Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen

Probing Heavy Majorana Neutrinos and the Weinberg Operator through Vector Boson Fusion Processes in Proton-Proton Collisions at sqrt[s]=13 TeV

The first search exploiting the vector boson fusion process to probe heavy Majorana neutrinos and the Weinberg operator at the LHC is presented. The search is performed in the same-sign dimuon final state using a proton-proton collision dataset recorded at sqrt[s]=13  TeV, collected with the CMS detector and corresponding to a total integrated luminosity of 138  fb^{-1}. The results are found to agree with the predictions of the standard model. For heavy Majorana neutrinos, constraints on the squared mixing element between the muon and the heavy neutrino are derived in the heavy neutrino mass range 50 GeV-25 TeV; for masses above 650 GeV these are the most stringent constraints from searches at the LHC to date. A first test of the Weinberg operator at colliders provides an observed upper limit at 95% confidence level on the effective μμ Majorana neutrino mass of 10.8 GeV.

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

The KamLAND-Zen experiment has provided stringent constraints on the neutrinoless double-beta (0νββ) decay half-life in ^{136}Xe using a xenon-loaded liquid scintillator. We report an improved search using an upgraded detector with almost double the amount of xenon and an ultralow radioactivity container, corresponding to an exposure of 970 kg yr of ^{136}Xe. These new data provide valuable insight into backgrounds, especially from cosmic muon spallation of xenon, and have required the use of novel background rejection techniques. We obtain a lower limit for the 0νββ decay half-life of T_{1/2}^{0ν}>2.3×10^{26}  yr at 90% C.L., corresponding to upper limits on the effective Majorana neutrino mass of 36-156 meV using commonly adopted nuclear matrix element calculations.

Determining g_{A}/g_{V} with High-Resolution Spectral Measurements Using a LiInSe_{2} Bolometer

Neutrinoless double beta decay (0νββ) processes sample a wide range of intermediate forbidden nuclear transitions, which may be impacted by quenching of the axial vector coupling constant (g_{A}/g_{V}), the uncertainty of which plays a pivotal role in determining the sensitivity reach of 0νββ experiments. In this Letter, we present measurements performed on a high-resolution LiInSe_{2} bolometer in a "source=detector" configuration to measure the spectral shape of the fourfold forbidden β decay of ^{115}In. The value of g_{A}/g_{V} is determined by comparing the spectral shape of theoretical predictions to the experimental β spectrum taking into account various simulated background components as well as a variety of detector effects. We find evidence of quenching of g_{A}/g_{V} at >5σ with a model-dependent quenching factor of 0.655±0.002 as compared to the free-nucleon value for the interacting shell model. We also measured the ^{115}In half-life to be [5.18±0.06(stat)_{-0.015}^{+0.005}(sys)]×10^{14}  yr within the interacting shell model framework. This Letter demonstrates the power of the bolometeric technique to perform precision nuclear physics single-β decay measurements, which along with improved nuclear modeling can help reduce the uncertainties in the calculation of several decay nuclear matrix elements including those used in 0νββ sensitivity calculations.

Final Result on the Neutrinoless Double Beta Decay of ^{82}Se with CUPID-0

CUPID-0, an array of Zn^{82}Se cryogenic calorimeters, was the first medium-scale demonstrator of the scintillating bolometers' technology. The first project phase (March 2017-December 2018) allowed the most stringent limit on the neutrinoless double beta decay half-life of the isotope of interest, ^{82}Se, to be set. After a six month long detector upgrade, CUPID-0 began its second and last phase (June 2019-February 2020). In this Letter, we describe the search for neutrinoless double beta decay of ^{82}Se with a total exposure (phase I+II) of 8.82  kg yr^{-1} of isotope. We set a limit on the half-life of ^{82}Se to the ground state of ^{82}Kr of T_{1/2}^{0ν}(^{82}Se)>4.6×10^{24}  yr (90% credible interval), corresponding to an effective Majorana neutrino mass m_{ββ}<(263-545)  meV. We also set the most stringent lower limits on the neutrinoless decays of ^{82}Se to the 0_{1}^{+}, 2_{1}^{+}, and 2_{2}^{+} excited states of ^{82}Kr, finding 1.8×10^{23}  yr, 3.0×10^{23}  yr, and 3.2×10^{23}  yr (90% credible interval) respectively.

Enrichment of 150Nd for neutrinoless double-beta decay detection

A 562 nm–627 nm–597 nm three-step resonant photoionization scheme has been studied using the density matrix formalism for the enrichment of ¹⁵⁰Nd in weighable quantities for the neutrinoless double-beta decay detection. The effect of bandwidth of the excitation laser and charge exchange collisions, on the production rates and degree of enrichment has also been studied. Optimum conditions for the efficient enrichment of ¹⁵⁰Nd isotope have been derived. It has been shown that it might be possible to produce 50 kg of 66% enriched ¹⁵⁰Nd isotope in about five months (16 h/day) using the conditions derived through this investigation. This enables to reach the 0νββ decay half-life limit of ≈ 1.8 × 10²⁵ year for the ¹⁵⁰Nd isotope.

Measurements of Rayleigh ratios in linear alkylbenzene

This paper describes a new experimental setup designed for the direct measurement of the Rayleigh ratio and Rayleigh scattering length for linear alkylbenzene, a solvent commonly used in liquid scintillator detectors for neutrino experiments. Using the new approach, the perpendicularly polarized Rayleigh ratio was determined to be (4.52 ± 0.28) × 10^-6 m^-1 sr^-1 at 405 nm and (3.82 ± 0.24) × 10^-6 m^-1 sr^-1 at 432 nm, and the corresponding Rayleigh scattering length was L_Ray = 22.9 ± 0.3(stat.) ± 1.7(sys.) m at 405 nm and L_Ray = 27.0 ± 0.9(stat.) ± 1.8(sys.) m at 432 nm. These results are consistent with both previous results determined using other experimental strategies and theoretical predictions.

Search for Majorana neutrinos exploiting millikelvin cryogenics with CUORE

The possibility that neutrinos may be their own antiparticles, unique among the known fundamental particles, arises from the symmetric theory of fermions proposed by Ettore Majorana in 1937¹. Given the profound consequences of such Majorana neutrinos, among which is a potential explanation for the matter–antimatter asymmetry of the universe via leptogenesis², the Majorana nature of neutrinos commands intense experimental scrutiny globally; one of the primary experimental probes is neutrinoless double beta (0νββ) decay. Here we show results from the search for 0νββ decay of ¹³⁰Te, using the latest advanced cryogenic calorimeters with the CUORE experiment³. CUORE, operating just 10 millikelvin above absolute zero, has pushed the state of the art on three frontiers: the sheer mass held at such ultralow temperatures, operational longevity, and the low levels of ionizing radiation emanating from the cryogenic infrastructure. We find no evidence for 0νββ decay and set a lower bound of the process half-life as 2.2 × 10²⁵ years at a 90 per cent credibility interval. We discuss potential applications of the advances made with CUORE to other fields such as direct dark matter, neutrino and nuclear physics searches and large-scale quantum computing, which can benefit from sustained operation of large payloads in a low-radioactivity, ultralow-temperature cryogenic environment.

The CUORE experiment finds no evidence for neutrinoless double beta decay after operating a large cryogenic TeO₂ calorimeter stably for several years in an extreme low-radiation environment at a temperature of 10 millikelvin.

Neutrino Mass Spectrum: Present Indication and Future Prospect

The fact that neutrinos are massive has been the most crucial evidence of physics beyond the Standard Model of elementary particles. To date, we still do not know how neutrinos get mass and why their mass is much smaller than that of their charged fermion cousins. The precise determination of the neutrino mass spectrum has become one of the central tasks of neutrino physics, providing critical input for understanding the nature of neutrino mass and extending our model. The present landscape of the neutrino mass spectrum is reviewed and explored in this article using data from the neutrino oscillation, cosmology, and beta decay. In addition, we discuss the possibility of relevant programs elucidating the neutrino mass spectrum in the coming decades.

Bolometric Double Beta Decay Experiments: Review and Prospects

This review aims to cover the history and recent developments on cryogenic bolometers for neutrinoless double beta decay (0ν2β) searches. A 0ν2β decay observation would confirm the total lepton charge non-conservation, which is related to a global U(1)LC symmetry. This discovery would also provide essential information on neutrino masses and nature, opening the door to new physics beyond the Standard Model. The bolometric technology shows good prospects for future ton-scale experiments that aim to fully investigate the inverted ordering region of neutrino masses. The big advantage of bolometers is the high energy resolution and the possibility of particle identification, as well as various methods of additional background rejection. The CUORE experiment has proved the feasibility of ton-scale cryogenic experiments, setting the most stringent limit on 130Te 0ν2β decay. Two CUPID demonstrators (CUPID-0 and CUPID-Mo) have set the most stringent limits on 82Se and 100Mo isotopes, respectively, with compatibly low exposures. Several experiments are developing new methods to improve the background in the region of interest with bolometric detectors. CUPID and AMoRE experiments aim to cover the inverted hierarchy region, using scintillating bolometers with hundreds of kg of 100Mo. We review all of these efforts here, with a focus on the different types of radioactive background and the measures put in place to mitigate them.

Searching for the Radiative Decay of the Cosmic Neutrino Background with Line-Intensity Mapping

We study the possibility to use line-intensity mapping (LIM) to seek photons from the radiative decay of neutrinos in the cosmic neutrino background. The Standard Model prediction for the rate for these decays is extremely small, but it can be enhanced if new physics increases the neutrino electromagnetic moments. The decay photons will appear as an interloper of astrophysical spectral lines. We propose that the neutrino-decay line can be identified with anisotropies in LIM clustering and also with the voxel intensity distribution. Ongoing and future LIM experiments will have-depending on the neutrino hierarchy, transition, and experiment considered-a sensitivity to an effective electromagnetic transition moment ∼10^{-12}-10^{-8}(m_{i}c^{2}/0.1  eV)^{3/2}μ_{B}, where m_{i} is the mass of the decaying neutrino and μ_{B} is the Bohr magneton. This will be significantly more sensitive than cosmic microwave background spectral distortions, and it will be competitive with stellar cooling studies. As a by-product, we also report an analytic form of the one-point probability distribution function for neutrino-density fluctuations, obtained from the quijote simulations using symbolic regression.

Neutrinoless Double Beta Decay with Germanium Detectors: 1026 yr and Beyond

In the global landscape of neutrinoless double beta (0νββ) decay search, the use of semiconductor germanium detectors provides many advantages. The excellent energy resolution, the negligible intrinsic radioactive contamination, the possibility of enriching the crystals up to 88% in the 76Ge isotope as well as the high detection efficiency, are all key ingredients for highly sensitive 0νββ decay search. The Majorana and Gerda experiments successfully implemented the use of germanium (Ge) semiconductor detectors, reaching an energy resolution of 2.53 ± 0.08 keV at the Qββ and an unprecedented low background level of 5.2×10−4 cts/(keV·kg·yr), respectively. In this paper, we will review the path of 0νββ decay search with Ge detectors from the original idea of E. Fiorini et al. in 1967, to the final recent results of the Gerda experiment setting a limit on the half-life of 76Ge 0νββ decay at T1/2>1.8×1026 yr (90% C.L.). We will then present the LEGEND project designed to reach a sensitivity to the half-life up to 1028 yr and beyond, opening the way to the exploration of the normal ordering region.

GERDA and LEGEND: Probing the Neutrino Nature and Mass at 100 meV and beyond

The Gerda (GERmanium Detector Array) project, located at Laboratori Nazionali del Gran Sasso (LNGS), was started in 2005, a few years after the claim of evidence for the neutrinoless double beta decay (0νββ) of 76Ge to the ground state of 76Se: it is an ultra-rare process whose detection would directly establish the Majorana nature of the neutrino and provide a measurement of its mass and mass hierarchy. The aim of Gerda was to confirm or disprove the claim by an increased sensitivity experiment. After establishing the new technology of Ge detectors operated bare in liquid Argon and since 2011, Gerda efficiently collected data searching for 0νββ of 76Ge, first deploying the 76Ge-enriched detectors from two former experiments and later new detectors with enhanced signal-to-background rejection, produced from freshly 76Ge-enriched material. Since then, the Gerda setup has been upgraded twice, first in 2013–2015 and later in 2018. The period before 2013 is Phase I and that after 2015 is Phase II. Both the Gerda setup and the analysis tools evolved along the project lifetime, allowing to achieve the remarkable average energy resolution of ∼3.6 and ∼2.6 keV for Coaxial Germanium (Coax) detectors and for Broad Energy Germanium (BEGe), respectively, and the background index of 5.2−1.3+1.6 · 10−4 cts/(keV·kg·yr) in a 230 keV net range centered at Qββ. No evidence of the 0νββ decay at Qββ = 2039.1 keV has been found, hence the limit of 1.8·1026 yr on the half-life (T1/20ν) at 90% C.L. was set with the exposure of 127.2 kg·yr. The corresponding limit range for the effective Majorana neutrino mass mee has been set to 79–180 meV. The Gerda performances in terms of background index, energy resolution and exposure are the best achieved so far by 76Ge double beta decay experiments. In Phase II, Gerda succeeded in operating in a background free regime and set a world record. In 2017, the Legend Collaboration was born from the merging of the Gerda and Majorana Collaborations and resources with the aim to further improve the Gerda sensitivity. First, the Legend200 project, with a mass of up to 200 kg of 76Ge-enriched detectors, aims to further improve the background index down to <0.6 · 10−3 cts/(keV·kg·yr) to explore the Inverted Hierarchy region of the neutrino mass ordering, then the Legend1000 (1 ton of 76Ge-enriched) will probe the Normal Hierarchy. In this paper, we describe the Gerda experiment, its evolution, the data analysis flow, a selection of its results and technological achievements, and finally the design, features and challenges of Legend, the Gerda prosecutor.

Scintillation in Low-Temperature Particle Detectors

Inorganic crystal scintillators play a crucial role in particle detection for various applications in fundamental physics and applied science. The use of such materials as scintillating bolometers, which operate at temperatures as low as 10 mK and detect both heat (phonon) and scintillation signals, significantly extends detectors performance compared to the conventional scintillation counters. In particular, such low-temperature devices offer a high energy resolution in a wide energy interval thanks to a phonon signal detection, while a simultaneous registration of scintillation emitted provides an efficient particle identification tool. This feature is of great importance for a background identification and rejection. Combined with a large variety of elements of interest, which can be embedded in crystal scintillators, scintillating bolometers represent powerful particle detectors for rare-event searches (e.g., rare alpha and beta decays, double-beta decay, dark matter particles, neutrino detection). Here, we review the features and results of low-temperature scintillation detection achieved over a 30-year history of developments of scintillating bolometers and their use in rare-event search experiments.

Status and Perspectives on Rare Decay Searches in Tellurium Isotopes

Neutrinoless double beta decay (0νββ) is a posited lepton number violating decay whose search is an increasingly active field in modern astroparticle physics. A discovery would imply neutrinos are Majorana particles and inform neutrino physics, cosmology and beyond-standard-model theories. Among the few nuclei where double beta decay (ββ) is allowed, tellurium isotopes stand for their high natural abundance and are currently employed in multiple experiments. The search for 0νββ will provide large exposure data sets in the coming years, paving the way for unprecedented sensitivities. We review the latest rare decay searches in tellurium isotopes and compare past results with theories and prospects from running experiments.

Characterization of inverted coaxial 76 Ge detectors in GERDA for future double- β decay experiments

Neutrinoless double- β decay of 76 Ge is searched for with germanium detectors where source and detector of the decay are identical. For the success of future experiments it is important to increase the mass of the detectors. We report here on the characterization and testing of five prototype detectors manufactured in inverted coaxial (IC) geometry from material enriched to 88% in 76 Ge. IC detectors combine the large mass of the traditional semi-coaxial Ge detectors with the superior resolution and pulse shape discrimination power of point contact detectors which exhibited so far much lower mass. Their performance has been found to be satisfactory both when operated in vacuum cryostat and bare in liquid argon within the Gerda setup. The measured resolutions at the Q-value for double- β decay of 76 Ge ( Q β β  = 2039 keV) are about 2.1 keV full width at half maximum in vacuum cryostat. After 18 months of operation within the ultra-low background environment of the GERmanium Detector Array (Gerda) experiment and an accumulated exposure of 8.5 kg · year, the background index after analysis cuts is measured to be 4 . 9 - 3.4 + 7.3 × 10 - 4 counts / ( keV · kg · year ) around Q β β . This work confirms the feasibility of IC detectors for the next-generation experiment Legend.

New Limit for Neutrinoless Double-Beta Decay of ^{100}Mo from the CUPID-Mo Experiment

The CUPID-Mo experiment at the Laboratoire Souterrain de Modane (France) is a demonstrator for CUPID, the next-generation ton-scale bolometric 0νββ experiment. It consists of a 4.2 kg array of 20 enriched Li_{2}^{100}MoO_{4} scintillating bolometers to search for the lepton-number-violating process of 0νββ decay in ^{100}Mo. With more than one year of operation (^{100}Mo exposure of 1.17  kg×yr for physics data), no event in the region of interest and, hence, no evidence for 0νββ is observed. We report a new limit on the half-life of 0νββ decay in ^{100}Mo of T_{1/2}>1.5×10^{24}  yr at 90% C.I. The limit corresponds to an effective Majorana neutrino mass ⟨m_{ββ}⟩<(0.31-0.54)  eV, dependent on the nuclear matrix element in the light Majorana neutrino exchange interpretation.

Toward Complete Leading-Order Predictions for Neutrinoless Double β Decay

The amplitude for the neutrinoless double β (0νββ) decay of the two-neutron system nn→ppe^{-}e^{-} constitutes a key building block for nuclear-structure calculations of heavy nuclei employed in large-scale 0νββ searches. Assuming that the 0νββ process is mediated by a light-Majorana-neutrino exchange, a systematic analysis in chiral effective field theory shows that already at leading order a contact operator is required to ensure renormalizability. In this Letter, we develop a method to estimate the numerical value of its coefficient (in analogy to the Cottingham formula for electromagnetic contributions to hadron masses) and validate the result by reproducing the charge-independence-breaking contribution to the nucleon-nucleon scattering lengths. Our central result, while derived in dimensional regularization, is given in terms of the renormalized amplitude A_{ν}(|p|,|p^{'}|), matching to which will allow one to determine the contact-term contribution in regularization schemes employed in nuclear-structure calculations. Our results thus greatly reduce a crucial uncertainty in the interpretation of searches for 0νββ decay.

Measurement of the 2νββ Decay Half-Life of ^{130}Te with CUORE

We measured two-neutrino double beta decay of ^{130}Te using an exposure of 300.7 kg yr accumulated with the CUORE detector. Using a Bayesian analysis to fit simulated spectra to experimental data, it was possible to disentangle all the major background sources and precisely measure the two-neutrino contribution. The half-life is in agreement with past measurements with a strongly reduced uncertainty: T_{1/2}^{2ν}=7.71_{-0.06}^{+0.08}(stat)_{-0.15}^{+0.12}(syst)×10^{20}  yr. This measurement is the most precise determination of the ^{130}Te 2νββ decay half-life to date.

The minimal seesaw and leptogenesis models

Given its briefness and predictability, the minimal seesaw-a simplified version of the canonical seesaw mechanism with only two right-handed neutrino fields-has been studied in depth and from many perspectives, and now it is being pushed close to a position of directly facing experimental tests. This article is intended to provide an up-to-date review of various phenomenological aspects of the minimal seesaw and its associated leptogenesis mechanism in neutrino physics and cosmology. Our focus is on possible flavor structures of such benchmark seesaw and leptogenesis scenarios and confronting their predictions with current neutrino oscillation data and cosmological observations. In this connection particular attention will be paid to the topics of lepton number violation, lepton flavor violation, discrete flavor symmetries, CP violation and antimatter of the Universe.

Left-Right Symmetry and Leading Contributions to Neutrinoless Double Beta Decay

We study the impact of the mixing (LR mixing) between the standard model W boson and its hypothetical, heavier right-handed parter W_{R} on the neutrinoless double beta decay (0νββ decay) rate. Our study is done in the minimal left-right symmetric model assuming a type-II dominance scenario with charge conjugation as the left-right symmetry. We then show that the 0νββ decay rate may be dominated by the contribution proportional to this LR mixing, which at the hadronic level induces the leading-order contribution to the interaction between two pions and two charged leptons. The resulting long-range pion exchange contribution can significantly enhance the decay rate compared to previously considered short-range contributions. Finally, we find that even if future cosmological experiments rule out the inverted hierarchy for neutrino masses, there are still good prospects for a positive signal in the next generation of 0νββ decay experiments.

The NUMEN Project: Toward New Experiments with High-Intensity Beams

The search for neutrinoless double-beta (0νββ) decay is currently a key topic in physics, due to its possible wide implications for nuclear physics, particle physics, and cosmology. The NUMEN project aims to provide experimental information on the nuclear matrix elements (NMEs) that are involved in the expression of 0νββ decay half-life by measuring the cross section of nuclear double-charge exchange (DCE) reactions. NUMEN has already demonstrated the feasibility of measuring these tiny cross sections for some nuclei of interest for the 0νββ using the superconducting cyclotron (CS) and the MAGNEX spectrometer at the Laboratori Nazionali del Sud (LNS.) Catania, Italy. However, since the DCE cross sections are very small and need to be measured with high sensitivity, the systematic exploration of all nuclei of interest requires major upgrade of the facility. R&D for technological tools has been completed. The realization of new radiation-tolerant detectors capable of sustaining high rates while preserving the requested resolution and sensitivity is underway, as well as the upgrade of the CS to deliver beams of higher intensity. Strategies to carry out DCE cross-section measurements with high-intensity beams were developed in order to achieve the challenging sensitivity requested to provide experimental constraints to 0νββ NMEs.

Enriched Crystal Scintillators for 2β Experiments

The investigation of 2β decay is an important issue in modern physics, allowing the test of the Standard Model of elementary particles and the study of the nature and properties of neutrinos. The crystal scintillators, especially made of isotopically-enriched materials, are powerful detectors for 2β decay experiments thanks to the high radiopurity level and the possibility to realize the calorimetric “source = detector” approach with a high detection efficiency. For the moment, the 2ν2β processes have been observed at the level of 1019–1024 years with enriched crystals; the sensitivity to the 0ν mode have reached the level of 1024–1026 years in some decay channels for different nuclides allowing one to calculate the upper limits on the effective mass of the Majorana neutrino at the level of 0.1–0.6 eV. The paper is intended to be a review on the latest results to investigate 2β processes with crystal scintillators enriched in 48Ca, 106Cd, and 116Cd.

What Is Matter According to Particle Physics, and Why Try to Observe Its Creation in a Lab?

The standard model of elementary interactions has long qualified as a theory of matter, in which the postulated conservation laws (one baryonic and three leptonic) acquire theoretical meaning. However, recent observations of lepton number violations—neutrino oscillations—demonstrate its incompleteness. We discuss why these considerations suggest the correctness of Ettore Majorana’s ideas on the nature of neutrino mass and add further interest to the search for an ultra-rare nuclear process in which two particles of matter (electrons) are created, commonly called neutrinoless double beta decay. The approach of the discussion is mainly historical, and its character is introductory. Some technical considerations, which highlight the usefulness of Majorana’s representation of gamma matrices, are presented in the appendix.

Bound on 3+1 Active-Sterile Neutrino Mixing from the First Four-Week Science Run of KATRIN

We report on the light sterile neutrino search from the first four-week science run of the KATRIN experiment in 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are analyzed by a high-resolution MAC-E filter down to 40 eV below the endpoint at 18.57 keV. We consider the framework with three active neutrinos and one sterile neutrino. The analysis is sensitive to the mass, m_{4}, of the fourth mass state for m_{4}^{2}≲1000  eV^{2} and to active-to-sterile neutrino mixing down to |U_{e4}|^{2}≳2×10^{-2}. No significant spectral distortion is observed and exclusion bounds on the sterile mass and mixing are reported. These new limits supersede the Mainz results for m_{4}^{2}≲1000  eV^{2} and improve the Troitsk bound for m_{4}^{2}<30  eV^{2}. The reactor and gallium anomalies are constrained for 100<Δm_{41}^{2}<1000  eV^{2}.

The Event Detection System in the NEXT-White Detector

This article describes the event detection system of the NEXT-White detector, a 5 kg high pressure xenon TPC with electroluminescent amplification, located in the Laboratorio Subterráneo de Canfranc (LSC), Spain. The detector is based on a plane of photomultipliers (PMTs) for energy measurements and a silicon photomultiplier (SiPM) tracking plane for offline topological event filtering. The event detection system, based on the SRS-ATCA data acquisition system developed in the framework of the CERN RD51 collaboration, has been designed to detect multiple events based on online PMT signal energy measurements and a coincidence-detection algorithm. Implemented on FPGA, the system has been successfully running and evolving during NEXT-White operation. The event detection system brings some relevant and new functionalities in the field. A distributed double event processor has been implemented to detect simultaneously two different types of events thus allowing simultaneous calibration and physics runs. This special feature provides constant monitoring of the detector conditions, being especially relevant to the lifetime and geometrical map computations which are needed to correct high-energy physics events. Other features, like primary scintillation event rejection, or a double buffer associated with the type of event being searched, help reduce the unnecessary data throughput thus minimizing dead time and improving trigger efficiency.

Final Results of GERDA on the Search for Neutrinoless Double-β Decay

The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-β (0νββ) decay of ^{76}Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in ^{76}Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2×10^{-4} counts/(keV kg yr) in the signal region and met the design goal to collect an exposure of 100 kg yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg yr of total exposure. A limit on the half-life of 0νββ decay in ^{76}Ge is set at T_{1/2}>1.8×10^{26}  yr at 90% C.L., which coincides with the sensitivity assuming no signal.

Present Status of Nuclear Shell-Model Calculations of 0νββ Decay Matrix Elements

Neutrinoless double beta (0νββ) decay searches are currently among the major foci of experimental physics. The observation of such a decay will have important implications in our understanding of the intrinsic nature of neutrinos and shed light on the limitations of the Standard Model. The rate of this process depends on both the unknown neutrino effective mass and the nuclear matrix element (M0ν) associated with the given 0νββ transition. The latter can only be provided by theoretical calculations, hence the need of accurate theoretical predictions of M0ν for the success of the experimental programs. This need drives the theoretical nuclear physics community to provide the most reliable calculations of M0ν. Among the various computational models adopted to solve the many-body nuclear problem, the shell model is widely considered as the basic framework of the microscopic description of the nucleus. Here, we review the most recent and advanced shell-model calculations of M0ν considering the light-neutrino-exchange channel for nuclei of experimental interest. We report the sensitivity of the theoretical calculations with respect to variations in the model spaces and the shell-model nuclear Hamiltonians.

Isotopic Dependence of Vapor Pressure in Xenon

The vapor pressure isotopic effect (VPIE) of xenon has been measured using cryogenic distillation. The still is calibrated with argon and krypton and yields a measurement of ln(p ₁₃₀/p ₁₃₆) ≃ (0.26 ± 0.04) × 10^-3, where p ₁₃₀ and p ₁₃₆ are the vapor pressures of ¹³⁰Xe and ¹³⁶Xe at the nominal boiling point, respectively. The dependence of the VPIE across the isotopes for the three elements is found to be approximately linear with atomic mass, and all values are consistent with theoretical expectations.

Cosmogenic Activation in Double Beta Decay Experiments

Double beta decay is a very rare nuclear process and, therefore, experiments intended to detect it must be operated deep underground and in ultra-low background conditions. Long-lived radioisotopes produced by the previous exposure of materials to cosmic rays on the Earth’s surface or even underground can become problematic for the required sensitivity. Here, the studies developed to quantify and reduce the activation yields in detectors and materials used in the set-up of these experiments will be reviewed, considering target materials like germanium, tellurium and xenon together with other ones commonly used like copper, lead, stainless steel or argon. Calculations following very different approaches and measurements from irradiation experiments using beams or directly cosmic rays will be considered for relevant radioisotopes. The effect of cosmogenic activation in present and future double beta decay projects based on different types of detectors will be analyzed too.

Precise Half-Life Values for Two-Neutrino Double-β Decay: 2020 Review

All existing positive results on two-neutrino double beta decay and two-neutrino double electron capture in different nuclei have been analyzed. Weighted average and recommended half-life values for 48Ca, 76Ge, 82Se, 96Zr, 100Mo, 100Mo - 100Ru (01+), 116Cd, 128Te, 130Te, 136Xe, 150Nd, 150Nd - 150Sm (01+), 238U, 78Kr, 124Xe and 130Ba have been obtained. Given the measured half-life values, effective nuclear matrix elements for all these transitions were calculated.

Neutrinos: Majorana or Dirac?

Are neutrinos with definite masses Majorana or Dirac particles? This is one of the most fundamental problems of modern neutrino physics. The solution to this problem could be crucial for understanding the origin of small neutrino masses. We review here basic arguments in favor of the Majorana nature of massive neutrinos. The phenomenological theory of 0νββ-decay is briefly discussed and recent experimental data and sensitivity of future experiments are presented.

Symmetries and Their Breaking in the Fundamental Laws of Physics

Symmetries in the Physical Laws of Nature lead to observable effects. Beyond the regularities and conserved magnitudes, the last few decades in particle physics have seen the identification of symmetries, and their well-defined breaking, as the guiding principle for the elementary constituents of matter and their interactions. Flavour SU(3) symmetry of hadrons led to the Quark Model and the antisymmetric requirement under exchange of identical fermions led to the colour degree of freedom. Colour became the generating charge for flavour-independent strong interactions of quarks and gluons in the exact colour SU(3) local gauge symmetry. Parity Violation in weak interactions led us to consider the chiral fields of fermions as the objects with definite transformation properties under the weak isospin SU(2) gauge group of the Unifying Electro-Weak SU(2) × U(1) symmetry, which predicted novel weak neutral current interactions. CP-Violation led to three families of quarks opening the field of Flavour Physics. Time-reversal violation has recently been observed with entangled neutral mesons, compatible with CPT-invariance. The cancellation of gauge anomalies, which would invalidate the gauge symmetry of the quantum field theory, led to Quark–Lepton Symmetry. Neutrinos were postulated in order to save the conservation laws of energy and angular momentum in nuclear beta decay. After the ups and downs of their mass, neutrino oscillations were discovered in 1998, opening a new era about their origin of mass, mixing, discrete symmetries and the possibility of global lepton-number violation through Majorana mass terms and Leptogenesis as the source of the matter–antimatter asymmetry in the universe. The experimental discovery of quarks and leptons and the mediators of their interactions, with physical observables in spectacular agreement with this Standard Theory, is the triumph of Symmetries. The gauge symmetry is exact only when the particles are massless. One needs a subtle breaking of the symmetry, providing the origin of mass without affecting the excellent description of the interactions. This is the Brout–Englert–Higgs Mechanism, which produces the Higgs Boson as a remnant, discovered at CERN in 2012. Open present problems are addressed with by searching the New Physics Beyond-the-Standard-Model.

Fluorescent bicolour sensor for low-background neutrinoless double β decay experiments

Observation of the neutrinoless double β decay is the only practical way to establish that neutrinos are their own antiparticles¹. Because of the small masses of neutrinos, the lifetime of neutrinoless double β decay is expected to be at least ten orders of magnitude greater than the typical lifetimes of natural radioactive chains, which can mimic the experimental signature of neutrinoless double β decay². The most robust identification of neutrinoless double β decay requires the definition of a signature signal-such as the observation of the daughter atom in the decay-that cannot be generated by radioactive backgrounds, as well as excellent energy resolution. In particular, the neutrinoless double β decay of ¹³⁶Xe could be established by detecting the daughter atom, ¹³⁶Ba²⁺, in its doubly ionized state^3-8. Here we demonstrate an important step towards a 'barium-tagging' experiment, which identifies double β decay through the detection of a single Ba²⁺ ion. We propose a fluorescent bicolour indicator as the core of a sensor that can detect single Ba²⁺ ions in a high-pressure xenon gas detector. In a sensor made of a monolayer of such indicators, the Ba²⁺ dication would be captured by one of the molecules and generate a Ba²⁺-coordinated species with distinct photophysical properties. The presence of such a single Ba²⁺-coordinated indicator would be revealed by its response to repeated interrogation with a laser system, enabling the development of a sensor able to detect single Ba²⁺ ions in high-pressure xenon gas detectors for barium-tagging experiments.

Improved Limit on Neutrinoless Double-Beta Decay in ^{130} Te with CUORE

We report new results from the search for neutrinoless double-beta decay in ^{130} Te with the CUORE detector. This search benefits from a fourfold increase in exposure, lower trigger thresholds, and analysis improvements relative to our previous results. We observe a background of (1.38±0.07)×10^{-2}  counts/(keV kg yr)) in the 0νββ decay region of interest and, with a total exposure of 372.5 kg yr, we attain a median exclusion sensitivity of 1.7×10^{25}  yr. We find no evidence for 0νββ decay and set a 90% credibility interval Bayesian lower limit of 3.2×10^{25}  yr on the ^{130} Te half-life for this process. In the hypothesis that 0νββ decay is mediated by light Majorana neutrinos, this results in an upper limit on the effective Majorana mass of 75-350 meV, depending on the nuclear matrix elements used.

Realistic shell model and nuclei around 132Sn

This contribution reports on a shell-model study of nuclei in the 132Sn region employing a realistic effective interaction derived from the CD-Bonn nucleon-nucleon potential renormalized through the use of the Vlow−k approach. We shall focus on some selected results for nuclei with a few valence particles and/or holes with respect to 132Sn, namely Sn isotopes with N > 82 and 130Te, which have, in part, been discussed in previous papers. Results are compared with experiments, and predictions that may provide guidance to future experiments are also discussed. It is the aim of this contribution to underline the importance of studying 132Sn neighbours to acquire a deep understanding of nuclear structure, that may be very useful also in other physics fields, and to show that the realistic shell model is a very effective tool to conduct these studies.

Search for Neutrinoless Double-β Decay with the Complete EXO-200 Dataset

A search for neutrinoless double-β decay (0νββ) in ^{136}Xe is performed with the full EXO-200 dataset using a deep neural network to discriminate between 0νββ and background events. Relative to previous analyses, the signal detection efficiency has been raised from 80.8% to 96.4±3.0%, and the energy resolution of the detector at the Q value of ^{136}Xe 0νββ has been improved from σ/E=1.23% to 1.15±0.02% with the upgraded detector. Accounting for the new data, the median 90% confidence level 0νββ half-life sensitivity for this analysis is 5.0×10^{25}  yr with a total ^{136}Xe exposure of 234.1 kg yr. No statistically significant evidence for 0νββ is observed, leading to a lower limit on the 0νββ half-life of 3.5×10^{25}  yr at the 90% confidence level.

Probing Majorana neutrinos with double-β decay

A discovery that neutrinos are Majorana fermions would have profound implications for particle physics and cosmology. The Majorana character of neutrinos would make possible the neutrinoless double-β (0νββ) decay, a matter-creating process without the balancing emission of antimatter. The GERDA Collaboration searches for the 0νββ decay of ⁷⁶Ge by operating bare germanium detectors in an active liquid argon shield. With a total exposure of 82.4 kg⋅year, we observe no signal and derive a lower half-life limit of T _1/2 > 0.9 × 10²⁶ years (90% C.L.). Our T _1/2 sensitivity, assuming no signal, is 1.1 × 10²⁶ years. Combining the latter with those from other 0νββ decay searches yields a sensitivity to the effective Majorana neutrino mass of 0.07 to 0.16 electron volts.

Final Result of CUPID-0 Phase-I in the Search for the ^{82}Se Neutrinoless Double-β Decay

CUPID-0 is the first pilot experiment of CUPID, a next-generation project for the measurement of neutrinoless double beta decay (0νDBD) with scintillating bolometers. The detector, consisting of 24 enriched and 2 natural ZnSe crystals, has been taking data at Laboratori Nazionali del Gran Sasso from June 2017 to December 2018, collecting a ^{82}Se exposure of 5.29  kg×yr. In this Letter we present the phase-I results in the search for 0νDBD. We demonstrate that the technology implemented by CUPID-0 allows us to reach the lowest background for calorimetric experiments: (3.5_{-0.9}^{+1.0})×10^{-3}  counts/(keV kg yr). Monitoring 3.88×10^{25} ^{82}Se nuclei×yr we reach a 90% credible interval median sensitivity of T_{1/2}^{0ν}>5.0×10^{24}  yr and set the most stringent limit on the half-life of ^{82}Se 0νDBD: T_{1/2}^{0ν}>3.5×10^{24}  yr (90% credible interval), corresponding to m_{ββ}<(311-638)  meV depending on the nuclear matrix element calculations.

Precision Analysis of the ^{136}Xe Two-Neutrino ββ Spectrum in KamLAND-Zen and Its Impact on the Quenching of Nuclear Matrix Elements

We present a precision analysis of the ^{136}Xe two-neutrino ββ electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino ββ rate allows us to measure the ratio of the leading and subleading 2νββ nuclear matrix elements (NMEs), ξ_{31}^{2ν}=-0.26_{-0.25}^{+0.31}. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ξ_{31}^{2ν} range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted ξ_{31}^{2ν} values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless ββ decay, ξ_{31}^{2ν} provides new insights toward reliable neutrinoless ββ NMEs.

Neutrinoless Double-β Decay with Nonstandard Majoron Emission

We present a novel mode of neutrinoless double-β decay with emission of a light Majoron-like scalar particle ϕ. We assume it couples via an effective seven-dimensional operator with a (V+A) lepton current and (V±A) quark currents leading to a long-range contribution that is unsuppressed by the light neutrino mass. We calculate the total double-β decay rate and determine the fully differential shape for this mode. We find that future double-β decay searches are sensitive to scales of the order Λ_{NP}≈1  TeV for the effective operator and a light scalar m_{ϕ}<0.2  MeV, based on ordinary double-β decay Majoron searches. The angular and energy distributions can deviate considerably from that of two-neutrino double-β decay, which is the main background. We point out possible ultraviolet completions where such an effective operator can emerge.