Caption

Trace the origin of the neutrino mass

by

    Julia Gehrlein

    • Department of Theoretical Physics, CERN, Geneva, Switzerland

• Physics 16, 20

The collider experiments have placed new direct limits on the existence of hypothetical heavy neutrinos, helping to define how ordinary neutrinos obtain their mass.

Figure 1: In the see-saw mechanism, a hypothetical neutrino (left) is “mixed” with an observed neutrino (right). This mixing is such that the masses of the two particles are inversely proportional: the heavier the hypothetical neutrino, the lighter the observed neutrino.In the see-saw mechanism, a hypothetical neutrino (left) is “mixed” with an observed neutrino (right). This mixing is such that the masses of the two particles are inversely proportional: the heavier the hypothetical neutrino, the lighter the observed neutrino… Show more

The discovery more than 20 years ago that neutrinos can oscillate from one type to another came as a surprise to particle physicists, as these oscillations require neutrinos to have mass, contrary to the standard model of particle physics. As multiple neutrino experiments continue to constrain oscillation rates and mass values ​​(see Viewpoint: Long-Base Neutrino Experiments Continue), a fundamental question remains unanswered: How do neutrinos get their mass? There are many theoretical models, but so far none of them have been confirmed experimentally. Now the CMS collaboration at CERN in Switzerland has presented the results of the search for a hypothetical heavy neutrino that could be linked to neutrino mass generation [1]. No traces of this particle have been found, placing new constraints on a popular model for the origin of neutrino mass, called the see-saw mechanism. These results open up a new way to probe the origin of neutrino mass at particle colliders in the future.

There are hints that neutrino masses might be special. Neutrinos have no electric charge, which makes them distinct from other particles of “matter,” such as electrons and quarks. Neutrinos are also unique in being observed with only one kind of handedness (a property that emerges from particle mass and spin). Other particles of matter can be left-handed or right-handed and obtain their mass through their interaction with the so-called Higgs field (see Focus: Nobel Prize – Why particles have mass). The observation that neutrinos are only left-handed might suggest that the Higgs mechanism does not apply to them.

There is another peculiarity of neutrinos: they are much lighter than all other elementary particles. The experiments force neutrinos to be at least a million times lighter than electrons, the next lightest particles. This disparity suggests that something is “pushing” the neutrino mass towards small values. The swing mechanism includes such a push [2–7]. Just like in a playground, two players are involved in this metaphorical see-saw: a neutrino observed from one side and a hypothetical neutrino from the other (Fig. 1). The quantum states of these two particles are mixed, meaning that one particle can potentially oscillate into the other. This mixing leads to an inverse relationship between the masses of the two actors: the heavier the hypothetical particle, the lighter the observed neutrino.

The hypothetical heavy neutrinos are “sterile” as they do not participate in any of the known fundamental interactions. Another crucial feature of the seesaw mechanism is that neutrinos must be their own antiparticles, so evidence that neutrinos self-annihilate would lend support to this mechanism.

Researchers have been looking for other neutrinos for many years. One probe involves observing neutrino oscillations and looking for signs of “missing” neutrinos. This scenario could occur, for example, if some of the electron neutrinos from a nuclear reactor turn into sterile neutrinos that cannot be detected. Some experiments have seen hints of sterile neutrinos (see Point of View: Neutrino Mystery Endures), but these eventually observed sterile neutrinos are much lighter than the sterile neutrinos predicted by the seesaw mechanism. At higher masses, global studies of neutrino data have discovered no signs of missing neutrinos, allowing physicists to derive constraints on extra neutrinos that extend to extremely high masses of 1015 GeV/c2.

Figure 2: One way to test the seesaw mechanism is to look for lepton number violation events in proton-proton collisions from the LHC. This diagram shows an example of such an event, where two quarks from each proton interact w bosons. This interaction produces a pair of heavy sterile neutrinos (No) and a pair of muons (µ). The heavy and sterile neutrinos annihilate each other, leaving only the muon pair. The search for these events came up empty, placing new constraints on sterile heavy neutrinos.One way to test the seesaw mechanism is to look for lepton number violation events in proton-proton collisions from the LHC. This diagram shows an example of such an event, where two quarks from each proton interact w bosons. This in… Show more

A more direct search for more neutrinos relies on their production in high-energy experiments. The CMS collaboration looked for signs of extra neutrinos in collision data from the Large Hadron Collider (LHC) at CERN. The team’s target signal was a violation of the lepton number, which is a kind of ‘charge’ related to electrons, muons, tauons and neutrinos (Fig. 2). So far, physicists have only seen processes that conserve number of leptons (see Point of View: The Hunt for No Neutrinos) [8]but dedicated high-energy searches have been rare [9]. In this new study, the CMS collaboration looked for collisions between protons (lepton number 0) that produced a muon pair (lepton number +2) or an antimuon pair (lepton number -2). The observation of such dimuon events would imply that the muon neutrinos annihilated themselves through their coupling with sterile neutrinos.

The CMS team found no evidence of lepton number violation in the muon data, which allowed the researchers to derive new limits on the mixing of sterile neutrinos with muon neutrinos. These limits improve on existing limits for sterile neutrino masses greater than 650 GeV/c2and represent the strongest limits from colliders for sterile neutrino masses up to 25 TeV/c2. (It should be noted that the swing experiments provide stronger bounds on sterile neutrinos, but these bounds do not include lepton number violation, and thus the relationship to the swing mechanism is not simple [10].)

Putting these results in the context of the neutrino mass mechanism theoretical landscape, the CMS constraints on the mixing parameters are about 10 orders of magnitude weaker than the predicted values ​​of these parameters in the simplest version of the see-saw mechanism. Collider experiments are therefore unlikely to probe this simple version even in the future with more statistics and updated particle accelerators. However, these results provide important constraints on popular variants of the seesaw mechanism, which generally exhibit larger mixtures. Indeed, these variants will continue to be a target of opportunity for future collider experiments, as well as for studies combining lepton number violation searches and sterile neutrino searches.

While neutrino oscillation experiments are getting close to determining all oscillation parameters, understanding the mechanism behind neutrino mass generation requires research beyond oscillation experiments. These neutrino mass studies are progressing on several fronts, but have so far only provided constraints, rather than a discovery of the origin of neutrino masses.

References

  1. A. Tumasyan et al. (CMS Collaboration), Probing Majorana heavy neutrinos and the Weinberg operator through vector boson fusion processes in proton-proton collisions at St = 13TeV, Phys. Rev. Lett. 131011803 (2023).
  2. P.Minkowski, And at a rate of one in 109 does the muon decay? Phys. Lett. B 67 (1977).
  3. P. Ramond, The family group in grand unified theories, (1979), arXiv:hep-ph/9809459.
  4. M. Gell-Mann et al.Complex spinors and unified theories, (1979), Conf. Proc. C 790927, arXiv:1306.4669.
  5. T. Yanagida, Horizontal gauge symmetry and neutrino masses, Conf. Proc. C 7902131 (1979) https://inspirehep.net/literature/143150.
  6. RN Mohapatra and G. Senjanovi, neutrino mass and non-conservation of spontaneous parity, Phys. Rev. Lett. 44 (1980).
  7. J. Schechter and JWF Valle, neutrino masses in SU(2) U theories(1), Phys. Rev. D 22 (1980).
  8. St. Abe et al. (KamLAND-Zen Collaboration), First investigation of the Majorana nature of neutrinos in the inverted mass ordering region with KamLAND-Zen, (2022), arXiv:2203.02139.
  9. B. Fuks et al.By probing the Weinberg operator at the colliders, Phys. Rev. D 103 (2021).
  10. E. Fernandez-Martinez et al.Global constraints on heavy neutrino mixing, J. High Energy Physics. 2016 (2016).

About the author

Image by Julia Gehrlein

Julia Gehrlein is a senior researcher in the department of theoretical physics at CERN, Switzerland. She received her PhD in 2019 from the Autonomous University of Madrid, where she was funded by an Early Career Junior Fellowship of the European Marie Curie project ITN Elusives. She then moved to Brookhaven National Laboratory, New York, as a research associate before joining CERN in 2022. Her research focuses on all aspects of neutrino physics, including neutrino mass models, new physical signatures in the neutrino sector and neutrino connections to other open problems of the standard model.

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