Neutrino

Neutrino

Neutrinos are electrically neutral elementary particles with very low mass. In the Standard Model of elementary particle physics, there are three types ( generations) of neutrinos: electron, muon and tau neutrinos. Each neutrino generation consists of the neutrino itself and its anti-neutrino. The name neutrino was proposed by Enrico Fermi for the first discovered electron-neutrino and means ( according to the Italian diminutive ino ) small neutron.

For interaction of neutrinos with matter, see, unlike the other known elementary particles, only the weak interaction processes take place. Reactions thus take place in comparison to the electromagnetic and strong interactions relatively rare. A single event - if it occurs - can still release large amounts of energy. A beam of neutrinos also goes by thick layers - for example, through the whole earth - almost through without weakening. Accordingly demanding is the detection of neutrinos in experiments.

• 3.1 generating reactions

• 4.1 Astrophysics
• 4.2 Neutrino detectors

History of Research

When radioactive beta-minus decay initially only one emitted electron was observed. Together with the remaining core it seemed therefore to be a two-body problem (see also kinematics ( Teilchenprozesse ) ). Thus, the continuous energy spectrum of the beta electrons could only be explained if one assumed a violation of conservation of energy. This led Wolfgang Pauli to adopt a new elementary particle, which is emitted by the detectors unobserved simultaneously with the electron from the nucleus. This particle carries a portion of the energy released during the decay of it. In this way, the electrons of the beta radiation to different received much kinetic energy, without the energy condition is violated.

Pauli suggested in a letter dated December 4, 1930 in front of this hypothetical particle, which he called first neutron. Enrico Fermi, who worked out a theory of the basic properties and interactions of this particle, called it in neutrino (Italian for " little neutron ", " Neutrönchen " ) in order to avoid a conflict with the particles now known under the same name. It was not until 1933 Pauli presented his hypothesis to a wider audience and raised the question of a possible experimental verification.

Pauli assumed that the neutrino is extremely difficult to prove. In fact, the first observation was only 23 years later, they succeeded in 1956 in one of the first major nuclear reactors with the Cowan - Reines neutrino experiment. The researchers sent on June 14, 1956 Wolfgang Pauli a telegram with the success message after Zurich. A nuclear reactor emitted by the beta decay of the fission products neutrinos (more precisely, electron antineutrinos ) with much higher flux density than would be achievable with a radioactive preparation. Pure and Cowan used to detect the antineutrinos the following particle reaction (so-called inverse beta decay ):

An anti- neutrino strikes a proton, producing a positron and a neutron. These reaction products are both relatively easily observable. For this discovery Reines received the 1995 Nobel Prize in Physics.

The muon neutrino was discovered in 1962 by Jack Steinberger, Melvin Schwartz and Leon Max Lederman with the first produced at the accelerator neutrino beam. They received the Nobel prize for the year 1988. Using the muon neutrino, a second -generation neutrino was known, which is the analogue of the electron-neutrino for muons. Short term was for the muon neutrino, the term Neutretto in use ( - etto is also an Italian diminutive form ), but was not widely prevalent. When in 1975 the tauon was discovered, physicists expected also an associated neutrino generation, tauon - neutrino. The first signs of the existence of which was the continuous spectrum in tauon decay, similar to the beta decay. In 2000, the tau neutrino was detected for the first time directly on the DONUT experiment.

The 1993 current and 1998 LSND experiment at Los Alamos has been interpreted as evidence for the existence of sterile neutrinos, however, was controversial. After the Karlsruhe- Rutherford Medium Energy Neutrino ( KARMEN ) experiment under the leadership of the Forschungszentrum Karlsruhe in the UK Rutherford laboratory, the results could not reproduce this interpretation is valid, since 2007, the first results of mini Boone ( miniature booster neutrino experiment at the Fermi National Accelerator Laboratory) as open.

Properties

Three generations of neutrinos and antineutrinos

There are known three generations of leptons. Each of them consists of an electrically charged particles - electron, muon or tauon - and in each case an electrically neutral neutrino, electron-neutrino (), muon neutrino () or dew or tauon neutrino (). In addition, the corresponding six antiparticles. All leptons have a weak charge and spin ½.

According to recent findings neutrinos can transform into each other. This leads to an alternative description as three different states, and each have a sharp certain (but unknown ) mass. The charged leptons associated with the electron, muon and tau neutrinos are quantum mechanical superpositions of three mass eigenstates.

The number of neutrino species with a mass that is less than half the mass of the Z boson, was determined in precision experiments, inter alia, on the L3 detector at CERN to exactly three.

There is currently no evidence of neutrinoless double beta decay. Previous work, which suggested this was refuted by more accurate measurements. A neutrino -less double beta decay would mean that either the conservation of lepton number violated or the neutrino would be its own antiparticle. In the quantum field theoretical description that would mean ( contrary to the current standard model) that the neutrino Dirac spinor field no, but a Majorana spinor would.

The physicist Lee and Yang provided the impetus for an experiment to study the spin of neutrinos and antineutrinos. This was carried out in 1956 by Chien- Shiung Wu and brought the result that parity conservation does not apply without exception:

The neutrino was found to be " left-handed ", its spin is its movement direction opposite ( antiparallel; see handedness ). This provides an objective explanation of the left and right is possible. In the weak interaction must therefore in the transition from one particle to its antiparticle not only the electric charge, but also the parity, ie the spin, are interchanged. The weak interaction that is different from the electromagnetic interaction by linking the weak charge with the right-or left - handedness of a particle. The leptons and quarks only the left-handed particles and their antiparticles have a right-handed weak charge. In contrast, the right-handed particles and their antiparticles are left-handed neutral with respect to the weak charge. We call this phenomenon as the maximum parity violation.

This is also understandable that neutrinos could be their own antiparticles, although neutrinos and anti- neutrinos in the experiment behave differently: known from the experiment as an anti- neutrino particles would simply neutrinos whose spin is parallel to the direction of movement. You can not just reverse the direction of motion of the neutrinos experimentally; also you can currently perform any experiments in which a neutrino is overtaken by a faster particles and with this interacts so that the direction of movement is opposite in the reference system of the interaction of gravity of the movement direction in the reference frame of the laboratory.

Neutrino mass

In today's standard model of particle physics neutrinos have no mass. There are extensions of the standard model and also some Grand Unified theories that predict a non-zero mass.

Methods for the determination of the neutrino mass divided into four groups:

• Direct determination of the mass of the missing energy in beta decay,
• The observation of neutrino oscillations, ie conversions of a neutrino into another,
• The search for neutrinoless double beta decays, and
• Indirect inferences from other observations, particularly from observational cosmology.

All published results are evaluated by the Particle Data Group and are included in the Review of Particle Physics published annually.

Direct measurements of the end point of the beta spectrum of tritium was until 2006 the possible mass of the electron neutrino with 2 eV / c ² upward limit. A better upper limit is hoped that by more accurate measurements of the KATRIN experiment at the Karlsruhe Institute of Technology, which is expected to reach an upper limit of 0.2 eV / c ². Previous measurements could not exclude the possibility that electron neutrinos are massless, and without an improvement in the measurement accuracy by several orders of magnitude, this is also not expected.

The observation of neutrino oscillations is an indirect measurement of mass differences between neutrinos. They prove that neutrinos actually have one very small, nonzero rest mass (compared to the associated charged leptons ). The resulting very small mass differences also mean that the above mass limit for the electron neutrino is also the limit for all types of neutrinos.

The hypothetical neutrino -less double beta decay is only possible if neutrinos are their own antiparticles. Then it can sometimes come to the annihilation of two virtual neutrinos rather than for the emission of two (real) neutrinos in the simultaneous beta decay of two neutrons in a nucleus. Since the neutrinos are even hard to measure, you measure the total energy of the 2 produced during the process electrons: Come neutrinoless decays before, the electron energy spectrum has a local maximum near the decay energy, because almost all the decay energy now dissipated by the electrons is (a little rest goes into kinetic energy of the atomic nucleus on ).

The cosmological access to determine the neutrino masses based on the observation of the anisotropy of the cosmic background radiation by WMAP and other observations that determine the parameters of the Lambda - CDM model, the current standard model of cosmology. Through the influence that neutrinos on structure formation in the universe and on the primordial nucleosynthesis, can (as of 2007 ) as the upper limit for the sum of the three neutrino masses of 0.2 eV / c ² are accepted.

Speed

Due to their low mass, it is expected that neutrinos generated move in particle physics processes with near vacuum speed of light. In several experiments, the speed of neutrinos was measured and observed agreement within the measurement accuracy with the theoretical prediction.

The measurement of the neutrino mass, neutrino speed and neutrino oscillations addition, they are ways to check the validity of the Lorentz invariance of special relativity. Measurement results of the OPERA experiment in 2011, according to which neutrinos should have moved faster than light, could be attributed to measurement error. A new measurement by ICARUS and a new analysis of the OPERA data have shown similarities with the speed of light.

Penetrability

The penetration depends on the energy of the neutrino. With increasing energy, the cross section of neutrinos increases and the Mean free path accordingly.

Example: At 1000 TeV energy the mean free path of the neutrinos in the earth is approximately a diameter of Earth. This means that during a flight interact vertically through the earth about two-thirds of such neutrinos, while one-third by flying freely through the earth .. At 11 MeV the mean free path in lead is already 350 billion kilometers, and in the earth would benefit from about three billion neutrinos received on average an interaction, while the remaining fly through unhindered.

By comparison, the largest particle accelerator in the world, the LHC, particles with an energy of 7 TeV per nucleon, the sun mainly produces neutrinos with energies below 10 MeV.

A recent review of the cross section of neutrinos in various reactions and energies is available on the internet.

Neutrino and antineutrino reactions

All neutrino reactions proceed via the weak interaction. Neutrinos are also subject to gravity, but this is so weak that it virtually has no meaning. Neutrino reactions can be classified as all of the reactions of the weak nuclear force into three categories:

• Elastic scattering: A neutrino exchanges with the associated lepton energy and momentum. The reactants remain otherwise unchanged, ie, it is a total no conversion to other elementary place.
• Charged Current: An elementary particle coupled via an electrically charged W boson to a neutrino. Here, the particles involved are transformed into others. The Austauschboson is positively or negatively charged depending on the reaction, to ensure the conservation of charge. Strictly speaking, it is also in the elastic scattering of a reaction from this category, because there is an exchange of a W boson instead. However, because the particles at the beginning and end are the same, they can be described as a classical scattering usually simple.
• Neutral current: An elementary particle coupled via an electrically neutral Z boson at a neutrino. Here, the particles involved can be maintained, and the reaction is like an elastic collision, which can take place with any leptons or quarks. If the energy transfer is large enough, yet subsequent particle transformations can take place to hit atomic nuclei.

Generating reactions

The simplest reactions involving the participation of neutrinos, the radioactive beta decays. They occur in unstable nuclei spontaneously and do not require excitation by other particles.

When β - decay (beta- minus decay ) a neutron converts into a proton to where an electron and an electron antineutrino emerge. At the quantum level the emission of one of the two down quarks of the neutron the W, which is transformed intermediate vector boson in an up quark. The emitted W boson finally disintegrates into an electron and an electron antineutrino. It is the " charged current ". This decay occurs for example when free neutrons, but also in atomic nuclei, which have a large neutron excess.

Conversely, changing the β -decay ( beta plus decay) a proton into a neutron and sends by the decay of the resulting W boson, a positron and an electron neutrino from. The process occurs in excess proton in the nucleus. Since the reaction products are heavier than the original proton, the mass difference to be applied on the binding energy of the core.

Important neutrino sources are also cosmic nuclear fusion processes, for example in the sun. An example is the proton / proton reaction is particularly useful for small stars important. There are two hydrogen nuclei fuse under extremely high temperature to form a deuterium nucleus, which will be released by the conversion of a proton into a neutron, a positron and an electron neutrino.

On the quantum level is the reaction equivalent to β decay. Because in the sun but enormously held many mergers per second and thus an enormous number of neutrinos are released, the proton / proton reaction has the greater importance in neutrino research. In the sun and stars heavier electron neutrinos also result in a further fusion process, the Bethe- Weizsäcker cycle. The observation of the so-called solar neutrinos is important to understand the exact processes inside the Sun and the fundamental interactions of physics.

Neutrino research

Although the low reactivity of the neutrinos makes their detection difficult, you can take advantage of the penetrating power of neutrinos in the research also: neutrinos from cosmic events reach the Earth, while electromagnetic radiation or other particles are screened in interstellar matter.

Astrophysics

First neutrinos were used to explore the interior of the sun. Direct visual observation of the core is possible due to the diffusion of electromagnetic radiation in the surrounding plasma layers. The neutrinos, however, caused in large numbers in the fusion reactions in the solar interior, interact only weakly and the plasma can penetrate virtually unhindered. A photon typically requires some 1,000 years until it diffuses to the surface of the sun; a neutrino, requiring just a few seconds.

Later used for the observation of cosmic neutrinos also objects and events beyond our solar system. They are the only known particles that are not significantly affected by interstellar matter. Electromagnetic signals can be shielded from dust and gas clouds or be covered in the detection of cosmic rays on Earth. Cosmic rays in turn, in the form of super-fast protons and nuclei can not propagate further than 100 megaparsecs due to the GZK cutoff (interaction with background radiation). Also, the center of our galaxy is excluded from direct observation because of dense gas and countless bright stars. However, it is likely that neutrinos can be measured in the near future on Earth from the galactic center. Also play an important role in observing neutrinos from supernovae which release about 99 % of their energy in a neutrino flash. The resulting neutrinos can be detected on Earth and provide information on the processes during a supernova. So in 1987, neutrinos were detected, which originated from the supernova 1987A in the Large Magellanic Cloud, 11 in the Kamiokande, 8 in Irvine Michigan Brookhaven experiment, 5 in the Mont Blanc Underground Neutrino Observatory and possibly 5 in the Baksan detector. These are to date the only proven neutrinos, which certainly come from a supernova, because it was observed a few hours later with telescopes.

Experiments such as IceCube, Amanda, Antares and Nestor have proof of cosmogenic neutrinos to the destination. IceCube is currently the largest neutrino experiment.

Neutrino detectors

The mentioned in the previous section astrophysics experiment IceCube is a high-energy neutrino observatory with about 260 employees. It was placed in the ice of the South Pole completed in 2010 and has a volume of 1 km ³. The response of high-energy neutrinos with the elementary particles of ice is observed with this detector and evaluated.

Known neutrino detectors remain or one hand, the radiochemical detectors (eg the chlorine experiment in the Homestake Gold Mine, USA or the GALLEX detector in the Gran Sasso tunnel in Italy), on the other hand based on the Cherenkov effect detectors here Sudbury Neutrino Observatory, especially the ( SNO ) and Super - Kamiokande. They have solar and atmospheric neutrinos by and permit, inter alia, the measurement of neutrino oscillations and thus conclusions on the differences of the neutrino masses, since the processes taking place in the solar interior reactions and thus the emission of neutrinos from the sun are well known. Experiments such as the Double Chooz experiment, or working since 2002 KamLAND detector in Kamioka Neutrino Observatory are able to prove via the inverse beta decay Geoneutrinos and reactor neutrinos, and provide complementary information from an area which is not covered by solar neutrino detectors.

One of the currently largest neutrino detectors called MINOS is underground in an iron mine in the USA, 750 km away from Fermilab research center. From this research, a neutrino beam is radiated in the direction of the detector, where it is then counted how many of the neutrinos are converted during the underground flight.

The CNGS experiment ( CERN Neutrinos to Gran Sasso ) has been studied since 2007, the physics of neutrinos. For this purpose, a neutrino beam from CERN is sent over a distance of 732 km through the Earth's crust to the Gran Sasso laboratory in Italy where it is detected. Some of the muon neutrinos transform themselves go into other neutrino species (almost exclusively tau neutrinos ) to which are detected by the OPERA detector ( Oscillation Project with Emulsion - Tracking Apparatus). For the related velocity measurements, see the section speed.

Application

Researchers at the Sandia National Laboratories want to use the detection of anti-neutrinos to measure the production of plutonium in nuclear reactors, so that the IAEA can no longer rely on estimates and no one can branch off a little for the construction of nuclear weapons. Because of the extremely high production rate of anti-neutrinos in nuclear reactors, a detector with 1 m³ liquid detector would be enough before the nuclear power plant.

Researchers at the University of Rochester and North Carolina State University in 2012, it is possible for the first time to send a message with the help of neutrinos through solid matter. A proton accelerator produced a neutrino beam which is 100 meters was recorded under the ground of a neutrino detector.