Professor Barbeau's research interests are predominantly in the fields of neutrino and astroparticle physics. His active efforts are focused on (but not limited to) three major areas of research: work towards the first observation of coherent neutrino-nucleus scattering; novel searches for the dark matter in our universe; and searches for zero neutrino double beta decay. The unifying aspect of the work is the common need for new and creative detector development in order to solve some of the "hard" problems in low-background rare-event detection.
The measurement of coherent neutrino-nucleus scattering has been a holy grail in neutrino physics since its prediction almost 40 years ago. The scattering process is analogous to the coherent forward scattering of photons on atoms. This elastic scattering proceeds via the neutral weak current and benefits from a coherent enhancement to the cross-section. The enhancement, a purely quantum mechanical effect, is approximately proportional to the square of the number neutrons in the nucleus. The full coherence condition, when the wavelength of the scattering is longer than the size of the nucleus, is guaranteed for nearly all nuclear targets when neutrino energies are <10's of MeV. Such neutrinos are produced in copious amounts at nuclear power reactors; a fact which suggests that a deployment to a reactor is an ideal location for such a search. Neutrinos with slightly higher energies (>10's of MeV) are also available at stopped pion beams, a background-suppressing pulsed neutrino source.
In supernova dynamics, it is the coherent neutrino scattering process which has the larget cross-section, helping to expel material outward. As such, it should be measured in order to validate astrophysical models. A large enough detector can be used to efficiently detect neutrinos of all species from a nearby supernova, allowing a determination of the oscillation pattern and thus the total energy and temperature of the event. Also, a number of experimental searches for physics beyond the standard model can be performed with a coherent neutrino scattering detector. A flavor blind detector (10's of kg) operated at a nuclear reactor can perform searches for sterile neutrinos, provide a sensitive test of the weak nuclear charge and search for supersymmetric extensions to the standard model. Detectors that incorporate several target nuclei can probe the effect of non-standard neutrino interactions such as flavor changing or non-universal neutral current scattering. The coherent scatter cross-section is also highly sensitive to the neutrino magnetic moment. An experiment located at a stopped pion beam, such as the SNS in Oak Ridge Tennessee, can access neutrinos with slightly higher energies for which the coherence condition begins to break down. Such an experiment would then be able to probe the neutron structure of the target nuclei using neutrinos.
A question of detector development
To date, the inability to measure the coherent neutrino-nucleus scattering cross-section, for which the signature is typically a sub-keV nuclear recoil, can be attributed to the scarcity of available technology. What is needed is a detector with a large enough mass (>1 kg), low enough backgrounds and a low enough threshold (<1 keV). The experimental problem is really one of detector development. This is what we do. The PPC high purity germanium detector was developed to solve this impasse and resulted in the genesis of the Coherent Germanium Neutrino Technology (CoGeNT) collaboration.
More details on the status of this effort can be found here.
The current cosmological paradigm explains that baryonic matter (the stuff that we are made of) accounts for only a small fraction of the total mass of the Universe. The vast majority of the Universe's matter is composed of dark matter, so called because the only evidence for its existence has come via its gravitaional interaction. Today, evidence for the existence of dark matter is widespread: from the velocity dispersions of galaxy clusters, the rotation curves of galaxies, measurements of the mass distributions of colliding astrophysical objects like the Bullet cluster, to measurements of the Cosmic Microwave Background Radiation. We may even be beginning to see the first putative hints of dark matter annihilation from space based indirect detection experiments.
As it turns out
The low threshold, large mass and low backgrounds that have been demonstrated with PPC detectors make them especially sensitive to a few novel dark matter candidates. These candidates have interesting or obscure names like "light WIMPs" (for Weakly Interacting Massive Partiles), "axions" and "dark pseudoscalars", among many others. In 2009, the CoGeNT collaboration redeployed a PPC detector from a nuclear reactor to the Soudan Mine in Minnesota. Subsequent data have shown tantalizing hints of a potential signature of light WIMP dark matter in the form of an annual modulation of the interaction rate.
See how we are investigating these hints here.
With the resolution of the Solar Neutrino problem along with definitive measurements from atmospheric and long baseline neutrino experiments over the last 15 years, the physics community has been able to confirm the hypothesis that neutrinos oscillate from one type (flavor) to another. This means that neutrinos must have a finite, non-zero mass.
Neutrino oscillation measurements allow us to measure the differences (squared) between neutrino species. They do not, however, provide a measurement of the absolute neutrino mass scale, and it is experimentally difficult to determine the ordering (hierarchy) of the three known neutrino mass eigenstates.
Nature can help us
The double beta decay of certain even-even nuclei with the emission of two electrons and two neutrinos is an allowed standard model process, where the single beta decay of the nucleus is energetically forbidden. Zero neutrino double beta decay can only occur if the participating neutrinos (which do not escape the nucleus) have a Majorana nature (that is, if they are their own anti-particles). This violation of lepton number, which is a fundamental symmetry of nature, is an open quesiton in particle physics. The zero neutrino double beta decay rate is proportaional, with some model variance, to a composite measure of the masses of the neutrinos. An observation of this decay would decide the Majorana nature of neutrinos (that lepton number is not confirmed), and can help set the neutrino mass scale and hierarchy.
The EXO collaboration searches for zero neutrino double beta decay using a suite of experiments that contain 136Xe, which serves as both the source of the decay and the detector medium to detect the decay. EXO-200 is a low background liquid xenon detector that is currently operating underground in the Waste Isolation Pilot Plant (WIPP) site, NM. The EXO-200 prototype detector disovered the two neutrino decay mode of 136Xe, has produced the most accurate measurement of any two-neutrino double beta decay, and has obtained one of the two most stringent constraints on the double beta decay mass term. The collaboration is currently analyzing more data and upgrading the detector to improve its sensitivity. The collaboration is also looking ahead, developing the next EXO detector concept (nEXO) which will scale up to 5 tons of enriched liquid xenon. While it will initially be operated similarly to the EXO-200 prototype, taking advantage of the low backgrounds already demonstrated, nEXO is being designed such that it can be upgraded to have the capability to extract and tag the 136Ba daughter isotope in coincidence with an observed double beta decay.
Learn about EXO-200 and nEXO here.
Other Searches for Exotic Physics
Oftentimes in particle physics, many unplanned searches for new physics are made possible when a new detector technology is developed. A few examples for the above mentioned detectors include: Di- and Tri-Nucleon decay, electron decay, nuclear transitions into superdense states, Pauli Exclusion Principle Violation, Solar axions and SIMPs (Strongly Interacting Massive Particles).