Evidence of the existence of a new fundamental particle

The experiment in the national accelerator laboratory. Enrico Fermi (Fermilab) near Chicago discovered much more electron neutrinos than was predicted. This event may be the herald of a completely new elementary particle, a sterile neutrino , although many physicists remain skeptical.

In the MiniBooNE reservoir , photocells capture light that appears when neutrinos interact with atomic nuclei.

Physicists are surprised and shocked by the new Fermlab neutrino experiment report . The MiniBooNE experiment found far more neutrinos of a certain type than expected - and the easiest way to explain this phenomenon is the existence of a new elementary particle: a sterile neutrino, an even more secretive and strange particle than the three known types of neutrinos. The result, apparently, confirms the anomalous results of the old experiment, to confirm which MiniBooNE was built.

The persistence of the neutrino anomaly makes physicists very happy, said Scott Dodelson of Carnegie Mellon University. This "says that something really interesting is happening ," added Anzi Slozar from the National Laboratory in Brookhaven.

But what exactly, no one can say.

“I was very interested in the result, but I’m not ready to shout“ Eureka! ”Said Janet Conrad , an expert in neutrino physics from MIT, and a member of the MiniBooNE collaboration.

The existence of sterile neutrinos would revolutionize physics at all levels. It would finally break the Standard Model of Particle Physics, which has been held since the 1970s. It would also require "a new standard cosmology model," said Dodelson. “There are other cracks in the standard picture of the world,” he added. “The neutrino paradox can open the way for a new, improved model.”

Neutrinos are tiny particles, and billions of them pass through our bodies every second, but rarely interact with them somehow. They constantly oscillate between three "grades" - electronic, muon and tau. In the MiniBooNE experiment, a beam of muon neutrinos fires in the direction of a giant oil tank. On the way to the tank, some of the muon neutrinos should be converted into electron with a speed determined by their mass difference. Then MiniBooNE tracks the arrival of electron neutrinos that produce characteristic bursts of radiation in those rare cases when they interact with oil molecules. For 15 years of operation, MiniBooNE has registered several hundred more electron neutrinos than expected.

The simplest explanation for this unexpectedly large number is that some muon neutrinos oscillate into another, heavier neutrino sort — sterile, which does not interact with anything other than neutrinos — and that some of these heavy sterile neutrinos then oscillate into electronic ones. The large difference in mass leads to more oscillations and more detections.


The MiniBooNE tank is 12 meters in diameter and lined with 1520 photocells

A liquid neutrino scintillation counter (LSND) at Los Alamos discovered a similar anomaly in the 1990s, which is why the construction of the MiniBooNE was required. However, in other experiments with neutrinos, which operate according to principles other than LSND and MiniBooNE, they could not find clear signs of the presence of supposed sterile neutrinos. “This is the curse of our work - some experiments see something, while others do not see it,” said Werner Rodejohan from the Max Planck Institute for Nuclear Physics.

If the explanation of the new results will indeed be sterile neutrinos, then physicists cannot yet understand how to combine the properties of these new particles with everything else that we know. Perhaps the most unpleasant fact is that when observing light coming to us from the early Universe, it is said that at that time there were only three kinds of neutrinos. To understand the meaning of the results of LSND, MiniBooNE and other experiments, “some kind of completely new theoretical platform is required,” said Slozar.

Moreover, specifically those sterile neutrinos that are hypothetically able to fit into the data obtained on the MiniBooNE do not solve any riddles, because of which physicists in general began to build theories about the existence of such particles. Sterile neutrinos, being quite heavy, could explain the invisible "dark matter", which, apparently, envelops the galaxies. They would explain why electron, muon and tau neutrinos are so light, with the help of a mathematical trick called the seesaw mechanism. But with a mass of less than 1 eV, the alleged sterile neutrinos on the MiniBooNE lack the masses for the described purposes. “We wouldn’t have reason to expect the presence of sterile neutrinos of [mass in] 1 eV,” said Matthew Buckley , an expert in particle physics from Rutgers University. “But it’s not that the Universe in the past stopped it from adding new particles to us.”

The confusion has led many experts to hold back optimism and begin to suspect that MiniBooNE and LSND have fallen victim to an unknown error. Freya Bleckman , a physicist at the Free University of Brussels, argues that the experiments could systematically underestimate the rate at which neutral peonies disintegrate in the MiniBooNE oil tank — and these events mimic signals from electronic neutrinos.

"It is clear that there is a need to sort out something, and I hope that this will be the fourth neutrino," said Neil Weiner , a theoretical physicist at New York University. “Considering this, let us remember that this would be the first particle discovered outside the Standard Model, therefore the threshold for evidence of its existence is obviously very high.” So far, he said: “I am inclined to the“ wait and see ”approach.

A more definite answer will appear in future experiments, including IsoDAR , proposed by Konrad and many of her colleagues. Instead of counting the number of neutrinos of a certain sort at the end of the beam, he will see how the neutrinos oscillate here and there, between different varieties, during the journey, which will give a more complete picture of the oscillations. “I would not yet put my money on this project, since this excess of neutrinos is just a spot on the graph,” said Konrad. “What if the stain could be caused by something else?” To truly see this, I need to see these predicted fluctuations with good statistical significance. ”