Neutrinoless Double Beta Decay Experiments

• Neutrinoless double beta decay is a lepton-number-violating process that signals Majorana neutrinos and offers insights into the absolute neutrino mass scale.
• Experiments employ homogeneous calorimetric detectors and tracking systems with ultra-low background techniques and high energy resolution to isolate rare decay events.
• Innovative methods, including multi-isotope measurements and advanced event topology, enhance sensitivity and help distinguish the mass mechanism from exotic new physics.

Neutrinoless double beta decay experiments are at the forefront of experimental particle physics, aiming to resolve key questions about the nature of neutrinos, lepton number conservation, and the absolute mass scale of neutrinos. These experiments search for the lepton-number-violating process $(A,Z) \rightarrow (A, Z+2) + 2e^-$, a transition forbidden in the Standard Model unless neutrinos are Majorana fermions. The observation of such a process would not only confirm the Majorana nature of neutrinos, but would also have profound implications for theories of mass generation, the origin of matter-antimatter asymmetry, and physics beyond the Standard Model.

1. Theoretical Motivation and Decay Mechanism

The search for neutrinoless double beta decay ($0\nu\beta\beta$) is motivated by its direct connection to physics beyond the Standard Model:

• Majorana Nature of Neutrinos: Observation of $0\nu\beta\beta$ would prove that neutrinos are their own antiparticles. The process requires a virtual neutrino propagator that must be Majorana to allow the transformation without emitting neutrinos.
• Lepton Number Violation: The decay would signify $\Delta L = 2$ violation, providing experimental evidence for lepton number non-conservation.
• Neutrino Mass Scale and Hierarchy: The half-life of $0\nu\beta\beta$ is inversely proportional to the square of the effective Majorana mass $m_{\beta\beta} = |\sum U_{ei}^2 m_i|$, linking it to the absolute neutrino mass scale and potentially discerning between normal and inverted hierarchies.
• New Physics Contributions: Beyond the light Majorana neutrino exchange, alternative mechanisms (e.g., right-handed currents, supersymmetric contributions) may also mediate $0\nu\beta\beta$ decay, parameterized by effective couplings such as $\epsilon$. Multiple isotopes are required to disentangle such mechanisms (Agostini et al., 2022).

The general relation connecting the half-life to physics parameters reads:

$$\frac{1}{T_{1/2}^{0\nu}} = G^{0\nu}(Q,Z) \, |M^{0\nu}|^2 \left( \frac{m_{\beta\beta}}{m_e} \right)^2$$

where $G^{0\nu}$ is the phase space factor, $M^{0\nu}$ the nuclear matrix element (NME), and $m_e$ the electron mass (Ejiri, 2020).

2. Experimental Strategies and Detection Technologies

Experimental approaches to $0\nu\beta\beta$ searches fall into two broad classes:

• Homogeneous Calorimetric Detector Experiments: The $\beta\beta$-emitting isotope is both the decay source and the detection medium, maximizing efficiency and energy resolution. Key examples include:
  • HPGe arrays (e.g., GERDA, Majorana Demonstrator, LEGEND) using $^{76}$Ge with superb energy resolution ($\sim 0.1$–$0.2\%$ at $Q_{\beta\beta}$).
  • Bolometric detectors (e.g., CUORE for $^{130}$Te) offering high mass, excellent energy resolution, and scalability.
  • High-pressure xenon time projection chambers (HPXe TPCs), e.g., NEXT, combining calorimetry with event topology (Martín-Albo, 2019).
• Heterogeneous (Tracking) Detector Experiments: The source is distinct from the detector, allowing explicit tracking of the two emitted electrons (NEMO-3/SuperNEMO for $^{82}$Se). Such designs can reconstruct event kinematics but at the cost of lower efficiency and energy resolution (Cremonesi, 2012).

Common design goals are:

• Ultra-low Background: Achieved via deep underground operation, material radiopurity, active background veto systems (e.g., LAr veto in LEGEND), and advanced pulse shape/event topology discrimination.
• Energy Resolution: High energy resolution sharply defines the signal region around $Q_{\beta\beta}$ and suppresses background from $2\nu\beta\beta$ tails and intrinsic radioactivity.

Representative Experiments and Isotopes

Experiment	Isotope	Mass (typical scale)	Energy Resolution (FWHM)	Key Background Control
LEGEND-200/1000	$^{76}$Ge	200–1000 kg	$\sim 0.1\%$	LAr veto, electroformed Cu, PSD
GERDA	$^{76}$Ge	35–40 kg	$2.5$ keV @ 2039 keV	LAr veto, PSD, active water veto
Majorana Demo.	$^{76}$Ge	40 kg	$2.5$ keV @ 2039 keV	Coincidence/PSD, underground Cu
NEXT-100	$^{136}$Xe	100 kg	$<1\%$ at 2.5 MeV	Event topology, radiopure SiPMs
CUORE	$^{130}$Te	741 kg ($\sim$206 kg $^{130}$Te)	$0.2\%$	Cryogenic bolometry, surface rejection

3. Sensitivity, Backgrounds, and Data Analysis

The sensitivity of a $0\nu\beta\beta$ experiment is a function of several experimental and nuclear parameters (Ejiri, 2020):

• Detector Sensitivity:

$$m_m = m_0 \cdot d \ \ \text{with} \ \ d = d_0 \eta^{-1/2} \epsilon^{-1/2} (NT/B)^{-1/4}$$

where $m_0$ is the nuclear sensitivity mass, $\eta$ isotopic enrichment, $\epsilon$ efficiency, $N$ isotope mass, $T$ exposure time, $B$ background index [counts/t·yr], and $d_0 \sim 1.4$.

• Exposure and Background Index: Increasing exposure and minimizing $B$ are equally critical; reducing $B$ by an order of magnitude can have as much impact as an equivalent increase in exposure (Brugnera, 17 Jan 2025).
• Background-Free vs Background-Limited Regime: As backgrounds decrease towards $\sim 1$ event in the region of interest (ROI), sensitivity scales linearly with exposure.

Current leading experiments achieve background indices as low as $2 \times 10^{-4}$ counts/(keV·kg·yr) (LEGEND-200), with energy resolutions at $Q_{\beta\beta}$ of about $0.1\%$ FWHM (Brugnera, 17 Jan 2025).

4. Experimental Status and Results

Germanium-Based Experiments: GERDA, Majorana Demonstrator, LEGEND

• GERDA Phase II: Achieved a half-life sensitivity of $T_{1/2} > 1.8 \times 10^{26}$ yr for $^{76}$Ge (Brugnera, 17 Jan 2025), with background indices enabling "background-free" operation up to its design exposure.
• Majorana Demonstrator: Demonstrated ultra-low backgrounds, energy resolution of $2.5$ keV @ 2039 keV, and a half-life sensitivity of $T_{1/2} > 0.8 \times 10^{26}$ yr (Brugnera, 17 Jan 2025).
• LEGEND-200: In the first year, acquired 76.2 kg·yr exposure, with the “golden” data set of 48.3 kg·yr used for a $0\nu\beta\beta$ search. Background index near $2 \times 10^{-4}$ counts/(keV·kg·yr). Combined with previous data, achieved

$$T_{1/2}^{0\nu} > 2.8 \times 10^{26} \text{ yr (median sensitivity)}, \ T_{1/2}^{0\nu} > 1.9 \times 10^{26} \text{ yr (limit at 90\% CL)}.$$

LEGEND-1000 (future): Targets $>$1 tonne $^{76}$Ge, background index $\sim 10^{-5}$ counts/(keV·kg·yr), and half-life sensitivities in excess of $10^{28}$ yr (Brugnera, 17 Jan 2025, Guinn et al., 2019, López-Castaño et al., 2019, Myslik, 2018).

Xenon-Based Experiments: NEXT, nEXO

• NEXT-White (5 kg $^{136}$Xe): Demonstrated $<1\%$ FWHM energy resolution at $Q_{\beta\beta}$.
• NEXT-100: 100 kg HPXe TPC, predicted sensitivity $T_{1/2}^{0\nu} > 1.0 \times 10^{26}$ yr after 500 kg·yr exposure (Cebrian, 2020). Strong background rejection using event topology (double-electron "blobs") and radiopure design; future tonne-scale plans include barium tagging for near background-free operation (Martín-Albo, 2019).
• nEXO: Multi-tonne liquid Xe TPC, aims for half-life sensitivity $>10^{27}$ yr (Agostini et al., 2022).

Te- and Mo-based Experiments: CUORE, CUPID, CROSS

• CUORE: Uses bolometric detection with $^{130}$Te, designed sensitivity $T_{1/2}^{0\nu} > 9.5 \times 10^{25}$ yr (Maneschg, 2017).
• CUPID, CROSS: Next-generation bolometer arrays deploying scintillating or surface-sensitive techniques for $^{100}$Mo and $^{130}$Te, aiming to suppress $\alpha$ surface backgrounds and reach half-life sensitivity $\sim 10^{26}$–$10^{27}$ yr (Cebrian, 2020, Ejiri, 2020).

5. Multi-Isotope and Mechanism-Resolving Approaches

A robust discovery of $0\nu\beta\beta$ and elucidation of its underlying mechanism require measurements across multiple isotopes with uncorrelated systematics:

• Degeneracy Breaking: The effective Majorana mass and possible exotic-physics parameters (e.g., short-range coupling $\epsilon$) have distinct nuclear matrix element dependencies. Measuring half-lives in three isotopes (e.g., $^{76}$Ge, $^{100}$Mo, $^{136}$Xe) allows for a unique determination of underlying physics parameters, resolving degeneracies between standard and exotic mechanisms (Agostini et al., 2022).
• NME Uncertainties: The propagation of correlated nuclear matrix element uncertainties is handled via global Bayesian analyses and impacts the constraints derivable from experimental data.

6. Key Challenges and Technical Innovations

• Background Rejection: Innovations include active LAr veto (LEGEND), pulse-shape discrimination (PPC, BEGe, ICPC detectors), event topology in TPCs (NEXT).
• Energy Resolution: Achieving $0.1$–$0.2\%$ FWHM at $Q_{\beta\beta}$ is critical for separating signal from $2\nu\beta\beta$ backgrounds.
• Detector Scalability: Current and future experiments aim to scale to tonne-scale target masses, requiring advances in enrichment, mechanical design, data acquisition, and material purity.
• Material Radiopurity: Extensive screening and new production methods for copper, scintillators, and crystal growth are a routine part of experiment preparation (Collaboration et al., 2013, Pandola, 2014).
• Surface Sensitivity: Projects like CROSS employ superconducting films for bolometric detectors to suppress surface alpha backgrounds with $>99.9\%$ rejection (Cebrian, 2020).

7. Future Prospects

• Probing Neutrino Mass Hierarchies: Large-scale experiments with sensitivities $T_{1/2}^{0\nu} \gtrsim 10^{28}$ yr are required to fully explore the inverted ordering ($m_{\beta\beta}\sim17$ meV). Probing the normal hierarchy ($m_{\beta\beta}\sim$ few meV) will demand exposures of $\sim100$ ton-year and fundamentally new technological strategies (Ejiri, 2020).
• Mechanism Identification: Combining results from multiple isotopes and using global fits, as well as improved nuclear theory, will enable the separation of mass mechanism from exotic contributions—with significant consequences for underlying new physics models (Agostini et al., 2022).
• Upcoming Facilities: Ongoing and projected experiments—LEGEND-1000, nEXO, CUPID, NEXT-HD/NEXT-BOLD, SNO+ upgrades—are designed to push backgrounds well below $<10^{-4}$ counts/(keV·kg·yr) and deploy multitonne target masses (Brugnera, 17 Jan 2025, Martín-Albo, 2019, Paton, 2019).

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Neutrinoless double beta decay experiments now operate at the limit of ultra-rare event detection. Achieving backgrounds below $10^{-4}$ counts/(keV·kg·yr) and scaling to tonne-mass targets will determine whether the inverted neutrino hierarchy, and thus lepton number violation and the Majorana nature of the neutrino, is decisively probed within the next decade. The integration of improved detector technologies and a multi-isotope, multi-experiment strategy is essential both for unambiguous discovery and for elucidating the decay mechanism if a positive observation is made (Brugnera, 17 Jan 2025, Agostini et al., 2022).