Additional Neutrino Research Articles

Constraining the mass-spectra in the presence of a light sterile neutrino from absolute mass-related observables

The framework of three-flavor neutrino oscillation is a well-established phenomenon, but results from the short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and MiniBooster Neutrino Experiment (MiniBooNE), hint at the potential existence of an additional light neutrino state characterized by a mass-squared difference of approximately 1 eV2. The new neutrino state is devoid of all Standard Model (SM) interactions, commonly referred to as a “sterile” state. In addition, a sterile neutrino with a mass-squared difference of 10−2 eV2 has been proposed to improve the tension between the results obtained from the Tokai to Kamioka (T2K) and the NuMI Off-axis νe Appearance (NOνA) experiments. Further, the nonobservation of the predicted upturn in the solar neutrino spectra below 8 MeV can be explained by postulating an extra light sterile neutrino state with a mass-squared difference around 10−5 eV2. The hypothesis of an additional light sterile neutrino state introduces four distinct mass spectra depending on the sign of the mass-squared difference. In this paper, we discuss the implications of the above scenarios on the observables that depend on the absolute mass of the neutrinos; namely, the sum of the light neutrino masses (Σ) from cosmology, the effective mass of the electron neutrino from beta decay (mβ), and the effective Majorana mass (mββ) from neutrinoless double beta decay. We show that some scenarios can be disfavored by the current constraints of the above variables. The implications for projected sensitivity of Karlsruhe Tritium Neutrino Experiment (KATRIN) and future experiments like Project-8, next Enriched Xenon Observatory (nEXO) are discussed. Published by the American Physical Society 2024

Testing the number of neutrino species with a global fit of neutrino data

We present the first experimental constraints on models with many additional neutrino species with a similar Yukawa coupling with their right-handed partners among the additional Higgsed sectors by an analysis of current neutrino data. These types of models are motivated as a solution to the hierarchy problem by lowering the species scale of gravity to TeV. Additionally, they offer a natural mechanism to generate small neutrino masses and provide interesting dark matter candidates. This study analyzes data from DayaBay, KamLAND, MINOS, NOνA, and KATRIN. We do not find evidence for the presence of any additional neutrino species, therefore we report lower bounds on the allowed number of neutrino species realized in nature. For the normal/inverted neutrino mass ordering, we can give a lower bound on the number of neutrino species of O(30) and O(100), respectively, over a large range of the parameter space. Published by the American Physical Society 2024

How many dark neutrino sectors does cosmology allow?

We present the very first constraints on the number of Standard Model (SM) copies with an additional Dirac right-handed neutrino. From cosmology, we are able to pose strong limits on large regions of the parameter space. Moreover, we show that it is possible to account for the right dark matter density in form of stable particles from the dark sectors.

Dark radiation from the primordial thermal bath in momentum space

Motivated by the stunning projections for future CMB surveys, we evaluate the amount of dark radiation produced in the early Universe by two-body decays or binary scatterings with thermal bath particles via a rigorous analysis in momentum space. We track the evolution of the dark radiation phase space distribution, and we use the asymptotic solution to evaluate the amount of additional relativistic energy density parameterized in terms of an effective number of additional neutrino species ΔN eff. Our approach allows for studying light particles that never reach equilibrium across cosmic history, and to scrutinize the physics of the decoupling when they thermalize instead. We incorporate quantum statistical effects for all the particles involved in the production processes, and we account for the energy exchanged between the visible and invisible sectors. Non-instantaneous decoupling is responsible for spectral distortions in the final distributions, and we quantify how they translate into the corresponding value for ΔN eff. Finally, we undertake a comprehensive comparison between our exact results and approximated methods commonly employed in the existing literature. Remarkably, we find that the difference can be larger than the experimental sensitivity of future observations, justifying the need for a rigorous analysis in momentum space.

Bounds on lepton non-unitarity and heavy neutrino mixing

We present an updated and improved global fit analysis of current flavour and electroweak precision observables to derive bounds on unitarity deviations of the leptonic mixing matrix and on the mixing of heavy neutrinos with the active flavours. This new analysis is motivated by new and updated experimental results on key observables such as Vud, the invisible decay width of the Z boson and the W boson mass. It also improves upon previous studies by considering the full correlations among the different observables and explicitly calibrating the test statistic, which may present significant deviations from a χ2 distribution. The results are provided for three different Type-I seesaw scenarios: the minimal scenario with only two additional right-handed neutrinos, the next to minimal one with three extra neutrinos, and the most general one with an arbitrary number of heavy neutrinos that we parametrise via a generic deviation from a unitary leptonic mixing matrix. Additionally, we also analyze the case of generic deviations from unitarity of the leptonic mixing matrix, not necessarily induced by the presence of additional neutrinos. This last case relaxes some correlations among the parameters and is able to provide a better fit to the data. Nevertheless, inducing only leptonic unitarity deviations avoiding both the correlations implied by the right-handed neutrino extension as well as more strongly constrained operators is challenging and would imply significantly more complex UV completions.

Neutrino follow-up with the Zwicky transient facility: results from the first 24 campaigns

ABSTRACT The Zwicky Transient Facility (ZTF) performs a systematic neutrino follow-up programme, searching for optical counterparts to high-energy neutrinos with dedicated Target-of-Opportunity (ToO) observations. Since first light in March 2018, ZTF has taken prompt observations for 24 high-quality neutrino alerts from the IceCube Neutrino Observatory, with a median latency of 12.2 h from initial neutrino detection. From two of these campaigns, we have already reported tidal disruption event (TDE) AT 2019dsg and likely TDE AT 2019fdr as probable counterparts, suggesting that TDEs contribute >7.8 per cent of the astrophysical neutrino flux. We here present the full results of our programme through to December 2021. No additional candidate neutrino sources were identified by our programme, allowing us to place the first constraints on the underlying optical luminosity function of astrophysical neutrino sources. Transients with optical absolutes magnitudes brighter that −21 can contribute no more than 87 per cent of the total, while transients brighter than −22 can contribute no more than 58 per cent of the total, neglecting the effect of extinction and assuming they follow the star formation rate. These are the first observational constraints on the neutrino emission of bright populations such as superluminous supernovae. None of the neutrinos were coincident with bright optical AGN flares comparable to that observed for TXS 0506+056/IC170922A, with such optical blazar flares producing no more than 26 per cent of the total neutrino flux. We highlight the outlook for electromagnetic neutrino follow-up programmes, including the expected potential for the Rubin Observatory.

Inelastic nuclear scattering from neutrinos and dark matter

Neutrinos with energy of order 10~MeV, such as from pion decay-at-rest sources, are an invaluable tool for studying low-energy neutrino interactions with nuclei -- previously enabling the first measurement of coherent elastic neutrino-nucleus scattering. Beyond elastic scattering, neutrinos and dark matter in this energy range also excite nuclei to its low-lying nuclear states, providing an additional physics channel. Here, we consider neutral-current inelastic neutrino-nucleus and dark matter(DM)-nucleus scattering off $^{40}$Ar, $^{133}$Cs, and $^{127}$I nuclei that are relevant to a number of low-threshold neutrino experiments at pion decay-at-rest facilities. We carry out large scale nuclear shell model calculations of the inelastic cross sections considering the full set of electroweak multipole operators. Our results demonstrate that Gamow-Teller transitions provide the dominant contribution to the cross section and that the long-wavelength limit provides a reasonable approximation to the total cross section for neutrino sources. We show that future experiments will be sensitive to this channel and thus these results provide additional neutrino and DM scattering channels to explore at pion decay-at-rest facilities.

Light sterile neutrino and leptogenesis

We studied models of leptogenesis where three right-handed Majorana neutrinos are involved and the minimal-extended seesaw mechanism including an additional singlet field produces four light neutrinos. This study shows that the type of mass ordering and heavy Majorana scales can be determined by inputting the simplest orthogonal matrix into the Casas-Ibarra(CI) representation of seesaw. The CP asymmetry produced from the decays of heavy neutrinos and the dilution mass are predicted in terms of the mass and mixing elements of the fourth neutrino. Upon the choice of CI matrix, the existence of a light sterile neutrino is required to explain the high-energy lepton asymmetry in light of phenomenological measurements. Although there are several free parameters attributable to an additional neutrino, the model can be in part constrained by low-energy experiments such as sterile neutrino searches and neutrinoless double-beta decays, as well as the observed baryon asymmetry in the universe.

Charged lepton flavor violating processes in the Grimus-Neufeld model

Charged Lepton Flavour Violating (cLFV) decays constrain the relationship between the neutrino and the scalar sectors of the Grimus-Neufeld model (GNM), an appealing minimal model of neutrino masses. It turns out, that in the scenario, where the seesaw scale is lower than the electroweak one, cLFV is completely defined by the new Yukawa interactions between the additional single heavy Majorana neutrino, the second Higgs doublet and the lepton doublets. Therefore, we derive a useful parameterization for the Yukawa couplings which reproduces by construction the correct PMNS matrix and the correct neutrino masses for both Normal and Inverted ordering at one-loop level. We embed this scenario in the FlexibleSUSY spectrum-generator generator to perform parameter scans. Focusing on the tiny seesaw scale, we show that current μ → eγ limits provide significant constraints on the scalar sector, and we evaluate the impact of future cLFV τ-decay searches for the cases of discovery or non-discovery. The tiny seesaw scale makes the neutrino sector and the cLFV processes in the GNM similar to the scotogenic and the scoto-seesaw models, so we provide constraints for these models as well.

Geometry of the neutrino mixing space

We study the geometric structure of the physical region of neutrino mixing matrices as part of the unit ball of the spectral norm. Each matrix from the geometric region is a convex combination of unitary PMNS matrices. The disjoint subsets corresponding to a different minimal number of additional neutrinos are described as relative interiors of faces of the unit ball. We determined the Carath\'eodory's number showing that at most four unitary matrices of dimension three are necessary to represent any matrix from the neutrino geometric region. For matrices which correspond to scenarios with one and two additional neutrino states, the Carath\'eodory's number is two and three, respectively. Further, we discuss the volume associated with different mathematical structures, particularly with unitary and orthogonal groups, and the unit ball of the spectral norm. We compare the obtained volumes to the volume of the region of physically admissible mixing matrices for both the $CP$-conserving and $CP$-violating cases in the present scenario with three neutrino families and scenarios with the neutrino mixing matrix of dimension higher than three.

Robustness of ARS leptogenesis in scalar extensions

Extensions of the Standard Model (SM) with sterile neutrinos are well motivated from the observed oscillations of the light neutrinos and they have shown to successfully explain the Baryon Asymmetry of the Universe (BAU) through, for instance, the so-called ARS leptogenesis. Sterile neutrinos can be added in minimal ways to the SM, but many theories exist where sterile neutrinos are not the only new fields. Such theories often include scalar bosons, which brings about the possibility of further interactions between the sterile neutrinos and the SM. In this paper we consider an extension of the SM with two sterile neutrinos and one scalar singlet particle and investigate the effect that an additional, thermalised, scalar has on the ARS leptogenesis mechanism. We show that in general the created asymmetry is reduced due to additional sterile neutrino production from scalar decays. When sterile neutrinos and scalars are discovered in the laboratory, our results will provide information on the applicability of the ARS leptogenesis mechanism.

Possible Neutrino-Antineutrino Production during Gamma Ray e−e+ Pair Production: Monte Carlo Simulation Study

An alternative Feynman diagram for electron-positron pair production, in which neutrino and antineutrino are also produced on the same pathway, is introduced here. In the proposed pair production process, a portion of the momentum is carried by neutrinos and antineutrinos, allowing the rest of the momentum for the electron-positron pair. Simulations to inspect the proposed pair production process were conducted in this research using the EGS5 code system while modifying its subroutine “PAIR”. Liquid Xenon detector was then positioned in the path of various mono-energetic photon beams ranging from 2.6 to 12 MeV. These simulations were intended to inspect the detectability of the alternative pair production effects on radiation measurements in order to assess the detection conditions. Simulation results provided a comparison between the original pair production process and the proposed pair production process. Spectral results showed that changes in the region around 1 - 2 MeV and in the photopeak region were remarkable, therefore detectable. Further experimental research is recommended based on simulation findings. The alternative pair production process, firstly introduced in this paper, led to production of a larger flux of neutrinos from gamma radiation. This additional neutrino production and its contribution to non-baryonic dark matter are discussed.

Multi-messenger emission from the parsec-scale jet of the flat-spectrum radio quasar PKS 1502+106 coincident with high-energy neutrino IceCube-190730A

On July 30th, 2019 IceCube detected a high-energy astrophysical muon neutrino candidate, IC-190730A with a 67% probability of astrophysical origin. The flat spectrum radio quasar (FSRQ) PKS 1502 +106 is in the error circle of the neutrino. Motivated by this observation, we study PKS 1502+106 as a possible source of IC-190730A. PKS 1502+106 was in a quiet state in terms of UV/optical/X-ray/γ-ray flux at the time of the neutrino alert, we therefore model the expected neutrino emission from the source during its average long-term state, and investigate whether the emission of IC-190730A as a result of the quiet long-term emission of PKS 1502+106 is plausible. We analyse UV/optical and X-ray data and collect additional observations from the literature to construct the multi-wavelength spectral energy distribution of PKS 1502+106. We perform leptohadronic modelling of the multi-wavelength emission of the source and determine the most plausible emission scenarios and the maximum expected accompanying neutrino flux. A model in which the multi-wavelength emission of PKS 1502+106 originates beyond the broad-line region and inside the dust torus is most consistent with the observations. In this scenario, PKS 1502+106 can have produced up to of order one muon neutrino with energy exceeding 100 TeV in the lifetime of IceCube. An appealing feature of this model is that the required proton luminosity is consistent with the average required proton luminosity if blazars power the observed ultra-high-energy-cosmic-ray flux and well below the source's Eddington luminosity. If such a model is ubiquitous among FSRQs, additional neutrinos can be expected from other bright sources with energy ≳ 10 PeV.

Formula omitted] and non-standard neutrino interactions

We discuss the modes B→K(⁎)νν¯ in the context of non-standard neutrino interactions that add incoherently to the SM rates. We consider two scenarios: an additional light neutrino; and neutrino lepton flavour violation. We find that an additional light neutrino that interacts with SM fields via a non-universal Z′ can increase RK(⁎)ν by up to a factor of two without conflicting with Bs−B¯s mixing. This model then predicts rates for Bs→τ+τ− up to six times larger than the SM. In the context of neutrino lepton flavour violation mediated by leptoquarks we find that the current experimental upper bounds on RK(⁎)ν are already more constraining than direct bounds from Bs→τℓ and B→K(⁎)τℓ modes for ℓ=e,μ.

From Peccei Quinn symmetry to mass hierarchy problem

We propose a non-universal U(1) X gauge extension to the standard model (SM) and an additional Peccei–Quinn (PQ) global symmetry to study the mass hierarchy and strong CP problem. The scheme allows us to distinguish among fermion families and to generate the fermionic mass spectrum of particles of the SM. The symmetry breaking is performed by two scalar Higgs doublets and two scalar Higgs singlets, where one of these has the axion which turns out to be a candidate for cold dark matter. The exotic sector is composed by one up-like T and two down-like J 1,2 heavy quarks, two heavy charged leptons , one additional right-handed neutrino per family , and an invisible axion a. In addition, the large energy scale associated to the breaking of the PQ-symmetry gives masses to the right-handed neutrinos in such a way that the active neutrinos acquire eV-mass values due to the see-saw mechanism. On the other hand, from the non-linear effective Lagrangian, the flavour changing of the down quarks and charged leptons with the axion are considered.

Search for signatures of sterile neutrinos with Double Chooz

We present a search for signatures of neutrino mixing of electron anti-neutrinos with additional hypothetical sterile neutrino flavors using the Double Chooz experiment. The search is based on data from 5 years of operation of Double Chooz, including 2 years in the two-detector configuration. The analysis is based on a profile likelihood, i.e. comparing the data to the model prediction of disappearance in a data-to-data comparison of the two respective detectors. The analysis is optimized for a model of three active and one sterile neutrino. It is sensitive in the typical mass range {5 times 10^{-3}},mathrm{eV}^2 lesssim varDelta m^2_{41} lesssim {3 times 10^{-1}},mathrm{eV}^2 for mixing angles down to sin ^2 2theta _{14} gtrsim {0.02} . No significant disappearance additionally to the conventional disappearance related to theta _{13} is observed and correspondingly exclusion bounds on the sterile mixing parameter theta _{14} as a function of varDelta m^2_{41} are obtained.

Dark radiation from inflationary fluctuations

Light new vector bosons can be produced gravitationally through quantum fluctuations during inflation; if these particles are feebly coupled and cosmologically metastable, they can account for the observed dark matter abundance. However, in minimal anomaly-free $U(1)$ extensions to the Standard Model, these vectors generically decay to neutrinos if at least one neutrino mass eigenstate is sufficiently light. If these decays occur between neutrino decoupling and cosmic microwave background (CMB) freeze-out, the resulting radiation energy density can contribute to $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}$ at levels that can ameliorate the Hubble tension and be discovered with future CMB and relic neutrino detection experiments. Since the additional neutrinos are produced from vector decays after Big Bang Nucleosynthesis (BBN), this scenario predicts $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}>0$ at recombination, but $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}=0$ during BBN. Furthermore, due to a fortuitous cancellation, the contribution to $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}$ is approximately mass independent.

Production of thermal axions across the electroweak phase transition

Light axions can potentially leave a cosmic background, just like neutrinos. We complete the study of thermal axion production across the electroweak scale by providing a smooth and continuous treatment through the two phases. Focusing on both flavor conserving and violating couplings to third generation quarks, we compute the amount of axions produced via scatterings and decays of thermal bath particles. We perform a model independent analysis in terms of axion effective couplings, and we also make predictions for specific microscopic QCD axion scenarios. This observable effect, parameterized as it is conventional by an effective number of additional neutrinos, is above the 1σ sensitivity of future CMB-S4 surveys. Moreover, if one assumes no large hierarchies among dimensionless axion couplings to standard model particles, future axion helioscopes will provide a complementary probe for the parameter region we study.

Status of low mass LSP in SUSY

In this article we review the case for a light ($< m_{h_{125}}/2$) neutralino and sneutrino being a viable Dark Matter (DM) candidate in Supersymmetry(SUSY). To that end we recapitulate, very briefly, three issues related to the DM which impact the discussions : calculation of DM relic density, detection of the DM in Direct and Indirect experiments and creation /detection at the Colliders. In case of SUSY, the results from Higgs and SUSY searches at the colliders also have implications for the DM mass and couplings. In view of the constraints coming from all these sources, the possibility of a light neutralino is all but ruled out for the constrained MSSM : cMSSM. The pMSSM, where the gaugino masses are not related at high scale, is also quite constrained and under tension in case of thermal DM and will be put to very stern test in the near future in Direct Detection (DD) experiments as well as by the LHC analyses. However in the pMSSM with modified cosmology and hence non-thermal DM or in the NMSSM, a light neutralino is much more easily accommodated. A light RH sneutrino is also still a viable DM candidate although it requires extending the MSSM with additional singlet neutrino superfields. All of these possibilities can be indeed tested jointly in the upcoming SUSY-electroweakino and Higgs searches at the HL/HE luminosity LHC, the upcoming experiments for the Direct Detection (DD) and indirect detection for the DM as well as the high precision electron-positron colliders under planning.

Complete leading-order standard model corrections to quantum leptogenesis

Thermal leptogenesis, in the framework of the standard model with three additional heavy Majorana neutrinos, provides an attractive scenario to explain the observed baryon asymmetry in the universe. It is based on the out-of-equilibrium decay of Majorana neutrinos in a thermal bath of standard model particles, which in a fully quantum field theoretical formalism is obtained by solving Kadanoff-Baym equations. So far, the leading two-loop contributions from leptons and Higgs particles are included, but not yet gauge corrections. These enter at three-loop level but, in certain kinematical regimes, require a resummation to infinite loop order for a result to leading order in the gauge coupling. In this work, we apply such a resummation to the calculation of the lepton number density. The full result for the simplest “vanilla leptogenesis” scenario is by mathcal{O} (1) increased compared to that of quantum Boltzmann equations, and for the first time permits an estimate of all theoretical uncertainties. This step completes the quantum theory of leptogenesis and forms the basis for quantitative evaluations, as well as extensions to other scenarios.