AMoRE experiment sets new limits on neutrinoless double beta decay of ¹⁰⁰Mo

In recent years, some large physics experiments worldwide have been trying to gather evidence of a nuclear process known as neutrinoless double beta (0νββ) decay. This is a rare process that entails the simultaneous decay of two neutrons in a nucleus into two protons, without resulting in the emission of neutrinos, which is instead associated with standard double beta decay.

The observation of neutrinoless double beta decay could have important implications for the study of matter and antimatter. In fact, it would confirm that neutrinos and their antiparticles (i.e., antineutrinos) are in fact the same, a hypothesis first introduced by theoretical physicist Ettore Majorana in 1937.

The AMoRE (advanced Mo-based rare process experiment) collaboration, a large international research team, has been searching for neutrinoless double beta decay using molybdate scintillating crystals operating at milli-Kelvin temperatures.

In a paper published in Physical Review Letters, the researchers published the results of their most recent search, setting new constraints that could guide future efforts aimed at observing this rare nuclear process.

“The neutrino is one of the elementary particles in the Standard Model. It was ‘invented’ by Wolfgang Pauli about a hundred years ago and discovered a couple of decades later than that,” Yoomin Oh, corresponding author of the paper, told Phys.org.

“It is the most abundant particle in the universe but many of its properties, including its mass, are still shrouded and not well explained by the Standard Model.”

The primary objective of the AMoRE experiment is to measure the mass of neutrinos and answer key research questions regarding the symmetry between matter and antimatter.

To achieve this, the AMoRE collaboration tried to observe neutrinoless double beta decay using molybdenum-100 (¹⁰⁰Mo), a radioactive isotope of molybdenum with an atomic number of 42 and a mass number of 100.

“If the neutrino and its antiparticle (antineutrino) are the same, as first suggested by Majorana, the decay process could occur without emitting neutrinos,” explained Yoomin.

“Because the probability of such decay is really low, one basically has to prepare a lot of the decaying isotopes and wait for the decay signal to be detected in a low background environment. This is how most double beta decay measurements are collected, including those we collect as part of the AMoRE experiment.”

To conduct its most recent search for neutrinoless double beta decay, the AMoRE collaboration prepared several kgs of molybdenum, enriched in ¹⁰⁰Mo, in the form of scintillating crystals. A particle interaction in these crystals produces a heat and light signal. A low-temperature detector system encapsulating the crystals was located 700-meter underground at the Yangyang Underground Laboratory in Korea.

“We performed the AMoRE-I experiment with the best sensitivity ever for observing neutrinoless double beta decay of molybdenum-100, but we have not found any evident signal,” said Yoomin. “The background-only result led us to set the most improved limit on the decay halflife of Mo-100.”

The new limits that the AMoRE collaboration set on neutrinoless double beta decay could help to better target future searches for this elusive process.

Meanwhile, the AMoRE collaboration plans to conduct further searches using detection systems at a newly built laboratory in Korea, dubbed Yemilab, which is 1000m underground.

“The next phase of AMoRE, AMoRE-II, is under preparation in Yemilab to start its data taking in a year,” added Yoomin.

“It is challenging to use about 100 kg of molybdenum-based crystal detectors in a large ultra-low temperature detector and achieve very low background at the same time. This upcoming experiment will be one of the most sensitive searches for neutrinoless double beta decay in the world.”

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