What is the most common subatomic particle
|The standard model of particle physics|
Neutrinos are subatomic particles that are created when radioactive elements decay and that do not carry any electrical charge. After the light particles, they are the most common particles in the universe: every cubic centimeter of the cosmos contains 340 neutrinos, which are relics of the Big Bang. Their unique advantage stems from a fundamental property: they are only affected by the weakest of the forces of nature (other than gravity).
The weak interaction of neutrinos with matter makes them particularly valuable as astronomical messengers. In contrast to photons or charged particles, neutrinos can escape deep inside their sources and traverse the entire universe unhindered. They are neither deflected by interstellar magnetic fields nor absorbed by intervening matter. But it is precisely because of this property that cosmic neutrinos are so difficult to detect. You need huge devices if you want to catch them in sufficient numbers and trace their origins.
|Detectors show the types of neutrinos νe , νµ , νt (the so-called flavors), which in turn mixes|
of mass states (ν1 , ν2 , ν3) are. The differences between the masses have been measured,
but the masses themselves are not known. Nor is it known whether the biggest difference is between
the most severe and the second most severe or between the second most severe and the lightest condition.
(Source: Berkeley Lab).
We know three types (Flavors - from English: taste) of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. In the standard model, neutrinos have no mass. A major breakthrough in the last decade was the discovery that neutrinos do have mass. The assumption of massless neutrinos became questionable when electron neutrinos were shown to disappear on their way from the sun to earth. Today we know that electron neutrinos transform into one of the other varieties, and that other variety originally escaped detection. A similar effect occurs in the earth's atmosphere: neutrinos produced by cosmic rays change their identity on their journey to the detection device. This change is called neutrino oscillation or mixing, and it can only occur when neutrinos have mass. Therefore, these oscillations are the first physical harbingers that point beyond the standard model. Experiments on nuclear reactors and accelerators have confirmed these findings and also demonstrated regardless of the strength with which the three types of neutrinos mix with one another. Unfortunately, these oscillations do not give us the values of these three masses, but only the squares of the mass differences.
| The standard beta decay with a tritium atom is shown on the left. In the standard double beta decay (center), two neutrons are converted into protons, with the emission of two electrons (beta particles) and two neutrinos. If the neutrinos are their own antiparticles, however, an antineutrino emitted during one decay can be absorbed as a neutrino in the second decay process and then lead to a neutrino-free double beta decay (right).|
(Left side, source: KATRIN collaboration website)
(Middle and right, source: Berkeley Lab)
The mass differences between different neutrino states can be derived from the oscillation pattern, but not the absolute values of their masses and also not the mass hierarchy (is?3 the heaviest or the lightest?). One tries to determine the absolute values of the neutrino mass by measuring the electrons that are formed during the beta decay of tritium, in which a neutron within a nucleus is converted into a proton, an electron and an antineutrino. From these experiments we know that the heaviest of the three neutrinos is lighter than 2.3 eV, that is about a four-millionth of the electron mass.
Neutrinos as their own antiparticles: Majorana neutrinos?
Another group of experiments is looking for the “neutrino-free double beta decay”. In doing so, we may be able to find out whether neutrinos are their own antiparticles (Majorana neutrinos) - a discovery that goes far beyond the precision measurement of their absolute mass. If there really were Majorana neutrinos, the consequences would be dire. For example, Majorana neutrinos are a prerequisite for the creation of an excess of matter over antimatter in the cosmos in the early universe via a process known as leptogenesis.
|Star rings around SN 1987A - in the middle of the inner one|
Around the ejection of the supernova explosion
(Source: The Hubble Heritage Team (AURA / STScI / NASA))
(Close-up: Dr. Christopher Burrows, ESA / STScI and NASA)
High-energy neutrinos must arise as a by-product of high-energy collisions between charged cosmic rays and matter. Since they can escape from celestial bodies that are so dense that light cannot come out, they may indicate processes that traditional astronomy cannot. Nevertheless, their extremely low probability of interaction makes their detection extremely difficult at the same time.
Solar neutrino detectors are placed deep underground to shield them from noise that might simulate their rare interactions. Only neutrinos can penetrate deep enough to reach these devices undisturbed. Although neutrino astronomy is in the energy range of 1 MeV (1 MeV = 106 eV) through the impressive observation of solar neutrinos and neutrinos from the supernova SN 1987A, but neutrinos with energies of 1 GeV (1 GeV = 109 eV) and more that have to occur during the formation of high-energy cosmic rays have not yet been discovered.
Around the weak fluxes from the presumed distant sources of high-energy neutrinos of 1 TeV (1 TeV = 1012 eV) and 1 PeV (1 PeV = 1015 eV) requires huge detectors with a volume of one cubic kilometer or more. Since they cannot be set up underground, these expandable arrangements are set up deep in open water - seas or lakes - or in the ice, where there is enough space.
|The IceCube Neutrino Observatory at the South Pole|
(Source: Danielle Vevea / NSF & Jamie Yang / NSF)
|The ANTARES detector in the Mediterranean|
(Source: J.A. Aguilar)
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