Physics Division Research Highlights
Neutrinoless double-β-decay: The impact of new nuclear data
A definitive observation of neutrinoless double-β-decay (0νββ) would have profound implications on our understanding of nature. Not only would such a discovery demonstrate that the neutrino is its own anti-particle (e.g, the neutrino would be a Majorana particle), but it would also constrain the neutrino mass hierarchy and scale, and provide a mechanism for lepton-number violation. To date, it is generally accepted by the research community that the best constraints on the observation of 0νββ will come from searches in two nuclei: 130Te and 76Ge. New experiments, to be installed in underground laboratories, are currently under development in the U.S. and elsewhere. Two recent experiments are assisting in providing high-precision nuclear physics data essential for the planned and on-going 0νββ searches using 130Te.
An essential input into the 0νββ process is the difference in mass between the parent and the daughter nucleus, the so-called Q value. The 76Ge Q value is known with sufficient precision, but the 130Te one is less precise than the current and anticipated energy resolution of detectors in 0νββ-search experiments. Q values are measured at ATLAS with the Canadian Penning Trap by determining the cyclotron frequency of ions within a stable, homogeneous magnetic field. For each ion bunch confined in the trap, the ions are subjected to a quadrupole radiofrequency field. Upon ejection from the Penning trap, ions that were resonantly excited at or near their cyclotron frequency arrive at a detector in less time than ions subjected to other frequencies. The masses of the parent-daughter pair 130Te-130Xe were determined this way . Sample spectra are shown in Fig. 1. The results indicate that the 130Te ββ-decay Q value (2527.01 ± 0.32 keV) is 3.3 keV smaller than previously evaluated. This puts the expected 0νββ signal close to a contaminant (60Co) sum-peak, but is at least 5 keV away from essentially all γ rays emitted from natural decay chains. It is worth noting that a measurement of such high precision is only possible because of the use of a Penning trap
Fig. 1: Time-of-flight spectra of 130Te (a) and 130Xe (b) taken with the Canadian Penning Trap mass spectrometer. The theoretical lineshape is fit to the data from which the cyclotron frequency of the ions can be determined.
Another important ingredient into the problem is related to nuclear structure. Indeed, the decay rate for the 0νββ process depends on the matrix element for the decay. This matrix element in turn relies on nuclear theory to provide the wave functions of the states involved. One of the main uncertainties in the latter is the description of so-called pair correlations, the force responsible for coupling nucleons inside the nucleus in pairs with spin up and spin down. The best experimental probe of such pair correlations are pair-transfer reactions such as (p,t) and (3He,n) in which a localized pair of neutrons or protons is removed or added. The former reaction has now been studied  by an ANL-LBNL-Yale-Manchester collaboration. Representative spectra are given in figure 2. From the available data it appears that there may be a serious problem with the approximations inherent to contemporary model descriptions; i.e., transitions are observed to occur that theory forbids from basic assumptions. This could significantly affect the matrix elements predicted for the decay of 130Te and needs further theoretical and experimental investigation. A complementary study of the (3He,n) is scheduled to start in early 2011.
Fig. 2: Triton spectra from neutron pair transfer reactions on 128Te and 130Te. The excitation energies and angular momentum transfer (l) are labeled for states of interest and l=0 states are shaded.
- N. D. Scielzo, et al., Phys. Rev. C 80, 025501 (2009)
- T. Bloxham, et al., Phys. Rev. C 82, 027308 (2010)