Majorana neutrinos remain elusive

10 November 2017

Researchers at the Cryogenic Underground Observatory for Rare Events (CUORE), located at Gran Sasso National Laboratories (LNGS) in Italy, have reported the latest results in their search for neutrinoless double beta-decay based on CUORE’s first full data set. This exceedingly rare process, which is predicted to occur less than once about every 1026 years in a given nucleus, if it occurs at all, involves two neutrons in an atomic nucleus simultaneously decaying into two protons with the emission of two electrons and no neutrinos. This is only possible if neutrinos and antineutrinos are identical or “Majorana” particles, as posited by Ettore Majorana 80 years ago, such that the two neutrinos from the decay cancel each other out.

The discovery of neutrinoless double beta-decay (NDBD) would demonstrate that lepton number is not a symmetry of nature, perhaps playing a role in the observed matter–antimatter asymmetry in the universe, and constitute firm evidence for physics beyond the Standard Model. Following the discovery two decades ago that neutrinos have mass (a necessary condition for them to be Majorana particles), several experiments worldwide are competing to spot this exotic decay using a variety of techniques and different NDBD candidate nuclei.

CUORE is a tonne-scale cryogenic bolometer comprising 19 copper-framed towers that each house a matrix of 52 cube-shaped crystals of highly purified natural tellurium (containing more than 34% tellurium-130). The detector array, which has been cooled below a temperature of 10 mK and is shielded from cosmic rays by 1.4 km of rock and thick lead sheets, was designed and assembled over a 10 year period. Following initial results in 2015 from a CUORE prototype containing just one tower, the full detector with 19 towers was cooled down in the CUORE cryostat one year ago and the collaboration has now released its first publication, submitted to Physical Review Letters, with much higher statistics. The large volume of detector crystals greatly increases the likelihood of recording a NDBD event during the lifetime of the experiment.

Based on around seven weeks of data-taking, alternated with an intense programme of commissioning of the detector from May to September 2017 and corresponding to a total tellurium exposure of 86.3 kg per year, CUORE finds no sign of NDBD, placing a lower limit of the decay half-life of NDBD in tellurium-130 of 1.5 × 1025 years (90% C.L.). This is the most stringent limit to date on this decay, says the team, and suggests that the effective Majorana neutrino mass is less than 140−400 meV, where the large range results from the nuclear matrix-element estimates employed. “This is the first preview of what an instrument this size is able to do,” says CUORE spokesperson Oliviero Cremonesi of INFN. “Already, the full detector array’s sensitivity has exceeded the precision of the measurements reported in April 2015 after a successful two-year test run that enlisted one detector tower.”

Over the next five years CUORE will collect around 100 times more data. Combined with search results in other isotopes, the possible hiding places of Majorana neutrinos will shrink much further.

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