Right Handed NeutrinoEdit
A right-handed neutrino is a hypothetical fermion that completes the standard picture of neutrino masses in many extensions of the Standard Model. In the minimal formulation of the Standard Model, only left-handed neutrinos participate in the weak interaction, and there is no need for a right-handed partner. The introduction of one or more gauge-singlet right-handed neutrinos allows neutrinos to acquire mass through new terms in the Lagrangian. These states are often called sterile neutrinos in the literature because they do not interact via the Standard Model gauge forces, aside from possible mixings with the active neutrinos. The idea is widely used to explain the observed pattern of neutrino masses and mixings, to connect the microscopic origin of mass with the generation of the matter-antimatter asymmetry of the universe, and to explore possible dark matter candidates and new collider phenomenology.
Right-handed neutrinos are a common feature of many theories beyond the Standard Model. Their defining characteristic is that they are gauge singlets under the electroweak group, so they do not couple to the W and Z bosons in the same way as the active, left-handed neutrinos. This allows them to have a Majorana mass term on its own, in addition to a Dirac-type mass term that couples left- and right-handed neutrinos through the Higgs field. The interplay of these mass terms can produce observable consequences for the light, active neutrinos and for potential new physics at higher energy scales or in cosmology.
Theoretical basis
Lagrangian and mass terms
In a typical extension, the neutrino sector includes Yukawa couplings between the left-handed lepton doublets, the Higgs field, and the right-handed neutrinos, as well as a possible Majorana mass term for the right-handed states. After electroweak symmetry breaking, the Dirac mass matrix m_D arises from the Yukawa couplings and the Higgs vacuum expectation value, while the Majorana mass matrix M_R governs the intrinsic mass of the right-handed neutrinos. In the simplest Type I seesaw setup, the light neutrino masses emerge from a mass matrix of the form
m_ν ≈ - m_D M_R^{-1} m_D^T,
with the heavy right-handed neutrinos having masses approximately equal to M_R. This framework provides a natural mechanism to explain why the observed neutrino masses are so small compared with other fermions, given that the scale of M_R can be much larger than the electroweak scale.
Seesaw variants and model-building
The idea of right-handed neutrinos is embedded in several broader schemes. The Type I seesaw is the most widely discussed, but there are related constructions such as:
- Type II seesaw, which introduces scalar triplets that directly contribute to light-neutrino masses.
- Type III seesaw, which uses fermionic triplets with different gauge properties.
- Left-right symmetric models, where right-handed neutrinos are part of SU(2)R doublets and parity symmetry is restored at high energies.
- The nuMSM (neutrino Minimal Standard Model), which posits a small number of right-handed neutrinos with a range of masses to address neutrino masses, dark matter, and baryogenesis within a single framework.
In left-right and related constructions, the right-handed neutrinos are not just inert singlets; they can participate in extended gauge interactions and carry testable implications at high-energy colliders or in precision measurements. The mass scale of M_R in these models can span many orders of magnitude, from eV-scale to well above the TeV scale, producing a wide range of experimental consequences.
Mixing with active neutrinos
A key phenomenological feature is the mixing between active neutrinos and the right-handed (sterile) states, parametrized by mixing angles U_{αN} (α = e, μ, τ). These mixings determine how strongly the right-handed neutrinos can be produced or observed in experiments that probe the lepton sector. When the mixings are small, the right-handed neutrinos are long-lived on detector scales and can leave distinctive displaced signatures in colliders or fixed-target experiments. When the masses are light enough, they can participate in neutrino oscillations and affect short-baseline experiments.
Cosmology and leptogenesis
Right-handed neutrinos can play a central role in cosmology. If they decay out of equilibrium in the early universe and with CP-violating interactions, they can generate a lepton asymmetry. This lepton asymmetry can be partially converted into a baryon asymmetry by nonperturbative electroweak processes (sphalerons), offering a mechanism for the observed matter–antimatter asymmetry of the universe known as leptogenesis. The viability of leptogenesis depends on the mass spectrum, CP-violating phases, and the strength of interactions, and it connects neutrino physics to early-universe dynamics in a concrete way.
Right-handed neutrinos can also influence cosmology and structure formation through their contribution to the radiation content of the universe (the effective number of relativistic species, N_eff), their decay products, and, in certain mass ranges, as candidates for dark matter. In particular, keV-scale right-handed neutrinos have been discussed as warm dark matter candidates, while lighter or heavier spectra lead to different cosmological footprints. These possibilities are actively explored in conjunction with data from the cosmic microwave background, large-scale structure, and X-ray observations.
Experimental status and searches
Direct searches and collider phenomenology
If right-handed neutrinos lie at accessible energies, they can be produced in high-energy collisions and observed through characteristic signatures such as displaced vertices, lepton-number-violating decays, or anomalous events with leptons and missing energy. Experiments at the Large Hadron Collider (LHC) and earlier colliders have searched for heavy neutral leptons with varying assumptions about their masses and couplings. Fixed-target and beam-dump experiments, as well as proposed facilities like SHiP, target the possibility of long-lived sterile neutrinos that can be produced and decay away from the interaction point. The absence of a definitive discovery so far places bounds on the mixing parameters U_{αN} and on the mass scale M_R across different regions of parameter space.
Indirect constraints and flavor physics
Even if right-handed neutrinos do not couple strongly enough to be produced directly, they can leave indirect imprints. Precision electroweak measurements constrain new contributions to lepton couplings, while searches for lepton flavor violation (such as μ → eγ or μ → 3e) place limits on the structure of the neutrino Yukawa couplings and the mass spectrum. In certain models, the presence of right-handed neutrinos modifies neutrinoless double beta decay rates and other rare processes, linking laboratory experiments with the Majorana nature of neutrinos and the overall neutrino-mass mechanism.
Neutrinoless double beta decay and Majorana nature
A central question concerns whether neutrinos are Majorana particles. If neutrinos are Majorana fermions, neutrinoless double beta decay could occur, with a rate that depends on the Majorana masses of light neutrinos and potentially on exchange with heavier right-handed neutrinos in extended models. Observations of this process would have profound implications for the neutrino mass mechanism and for the symmetries of the lepton sector. See neutrinoless double beta decay for details and context.
Astrophysical and cosmological constraints
KeV-scale right-handed neutrinos as dark matter candidates are constrained by X-ray observations searching for radiative decays and by requirements from structure formation. In the eV to sub-GeV range, sterile neutrinos affect oscillation phenomenology and the cosmic radiation budget, with bounds coming from Big Bang nucleosynthesis and the cosmic microwave background data. The interplay between laboratory searches and cosmology helps carve out the viable regions in mass-mmixing space.
Implications for broader physics
Neutrino masses and the flavor puzzle
Right-handed neutrinos provide a natural framework for understanding the hierarchy of neutrino masses and the pattern of mixing angles observed in solar, atmospheric, reactor, and accelerator experiments. The seesaw relations tie the smallness of active-neutrino masses to the scale and structure of the right-handed sector, offering a coherent narrative that connects low-energy neutrino data to high-energy model-building.
Testable predictions and future prospects
A variety of experimental programs aim to probe the right-handed neutrino sector across mass scales. At the high end, collider experiments search for heavy neutral leptons and related gauge bosons in left-right symmetric models. In the intermediate and low-mass regimes, precision neutrino experiments, short-baseline oscillation studies, and dedicated fixed-target facilities look for signatures of sterile neutrinos, displaced decays, or deviations in lepton-flavor observables. Cosmological data continue to constrain the number and properties of additional neutrino-like states, shaping the accessible parameter space for these theories.