Wonseok Bae, Ph.D.
Postdoctoral Scholar in Rare-Event Searches: Dark Matter, Neutrino, and Nuclear Physics
Northwestern University — Department of Physics & Astronomy
Research
Neutrinoless double beta decay
Neutrinoless double beta decay
One of the deepest puzzles in physics is why the universe is dominated by matter, with almost no antimatter. Matter and antimatter should have been created in nearly equal amounts in the early universe and then annihilated almost completely — yet a tiny excess of matter, about one part in a billion, survived to form everything we see today (Figure 1). Leptogenesis offers an explanation: if neutrinos are Majorana particles — their own antiparticles — heavy Majorana neutrinos in the early universe could have decayed to generate a lepton asymmetry, which Standard-Model processes later converted into the matter–antimatter imbalance we observe. That same Majorana nature would reveal itself in a single rare nuclear decay.
That decay is neutrinoless double beta decay. In some even–even nuclei, two neutrons convert into two protons at once, emitting two electrons. Every such double beta decay seen so far also releases two antineutrinos (2νββ) — a rare process still allowed by the Standard Model. If the neutrino is a Majorana particle, the same transition can happen with no neutrinos at all: neutrinoless double beta decay, 0νββ (Figure 2). It has never been observed.
The two modes are told apart by the summed energy of the electrons (Figure 3): 2νββ gives a broad continuum, while 0νββ would appear as a single sharp line at the decay’s Q-value (Qββ ≈ 2039 keV for 76Ge). Detecting that line would prove lepton number is violated and bear on the matter–antimatter question above.
The measured half-life maps onto the effective Majorana mass mββ. As Figure 4 shows, today’s searches are pushing toward the band predicted for the inverted mass ordering; LEGEND-1000 aims to cover it and reach into the normal-ordering region.
Many isotopes and detector technologies have been applied to this search — in past experiments, in those running now, and in others planned for the future (Figure 5). 76Ge stands out: germanium detectors have the best energy resolution (~0.1% at Qββ), the crystal is both source and detector, and no known background makes a peak near Qββ.
The LEGEND experiment
LEGEND — the Large Enriched Germanium Experiment for Neutrinoless ββ Decay — searches for 0νββ in 76Ge, building on the earlier GERDA and MAJORANA Demonstrator programs. It runs deep under the Gran Sasso mountains in central Italy (Figure 6), where about 3600 m of water-equivalent rock overhead shields it from cosmic rays.
Germanium detector strings hang inside a liquid-argon cryostat that itself sits in a large water tank (Figure 7). Liquid argon acts as both a passive shield and an active veto: particles scattering in it produce 127 nm scintillation light — shifted by TPB and green WLS fibers and read out by SiPMs. Figure 7 also zooms into a single detector string, showing the WLS fiber shrouds and an HPGe detector on its support plate.
LEGEND runs in two stages. LEGEND-200 (~200 kg of enriched 76Ge) has taken data since 2023 and targets half-lives near 1027 yr. Its first search, combined with GERDA and MAJORANA, set a limit of T1/2 > 1.9×1026 yr (90% C.L.), i.e. mββ < 75–200 meV. Background is beaten down by a layered veto — a muon veto, a multiplicity cut, pulse-shape discrimination, and the liquid-argon veto — whose effect on the spectrum is shown in Figures 8 and 9.
LEGEND-1000 scales up to ~1000 kg and aims beyond 1028 yr, which demands another order-of-magnitude cut in background (Figure 10). Key upgrades: underground-sourced argon (far less 42Ar), an internal neutron moderator, and new light-guide fibers. Figure 11 shows the projected background and signal near Qββ from a Monte Carlo simulation.
Direct dark-matter search · other rare event searches
Dark matter and WIMPs
The universe contains far more mass than we can see. Its gravity shapes how galaxies rotate, binds clusters together, and molds the large-scale structure of the cosmos — yet it emits no light and has never been detected directly. This dark matter accounts for about a quarter of the total energy budget of the universe (Figure 1), and identifying it is one of the central goals of modern physics.
A leading candidate is the weakly interacting massive particle (WIMP) — a heavy, electrically neutral particle that barely couples to ordinary matter. If WIMPs exist, they should occasionally strike an atomic nucleus (Figure 2) and deposit a tiny amount of energy. Direct-detection experiments try to catch those rare recoils inside a shielded, ultra-quiet detector deep underground.
The LZ experiment
LUX-ZEPLIN (LZ) is one of the world’s most sensitive WIMP searches. It runs about 1.5 km underground at the Sanford Underground Research Facility (SURF) in South Dakota (Figure 3), where the rock overhead shields it from cosmic rays.
At its heart is a dual-phase xenon time projection chamber (TPC) holding seven tonnes of liquid xenon. When a particle scatters in the liquid it makes a prompt scintillation flash — the S1 signal. The ionization electrons it frees drift upward in an electric field into a thin gas layer, where they make a second, larger pulse — the S2 signal (Figure 4). Together, the S1–S2 delay gives the interaction’s depth, while the S1 and S2 light patterns give its horizontal position and energy; combining the S1 and S2 signals is what picks out the WIMP-like signal.
Two arrays of photomultiplier tubes (PMTs) above and below the xenon collect this light (Figure 5, Figure 6). LZ wraps the TPC in nested vetoes: a xenon “skin” region that tags gamma rays, and an outer detector of gadolinium-loaded liquid scintillator that catches neutrons, and PMTs in the surrounding water tank that flag cosmic muons by their Cherenkov light.
With this layered design, LZ has set among the world’s strongest limits on WIMP dark matter across a wide mass range (Figure 7).
Scintillator & wavelength-shifting materials
Detector material characterization
Scintillators and light-shifting materials are widely used in experimental particle and nuclear physics. Plastic scintillators form the trigger counters, hodoscopes, and calorimeter tiles of collider and fixed-target experiments, and the active planes of many neutrino and cosmic-ray detectors. Liquid scintillators instrument large neutrino observatories and fast-neutron spectrometers. Wavelength shifters convert hard-to-collect ultraviolet scintillation light into a detectable band in noble-liquid time projection chambers and calorimeters. Scintillating fibers build high-resolution tracking detectors and beam monitors. The same materials run throughout applied radiation detection as well — medical imaging, nuclear security, and space instrumentation.
This research program characterizes such materials — plastic and liquid scintillators, wavelength shifters, and scintillating and wavelength-shifting fibers (Figure 1).
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The measurements span light yield, quenching factor, absorption and emission spectra, light attenuation length, pulse-shape-discrimination (PSD) capability, and how these vary with temperature and dopant concentration.
The aim is not a single material but a systematic, long-term survey: characterizing a broad library of candidates — hundreds of them over time — and reporting useful, reproducible benchmarks to the research community and to industry.
These benchmarks also benefit large rare-event search programs, whose veto detectors rely on well-understood light-collection materials. The characterization setup is compact — a light or radiation source, the material under test, and a DAQ system — and an excellent platform for training students across the full arc of an experiment, from hardware to analysis.
Case study: scintillating and wavelength-shifting fibers
Plastic scintillating and wavelength-shifting (WLS) fibers have been used since the 1980s — WLS fibers in MINOS, NOvA, GERDA, and LEGEND-200, scintillating fibers in DØ and the LHCb SciFi Tracker. Near ultra-low-background detectors, though, fiber choice is set by radiopurity as much as by optical performance, since radio-impurities in the fiber itself generate background radiation. Scintillating-wavelength-shifting (Sci-WLS) fibers, a recently developed class, are attractive because they add scintillation to wavelength shifting and light guiding — raising the possibility of self-tagging fiber-borne backgrounds in a veto, which is valuable for rare-event searches such as LEGEND-1000.
Two papers benchmark these fibers: their optical behaviour — emission and absorption spectra, attenuation length — in Bae et al., JINST 21, P01027 (2026), and their response to ionizing radiation, the signal that makes self-tagging possible, in Bae et al., JINST 21, P03053 (2026).
- Response of wavelength-shifting and scintillating-wavelength-shifting fibers to ionizing radiationLead author
- Optical characterization of wavelength-shifting and scintillating-wavelength-shifting fibersLead author
- First results on the search for lepton-number-violating neutrinoless double beta decay with the LEGEND-200 experiment
- The Large Enriched Germanium Experiment for Neutrinoless ββ Decay: LEGEND-1000 preconceptual design report
- Interaction position, time, and energy resolution in organic scintillator bars with dual-ended readout
Talks
Selected presentations
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Notes
Notes
Occasional writing and short videos on physics.
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