Results on neutrinoless double-β decay of 76Ge from phase I of the GERDA experiment

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.

Calibration of the Gerda experiment

The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double- β decay in76 Ge with an array of about 40 high-purity isotopically-enriched germanium detectors. The experimental signature of the decay is a monoenergetic signal at Q β β = 2039.061 ( 7 )  keV in the measured summed energy spectrum of the two emitted electrons. Both the energy reconstruction and resolution of the germanium detectors are crucial to separate a potential signal from various backgrounds, such as neutrino-accompanied double- β decays allowed by the Standard Model. The energy resolution and stability were determined and monitored as a function of time using data from regular228 Th calibrations. In this work, we describe the calibration process and associated data analysis of the full Gerda dataset, tailored to preserve the excellent resolution of the individual germanium detectors when combining data over several years.

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.

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.

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.

Read-out Electronics and Signal Processing in GERDA and Future Prospects

The GERDA experiment searches for the neutrinoless double beta decay of 76Ge. The experiment is using 36 kg of high-purity germanium detectors, simultaneously as source and detector, deployed into ultra-pure cryogenic liquid argon. GERDA is one the leading experiment in the field, reporting the highest sensitivity on the half-life of 0νββ decay with 1.1·1026 yr, the lowest background index with 6·10−4 cts/(keV·kg·yr) and an excellent energy resolution of 0.12% (FWHM).

The search for the 0νββ decay of the isotope 76Ge will be continued in the next years by the LEGEND-200 experiment, that aims to reach a sensitivity up to 1027 yr using 200 kg of enriched HPGe detectors. The preparation of this experiment already started.

The basic concepts of the GERDA read-out electronics, obeying both the severe requirements of ultra high radio-purity and cryogenic operation, are summarized. For LEGEND-200 a new electronics design, including a separation of the preamplifier in two stages, has been already designed and realized: results from tests are presented.

Additionally, we will introduce the digital signal processing adopted for the energy reconstruction in GERDA and a new implementation of an optimum digital filter by means of the DPLMS method. This method are discussed and the first application to GERDA data are presented.

Neutrinoless Double-Beta Decay and Realistic Shell Model

We report on the calculation of the neutrinoless double-ß decay nuclear matrix element for 76Ge within the framework of the realistic shell model. The effective shell-model Hamiltonian and the two-body transition operator describing the decay are derived by way of many-body perturbation theory. Particular attention is focused on the role played by the so-called Pauli blocking effect in the derivation of the effective operator.

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.

Evidence for Rigid Triaxial Deformation in ^{76}Ge from a Model-Independent Analysis

An extensive, model-independent analysis of the nature of triaxial deformation in ^{76}Ge, a candidate for neutrinoless double-beta (0νββ) decay, was carried out following multistep Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and γ bands. Both sequences were determined to be characterized by the same β and γ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in ^{76}Ge was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured E2 matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to 0νββ.

Neutrino Masses from Planck-Scale Lepton Number Breaking

We consider an extension of the standard model by right-handed neutrinos and we argue that, under plausible assumptions, a neutrino mass of O(0.1)  eV is naturally generated by the breaking of the lepton number at the Planck scale, possibly by gravitational effects, without the necessity of introducing new mass scales in the model. Some implications of this framework are also briefly discussed.

Light-Neutrino Exchange and Long-Distance Contributions to 0ν2β Decays: An Exploratory Study on ππ→ee

We present an exploratory lattice QCD calculation of the neutrinoless double beta decay ππ→ee. Under the mechanism of light-neutrino exchange, the decay amplitude involves significant long-distance contributions. The calculation reported here, with pion masses m_{π}=420 and 140 MeV, demonstrates that the decay amplitude can be computed from first principles using lattice methods. At unphysical and physical pion masses, we obtain that amplitudes are 24% and 9% smaller than the predication from leading order chiral perturbation theory. Our findings provide the lattice QCD inputs and constraints for effective field theory. A follow-on calculation with fully controlled systematic errors will be possible with adequate computational resources.

Heavy Physics Contributions to Neutrinoless Double Beta Decay from QCD

Observation of neutrinoless double beta decay, a lepton number violating process that has been proposed to clarify the nature of neutrino masses, has spawned an enormous world-wide experimental effort. Relating nuclear decay rates to high-energy, beyond the standard model (BSM) physics requires detailed knowledge of nonperturbative QCD effects. Using lattice QCD, we compute the necessary matrix elements of short-range operators, which arise due to heavy BSM mediators, that contribute to this decay via the leading order π^{-}→π^{+} exchange diagrams. Utilizing our result and taking advantage of effective field theory methods will allow for model-independent calculations of the relevant two-nucleon decay, which may then be used as input for nuclear many-body calculations of the relevant experimental decays. Contributions from short-range operators may prove to be equally important to, or even more important than, those from long-range Majorana neutrino exchange.

New Leading Contribution to Neutrinoless Double-β Decay

Within the framework of chiral effective field theory, we discuss the leading contributions to the neutrinoless double-beta decay transition operator induced by light Majorana neutrinos. Based on renormalization arguments in both dimensional regularization with minimal subtraction and a coordinate-space cutoff scheme, we show the need to introduce a leading-order short-range operator, missing in all current calculations. We discuss strategies to determine the finite part of the short-range coupling by matching to lattice QCD or by relating it via chiral symmetry to isospin-breaking observables in the two-nucleon sector. Finally, we speculate on the impact of this new contribution on nuclear matrix elements of relevance to experiment.

Improved Limit on Neutrinoless Double-β Decay of ^{76}Ge from GERDA Phase II

The GERDA experiment searches for the lepton-number-violating neutrinoless double-β decay of ^{76}Ge (^{76}Ge→^{76}Se+2e^{-}) operating bare Ge diodes with an enriched ^{76}Ge fraction in liquid argon. The exposure for broad-energy germanium type (BEGe) detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of 1.0_{-0.4}^{+0.6}×10^{-3}  counts/(keV kg yr) has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any 0νββ experiment. No signal is observed and a new 90% C.L. lower limit for the half-life of 8.0×10^{25}  yr is placed when combining with our previous data. The expected median sensitivity assuming no signal is 5.8×10^{25}  yr.

Characterization of the ^{163}Ho Electron Capture Spectrum: A Step Towards the Electron Neutrino Mass Determination

The isotope ^{163}Ho is in many ways the best candidate to perform experiments to investigate the value of the electron neutrino mass. It undergoes an electron capture process to ^{163}Dy with an energy available to the decay, Q_{EC}, of about 2.8 keV. According to the present knowledge, this is the lowest Q_{EC} value for such transitions. Here we discuss a newly obtained spectrum of ^{163}Ho, taken by cryogenic metallic magnetic calorimeters with ^{163}Ho implanted in the absorbers and operated in anticoincident mode for background reduction. For the first time, the atomic deexcitation of the ^{163}Dy daughter atom following the capture of electrons from the 5s shell in ^{163}Ho, the OI line, was observed with a calorimetric measurement. The peak energy is determined to be 48 eV. In addition, a precise determination of the energy available for the decay Q_{EC}=(2.858±0.010_{stat}±0.05_{syst})  keV was obtained by analyzing the intensities of the lines in the spectrum. This value is in good agreement with the measurement of the mass difference between ^{163}Ho and ^{163}Dy obtained by Penning-trap mass spectrometry, demonstrating the reliability of the calorimetric technique.

A Compton scattering setup for pulse shape discrimination studies in germanium detectors

Pulse shape discrimination is an important handle to improve sensitivity in low background experiments. A dedicated setup was built to investigate the response of high-purity germanium detectors to single Compton scattered events. Using properly collimated γ-ray sources, it is possible to select events with known interaction location. The aim is to correlate the position dependent signal shape with geometrical and electrical properties of the detector. We report on design and performance of the setup with a first look on data.

Status and future of nuclear matrix elements for neutrinoless double-beta decay: a review

The nuclear matrix elements that govern the rate of neutrinoless double beta decay must be accurately calculated if experiments are to reach their full potential. Theorists have been working on the problem for a long time but have recently stepped up their efforts as ton-scale experiments have begun to look feasible. Here we review past and recent work on the matrix elements in a wide variety of nuclear models and discuss work that will be done in the near future. Ab initio nuclear-structure theory, which is developing rapidly, holds out hope of more accurate matrix elements with quantifiable error bars.

Studies of surface and bulk 210Po in metals using an ultra-low background large surface alpha spectrometer

First measurements of natural surface and bulk ²¹⁰Po specific activities for metals are reported. If covered with protective foils, the surfaces did not show indications of ²¹⁰Po and the obtained upper limits are in the range of single mBqm^-2. Weak bulk activities, in the range of 50 - 280mBqkg^-1, were registered for Stainless Steel and Copper, while significant amounts of ²¹⁰Po, ∼1.5Bqkg^-1, were detected in Titanium. One special Teflon sample was investigated with respect to its bulk ²¹⁰Po.

A facility to search for hidden particles at the CERN SPS: the SHiP physics case

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

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

We present an improved search for neutrinoless double-beta (0νββ) decay of ^{136}Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the ^{110m}Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νββ decay half-life of T_{1/2}^{0ν}>1.07×10^{26}  yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.

Large-Scale Shell-Model Analysis of the Neutrinoless ββ Decay of ^{48}Ca

We present the nuclear matrix element for the neutrinoless double-beta decay of ^{48}Ca based on large-scale shell-model calculations including two harmonic oscillator shells (sd and pf shells). The excitation spectra of ^{48}Ca and ^{48}Ti, and the two-neutrino double-beta decay of ^{48}Ca are reproduced in good agreement to the experimental data. We find that the neutrinoless double-beta decay nuclear matrix element is enhanced by about 30% compared to pf-shell calculations. This reduces the decay lifetime by almost a factor of 2. The matrix-element increase is mostly due to pairing correlations associated with cross-shell sd-pf excitations. We also investigate possible implications for heavier neutrinoless double-beta decay candidates.

Search for Neutrinoless Double-Beta Decay of (130)Te with CUORE-0

We report the results of a search for neutrinoless double-beta decay in a 9.8 kg yr exposure of (130)Te using a bolometric detector array, CUORE-0. The characteristic detector energy resolution and background level in the region of interest are 5.1±0.3  keV FWHM and 0.058±0.004(stat)±0.002(syst)counts/(keV kg yr), respectively. The median 90% C.L. lower-limit half-life sensitivity of the experiment is 2.9×10(24)  yr and surpasses the sensitivity of previous searches. We find no evidence for neutrinoless double-beta decay of (130)Te and place a Bayesian lower bound on the decay half-life, T(1/2)(0ν)>2.7×10(24)  yr at 90% C.L. Combining CUORE-0 data with the 19.75 kg yr exposure of (130)Te from the Cuoricino experiment we obtain T(1/2)(0ν)>4.0×10(24)  yr at 90% C.L. (Bayesian), the most stringent limit to date on this half-life. Using a range of nuclear matrix element estimates we interpret this as a limit on the effective Majorana neutrino mass, m(ββ)<270-760  meV.

Lepton Number Violation in Higgs Decay at LHC

We show that within the left-right symmetric model, lepton number violating decays of the Higgs boson can be discovered at the LHC. The process is due to the mixing of the Higgs boson with the triplet that breaks parity. As a result, the Higgs boson can act as a gateway to the origin of the heavy Majorana neutrino mass. To assess the LHC reach, a detailed collider study of the same-sign dileptons plus jets channel is provided. This process is complementary to the existing nuclear and collider searches for lepton number violation and can probe the scale of parity restoration even beyond other direct searches.