keV-scale Sterile Neutrinos Research Articles

Development of a silicon drift detector array to search for keV-scale sterile neutrinos with the KATRIN experiment

Sterile neutrinos in the keV mass range present a viable candidate for dark matter. They can be detected through single β -decay, where they cause small spectral distortions. The Karlsruhe Tritium Neutrino (KATRIN) experiment aims to search for keV-scale sterile neutrinos with high sensitivity. To achieve this, the KATRIN beamline will be equipped with a novel multi-pixel silicon drift detector focal plane array named TRISTAN. In this study, we present the performance of a TRISTAN detector module, a component of the eventual 9-module system. Our investigation encompasses spectroscopic aspects such as noise performance, energy resolution, linearity, and stability.

Exotic Dark Matter Search with the Majorana Demonstrator

With excellent energy resolution and ultralow-level radiogenic backgrounds, the high-purity germanium detectors in the enable searches for several classes of exotic dark matter (DM) models. In this work, we report new experimental limits on keV-scale sterile neutrino DM via the transition magnetic moment from conversion to active neutrinos νs→νa. We report new limits on fermionic dark matter absorption (χ+A→ν+A) and sub-GeV DM-nucleus 3→2 scattering (χ+χ+A→ϕ+A), and new exclusion limits for bosonic dark matter (axionlike particles and dark photons). These searches utilize the (1–100)-keV low-energy region of a 37.5-kg y exposure collected by the between May 2016 and November 2019 using a set of Ge76-enriched detectors whose surface exposure time was carefully controlled, resulting in extremely low levels of cosmogenic activation. Published by the American Physical Society 2024

Neutrino mass measurement and sterile neutrinos search with the KATRIN experiment

In this proceeding, we report on the latest results of the KATRIN experiment on the effective electron anti-neutrino mass and the search for sterile neutrinos. KATRIN performed a high resolution and high statistics spectroscopy measurement of the tritium β-decay spectrum around its endpoint energy. The analysis of ~ 1265 h of data acquired during the first two measurement campaigns of 2019 led to the most stringent limit on the neutrino mass with mν < 0.8 eV- (90% CL). We also report on the improved KATRIN exclusion limits for the eV- and keV-scale sterile neutrinos search. Finally, the future of KATRIN with the TRISTAN detector aiming at further improve the laboratory-based sensitivity to keV-scale sterile neutrino is discussed.

A Lab-Scale Experiment for keV Sterile Neutrino Search

We developed a simple small-scale experiment to measure the beta decay spectrum of ^{3}H. The aim of this research is to investigate the presence of sterile neutrinos in the keV region. Tritium nuclei were embedded in a 1times1times1 cm^3 LiF crystal from the ^6Li(n,alpha)^3H reaction. The energy of the beta electrons absorbed in the LiF crystal was measured with a magnetic microcalorimeter at 40 mK. We report a new method of sample preparation, experiments, and analysis of ^3H beta measurements. The spectrum of a 10-hour measurement agrees well with the expected spectrum of ^3H beta decay. The analysis results indicate that this method can be used to search for keV-scale sterile neutrinos.

Characterization measurements of the TRISTAN multi-pixel silicon drift detector

Sterile neutrinos are a minimal extension of the standard model of particle physics. A laboratory-based approach to search for this particle is via tritium β-decay, where a sterile neutrino would cause a kink-like spectral distortion. The Karlsruhe Tritium Neutrino (KATRIN) experiment extended by a multi-pixel Silicon Drift Detector system has the potential to reach an unprecedented sensitivity to the keV-scale sterile neutrino in a lab-based experiment. The new detector system combines good spectroscopic performance with a high rate capability. In this work, we report about the characterization of charge-sharing between pixels and the commissioning of a 47-pixel prototype detector in a MAC-E filter.

Neutrino bound states and bound systems

Yukawa interactions of neutrinos with a new light scalar boson ϕ can lead to formation of stable bound states and bound systems of many neutrinos (ν-clusters). For allowed values of the coupling y and the scalar mass mϕ, the bound state of two neutrinos would have the size larger than 1012 cm. Bound states with sub-cm sizes are possible for keV scale sterile neutrinos with coupling y > 10−4. For ν-clusters we study in detail the properties of final stable configurations. If there is an efficient cooling mechanism, these configurations are in the state of degenerate Fermi gas. We formulate and solve equations of the density distributions in ν-clusters. In the non-relativistic case, they are reduced to the Lane-Emden equation. We find that (i) stable configurations exist for any number of neutrinos, N; (ii) there is a maximal central density ∼ 109 cm−3 determined by the neutrino mass; (iii) for a given mϕ there is a minimal value of Ny3 for which stable configurations can be formed; (iv) for a given strength of interaction, Sϕ = (ymν/mϕ)2, the minimal radius of ν-clusters exists. We discuss the formation of ν-clusters from relic neutrino background in the process of expansion and cooling of the Universe. One possibility realized for Sϕ > 700 is the development of instabilities in the ν-background at T < mν which leads to its fragmentation. For Sϕ ∈ [70, 700]) they might be formed via the growth of initial density perturbations in the ν-background and virialiazation, in analogy with the formation of Dark Matter halos. For allowed values of y, cooling of ν-clusters due to ϕ-bremsstrahlung and neutrino annihilation is negligible. The sizes of ν-clusters may range from ∼ km to ∼ 5 Mpc.

Towards the TRISTAN detector: Characterization of a 47-pixel monolithic SDD array

Multi-pixel Silicon Drift Detectors (SDD), widely employed in X-ray applications, can also be applied to electron spectroscopy experiments. The objective of the TRISTAN project is to equip the KATRIN beam line with a multi-pixel SDD focal plane array. This would allow to extend the physics program of KATRIN, whose main goal is the determination of the neutrino mass via a precise measurement of the tritium beta decay spectrum close to the endpoint. An extension of the measurement interval would enable a sensitive search for physics beyond the standard model, in particular keV-scale sterile neutrinos, a prime dark matter candidate. The SDD technology guarantees excellent energy resolution and high throughput capabilities. A custom 47-pixel prototype, based on a monolithic SDD tile, has been developed and tested for the TRISTAN project. The full functionality of the system has been verified for all 47 channels. Good spectroscopic performance has been observed with a 55Fe X-ray source. This is a crucial step towards the realization of the final 21-fold 166-pixel TRISTAN focal plane detector.

Sterile neutrino dark matter, neutrino secret self-interactions and extra radiation

keV scale sterile neutrinos are excellent warm dark matter candidates. In the early Universe, these can be produced from oscillations and scatterings of active neutrinos. However, in the absence of any new physics, this mechanism is in severe tension with observations from X-ray searches. In this work, we show that secret self-interactions of active neutrinos, mediated by a scalar, can efficiently produce these sterile neutrinos without being ruled out by X-ray observations. These neutrino self-interactions are also testable: for a mediator mass greater than a few MeV, these self-interactions can give signatures in laboratory based experiments, while for lighter mediators, there will be observable consequences for upcoming cosmology probes.1

Neutrino mass, leptogenesis and sterile neutrino dark matter in inverse seesaw framework

We study [Formula: see text] flavor symmetric inverse seesaw model which has the possibility of simultaneously addressing neutrino phenomenology, dark matter (DM) and baryon asymmetry of the universe (BAU) through leptogenesis. The model is the extension of the standard model by the addition of two (RH) neutrinos and three sterile fermions leading to a keV scale sterile neutrino DM and two pairs of quasi-Dirac states. The CP violating decay of the lightest quasi-Dirac pair present in the model generates lepton asymmetry which then converts to BAU. Thus, this model can provide a simultaneous solution for nonzero neutrino mass, DM content of the universes and the observed baryon asymmetry. The [Formula: see text] flavor symmetry in this model is augmented by additional [Formula: see text] symmetry to constrain the Yukawa Lagrangian. A detailed numerical analysis has been carried out to obtain DM mass, DM-active mixing as well as BAU both for normal hierarchy as well as inverted hierarchy. We try to correlate the two cosmological observables and found a common parameter space satisfying the DM phenomenology and BAU. The parameter space of the model is further constrained from the latest cosmological bounds on the observables.

Phenomenology of keV sterile neutrino in minimal extended seesaw

We explore the possibility of a single generation of keV scale sterile neutrino [Formula: see text] as a dark matter candidate within the minimal extended seesaw (MES) framework and its influence in neutrinoless double beta decay [Formula: see text] study. Three hierarchical right-handed neutrinos were considered to explain neutrino mass. We also address baryogenesis via the mechanism of thermal leptogenesis considering the decay of the lightest RH neutrino to a lepton and Higgs doublet. A generic model based on [Formula: see text] flavor symmetry is constructed to explain both normal and inverted hierarchy mass pattern of neutrinos. Significant results on effective neutrino masses are observed in presence of sterile mass [Formula: see text] and active-sterile mixing [Formula: see text] in [Formula: see text]. Results from [Formula: see text] give stringent upper bounds on the active-sterile mixing matrix element. To establish sterile neutrino as dark matter within this model, we checked decay width and relic abundance of the sterile neutrino, which restricted sterile mass [Formula: see text] within some definite bounds. Constrained regions on the CP phases and Yukawa couplings are obtained from [Formula: see text] and baryogenesis results. Co-relations among these observable are also established and discussed within this framework.

Sterile neutrino dark matter via coinciding resonances

It has been proposed that two resonances could coincide in the early universe at temperatures T ∼ 0.2 … 0.5 GeV: one between two nearly degenerate GeV-scale sterile neutrinos, producing a large lepton asymmetry through freeze-out and decays; another between medium-modified active neutrinos and keV-scale sterile neutrinos, converting the lepton asymmetry into dark matter. Making use of a framework which tracks three sterile neutrinos of both helicities as well as three separate lepton asymmetries, and scanning the parameter space of the GeV-scale species, we establish the degree of fine-tuning that is needed for realizing this scenario.

Scalar induced resonant sterile neutrino production in the early Universe

It has been recently suggested \cite{Bezrukov:2017ike,Bezrukov:2018wvd} that a cosmic scalar field can completely change the keV-scale sterile neutrino production in the early Universe. Its effect may, for various parameter choices, either suppress sterile neutrino production and make moderate active-sterile mixing cosmologically acceptable, or increase the production and generate considerable dark matter component out of sterile neutrino with otherwise negligible mixing with SM. In this paper we provide analytic estimates complementing and providing details of the numerical calculations performed in \cite{Bezrukov:2018wvd} in the case of resonant amplification of the sterile neutrino production. We also discuss phenomenological and theoretical issues related to the successful implementation of this idea in fully realistic extensions of the Standard Model of particle physics.

Design and status of the Micro-X microcalorimeter sounding rocket

The Micro-X High Resolution Microcalorimeter X-Ray Imaging Rocket is a sounding rocket mission that will observe Supernova Remnants and search for keV-scale sterile neutrino dark matter. Micro-X will combine the excellent energy resolution of Transition Edge Sensor microcalorimeters with the imaging capabilities of a conical imaging mirror to map extended and point X-ray sources with an unprecedented combination of energy and spatial resolution. The payload has been designed to operate in the challenging conditions of a sounding rocket flight and to achieve sensitive results, in a single five-minute exposure, for each of these science goals. Micro-X’s unique design considerations are presented here, along with the status of the instrument and projections for the upcoming flights. The first Micro-X flight in 2018 will observe the Puppis A supernova remnant, where it will attain nearly 13,000 counts in the 300 s exposure. The second Micro-X flight will observe the Galactic Center to search for keV-scale dark matter and explore the nature of the unexplained 3.5 keV line observed by X-ray satellites.

Prospects for finding sterile neutrino dark matter at KATRIN

We discuss under what circumstances a signal in upcoming laboratory searches for keV-scale sterile neutrinos would be compatible with those particles being a sizable part or all of dark matter. In the parameter space that will be experimentally accessible by KATRIN/TRISTAN, strong X-ray limits need to be relaxed and dark matter overproduction needs to be avoided. We discuss postponing the dark matter production to lower temperatures, a reduced sterile neutrino contribution to dark matter, and a reduction of the branching ratio in photons and active neutrinos through cancellation with a new physics diagram. Both the Dodelson-Widrow and the Shi-Fuller mechanisms for sterile neutrino dark matter production are considered. As a final exotic example, potential consequences of CPT violation are discussed.

Internal Ionization of an Atom in the β Decay of Tritium

The problem of internal ionization in the beta decay of tritium due to the scattering of a β electron off a bound electron is discussed. The total probability of the process per a single decay event is calculated. The distributions over the momentum and the kinetic energy of a liberated electron are plotted. A correction to the β spectrum produced by internal ionization is determined. The identity of electrons is taken into account in all calculations. The results of calculations of the modified spectrum are of interest for the KATRIN experiment aimed at measuring the mass of an electron antineutrino. The high-luminosity source also makes it possible to search for keV-scale sterile neutrinos in the middle part of the β spectrum, where the internal ionization effect is significant.

Type I seesaw mechanism as the common origin of neutrino mass, baryon asymmetry, and the electroweak scale

The type-I seesaw represents one of the most popular extensions of the Standard Model. Previous studies of this model have mostly focused on its ability to explain neutrino oscillations as well as on the generation of the baryon asymmetry via leptogenesis. Recently, it has been pointed out that the type-I seesaw can also account for the origin of the electroweak scale due to heavy-neutrino threshold corrections to the Higgs potential. In this paper, we show for the first time that all of these features of the type-I seesaw are compatible with each other. Integrating out a set of heavy Majorana neutrinos results in small masses for the Standard Model neutrinos; baryogenesis is accomplished by resonant leptogenesis; and the Higgs mass is entirely induced by heavy-neutrino one-loop diagrams, provided that the tree-level Higgs potential satisfies scale-invariant boundary conditions in the ultraviolet. The viable parameter space is characterized by a heavy-neutrino mass scale roughly in the range $10^{6.5\cdots7.0}$ GeV and a mass splitting among the nearly degenerate heavy-neutrino states up to a few TeV. Our findings have interesting implications for high-energy flavor models and low-energy neutrino observables. We conclude that the type-I seesaw sector might be the root cause behind the masses and cosmological abundances of all known particles. This statement might even extend to dark matter in the presence of a keV-scale sterile neutrino.

Self-interacting sterile neutrino dark matter: The heavy-mediator case

For active-sterile mixing to be responsible for the full relic abundance of dark matter additional new physics is needed beyond the keV-scale sterile neutrino itself. The extra ingredient we consider here is the presence of self-interactions among the sterile neutrinos. We examine whether active-to-sterile conversion is amplified enough in this scenario that the observed abundance of dark matter can be obtained with a subconstraint mixing angle. This turns out never to be the case in the region we explore: either self-interactions have too small an impact and cannot escape bounds on the mass and mixing angle, or they have too great an impact and cause dark matter to be overproduced. The sharp transition from marginal to excessive effectiveness occurs because a resonance criterion is met in the effective in-medium mixing angle. Once the system goes resonant the game is as good as over, as nonlinearity in the Boltzmann equation leads to runaway production of sterile neutrinos, beginning at a plasma temperature of a few hundred MeV and typically ending at a few tens of MeV. The scenario is therefore ruled out largely by its own dynamics. In this study, we focus exclusively on mediators heavier than $\ensuremath{\sim}1\text{ }\text{ }\mathrm{GeV}$; future work will extend the analysis to lighter mediators, allowing for contact to be made with the kinds of scenarios motivated by issues of small-scale structure.

A novel detector system for KATRIN to search for keV-scale sterile neutrinos

Sterile neutrinos appear in minimal extensions of the Standard Model of particle physics. If their mass is in the keV regime, they are viable dark matter candidates. One way to search for sterile neutrinos in a laboratory-based experiment is via the analysis of β-decay spectra, where the new neutrino mass eigenstate would manifest itself as a kink-like distortion of the β-decay spectrum. The objective of the TRISTAN project is to extend the KATRIN setup with a new multi-pixel silicon drift detector system to search for a keV-scale sterile neutrino signal. In this paper we describe the requirements of such a new detector, and present first characterization measurement results obtained with a 7 pixel prototype system.

Roles of sterile neutrinos in particle physics and cosmology

The impacts of the light sterile neutrino hypothesis in particle physics and cosmology are reviewed. The observed short baseline neutrino anomalies challenging the standard explanation of neutrino oscillations within the framework of three active neutrinos are addressed. It is shown that they can be interpreted as the experimental hints pointing towards the existence of sterile neutrino at the eV scale. While the electron neutrino appearance and disappearance data are in favor of such a sterile neutrino, the muon disappearance data disfavor it, which gives rise to a strong appearance–disappearance tension. After a brief review on the cosmological effects of light sterile neutrinos, proposed signatures of light sterile neutrinos in the existing cosmological data are discussed. The keV-scale sterile neutrinos as possible dark matter candidates are also discussed by reviewing different mechanisms of how they can be produced in the early Universe and how their properties can be constrained by several cosmological observations. We give an overview of the possibility that keV-scale sterile neutrino can be a good DM candidate and play a key role in achieving low-scale leptogenesis simultaneously by introducing a model where an extra light sterile neutrino is added on top of type I seesaw model.

Development of a silicon drift detector system for the TRISTAN project—Future search for sterile neutrinos

Sterile neutrinos are a minimal extension of the Standard Model of Particle Physics. If their mass is in the kilo-electron-volt regime, they are viable dark matter candidates. One way to search for sterile neutrinos in a laboratory-based experiment is via tritium beta decay, where the new neutrino mass eigenstate would manifest itself as a kink-like distortion of the spectrum. The objective of the TRISTAN project is to extend the KATRIN setup with a novel multi-pixel silicon drift detector system to search for a keV-scale sterile neutrino signal. First seven-pixel prototype detectors were produced and characterized with radioactive X-ray and electron sources. The next prototype generation with 166 pixels is currently in production and will be available beginning of 2019. In this work, we describe the requirements of the novel TRISTAN detector system and present the technical realization of the first prototypes.