Search

We propose a simple scenario that directly connects the dark matter (DM) and neutrino mass scales. Based on an interaction between the DM particle x and the neutrino v of the form XXvv/Λ², the DM annihilation cross section into the neutrino is determined and a neutrino mass is radiatively induced. Using the observed neutrino mass scale and the DM relic density, the DM mass and the effective scale A are found to be of the order MeVand GeV, respectively. We construct an ultraviolet-complete toy model based on the inverse seesaw mechanism which realizes this potential connection between DM and neutrino physics. [ABSTRACT FROM AUTHOR]

Heavy right-handed neutrinos are a natural addition to the particle content of the Standard Model. Within the high-scale seesaw mechanism they provide a natural explanation for the lightness of the active neutrinos. We here briefly discuss a few possible modifications of the standard seesaw scenario that permit relatively light heavy neutrinos at or below the electroweak scale. Such scenarios are experimentally testable and we touch upon the corresponding phenomenology at colliders and in meson decays.

We propose a class of left-right symmetric models (LRSMs) with spontaneous D-parity breaking, where SU(2)R breaks at the TeV scale while discrete left-right symmetry breaks around 109 GeV. By embedding this framework in a nonsupersymmetric SO(10) grand unified theory (GUT) with Pati-Salam symmetry as the highest intermediate breaking step, we obtain gR/gL≈0.6 between the right- and left-handed gauge couplings at the TeV scale. This leads to a suppression of beyond the Standard Model phenomena induced by the right-handed gauge coupling. Here we focus specifically on the consequences for neutrinoless double beta decay, low-energy lepton flavor violation, and LHC signatures due to the suppressed right handed currents. Interestingly, the reduced gR allows us to interpret an excess of events observed recently in the range of 1.9 to 2.4 TeV by the CMS group at the LHC as the signature of a right-handed gauge boson in LRSMs with spontaneous D-parity breaking. Moreover, the reduced right-handed gauge coupling also strongly suppresses the nonstandard contribution of heavy states to the neutrinoless double beta decay rate as well as the amplitude of low-energy lepton flavor violating processes. In a dominant type-II seesaw mechanism of neutrino mass generation, we find that both sets of observables provide stringent and complimentary bounds which make it challenging to observe the scenario at the LHC. [ABSTRACT FROM AUTHOR]

We point out that the recent excess observed in searches for a right-handed gauge boson WR at CMS can be explained in a left-right symmetric model with D-parity violation. In a class of SO(10) models, in which D parity is broken at a high scale, the left-right gauge symmetry breaking scale is naturally small, and at a few TeV the gauge coupling constants satisfy gR ≈ 0.6gL. Such models therefore predict a right-handed charged gauge boson WR in the TeV range with a suppressed gauge coupling as compared to the usually assumed manifest left-right symmetry case gR = gL. The recent CMS data show excess events which are consistent with the cross section predicted in the D-parity breaking model for 1.9 TeV < MWR < 2.4 TeV. If the excess is confirmed, it would in general be a direct signal of new physics beyond the Standard Model at the LHC. A TeV scale WR would for example not only rule out SU(5) grand unified theory models. It would also imply B - L violation at the TeV scale, which would be the first evidence for baryon or lepton number violation in nature, and it has strong implications on the generation of neutrino masses and the baryon asymmetry in the Universe. [ABSTRACT FROM AUTHOR]

We study the impact of minimal nonsupersymmetric models of resonant leptogenesis on charged-lepton flavor violation and the neutrino-mixing angle θ13. Possible low-scale flavor realizations of resonant τ-, μ- and e-leptogenesis provide very distinct and predictive frameworks to explain the observed baryon asymmetry in the Universe by sphaleron conversion of an individual τ-, μ- and e-lepton-number asymmetry which gets resonantly enhanced via out-of-equilibrium decays of nearly degenerate heavy Majorana neutrinos. Based on approximate flavor symmetries, we construct viable scenarios of resonant τ-, μ- and e-leptogenesis compatible with universal right-handed neutrino masses at the grand unified theory scale, where the required heavy-neutrino mass splittings are generated radiatively. The heavy Majorana neutrinos in such scenarios can be as light as 100 GeV and their couplings to two of the charged leptons may be large. In particular, we explicitly demonstrate the compelling role that the three heavy Majorana neutrinos play, in order to obtain successful leptogenesis and experimentally testable rates for lepton-flavor violating processes, such as μ→eγ and μ→e conversion in nuclei. [ABSTRACT FROM AUTHOR]

Bayesian parameter inference techniques require a choice of prior distribution, which can strongly impact the statistical conclusions drawn. We discuss the construction of least-informative priors for neutrinoless double beta decay searches. Such priors attempt to be objective by maximizing the information gain from an experimental setup. In a parametrization using the lightest neutrino mass ml and an effective Majorana phase parameter Φ, we construct such a prior using two different approaches and compare them with the standard flat and logarithmic priors in ml. [ABSTRACT FROM AUTHOR]

Usually considered a background for experimental searches for the hypothetical neutrinoless double beta decay process, two-neutrino double beta decay nevertheless provides a complementary probe of physics beyond the Standard Model. In this paper we investigate how the presence of a sterile neutrino, coupled to the Standard Model either via a left-handed or right-handed current, affects the energy distribution and angular correlation of the outgoing electrons in two-neutrino double beta decay. We pay particular attention on the behavior of the energy distribution at the kinematic end point and we estimate the current limits on the active-sterile mixing and effective right-handed coupling using current experimental data as a function of the sterile neutrino mass. We also investigate the sensitivities of future experiments. Our results complement the corresponding constraints on sterile neutrinos from single beta decay measurements in the 0.1-10 MeV mass range. [ABSTRACT FROM AUTHOR]

Neutrinoless double beta decay (0νββ) is a crucial test for lepton number violation. Observation of this process would have fundamental implications for neutrino physics, theories beyond the Standard Model and cosmology. Focusing on so-called short-range operators of 0νββ and their potential interplay with the standard light Majorana neutrino exchange, we present the first complete calculation of the relevant nuclear matrix elements, performed within the interacting boson model (IBM-2). Furthermore, we calculate the relevant phase space factors using exact Dirac electron wave functions, taking into account the finite nuclear size and screening by the electron cloud. The obtained numerical results are presented together with up-to-date limits on the standard mass mechanism and effective 0νββ short-range operators in the interacting boson model framework. Finally, we interpret the limits in the particle physics scenarios incorporating heavy sterile neutrinos, left-right symmetry and R-parity violating supersymmetry. [ABSTRACT FROM AUTHOR]

Neutrino self-interactions (νSI) beyond the Standard Model are an attractive possibility to soften cosmological constraints on neutrino properties and also to explain the tension in late and early time measurements of the Hubble expansion rate. The required strength of νSI to explain the 4σ Hubble tension is in terms of a pointlike effective four-fermion coupling that can be as high as 109GF, where GF is the Fermi constant. In this work, we show that such strong νSI can cause significant effects in two-neutrino double beta decay, leading to an observable enhancement of decay rates and to spectrum distortions. We analyze self-interactions via an effective operator as well as when mediated by a light scalar. Data from observed two-neutrino double beta decay are used to constrain νSI, which rules out the regime around 109GF. [ABSTRACT FROM AUTHOR]

We investigate the potential of LHC resonance searches in leptonic final states to probe the Z' in the minimal U(1)B-L model. Considering the current constraints on the Z' in terms of its mass mZ' and the associated gauge coupling gB-L as well as constraints in the Higgs sector, we analyze the potential of dilepton and four lepton final states for Z' production. This includes Drell-Yan production, Higgs mediated decays and final state radiation processes concentrating only on the ATLAS and CMS detectors at the LHC. We show that the four-lepton final state is sensitive to mZ' as low as 0.25 GeV. Furthermore, setting the Higgs mixing to sinα=0.3, this final state has a strong sensitivity and it probes regions of parameter space where the Z' is long-lived. We demonstrate the sensitivity at the High Luminosity LHC and comment on the potential of probing displaced vertices due to long-lived Z'. Finally, we also comment on the strength of Z' and Higgs mediated heavy neutrino processes by taking into account the constraints derived. [ABSTRACT FROM AUTHOR]

We propose an alternative formulation of a left-right symmetric model where the difference between baryon number (B) and lepton number (L) remains an unbroken symmetry. This is unlike the conventional formulation, where B - L is promoted to a local symmetry and is broken explicitly in order to generate Majorana neutrino masses. In our case B - L remains a global symmetry after the left-right symmetry breaking, allowing only Dirac mass terms for neutrinos. In addition to parity restoration at some high scale, this formulation provides a natural framework to explain B - L as an anomaly-free global symmetry of the Standard Model and the nonobservation of (B - L)-violating processes. Neutrino masses are purely Dirac type and are generated either through a two-loop radiative mechanism or by implementing a Dirac seesaw mechanism. [ABSTRACT FROM AUTHOR]

We investigate the pair production of right-handed neutrinos from the decay of an additional neutral Z' boson in the gauged B - L model. Taking into account current constraints on the Z' mass and the associated gauge coupling g1', we analyse the sensitivity of proposed experiments at the lifetime frontier, FASER 2, CODEX-b, MATHUSLA as well as a hypothetical version of the MAPP detector to a long lived heavy neutrino N originating in the decays of the Z'. We further complement this study with determining the reach of LHCb and a CMS-type detector for the high-luminosity LHC run. We demonstrate that in a backgroundfree scenario with g1' = 10-3 near the current limit, FASER 2 is sensitive to the active-sterile neutrino mixing down to VμN ≈ 10-4, while a reach of VμN ≈ 10-5 can be obtained for CODEX-b and LHCb, in a mass regime of mN ≈ 5 - 20 GeV and mZ' ≈ 20 - 70 GeV. Finally, MATHUSLA can probe VμN ≈ 10-7 and cover the mixing regime expected in a canonical seesaw scenario of light neutrino mass generation. [ABSTRACT FROM AUTHOR]

We discuss lepton number violating processes in the context of long-baseline neutrino oscillations. We summarize and compare neutrino flavor oscillations in quantum mechanics and quantum field theory, both for standard oscillations and for those that violate lepton number. When the active neutrinos are Majorana in nature, the required helicity reversal gives a strong suppression by the neutrino mass over the energy, (mν/Eν)². Instead, the presence of nonstandard lepton number violating interactions incorporating right-handed lepton currents at production or detection alleviate the mass suppression while also factorizing the oscillation probability from the total rate. Such interactions arise from dimension-six operators in the low energy effective field theory of the Standard Model. We derive general and simplified expressions for the lepton number violating oscillation probabilities and use limits from MINOS and KamLAND to place bounds on the interaction strength in interplay with the unknown Majorana phases in neutrino mixing. We compare the bounds with those from neutrinoless double beta decay and other microscopic lepton number violating processes and outline the requirements for future short- and long-baseline neutrino oscillation experiments to improve on the existing bounds. [ABSTRACT FROM AUTHOR]

Neutrinoless double beta decay can significantly help to shed light on the issue of nonzero neutrino mass, as the observation of this lepton-number-violating process would imply that neutrinos are Majorana particles. However, the underlying interaction does not have to be as simple as the standard neutrino mass mechanism. The entire variety of neutrinoless double beta decay mechanisms can be approached effectively. In this work, we focus on a theoretical description of short-range effective contributions to neutrinoless double beta decay, which are equivalent to nine-dimensional effective operators incorporating the appropriate field content. We give a detailed derivation of the nuclear matrix elements and phase-space factors corresponding to individual terms of the effective Lagrangian. Using these, we provide general formulas for the neutrinoless double beta decay half-life and angular correlation of the outgoing electrons. [ABSTRACT FROM AUTHOR]

We discuss the impact of the observation of neutrinoless double beta decay on the washout of lepton number in the early universe. Neutrinoless double beta decay can be triggered by a large number of mechanisms that can be encoded in terms of standard model effective operators which violate lepton number by two units. We calculate the contribution of such operators to the rate of neutrinoless double beta decay and correlate it with the washout of lepton number induced by the same operators in the early universe. We find that the observation of a nonstandard contribution to neutrinoless double beta decay, i.e., not induced by the standard mass mechanism of light neutrino exchange, would correspond to an efficient washout of lepton number above the electroweak scale for many operators up to mass dimension 11. Combined with standard model sphaleron transitions, this would render many baryogenesis mechanisms at higher scales ineffective. [ABSTRACT FROM AUTHOR]

We discuss a class of left-right symmetric theories with a universal seesaw mechanism for fermion masses and mixing and the implications for neutrinoless double beta (0νββ) decay where neutrino masses are governed by natural type-II seesaw dominance. The scalar sector consists of left- and right-handed Higgs doublets and triplets, while the conventional Higgs bidoublet is absent in this scenario. We use the Higgs doublets to implement the left-right and the electroweak symmetry breaking. On the other hand, the Higgs triplets with induced vacuum expectation values can give Majorana masses to light and heavy neutrinos and mediate 0νββ decay. In the absence of the Dirac mass terms for the neutrinos, this framework can naturally realize type-II seesaw dominance even if the right-handed neutrinos have masses of a few TeV. We study the implications of this framework in the context of 0νββ decay. [ABSTRACT FROM AUTHOR]

Grand unified theories (GUTs) are a very well motivated extensions of the Standard Model (SM), but the landscape of models and possibilities is overwhelming, and different patterns can lead to rather distinct phenomenologies. In this work we present a way to automatize the model building process, by considering a top to bottom approach that constructs viable and sensible theories from a small and controllable set of inputs at the high scale. By providing a GUT scale symmetry group and the field content, possible symmetry breaking paths are generated and checked for consistency, ensuring anomaly cancellation, SM embedding and gauge coupling unification. We emphasize the usefulness of this approach for the particular case of a nonsupersymmetric SO(10) model with an intermediate left-right symmetry, and we analyze how low-energy observables such as proton decay and lepton flavor violation might affect the generated model landscape. [ABSTRACT FROM AUTHOR]