[Paper Review] Search For Neutrinoless Double Beta Decay With Enriched 76Ge 1990-2003 -- HEIDELBERG-MOSCOW-Experiment

H. V. Klapdor‐Kleingrothaus, I. V. Krivosheina+4 more|arXiv (Cornell University)|Apr 6, 2004

Neutrino Physics Research3 references5 citations

TL;DR

The HEIDELBERG-MOSCOW experiment reports evidence for neutrinoless double beta decay (0νββ) in enriched ⁷⁶Ge, analyzing data through May 20, 2003, with a 4.2σ significance. It reports a half-life of T₁/₂⁰ν = (0.69–4.18)×10²⁵ y at 99.73% confidence level, implying an effective neutrino mass of ⟨mν⟩ = (0.24–0.58) eV, supporting a degenerate neutrino mass scenario and indicating the neutrino is a Majorana particle.

ABSTRACT

The HEIDELBERG-MOSCOW experiment, which is the most sensitive double beta decay experiment since ten years has been regularly continued until end of November 2003. An analysis of the data has been performed already until May 20, 2003. The experiment yields now, on a 4.2 sigma level, evidence for lepton number violation and proves that the neutrino is a Majorana particle.It further shows that neutrino masses are degenerate. In addition it puts several stringent constraints on other physics beyond the Standard Model. Among others it opens the door to test various supersymmetric theory scenarios, for example it gives the sharpest limit on the parameter lambda'_{111} in the R-parity violating part of the superpotential, and gives information on the splitting of the sneutrino-antisneutrino system. The result from the HEIDELBERG-MOSCOW experiment is consistent with recent results from CMB investigations, with high energy cosmic rays, with the result from the g-2 experiment and with recent theoretical work. It is indirectly supported by the analysis of other Ge double beta experiments. Recent criticism of various kind has been shown to be wrong, among others by measurements performed in 2003 with a 214Bi source (226Ra), by simulation of the background in the range of Q_(beta-beta) by GEANT4, and by deeper investigation of statistical features such as sensitivity of peak search, and relevance of width of window of analysis.

Motivation & Objective

• To search for neutrinoless double beta decay (0νββ) in enriched ⁷⁶Ge as a probe of lepton number violation.
• To determine the absolute scale of neutrino masses through the effective Majorana mass ⟨mν⟩.
• To test the Majorana nature of neutrinos and constrain beyond-Standard Model physics, including R-parity violating supersymmetry.
• To validate the experimental signal against background and statistical challenges through dedicated measurements and simulations.
• To provide a stringent test of theoretical models involving lepton number violation and neutrino mass generation mechanisms.

Proposed method

• Utilized high-purity germanium (HPGe) detectors enriched in ⁷⁶Ge to detect the 2νββ and 0νββ decay signatures.
• Measured the energy spectrum of decay products with high energy resolution (ΔE) to identify the 0νββ peak at Qββ = 2034 keV.
• Applied a windowed analysis method with optimized energy window and background modeling using GEANT4 simulations.
• Calculated the half-life limit using the formula T₁/₂⁰ν ∼ ε × a × √(Mt / (ΔE × B)), where ε is efficiency, a is isotopic abundance, M is mass, t is time, ΔE is resolution, and B is background rate.
• Performed statistical analysis including peak search sensitivity and window width relevance to assess significance.
• Validated results with ²¹⁴Bi and ²²⁶Ra source measurements in 2003 to rule out instrumental or background artifacts.

Experimental results

Research questions

• RQ1 Is there evidence for neutrinoless double beta decay in ⁷⁶Ge at a significance level exceeding 4σ?
• RQ2 What is the effective Majorana neutrino mass ⟨mν⟩ derived from the 0νββ decay rate?
• RQ3 Does the observed signal support the Majorana nature of the neutrino and a degenerate neutrino mass spectrum?
• RQ4 What constraints does the result place on R-parity violating couplings, such as λ′₁₁₁, in supersymmetric models?
• RQ5 How robust is the signal against background contributions and statistical fluctuations, and are alternative explanations ruled out?

Key findings

• The experiment reports a 4.2σ evidence for neutrinoless double beta decay in ⁷⁶Ge, with a half-life T₁/₂⁰ν = (0.69–4.18)×10²⁵ y at 99.73% confidence level.
• The best-fit half-life is 1.19×10²⁵ y, implying an effective neutrino mass of ⟨mν⟩ = 0.44 eV, with a 99.73% confidence interval of (0.24–0.58) eV.
• Allowing for ±50% uncertainty in nuclear matrix elements, the effective mass range widens to (0.1–0.9) eV.
• The result supports a degenerate neutrino mass scenario with a common mass eigenvalue m_com = (0.14–3.6) eV at 99.73% confidence level.
• The signal provides the sharpest existing limit on the R-parity violating Yukawa coupling λ′₁₁₁, constraining its value more stringently than any other experiment.
• The result is consistent with CMB data, g-2 experiment results, and theoretical work, and is indirectly supported by other Ge double beta experiments.