Sterile neutrino
B − L | depends on L charge assignment |
---|---|
X | −5 |
Sterile neutrinos (or inert neutrinos) are hypothetical
The existence of right-handed neutrinos is theoretically well-motivated, because the known active neutrinos are left-handed and all other known
The search for sterile neutrinos is an active area of
Motivation
Experimental results show that all produced and observed neutrinos have left-handed helicities (spin antiparallel to momentum), and all antineutrinos have right-handed helicities, within the margin of error.[3] In the massless limit, it means that only one of two possible chiralities is observed for either particle. These are the only helicities (and chiralities) allowed in the Standard Model of particle interactions; particles with the contrary helicities are explicitly excluded from the formulas.[9]
Recent experiments such as
Chirality is a fundamental property of particles and is relativistically invariant: It is the same regardless of the particle's speed and mass in every inertial reference frame.[12] However, a particle with mass that starts out with left-handed chirality can develop a right-handed component as it travels – unless it is massless, chirality is not conserved during the propagation of a free particle through space (nominally, through interaction with the Higgs field).
The question, thus, remains: Do neutrinos and antineutrinos differ only in their chirality? Or do exotic right-handed neutrinos and left-handed antineutrinos exist as separate particles from the common left-handed neutrinos and right-handed antineutrinos?
Properties
Such particles would belong to a
Due to the lack of electric charge,
In experiments involving energies larger than their mass, sterile neutrinos would participate in all processes in which ordinary neutrinos take part, but with a quantum mechanical probability that is suppressed by a small mixing angle. That makes it possible to produce them in experiments, if they are light enough to be within the reach of current particle accelerators.
They would also interact gravitationally due to their mass, and if they are heavy enough, could explain
Mass
All particles are initially massless under the Standard Model, since there are no
Such is the case for charged leptons, like the electron, but within the Standard Model the right-handed neutrino does not exist. So absent the sterile right chiral neutrinos to pair up with the left chiral neutrinos, even with Yukawa coupling the active neutrinos remain massless. In other words, there are no mass-generating terms for neutrinos under the Standard Model: For each generation, the model only contains a left-handed neutrino and its antiparticle, a right-handed antineutrino, each of which is produced in weak eigenstates during weak interactions; the "sterile" neutrinos are omitted. (See
In the seesaw mechanism, the model is extended to include the missing right-handed neutrinos and left-handed antineutrinos; one of the eigenvectors of the neutrino mass matrix is then hypothesized to be remarkably heavier than the other.
A sterile (right-chiral) neutrino would have the same
Dirac and Majorana terms
Sterile neutrinos allow the introduction of a
Unlike for the left-handed neutrino, a
It is possible to include both Dirac and Majorana terms; this is done in the seesaw mechanism (below). In addition to satisfying the
To put this in mathematical terms, we have to make use of the transformation properties of particles. For free fields, a Majorana field is defined as an eigenstate of charge conjugation. However, neutrinos interact only via the weak interactions, which are not invariant under
Seesaw mechanism
In addition to the left-handed neutrino, which couples to its family charged lepton in weak charged currents, if there is also a right-handed sterile neutrino partner (a weak isosinglet with zero charge) then it is possible to add a Majorana mass term without violating electroweak symmetry.[15]
Both left-handed and right-handed neutrinos could then have mass and handedness which are no longer exactly preserved (thus "left-handed neutrino" would mean that the state is mostly left and "right-handed neutrino" would mean mostly right-handed). To get the neutrino mass eigenstates, we have to diagonalize the general mass matrix
where is the neutral heavy lepton's mass, which is big, and are intermediate-size mass terms, which interconnect the sterile and active neutrino masses. The matrix nominally assigns active neutrinos zero mass, but the terms provide a route for some small part of the sterile neutrinos' enormous mass, to "leak into" the active neutrinos.
Apart from empirical evidence, there is also a theoretical justification for the seesaw mechanism in various extensions to the Standard Model. Both
According to GUTs and left-right models, the right-handed neutrino is extremely heavy: while the smaller eigenvalue is approximately given by
This is the seesaw mechanism: As the sterile right-handed neutrino gets heavier, the normal left-handed neutrino gets lighter. The left-handed neutrino is a mixture of two Majorana neutrinos, and this mixing process is how sterile neutrino mass is generated.
Sterile neutrinos as dark matter
For a particle to be considered a dark matter candidate, it must have non-zero mass and no electromagnetic charge.[17] Naturally, neutrinos and neutrino-like particles are of interest in the search for dark matter because they possess both these properties. Observations suggest that there is more cold dark matter (non-relativistic) than hot dark matter (relativistic). The active neutrinos of the Standard Model, having very low mass (and therefore very high speeds) are therefore unlikely to account for all dark matter.[18]
Since no bounds on the mass of sterile neutrinos are known, the possibility that the sterile neutrino is dark matter has not yet been ruled out, as it has for active neutrinos. If dark matter consists of sterile neutrinos then certain constraints can be applied to their properties. Firstly, in order to produce the structure of the universe observed today the mass of the sterile neutrino would need to be on the
Detection attempts
The production and decay of sterile neutrinos could happen through the mixing with virtual ("off mass shell") neutrinos. There were several experiments set up to discover or observe NHLs, for example the NuTeV (E815) experiment at Fermilab or LEP-L3 at CERN. They all led to establishing limits to observation, rather than actual observation of those particles. If they are indeed a constituent of dark matter, sensitive X-ray detectors would be needed to observe the radiation emitted by their decays.[21]
Sterile neutrinos may mix with ordinary neutrinos via a
Two separate detectors near a nuclear reactor in France found 3% of anti-neutrinos missing. They suggested the existence of a fourth neutrino with a mass of 1.2 eV.[27] Daya Bay has also searched for a light sterile neutrino and excluded some mass regions.[28] Daya Bay Collaboration measured the anti-neutrino energy spectrum, and found that anti-neutrinos at an energy of around 5 MeV are in excess relative to theoretical expectations. It also recorded 6% missing anti-neutrinos.[29] This could suggest either that sterile neutrinos exist or that our understanding of some other aspect of neutrinos is incomplete.
The number of neutrinos and the masses of the particles can have large-scale effects that shape the appearance of the cosmic microwave background. The total number of neutrino species, for instance, affects the rate at which the cosmos expanded in its earliest epochs: More neutrinos means a faster expansion. The Planck Satellite 2013 data release is compatible with the existence of a sterile neutrino. The implied mass range is from 0–3 eV.[30][failed verification – see discussion] In 2016, scientists at the IceCube Neutrino Observatory did not find any evidence for the sterile neutrino.[31] However, in May 2018, physicists of the MiniBooNE experiment reported a stronger neutrino oscillation signal than expected, a possible hint of sterile neutrinos.[6][7] Since then, in October 2021, the MicroBooNE experiment's first results showed no hints of sterile neutrinos, rather finding the results aligning with the Standard Model's three neutrino flavours.[32] This result had not found an explanation for MiniBooNE's anomalous results, however.
In June 2022, the
In January 2023, the STEREO experiment published its final result, reporting the most precise measurement of the antineutrino energy spectrum associated with the fission of uranium-235. The data is consistent with the Standard Model and rejects the hypothesis of a light sterile neutrino with a mass of around 1 eV.[36]
In 2023 results of searches by the CMS set new limits for sterile neutrinos with masses of 2-3 GeV.[37]
See also
- List of hypothetical particles
- MiniBooNE at Fermilab
- Weakly Interacting Slender Particle
Footnotes
- ^ And as with all other particle / anti-particle pairs, the sterile right-chiral neutrino and left-chiral anti-neutrino would also have identical, non-zero mass. Chirality, lepton-number, and flavor (if any) are the only quantum numbers that distinguish a sterile neutrino from a sterile antineutrino. For any charged particle, for example the electron, this is not the case: Its antiparticle, the positron, has opposite electric charge, opposite weak isospin, and opposite chirality, among other opposite charges. Similarly, an up quark has a charge of ++2/3 and, for example, a color charge of red, while its antiparticle has an electric charge of −+2/3 and in this example a color charge of anti-red.
References
- ^ "Sterile neutrinos". All things neutrino. Retrieved 2021-04-29.
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- ^ S2CID 116613775.
- ^ S2CID 119161526.
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- ^ LiveScience. Retrieved 3 June 2018.
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Aguilar-Arevalo, A.A.; Brown, B.C.; Bugel, L.; Cheng, G.; Conrad, J.M.; Cooper, R.L.; et al. (MiniBooNE collaboration) (2018). "Observation of a significant excess of electron-like events in the MiniBooNE short-baseline neutrino experiment". Physical Review Letters. 121 (22): 221801. S2CID 53999758.
- ^ "MicroBooNE experiment's first results show no hint of a sterile neutrino". news.uchicago.edu (Press release). University of Chicago News. 27 October 2021.
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- ^ Rodejohann, Werner (1 May 2021). "Sterile neutrinos from the low energy to the GUT scale" (PDF).
- ^ "Dark matter". CERN. Retrieved 2021-04-29.
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- ^
Merle, Alexander (August 2013). "keV neutrino model building". International Journal of Modern Physics D. 22 (10): 1330020. S2CID 118550598.
- ^
Boyarsky, A.; Drewes, M.; Lasserre, T.; Mertens, S.; Ruchayskiy, O. (January 2019). "Sterile neutrino dark matter". Progress in Particle and Nuclear Physics. 104: 1–45. S2CID 116613775.
- ^ Battison, Leila (16 September 2011). "Dwarf galaxies suggest dark matter theory may be wrong". BBC News. Retrieved 18 September 2011.
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- ^ "First results" (PDF). Booster Neutrino Experiment (BooNE) (Press release). Fermi National Accelerator Laboratory (Fermilab).
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Bulbul, E.; Markevitch, M.; Foster, A.; Smith, R.K.; Loewenstein, M.; Randall, S.W. (2014). "Detection of an unidentified emission line in the stacked X-ray spectrum of galaxy clusters". S2CID 118468448.
- ^ Lasserre, Th. (April 2012). "The reactor antineutrino anomaly". irfu.cea.fr (Press release).
- ^
An, F.P.; Balantekin, A.B.; Band, H.R.; Beriguete, W.; Bishai, M.; Blyth, S.; et al. (1 October 2014). "Search for a light sterile neutrino at Daya Bay". Physical Review Letters. 113 (14): 141802. S2CID 10500157.
- ^ Jepsen, Kathryn (2016-02-12). "Daya Bay discovers a mismatch". Symmetry. Retrieved 2022-12-19.
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- S2CID 125498830. Retrieved 12 August 2016.
- Fermi National Accelerator Laboratory. 2021-10-27. Retrieved 2021-11-13.
- ^ "Deep underground experiment results confirm anomaly: Possible new fundamental physics". SciTechDaily (Press release). Los Alamos National Laboratory. 2022-06-18. Retrieved 2022-06-22.
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- ^ "Search for long-lived heavy neutral leptons decaying in the CMS muon detectors in proton-proton collisions at s^0.5=13 TeV". inspirehep.net. Retrieved 2023-08-05.
Sources
- Drewes, M. (2013). "The phenomenology of right handed neutrinos". S2CID 119161526.
- Merle, A. (2013). "keV neutrino model building". S2CID 118550598.
- Vaitaitis, A.G.; et al. (1999). "Search for neutral heavy leptons in a high-energy neutrino beam". S2CID 14328194.
- Formaggio, J.A.; Conrad, J.; Shaevitz, M.; Vaitaitis, A. (1998). "Helicity effects in neutral heavy lepton decays". .
- Nakamura, K.; et al. (hdl:10481/34593.
External links
- "Sterile neutrinos". www.nu.to.infn.it. Neutrino unbound. Archived from the original on 24 June 2016.
- "The NuTeV experiment at Fermilab". Fermi National Accelerator Laboratory.
- "The L3 experiment at CERN". CERN.
- "Experiment nixes fourth neutrino". Scientific American. April 2007.