Search for the Majorana Nature of Neutrinos in the Inverted Mass Ordering Region with KamLAND-Zen

Direct neutrino-mass measurement based on 259 days of KATRIN data

That neutrinos carry a nonvanishing rest mass is evidence of physics beyond the Standard Model of elementary particles. Their absolute mass holds relevance in fields from particle physics to cosmology. We report on the search for the effective electron antineutrino mass with the KATRIN experiment. KATRIN performs precision spectroscopy of the tritium β-decay close to the kinematic endpoint. On the basis of the first five measurement campaigns, we derived a best-fit value of [Formula: see text] eV2, resulting in an upper limit of mν < 0.45 eV at 90% confidence level. Stemming from 36 million electrons collected in 259 measurement days, a substantial reduction of the background level, and improved systematic uncertainties, this result tightens KATRIN's previous bound by a factor of almost two.

Supramolecular chemistry in solution and solid-gas interfaces: synthesis and photophysical properties of monocolor and bicolor fluorescent sensors for barium tagging in neutrinoless double beta decay

Translation of photophysical properties of fluorescent sensors from solution to solid-gas environments via functionalized surfaces constitutes a challenge in chemistry. In this work, we report on the chemical synthesis, barium capture ability and photophysical properties of two families of monocolor and bicolor fluorescent sensors. These sensors were prepared to capture barium cations that can be produced in neutrinoless double beta decay of Xe-136. These sensors incorporate crown ether units, two different fluorophores, aliphatic spacers of different lengths, and a silatrane linker that forms covalent bonds with indium tin oxide (ITO) surfaces. Both species shared excellent Ba2+ binding abilities. Fluorescent monocolor indicators (FMIs), based on naphthyl fluorophores, showed an off-on character in solution controlled by photoinduced electron transfer. Fluorescent bicolor indicators (FBIs), based on benzo[a]imidazo[5,1,2-cd] fluorophores, exhibited a significant change in their emission spectra on going from the free to the barium-bound state. Both FMIs and FBIs showed similar photophysics in solution and on ITO. However, their performance on ITO was found to be attenuated, but not fully extinguished, with respect to the values obtained in solution, both in terms of intensity and selectivity between the free and Ba2+-bound states. Despite this issue, improved performance of the FBIs based on confocal microscopy of the directly attached molecules was observed. These selective FMI and FBI chemosensors installed on tailor-made functionalized surfaces are promising tools to capture the barium cations produced in the double beta decay of Xe-136. The identification of this capture would boost the sensitivity of the experiments searching for the Xe-136-based neutrinoless double beta decay, as backgrounds would be almost totally suppressed.

Fluorescence imaging of individual ions and molecules in pressurized noble gases for barium tagging in 136Xe

The imaging of individual Ba2+ ions in high pressure xenon gas is one possible way to attain background-free sensitivity to neutrinoless double beta decay and hence establish the Majorana nature of the neutrino. In this paper we demonstrate selective single Ba2+ ion imaging inside a high-pressure xenon gas environment. Ba2+ ions chelated with molecular chemosensors are resolved at the gas-solid interface using a diffraction-limited imaging system with scan area of 1 × 1 cm2 located inside 10 bar of xenon gas. This form of microscopy represents key ingredient in the development of barium tagging for neutrinoless double beta decay searches in 136Xe. This also provides a new tool for studying the photophysics of fluorescent molecules and chemosensors at the solid-gas interface to enable bottom-up design of catalysts and sensors.

Limits on W_{R} from Meson Decays

In this Letter we show that pseudoscalar meson leptonic decay data can be used to set stringent limits on the mass m_{W_{R}} of a right-handed vector boson, such as the one that appears in left-right symmetric models. We have shown that for a heavy neutrino with a mass m_{N} in the range 50 Collapse

Key Words Collapse

MESH Headings Collapse

Grants Collapse

Affiliation(s)

Gustavo F S Alves Fermilab, Instituto de Física, Universidade de São Paulo, C.P. 66.318, 05315-970 São Paulo, Brazil and Particle Theory Department, P.O. Box 500, Batavia, Illinois 60510, USA
Chee Sheng Fong Universidade Federal do ABC, Centro de Ciências Naturais e Humanas , 09.210-170, Santo André, Brazil
Luighi P S Leal Instituto de Física, Universidade de São Paulo, C.P. 66.318, 05315-970 São Paulo, Brazil
Renata Zukanovich Funchal Instituto de Física, Universidade de São Paulo, C.P. 66.318, 05315-970 São Paulo, Brazil

Collapse

Simultaneous Measurement of the Half-Life and Spectral Shape of ^{115}In β Decay with an Indium Iodide Cryogenic Calorimeter

Current bounds on the neutrino Majorana mass are affected by significant uncertainties in the nuclear calculations for neutrinoless double-beta decay. A key issue for a data-driven improvement of the nuclear theory is the actual value of the axial coupling constant g_{A}, which can be investigated through forbidden β decays. We present the first measurement of the 4th-forbidden β decay of ^{115}In with a cryogenic calorimeter based on indium iodide. Exploiting the enhanced spectrum-shape method for the first time to this isotope, our study accurately determines simultaneously spectral shape, g_{A}, and half-life. The interacting shell model, which best fits our data, indicates a half-life for this decay at T_{1/2}=(5.26±0.06)×10^{14}  yr.

Experimental neutrino physics in a nuclear landscape

There are profound connections between neutrino physics and nuclear experiments. Exceptionally precise measurements of single and double beta-decay spectra illuminate the scale and nature of neutrino mass and may finally answer the question of whether neutrinos are their own anti-matter counterparts. Neutrino-nucleus scattering underpins oscillation experiments and probes nuclear structure, neutrinos offer a rare vantage point into collapsing stars and nuclear fission reactors and techniques pioneered in neutrino nuclear physics experiments are advancing quantum sensing technologies. In this article, we review current and planned efforts at the intersection of neutrino and nuclear experiments. This article is part of the theme issue 'The liminal position of Nuclear Physics: from hadrons to neutron stars'.

Neutrino mass and mixing with modular symmetry

This is a review article about neutrino mass and mixing and flavour model building strategies based on modular symmetry. After a brief survey of neutrino mass and lepton mixing, and various Majorana seesaw mechanisms, we construct and parameterise the lepton mixing matrix and summarise the latest global fits, before discussing the flavour problem of the Standard Model. We then introduce some simple patterns of lepton mixing, introduce family (or flavour) symmetries, and show how they may be applied to direct, semi-direct and tri-direct CP models, where the simple patterns of lepton mixing, or corrected versions of them, may be enforced by the full family symmetry or a part of it, leading to mixing sum rules. We then turn to the main subject of this review, namely a pedagogical introduction to modular symmetry as a candidate for family symmetry, from the bottom-up point of view. After an informal introduction to modular symmetry, we introduce the modular group, and discuss its fixed points and residual symmetry, assuming supersymmetry throughout. We then introduce finite modular groups of levelNand modular forms with integer or rational modular weights, corresponding to simple geometric groups or their double or metaplectic covers, including the most general finite modular groups and vector-valued modular forms, with detailed results forN=2,3,4,5. The interplay between modular symmetry and generalized CP symmetry is discussed, deriving CP transformations on matter multiplets and modular forms, highlighting the CP fixed points and their implications. In general, compactification of extra dimensions generally leads to a number of moduli, and modular invariance with factorizable and non-factorizable multiple moduli based on symplectic modular invariance and automorphic forms is reviewed. Modular strategies for understanding fermion mass hierarchies are discussed, including the weighton mechanism, small deviations from fixed points, and texture zeroes. Then examples of modular models are discussed based on single modulusA4models, a minimalS4'model of leptons (and quarks), and a multiple moduli model based on threeS4groups capable of reproducing the Littlest Seesaw model. We then extend the discussion to include Grand Unified Theories based on modular (flipped)SU(5) andSO(10). Finally we briefly mention some issues related to top-down approaches based on string theory, including eclectic flavour symmetry and moduli stabilisation, before concluding.

Searching for Two-Neutrino and Neutrinoless Double Beta Decay of ^{134}Xe with the PandaX-4T Experiment

^{134}Xe is a candidate isotope for neutrinoless double beta decay (0νββ) search. In addition, the two-neutrino case (2νββ) allowed by the standard model of particle physics has not yet been observed. With the 656-kg natural xenon in the fiducial volume of the PandaX-4T detector, which contains 10.4% of ^{134}Xe, and its initial 94.9-day exposure, we have established the most stringent constraints on 2νββ and 0νββ of ^{134}Xe half-lives, with limits of 2.8×10^{22}  yr and 3.0×10^{23}  yr at 90% confidence level, respectively. The 2νββ (0νββ) limit surpasses the previously reported best result by a factor of 32 (2.7), highlighting the potential of large monolithic natural xenon detectors for double beta decay searches.

Dual-phase xenon time projection chambers for rare-event searches

In the past decade, dual-phase xenon time projection chambers (Xe-TPCs) have emerged as some of the most powerful detectors in the fields of astroparticle physics and rare-event searches. Developed primarily towards the direct detection of dark matter particles, experiments presently operating deep underground have reached target masses at the multi-tonne scale, energy thresholds of approximately 1 keV and radioactivity-induced background rates similar to those from solar neutrinos. These unique properties, together with demonstrated stable operation over several years, allow for the exploration of new territory via high-sensitivity searches for a plethora of ultra-rare interactions. These include searches for particle dark matter, for second-order weak decays, and the observation of astrophysical neutrinos. We first review some properties of xenon as a radiation detection medium and the operation principles of dual-phase Xe-TPCs together with their energy calibration and resolution. We then discuss the status of currently running experiments and of proposed next-generation projects, describing some of the technological challenges. We end by looking at their sensitivity to dark matter candidates, to second-order weak decays and to solar and supernova neutrinos. Experiments based on dual-phase Xe-TPCs are difficult and, like all good experiments, they are constantly pushed to their limits. Together with many other endeavours in astroparticle physics and cosmology they will continue to push at the borders of the unknown, hopefully to reveal profound new knowledge about our cosmos. This article is part of the theme issue 'The particle-gravity frontier'.

Axion Paradigm with Color-Mediated Neutrino Masses

We propose a generalized Kim-Shifman-Vainshtein-Zakharov-type axion framework in which colored fermions and scalars act as two-loop Majorana neutrino-mass mediators. The global Peccei-Quinn symmetry under which exotic fermions are charged solves the strong CP problem. Within our general proposal, various setups can be distinguished by probing the axion-to-photon coupling at helioscopes and haloscopes. We also comment on axion dark-matter production in the early Universe.

Measurement of the 2νββ Decay Half-Life of ^{82}Se with the Global CUPID-0 Background Model

We report on the results obtained with the global CUPID-0 background model, which combines the data collected in the two measurement campaigns for a total exposure of 8.82  kg×yr of ^{82}Se. We identify with improved precision the background sources within the 3 MeV energy region, where neutrinoless double β decay of ^{82}Se and ^{100}Mo is expected, making more solid the foundations for the background budget of the next-generation CUPID experiment. Relying on the excellent data reconstruction, we measure the two-neutrino double β-decay half-life of ^{82}Se with unprecedented accuracy: T_{1/2}^{2ν}=[8.69±0.05(stat)_{-0.06}^{+0.09}(syst)]×10^{19}  yr.

Tritium Beta Spectrum Measurement and Neutrino Mass Limit from Cyclotron Radiation Emission Spectroscopy

The absolute scale of the neutrino mass plays a critical role in physics at every scale, from the subatomic to the cosmological. Measurements of the tritium end-point spectrum have provided the most precise direct limit on the neutrino mass scale. In this Letter, we present advances by Project 8 to the cyclotron radiation emission spectroscopy (CRES) technique culminating in the first frequency-based neutrino mass limit. With only a cm^{3}-scale physical detection volume, a limit of m_{β}<155  eV/c^{2} (152  eV/c^{2}) is extracted from the background-free measurement of the continuous tritium beta spectrum in a Bayesian (frequentist) analysis. Using ^{83m}Kr calibration data, a resolution of 1.66±0.19  eV (FWHM) is measured, the detector response model is validated, and the efficiency is characterized over the multi-keV tritium analysis window. These measurements establish the potential of CRES for a high-sensitivity next-generation direct neutrino mass experiment featuring low background and high resolution.

Observation of Low-Lying Isomeric States in ^{136}Cs: A New Avenue for Dark Matter and Solar Neutrino Detection in Xenon Detectors

We report on new measurements establishing the existence of low-lying isomeric states in ^{136}Cs using γ rays produced in ^{136}Xe(p,n)^{136}Cs reactions. Two states with O(100)  ns lifetimes are placed in the decay sequence of the ^{136}Cs levels that are populated in charged-current interactions of solar neutrinos and fermionic dark matter with ^{136}Xe. Xenon-based experiments can therefore exploit a delayed-coincidence tag of these interactions, greatly suppressing backgrounds to enable spectroscopic studies of solar neutrinos and dark matter.

^{138}Ba(d,α) Study of States in ^{136}Cs: Implications for New Physics Searches with Xenon Detectors

We used the ^{138}Ba(d,α) reaction to carry out an in-depth study of states in ^{136}Cs, up to around 2.5 MeV. In this Letter, we place emphasis on hitherto unobserved states below the first 1^{+} level, which are important in the context of solar neutrino and fermionic dark matter (FDM) detection in large-scale xenon-based experiments. We identify for the first time candidate metastable states in ^{136}Cs, which would allow a real-time detection of solar neutrino and FDM events in xenon detectors, with high background suppression. Our results are also compared with shell-model calculations performed with three Hamiltonians that were previously used to evaluate the nuclear matrix element (NME) for ^{136}Xe neutrinoless double beta decay. We find that one of these Hamiltonians, which also systematically underestimates the NME compared with the others, dramatically fails to describe the observed low-energy ^{136}Cs spectrum, while the other two show reasonably good agreement.