Are neutrinos massive

The neutrino is an electrically neutral elementary particle. It belongs to the leptons and is only influenced by the weak interaction and, like every elementary particle, according to general relativity, also by gravity. As a fermion in the standard model, the neutrino has a spin of and negative helicity. The interaction probability of the neutrino is extremely small. Its detection is only possible via the charged and neutral current, the Z boson and the W boson as exchange particles of the weak interaction, and therefore difficult. The symbol for the neutrino is the Greek letter ν. The name was suggested by Enrico Fermi and means (according to the Italian diminutive in O) little neutron.

For typical solar neutrinos (energy of a few hundred keV) a lead wall about one light year thick would be needed (about 1016 m) to capture half of them. Around 70 billion solar neutrinos move on average through one square centimeter of the earth's surface per second.[1]


Three generations of neutrinos and antineutrinos

Three generations of leptons are known, each consisting of a pair of an electrically charged particle (electron, muon and tauon) and an electrically neutral, associated neutrino. One speaks of Electron neutrinoe), Muon neutrinoμ) and dew or Tauon neutrinoτ). All leptons carry the so-called "weak charge" and spin 1/2. There is also an antiparticle for every neutrino, the Antineutrino, So a Electron antineutrino (), Muon antineutrino () and Tauon antineutrino ().

The leptons differ from generation to generation only in the different masses of the electrically charged leptons, while only upper limits are known of the masses of neutrinos.

The number of types of neutrinos with a neutrino mass that is less than half the mass of the Z boson has been shown in precision experiments, inter alia. determined to be exactly three at the L3 detector at CERN.

There is evidence of neutrino-free double beta decay. This would mean that either the conservation of lepton number is violated or the neutrino would be its own antiparticle. In the description of the quantum field theory, this would mean that the neutrino field would not be a Dirac spinor, but a Majorana spinor, in contradiction to the current standard model.

The physicists Lee and Yang initiated an experiment to study the spins of neutrinos and antineutrinos. This was carried out by Chien-Shiung Wu in 1956 and resulted in the fact that parity maintenance does not apply without exception. The neutrino turned out to be "left-handed," which means that it rotates counterclockwise with respect to its direction of movement. This enables an objective explanation of “left” and “right”. In the area of ​​weak interaction, therefore, not only the electrical charge, but also the parity, i.e. the parity, must be required for the transition from a particle to its antiparticle. H. the spin, to be swapped. The weak interaction differs from the electromagnetic interaction in that the “weak charge” is linked to the right- or left-handedness of a particle. In the case of leptons and quarks, only the left-handed particles and their right-handed antiparticles have a weak charge. In contrast, the right-handed particles and their left-handed antiparticles are neutral towards the weak charge. Particles with a weak charge can emerge from the vacuum and then disappear again. This phenomenon is called "spontaneous (mirror) symmetry breaking".

Neutrino mass

The complete Hamilton function of the quantum eigenstates in the Glass show salam vineyard theory (see Electroweak Interaction) contains mass terms for the neutrinos without giving any indication of the size of the mass. However, because the experimental upper limits of the neutrino masses are several orders of magnitude below the masses of the associated charged leptons, it is permissible to set these to zero in many calculations.

In the standard model of elementary particles, the neutrino mass is set to zero when the mass eigenstates of the fermions (leptons and quarks) are derived from the quantum eigenstates, so that the detection of a neutrino mass requires this model to be completed. There are extensions to today's Standard Model and also some interesting Great Unified Theories that predict massive neutrinos.

The latest measurements show that neutrinos actually have a rest mass that is different from zero (compared to the associated charged leptons), because mass differences between the neutrino generations are a prerequisite for being able to convert from one type of neutrino to another (neutrino oscillation). In 2002, oscillations of solar neutrinos were detected by the SNO.

For cosmology, neutrinos are therefore a candidate for part of dark matter, but they can at best represent hot dark matter.

The best upper limit for the electron neutrino mass is currently the value of 2.3 eV obtained from the direct determination of the neutrino mass by measuring the end point of the beta spectrum of tritium in the Mainz neutrino experiment [2][3].

Tritium Beta Decay:

It is hoped that a better upper limit will be achieved through even more precise measurements by the follow-up experiment KATRIN at the Karlsruhe Research Center, which should provide an upper limit of 0.2 eV.

For comparison: An electron has a rest energy of 511 keV = 511,000 eV

Neutrino and antineutrino reactions


All neutrino reactions take place via the weak interaction, which is why neutrinos only very rarely take part in reactions. They are also subject to gravity, but this is so weak that it has practically no meaning. Neutrino reactions (or in general reactions of the weak nuclear force) can be divided into three categories:

  • elastic scattering: A neutrino exchanges energy and momentum with the associated lepton. The reaction partners remain otherwise unchanged, that is, there is no overall conversion into other elementary particles.
  • charged electricity: An elementary particle couples to a neutrino via an electrically charged W boson. Here the particles involved are transformed into others. The exchange boson is positively or negatively charged, depending on the reaction, in order to ensure that the charge is retained. Strictly speaking, elastic scattering is also a reaction from this category, because there, too, an exchange of a W boson takes place. However, because the particles are the same at the beginning and the end, they can usually be described simply as a classic scattering.
  • neutral electricity: An elementary particle couples to a neutrino via an electrically neutral Z boson. The particles involved also transform into others, but the exchange boson is not charged.

Generative reactions

The simplest reactions in which neutrinos participate are the radioactive beta decays. They occur spontaneously in unstable nuclei and do not require any excitation from other particles.

At the β-Decay (beta-minus-decay) converts a neutron into a proton, creating an electron and an electron antineutrino. At the quantum level, one of the two down quarks of the neutron emits the intermediate vector boson W.-whereby it turns into an up-curd. The emitted W boson eventually decays into an electron and an antineutrino. So it is the “charged current”. This decay occurs, for example, with free neutrons, but also with atomic nuclei that have a large excess of neutrons.

A nuclide emits an electron and an anti-electron neutrino
into a daughter kernel, which has an atomic number increased by 1.

Conversely, the β changes+-Decay (beta-plus-decay) converts a proton into a neutron and sends out a positron and an electron neutrino. The process occurs when there is an excess of protons in the nucleus. Since the reaction products are heavier than the original proton, the mass difference has to be applied by the binding energy of the nucleus.

A nuclide emits a positron and an electron neutrino
into a daughter kernel, which has an ordinal number reduced by 1.

Important sources of neutrinos are also nuclear fusion processes, such as those that occur in the sun. One example is the proton-proton reaction, which is particularly important for small stars. Two hydrogen nuclei fuse at extremely high temperatures to form a deuterium nucleus, whereby a positron and an electron neutrino are released through the conversion of a proton into a neutron.

At the quantum level the reaction is equivalent to β+-Decay. But because an enormous number of fusions take place in the sun per second and an enormous number of neutrinos are released as a result, the proton-proton reaction is of greater importance in neutrino research. In the sun and heavier stars, electron neutrinos are also produced in the Bethe-Weizsäcker cycle, which is another fusion process. Observing the so-called solar neutrinos is important in order to understand the exact processes inside the sun and the fundamental interactions in physics.

Neutrino research

Although the low reactivity of neutrinos makes their detection difficult, the penetrative nature of neutrinos can be exploited in research. This is how neutrinos from cosmic events reach the earth, while electromagnetic radiation or other particles are shielded in interstellar matter.


First, neutrinos were used to explore the interior of the sun. Direct optical observation of the core is not possible due to the diffusion of electromagnetic radiation in the surrounding plasma layers. The neutrinos, however, which are produced in large numbers during the fusion reactions in the sun's interior, only interact weakly and can penetrate the plasma practically unhindered. A photon typically takes a few thousand years to diffuse to the surface of the sun; a neutrino only needs a few seconds for this.

Later neutrinos were also used to observe cosmic 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 covered by cosmic radiation when detected on earth. The cosmic radiation, in the form of super-fast protons and atomic nuclei, cannot spread further than 100 megaparsecs due to the GZK cutoff (interaction with background radiation). The center of our galaxy is also excluded from direct observation because of dense gas and countless bright stars. However, it is likely that neutrinos from the galactic center will be able to be measured on Earth in the near future. Neutrinos also play an important role in the observation of supernovae, which release around 99% of their energy in a neutrino flash. The resulting neutrinos can be detected on earth and give information about the processes during a supernova. In 1987 neutrinos were detected that came from the supernova 1987A from the Large Magellanic Cloud, 11 in the Kamiokande[4], 8 in the Irvine Michigan Brookhaven Experiment[5], 5 at the Mont Blanc Underground Neutrino Observatory[6] and possibly 5 in the Baksan detector[7][8] To date, these are the only neutrinos that have been detected that have definitely come from a supernova, as this was observed with telescopes a few hours later.

Experiments like Amanda, Antares and Nestor aim to detect cosmogenic neutrinos.

Particle physics

In particle physics, neutrinos are important because they are the lightest elementary particles in the extensions of the Standard Model. For example, assuming a fourth generation of fermions (besides electron, muon, and tauon), the associated neutrino would be the easiest to generate. Neutrinos could also be used to study quantum gravity effects. Because they do not appear in composite particles (e.g. protons and neutrons) or because they decay after a short time, it might be possible to isolate and measure such effects.

The CNGS experiment (CERN neutrinos to Gran Sasso) should bring further clarification about the physics of neutrinos from 2007. A neutrino beam will travel from CERN over a distance of 730 km through the interior of the earth to the Gran Sasso Laboratory in Italy and will be detected there. Some of the muon neutrinos will transform into other types of neutrinos (almost exclusively tau neutrinos) on the way, which will then be detected by the OPERA detector.

Neutrino detectors

Main article: Neutrino detector

Well-known neutrino detectors are on the one hand the radiochemical detectors (e.g. the chlorine experiment in the Homestake gold mine, USA or the GALLEX detector in the Gran Sasso tunnel (Italy)), on the other hand the detectors based on the Cherenkov effect, especially here the Sudbury Neutrino Observatory (SNO) and Super-Kamiokande. They detect solar and atmospheric neutrinos and allow, among other things. the measurement of neutrino oscillations and thus inferences about the differences of the neutrino masses, since the reactions taking place inside the sun and thus the neutrino emission of the sun are well known. Experiments like Chooz or Kamland are able to detect geoneutrinos and reactor neutrinos via the inverse beta decay and provide complementary information from an area that is not covered by solar neutrino detectors.

The currently largest neutrino detector, called MINOS, is located underground in an iron mine in the USA, 750 kilometers from the FERMILAB research center. A neutrino beam is emitted from this research center in the direction of the detector, where it is then to be counted how many of the neutrinos change during the underground flight.


Researchers at Sandia National Laboratories want to use antineutrinos to measure the production of plutonium in nuclear reactors so that the IAEA no longer has to rely on estimates and no one can divert anything for the construction of nuclear weapons. Because of the extremely high production rate of antineutrinos in nuclear reactors, a detector with 1 m³ of detector liquid in front of the nuclear power plant would be sufficient.[9]

Research history

Until 1930, the radioactive beta decay was not understood. With such a decay, only one emitted electron was observed. Together with the remaining core, it would be a two-body problem, with which, however, the continuous spectrum of the beta decay could not be explained without assuming a violation of the law of conservation of energy. This led Wolfgang Pauli to postulate a previously unobserved elementary particle that, along with the electron and the nucleus, should also take part in the process. This particle should carry part of the energy released during the decay and thus ensure the conservation of energy and momentum. Pauli first named his hypothetical particle, postulated in a private letter on December 4, 1930, as a neutron; Enrico Fermi, who developed a theory about the basic properties and interactions of this particle, named it in neutrino (Italian for “small neutron”, “little neutron”) in order to avoid a conflict with the particle known today under the same name. It was not until 1933 that Pauli presented his hypothesis to a wider audience and asked about possible experimental evidence.

As Pauli already assumed, the neutrino would have to be extremely difficult to detect. In fact, the first observation was made 23 years later: in 1956, the group around Clyde L. Cowan and Frederick Reines succeeded in demonstrating the inverse beta decay in a nuclear reactor, which causes a significantly higher neutrino flux than radioactive elements during beta decay.

An antineutrino meets a proton and creates a positron and a neutron.

Both reaction products are comparatively easy to observe. For their discovery, Cowan and Reines were awarded the Nobel Prize in Physics in 1995. Today we know that the particle involved was an antineutrino.

The muon neutrino was discovered in 1962 by Jack Steinberger, Melvin Schwartz and Leon Max Lederman with the first neutrino beam produced at the accelerator. For this they received the Nobel Prize in Physics in 1988. With the muon neutrino, a second generation of neutrinos became known, which is the analogue of the electron neutrino for muons. For a short time the term neutretto was used for the muon neutrino (-net is also an Italian diminutive), which, however, was not widely used. When the tauon was discovered in 1975, physicists also expected a corresponding generation of neutrinos, the tauon neutrino. The first signs of its existence were given by the continuous spectrum in the Tauon decay, similar to that in the beta decay. In 2000, the Tau neutrino was then directly detected for the first time in the DONUT experiment.

The LSND experiment, which ran from 1993 to 1998, was interpreted as an indication of the existence of sterile neutrinos, but it was controversial. After KARMEN was not able to reproduce the results, this interpretation has been valid since 2007 due to the first results of MiniBooNE (miniature booster neutrino experiment at the Fermi National Accelerator Laboratory) as refuted.[10]

  • Video from a lecture at the University of Tübingen and others. about the neutrino
  • [1] Heinrich Päs, Sandip Pakvasa, Thomas J. Weiler (November 20, 2006). "Shortcuts in extra dimensions and neutrino physics". Retrieved on 2007-07-23.

Individual evidence

  1. Claus groups: Astroparticle physics. Vieweg Verlag, Braunschweig / Wiesbaden 2000, ISBN 3-528-03158-1, P. 69.
  2. Ch. Kraus et al .: Final results from phase II of the Mainz neutrino mass search in tritium β-decay in Eur. Phys. J. C 40: 447-468 (2005), doi: 10.1140 / epjc / s2005-02139-7; PDF on
  4. Hirata, K. et al. (KAMIOKANDE-II Collabration): Observation of a Neutrino Burst from the Supernova SN 1987a in phys. Rev. Lett. 58 (1987), 1490-1493 doi: 10.1103 / PhysRevLett.58.1490
  5. R.M. Bionta et al .: Observation of a Neutrino Burst in Coincidence with Supernova SN 1987a in the Large Magellanic Cloud in phys. Rev. Lett. 58 (1987), 1494 doi: 10.1103 / PhysRevLett.58.1494
  6. M. Aglietta et al .: On the Event Observed in the Mont Blanc Underground Neutrino Observatory during the Occurrence of Supernova 1987a in Europhys. Lett. 3 (1987), 1315-1320 doi: 10.1209 / 0295-5075 / 3/12/011
  7. E.N. Alexeyev et al. in sov. JETP Lett. 45: 461 (1987)
  8. Kai Zuber: Neutrino Physics. Institute of Physics Publishing, Bristol and Philadelphia 2004, ISBN 0-7503-0750-1.
  9. Antineutrinos monitor plutonium production
  10. MiniBooNE Collaboration - "A Search for Electron Neutrino Appearance", Phys. Rev. Lett. 98, 231801 (2007). (available online at, pdf file, 194 KB)

Category: elementary particles