Double Beta Decay Processes Research Articles

The TITAN in-trap decay spectroscopy facility at TRIUMF

This paper presents an upgraded in-trap decay spectroscopy apparatus which has been developed and constructed for use with TRIUMF׳s Ion Trap for Atomic and Nuclear science (TITAN). This device consists of an open-access electron-beam ion-trap (EBIT), which is surrounded radially by seven low-energy planar Si(Li) detectors. The environment of the EBIT allows for the detection of low-energy photons by providing backing-free storage of the radioactive ions, while guiding charged decay particles away from the trap centre via the strong (up to 6T) magnetic field. In addition to excellent ion confinement and storage, the EBIT also provides a venue for performing decay spectroscopy on highly charged radioactive ions. Recent technical advancements have been able to provide a significant increase in sensitivity for low-energy photon detection, towards the goal of measuring weak electron-capture branching ratios of the intermediate nuclei in the two-neutrino double beta (2νββ) decay process. The design, development, and commissioning of this apparatus are presented together with the main physics objectives. The future of the device and experimental technique are discussed.

Double Beta Decay Experiments: Present Status and Prospects for the Future

The review of modern experiments on search and studying of double beta decay processes is done. Results of the most sensitive current experiments are discussed. The main attention is paid to EXO-200, KamLAND-Zen, GERDA-I and CUORE-0 experiments. Modern values of T1/2(2ν) and best present limits on neutrinoless double beta decay and double beta decay with Majoron emission are presented. Conservative limits on effective mass of a Majorana neutrino (<mν> < 0.46eV) and a coupling constant of Majoron to neutrino (<gee> < 1.3 × 10−5) are obtained. In the second part of the review prospects of search for the neutrinoless double beta decay in new experiments with sensitivity to <mν> at the level of ∼ (0.01-0.1) eV are discussed. The main attention is paid to experiments of CUORE, GERDA, MAJORANA, EXO, KamLAND-Zen-2, SuperNEMO and SNO+. Possibilities of low-temperature scintillating bolometers on the basis of inorganic crystals (ZnSe, ZnMoO4, Li2MoO4, CaMoO4 and CdWO4) are considered too.

New Nuclear Structure and Decay Results in the 76Ge–76As System

The process of neutrinoless double beta decay (0νββ) plays a key role in modern neutrino physics. Experiments on 76Ge 0νββ-decay using germanium semiconductors are at the forefront in this field. Due to the extremely low count rates expected for this rare decay, any kind of background event in the detector, especially at energies close to Qββ=2039.006 keV must be avoided. Therefore, a careful investigation on the neutron-induced background was carried out. In this scope experiments investigating the inelastic neutron excitation of the lower lying and the 69th excited level of 76Ge have been performed. The existence of a 2040.7 keV gamma-ray, that occurs by the de-excitation of the 69th excited level, was confirmed. Interfering background from 68Ge was studied via cross-section measurements of the natGe(n,jn)68Ge reaction using quasi-monoenergetic neutrons and accelerator mass specrometry for 68Ge detection. In order to explore the matrix element for the transition between the 76Ge-76As ground states, the electron–capture of 76As has been measured for the first time.

Searching for the Dirac Nature of Neutrinos: Combining Neutrinoless Double Beta Decay and Neutrino Mass Measurements

We studied the neutrinoless double beta decay process to tackle the issue about the nature of neutrino. Establishing the nature of neutrinos, whether they are Dirac or Majorana particles is one of the fundamental questions we need to answer in particle physics, and is related to the conservation of lepton number. Neutrinoless double beta decay ((ββ)0ν) is the tool of choice for testing the Majorana nature of neutrinos. However, up to now, this process has not been observed, but a wide experimental effort is taking place worldwide and soon new results will become available.Different mechanisms can induce (ββ)0ν-decay and might interfere with each other, potentially leading to suppressed contributions to the decay rate. This possibility would become of great interest if upcoming neutrino mass measurements from KATRIN and cosmological observations found that mν>0.2eV but no positive signal was observed in (ββ)0ν-decay experiments. We focus on the possible interference between light Majorana neutrino exchange with other mechanisms, such as heavy sterile neutrinos and R-parity violating supersymmetric models. We show that in some cases the use of different nuclei would allow to disentangle the different contributions and allow to test the hypothesis of destructive interference. Finally, we present a model in which such interference can emerge and we discuss the range of parameters which would lead to a significant suppression of the decay rate.

Radioactive contamination of BaF2crystal scintillator

Barium fluoride (BaF2) crystal scintillators are promising detectors to search for double beta decay processes in 130 Ba (Q2 = 2619(3) keV) and 132 Ba (Q2 = 844(1) keV). The 130 Ba isotope is of particular interest because of the indications on 2 decay found in two geochemical experiments. The radioactive contamination of BaF2 scintillation crystal with mass of 1.714 kg was measured over 113.4 hours in a low-background DAMA/R&D set-up deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of INFN (LNGS, Italy). The half-life of 212 Po (present in the crystal scintillator due to contamination by radium) was estimated as T1/2 = 298.8 ± 0.8(stat.) ± 1.4(syst.) ns by analysis of the events pulse profiles.

Ultra-low gamma-ray measurement system for neutrinoless double beta decay

An experiment for the detection of 0νβ+/EC and 0νEC/EC in 92Mo nuclei has been carried out with a scintillating crystal, CaMoO4, in coincidence with the HPGe detector. We study the background events inside the event selection window for 0ν β+/EC decays of CaMoO4 detector. For 51.2 days of data taking period, we didn't observe any event in the neutrinoless EC/EC decay event window. The 92Mo 0νβ+/EC decay half-life limit was set to 0.61×1020 years with a 90% confidence by method of Feldman and Cousins. This ultra-low gamma ray measurement utilizing coincidence technique can be used for the resonant EC/EC decay process of some nuclei which is potentially important for neutrinoless double beta decay process.

Search for2βdecays of96Ru and104Ru by ultralow-background HPGeγspectrometry at LNGS: Final results

An experiment to search for double beta decay processes in 96Ru and 104Ru, which are accompanied by gamma rays, has been realized in the underground Gran Sasso National Laboratories of the I.N.F.N. (Italy). Ruthenium samples with masses of about (0.5-0.7) kg were measured with the help of ultra-low background high purity Ge gamma ray spectrometry. After 2162 h of data taking the samples were deeply purified to reduce the internal contamination of 40K. The last part of the data has been accumulated over 5479 h. New improved half life limits on 2\beta+/\epsilon \beta+/2\epsilon\ processes in 96Ru have been established on the level of 10^{20} yr, in particular for decays to the ground state of 96Mo: T1/2(2\nu 2\beta+) > 1.4 10^{20} yr, T1/2(2\nu \epsilon\beta+) > 8.0 10^{19} yr and T1/2(0\nu 2K) > 1.0 10^{21} yr (all limits are at 90% C.L.). The resonant neutrinoless double electron captures to the 2700.2 keV and 2712.7 keV excited states of 96Mo are restricted as: T1/2(0\nu KL) > 2.0 10^{20} yr and T1/2(0\nu 2L) > 3.6 10^{20} yr, respectively. Various two neutrino and neutrinoless 2\beta\ half lives of 96Ru have been estimated in the framework of the QRPA approach. In addition, the T1/2 limit for 0\nu 2\beta- transitions of 104Ru to the first excited state of 104Pd has been set as > 6.5 10^{20} yr.

Systematic decomposition of the neutrinoless double beta decay operator

We discuss the systematic decomposition of the dimension nine neutrinoless double beta decay operator, focusing on mechanisms with potentially small contributions to neutrino mass, while being accessible at the LHC. We first provide a (d=9 tree-level) complete list of diagrams for neutrinoless double beta decay. From this list one can easily recover all previously discussed contributions to the neutrinoless double beta decay process, such as the celebrated mass mechanism or "exotics", such as contributions from left-right symmetric models, R-parity violating supersymmetry and leptoquarks. More interestingly, however, we identify a number of new possibilities which have not been discussed in the literature previously. Contact to earlier works based on a general Lorentz-invariant parametrisation of the neutrinoless double beta decay rate is made, which allows, in principle, to derive limits on all possible contributions. We furthermore discuss possible signals at the LHC for mediators leading to the short-range part of the amplitude with one specific example. The study of such contributions would gain particular importance if there were a tension between different measurements of neutrino mass such as coming from neutrinoless double beta decay and cosmology or single beta decay.

Neutrinoless double beta decay process in left-right symmetric models without scalar bidoublet

We present an alternative formulation of the left-right symmetric theory where the scalar sector consists of two Higgs doublets which differs from the standard version of the left-right model that makes use of $L-$ and $R-$ Higgs triplets and a Higgs bi-doublet. The basic idea is to consider few extra charged iso-singlet fields and the fermion masses can be realized by integrating out these heavy isosinglet fields. We also give a detailed discussion on neutrinoless double beta decay in this particular left-right symmetric theory where the right-handed Majorana neutrino can be of MeV range. With this right-handed Majorana mass around MeV scale, the contribution to neutrinoless double beta decay coming from the right-handed current can be comparable with the contributions coming from the standard left-handed sector only if the right-handed gauge boson mass is around 5 TeV, and with this operative scale of $W_R$ around few TeV, it is possible to probe at LHC. We have briefly commented on cosmological constraints coming from the big-bang nucleosynthesis and Universe cosmology to the right-handed neutrinos involved in this discussion.

Discovering neutrinoless double beta decay with NEXT100 detector

NEXT is a new experiment to search for neutrinoless double beta decay processes that will start operation at the LSC laboratory (Canfranc, Spain) in 2013. The apparatus is a high pressure gas xenon chamber (HPGXe) filled with 100-150 kg of gas Xenon enriched at 90% in the 136Xe isotope. NEXT proposes a novel detection technique called SOFT (Separated Optimized Function TPC) which optimizes both the energy resolution and the measurement of the topological signature of the event. This results in a powerful background rejection, which, combined with a carefully screened radiopure detector will allow NEXT to be competitive with existing proposals for next-generation neutrinoless double-beta decay experiments. First prototypes have been operating successfully in different laboratories. First results with large-scale prototypes measure a resolution of 1% FWHM at the 137Cs photopeak. This extrapolates to a resolution better than 0.5% FWHM at Qββ.

LeptonicCPviolation

Several topics on $CP$ violation in the lepton sector are reviewed. A few theoretical aspects concerning neutrino masses, leptonic mixing, and $CP$ violation will be covered, with special emphasis on seesaw models. A discussion is provided on observable effects which are manifest in the presence of $CP$ violation, particularly, in neutrino oscillations and neutrinoless double beta decay processes, and their possible implications in collider experiments such as the LHC. The role that leptonic $CP$ violation may have played in the generation of the baryon asymmetry of the Universe through the mechanism of leptogenesis is also discussed.

Contributions of the isobar analogue states to the two neutrino double beta decay process

The two neutrino double beta decay of 128, 130Te isotopes to the ground state of the 128, 130Xe isotopes has been calculated by including the contributions coming from the isobar analog states in the intermediate nuclei. The nuclear matrix elements have been computed by using the proton–neutron quasi-particle random phase approximation (pnQRPA). The broken isospin invariance of the nuclear part of the total nucleus Hamiltonian has been restored for the study of the isobar analogue excitations in the intermediate nuclei. The SU(4) symmetry violation in the shell model approximation has been considered for the investigation of the Gamow–Teller states.

NEMO-3 and SuperNEMO: A search for zero neutrino double beta decay

The NEMO-3 experiment is located in the Modane Underground Laboratory (LSM) and has been taking data with seven isotopes from 2003. It is searching for the double beta decay process with two ( 2 ν β β ) or zero ( 0 ν β β ) neutrinos emitted in the final state. Precision measurements of the half-life of the isotopes due to 2 ν β β decay have been performed and updated results for all of the isotopes will be published. In particular T 1 / 2 2 ν ( Mo 100 ) = [ 7.17 ± 0.01 ( stat ) ± 0.54 ( syst ) ] ⋅ 10 18 y and the nuclear matrix elements (NME) 0.126 ± 0.006 is presented here. No evidence for 0 ν β β decay has been found and a 90% Confidence Level lower limit of T 1 / 2 0 ν ( Mo 100 ) > [ 1 ⋅ 10 24 y ] on the half-life of this process is derived. From this an upper limit can be set on the effective Majorana neutrino mass using nuclear matrix element (NME) calculations. NEMO-3 will stop in 2010. A four year R&D programme has been undertaken for SuperNEMO, the successor to NEMO-3. The construction of a demonstrator has begun and will be situated on the site of NEMO-3.

Background capabilities of pixel detectors for double beta decay measurements

We discuss the possible use of a progressive detection technique based on pixel detectors for the study of double beta decay (ββ) processes. A series of background measurements in various environments (surface laboratory, underground laboratory, with and without Pb shielding) was performed using the TimePix silicon hybrid pixel device. The pixel detector response to the natural background and intrinsic background properties measured by a low-background HPGe detector are presented.

Inverse neutrinoless double beta decay revisited: Neutrinos, Higgs triplets, and a muon collider

We revisit the process of inverse neutrinoless double beta decay (${e}^{\ensuremath{-}}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{W}^{\ensuremath{-}}{W}^{\ensuremath{-}}$) at future linear colliders. The cases of Majorana neutrino and Higgs triplet exchange are considered. We also discuss the processes ${e}^{\ensuremath{-}}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\rightarrow}{W}^{\ensuremath{-}}{W}^{\ensuremath{-}}$ and ${\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\rightarrow}{W}^{\ensuremath{-}}{W}^{\ensuremath{-}}$, which are motivated by the possibility of muon colliders. For heavy neutrino exchange, we show that masses up to ${10}^{6}$ $({10}^{5})\text{ }\text{ }\mathrm{GeV}$ could be probed for $ee$ and $e\ensuremath{\mu}$ machines, respectively. The stringent limits for mixing of heavy neutrinos with muons render ${\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\rightarrow}{W}^{\ensuremath{-}}{W}^{\ensuremath{-}}$ less promising, even though this process is not constrained by limits from neutrinoless double beta decay. If Higgs triplets are responsible for inverse neutrinoless double beta decay, observable signals are only possible if a very narrow resonance is met. We also consider unitarity aspects of the process in case both Higgs triplets and neutrinos are exchanged. An exact seesaw relation connecting low energy data with heavy neutrino and triplet parameters is found.

Shell model analysis of the neutrinoless double-βdecay ofCa48

The neutrinoless double beta ($0\nu\beta\beta$) decay process could provide crucial information to determine the absolute scale of the neutrino masses, and it is the only one that can establish whether neutrino is a Dirac or a Majorana particle. A key ingredient for extracting the absolute neutrino masses from $0\nu\beta\beta$ decay experiments is a precise knowledge of the nuclear matrix elements (NME) describing the half-life of this process. We developed a new shell model approach for computing the $0\nu\beta\beta$ decay NME, and we used it to analyze the $0\nu\beta\beta$ mode of $^{48}$Ca. The dependence of the NME on the short range correlations parameters, on the average energy of the intermediate states, on the finite-size cutoff parameters, and on the effective interaction used for the many-body calculations is discussed.

LHC and B physics probes of neutrinoless double beta decay in supersymmetry without R-parity

In the event of an observation of neutrinoless double beta decay, a relevant question would be: what lepton number violating physics is responsible for the decay? The exchange of Majorana neutrinos and/or supersymmetric particles may contribute. We point out that measurements of supersymmetric signals at the LHC, including single slepton production, could be used to help bound some supersymmetric processes contributing to neutrinoless double beta decay. LHC information about the supersymmetric spectrum could be combined with Bd-Bd bar mixing data in order to bound a competing neutrinoless double beta decay process involving sbottom exchange.

Possibility of detecting neutrinoless double beta decays using nuclear emulsions

A complementary physics to the LHC subjects comes from the neutrino sector which has already opened a window beyond the standard model. Neutrino oscillations have already proved that neutrinos have non zero masses. But, neutrino nature and absolute mass scale are still major questions which answers could have big effects at the cosmological level. Several neutrinoless double beta decay experiments are underway or planed for the near future trying to address the above questions. These experiments mainly use bolometric techniques or gaseous counters coupled with scintillator detectors. Energy resolution is significantly better in bolometric experiments but experiments coupling tracking with calorimetry have much better background rejection by identifying the two electrons produced by the double beta decay process. Another technique of detecting the electrons with reasonable energy resolution without using electronic detectors is the utilisation of nuclear emulsions. This technique has the advantage of precise tracking and vertexing even for low energy electrons, allowing a high background rejection. This possibility is presented in this paper.

Results from NEMO-3

The NEMO-3 experiment is located in the Modane Underground Laboratory (LSM) and has been taking data since 2003 with seven isotopes. It is searching for the double beta decay process with two (2νββ) or zero (0νββ) neutrinos emitted in the final state. Precision measurements of the half-life of the isotopes due to 2νββ decay have been performed and new results for T2ν1/2(96Zr) = [2.3 ± 0.2(stat) ± 0.3(syst)] ∣ 1019y, T2ν1/2(48Ca) = [4.4+0.5-0.4(stat) ± 0.4(syst)|·1019y and T2ν1/2(150Nd) = [0.920+0.025-0.022(stat)± 0.072 (syst)]· 1019y are presented here. Measurements of this process are important for reducing the uncertainties on the nuclear matrix elements. No evidence for 0νββ decay has been found and a 90% Confidence Level lower limit on the half-life of this process is derived. From this an upper limit can be set on the effective Majorana neutrino mass using the most recent nuclear matrix elements (NME) calculations.

Single- and low-lying-states dominance in two-neutrino double-beta decay

A theoretical analysis of the single-state dominance hypothesis for the two-neutrino double-beta decay rates is performed on the examples of the double-beta decays of 100Mo, 116Cd and 128Te. We also test the validity of an extended low-lying-state dominance that takes into account the contributions of the low-lying excited states in the intermediate nucleus to the double-beta decay rates. This study has been accomplished for all the double-beta emitters for which we have experimental information on their half-lives. The theoretical framework is a proton–neutron quasiparticle random-phase approximation based on a deformed Skyrme Hartree–Fock mean field with pairing correlations. Our calculations indicate that there are no clear evidences for single- or low-lying-state dominance in the two-neutrino double-beta decay. Finally, we investigate the single-electron energy distributions of the outgoing electrons in the double-beta decay processes with an exact treatment of the energy denominators, which could help for a more comprehensive analysis of NEMO-3 data.