Double Beta Decay
The AMoRE Experiment
Advanced Mo-based Rare process Experiment
Why are we interested in neutrinoless double beta decay?
The recent discovery of neutrino oscillations clearly indicates that neutrinos do have mass, which is compelling evidence of physics beyond the standard model of particle physics. However, while those observations provide information about the differences in the neutrino masses, they do not provide a direct measurement of the absolute mass scale and they leave un-answered questions about the properties of neutrinos.
To answer to those questions, the AMoRE experiment will explore one of the most sensitive searches for “neutrinoless double beta decay” in which only two electrons are emerged from the nuclear-beta decays. If this decay were observed, it would be a clear signal that neutrinos have a different mass structure than other elementary particles and would provide a measurement of the absolute neutrino mass scale.
In the beta decay, one electron and one neutrino are emitted when a neutron is converted to a proton. In the two-neutrino double beta decay, two electrons are emitted with neutrinos when two neutrons are simultaneously converted to two protons. In neutrinoless double beta decay the two neutrinos are virtual and annihilate without being emitted.
What is the AMoRE experiment?
The AMoRE experiment is an international collaboration with about 90 collaborators from 16 institutions in 7 countries, China, Germany, Korea, Pakistan, Russia, Thailand and Ukraine. This experiment aims to search for the extremely rare process of neutrinoless double beta decay of 100Mo isotope using 40Ca100MoO4 crystal in a deep underground laboratory to minimize possible cosmic-ray backgrounds. The decays are observed by detecting tiny amounts of heat (phonons) and scintillation light (photons) emitted by the crystal in response to the energy of the emitted electrons. Among molybdenum-containing crystals, the CaMoO4 crystal shows the brightest scintillation light at room and cryogenic temperatures.
The current R&D detector (bottom) for the AMoRE experiment, we achieved about 10 keV FWHM of energy resolution at 2,615 keV from Th-232 source.
AMoRE-Pilot, the pilot phase of the AMoRE project, is an experiment that has been running in the 700-m-deep Yangyang underground Laboratory (Y2L) since summer 2015 using 100Mo-enriched, 48Ca-depleted 40Ca100MoO4 scintillating crystals. The experiment started and ran with five crystals, with a total mass of about 1.6 kg, until a sixth calcium molybdate crystal was installed in spring 2017, resulting in a total crystal mass of about 1.9 kg. Each crystal is equipped with a heat (phonon) detector and a light (photon) detector for a simultaneous measurement. When a particle interacts with the crystal, the produced heat and light are measured using metallic magnetic calorimeters (MMCs), which are highly sensitive temperature sensors operating at millikelvin temperatures. The MMC signals are then read via a superconducting quantum interference device (SQUID). MMCs are useful in particle physics experiments searching for rare events as they offer a fast response, high energy and timing resolutions, and good particle discrimination. A dilution refrigerator allows us to reach operating temperatures as low as 8 mK. After several runs and subsequent detector setup upgrades, the first long run of AMoRE-Pilot is currently underway.