My research is focused on finding experimental answers to some of the most fundamental questions about the nature of
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?
A common theme in my research is performing experiments at deep underground locations to reduce or eliminate the ionizing
radiation backgrounds from high-energy cosmic-rays (primarily muons) at the earth's surface. These cosmic-rays are extremely
penetrating and require miles of rock to shield against; hence the most sensitive experiments have to be located deep
underground in mines.
My research is also collaborative and I am a member of the
Experimental Nuclear and Particle Astrophysics (ENAP)
group at UNC.
I also carry out a significant fraction of my work at the
Triangle Universities Nuclear Laboratory (TUNL)
To address the questions listed above, I am working in several areas:
Neutrinoless double-beta decay
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.
There exist several compelling theoretical arguments for why neutrinos should have this unique property.
Proof of the Majorana nature of neutrinos would also imply physics beyond the Standard Model of particle
physics and force us the reconsider the fundamental nature of matter. NDBD is the only practical
experimental technique to determine if the neutrino is a Majorana fermion.
- 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.
My main area of involvement is with the
MAJORANA is an international, collaborative (~100 scientists, 20 institutions)
will search for NDBD in the Ge-76 nucleus using arrays of enriched HPGe detectors deployed in ultra-low background electroformed copper cryostats and
The first phase of MAJORANA is currently under construction almost a mile underground at the
Sanford Underground Research Facility (SURF)
in Lead, South Dakota.
In the past I have played a leading role in different parts of MAJORANA related to simulations, analysis,
underground infrastructure, and overall construction. I am currently leading the testing and commissioning task of the experiment and serve on the
executive board. I am also involved with research and development for a tonne-scale germanium experiment, with special emphasis on
physics other than NDBD that can be done with such an experiment (see below).
A well-known problem in contemporary cosmology is that of so-called dark matter, where a wide variety of astronomical observations indicate the existence
of large amounts of non-luminous matter in the universe.
The nature of this dark matter is a mystery and there currently exist many candidates.
There is strong reason to believe that dark matter is non-baryonic (ie. exotic), and that the mass of
non-baryonic matter in the universe is about 5 times that of normal baryonic matter.
One candidate for dark matter would be an unknown but stable particle or particles that currently exist in
cosmologically significant quantities and that interact at the weak scale.
These are called Weakly Interacting Massive Particles or WIMPs.
One method to search for WIMPs is via “direct detection”. If the WIMP hypothesis is correct,
then the earth is continuously plowing through a cloud of WIMPs that is trapped by the galaxy’s
gravitational well. Very rarely, a WIMP particle might scatter off an atomic nucleus in a
dedicated dark matter detector on earth, causing the nucleus to recoil. There are many different
experimental techniques for detecting this recoiling atomic nucleus and there are severa
l large and international experimental efforts underway using different experimental approaches
attempting to directly detect this dark matter. The MAJORANA experiment will be sensitive to light WIMPS with masses < 10 GeV and
will be complementary to some of these other searches.
We have deployed a prototype experiment to quantify backgrounds and test MAJORANA technologies to search for light WIMPS.
This experiment, called MALBEK (MAJORANA Low-background BEGe experiment at KURF),
was deployed at the
Kimballton Underground Research Facility (KURF)
and has been in operation since 2010.
Our first publication will be out soon and will help lay the ground work for a dark matter search
with MAJORANA and elucidate some of the backgrounds observed by previous experiments in this
mass range. One of my graduate students, Paddy Finnerty, wrote and defended his PhD thesis related to initial work on this experiment.
Prior to arriving at UNC I was involved with the Sudbury Neutrino Observatory
and the Alpha Magnetic Spectrometer (AMS)
experiments. Other projects that I have recently worked on or are in the planning phases include:
- Sean MacMullin, one of my graduate students, measured neutron scattering cross-sections
in neon and argon with collaborators from TUNL and Los Alamos.
These measurements are important for quantifying backgrounds in dark matter experiments that use Ne and Ar as detection mediums.
We performed the first measurement of the
scattering cross-section of neon and improved the existing dataset in argon, leading to three publications.
- With funding from the National Science Foundation we have deployed two low-background HPGe detectors underground at KURF to use for materials
assay. We are operating these detectors as a training facility for students and to perform assay for materials and parts in our experiments.
- I am also interested in using MAJORANA to search for solar axions. For this project it is
important to determine the orientation of crystal axes relative to the sun and
this is a problem we are working on at UNC. The background issues are similar to that encountered for a light WIMP search.
- I have recently become interested in looking for symmetry violations in positronium decays and am looking into possible experiments
we can do at TUNL.
Stay tuned for more updates on these projects!
last update: 11/20/2013