CUORE, the Cryogenic Underground Observatory for Rare Events, has the goal of detecting neutrinoless double beta decay in 130Te, an isotope of the element tellurium. The experiment is located deep underground at the Laboratori Nazionali del Gran Sasso (LNGS) in Assergi, Italy, and is a collaboration between several European and American institutions.
Neutrinos are electrically neutral fundamental particles, known to interact with other particles only through gravity and the weak interaction. Originally thought to be massless, neutrinos have been shown to have mass through the discovery of neutrino oscillations in solar, atmospheric, and reactor experiments. Mass differences between the neutrino flavors, as opposed to the masses themselves, are responsible for the neutrino oscillations, and as such the absolute neutrino masses remain unknown.
All known elementary fermions other than the neutrino are known to have a distinct antiparticle. The nature of the neutrino is not yet settled, and it may be its own antiparticle; such a particle would be known as a Majorana fermion. A confirmation of the Majorana nature of neutrinos would have far-reaching effects and require significant modifications to the Standard Model, which is the foundation of our understanding of modern particle physics. A Majorana neutrino would imply that lepton number, an important symmetry of the Standard Model, is not conserved. This symmetry violation could be the key to understanding why matter dominated over antimatter in the early universe, and to understanding the large-scale structure of the universe today.
The search for neutrinoless double beta decay (0νββ) is the only currently feasible way to test the hypothesis that neutrinos are Majorana particles. Double beta decay is a rare second-order process involving the emission of two electrons from a nucleus simultaneously. In ordinary double beta decay (2νββ), which has been observed in many nuclides, two electron antineutrinos are released along with the electrons. In 0νββ, which can only occur if the neutrino is its own antiparticle, a virtual neutrino is exchanged in the process, but no neutrinos are emitted. 0νββ has never been observed. Experimentally, this process could be detected by looking at the energy spectrum of electrons emitted in many events. The emitted neutrinos cannot be detected, so the total recorded energy of 2νββ events will be spread over a large range, while the energy of 0νββ events will all be at a specific energy value, known as the Q-value, which is the difference in binding energy between the initial and final nuclei. If the 0νββ half-life can be measured for a specific nuclide, the effective Majorana mass of the electron neutrino can be deduced, albeit with significant uncertainties, by way of various theoretical models.
CUORE, a neutrinoless double beta search, is composed of 988 TeO2 crystals, and uses these crystals as both decay sources and bolometric detectors. The tellurium in the crystals is 33.8% 130Te, which is its natural isotopic abundance. Each crystal is a 5 × 5 × 5 cm3 cube and has a mass of 750 g; the entire detector has a mass of 742 kg, corresponding to 203 kg of 130Te. The crystals are arranged into 19 vertical towers and cooled to 10 mK (one hundredth of a degree above absolute zero) in one of the most powerful dilution refrigerators ever constructed. The crystals act as bolometers, ultra-low temperature calorimeters that rise in temperature when energy is deposited in them. Because they are so cold and therefore have such a low heat capacity, a 0νββ event inside a crystal will cause a measurable rise in temperature, which we use to determine the energy of such an event.
There are several advantages to the CUORE experimental scheme over other 0νββ searches. The natural isotopic abundance of 130Te is high, significantly higher than the natural abundance of other 0νββ candidate isotopes, so no isotopic enrichment is required. This makes a ton-scale experiment technically and financially feasible. The Q-value of 130Te 0νββ is also very well-established; it is fortuitously located above the Compton edge of the detector, so the electromagnetic background is minimal, and near but distinctly below a major background source (unavoidable 208Tl decay), which can be used for calibration. In addition, TeO2 bolometers are well-researched and provide high energy resolution, approximately 4 keV at the Q-value of 2527 keV, significantly greater than that of liquid scintillator detectors. All of these factors combined make CUORE one of the most promising of the current generation of 0νββ experiments.
Doctoral Dissertation
- A search for neutrinoless double-beta decay in tellurium-130 with CUORE, Jeremy Stein Cushman, Yale University, December 2017
Slides
- Dissertation Defense, Yale University, December 2017
- 2017 Fall Meeting of the APS Division of Nuclear Physics, Pittsburgh, PA, October 2017
- 2016 Fall Meeting of the APS Division of Nuclear Physics, Vancouver, BC, October 2016
- 2015 Fall Meeting of the APS Division of Nuclear Physics, Santa Fe, NM, October 2015
- 2015 American Physical Society April Meeting, Baltimore, MD, April 2015
- Weak Interaction Discussion Group Seminar, Yale University, February 2015
Posters
- 2017 International Conference on Topics in Astroparticle and Underground Physics (TAUP), Sudbury, ON, July 2017
- 2014 International Conference on Neutrino Physics and Astrophysics, Boston, MA, June 2014