because neutrinos interact so weakly and very little data is available. However, the experimentalists calibrate their detectors with other radioactive sources in near vicinity, and no bumps have been seen in these reference measurements. This strongly speaks against detector shortcomings as an explanation.

Travel: An overall lack of particles could be explained with oscillation into a so-far undiscovered new type of ‘sterile’ neutrino. However, such an oscillation cannot account for a bump in the spectrum. This could thus at best be a partial explanation, though an intriguing one.

Production: The missing neutrinos and the bump in the spectrum are inferred relative to the expected neutrino flux from the power plant. To calculate the emission spectrum, the physicists rely on nuclear models. The isotopes in the power plant’s core are among the best studied nuclei ever, but still this is a likely source of error. Most research studies of radioactive nuclei investigate them in small numbers, whereas in a reactor a huge number of different nuclei are able to interact with each other. A few proposals have been put forward that mostly focus on the decay of Rubidium and Yttrium isotopes because these make the main contribution to the high energy tail of the spectrum. But so far none of the proposed explanations has been entirely convincing.

Background: Daya Bay and RENO both state that the signal is correlated with the reactor power which makes it implausible that it’s a background effect. There aren’t many details in the paper about the time-dependence of the emission though. It would seem possible to me that reactor power depends on the time of the day or on the season, both of which could also be correlated with background. But this admittedly seems like a long shot.

Thus, at the moment the most conservative explanation is a lacking understanding of processes taking place in the nuclear power plant. It presently seems very unlikely to me that there is fundamentally new physics involved in this – if the signal is real to begin with. It looks convincing to me, but I asked fellow blogger Tommaso Dorigo for his thoughts: “Their signal looks a bit shaky to me - it is very dependent on the modeling of the spectrum and the p-value is unimpressive, given that there is no reason to single out the 5 MeV region a priori. I bet it's a modeling issue.”

Whatever the origin of the reactor antineutrino anomaly, it will require further experiments. As Anna Hayes, a nuclear theorist at Los Alamos National Laboratory, told Fermilab’s Symmetry Magazine: “Nobody expected that from neutrino physics. They uncovered something that nuclear physics was unaware of for 40 years.”


Shantanu said...

Note that Fred Reines was in very poor health when he got the nobel Prize. In fact the actual Nobel lecture
was given by Bill Kropp (in his place for him).

Ervin Goldfain said...

I tend to agree with Tommaso that the Reactor Neutrino Anomaly may be an artifact of the fissile model. No apriori reason for why there is an excess at 5 MeV and a deficit elsewhere in the spectrum. Besides, neutrino physics is the least understood sector of the Standard Model, with a host of open questions on the mass hierarchy, precise knowledge of mixing angles, Dirac vs. Majorana, the reality of sterile neutrinos and so on. But I also think that it is too early to discard the possibility of a breakthrough, which is what everyone in hep-th is hoping for.

Uncle Al said...

Adopt the String Theory paradigm:
1) Postulate. A tau, muon, electron neutrino is a tau, muon, electron less its charge and self-energy. Given many fulcrums, there must be a see-saw mechanism.
2) Write theory, much theory, with a few hundred tuning parameters. When a contingency threatens to be detectable but isn't, draw a Yukawa potential graph and demand funding to probe below fundamental noise levels. Retune and recalculate to a few orders of magnitude lower signal. Repeat.
3)Demand the elegance and naturalness of theory is preponderant against allowing a Popper or even Bayes falsification.
4)Complain that the field lacks diversity, social justice, and biffies for ambiguous gender assignments.

One could seek a 5 MeV neutrino resonance or a forbidden transition being tickled, but that risks failure and is thus unfundable.

Tom Andersen said...

It looks interesting. But there are lots of anomalous results - especially with delicate neutrino measurements.

We had a 17 keV neutrino in our lab for a few years! - As measured by beta decay spectrum anomalies that persisted through various attempts at killing it.

akidbelle said...

Uncle Al:

Then should any real breakthrough be unfunded?

It reminds me of Einstein's magic year or even the thesis of a couple of bright students (de Broglie, Feynman). None of them had learnt the principles of their theories. Or was it just the luck of the beginner?


SteveB said...

Thanks for the interesting note.

You wrote: It would seem possible to me that reactor power depends on the time of the day or on the season, both of which could also be correlated with background.

Nuclear Power plants tend to be run at constant power providing a base load to a utility's overall electricity demand. They tend to use hydroelectric and gas turbines (perhaps purchased from a neighbor) to follow the changing load.

Pressurized water reactors fine tune their power output using chemical additives in the coolant and can change power only slowly. (Moving the control rods messes up the overall planned consumption of the fuel and would require many hours of recomputing the local burn up.) Boiling water reactors can do some load following and are known to do some seasonal adjusting.

Regardless there should be extremely accurate power production statistics. Also, every reactor shuts down for refuelling for several weeks every few years, rapidly dropping neutrino production to near zero, and that should be noticeable in the data.


Uncle Al said...

@akidbelle New folks find better theory when theory leaks exceptions. Aristotle vs. Galileo, Euclid vs. Bolyai then Thurston, Newton vs. relativity, quantum mechanics, and statistical mechanics. Otto Stern, Dirac equation failing for proton magnetic moment, then quarks. Penzias and Wilson, a microwave horn obserbvng empty sky, then cosmic microwave background. Particle theory was mirror-symmetric, then Yang and Lee. Davis, Bahcall, 100,00 gallons of perchloroethylene, and 2/3 too few neutrinos detected. Look.

Islam accepted all worldviews while Europe rotted within Roman Catholicism. Capitalism trouncing socialism. It was then administratively preponderant to adopt losers' strategies for their powers of physical and intellectual penury wielded over polities. Failure must be changed not enforced .

General relativity has three recent victories. Gravitational lensing is spot on, arXiv:1512.04654, doi:10.3847/2041-8205/819/1/l8. Kerr black holes exist to spec and gravitational waves work to spec, arXiv:1602.03837, doi:10.1103/PhysRevLett.116.061102, including trilateration to a simultaneous gamma pulse, arxiv:1602.03920.

Improving GR, string theory, quantized gravitation, SUSY, standard model, dark matter requires observing outside postulates - unfundable because this violates postulates. Empirical reality is not a business model.