Neutrino

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The neutrino (a term which in Italian means 'small neutron'), discovered by Clyde Cowman and Federick Reines, is a subatomic particle of the fermionic type, without charge and with spin ½. Since the beginning of the XXI century, after several experiments carried out at the facilities of the Sudbury Neutrino Observatory (SNO), in Canada, and in the Super-Kamiokande in Japan, among others, it is known, contrary to the electroweak model, that these particles have mass, but it is very small, and that it is very difficult to measure it. As of 2016, the upper bound of neutrino mass is 5.5 eV/c², which is less than one billionth of the mass of a carbon atom. hydrogen. Their conclusion is based on the analysis of the distribution of galaxies in the universe and is, according to these scientists, the most accurate measurement so far of the mass of the neutrino. In addition, their interaction with other particles is minimal, so they pass through ordinary matter with hardly any disturbance.

A phenomenon known as neutrino oscillation, that is, the possibility of transformations between the three existing types of neutrinos, implies that neutrinos have mass, and this has important consequences in the standard model of particle physics. In the standard model neutrinos have no mass and therefore must be modified.

In any case, neutrinos are not affected by the electromagnetic force or the strong nuclear force, but they are affected by the weak nuclear force and by the gravitational force.^{[citation needed]}

History

The existence of the neutrino was proposed in 1930 by the physicist Wolfgang Pauli to compensate for the apparent loss of energy and linear momentum in the β-decay of neutrons according to the following equation:

{displaystyle mathrm {n} rightarrow mathrm {p} ^{+}+mathrm {e} ^{-}+{bar {nu }}_{e},}

Wolfgang Pauli interpreted that both mass and energy would be conserved if a hypothetical particle called "neutrino" participated in disintegration by incorporating lost quantities. Unfortunately, this hypothetically planned particle had to be without mass, neither load, nor strong interaction, so it could not be detected with the media of the time. This was the result of a very small effective section ( ${displaystyle sigma _{mu }sim 10^{-44}{text{cm}}^{2}}$ ). For 25 years, the idea of the existence of this particle was only hypothetically established.

In fact, the possibility of a neutrino interacting with matter is very small since, according to quantum physics calculations, a block of lead with a length of one light year (9.46 trillion kilometers) would be necessary. to stop half the neutrinos going through it.

In 1956 Clyde Cowan and Frederick Reines demonstrated its existence experimentally. They did this by bombarding pure water with a beam of 10¹⁸ neutrons per second. They observed the subsequent emission of photons, thus determining their existence. This trial is called the neutrino experiment.

In 1962 Leon Max Lederman, Melvin Schwartz, and Jack Steinberger showed that there was more than one type of neutrino when they first detected the muon neutrino. In the year 2000 the discovery of the tauonic neutrino was announced by the DONUT Collaboration at Fermilab. Its existence had already been predicted, since the results of the decay of the Z boson measured by LEP at CERN were compatible with the existence of 3 neutrinos.

In September 2011, the OPERA collaboration announced that analysis of measurements for the speed of neutrinos in their experiment yielded superluminal values. In particular, the speed of a certain class of neutrino could be 0.002% greater than that of light, constituting the superluminal neutrino anomaly, in contradiction with the theory of relativity. However, the same body recognized months later that at the time of measuring the distance traveled by the neutrinos there was a failure in the positioning system (GPS), having a disconnected cable, so the measurement of the superluminal speed has been discarded.

Classes

There are three types of neutrinos associated with each of the leptonic families (or flavors): electronic neutrino ( ${displaystyle nu _{e}}$ ), muonic neutrino ( ${displaystyle nu _{mu }}$ ) and tauonic neutrino ( ${displaystyle nu _{tau }}$ ) plus their respective anti-particles.

Neutrinos can pass from one family to another (ie, change flavor) in a process known as neutrino oscillation. The oscillation between the different families occurs randomly, and the probability of change seems to be higher in a material medium than in a vacuum. Given the randomness of the process, the proportions between each of the flavors tend to spread equally (1/3 of the total for each type of neutrino) as successive oscillations occur. It was this fact that allowed us to consider the oscillation of neutrinos for the first time, since when observing the neutrinos coming from the Sun (which should be mainly electronic) it was found that only a third of those expected arrived. The missing two-thirds had oscillated to the other two flavors and were therefore not detected. This is the so-called "solar neutrino problem."

The oscillation of neutrinos directly implies that they have a non-zero mass, since the transition from one flavor to another can only occur in massive particles. This is due to the fact that, for particles of zero mass, the proper time is zero, which implies that from the point of view of the particle the entire trajectory is traveled at the same instant of time, which does not allow for a change. of state at some point in it.

Astrophysical implications of its mass

In the standard model, the neutrino was initially considered a massless particle. In fact, in many ways it can be considered zero mass, since it is at least ten thousand times less than that of the electron. This implies that neutrinos travel at speeds very close to the speed of light. For this reason, in cosmological terms the neutrino is considered hot matter, or relativistic matter. In contrast, cold matter would be non-relativistic matter.

In 1998, during the 0-mass neutrino conference, the first papers showing that these particles have a negligible mass were presented. Prior to these works, it had been considered that the hypothetical mass of neutrinos could have an important contribution within the dark matter of the universe. However, it turned out that the mass of the neutrino was insufficient, too small to even be taken into account in the enormous amount of dark matter that is estimated to be in the universe. On the other hand, the models of cosmological evolution did not agree with the observations if hot dark matter was introduced. In this case the structures were formed from larger to smaller scale. While observations seemed to indicate that clumps of gas formed first, then stars, then protogalaxies, then clusters, clusters of clusters, etc. The observations, then, fit with a model of cold dark matter. For these two reasons, the idea that the neutrino contributed significantly to the total mass of the universe was discarded.

Fonts

The Sun

Generation of solar neutrinos in proton-proton chains.

Artificial sources

The main sources of artificial neutrinos are nuclear power plants, which can generate about 5·10²⁰ anti-neutrinos per second, and to a lesser extent, particle accelerators.

Astrophysical phenomena

SN 1987A.

Cosmic background radiation

It is believed that, like the background microwave radiation from the Big Bang, there is a background of low-energy neutrinos in our Universe. In the 1980s it was proposed that these may be the explanation for the dark matter thought to exist in the universe. Neutrinos have one important advantage over most dark matter candidates: we know they exist. However, they also have serious problems.

From particle experiments, neutrinos are known to be very light. This means that they move at speeds close to the speed of light. Thus, dark matter made of neutrinos is called "hot dark matter." The problem is that, being in rapid motion, neutrinos would have tended to spread uniformly in the Universe, before cosmological expansion cooled them enough to concentrate in clusters. This would cause the part of dark matter made of neutrinos to expand, unable to form the large galactic structures we see.

In addition, these same galaxies and galaxy groups appear to be surrounded by dark matter that is not fast enough to escape these galaxies. Presumably, this matter provided the gravitational core for the formation of these galaxies. This implies that neutrinos make up only a small part of the total amount of dark matter.

From cosmological arguments, relic neutrinos (from the low energy background) are estimated to have a density of 56 per cubic centimeter, and to have a temperature of 1.9 K (1.7×10−4 eV), that is, if they do not have mass. Otherwise, they would be much colder if their mass exceeds 0.001 eV. Although its density is quite high, due to the extremely low neutrino cross sections at energies below 1 eV, the low-energy neutrino background has not yet been observed in the laboratory.

In contrast, boron-8 solar neutrinos, which are emitted with higher energy, have been definitely detected despite having a lower spatial density than relic neutrinos, by about 6 orders of magnitude.

Earth and atmosphere

Beta-decay reactions of terrestrial radioactive isotopes provide a small source of neutrinos, which are produced as a consequence of the natural background radiation. In particular, the decay chains of 238.92U and 232.90Th, as well as 40.19K, include beta-decays that emit anti-neutrinos. These so-called geoneutrinos can provide valuable information about the interior of the Earth. A first indication of geoneutrinos was found by the KamLAND experiment in 2005. KamLAND's main antecedents in geoneutrino measurement are anti-neutrinos coming from reactors. Several future experiments aim to improve geoneutrino measurement and these will necessarily have to be far from the reactors.

Detectors

When the nuclear reactions that occur in the Sun were known exactly, it was calculated that an appreciable flux of solar neutrinos had to cross the Earth at every instant. This flux is enormous, but neutrinos barely interact with ordinary matter. Even the conditions inside the Sun are "transparent" to these. In fact, a human being is traversed by billions of these tiny particles every second without being aware of it. So it was difficult to conceive of any system that could detect them.

Scintillator-based detectors

The first particles of this type ever detected were electron antineutrinos emitted by the nuclear reactor at the Savannah River plant in Georgia (USA), which thanks to the experiment of Frederick Reines and Clyde Cowan (Reines-Cowan experiment), could be directly observed by using two “targets” of cadmium chloride dissolved in water. The protons in the water were the targets of the (anti)neutrinos: if they had an energy of more than 1.8 MeV they were capable of causing a charged current interaction (CC) called “inverse beta decay”, which would result in positrons and neutrons:

${displaystyle {bar {nu _{e}}}+p^{+}longrightarrow n^{0}+e^{+}}$

The positrons would rapidly annihilate with electrons from the environment, giving rise to a fast signal consisting of two coincident 511 keV photons. This was the fast scintillation signal, and could be detected with two scintillation detectors placed above and below the 'target' tank. The dissolved cadmium ions in the water were targeted by the neutrons, which once thermalized had a high probability of being captured by said atomic nuclei, resulting in a "delayed" signal (relative to the fast positrons)., with emission of gamma rays of about 8 MeV, which were detected a few microseconds after the positron annihilation signal. The experiment proved the existence of neutrinos, but did not aim to measure the total flux, since only about 3% of the antineutrinos produced by a typical nuclear reactor have sufficient energy (>1.8 MeV) enough to give rise to an inverse beta decay reaction.

More recently, much larger and more sophisticated detectors use the scintillation system, not only in observing neutrinos but also for other targets. KamLAND, for example, uses scintillation detection to study the antineutrino oscillations of 53 Japanese nuclear reactors. Borexino is a detector that uses the liquid organic scintillator (pseudocumene with diphenyloxazol) with the lowest concentration of radioactive elements of any material in the world. Thanks to it, it is capable of detecting and separating neutrino components from the Sun (the most important natural source of neutrinos), through elastic scattering of low-energy neutrinos. against delocalized electrons in the benzene ring orbitals of the aromatic molecules of their scintillator, mediated by charged current interactions (CC, mediated by W^± bosons) for electron neutrinos, and in to a lesser extent by neutral current interactions (NC, mediated by the Z0 neutral boson) for the other neutrino flavors (muon and tauon):

${displaystyle nu _{e}+e^{-}longrightarrow nu _{e}+e^{-}}$ (charged current interaction)

${displaystyle nu _{x}+e^{-}longrightarrow nu _{x}+e^{-}}$ (neutral current interaction, where x=e,μ,τ)

Borexino is also sensitive to the inverse beta decay reaction to observe antineutrinos from nuclear reactors around the world, coming from within the Earth itself, or from radioactive material concentrated near the detector, such as the antineutrino generator for the study of the short-distance oscillations of his experimental SOX program.

Detectors based on radioactive (radiochemical) processes

However, in 1967 Raymond Davis managed to come up with a detection system. He observed that chlorine-37 was capable of absorbing a neutrino to become argon-37 as shown in the following equation:

{displaystyle {}^{37}mathrm {Cl} +nu _{e}rightarrow {}^{37}mathrm {Ar} +mathrm {e} ^{-},}

Of course, this was not the only reaction between neutrinos and ordinary matter. What was special about chlorine-37 is that it met certain requirements for use in a future detector.

(a) The effective section of chlorine-37 interaction with a neutrino is quite large which implies a greater likelihood that such a reaction occurs
(b) The argon-37 is radioactive so it is possible to detect its presence due to its emissions
(c) The chlorine-37, although not the most abundant chlorine isotope, is very easy to obtain.

Normally chlorine-37 appears mixed with other isotopes. Particularly with chlorine-35, the most abundant. In addition, it can be mixed with other atoms or molecules, always knowing their proportion. To avoid false measurements due to argon-37 already present in the mix, the first step was to clean the product. Once this was done, the chlorine-37 mixture should be left to rest for a few months until it reached a stationary situation. This is when the amount of argon that decays equals the amount that is formed. The equilibrium moment will be determined by the half-life.

To protect the detector from background noise produced by cosmic radiation, tank¹ of the chlorinated mixture was buried deep in a South Dakota gold mine. However, the first observations only gave upper bounds, still compatible with zero². The results were lower than expected and were confused by the noise. After repeated increases in the sensitivity of the instruments and in the purity of the chlorine-37 mixture, it was finally possible to calculate that approximately one third of the expected flow ³ reached us. These results were not taken very seriously at first, so they continued to experiment with better but also more expensive mixtures based on gallium or boron.

¹The tank contained 380,000 litres of tetrachloroethylene, a fluid frequently used in dry cleaners.

²The initial sensitivity of the detector was intended to detect the expected flow of solar neutrinos. But being this below the accuracy of the system initially only a top quote was obtained.

³An average of one and a half neutrino was expected captured every day. But the result was only half a neutrino a day.

Detectors based on the Cherenkov effect

Doubts about the methods used by Davis encouraged the search for alternatives for the detection of such elusive particles. Thus a new line of detectors arose that were based on the collision of neutrinos with electrons contained in an aqueous medium.

{displaystyle nu _{e}+e^{-}rightarrow e^{-}+nu _{e},}

These detectors are based on the fact that when the neutrino hits an electron, it transmits part of its momentum, giving it a speed sometimes greater than that of light in that same aqueous medium. It is at this moment that a characteristic light emission occurs, known as Cherenkov radiation, which is captured by the photomultipliers that cover the walls of the container. So that when the neutrino impacts against the electron that orbits the atom of the interaction matter, it generates a moment of inertia, causing an instantaneous acceleration that, in the face of friction due to collision, causes an instantaneous power differential that produces said momentary light emission. Since what is observed is a transmission of linear momentum, we can approximately infer their mass and the direction from which they come, while with the previous detection system we could only calculate the neutrino flux.

Super Kamiokande

It is the most famous neutrino detector. It receives its name from the Japanese mine in which Kamioka is located at a depth of 1,000 meters. It consists of a cylinder 39.3 meters in diameter and 41 meters high whose walls are covered with 11,200 multipliers to detect the light of the Cherenkov effect. It is filled with 50,000 tons of pure water that serve to cause interaction with neutrinos. The first thing that was done was to detect the neutrinos coming from supernova 1987A. Then the flux of solar neutrinos was measured, corroborating the results of the Davis detector. It was with the supernova experiment with which the laboratory became famous by being able to determine that the mass of the neutrino was not null, limiting its value (not measuring it exactly) from the measurement of the delay with which the neutrinos arrived. coming from the explosion. If these had lacked mass, they would have arrived together with the photons (the light of the supernova). But what has brought them world fame have been the experiments that demonstrate the oscillation of neutrinos and for which their director Takaaki Kajita received the 2015 Nobel Prize in Physics together with the director of the Subdury Neutrino Observatory in Canada.

Sudbury Neutrino Observatory (SNO)

This neutrino detector consists of a 17.8-meter-diameter sphere located at a depth of 2,100 meters in the Creighton mine, in Subdury, Ontario, Canada. Instead of conventional water, heavy water is used because it is more likely to interact with neutrinos, enclosed in an acrylic sphere 12 meters in diameter and with a capacity of 1000 tons. Around this container, until the detector is filled, there is pure normal water to give it flotation and as an anti-radiation shield. Its results also demonstrate the phenomenon of neutrino oscillation, for which its director Arthur B. McDonald also received the Nobel Prize in Physics in 2015.

Superluminal neutrino anomaly

In 2011, the OPERA experiment mistakenly observed that neutrinos seemed to travel faster than light. Even before the error was discovered, the result was considered anomalous because speeds greater than the speed of light in a vacuum are believed to violate special relativity, a cornerstone of modern understanding of physics for more than a century.

After discovering errors that could have affected the speed measurement, the results were refuted. In 2012, experiments were carried out again that concluded that the speed of the neutrino was consistent with that of light.

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