Search for keV-scale sterile neutrinos with the first KATRIN data

Physical Review D

We discuss under what circumstances a signal in upcoming laboratory searches for keVscale 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 KA-TRIN/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. a cristina.benso@mpi-hd.mpg.de b vedran.brdar@mpi-hd.mpg.de c manfred.lindner@mpi-hd.mpg.de d werner.rodejohann@mpi-hd.mpg.de 1 In what follows, for brevity, we will refer to such particles simply as "sterile neutrinos". We note that the sterile neutrinos at eV-scale are also well studied in the context of short-baseline neutrino oscillation anomalies [6], but consideration of such states is beyond the scope of this work. 2 see also where this is achieved in the framework of scotogenic model.

2004

* Full texts of the report of the working group. For the summary report of the APS Multidivisional Neutrino Study, 'The Neutrino Matrix', see physics/0411216 0νββ decay, independent of its rate, would show that neutrinos, unlike all the other constituents of matter, are their own antiparticles. There is no other realistic way to determine the nature-Dirac or Majorana, of massive neutrinos. This would be a discovery of major importance, with impact not only on this fundamental question, but also on the determination of the absolute neutrino mass scale, and on the pattern of neutrino masses, and possibly on the problem of CP violation in the lepton sector, associated with Majorana neutrinos. There is a consensus on this basic point which we translate into the recommendations how to proceed with experiments dedicated to the search of the 0νββ decay, and how to fund them. • To reach our conclusion, we have to consider past achievements, the size of previous experiments, and the existing proposals. There is a considerable community of physicists worldwide as well as in the US interested in pursuing the search for the 0νββ decay. Past experiments were of relatively modest size. Clearly, the scope of future experiments should be considerably larger, and will require advances in experimental techniques, larger collaborations and additional funding. In terms of m ββ , the effective neutrino Majorana mass that can be extracted from the observed 0νββ decay rate, there are three ranges of increasing sensitivity, related to known neutrino-mass scales of neutrino oscillations. • The ∼100-500 meV m ββ range corresponds to the quasi-degenerate spectrum of neutrino masses. The motivation for reaching this scale has been strengthened by the recent claim of an observation of 0νββ decay in 76 Ge; a claim that obviously requires further investigation. To reach this scale and perform reliable measurements, the size of the experiment should be approximately 200 kg of the decaying isotope, with a corresponding reduction of the background. This quasi-degenerate scale is achievable in the relatively near term, ∼ 3-5 years. Several groups with considerable US participation have well established plans to build ∼ 200-kg devices that could scale straightforwardly to 1 ton (Majorana using 76 Ge, Cuore using 130 Te, and EXO using 136 Xe). There are also other proposed experiments worldwide which offer to study a number of other isotopes and could reach similar sensitivity after further R&D. Several among them (e.g. Super-NEMO, MOON) have US participation. By making measurements in several nuclei the uncertainty arising from the nuclear matrix elements would be reduced. The development of different detection techniques, and measurements in several nuclei, is invaluable for establishing the existence (or lack thereof) of the 0νββ decay at this effective neutrino mass range. • The ∼20-55 meV range arises from the atmospheric neutrino oscillation results. Observation of m ββ at this mass scale would imply the inverted neutrino mass hierarchy or the normal-hierarchy ν mass spectrum very near the quasidegenerate region. If either this or the quasi-degenerate spectrum is established, it would be invaluable not only for the understanding of the origin of neutrino mass, but also as input to the overall neutrino physics program (long baseline oscillations, search for CP violations, search for neutrino mass in tritium beta decay and astrophysics/cosmology, etc.) To study the 20-50 meV mass range will require about 1 ton of the isotope mass, a challenge of its own. Given the importance, and the points discussed above, more than one experiment of that size is desirable. • The ∼2-5 meV range arises from the solar neutrino oscillation results and will almost certainly lead to the 0νββ decay, provided neutrinos are Majorana particles. To reach this goal will require ∼100 tons of the decaying isotope, and no current technique provides such a leap in sensitivity. • The qualitative physics results that arise from an observation of 0νββ decay are profound. Hence, the program described above is vital and fundamentally important even if the resulting m ββ would be rather uncertain in value. However, by making measurements in several nuclei the uncertainty arising from the nuclear matrix elements would be reduced. • Unlike double-beta decay, beta-decay endpoint measurements search for a kinematic effect due to neutrino mass and thus are "direct searches" for neutrino mass. This technique, which is essentially free of theoretical assumptions about neutrino properties, is not just complementary. In fact, both types of measurements will be required to fully untangle the nature of the neutrino mass. Excitingly, a very large new beta spectrometer is being built in Germany. This KATRIN experiment has a design sensitivity approaching 200 meV. If the neutrino masses are quasi-degenerate, as would be the case if the recent double-beta decay claim proves true, KATRIN will see the effect. In this case the 0νββ-decay experiments can provide, in principle, unique information about CP-violation in the lepton sector, associated with Majorana neutrinos. • Cosmology can also provide crucial information on the sum of the neutrino masses. This topic is summarized in a different section of the report, but it should be mentioned here that the next generation of measurements hope to be able to observe a sum of neutrino masses as small as 40 meV. We would like to emphasize the complementarity of the three approaches, 0νββ , β decay, and cosmology. Recommendations: We conclude that such a double-beta-decay program can be summarized as having three components and our recommendations can be summarized as follows:

Vacuum

Figure 1: Overview of the Katrin experiment. Tritium gas is injected in the source (WGTS) and pumped out in adjacent pumping sections (DPS1/2, CPS). Electrons from β-decay are magnetically guided to the energy analysing spectrometer section and are counted at the detector.

Progress in Particle and Nuclear Physics

A number of anomalous results in short-baseline oscillation may hint at the existence of one or more light sterile neutrino states in the eV mass range and have triggered a wave of new experimental efforts to search for a definite signature of oscillations between active and sterile neutrino states. The present paper aims to provide a comprehensive review on the status of light sterile neutrino searches in mid-2019: we discuss not only the basic experimental approaches and sensitivities of reactor, source, atmospheric, and accelerator neutrino oscillation experiments but also the complementary bounds arising from direct neutrino mass experiments and cosmological observations. Moreover, we review current results from global oscillation analyses that include the constraints set by running reactor and atmospheric neutrino experiments. They permit to set tighter bounds on the active-sterile oscillation parameters but as yet are not able to provide a definite conclusion on the existence of eV-scale sterile neutrinos.

Physical Review D, 2013

Medium baseline reactor experiments (Double Chooz, Daya Bay and RENO) provide a unique opportunity to test the presence of light sterile neutrinos. We analyze the data of these experiments in the search of sterile neutrinos and also test the robustness of θ 13 determination in the presence of sterile neutrinos. We show that existence of a light sterile neutrino state improves the fit to these data moderately. We also show that the measured value of θ 13 by these experiments is reliable even in the presence of sterile neutrinos, and the reliability owes significantly to the Daya Bay and RENO data. From the combined analysis of the data of these experiments we constrain the mixing of a sterile neutrino with ∆m 2 41 ∼ (10 −3 − 10 −1) eV 2 to sin 2 2θ 14 0.1 at 95% C.L..

Journal of Physics: Conference Series, 2017

NOvA is the current United States flagship long-baseline neutrino experiment designed to study the properties of neutrino oscillations. It consists of two functionally identical detectors each located 14.6 mrad off the central axis from the Fermilab NuMI neutrino beam. The Near Detector is located 1 km downstream from the beam source, and the Far Detector is located 810 km away in Ash River, Minnesota. This long baseline, combined with the ability of the NuMI facility to switch between nearly pure neutrino and anti-neutrino beams, allows NOvA to make precision measurements of neutrino mixing angles, potentially determine the neutrino mass hierarchy, and begin searching for CP violating effects in the lepton sector. However, NOvA can also probe more exotic scenarios, such as oscillations between the known active neutrinos and new sterile species. We will showcase the first search for sterile neutrinos in a 3 + 1 model at NOvA. The analysis presented searches for a deficit in the rate of neutral current events at the Far Detector using the Near Detector to constrain the predicted spectrum. This analysis was performed using data taken between February 2014 and May 2016 corresponding to 6.05 × 10 20 protons on target.

arXiv (Cornell University), 2022

Recently announced results from the KATRIN collaboration imply an upper bound on the effective electron anti-neutrino mass mν e , mν e < 0.8 eV/c 2 . Here we explore the implications of combining the KATRIN upper bound using a previously inferred lower bound on the smallest neutrino mass state, mi,min 0.4 eV/c 2 implied by the stability of white dwarfs and neutron stars in the presence of long-range many-body neutrino-exchange forces. By combining a revised lower bound estimate with the expected final upper bound from KATRIN, we find that the available parameter space for mν e may be closed completely within the next few years. We then extend the argument when a single light sterile neutrino flavor is present to set a lower mass limit on sterile neutrinos.

2021

We report the results of the second measurement campaign of the Karlsruhe Tritium Neutrino (KATRIN) experiment. KATRIN probes the effective electron anti-neutrino mass, mν, via a high-precision measurement of the tritium β-decay spectrum close to its endpoint at 18.6 keV. In the second physics run presented here, the source activity was increased by a factor of 3.8 and the background was reduced by 25% with respect to the first campaign. A sensitivity on mν of 0.7 eV/c2 at 90% confidence level (CL) was reached. This is the first sub-eV sensitivity from a direct neutrino-mass experiment. The best fit to the spectral data yields mν2=(0.26±0.34) eV2/c4, resulting in an upper limit of mν&lt;0.9 eV/c2 (90% CL). By combining this result with the first neutrino mass campaign, we find an upper limit of mν&lt;0.8 eV/c2 (90% CL).

Physical review letters, 2016

This Letter reports an improved search for light sterile neutrino mixing in the electron antineutrino disappearance channel with the full configuration of the Daya Bay Reactor Neutrino Experiment. With an additional 404 days of data collected in eight antineutrino detectors, this search benefits from 3.6 times the statistics available to the previous publication, as well as from improvements in energy calibration and background reduction. A relative comparison of the rate and energy spectrum of reactor antineutrinos in the three experimental halls yields no evidence of sterile neutrino mixing in the 2×10^{-4}≲|Δm_{41}^{2}|≲0.3 eV^{2} mass range. The resulting limits on sin^{2}2θ_{14} are improved by approx imately a factor of 2 over previous results and constitute the most stringent constraints to date in the |Δm_{41}^{2}|≲0.2 eV^{2} region.

Physical Review D, 2010

A search for depletion of the combined flux of active neutrino species over a 735 km baseline is reported using neutral-current interaction data recorded by the MINOS detectors in the NuMI neutrino beam. Such a depletion is not expected according to conventional interpretations of neutrino oscillation data involving the three known neutrino flavors. A depletion would be a signature of oscillations or decay to postulated noninteracting sterile neutrinos, scenarios not ruled out by existing data. From an exposure of 3.18 × 10 20 protons on target in which neutrinos of energies between ∼500 MeV and 120 GeV are produced predominantly as νµ, the visible energy spectrum of candidate neutral-current reactions in the MINOS far-detector is reconstructed. Comparison of this spectrum to that inferred from a similarly selected near-detector sample shows that of the portion of the νµ flux observed to disappear in charged-current interaction data, the fraction that could be converting to a sterile state is less than 52% at 90% confidence level (C.L.). The hypothesis that active neutrinos mix with a single sterile neutrino via oscillations is tested by fitting the data to various models. In the particular four-neutrino models considered, the mixing angles θ24 and θ34 are constrained to be less than 11 • and 56 • at 90% C.L., respectively. The possibility that active neutrinos may decay to sterile neutrinos is also investigated. Pure neutrino decay without oscillations is ruled out at 5.4 standard deviations. For the scenario in which active neutrinos decay into sterile states concurrently with neutrino oscillations, a lower limit is established for the neutrino decay lifetime τ3/m3 > 2.1 × 10 −12 s/eV at 90% C.L.

Journal of Physics: Conference Series

2016

In this work, we present a realistic analysis of the potential of the presentday reactor experiments Double Chooz, Daya Bay and RENO for probing the existence of sterile neutrinos. We present exclusion regions for sterile oscillation parameters for each of these experiments, using simulations with realistic estimates of systematic errors and detector resolutions, and compare the sterile parameter sensitivity regions we obtain with the existing bounds from other reactor experiments. We find that these experimental setups give significant bounds on the parameter Θ ee especially in the low sterile oscillation region 0.01 < ∆m 2 41 < 0.05 eV 2. These bounds can add to our understanding of the sterile neutrino sector since there is still a tension in the allowed regions from different experiments for sterile parameters.

Nuclear and Particle Physics Proceedings, 2015

Physical review letters, 2017

An experiment to search for light sterile neutrinos is conducted at a reactor with a thermal power of 2.8 GW located at the Hanbit nuclear power complex. The search is done with a detector consisting of a ton of Gd-loaded liquid scintillator in a tendon gallery approximately 24 m from the reactor core. The measured antineutrino event rate is 1976 per day with a signal to background ratio of about 22. The shape of the antineutrino energy spectrum obtained from the eight-month data-taking period is compared with a hypothesis of oscillations due to active-sterile antineutrino mixing. No strong evidence of 3+1 neutrino oscillation is found. An excess around the 5 MeV prompt energy range is observed as seen in existing longer-baseline experiments. The mixing parameter sin^{2}2θ_{14} is limited up to less than 0.1 for Δm_{41}^{2} ranging from 0.2 to 2.3 eV^{2} with a 90% confidence level.

2009

In this talk we review the current status of sterile neutrino searches and discuss the potential of future long baseline experiments to study their properties.