The National Science Foundation Grant (#0605119), supported the role of

the University of South Carolina in the construction of a detector designed to

search for an exotic radioactive decay to test an important conservation law in

elementary particle physics. Several issues frustrating the completion of the

Standard-Model of Elementary Particle Physics, stem from the lack of detailed

knowledge of the most prolific elementary particle in the universe, the neutrino.

Because neutrinos interact very weakly with ordinary matter, they are very

difficult to detect. Neutrinos are emitted in radioactive beta decay; also they are

produced in a number of nuclear and elementary particle reactions. They are

emitted in the decay of fission products in nuclear reactors, in high-energy

particle collisions and in reactions in the stars, our sun for example. Experiments

that detect neutrinos from the sun, from cosmic rays, from reactors, and from

accelerator experiments, clearly demonstrate that neutrinos have mass;

however, these experiments are not capable of quantifying the mass scale.

Another critically important issue in elementary particle physics concerns

conservation laws. For example, throughout any physical process total energy

must be conserved, including mass energy. The same is true of angular

momentum. Other conservation laws involve particle families. Protons and

neutrons (baryons) are assigned a “baryon” number, +1 for protons and

neutrons and -1 for their anti-particles. Quarks have baryon number +1/3, while

anti-quarks have baryon number -1/3. Throughout all particle interactions, we

observe that baryon number is always conserved. The light particles (leptons),

electrons and positrons (anti-electrons), and their corresponding neutrinos, are

assigned “lepton” numbers. Electrons and neutrinos have lepton number +1,

while positrons and anti-neutrinos have lepton number -1. Thus far, lepton

number conservation is found to hold. Electric charge is always conserved

throughout all reactions. However, neutrinos do not have electric charge, so it

might be possible for reactions involving neutrinos to violate lepton number

symmetry while not violating other conservation laws. Because they have no

electric charge, neutrinos could be their own anti-particles. Testing these

symmetries is very important because our Standard Model of elementary

particles is based on symmetries. In beta decay, a nucleus emits an electron

(lepton number +1) and an anti-neutrino (lepton number -1). The lepton number

before the decay was zero and is also zero after the decay; therefore, lepton

number is conserved in beta decay. Is it ever violated? That is an important

question.

The only practical way to test lepton number conservation and at the

same time determine if neutrinos are their own anti-particles, is with a

hypothetical exotic radioactive decay called neutrino-less double beta decay. In

this process, a nucleus would emit two electrons and no anti-neutrinos, thereby

violating lepton number by 2. In our picture of neutrino-less double-beta decay, a

neutron emits an electron and an anti-neutrino, but that anti-neutrino is

absorbed by another neutron in the same nucleus, which then emits a second

electron. That changes two neutrons into protons, and violates lepton number

conservation. Also, for the re-absorption of the neutrino in the second neutron,

neutrinos must be their own antiparticles. In addition, the measurement of the

rate of the decay would establish the mass scale of neutrinos. Knowledge of

neutrino mass is extremely important for understanding the evolution of our

universe.

To address these problems, the PI joined Professor Ettore Fiorini of the

University of Milan-Bicocca in establishing the project called CUORE (Cryogenic

Underground Observatory for Rare Events). CUORE is an array of 988 thermal

detectors fabricated with tellurium dioxide (TeO2). Tellurium has a natural

abundance of about 34% tellurium-130 (130Te), a prime candidate isotope for

neutrino-less double-beta decay. The thermal detectors operate at about 10-one

thousandths of a degree absolute. To cool such a large mass to that temperature

requires an enormous dilution refrigerator and cryostat. This constitutes new

technology and a very large infrastructure, housed in a three-story building

underground in the Gran Sasso Laboratory in Assergi, Italy. The detector

construction is now complete and in the commissioning phase. CUORE will begin

taking data in the spring of 2017. The University of South Carolina group,

supported by this grant, was responsible for the construction, testing and

delivery of all of the front-end electronics. USC was also responsible for the

research and development to determine the feasibility and cost of reconstructing

CUORE with detectors fabricated from tellurium enriched to 95% in the isotope

130Te. The results of the enrichment study have recently been published. These

accomplishments by the USC group, as well as those of the entire CUORE

Collaboration, have made CUORE technology a solid candidate for the next

generation searches for neutrino-less double-beta decay. The CUORE

Collaboration has grown to 125 members representing 36 institutions

internationally and is poised to make major contributions to this important area

of physics.

Last Modified: 11/27/2016
Modified by: Frank T Avignone