Research Overview
My research is focused on finding experimental answers to some of the most fundamental questions about the nature of matter. Over the past few decades physicists have developed and experimentally confirmed to high precision the so-called "Standard Model" of particle interactions. Although this model is extraordinarily succesful, we know that it has to be incomplete. Three of the major issues that it does not address are the following:
- Why is there so much more matter than anti-matter in the universe?
- Is the neutrino, a type of fundamental particle, its own anti-particle?
- What is the nature of the dark matter that pervades our universe?
I am a member of the Experimental Nuclear and Particle Astrophysics (ENAP) group at UNC. I also work at the Triangle Universities Nuclear Laboratory (TUNL).
MAJORANA/LEGEND
Neutrinoless double-beta decay (NDBD) is a hypothetical nuclear decay associated with the emission of two electrons and no neutrinos from an atomic nucleus. The discovery of this currently unobserved decay would have the following significant implications:
- It would imply that the neutrino is a Majorana fermion; in other words it is its own antiparticle.
- It would be the first evidence of total lepton number violation.
- It would provide further evidence that at least two of the three known neutrinos have mass.
- The measured half-life of the decay could provide a measurement of the neutrino mass-scale.
The existence of Majorana neutrinos also has significant cosmological implications, since it is a general requirement of most
leptogenesis theories that may explain the baryon asymmetry in the universe.
It would also provide a key component to physical theories that attempt to explain the dominance of matter over antimatter in the universe.
I'm involved with two NDBD projects.
The
MAJORANA experiment
is searching for NDBD in the Ge-76 nucleus using arrays of enriched HPGe detectors deployed in ultra-low background electroformed copper cryostats and shield. It is operating almost a mile underground at the
Sanford Underground Research Facility (SURF) in Lead, South Dakota.
Schematic of the MAJORANA DEMONSTRATOR
Installation of HPGE Detectors in MAJORANA
I'm also a member of the LEGEND collaboration. We are funded to deploy a 200-kg array of enriched HPGe detectors underground at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. We are are also pursuing funding for a tonne-scale version of LEGEND. I'm particularly interested physics other than NDBD that can be done with these experiments, such as searches for dark matter.
Axion Dark Matter
The nature of dark matter (DM) remains one of the universe's greatest mysteries. As the gravitational evidence for DM continues to accumulate, the lack of laboratory evidence for DM becomes ever more acute. Axions belong to a broad class of well-motivated dark matter models consisting of light pseudoscalar particles coupled weakly to ordinary matter. The most famous example is the QCD axion, a consequence of the Peccei-Quinn (PQ) mechanism originally proposed to solve the strong-CP problem: the neutron electric dipole moment (EDM) is much smaller than dimensional analysis would suggest, suggesting a fine-tuning of parameters. In the PQ model, the axion couples to the strong nuclear force (quantum chromodynamics, or QCD) so that the lowest-energy field configuration is one where the neutron EDM vanishes. The QCD axion also acquires a small mass that is proportional to its couplings to ordinary matter. The axion is an economical and elegant solution to two seemingly unrelated problems, the nature of DM and the smallness of the neutron EDM.
Rendering and picture of ABRA-10cm prototype.
ABRACADABRA (A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus) is a new search for sub ueV axions and axion-like particles. It relies on a novel toroidal magnet design in which the axion dark matter would couple to the magnetic field and induce a very small oscillating magnetic field in the center of the torus. This field can be detected using SQUIDs or other quantum sensors. We published our first results in 2018 and developing proposals for the next phases
Other Work
At UNC and TUNL I have worked on measurements of neutron scattering differential cross-sections in neon and argon. These are germane to dark matter experiments and LEGEND. I have also worked on searching for symmetry violating effects in ortho-positronium decays. Prior to arriving at UNC I was involved with the Sudbury Neutrino Observatory and the Alpha Magnetic Spectrometer (AMS) experiments.
