Lepton Number Research Articles

Lepton and baryon numbers as local gauge symmetries

A simple theory where the total lepton number is a local gauge symmetry is proposed. In this context, the gauge anomalies are canceled with the minimal number of extra fermionic fields and one predicts that the neutrinos are Majorana fermions. The properties of the neutrino sector are discussed, showing that this theory predicts a 3+2 light neutrino sector. We show that using the same fermionic fields one can gauge the baryon number and define a simple theory where the lepton and baryon numbers can be spontaneously broken at the low scale in agreement with experiments. Published by the American Physical Society 2024

Dynamical origin of neutrino masses and dark matter from a new confining sector

A dynamical mechanism, based on a confining non-Abelian dark symmetry, which generates Majorana masses for hyperchargeless fermions, is proposed. We apply it to the inverse seesaw scenario, which allows us to generate light neutrino masses from the interplay of TeV-scale pseudo-Dirac mass terms and a small explicit breaking of lepton number. A single generation of vectorlike dark quarks, transforming under a SU(3)D gauge symmetry, is coupled to a real singlet scalar, which serves as a portal between the dark quark condensate and three generations of heavy sterile neutrinos. Such a dark sector and the Standard Model (SM) are kept in thermal equilibrium with each other via sizable Yukawa couplings to the heavy neutrinos. In this framework, the lightest dark baryon, which has spin 3/2 and is stabilized at the renormalizable level by an accidental dark baryon number symmetry, can account for the observed relic density via thermal freeze-out from annihilations into the lightest dark mesons. These mesons, in turn, decay to heavy neutrinos, which produce SM final states upon decay. This model may be probed by next generation neutrino telescopes via neutrino lines produced from dark matter annihilations. Published by the American Physical Society 2024

Enhanced muonization by active-sterile neutrino mixing in protoneutron stars

We study νμ−νs and ν¯μ−ν¯s mixing in the protoneutron star (PNS) created in a core-collapse supernova (CCSN). We point out the importance of the feedback on the general composition of the PNS in addition to the obvious feedback on the νμ lepton number. We show that for our adopted mixing parameters δm2∼102 keV2 and sin22θ consistent with the current constraints, sterile neutrino production is dominated by the Mikheyev–Smirnov–Wolfenstein conversion of ν¯μ into ν¯s and that the subsequent escape of ν¯s increases the νμ lepton number, which in turn enhances muonization of the PNS primarily through νμ+n→p+μ−. While these results are qualitatively robust, their quantitative effects on the dynamics and active neutrino emission of core-collapse supernovae should be evaluated by including νμ−νs and ν¯μ−ν¯s mixing in the simulations. Published by the American Physical Society 2024

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

The CUPID neutrinoless double-beta decay experiment

Neutrinoless double-beta decay (0νββ) is a key process to address some of the major outstanding issues in particle physics, such as the lepton number conservation and the Majorana nature of the neutrino. The next-generation of experiments aims at covering the Inverted-Ordering region of the neutrino mass spectrum, with sensitivities on the half-lives greater than 1027 years. Among the exploited techniques, low-temperature calorimetry has proved to be a very promising one, and will keep its leading role in the future thanks to the CUPID experiment. CUPID will search for the neutrinoless double-beta decay of 100Mo. By deploying Li2MoO4 scintillating crystals enriched in 100Mo and cryogenic light detectors, CUPID will perform simultaneous readout of heat and light signals, allowing for particle identification, and thus a powerful rejection technique for α background. To meet the new sensitivity target, the CUORE readout system must be upgraded. The readout system for each channel will consist of a pre-amplifier, a Programmable Gain Amplifier (PGA), a low-pass filter (Bessel filter) and a Digital AcQuisition System (DAQ). Finally, there is a pulser to generate thermal stabilization energy in the crystals. The motivation and expected performance of the technical upgrades are described in the context of the goals of the CUPID experiment.

Nuclear matrix elements of neutrinoless double-beta decay in covariant density functional theory with different mechanisms

Nuclear matrix elements (NMEs) for neutrinoless double-beta (0νββ) decay in candidate nuclei play a crucial role in interpreting results from current experiments and in designing future ones. Accurate NME values serve as important nuclear inputs for constraining parameters in new physics, such as neutrino mass and the Wilson coefficients of lepton-number-violating (LNV) operators. In this study, we present a comprehensive calculation of NMEs for 0νββ decay in 76Ge, 82Se, 100Mo, 130Te, and 136Xe, using nuclear wave functions obtained from multi-reference covariant density functional theory (MR-CDFT). We employ three types of transition potentials at the leading order in chiral effective field theory. Our results, along with recent data, are utilized to constrain the coefficients of LNV operators. The results demonstrate that the combined NMEs based on the Feynman diagrams at the hadronic scale for the nonstandard mechanisms leads to the uncertainty by different nuclear models comparable to that for the standard mechanism. The use of NMEs from various nuclear models does not dramatically change the parameter space intervals for the coefficients, although MR-CDFT yields the most stringent constraint. Furthermore, our NMEs can also be used to perform a more comprehensive analysis with multiple isotopes.

Searching for a new light gauge boson with axial couplings in muon beam dump experiments

We present a formalism for new U(1) interactions involving weak hypercharge, baryon, and lepton numbers, and a possible axial symmetry generator FA in the presence of a second Brout-Englert-Higgs doublet. The resulting U boson, after mixing with the Z, interpolates between a generalised dark photon, a dark Z, and an axially coupled gauge boson. We especially focus on the axial couplings originating from FA or from mixing with the Z, determined by the scalar sector via parameters like tan β and the v.e.v. of an extra dark singlet.We explore the distinctive features of axially coupled interactions, especially in the ultrarelativistic limit, where the U boson behaves much as an axion-like particle, with enhanced interactions to quarks and leptons. This enhancement is particularly relevant for future muon beam dump experiments, since the muon mass considerably increases the effective coupling, proportional to 2mμ/mU, compared to analogous experiments with electrons.We also analyse the shape of the expected beam dump exclusion or discovery regions, influenced by U boson interactions and the experiment geometry. Different situations are considered, limited in particular by cases for which the U decays before reaching the detector, or has too small couplings to produce detectable events. We also compare to vectorially coupled bosons and axion-like pseudoscalars, highlighting the importance of understanding the parameter space for future experiment design and optimisation.

The Type-I Seesaw family

We provide a comprehensive analysis of the Type-I Seesaw family of neutrino mass models, including the conventional type-I seesaw and its low-scale variants, namely the linear and inverse seesaws. We establish that all these models essentially correspond to a particular form of the type-I seesaw in the context of explicit lepton number violation. We then focus into the more interesting scenario of spontaneous lepton number violation, systematically categorizing all inequivalent minimal models. Furthermore, we identify and flesh out specific models that feature a rich majoron phenomenology and discuss some scenarios which, despite having heavy mediators and being invisible in processes such as μ → eγ, predict sizable rates for decays including the majoron in the final state.

Majorana neutrinos and dark matter from anomaly cancellation

We discuss a simple theory for neutrino masses where the total lepton number is a local gauge symmetry spontaneously broken below the multi-TeV scale. In this context, the neutrino masses are generated through the canonical seesaw mechanism and a Majorana dark matter candidate is predicted from anomaly cancellation. We discuss in great detail the dark matter annihilation channels and find out the upper bound on the symmetry-breaking scale using the cosmological bounds on the relic density. Since in this context the dark matter candidate has suppressed couplings to the Standard Model quarks, one can satisfy the direct detection bounds even if the dark matter mass is close to the electroweak scale. This theory predicts a light pseudo-Nambu-Goldstone boson (the Majoron) associated to the mechanism of neutrino mass. We discuss briefly the properties of the Majoron and the impact of the big bang nucleosynthesis bounds. Published by the American Physical Society 2024

Model-independent estimates for loop-induced baryon-number-violating nucleon decays

Baryon number is an accidental symmetry of the Standard Model (SM) Lagrangian that so far has been measured to be exactly preserved, although it is expected to be violated at higher energies. In this work we compute order-of-magnitude estimates for the matching contributions of generic ultraviolet models to effective operators that generate nucleon decay processes. This is done in a systematic and automated way using operators constructed from SM fields up to dimension nine and working in a framework that has proved useful in the study of lepton-number violation. For each of the operators we derive estimates for the rates of different nucleon-decay channels. These allow us to establish model-independent lower bounds on the underlying new-physics scale and identify potential correlations between the various decay modes. The results are most relevant for families of models that generate the considered operator. This analysis is especially timely given the expected future sensitivities in numerous experiments such as Hyper-K, DUNE, JUNO and THEIA.

Probing lepton number violation and Majorana nature of neutrinos at the LHC

Observation of lepton number (L) violation by two units at colliders would provide evidence for the Majorana nature of neutrinos. We study signals of L-violation in the context of two popular models of neutrino masses, the type-II seesaw model and the Zee model, wherein small neutrino masses arise at the tree-level and one-loop level, respectively. We focus on L-violation signals at the LHC arising through the process pp → ℓ±ℓ′± + jets within these frameworks. We obtain sensitivity to L-violation in the type-II seesaw model for triplet scalar masses up to 700 GeV and in the Zee model for charged scalar masses up to 4.8 TeV at the high-luminosity LHC with an integrated luminosity of 3 ab−1.

Contribution of Majoron to Hubble Tension in Gauged U(1)Lμ – Lτ Model

Abstract In this paper, we analyze parameter regions that can alleviate the Hubble tension in the U(1)$_{L_\mu - L_\tau }$ model with broken lepton number U(1)$_L$ symmetry. As new particles, this model has a U(1)$_{L_\mu - L_\tau }$ gauge boson $Z^{\prime }$ and a Majoron $\phi$, which can affect the early universe and the effective number of neutrino species $N_\textrm{eff}$. If $Z^{\prime }$ and $\phi$ simultaneously exist in the early universe, $Z^{\prime }\,$–$\, \phi$ direct reaction processes such as $Z^{\prime }\nu _\alpha \leftrightarrow \phi \bar{\nu }_\beta$ and $Z^{\prime } \phi \leftrightarrow \bar{\nu }_\alpha \bar{\nu }_\beta$ occur. The comparison of $N_\textrm{eff}$ between the cases with and without the $Z^{\prime }\,$–$\, \phi$ direct reaction processes shows that these processes make a small contribution of $\mathcal {O}(10^{-4})$ to $N_\textrm{eff}$, and it does not need to be considered for the alleviation of the Hubble tension. Based on these facts, we calculated $N_\textrm{eff}$ for various Majoron parameters without the $Z^{\prime }\,$–$\, \phi$ direct reaction processes to search for parameters that could alleviate the Hubble tension. As a result, we found that the U(1)$_{L_\mu - L_\tau }$ gauge boson and Majoron can alleviate the Hubble tension in some parameter regions, and there is a nontrivial indirect synergy contribution between $Z^{\prime }$ and $\phi$, although the contribution from $Z^{\prime }\,$–$\, \phi$ direct reaction processes is negligible.

Dyonium induced fermion number violation

Dyonium induced fermion number violation is studied in a SU(2)L×U(1)Y gauge theory with a doublet Higgs field. Dyonium is a generalization of Nambu's monopolium, having finite sized pair of dyon and anti-dyon, connected with a thin string under the linear plus Coulomb force.1This is a follow-up of the author's paper on monopolium induced neutrino mass (including the lepton and baryon number violations), studied with a SO(10) Grand Unified model in 1983. To fulfill the requirement from the chiral anomaly or its index theorem, an electric field should be excited in parallel to the dipole magnetic field. This crucial dynamical problem, not fully answered then, has been solved, by considering not monopolium but dyonium.The fermionic zero modes in the dyonium background fields are necessary in evaluating the transition rate of the fermion number violation processes. Their Dirac equation can be reduced to a single component partial differential equation, similar to the renormalization group equation, which may be useful to estimate the reaction rates.

Heavy neutral leptons in gauged U(1) L µ −L τ at muon collider* *Supported by the National Natural Science Foundation of China (12375074, 11805081) and the Natural Science Foundation of Shandong Province, China (ZR2019QA021)

Heavy neutral leptons N are the most appealing candidates to generate tiny neutrino masses. We studied the signature of heavy neutral leptons in gauged at a muon collider. Charged under the symmetry, the heavy neutral leptons can be pair produced via the new gauge boson at the muon collider as and . We then performed a detailed analysis on the lepton number violation signature and at the 3 TeV muon collider, where the hadronic decays of W boson are treated as fat-jets J. These lepton number violation signatures have quite clean backgrounds at the muon collider. Our simulation shows that a wide range of viable parameter space is within the reach of the 3 TeV muon collider. For instance, with new gauge coupling and an integrated luminosity of 1000 fb , the signal could probe TeV. Meanwhile, if the gauge boson mass satisfies , the signature would be more promising than the signature.

Probing lepton number violation: a comprehensive survey of dimension-7 SMEFT

Observation of lepton number violation would represent a groundbreaking discovery with profound consequences for fundamental physics and as such, it has motivated an extensive experimental program searching for neutrinoless double beta decay. However, the violation of lepton number can be also tested by a variety of other observables. We focus on the possibilities of probing this fundamental symmetry within the framework of the Standard Model Effective Field Theory (SMEFT) beyond the minimal dimension-5. Specifically, we study the bounds on ∆L = 2 dimension-7 effective operators beyond the electron flavor imposed by all relevant low-energy observables and confront them with derived high-energy collider limits. We also discuss how the synergy of the analyzed multi-frontier observables can play a crucial role in distinguishing among different dimension-7 SMEFT operators.

Three-loop inverse scotogenic seesaw models

We propose a class of models providing an explanation of the origin of light neutrino masses, the baryon asymmetry of the Universe via leptogenesis and offering viable dark matter candidates. In these models the Majorana masses of the active neutrino are generated by the inverse seesaw mechanism with the lepton number violating right-handed Majorana neutrino masses μ arising at three loops. The latter is ensured by the preserved discrete symmetries, which also guarantee the stability of the dark matter candidate. We focus on one of these models and perform a detailed analysis of the phenomenology of its leptonic sector. The model can successfully accommodate baryogenesis through leptogenesis in both weak and strong washout regimes. The lightest heavy fermion turns out to be a viable dark matter candidate, provided that the entries of the Majorana submatrix μ are in the keV to MeV range. The solutions are consistent with the experimental constraints, accommodating both mass orderings for active neutrinos, in particular charged-lepton flavor violating decays μ → eγ, μ → eee, and the electron-muon conversion processes get sizable rates within future sensitivity reach.

MeV gamma rays from Q -ball decay

We study the supersymmetric Q balls which decay at present and find that they create a distinctive spectrum of gamma rays at around O(10) MeV. The charge of the Q ball is lepton numbers in order for the lifetime to be as long as the present age of the Universe, and the main decay products are light leptons. However, as the charge of the Q ball decreases, the decay channel into pions becomes kinematically allowed towards the end of the decay, and the pions are produced at rest. Immediately, π0 decays into two photons with the energy of 67.5 MeV, half the pion mass, which exhibits a unique emission line. In addition, π± decay into μ±, which further decay with emitting internal bremsstrahlung, whose spectrum has a sharp cutoff at ∼50 MeV. If the observations find these peculiar features of the gamma-ray spectrum in the future, it could be a smoking gun of the supersymmetric Q-ball decay at present. Published by the American Physical Society 2024

AbInitio Uncertainty Quantification of Neutrinoless Double-Beta Decay in ^{76}Ge.

The observation of neutrinoless double-beta (0νββ) decay would offer proof of lepton number violation, demonstrating that neutrinos are Majorana particles, while also helping us understand why there is more matter than antimatter in the Universe. If the decay is driven by the exchange of the three known light neutrinos, a discovery would, in addition, link the observed decay rate to the neutrino mass scale through a theoretical quantity known as the nuclear matrix element (NME). Accurate values of the NMEs for all nuclei considered for use in 0νββ experiments are therefore crucial for designing and interpreting those experiments. Here, we report the first comprehensive abinitio uncertainty quantification of the 0νββ-decay NME, in the key nucleus ^{76}Ge. Our method employs nuclear strong and weak interactions derived within chiral effective field theory and recently developed many-body emulators. Our result, with a conservative treatment of uncertainty, is an NME of 2.60_{-1.36}^{+1.28}, which, together with the best-existing half-life sensitivity and phase-space factor, sets an upper limit for effective neutrino mass of 187_{-62}^{+205} meV. The result is important for designing next-generation germanium detectors aiming to cover the entire inverted hierarchy region of neutrino masses.

Triplet scalar flavored leptogenesis with spontaneous CP violation

The inclusion of two triplet scalars in the Standard Model (SM) enables to accommodate neutrino mass generation as well as baryogenesis through leptogenesis. One of theessential ingredients of leptogenesis is the violation ofcharge conjugation and parity (CP) symmetry in lepton number violating decays of the triplet scalars. We work on the promising sector of spontaneous CP violation (SCPV) which is manifested by the involvement of one scalar singlet and two scalar fields, added to the SM. The predictiveaspect of the model is accomplished by imposing A 4 × Z 4 symmetry which results in the traditional tribimaximal mixing pattern. With updated data on neutrino oscillation, we study the parameter space of the model.The phase of the complex vacuum expectation value (VEV) of the singlet scalar acts as the common source ofCP violation in both low and high energy sectors.Due to the flavor symmetry of the model, required baryon asymmetry cannot be accomplished via unflavored leptogenesis. In the temperature regime, [109, 1012] GeV when flavor effects become important in the study of leptogenesis, it is shown that baryogenesis is achievable. The rich flavor interplay is explored through the study of the density matrix equations. We also study the interplay of hierarchical branching ratios of the decay of the triplet scalars and SCPV phase to accommodate the required CP asymmetry to account for the final baryon asymmetry in the observational range. Considering all possible mass hierarchies among the triplet scalars, the flavor structure of the triplet Yukawa couplings results in different scales of leptogenesis.

Mutually elusive: Vectorlike antileptons and leptoquarks

We study the properties of vectorlike fermions that have the same gauge charges as the Standard Model lepton doublets, but opposite lepton number. These antileptons undergo decays mediated by heavier scalar leptoquarks, while the symmetries of this renormalizable model protect the vectorlike fermions and the leptoquarks from standard decays probed so far at colliders. We derive upper limits on the new Yukawa couplings imposed by flavor-changing processes, including B→Kνν¯ and Bs−B¯s mixing, and show that they are compatible with prompt antilepton decays at the LHC for wide parameter ranges. If the new particles couple predominantly to second-generation quarks, then their collider probes involve multiple jets and two taus or neutrinos, and are hampered by large backgrounds. If couplings to third-generation quarks are large, then the collider signals involve top quarks, and can be probed more efficiently at the LHC. Even in that case, both the vectorlike fermion doublet and the leptoquarks remain more elusive than in models with standard decays. Published by the American Physical Society 2024