The search for rare decays is a window into physics beyond the Standard Model. The classic neutrinoless double beta decay (0ν2β) would point to the Majorana nature of neutrinos, but decades of searches have so far yielded nothing. In 2013, Heeck and Rodejohann proposed searching for neutrinoless quadruple beta decay (0ν4β), which is possible even for Dirac neutrinos. If 0ν4β were discovered in the absence of 0ν2β, that would be a strong argument in favor of the Dirac nature of neutrinos. The PandaX-4T work fits into the broader effort to search for lepton number violation with liquid xenon detectors, alongside studies of double beta decay of xenon-136 and double electron capture of xenon-124.
The analysis used data from two PandaX-4T runs: the commissioning run (Run0) and the first science run (Run1), with a combined exposure of 148.4 kg·yr on the xenon-136 isotope. The detector is a two-phase time projection chamber holding 3.7 tonnes of liquid xenon, located in the China Jinping Underground Laboratory at a depth of 2400 meters. The energy region of interest was widened to 20-140 keV in order to capture the expected peak at 79 keV. Backgrounds were modeled from detector materials, krypton-85 impurities, radon, double beta decay of xenon-136, double electron capture of xenon-124, and solar neutrinos. Calibration data and Geant4-based Monte Carlo simulations were used to estimate the background contributions.
No statistically significant excess over background was found in the spectrum. An upper limit on the signal rate of 43.75 events per tonne per year was obtained, corresponding to a lower limit on the half-life of neutrinoless quadruple beta decay of xenon-136: T₁/₂ > 6.01×10²⁴ years at the 90% confidence level. The median sensitivity was 5.5×10²⁴ years. The result is 1.6 times stronger than the previous XMASS limit (3.7×10²⁴ years) and is the most stringent for this process in xenon. The dominant backgrounds were activated xenon-133 in Run0 and lead-214 from the radon chain in Run1.
This work differs from neighboring searches in that it is the first to use the large fiducial mass of a liquid xenon detector to search for 0ν4β, whereas the previous limit came from the smaller XMASS detector. The new limit begins to constrain theoretical models with lepton number violation by four units. The absence of a signal does not yet allow a conclusion about the nature of neutrinos, but it shows that liquid xenon detectors can compete in this niche.
Further improvements in sensitivity are expected from the analysis of Run2 data and from the future PandaX-xT detector with a mass of tens of tonnes. Increased exposure and reduced backgrounds will either reveal 0ν4β or set limits that exclude a significant part of the parameter space of models with lepton quarticity.
The result will influence neutrino phenomenology, models with lepton number violation, and dark matter searches related to lepton quarticity.
Analysis of PandaX-4T Run2 data and preparation for the launch of PandaX-xT, where the sensitivity to 0ν4β will increase by orders of magnitude.
For neutrino physics, the key unsolved problem is the nature of neutrino mass: Majorana or Dirac. The search for 0ν4β is directly tied to this problem, because its discovery in the absence of 0ν2β would point to the Dirac scenario. The work also touches on the origin of the baryon asymmetry of the Universe through leptogenesis, which requires lepton number violation.
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What the work does
The PandaX-4T collaboration reports a search for neutrinoless quadruple beta decay of xenon-136 using data from the detector's first science run, with an exposure of 148.4 kg·yr. The analysis in the 20-140 keV energy region found no significant excess over background and sets a new lower limit on the half-life T₁/₂ > 6.01×10²⁴ years at 90% confidence level, which is 1.6 times stronger than the previous XMASS result.
Where its strength is
For the first time, a large fiducial mass liquid xenon detector has been used to search for 0ν4β, yielding the most stringent limit on this process in xenon. The result begins to constrain theoretical models with lepton number violation by four units, demonstrating the competitiveness of liquid xenon detectors in this niche.
Where it could go next
Worth trying
Why this may matter
Improving the limit on 0ν4β is important for testing models with lepton number violation by four units, which may be linked to the Dirac nature of neutrinos. Further development of techniques with liquid xenon detectors could make this search channel competitive with the traditional 0ν2β.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
A half-life of 6.01×10²⁴ years is 400 trillion times longer than the age of the Universe. If such a decay occurred, less than one atom per year would decay in a kilogram of xenon-136.
In Liu Cixin's novel The Three-Body Problem, neutrinos are used to communicate with an alien civilization. The search for exceedingly rare decays such as 0ν4β resembles attempts to catch nearly imperceptible signals from the depths of the microworld.