Neutrino

In 1930, physicist Wolfgang Pauli introduced a neutral particle in order to explain the problem of energy going missing in the beta decay. He called this particle “neutron,” a combination of the root of the word “neutral” with the suffix “-on.” However another neutral but heavier particle was discovered in 1932 which was also given the same name. In 1933, physicist Enrico Fermi distinguished it from the heavier neutron by adding the Italian diminutive suffix “ino” to its name.

It took 26 years to prove its existence. Indeed only in 1956, Clyde Cowan and Frederick Reines observed neutrinos, with a revolutionary experiment, using the Savannah River nuclear reactor in South Carolina as source of beta decay.

Neutrinos come in three types, called flavors, that correspond to their charged-lepton partners: electron, muon and tau, until 1962 only one type of neutrino had been detected. An experiment at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL) led to the identification of another distinct type: the muon neutrino. The final type: the tau neutrino, was later observed in 2000 at Fermilab in the DONUT Experiment.

Neutrinos are in many ways one of the most interesting elementary particles since they have a very weak interaction with matter. This property makes them a unique probe of the deep universe: arriving undeflected on earth, they will help to pinpoint distant sources of powerful radiation, like quasars or gamma-ray bursts. Additionally, neutrinos can help to further investigate and understand the causes of catastophic events that occur in the universe, such as gravitational waves and supernovae.

Most neutrino experiments work in the same way: they detect neutrinos indirectly looking for the particles produced in their interactions. By-products of neutrino interaction with matter are highly energetic particles which travel faster than light in the detector material, emitting the so-called Cherenkov radiation which is detected by photomultipliers that convert light into electric signals.

Neutrinos, interacting weakly, can travel through matter without being deflected from their trajectory giving the possibility to investigate their production mechanism inside the core of the stars. The Sun is the first natural star to look at. It produces energy by the nuclear fusion of four protons into one Helium atom with the emission of neutrinos. The first experiment to study solar neutrinos was the Homestake experiment. Headed by Raymond Davis, Jr. It started in 1964 and it was located in the Homestake Gold Mine in South Dakota (USA). It detected only about half of the neutrinos expected from the current solar models opening the so-called ‘solar neutrino problem’.
The proposed explanation, along with the possibility of a wrong prediction of the neutrino flux coming from the Sun, was the hypothesis that neutrino can convert into another one during its flight from the Sun to the Earth. Furthermore oscillation is only possible if neutrinos have non-zero masses, properties not predicted by the Standard Model of particle physics.

SNO detector during construction
The Sudbury Neutrino Observatory

The solar neutrino problem was solved by SNO (Sudbury Neutrino Observatory) in 2002, showing the consistency of the total neutrino flux with the Standard Solar Model and proving the neutrino oscillation. This experiment, located in the Sudbury Neutrino Observatory in Canada, used 1000 tons of ultra-pure heavy water contained in a spherical acrylic vessel surrounded by an ultra-pure shield Cherenkov detector. The heavy water is viewed by approximately 9,600 Hamamatsu R1408 photomultiplier tubes (PMTs) mounted on a sphere.

Super-Kamiokande was the first experiment to prove neutrino oscillation by observing atmospheric neutrinos: it measured significantly more muon neutrinos coming from the surface direction than those first traversing the Earth. This removed even the last doubts about neutrino oscillation. Located 1,000 m underground in the Mount Ikeno at the Kamioka Observatory of the Institute for Cosmic Ray Research in Japan, the detector has a cylindrical shape, 41.4 m tall and 39.3 m in diameter, holding 50 ktons faced by 11,146 Hamamatsu R3600 50 cm diameter PMTs. In 2015 Professor Takaaki Kajita along with Arthur B. McDonald were awarded of the Nobel Prize in Physics "for the discovery of neutrino oscillations, which demonstrates that neutrinos have mass".

Katrin spectrometer during the transportation to the Karlsruhe Institute of Technology.
KATRIN Karlsruhe Trinitum Neutrino Experiment

Oscillation experiments cannot provide information on the exact neutrino masses, unfortunately. The present knowledge of the masses comes from cosmology according to which the masses are smaller than 2eV. The Karlsruhe Tritium Neutrino (KATRIN) experiment will measure the electron neutrino mass with a direct measurement, studying tritium β-decay. Furthermore, the origin of neutrino mass remains unknown which is closely related to the question whether neutrinos are their own antiparticles or not: so-called Majorana or Dirac particles.

The answer to this question could be found looking at the decay of some isotopes that don’t undergo ordinary β-decay because of energy conservation. These isotopes can only decay by the simultaneous decay of two neutrons into protons via the emission of two electrons and two anti-electron neutrinos. If neutrinos are their own antiparticles, the first anti-neutrino can be absorbed as a neutrino inducing the second β-decay - resulting in the emission in the final state of only the two electrons. This decay is called neutrino-less double β-decay.

Neutrinos are also produced in very energetic processes, for example in Supernovae or in the center of galaxies. On February 23 1987, the neutrinos emitted from the supernova explosion SN1987A in the Large Magellanic Cloud, about 160,000 light-years away, reached the earth. Kamiokande, predecessor of Super-Kamiokande, detected 11 neutrinos coming from the SN. In 2002, Dr. Masatoshi Koshiba who led the Kamiokande experiment, was awarded of a Nobel Prize in Physics for this result. The number of events and the energies of the neutrinos it was possible to prove Supernovae models.

Another open question in particle physics and astrophysics is the origin of cosmic ray, mainly protons and nuclei with energies between 109 and 1021 eV. Neutrinos can be produced when cosmic rays interact with matter. So-called neutrino telescopes are built to investigate where the cosmic radiation comes from. Recent experiments use water or Antarctic ice such as ANTARES, KM3NeT or IceCube as a target material.

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