Neutral Fermions Research Articles

Characterizing heavy neutral fermions via their decays

Many extensions of the Standard Model of particle physics contain new electrically-neutral fermions. Should one of these particles be discovered, questions will naturally arise regarding its nature. For instance: is it a self-conjugate particle (i.e., is it a Dirac or a Majorana fermion)?, does it interact via the Standard Model force carriers or something else? One set of well-motivated particles in this class are Heavy Neutral Leptons (HNLs), Standard Model gauge-singlet fermions that mix with the neutrinos and may be produced in meson decays. We demonstrate that measuring the three body decays of the HNL (or phenomenologically similar heavy fermions) can help determine whether they are Majorana or Dirac fermions. We also investigate the ability to distinguish among different models for the physics responsible for the HNL decay. We compare the reach assuming full and partial event reconstruction, and propose experimental analyses. Should a new fermion be discovered, studying its three body decays provides a powerful diagnostic tool of its nature.

Vacuum instability due to the creation of neutral fermion with anomalous magnetic moment by magnetic-field inhomogeneities

We study neutral fermions pair creation with anomalous magnetic moment from the vacuum by time-independent magnetic-field inhomogeneity as an external background. We show that the problem is technically reduced to the problem of charged-particle creation by an electric step, for which the nonperturbative formulation of strong-field QED is used. We consider a magnetic step given by an analytic function and whose inhomogeneity may vary from a “gradual” to a “sharp” field configuration. We obtain corresponding exact solutions of the Dirac-Pauli equation with this field and calculate pertinent quantities characterizing vacuum instability, such as the differential mean number and flux density of pairs created from the vacuum, vacuum fluxes of energy and magnetic moment. We show that the vacuum flux in one direction is formed from fluxes of particles and antiparticles of equal intensity and with the same magnetic moments parallel to the external field. Backreaction to the vacuum fluxes leads to a smoothing of the magnetic-field inhomogeneity. We also estimate critical magnetic field intensities, near which the phenomenon could be observed.

Particle scattering by rotating trapped quantum gases at finite temperature

We have analytically explored the quantum phenomena of particle scattering by rotating trapped quantum gases of electrically neutral bosons and fermions for the short-ranged Fermi-Huang interactions between the incident particle and the scatterers. We have predicted differential scattering cross-sections and their temperature and angular velocity dependencies in this regard, in particular, for an ideal Bose gas in a rotating harmonic trap, an ideal Fermi gas in a rotating harmonic trap, and a weakly interacting Bose gas in a slow rotating harmonic trap. We have theoretically probed the lattice-pattern of the vortices in a rapidly rotating strongly interacting Bose–Einstein condensate by the particle scattering method. We also have obtained de Haas-van Alphen-like oscillations in the differential scattering cross-section for an ideal ultracold Fermi gas in a rotating harmonic trap. Our predictions on the differential scattering cross-sections can be tested within the present-day experimental setups.

Search for a light Z′ at LHC in a neutrinophilic U(1) model

We consider a neutrinophilic $U(1)$ extension of the standard model (SM) which couples only to SM isosinglet neutral fermions, charged under the new group. The neutral fermions couple to the SM matter fields through Yukawa interactions. The neutrinos in the model get their masses from a standard inverse-seesaw mechanism while an added scalar sector is responsible for the breaking of the gauged $U(1)$ leading to a light neutral gauge boson (${Z}^{\ensuremath{'}}$), which has minimal interaction with the SM sector. We study the phenomenology of having such a light ${Z}^{\ensuremath{'}}$ in the context of neutrinophilic interactions as well as the role of allowing kinetic mixing between the new $U(1)$ group with the SM hypercharge group. We show that current experimental searches allow for a very light ${Z}^{\ensuremath{'}}$ if it does not couple to SM fields directly and highlight the search strategies at the LHC. We observe that multilepton final states in the form of $(4\ensuremath{\ell}+{\overline{)E}}_{T})$ and $(3\ensuremath{\ell}+2j+{\overline{)E}}_{T})$ could be crucial in discovering such a neutrinophilic gauge boson lying in a mass range of 200--500 GeV.

FIMP DM in the extended hyperchargeless Higgs triplet model

We perform an exclusive study on the feebly interacting massive particle dark matter candidate in an extended hyperchargeless ($Y=0$) Higgs triplet model. The additional ${Z}_{2}$ odd neutral fermion singlet plays the role of dark matter with support from two other vectorlike fermion doublets. The mixing between the neutral component of a doublet and singlet fermions controls the current relic density through the freeze-in mechanism, whereas the additional doublet fermion helps to get the neutrino mass and mixing angles. We obtain a broad region of the parameter spaces satisfying the current relic density and neutrino mass and mixing angles.

A full parametrization of the 9 × 9 active-sterile flavor mixing matrix in the inverse or linear seesaw scenario of massive neutrinos

The inverse and linear seesaw scenarios are two typical extensions of the canonical seesaw mechanism, which contain much more sterile degrees of freedom but can naturally explain the smallness of three active neutrino masses at a sufficiently low energy scale (e.g., the TeV scale). To fully describe the mixing among three active neutrinos, three sterile neutrinos and three extra gauge-singlet neutral fermions in either of these two seesaw paradigms, we present the first full parametrization of the 9×9 flavor mixing matrix in terms of 36 rotation angles and 36 CP-violating phases. The exact inverse and linear seesaw formulas are derived, respectively; and possible deviations of the 3×3 active neutrino mixing matrix from its unitary limit are discussed by calculating the effective Jarlskog invariants and unitarity nonagons.

Cosmological and astrophysical probes of dark baryons

We examine the cosmological and astrophysical signatures of a ``dark baryon,'' a neutral fermion that mixes with the neutron. As the mixing is through a higher-dimensional operator at the quark level, production of the dark baryon at high energies is enhanced so that its abundance in the early universe may be significant. Treating its initial abundance as a free parameter, we derive new, powerful limits on the properties of the dark baryon. Primordial nucleosynthesis and the cosmic microwave background provide strong constraints due to the interconversion of neutrons to dark baryons through their induced transition dipole, and due to late decays of the dark baryon. Additionally, neutrons in a neutron star could decay slowly to dark baryons, providing a novel source of heat that is constrained by measurements of pulsar temperatures. Taking all the constraints into account, we identify parameter space where the dark baryon can be a viable dark matter candidate and discuss promising avenues for probing it.

Hidden Fermi liquidity and topological criticality in the finite temperature Kitaev model

The fate of exotic spin liquid states with fractionalized excitations at finite temperature (T) is of great interest, since signatures of fractionalization manifest in finite-temperature (T) dynamics in real systems, above the tiny magnetic ordering scales. Here, we study a Jordan–Wigner (JW) fermionized Kitaev spin liquid at finite T employing combined exact diagonalization and Monte Carlo simulation methods. We uncover (i) checkerboard or stripy-ordered flux crystals depending on density of flux, and (ii) establish, surprisingly, that: (a) the finite-T version of the T=0 transition from a gapless to gapped phases in the Kitaev model is a Mott transition of the fermions, belonging to the two-dimensional Ising universality class. These transitions correspond to a topological transition between a string condensate and a dilute closed string state (b) the Mott “insulator” phase is a precise realization of Laughlin’s gossamer (here, p-wave) superconductor (g-SC), and (c) the Kitaev Toric Code phase (TC) is adiabatically connected to the g-SC, and is a fully Gutzwiller-projected fermi sea of JW fermions. These findings establish the finite-T quantum spin liquid phases in the d=2 to be hidden Fermi liquid(s) of neutral fermions.

Scalar dark matter and leptogenesis in the minimal scotogenic model

We study the minimal scotogenic model constituting an additional inert Higgs doublet and three sets of right-handed neutrinos. The scotogenic model connects dark matter, baryon asymmetry of the Universe and neutrino oscillation data. In our work, we obtain baryogenesis by the decay of TeV scale heavy neutral singlet fermion (N2). We primarily focus on the intermediate-mass region of dark matter within MW<MDM≤550 GeV, where observed relic density is suppressed due to co-annihilation processes. We consider thermal as well as the non-thermal approach of dark matter production and explore the possibility of the lightest stable candidate being a dark matter candidate. Within the inert Higgs doublet (IHD) desert, we explore a new allowed region of dark matter masses for the non-thermal generation of dark matter with a mass splitting of 10 GeV among the inert scalars. We also see the variation of relic abundance for unequal mass splitting among the scalars. The KamLand-Zen bound on the effective mass of the active neutrinos is also verified in this study.

Quantum Oscillations and Charge Neutral Fermions in Kondo Insulators

Quantum Oscillations and Charge Neutral Fermions in Kondo Insulators

Tunable flux through a synthetic Hall tube of neutral fermions

Hall tube with a tunable flux is an important geometry for studying quantum Hall physics, but its experimental realization in real space is still challenging. Here, we propose to realize a synthetic Hall tube with tunable flux in a one-dimensional optical lattice with the synthetic ring dimension defined by atomic hyperfine states. We investigate the effects of the flux on the system topology and study its quench dynamics. Utilizing the tunable flux, we show how to realize topological charge pumping, where interesting charge flow and transport are observed in rotated spin basis. Finally, we show that the recently observed quench dynamics in a synthetic Hall tube can be explained by the random flux existing in the experiment.

Cosmic-ray signatures of dark matter from a flavor dependent gauge symmetry model with neutrino mass mechanism

We propose an extension to the Standard Model accommodating two families of Dirac neutral fermions and Majorana fermions under additional ${U(1)_{e-\mu} \times Z_3\times Z_2}$ symmetries where ${U(1)_{e-\mu}}$ is a flavor dependent gauge symmetry related to the first and second family of the lepton sector, which features a two-loop induced neutrino mass model. The two families are favored by minimally reproducing the current neutrino oscillation data and two mass difference squares and canceling the gauge anomalies at the same time. As a result, we have a prediction for neutrino masses. The lightest Dirac neutral fermion is a dark matter candidate with tree-level interaction restricted to electron, muon and neutrinos, which makes it difficult to detect in direct dark matter search as well as indirect search focusing on the ${\tau}$-channel, such as through ${\gamma}$-rays. It may however be probed by search for dark matter signatures in electron and positron cosmic rays, and allows interpretation of a structure appearing in the CALET electron+positron spectrum around 350-400 GeV as its signature, with a boost factor $\approx$40 Breit-Wigner enhancement of the annihilation cross section.

Thermodynamic instabilities in holographic neutron stars at finite temperature

We study the thermodynamics of a self-gravitating system of neutral fermions at finite temperature and analyze its backreaction in an asymptotically AdS space. We evaluate numerically the free entropy as a function of temperature, and perform a stability analysis applying the simpler and powerful graphical method referred as the Katz criterion. We found that for highly-enough degenerate fermionic solutions, the onset of thermodynamic instability arises, though prior to the turning point on the mass as a function of the central density. Our results for finite temperature fermions provide a novel and more general way to study the confinement to deconfinement phase transition in the holographic field theory, generalizing former conclusions developed for systems at zero temperature.

Neutrino mass, mixing and muon g \u2212 2 explanation in mathrm{U}{(1)}_{L_{mu }-{L}_{tau }} extension of left-right theory

We consider a gauged mathrm{U}{(1)}_{L_{mu }-{L}_{tau }} extension of the left-right symmetric theory in order to simultaneously explain neutrino mass, mixing and the muon anomalous magnetic moment. We get sizeable contribution from the interaction of the new light gauge boson Zμτ of the mathrm{U}{(1)}_{L_{mu }-{L}_{tau }} symmetry with muons which can individually satisfy the current bounds on muon (g − 2) anomaly (∆aμ). The other positive contributions to ∆aμ come from the interactions of singly charged gauge bosons WL, WR with heavy neutral fermions and that of neutral CP-even scalars with muons. The interaction of WL with heavy neutrino is facilitated by inverse seesaw mechanism which allows large light-heavy neutrino mixing and explains neutrino mass in our model. CP-even scalars with mass around few hundreds GeV can also satisfy the entire current muon anomaly bound. The results show that the model gives a small but non-negligible contribution to ∆aμ thereby eliminating the entire deviation in theoretical prediction and experimental result of muon (g − 2) anomaly. We have briefly presented a comparative study for symmetric and asymmetric left-right symmetric model in context of various contribution to ∆aμ. We also discuss how the generation of neutrino mass is affected when left-right symmetry breaks down to Standard Model symmetry via various choices of scalars.

Correlation functions of charged free boson and fermion systems

Using the idea of the quantum inverse scattering method, we introduce the operators B(x), C(x) and corresponding to the off-diagonal entries of the monodromy matrix T for the phase model and i-boson model in terms of bc fermions and neutral fermions respectively, thus giving alternative treatment of the KP and BKP hierarchies. We also introduce analogous operators B*(x) and C*(x) for the charged free boson system and show that they are in complete analogy to those of bc fermionic fields. It is proved that the correlation function ⟨0|C(xN)⋯C(x1)B(y1)⋯B(yN)|0⟩ in the bc fermionic fields is the inverse of the correlation function ⟨0|C*(xN)⋯C*(x1)B*(y1)⋯B*(yN)|0⟩ in the charged free bosons.

Fuzzy dark matter and neutron Majorana mass from gravitational instantons

We explore the possibility that a super-light candidate for fuzzy (or wave) Dark Matter (fDM) may be a new composite boson state of fermions, rather than an axion-like Bose-Einstein condensate. We start from string-inspired massless Majorana fermions χ thought as super-modulini fields from string compactifications. We show that massless (or nearly massless) fermions can form a Bose-Einstein condensate and they can acquire a tiny mass term from quantum-gravitational non-perturbative effects such as gravitational instantons. For having a successful candidate for fDM, the so generated Majorana mass is , or so; otherwise decoherence would suppress quantum-mechanical effects in the macroscopic limit. Gravity is democratically coupled with every standard model particles; any neutral fermions acquire a Majorana mass equal to . While for neutrinos, such a mass would be completely out of any testability, a neutron Majorana mass can also be envisaged. A gravitational Majorana mass for the neutron generates baryon violating neutron-antineutron oscillations. Contrary to other competitive models, here the transition is generated without adding any new particles or interactions to the standard model: it is a genuine quantum gravity effect. Current limits on imply a Majorana mass bound and, therefore, the very same limit on the Gravi-fuzzy DM (GfDM) candidate. The next generation of experiments searching for can probe up to the , potentially excluding Gravi-fuzzy DM up to it.

Hunting for scalar lepton partners at future electron colliders

New physics close to the electroweak scale is well motivated by a number of theoretical arguments. However, colliders, most notably the Large Hadron Collider (LHC), have failed to deliver evidence for physics beyond the Standard Model. One possibility for how new electroweak-scale particles could have evaded detection so far is if they carry only electroweak charge, i.e. are color neutral. Future $e^+e^-$ colliders are prime tools to study such new physics. Here, we investigate the sensitivity of $e^+e^-$ colliders to scalar partners of the charged leptons, known as sleptons in supersymmetric extensions of the Standard Model. In order to allow such scalar lepton partners to decay, we consider models with an additional neutral fermion, which in supersymmetric models corresponds to a neutralino. We demonstrate that future $e^+e^-$ colliders would be able to probe most of the kinematically accessible parameter space, i.e. where the mass of the scalar lepton partner is less than half of the collider's center-of-mass energy, with only a few days of data. Besides constraining more general models, this would allow to probe some well motivated dark matter scenarios in the Minimal Supersymmetric Standard Model, in particular the incredible bulk and stau co-annihilation scenarios.

Fermion–fermion duality in 3+1 dimensions

Dualities play a central role in both quantum field theories and condensed matter systems. Recently, a web of dualities has been discovered in 2+1 dimensions. Here, we propose in particular a generalization of the Son’s fermion–fermion duality to 3+1 dimensions. We show that the action of charged Dirac fermions coupled to an external electromagnetic field is dual to an action of neutral fermions minimally coupled to an emergent vector gauge field. This dual action contains also a further tensor (Kalb–Ramond) gauge field coupled to the emergent and electromagnetic vector potentials. We firstly demonstrate the duality in the massive case. We then show the duality in the case of massless fermions starting from a lattice model and employing the slave-rotor approach already used in the 2+1-dimensional duality [Burkov (2019)]. We finally apply this result to 3D Dirac semimetals in the low-energy regime. Besides the implications in topological phases of matter, our results shed light on the possible existence of a novel web of dualities in 3+1-dimensional (non-supersymmetric) quantum field theories.

Inverse seesaw accompanied by Dirac fermionic dark matter

We present an inverse seesaw mechanism by resorting to a U(1)B−L gauge symmetry. In order to cancel the gauge anomalies, we introduce nine neutral fermions among which six participate in the inverse seesaw, while the other three form a Dirac fermion and a massless fermion. In this inverse seesaw, three neutral fermions are the usual right-handed neutrinos while the other three have a small Majorana mass term. An additional seesaw mechanism for generating these small Majorana masses also explains the cosmic baryon asymmetry in association with the sphaleron processes. The Dirac fermion becomes a stable dark matter while the massless fermion decouples early.

Neutrino phenomenology and dark matter in an A_4 flavour extended B-L model

We present an {mathrm{A}}_4 flavor extended {mathrm{B-L}} model for realization of eV scale sterile neutrinos, motivated by the recent experimental hints from both particle physics and cosmology. The framework considered here is a gauged {mathrm{B-L}} extension of standard model without the introduction of right-handed neutrinos, where the gauge triangle anomalies are canceled with the inclusion of three exotic neutral fermions N_{i} (i=1,2,3) with {mathrm{B-L}} charges -,4,-,4 and 5. The usual Dirac Yukawa couplings between the SM neutrinos and the exotic fermions are absent and thus, the model allows natural realization of eV scale sterile-like neutrino and its mixing with standard model neutrinos by invoking {mathrm{A}}_4 flavor symmetry. We demonstrate how the exact tri-bimaximal mixing pattern is perturbed due to active-sterile mixing by analyzing 1+3 case in detail. We also show the implication of eV scale sterile-like neutrino on various observables in neutrino oscillation experiments and the effective mass in neutrinoless double beta decay. Another interesting feature of the model is that one of three exotic fermions is required to explain eV scale phenomena, while other two fermions form stable dark matter candidates and their total relic density satisfy the observed 3sigma limit of Planck data. We constrain the gauge parameters associated with U(1) gauge extension, using relic density and collider bounds.