What is Neutrinos: Definition and 329 Discussions

A neutrino ( or ) (denoted by the Greek letter ν) is a fermion (an elementary particle with spin of 1/2) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small (-ino) that it was long thought to be zero. The rest mass of the neutrino is much smaller than that of the other known elementary particles excluding massless particles. The weak force has a very short range, the gravitational interaction is extremely weak, and neutrinos do not participate in the strong interaction. Thus, neutrinos typically pass through normal matter unimpeded and undetected.Weak interactions create neutrinos in one of three leptonic flavors: electron neutrinos (νe), muon neutrinos (νμ), or tau neutrinos (ντ), in association with the corresponding charged lepton. Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different tiny values, but they do not correspond uniquely to the three flavors. A neutrino created with a specific flavor has an associated specific quantum superposition of all three mass states. As a result, neutrinos oscillate between different flavors in flight. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. Although only differences between squares of the three mass values are known as of 2019, cosmological observations imply that the sum of the three masses (< 2.14 × 10−37 kg) must be less than one millionth that of the electron mass (9.11 × 10−31 kg).For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has spin of 1/2 and no electric charge. Antineutrinos are distinguished from the neutrinos by having opposite signs of lepton number and right-handed instead of left-handed chirality. To conserve total lepton number (in nuclear beta decay), electron neutrinos only appear together with positrons (anti-electrons) or electron-antineutrinos, whereas electron antineutrinos only appear with electrons or electron neutrinos.Neutrinos are created by various radioactive decays; the following list is not exhaustive, but includes some of those processes:

beta decay of atomic nuclei or hadrons,
natural nuclear reactions such as those that take place in the core of a star
artificial nuclear reactions in nuclear reactors, nuclear bombs, or particle accelerators
during a supernova
during the spin-down of a neutron star
when cosmic rays or accelerated particle beams strike atoms.The majority of neutrinos which are detected about the Earth are from nuclear reactions inside the Sun. At the surface of the Earth, the flux is about 65 billion (6.5×1010) solar neutrinos, per second per square centimeter. Neutrinos can be used for tomography of the interior of the earth.Research is intense in the hunt to elucidate the essential nature of neutrinos, with aspirations of finding:

the three neutrino mass values
the degree of CP violation in the leptonic sector (which may lead to leptogenesis)
evidence of physics which might break the Standard Model of particle physics, such as neutrinoless double beta decay, which would be evidence for violation of lepton number conservation.

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  1. ergospherical

    I What temperature would 1 keV neutrinos have today?

    Suppose there was a 4th generation of neutrino (X, say) with mass m ~ 1 keV. The other three neutrino generations decouple at T ~ 1 MeV and are not heated during ##e^{\pm}## annihilation (whereas the plasma is heated, leading to a bookwork ##T_{\nu}/T_{\gamma}## factor due to conservation of...
  2. S

    I Can there be structures made from neutrinos with angular momentum?

    Would it be possible to eventually have structures made from neutrinos somewhere in the universe, as it is indicated in this question (https://physics.stackexchange.com/questions/80390/are-neutrino-stars-theoretically-possible), like halos of neutrino gas surrounding the center of galaxies...
  3. I

    I Neutrino-Nucleus Interactions and Half-Life Constraints

    At what half-life duration, if any, does the likelihood of a neutrino collision within a sample of radionuclide atoms exceed the likelihood of a decay event in those atoms over the same time period? Can they be efficiently excluded by their reaction products, or are they meaningfully difficult...
  4. Vanadium 50

    I Majorana Neutrinos: Different Physics than Oscillation

    [Moderator's note: Spin-off from previous thread due to topic change.] A Majorana mass term is completely different physics from a neutrino oscillating to an antineutrino.
  5. S

    I Do supernovae generate neutrinos or antineutrinos?

    This Wikipedia particle says that neutrinos are generated: https://en.wikipedia.org/wiki/Supernova But this article (about SN1987A) seems to say that both neutrinos & antineutrinos were detected: https://en.wikipedia.org/wiki/SN_1987A
  6. G

    A Beta Decay, why did they have to resort to Neutrinos?

    I'm reviewing history of subatomic physics. By 1931AD the nuclear physics community had decided to propose the neutrino because they couldn't explain beta decay without it. Alpha and Gamma decays were more confined wrt the energy they would extract from the nucleus i.e. they had energy bands. By...
  7. MichPod

    B Is the photon a gauge boson for neutrinos?

    If I put this in technically correct terms, to enable a local symmetry related to electron phase change, we need to introduce a spin 1 field which is identified as electromagnetic field. Yet there are other spin 1/2 particles. Say, what about neutrino? As it does not couple with electromagnetic...
  8. Simon Peach

    B Question about photons and neutrinos

    while the photon travels at light speed and a neutrino travel at just below light speed why then are photons stopped by an object and the neutrino can past through?
  9. G

    I Data Showing Dark Matter Is Not Cold Neutrinos?

    How do we know that cold neutrinos do not make up 100% or a large percentage of the dark matter content in the universe? In my mind, the only way to prove that dark matter is not simply cold neutrinos would be to measure the density of cold neutrinos in the universe and then calculate the...
  10. malawi_glenn

    A New results, BEST-Experiment, sterile neutrinos?

    (not sure if this is more suitable in the beyond standard model and/or this has been discussed previously in a recent time, I did some searching and could not find any) "Search for electron-neutrino transitions to sterile states in the BEST experiment"...
  11. Zuzana

    A Atmospheric electron neutrinos

    Hi, I would like to ask question about atmospheric electron neutrinos. It is known that atmospheric electron neutrinos originate from the decay of muon in the atmosphere, but we can also calculate that muon with energy more than 10 GeV is able to penetrate about 100 km, so it does not decay and...
  12. K

    I Modified Newtonian Dynamics + dark matter sterile neutrinos or PBH?

    what about Modified Newtonian Dynamics + dark matter both right and both correct Are sterile neutrinos consistent with clusters, the CMB and MOND? Garry W. Angus arXiv:0805.4014 [ If no sterile neutrinos then Modified Newtonian Dynamics + primordial black holes Primordial Black Holes...
  13. D

    B Quarks, electrons, neutrinos, and photons?

    Can someone please explain the four classes of fundamental particles? (Just the basics) I came here because I never learned any chem or physics in school so please explain like I’m five :)
  14. fresh_42

    I How Will FASERν Transform Our Understanding of Neutrinos at High Energies?

    https://journals.aps.org/prd/pdf/10.1103/PhysRevD.104.L091101 And here is the pop science summary: https://www.sciencealert.com/for-the-first-time-neutrinos-may-have-been-detected-at-the-large-hadron-collider Which results can be expected?
  15. G

    Photon and Neutrino detector and photon trawl device

    Summary:: Hello I am a writer and presently working on treatment for a science-fiction story. So I am not a scientist, just a neophyte interested in science and wanting to write a fiction that would not be too far-fetched and that would make some sense for everyone, including the scientific...
  16. DaveC426913

    B Speed of neutrinos - latest results?

    There was a lot of kerfuffle a decade ago about neutrinos possibly exceeding the speed of light, A re-analysis of events debunked this back in 2012 I believe, but I don't know if the issue is conclusively settled. Has there been any re-confirmation of the speed of neutrinos in any paper later...
  17. Phys pilot

    I Energy distribution plot of neutrinos in beta decay

    Hello, When you have a beta decay you get the typical continuos spectrum representing counts against the kinetic energy of the electron. But what's the shape and how I get the spectrum of the kinetic energy of the neutrinos? Thanks
  18. S

    I Are Cosmic Rays Actually Neutrinos?

    I had thought that cosmic rays were protons, but this NY Times article seems to say that cosmic rays are neutrinos: https://www.nytimes.com/2021/03/30/science/neutrinos-lake-baikal.html
  19. B

    Nuclear physics - solar neutrinos interacting

    For the first part I thought you'd have either, p + anti-v -> n + e+ and n + v -> p + e-, but I thought it'd probably be the latter as it's a 'normal' neutrino not an anti neutrino? But do I need to include the actual elements in the equation? For the second part I have multiplied the density...
  20. john baez

    I Open Questions about Neutrinos Today

    Merry Christmas! Neutrinos are mysterious. I just blogged about some of the big open questions involving neutrinos: Neutrino puzzles. In brief, nontechnical terms they are these: What is the correct theory of neutrinos? Why are they almost but not quite massless? Do all three known...
  21. vintagelover007

    Fermi Motion of Nucleons due to a Beam of Neutrinos

    Stable nuclei have radii that are approximately given by the formula: R = r0_A^1/3 Where r0 = 1.25 × 10−15m and A is the atomic mass number. In many experiments of interest to modern particle physics, beams of neutrinos scatter from nucleons within the nucleus. Even though the nucleus is at...
  22. B

    I What is the electric charge of a neutrino?

    Hello! I am a bit confused about neutrinos in the standard model. The vertex of the weak interaction charged current, implies that any neutrino interacting through the charged current must be left handed. However the neutral current allows coupling to the right handed particles, too (and we see...
  23. M

    I Majorana Neutrinos: Evidence of Particle vs. Antiparticle?

    Hello! I don't know much about this topic so I am sorry if my question is silly. As far as I understand if neutrinos are Majorana particles, one consequence is that neutrinos are their own antiparticles. This can be observed, for example, in neutrinoless double beta decay. However, if we take...
  24. P

    I Why can you not describe massive neutrinos with a temperature?

    In the Wikipedia article for CvB, it mentions the following: "The above discussion is valid for massless neutrinos, which are always relativistic. For neutrinos with a non-zero rest mass, the description in terms of a temperature is no longer appropriate after they become non-relativistic; i.e...
  25. Helena Wells

    A If Neutrinos are majorana particles, does this mean lepton number is not conserved?

    If neutrinos are majorana particles does this mean that lepton number is not conserved in particle reactions? And I only noticed neutrinos are only produced when the decay of a particle to some other particles is carried by the W bosons ( weak interaction ). Is it possible the weak interaction...
  26. DrChinese

    Physicists Discover Neutrinos Just Little Italian Neutrons

    https://www.theonion.com/physicists-hail-major-breakthrough-after-discovering-ne-1844363984 "Confirming the search for the mysterious Godfather particle was finally over, physicists at the University of Chicago hailed what they call a major breakthrough Monday after discovering neutrinos are...
  27. A

    How do scientists use water Cerenkov detectors to detect neutrinos?

    Assuming that this sphere has a radius of 50kpc, I've converted to m (1.543e21) and plugged into the area equation for a total area of 2.992e43 m^2. From here I've talked myself into circles, and I honestly don't know where to go next. Any help or guidance would be greatly appreciated!
  28. Wrichik Basu

    A IceCube rules out last SM explanation of ANITA’s anomalous neutrinos

    Quoting from IceCube Observatory's webpage, (Emphasis added.) The paper, available on arXiv, has been submitted to The Astrophysical Journal. I haven't read through the paper yet, but if this detection cannot be explained by the Standard Model, what could possibly be the cause? Is there any...
  29. Physics4Eva

    I Unraveling Neutrino Confusion: Ettore Majorana's Theory

    I was looking into neutrinos and found that a man named Ettore Majorana proposed neutrinos and antineutrinos are the same thing. If this could be true, antineutrinos emitted during double beta decay could annihilate one another and vanish. However, this would violate lepton number conservation...
  30. qnach

    A The time between when the neutrinos are emitted in double-beta decay?

    Double-beta decay will emit two neutrinos. But, are they simultaneously released or separated?
  31. P

    B Neutrinos: Fast or Slow? Can We Observe It?

    As far as I know neutrinos are normally fast/ connected with a velocity close to the speed of light. This seems to be characteristic. Can neutrinos also be in a state of slowness or rest? If yes, has there ever been an indirect observation of such cases?
  32. C

    B Why don't neutrinos pass through atoms?

    Summary: Neutrino A neutrino smashes into an atom and creates an electron - why doesn't the neutrino pass through the atom?
  33. K

    I What is the origin of the non-existence of right handed chiral neutrinos?

    Hello! I am a bit confused about the non-existence of right handed chiral neutrinos. Is this and experimental fact, or it follows from the theory directly? And if it is experimental, how can one test that? Can the chirality be measured? Thank you!
  34. J

    A Why do we write that there are no right-handed neutrinos?

    I'm puzzled why, now that we know that neutrinos have mass, we still read that there are only left-handed neutrinos, as far as we know. I understand that right-handed neutrinos do not interact by the weak force, so we would not detect them. My question is why we read that they might not /...
  35. mfb

    I Measurement of Earth's mass - with neutrinos

    At very high energies Earth absorbs a relevant fraction of neutrinos passing through it. Experiments receive more of these neutrinos from above than from below and the difference depends on the mass of Earth. So why not measure it? Neutrino tomography of Earth The uncertainty is very large, of...
  36. T

    How Does Earth's Electron Density Affect Neutrino Oscillations?

    Homework Statement I'm trying to solve problem 3 from http://www.ictp-saifr.org/wp-content/uploads/2018/07/hw_ICTP-SAIFR-2.pdf The problem is as follows: Assume that solar neutrinos arrive at the surface of the Earth in the ##\left| \nu_2 \right>## state (a mass eigenstate). Assume this...
  37. T

    Fraction of solar neutrinos arriving at the Earth

    Homework Statement Consider solar neutrinos of energy 1 MeV (EDIT: 10 MeV not 1 MeV) which are formed at the center of the sun in the ##\nu_2## eigenstate. What fraction of it do you expect to arrive at Earth as ##\nu_\mu## and what fraction as ##\nu_\tau##? Assume that it evolves adiabaticaly...
  38. M

    A Baroque neutrinos and Zee-Babu model

    https://arxiv.org/abs/1810.01000 MiniBooNE, MINOS+ and IceCube data imply a baroque neutrino sector Jiajun Liao, Danny Marfatia, Kerry Whisnant (Submitted on 1 Oct 2018) The 4.8σ anomaly in MiniBooNE data cannot be reconciled with MINOS+ and IceCube data within the vanilla framework of neutrino...
  39. S

    I Do black holes accumulate neutrinos?

    Do black holes accumulate neutrinos ? Do the neutrinos that fall in, decay in some way , maybe interact with the condensed matter, or just stay "parked" in perpetual loops ?
  40. B

    Questions about neutrinos compared to the nature of light

    First question: Within experimental error all measurements of neutrino and light speed in vacuum are consistent with c, but one way speed measurement of light is well proscribed, so is a one way speed measurement of neutrinos also proscribed? I have more questions, but maybe just clarify this...
  41. jedishrfu

    B Blazar as the Source of Cosmic Neutrinos Confirmed

    Read more at: https://phys.org/news/2018-07-century-old-riddle-resolveda-blazar-source.html and a second article from Ars Technica: https://arstechnica.com/science/2018/07/icecube-spots-a-neutrino-source-a-black-hole-jet-pointed-at-earth/ showing how multi-messenger astronomy is...
  42. jim mcnamara

    I MiniBooNE results at 6.1 sigma: Potential evidence for sterile neutrinos

    Bee Hossenfelder's blog has a discussion: http://backreaction.blogspot.com/ Preprint: https://arxiv.org/abs/1805.12028 New evidence in the search for the 'sterile' neutrino. Can someone comment on this please. @mfb
  43. K

    I Neutrino Mass: Exploring the Oscillation Phenomenon

    Why is there an assumption that if neutrinos didn't have mass they would move at the speed of light? and how does the fact they oscillate prove they have mass?
  44. jishnu

    I Emission of neutrinos and antineutrinos

    My question is what is the actual purpose for the emission of neutrinos and antineutrinos during the beta + and beta - decay of an atom respectively?
  45. Kajagoogooooooo

    I Majorana neutrinos, sphaleron reactions, baryon asymmetry

    I`ve spent some time reading about the baryon asymmetry and the Sakharov`s conditions, and there some things I didn't exactly get: 1. Interactions out of thermal equilibrium: isn't it trivial? our universe is expanding so, of course, it out of equilibrium. 2. CP violation: I`ve read that Cronin...
  46. ohwilleke

    I Can Majorana Neutrinos Oscillate Between Neutrino and Antineutrino States?

    Background The Standard Model conserves baryon number, which is equal to (quarks divided by three) minus (anti-quarks divided by three) and lepton number, which is equal to leptons minus antileptons, separately, except in sphaleron processes which conserve B-L, but not B or L separately. There...
  47. D

    B What particle(s) pass through Earth?

    What particles(s) can pass through hundreds of miles of the Earth? Is it only neutrinos, or is there some other particle(s)?
  48. C

    A Thermal Neutrinos: Detecting Ancient Particles in the Universe?

    Would neutrinos emitted in the distant and early universe be slowed enough to become thermal neutrinos? Could these be detected?
  49. cipi

    A Why Does Casimir's Trick Include Nonphysical Neutrino Helicities?

    Hello I was reading in Griffiths that in the case of electron neutrino scattering, one can apply Casimir's trick to sum over input and output spin states, including the neutrinos. My question is, how come you are allowed to include in the sum the nonphysical right handed helicity state ? And...
  50. C

    I Neutrino flavour eigenstates and expansion of the universe

    Neutrinos were flavor eigenstates at the time of their decoupling from baryonic matter. Since they were not pure mass eigenstates, how do you take this fact into account if you try to study how they evolved as the universe expanded? Could we determine if the heaviest neutrino could be non...