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The MiniBooNE detector has found new evidence supporting sterile neutrinos

Photo of MiniBooNE Photomultiplier

Ryan Patterson / Princeton / Fermilab

The MiniBooNE group recalculated the "advantages" of electron and antineutrino neutrinos, including in the analysis of data collected in 2013-2017. As a result, the statistical significance of the effect has grown to 6 sigma, that is, it cannot be attributed to randomness. These results indicate the presence of sterile neutrinos, which can increase the oscillation frequency. Article published on Physical Review Letter and available for free, briefly informing about it Physics.

In the Standard Model, neutrinos are considered to be massless and divided into three generations with a fixed number of leptons (electrons, muons, and neutrinos), but in reality they are far more complex. In weak interactions – for example, in the beta decay of radioactive nuclei – neutrinos participate as taste states (states with a fixed lepton number). However, in free space, it is more appropriate to represent them in the form of a quantum superposition ("mixture") of three components with a fixed mass – called Hugh stated. When particles move in space and time, the wave function of each component oscillates, and the frequency of oscillations is inversely proportional to the mass of the component (the wave function of the free particle (t,x) = Akexp[i[i[i[ikr – iEt / ħ]where energy E = ħ2k2/ 2 m). Therefore, the probability of detecting neutrinos in certain states of feeling at different points in different spacetime. It turns out that neutrinos, which are initially electronic, after some time will turn into muons or know neutrinos, and the probability of transformation is proportional to the square of the difference in mass mass. This is what is called an oscillating neutrino; In 2015, Takaqi Kajita and Arthur MacDonald, who discovered this process and solved the "lack" problem of solar neutrinos, won the Nobel Prize in Physics. You can read more about neutrino oscillations in the material "H means neutrinos" or listen to stories by physicist Mikhail Danilov.

Neutrino oscillations make it possible to explain most measurements related to the physics of these particles, but in some cases experiments deviate from theory. In 1995, the LSND detector (Liquid Scintillator Neutrino Detector) found that muon neutrinos were transformed into electronics that were slightly faster than those that would follow calculations with known mass mass. Ten years later, a similar difference was noted in the MiniBooNE detector, which had nothing to do with LSND. The easiest way to explain this difference is to add sterile neutrinos to theories that are not involved in the interaction of the Standard Model. In this case, one more will be added to the three existing mass states, and the oscillation frequency will change. However, in the past ten years, theoretical physicists have developed several alternative methods. In addition, several experiments on the search for sterile neutrinos are negative – for example, the IceCube detector does not include the possibility of their existence with a probability of around 99 percent. However, the reliability of this statement is less than three sigma, and in physics it is customary to wait for deviations of more than five sigma before confidently announcing the discovery. Therefore, the question about the existence of sterile neutrinos remains open.

The MiniBooNE group re-examined and improved their results, after increasing statistics on neutrino oscillations more than ten times. At present, the MiniBooNE detector is a large diameter ball of about 12 meters, which is filled with 820 tons of pure mineral oil (CH).2) and viewed 1500 photomultiplier tubes. When neutrinos collide with oil molecules, it "drops" the charged particle, which in turn produces Cherenkov radiation and isotropic light from the scintillator. Photomultipliers record both types of flares, and electronically restore energy, lepton loads and other neutrino parameters from them. The particle source for the detector is the main injector from the National Accelerator Laboratory named after Enrico Fermi (Fermilab), where high-energy protons (energy of about eight gigayelectronvolt) collide with lithium beryllium targets. Charged pawn, which was born at the collision, was focused by a magnetic field and rapidly decayed into muon neutrinos or antineutrinoes. Over fifteen years of observation, which lasted from 2002 to 2017, protons collided with a target of around 2.7 × 1021 times Scientists have collected most of this data over the past four years, so since previous publications, the number of registered neutrinos has almost doubled.

Using the same method as in the previous publication, the scientists found that the "excess" of electron neutrinos in the energy range of 200–1250 MeV was 381 ± 85 events. If we add to this "excess" number of electron antineutrinos, deviations from the prediction of the model with three neutrinos will be 461 ± 99 events. The statistical significance of this result is around 4.7 sigma. In addition, he agrees with the LSND detector measurement, and if you combine the two results, the significance of the deviation will increase to 6.0 sigma (the probability of error is around 10−9) This means that effects cannot be attributed to randomness, as can be done for the first experiment with a significance of 1.0 sigma sequence.

However, the authors of the article are still in no hurry to declare 100% confirmation of the sterile neutrino theory. The excess events observed can still be attributed to other, more exotic theories. For example, one hypothesis states that the "extra" flashes in the detector can produce neutral pawns, which are also born from neutrino scattering and decay into photons. This flash is very similar to flashing charged particles, and can be confusing. However, the MiniBooNE group takes this possibility into account, estimating the contribution of neutral pawns and showing that it cannot explain such strong deviations. Nevertheless, it is possible to speak with confidence about this only when contributions from neutral pawns will be independently studied in other detectors, for example, DUNE.

In April this year, the MiniBooNE group measured the neutrino parameters for the first time with the exact energy of 236 MeV. The monoenergetic neutrinos participating in the measurements were emitted as a result of the resting of the kaon. In previous experiments, neutrino energy was never known before; this time, the researchers were able to improve this energy because of the successful geometry of the experimental arrangement.

Dmitry Trunin

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