Neutral Fermions Research Articles

Imaging fermionic dark matter cores at the centre of galaxies

ABSTRACT Current images of the supermassive black hole (SMBH) candidates at the centre of our Galaxy and M87 have opened an unprecedented era for studying strong gravity and the nature of relativistic sources. Very-long-baseline interferometry data show images consistent with a central SMBH within General Relativity (GR). However, it is essential to consider whether other well-motivated dark compact objects within GR could produce similar images. Recent studies have shown that dark matter (DM) haloes modelled as self-gravitating systems of neutral fermions can harbour very dense fermionic cores at their centres, which can mimic the space–time features of a black hole (BH). Such dense, horizonless DM cores can satisfy the observational constraints: they can be supermassive and compact and lack a hard surface. We investigate whether such cores can produce similar observational signatures to those of BHs when illuminated by an accretion disc. We compute images and spectra of the fermion cores with a general-relativistic ray tracing technique, assuming the radiation originates from standard $\alpha$ discs, which are self-consistently solved within the current DM framework. Our simulated images possess a central brightness depression surrounded by a ring-like feature, resembling what is expected in the BH scenario. For Milky Way-like haloes, the central brightness depressions have diameters down to ${\sim} 35\, \mu \text{as}$ as measured from a distance of approximately $8\,$ kpc. Finally, we show that the DM cores do not possess photon rings, a key difference from the BH paradigm, which could help discriminate between the models.

Dark sink enhances the direct detection of freeze-in dark matter

We describe a simple dark sector structure which, if present, has implications for the direct detection of dark matter (DM); . A dark sink transports energy density from the DM into light dark-sector states that do not appreciably contribute to the DM density. As an example, we consider a light, neutral fermion ψ which interacts solely with DM χ via the exchange of a heavy scalar Φ. We illustrate the impact of a dark sink by adding one to a DM freeze-in model in which χ couples to a light dark photon γ′ which kinetically mixes with the Standard Model (SM) photon. This freeze-in model (absent the sink) is itself a benchmark for ongoing experiments. In some cases, the literature for this benchmark has contained errors; we correct the predictions and provide them as a public code. We then analyze how the dark sink modifies this benchmark, solving coupled Boltzmann equations for the dark-sector energy density and DM yield. We check the contribution of the dark sink ψ’s to dark radiation; consistency with existing data limits the maximum attainable cross section. For DM with a mass between MeV−O(10 GeV), adding the dark sink can increase predictions for the direct detection cross section all the way up to the current limits. Published by the American Physical Society 2024

A novel proton decay signature at DUNE, JUNO, and Hyper-K

Proton decay, although unobserved so far, is a natural expectation when attempting to explain the baryon asymmetry of the universe. p→K+ν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ p\ o {K}^{+}\\overline{\ u} $$\\end{document} or p→K+χ~10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ p\ o {K}^{+}{\\overset{\\sim }{\\chi}}_1^0 $$\\end{document}, with χ~10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\overset{\\sim }{\\chi}}_1^0 $$\\end{document} a light exotic neutral particle, represent possible decay channels achievable in models of physics beyond the Standard Model, such as the MSSM with trilinear R-parity-violating terms, or the Standard Model extended by a heavy neutral lepton. Among the decay products of these modes, the neutral fermions would typically appear as missing energy in collider searches. The present study considers how such decay modes could be differentiated in experimental settings, as the exotic χ~10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\overset{\\sim }{\\chi}}_1^0 $$\\end{document} may further decay if it is not protected by a symmetry (such as R-parity in the MSSM). We assess the detection prospects of the proposed experiments DUNE, JUNO and Hyper-K in this context.

Complete EFT operator bases for dark matter and weakly-interacting light particle

The standard model can be extended to include weakly-interacting light particle (WILP): real or complex singlet scalar with a conserved U(1) charge, Majorana or Dirac neutral fermion, neutral or hidden-charged vector boson, etc. Imposing the Z2 symmetry, these particles can be lifted as the weakly-interacting massive particle (WIMP), the candidate of dark matter. Instead, imposing the shift symmetry on the scalar components gives rise to the axion-like particle, dark photon, etc. Utilizing these light degrees of freedom along with the standard model particles and imposing different symmetries, we construct the complete and independent sets of effective operators up to dimension eight with the Young tensor technique, consistent with counting from the Hilbert series.

UC and BUC plane partitions

This paper is concerned with the investigation of UC and BUC plane partitions based upon the fermion calculus approach. We construct generalized the vertex operators in terms of free charged fermions and neutral fermions and present the interlacing (strict) 2-partitions. Furthermore, it is showed that the generating functions of UC and BUC plane partitions can be written as product forms.

The noncommutative Dirac oscillator with a permanent electric dipole moment in the presence of an electromagnetic field

In this work, we investigate the noncommutative Dirac oscillator (NCDO) with a permanent electric dipole moment (EDM) in the presence of an electromagnetic field. We consider a radial magnetic field generated by anti-Helmholtz coils and the uniform electric field of the Stark effect. Next, we determine the exact solutions of the system, given by the Dirac spinor and by the relativistic energy spectrum. We note that such a spinor is written in terms of the generalized Laguerre polynomials, and such a spectrum is a linear function on the potential energy U and depends on the quantum numbers n and m, spin parameter s, and of four angular frequencies: ω, ω¯ , ω θ , and ω η , where ω is the frequency of the DO, ω¯ is a type of ‘cyclotron frequency’, and ω θ and ω η are the NC frequencies of position and momentum. We graphically analyze the behavior of the spectrum as a function of these frequencies for three different values of n, with and without the influence of U. Besides, as an interesting result, another interpretation can be given for the origin of the nonminimal coupling of the DO, which would be through a neutral fermion with EDM in a radial magnetic field. Finally, we also analyze the nonrelativistic limit of our results, and comparing our problem with other works, we verified that our results generalize some particular cases in the literature.

Long-lived neutral fermions at the DUNE near detector

At the Deep Underground Neutrino Experiment (DUNE), a proton beam hits a fixed target leading to large production rates of mesons. These mesons can decay and potentially provide a source of long-lived neutral fermions. Examples of such long-lived fermions are heavy neutral leptons which can mix with the standard-model active neutrinos, and the bino-like lightest neutralino in R-parity-violating supersymmetry. We show that the Standard Model Effective Field Theory extended with right-handed singlet neutrinos can simultaneously describe heavy neutral leptons and bino-like neutralinos in a unified manner. We use the effective-field-theory framework to determine the sensitivity reach of the DUNE near detector in probing various scenarios of long-lived neutral fermions.

On the fermionic couplings of axionic dark matter

In the non-relativistic limit, two types of dark matter axion interactions with fermions are thought to dominate: one is induced by the spatial gradient of the axion field and called the axion wind, and the other by the time-derivative of the axion field, generating axioelectric effects. By generalizing Schiff theorem, it is demonstrated that this latter operator is actually strongly screened. For a neutral fermion, it can be entirely rotated away and is unobservable. For charged fermions, the only effect that can peek through the screening is an axion-induced electric dipole moment (EDM). These EDMs are not related to the axion coupling to gluons, represent a prediction of the Dirac theory analogous to the g=2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$g=2$$\\end{document} magnetic moments, are not further screened by the original Schiff theorem, and are ultimately responsible for inducing the usual axioelectric ionization. The two main phenomenological consequences are then that first the axion-induced nucleon EDM could be significantly larger than expected from the axion gluonic coupling, and second, that the electron EDM could also become available, and could actually be highly sensitive to relic axions.

Origin of primeval seed magnetism in rotating astrophysical bodies

We show that a primeval seed magnetic field arises due to spin-degeneracy breaking of fermions caused by the dragging of inertial frames in the curved spacetime of rotating astrophysical bodies. This seed magnetic field would arise even due to electrically neutral fermions such as neutrons. As examples, firstly we show that an ideal neutron star rotating at 500 revolutions per second, having mass 0.83 M⊙ and described by an ensemble of degenerate neutrons, would have 0.12 Gauss seed magnetic field at its center arising through the breaking of spin-degeneracy. Secondly, similar seed field at a proto-galactic stage for the Milky Way galaxy as implied by its observed rotation curve is estimated to be between 10-19–10-24 Gauss, a field strength which is known to be sufficient to produce presently observed microgauss magnetic field.

Solutions of the universal character hierarchy and BUC hierarchy by fermionic approach

In this paper, by means of the fermionic approach, we investigate the (m,n)-order τ-function of the universal character (UC) hierarchy which can be expressed as the determinants of the Schur functions. Meanwhile, we present the general τ-function for charged free fermions, from which soliton solutions of the UC hierarchy have been discussed. Furthermore, the algebraic structure and properties of the universal character hierarchy of B type (BUC hierarchy) have been developed. In terms of neutral free fermions and the generalized Q-function, the polynomial τ-functions and the extended formulas of the BUC and the 2-component BUC hierarchies have been analyzed. In addition, we derive the rational solutions of the BUC hierarchy. It is showed that soliton solutions of the BUC hierarchy can be expressed as multiplication of Pfaffians.

LFV Higgs and Z-boson decays: leptonic CPV phases and CP asymmetries

Heavy neutral leptons are motivated by several extensions of the Standard Model and their presence induces modifications in the lepton mixing matrix, including new Dirac and Majorana CP violating phases. It has been recently shown that these phases play an important role in lepton number and lepton flavour violating decays and transitions, with a striking impact for the predicted rates of certain observables. In this work, we now consider the potential role of the heavy neutral fermions and of the new CP violating phases in Higgs and Z-boson lepton flavour violating decays H,Zrightarrow ell _alpha ^pm ell _beta ^mp , as well as in the corresponding CP asymmetries. In order to allow for a thorough analysis, we derive the full analytical expressions of the latter observables, taking into account the effect of the (final state) charged lepton masses. A comprehensive exploration of lepton flavour violating Z and Higgs decays confirms that these are very sensitive to the presence of the additional heavy neutral leptons. Moreover, the new CPV phases have a clear impact on the decay rates, leading to both destructive and constructive interferences. Interestingly, in the mu tau sector, the Zrightarrow ell _alpha ^pm ell _beta ^mp rates are within reach of FCC-ee, with associated CP asymmetries that can potentially reach up to 20–30%.

Gravitationally localized states of two neutral fermions interacting with a Higgs field

We present localized ``particlelike'' states composed of a pair of neutral fermions interacting with a scalar Higgs field and the metric of spacetime, extending the Einstein-Dirac formalism introduced by Finster et al. [Particlelike solutions of the Einstein-Dirac equations, Phys. Rev. D 59, 104020 (1999)]. We demonstrate that, when the coupling between the fermions and the Higgs field is strong, there is a class of states in which the total (ADM) mass no longer increases proportionally to the mass of the constituent fermions; indeed it decreases. This phenomenon enables fermionic particles with much larger masses than in the Higgs-free case to form localized states.

Fermionic Dark Matter: Physics, Astrophysics, and Cosmology

The nature of dark matter (DM) is one of the most relevant questions in modern astrophysics. We present a brief overview of recent results that inquire into the possible fermionic quantum nature of the DM particles, focusing mainly on the interconnection between the microphysics of the neutral fermions and the macrophysical structure of galactic halos, including their formation both in the linear and non-linear cosmological regimes. We discuss the general relativistic Ruffini–Argüelles–Rueda (RAR) model of fermionic DM in galaxies, its applications to the Milky Way, the possibility that the Galactic center harbors a DM core instead of a supermassive black hole (SMBH), the S-cluster stellar orbits with an in-depth analysis of the S2’s orbit including precession, the application of the RAR model to other galaxy types (dwarf, elliptic, big elliptic, and galaxy clusters), and universal galaxy relations. All the above focus on the model parameters’ constraints most relevant to the fermion mass. We also connect the RAR model fermions with particle physics DM candidates, self-interactions, and galactic observable constraints. The formation and stability of core–halo galactic structures predicted by the RAR model and their relations to warm DM cosmologies are also addressed. Finally, we provide a brief discussion of how gravitational lensing, dynamical friction, and the formation of SMBHs can also probe the DM’s nature.

Origin of neutrino masses, dark matter, leptogenesis, and inflation in a seesaw model with triplets

We consider a new physics model, where the Standard Model (SM) is extended by hyperchargeless $Y=0$ triplet fermions and a Higgs triplet with hypercharge $Y=2$. The first two generation fermion triplets are even under the ${Z}_{2}$ transformation. In contrast, the third fermion triplet and scalar triplet are odd under the same ${Z}_{2}$ transformation. It is a unifying framework for the simultaneous explanation of neutrino mass and mixing, dark matter, baryogenesis, inflation, and reheating temperature of the Universe. The two ${Z}_{2}$ even neutral fermions explain the neutrino low energy variables, whereas the third one can serve as a viable dark matter candidate, explaining the exact relic density. The scalar triplet is coupled nonminimally to gravity and forms the inflaton. We calculate the inflationary parameters and find them consistent with the new Planck-2018 constraints. We also do the reheating analysis for the inflaton decays/annihilations to relativistic SM particles. The triplet fermions associated with ${Z}_{2}$ even sector can provide the observed baryon asymmetry of the Universe at the TeV scale.

Searches for heavy neutrinos at multi-TeV muon collider: a resonant leptogenesis perspective

In this work, the Standard Model (SM) is extended with two right-handed (RH) neutrinos and two singlet neutral fermions to yield active neutrino masses via (2,2) inverse see-saw mechanism. We first validate the multi-dimensional model parameter space with neutrino oscillation data, obeying the experimental bounds coming from the lepton flavor violating (LFV) decays: mu rightarrow e gamma ,~ tau rightarrow e gamma , ~ tau rightarrow mu gamma . Besides we also search for the portion of the parameter space which yield the observed baryon asymmetry of the universe via resonant leptogenesis. Further, we pick up a few benchmark points from the aforementioned parameter space with TeV scale heavy neutrinos and perform an exhaustive collider analysis of the final states: 2l + E_T !!!!!!/;, and 1,l + 2j + not ! E_T in multi-TeV muon collider.

Connected (n,m)-point functions of diagonal 2-BKP tau-functions and spin double Hurwitz numbers

We derive an explicit formula for connected (n, m)-point functions associated with an arbitrary diagonal tau-function of the 2-BKP hierarchy using the computation of neutral fermions and boson–fermion correspondence of type B and then apply this formula to the computation of connected spin double Hurwitz numbers. This is the type B analog of Wang and Yang [arXiv:2210.08712 (2022)].

A Portalino to the Twin Sector

Extensions of the Standard Model are often highly constrained by cosmology. New states in the theory can dramatically alter observed properties of the universe by the presence of additional matter or entropy. In particular, attempts to solve the hierarchy problem through naturalness invariably predict new particles near the weak scale which come into thermal equilibrium. Without a means to deposit this energy into the SM, these models are often excluded. Scenarios of “neutral naturalness” in particular, such as the Twin Higgs, frequently suffer from this. However, the Portalino, a singlet fermion that marries gauge neutral fermion operators, can naturally help provide a portal for entropy to return to the SM and to lift fermionic degrees of freedom in the Twin Sector. Together with spontaneous breaking of the Z2 SM ↔ Twin symmetry, there are new opportunities to confront the cosmological challenges of these models. Here, we attempt to develop such ideas. We shall show how one can lift many of the light fields by breaking Z2 with a U(1)Y scalar and its Twin partner. The introduction of Portalinos can lift the remaining degrees of freedom. We shall find that such models are highly constrained by precision SM measurements, motivating moderate extensions beyond this. We will discuss two, both of which include Z2 breaking, one with additional electroweak matter and another with additional colored matter. The electroweak model will involve simple dim-6 operators, which are easily UV completed. The strong model will involve the presence of new leptoquarks and diquarks. We will discuss the implications for the observed value of the muon anomalous magnetic moment, contributions to μ → eγ decay and possible colored signals even within these models of neutral naturalness, some of which might appear at the LHC or future colliders.

Leptogenesis and Dark Matter–Nucleon Scattering Cross Section in the SE6SSM

The E6-inspired extension of the minimal supersymmetric (SUSY) standard model (MSSM) with an extra U(1)N gauge symmetry, under which right-handed neutrinos have zero charge, involves exotic matter beyond the MSSM to ensure anomaly cancellation. We consider the variant of this extension (SE6SSM) in which the cold dark matter is composed of the lightest neutral exotic fermion and gravitino. The observed baryon asymmetry can be induced in this case via the decays of the lightest right-handed neutrino/sneutrino into exotic states even for relatively low reheating temperatures TR≲106−7GeV. We argue that there are some regions of the SE6SSM parameter space, which are safe from all current constraints, and discuss the implications of this model for collider phenomenology.

The present and future status of heavy neutral leptons

The existence of nonzero neutrino masses points to the likely existence of multiple Standard Model neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as heavy neutral leptons (HNLs). In this white paper, we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding of key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.

Inner skin effects on non-Hermitian topological fractals

Non-Hermitian (NH) crystals, quasicrystals, and amorphous network display an accumulation of a macroscopic number of states near one of its specific interfaces with vacuum, such as edge, surface, hinge, or corner. This phenomenon is known as the NH skin effect, which can only be observed with open boundary condition. In this regard self-similar fractals, manifesting inner boundaries in the interior of the system, harbor a novel phenomenon, the inner skin effect (ISE). Then the NH skin effect appears at the inner boundaries of the fractal lattice with periodic boundary condition. We showcase this observation by implementing prominent models for NH insulators and superconductors on representative planar Sierpinski carpet fractal lattices. They accommodate both first-order and second-order ISEs at inner edges and corners, respectively, for charged as well as neutral Majorana fermions. Furthermore, over extended parameter regimes ISEs are tied with nontrivial bulk topological invariants, yielding intrinsic ISEs. With the recent success in engineering NH topological phases on highly tunable metamaterial platforms, such as photonic and phononic lattices, as well as topolectric circuits, the proposed ISEs can be observed experimentally at least on fractal metamaterials with periodic boundary condition.