Left-handed Neutrinos Research Articles

Leptogenesis in the minimal flipped SU(5) unification

We study the prospects of thermal leptogenesis in the framework of the minimal flipped SU(5) unified model in which the right-handed (RH) neutrino mass scale emerges as a two-loop effect. Despite its strong suppression with respect to the unification scale that tends to disfavor leptogenesis in the standard Davidson-Ibarra regime (and a notoriously large washout of the N1-generated asymmetry owing to a toplike Yukawa entry in the Dirac neutrino mass matrix) the desired ηB∼6×10−10 can still be attained in several parts of the parameter space exhibiting interesting baryon and lepton number violation phenomenology. Remarkably enough, in all these regions the mass of the lightest left-handed (LH) neutrino is so low that it yields mβ≲0.03 eV for the effective neutrino mass measured in beta decay, i.e., an order of magnitude below the design sensitivity limit of the KATRIN experiment. This makes the model potentially testable in the near future. Published by the American Physical Society 2024

Estimate for the Neutrino Magnetic Moment from Pulsar Kick Velocities Induced at the Birth of Strange Quark Matter Neutron Stars

We estimate the magnetic moment of electron neutrinos by computing the neutrino chirality flip rate that can occur in the core of a strange quark matter neutron star at birth. We show that this process allows neutrinos to anisotropically escape, thus inducing the star kick velocity. Although the flip from left- to right-handed neutrinos is assumed to happen in equilibrium, the no-go theorem does not apply because right-handed neutrinos do not interact with matter and the reverse process does not happen, producing the loss of detailed balance. For simplicity, we model the star core as consisting of strange quark matter. We find that even when the energy released in right-handed neutrinos is a small fraction of the total energy released in left-handed neutrinos, the process describes kick velocities for natal conditions, which are consistent with the observed ones and span the correct range of radii, temperatures and chemical potentials for typical magnetic field intensities. The neutrino magnetic moment is estimated to be μν∼3.6×10−18μB, where μB is the Bohr magneton. This value is more stringent than the bound found for massive neutrinos in a minimal extension of the standard model.

Solar neutrinos and leptonic spin forces

We quantify the effects of light spin-zero particles with pseudoscalar couplings to leptons and scalar couplings to nucleons on the evolution of solar neutrinos. In this scenario the matter potential sourced by the nucleons in the Sun’s matter gives rise to spin precession of the relativistic neutrino ensemble. As such the effects in the solar observables are different if neutrinos are Dirac or Majorana particles. For Dirac neutrinos the spin-flavour precession results into left-handed neutrino to right-handed neutrino (i.e., active-sterile) oscillations, while for Majorana neutrinos it results into left-handed neutrino to right-handed antineutrino (i.e., active-active) oscillations. In both cases this leads to distortions in the solar neutrino spectrum which we use to derive constraints on the allowed values of the mediator mass and couplings via a global analysis of the solar neutrino data. In addition for Majorana neutrinos spin-flavour precession results into a potentially observable flux of solar electron antineutrinos at the Earth which we quantify and constrain with the existing bounds from Borexino and KamLAND.

Strong cosmological constraints on the neutrino magnetic moment

A sizable magnetic moment for neutrinos would be evidence of exotic physics. In the early Universe, left-handed neutrinos with a magnetic moment would interact with electromagnetic fields in the primordial plasma, flipping their helicity and producing a population of right-handed (RH) neutrinos. In this work, we present a new calculation of the production rate of RH neutrinos in a multicomponent primordial plasma and quantify their contribution to the total energy density of relativistic species at early times, stressing the implications of the dependence on the initial time for production. We find that current cosmological data exclude values of the magnetic moment μ≳1.6×10−11μB, while future cosmological experiments will be able to probe nonthermal production of RH neutrinos, becoming competitive with stellar limits. Published by the American Physical Society 2024

Long-distance nuclear matrix elements for neutrinoless double-beta decay from lattice QCD

Neutrinoless double-beta (0νββ) decay is a heretofore unobserved process which, if observed, would imply that neutrinos are Majorana particles. Interpretations of the stringent experimental constraints on 0νββ-decay half-lives require calculations of nuclear matrix elements. This work presents the first lattice quantum chromodynamics (LQCD) calculation of the matrix element for 0νββ decay in a multinucleon system, specifically the nn→ppee transition, mediated by a light left-handed Majorana neutrino propagating over nuclear-scale distances. This calculation is performed with quark masses corresponding to a pion mass of mπ=806 MeV at a single lattice spacing and volume. The statistically cleaner Σ−→Σ+ee transition is also computed in order to investigate various systematic uncertainties. The prospects for matching the results of LQCD calculations onto a nuclear effective field theory to determine a leading-order low-energy constant relevant for 0νββ decay with a light Majorana neutrino are investigated. This work, therefore, sets the stage for future calculations at physical values of the quark masses that, combined with effective field theory and nuclear many-body studies, will provide controlled theoretical inputs to experimental searches of 0νββ decay. Published by the American Physical Society 2024

Light Dirac neutrino portal dark matter with gauged U(1)B−L symmetry

We propose a gauged U(1)B−L version of the light Dirac neutrino portal dark matter. The U(1)B−L symmetry provides a UV completion by naturally accommodating three right-handed neutrinos from anomaly cancellation requirements which, in combination with the left-handed neutrinos, form the sub-eV Dirac neutrinos after electroweak symmetry breaking. The particle content and the gauge charges are chosen in such a way that light neutrinos remain purely Dirac and dark matter, a gauge singlet Dirac fermion, remain stable. We consider both thermal and nonthermal production possibilities of dark matter and correlate the corresponding parameter space with the one within reach of future cosmic microwave background (CMB) experiments sensitive to enhanced relativistic degrees of freedom ΔNeff. The interplay of dark matter, CMB, structure formation and other terrestrial constraints keep the scenario very predictive leading the U(1)B−L parameter space into tight corners. Published by the American Physical Society 2024

Analysis of anomalies using weak effective Hamiltonian with complex couplings and their impact on various physical observables

Abstract Recently, the experimental measurements of the branching ratios and different polarization asymmetries for processes occurring through flavor-changing-charged current transitions by BABAR, Belle, and LHCb have revealed some significant differences from the corresponding Standard Model (SM) predictions. This has triggered an interest to search for physics beyond the SM in the context of various new physics (NP) models and using the model-independent weak effective Hamiltonian (WEH). Assuming left-handed neutrinos, we add the dimension-six vector, (pseudo-)scalar, and tensor operators with complex Wilson coefficients (WCs) to the SM WEH. Using 60%, 30%, and 10% constraints resulting from the branching ratio of , we reassess the parametric space of these new physics WCs accommodating the current anomalies based on the most recent HFLAV data of and and Belle data of and . We find that the allowed parametric region of left-handed scalar couplings strongly depends on the constraints of the branching ratio, and the maximum pull from the SM predictions results from the <60% branching ratio limit. Also, the parametric region changes significantly if we extend the analysis by adding LHCb data of and . Furthermore, due to the large uncertainties in the measurements of and , we derive the sum rules which complement them with and . Using the best-fit points of the new complex WCs along with the latest measurements of , we predict the numerical values of the observable , , and from the sum rules. The simultaneous dependence of abovementioned physical observables on the NP WCs is established by plotting their correlation with and , which are useful to discriminate between various NP scenarios. We find that the most significant impact of NP results from the WC . Finally, we study the impact of these NP couplings on various angular and triple product asymmetries that could be measured in some ongoing and future experiments. The precise measurements of these observables are important to check the SM and extract the possible NP.

A common origin for the QCD axion and sterile neutrinos from SU(5) strong dynamics

We identify the QCD axion and right-handed (sterile) neutrinos as bound states of an SU(5) chiral gauge theory with Peccei-Quinn (PQ) symmetry arising as a global symmetry of the strong dynamics. The strong dynamics is assumed to spontaneously break the PQ symmetry, producing a high-quality axion and naturally generating Majorana masses for the right-handed neutrinos at the PQ scale. The composite sterile neutrinos can directly couple to the left-handed (active) neutrinos, realizing a standard see-saw mechanism. Alternatively, the sterile neutrinos can couple to the active neutrinos via a naturally small mass mixing with additional elementary states, leading to light sterile neutrino eigenstates. The SU(5) strong dynamics therefore provides a common origin for a high-quality QCD axion and sterile neutrinos.

Search for Heavy Neutral Leptons in Decays of W Bosons Using a Dilepton Displaced Vertex in sqrt[s]=13 TeV pp Collisions with the ATLAS Detector.

A search for a long-lived, heavy neutral lepton (N) in 139 fb^{-1} of sqrt[s]=13 TeV pp collision data collected by the ATLAS detector at the Large Hadron Collider is reported. The N is produced via W→Nμ or W→Ne and decays into two charged leptons and a neutrino, forming a displaced vertex. The N mass is used to discriminate between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the N with the left-handed neutrino states for the N mass range 3 GeV<m_{N}<15 GeV. For the first time, limits are given for both single-flavor and multiflavor mixing scenarios motivated by neutrino flavor oscillation results for both the normal and inverted neutrino-mass hierarchies.

Chiral perturbative relation for neutrino masses in the type-I seesaw mechanism

In this letter, we perform a perturbative analysis by the lightest singular value mD1 of the Dirac mass matrix mD in the type-I seesaw mechanism. A mass relation M1=mD12/|(mν)11| is obtained for the lightest mass M1 of the right-handed neutrino νR1 and the mass matrix of the left-handed neutrinos mν in the diagonal basis of mD. This relation is rather stable under renormalization because it is gauge-invariant in the SM and associates with the approximate chiral symmetry of νR1.If diagonalization of the Yukawa matrices of leptons Yν,e has only small mixings, the value (mν)11 is close to the effective mass mee of the neutrinoless double beta decay. By assuming mD1≃mu,e≃0.5 MeV, the lightest mass is about M1≳O(100) TeV in the normal hierarchy and M1∼O(10) TeV in the inverted hierarchy. Such a νR1 with a tiny Yukawa coupling yν1∼O(10−5) can indirectly influence various observations.On the other hand, the famous bound of the thermal leptogenesis M1≳109 GeV that requires mD1≳30 MeV seems to be difficult to reconcile with a simple unified theory without a special condition.

Cosmic birefringence from neutrino and dark matter asymmetries

In light of the recent measurement of the nonzero Cosmic Microwave Background (CMB) polarization rotation angle from the Planck 2018 data, we explore the possibility that such a cosmic birefringence effect is induced by coupling a fermionic current with photons via a Chern-Simons-like term. We begin our discussion by rederiving the general formulae of the cosmic birefringence angle with correcting a mistake in the previous study.We then identify the fermions in the current as the left-handed electron neutrinos and asymmetric dark matter (ADM) particles, since the rotation angle is sourced by the number density difference between particles and antiparticles. For the electron neutrino case, with the value of the degeneracy parameter ξν e recently measured by the EMPRESS survey, we find a large parameter space which can explain the CMB photon polarization rotations. On the other hand, for the ADM solution, we consider two benchmark cases with Mχ = 5 GeV and 5 keV. The former is the natural value of the ADM mass if the observed ADM and baryon asymmetry in the Universe are produced by the same mechanism, while the latter provides a warm DM candidate. In addition, we explore the experimental constraints from the CMB power spectra and the DM direct detections.

Neutrinoless double beta decay in a left-right symmetric model with a double seesaw mechanism

We discuss a left-right (L-R) symmetric model with the double seesaw mechanism at the TeV scale generating Majorana masses for the active left-handed (LH) flavour neutrinos $\nu_{\alpha L}$ and the heavy right-handed (RH) neutrinos $N_{\beta R}$, $\alpha,\beta = e,\mu,\tau$, which in turn mediate lepton number violating processes, including neutrinoless double beta decay. The Higgs sector is composed of two Higgs doublets $H_L$, $H_R$, and a bi-doublet $\Phi$. The fermion sector has the usual for the L-R symmetric models quarks and leptons, along with three $SU(2)$ singlet fermion $S_{\gamma L}$. The choice of bare Majorana mass term for these sterile fermions induces large Majorana masses for the heavy RH neutrinos leading to two sets of heavy Majorana particles $N_j$ and $S_k$, $j,k=1,2,3$, with masses $m_{N_j} \ll m_{S_k}$. Working with a specific version of the model in which the $\nu_{\alpha L} - N_{\beta R}$ and the $N_{\beta R} - S_{\gamma L}$ Dirac mass terms are diagonal, and assuming that $m_{N_j} \sim (1 - 1000)$ GeV and ${\rm max}(m_{S_k}) \sim (1 - 10)$ TeV, $m_{N_j} \ll m_{S_k}$, we study in detail the new ``non-standard'' contributions to the $0\nu\beta\beta$ decay amplitude and half-life arising due to the exchange of virtual $N_j$ and $S_k$. We find that in both cases of NO and IO light neutrino mass spectra, these contributions are strongly enhanced and are dominant at relatively small values of the lightest neutrino mass $m_{1(3)} \sim (10^{-4} - 10^{-2})$ eV over the light Majorana neutrino exchange contribution. In large part of the parameter space, the predictions of the model for the $0\nu\beta\beta$ decay generalised effective Majorana mass and half-life are within the sensitivity range of the planned next generation of neutrinoless double beta decay experiments LEGEND-200 (LEGEND-1000), nEXO, KamlAND-Zen-II, CUPID, NEXT-HD.

Fermion mass formulation in the Modified Left-Right Symmetry Model

The Modified Left Right Symmetry Model is an extension of the Standard Model. This model introduces left-handed neutrinos to the right sector and a doublet scalar field to the left sector. This model cannot yet explain the mass generation of fermions and neutrinos. This study is theoretical research using the literature review method. Generating the masses of fermions (quark up-down) and electrons through spontaneous symmetry breaking in Yukawa's Lagrangian term produces a particle mass in the left sector, the same as the calculations in the Standard Model. The masses of fermions (up-down quarks) and electrons for the right sector produced in this study are much more massive than those of fermions (up-down quarks) and the left sector. The neutrino masses produced in this study are by following the Seesaw Mechanism. That is, if one neutrino mass is massive, then the other neutrino masses will be light.©2022 JNSMR UIN Walisongo. All rights reserved.

Undemocratic Dirac seesaw

The standard model left-handed neutrinos and several right-handed neutrinos can obtain a tiny Dirac mass matrix through their mixings with relatively heavy Dirac fermions. In this Dirac seesaw scenario, the mixings involving the left-handed neutrinos can be allowed much larger than those involving the right-handed neutrinos. This undemocratic parameter choice is attractive to phenomenology. We show that the small mixings between the heavy Dirac fermions and the right-handed neutrinos can have a common origin with the observed baryon asymmetry in the universe. We also connect the introduction of right-handed neutrinos to the existence and stability of dark matter by a new U(1) gauge symmetry for dark photon or baryon-minus-lepton number. We then specify how to embed our scenario into a left-right symmetric theory or a grand unification theory.

A split seesaw model with hidden neutrinoless double beta decay but successful leptogenesis

In a paper by Asaka, Ishida and Tanaka (Phys. Rev. D 103:015014, 2021), they proposed a novel possibility (which will be referred to as the AIT ansatz) that, in spite of the Majorana nature of neutrinos, the neutrinoless double beta (0nu beta beta ) decay can be hidden. In the original AIT model, the AIT ansatz is realized in the minimal seesaw model with two right-handed neutrinos which have a hierarchical mass structure: the lighter and heavier right-handed neutrinos are respectively much lighter and heavier than the typical Fermi-momentum scale of nuclei. However, the original AIT model does not accommodate a successful leptogenesis. For this problem, in this paper we study a split seesaw model with one lighter right-handed neutrino but two heavier right-handed neutrinos which can realize the AIT ansatz and accommodate a successful leptogenesis simultaneously. We first give the condition on the neutrino Yukawa couplings for realizing the AIT ansatz, discuss its realization by employing an Abelian flavor symmetry, and study its implications for the mixing of the lighter right-handed neutrino with three left-handed neutrinos. We then successively study the implications for leptogenesis of the interesting scenarios where M_{{textrm{D}}} is a triangular matrix (which has maximally-restricted texture zeros, in line with the simplicity principle) or respects the mu -tau reflection symmetry (which is well motivated by the experimental results), on top of the AIT ansatz.

Sterile neutrino from D-brane models

We discuss the appearance of sterile neutrino in D-brane models. In the simplest case, the sterile neutrino (SN) appears from a 5-stack intersecting D6-brane model with a gauged baryon number. SM is mixed with a single generation of left handed active neutrinos and a right handed neutrino.

β-Nuclear-Recoil Correlation from ^{6}He Decay in a Laser Trap.

We report the first precise measurement of a β-recoil correlation from a radioactive noble gas (^{6}He) confined via a magneto-optical trap. The measurement is motivated by the search for exotic tensor-type contributions to the charged weak current. Interpreted as tensor currents with right-handed neutrinos, the measurements yield |C_{T}/C_{A}|^{2}≤0.022 (90%confidence limit, C.L.). On the other hand, for left-handed neutrinos the limits are 0.007<C_{T}/C_{A}<0.111 (90%C.L.). The sensitivity of the present measurement is mainly limited by experimental uncertainties in determining the time response properties and the distance between the atom cloud and the microchannel plate used for recoil ion detection.

Elusive muonic WIMP

The Weakly Interacting Massive Particle (WIMP) paradigm is one of the most popular scenarios for Dark Matter (DM) theories that however is strongly constrained, in particular by direct detection experiments. We stick with the WIMP hypothesis and consider a Dirac fermion candidate for DM that interacts with the Standard Model (SM) via a spin-1 $Z'$, arising from the spontaneous breaking of an Abelian $U(1)'_{\mu}$ gauge symmetry, under which only second generation leptons and the DM are appropriately charged. Due to the charge assignment, the model is gauge anomalous and can only be interpreted as an effective field theory (EFT) at low energy. The $Z'$ couples at tree level only to the vector DM current, to the axial muon current and to left-handed muonic neutrinos, so the WIMP-nucleon cross section is beyond the experimental reach of spin-independent (SI) direct detection searches. We study the current bounds on this model coming from direct and indirect detection of DM, collider searches, contributions to $(g-2)_{\mu}$ and to neutrino trident production. We find that large regions of the parameter space remains to be explored. In the context of LHC searches, we study the impact of a muon-exclusive signal region for the $3\mu$ + ${E}^{{\rm miss}}_T$ channel with an invariant mass window around $m_{Z'}$. We show that this search can significantly improve the current collider bounds. Finally, from the anomalous nature of our EFT, there remain at low energy triboson anomalous interactions between the $Z'$ and the electroweak (EW) SM gauge bosons. We explore the possibilities of probing these interactions at the LHC and at a 100 TeV proton collider finding it extremely challenging. On the other hand, for a muon collider the resonant channel $\mu^{+}\mu^{-}\to Z'\to ZZ$ could be discovered in the most promising scenario with luminosity of $\mathcal{O}({\rm few}\; 10)$ ${\rm fb}^{-1}$.

New physics effects on Λb→Λc*τν¯τ decays

We benefit from a recent lattice determination of the full set of vector, axial and tensor form factors for the ${\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Lambda}}}_{c}^{*}(2595)\ensuremath{\tau}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$ and ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}(2625)\ensuremath{\tau}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$ semileptonic decays to study the possible role of these two reactions in lepton flavor universality violation studies. Using an effective theory approach, we analyze different observables that can be accessed through the visible kinematics of the charged particles produced in the tau decay, for which we consider the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\nu}}_{\ensuremath{\tau}},{\ensuremath{\rho}}^{\ensuremath{-}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ and ${\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ channels. We compare the results obtained in the Standard Model and other schemes containing new physics (NP) interactions, with either left-handed or right-handed neutrino operators. We find a discriminating power between models similar to the one of the ${\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Lambda}}}_{c}$ decay, although somewhat hindered in this case by the larger errors of the ${\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Lambda}}}_{c}^{*}$ lattice form factors. Notwithstanding this, the analysis of these reactions is already able to discriminate between some of the NP scenarios and its potentiality will certainly improve when more precise form factors are available.

Chiral gravitational waves from thermalized neutrinos in the early Universe

We investigate polarized gravitational waves generated by chiral fermions in the early Universe. In particular, we focus on the contribution from left-handed neutrinos in thermal equilibrium with finite temperature and chemical potential in the radiation dominated era. We compute the correlation functions of gravitational fields pertinent to the Stokes parameter V characterizing the circular polarization of gravitational waves in the Minkowski and expanding spacetime backgrounds. In the expanding universe, we find that the thermalized neutrinos induce a non-vanishing V linear to the neutrino degeneracy parameter and wavenumber of gravitational waves in the long wavelength region. While the magnitude of the gravitational waves generated by thermal neutrinos is too small to be detectable by current and planned third generation gravitational wave detectors, their observations by future generation detectors for ultra-high frequency regimes could provide information on the neutrino degeneracy parameter in the early Universe.