Wonseok Bae, Ph.D.

Postdoctoral Scholar in Rare-Event Searches: Dark Matter, Neutrino, and Nuclear Physics

Northwestern University — Department of Physics & Astronomy

Google Scholar INSPIRE-HEP LinkedIn CV

Research

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.

Timeline of the universe from the Big Bang to today

Figure 1. The history of the universe. A tiny matter–antimatter asymmetry in the early universe left behind the matter that makes up everything we see today; leptogenesis, enabled by Majorana neutrinos, is a leading explanation for how that asymmetry arose.Image: NASA / WMAP Science Team. Not subject to copyright (NASA media usage guidelines); credit required.

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.

Feynman diagrams of 2-neutrino and neutrinoless double beta decay

Figure 2. 2νββ (left) emits two antineutrinos; 0νββ (right) proceeds via light Majorana-neutrino exchange, with no neutrinos in the final state.

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 (Q_ββ ≈ 2039 keV for ⁷⁶Ge). Detecting that line would prove lepton number is violated and bear on the matter–antimatter question above.

Summed electron energy spectrum: 2-neutrino continuum and 0-neutrino peak at Q-beta-beta

Figure 3. Summed electron energy: the 2νββ continuum versus the monoenergetic 0νββ peak at Q_ββ (linear, left; log, right). Not to scale.

The measured half-life maps onto the effective Majorana mass m_ββ. 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.

Effective Majorana mass versus lightest neutrino mass with LEGEND sensitivity bands

Figure 4. Effective Majorana mass vs lightest neutrino mass. Green = inverted ordering, red = normal; gray bands show the LEGEND-200 and LEGEND-1000 reach (NuFIT 6.0; KATRIN and Planck exclusions shown).

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 Q_ββ), the crystal is both source and detector, and no known background makes a peak near Q_ββ.

Landscape of 0-neutrino-double-beta-decay experiments: background versus exposure

Figure 5. The 0νββ landscape — background vs exposure. Dashed lines mark discovery-sensitivity half-lives; LEGEND-1000 targets beyond 10²⁸ yr.From M. Agostini, G. Benato, J. A. Detwiler, J. Menéndez and F. Vissani, arXiv:2202.01787 — Rev. Mod. Phys. 95, 025002 (2023). Licensed under CC BY 4.0.

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.

Geological cross-section and hall layout of the LNGS underground laboratory beneath the Gran Sasso mountains

Figure 6. The LNGS underground laboratory beneath the Gran Sasso mountains: a geological cross-section (top) and the layout of the three experimental halls (bottom). LEGEND-200 is housed in Hall A; the future LEGEND-1000 will be sited in Hall C.Figure adapted from R. Guidotti & A. Castellani, “Tunnel Vaults under Seismic Excitation”, IntechOpen (2018), Fig. 3. Licensed under CC BY 3.0. LEGEND-200 and LEGEND-1000 labels added by the author.

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-200 setup rendering and a zoomed view of an HPGe detector string with WLS fiber shrouds

Figure 7. Left: the LEGEND-200 setup, with the germanium strings and wavelength-shifting reflector (WLSR) inside the liquid-argon cryostat, surrounded by the water-tank PMT veto. Right: a detector string showing the WLS fiber shrouds and a single HPGe detector on its PEN support plate.From N. Burlac (on behalf of the LEGEND Collaboration), Nucl. Instrum. Methods A 1080, 170779 (2025). Licensed under CC BY 4.0.

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 T_1/2 > 1.9×10²⁶ yr (90% C.L.), i.e. m_ββ < 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-200 energy spectrum before and after veto cuts

Figure 8. LEGEND-200 spectrum (61 kg·yr): white = after muon + multiplicity cuts; red = after adding the LAr and PSD cuts. Inset zooms on Q_ββ.Reproduced from Fig. 1 of H. Acharya et al. (LEGEND Collaboration), “First Results on the Search for Lepton Number Violating Neutrinoless Double-β Decay with the LEGEND-200 Experiment,” Phys. Rev. Lett. 136, 022701 (2026), DOI: 10.1103/25tk-nctn; arXiv:2505.10440. Licensed under CC BY 4.0.

LEGEND-200 analysis window around Q-beta-beta

Figure 9. The Q_ββ analysis window; the dark line is the combined ⁷⁶Ge limit with GERDA and MAJORANA.Adapted from Fig. 2 (top panel) of H. Acharya et al. (LEGEND Collaboration), “First Results on the Search for Lepton Number Violating Neutrinoless Double-β Decay with the LEGEND-200 Experiment,” Phys. Rev. Lett. 136, 022701 (2026), DOI: 10.1103/25tk-nctn; arXiv:2505.10440. Licensed under CC BY 4.0. Changes: cropped to top panel.

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 Q_ββ from a Monte Carlo simulation.

Figure 10. LEGEND-1000: germanium strings in underground-argon reentrant tubes, inside an argon cryostat with a neutron moderator, all within a water-Cherenkov veto.CAD rendering by P. Krause / LEGEND Collaboration, from the collaboration's internal wiki (legend-exp.atlassian.net/wiki), where the original is used in LEGEND-1000 design-overview documentation shared within the collaboration. Labels added by the author.

Projected LEGEND-1000 background spectrum near Q-beta-beta

Figure 11. Projected LEGEND-1000 background and signal near Q_ββ from a Monte Carlo simulation, showing a hypothetical 0νββ peak at T_1/2 = 10²⁸ yr.From LEGEND Collaboration, arXiv:2107.11462 — LEGEND-1000 Preconceptual Design Report (2021). Licensed under CC BY 4.0.

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.

Pie chart of the energy budget of the universe

Figure 1. The energy budget of the universe: dark energy (Λ, 68%), dark matter (χ, 27%), and ordinary baryonic matter (B, 5%); neutrinos and photons make up the small remainder.From M. Cirelli, A. Strumia and J. Zupan, Dark Matter, arXiv:2406.01705 — SciPost Phys. Rev. 1 (2026). Licensed under CC BY 4.0.

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.

Feynman diagram of a WIMP scattering elastically off an atomic nucleus

Figure 2. A WIMP (χ) scatters elastically off an atomic nucleus (N) by exchanging a mediator, transferring a small recoil energy — the signal that direct-detection experiments try to catch.

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.

Sanford Underground Research Facility location, deep underground in South Dakota

Figure 3. LZ operates about 1.5 km underground at the Sanford Underground Research Facility (SURF) in South Dakota.Illustration: Greg Stewart / SLAC National Accelerator Laboratory.

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.

Dual-phase xenon TPC working principle: S1 and S2 signals

Figure 4. Dual-phase xenon TPC principle: a particle makes a prompt flash (S1); the freed electrons drift up and produce a second, larger flash (S2) in the gas.Adapted from LZ Collaboration, The Data Acquisition System of the LZ Dark Matter Detector: FADR, arXiv:2405.14732 — Nucl. Instrum. Meth. A 1068, 169712 (2024). Licensed under CC BY 4.0. Relabelled by the author.

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.

Figure 5. The LZ detector. A 7-tonne liquid-xenon TPC (1) sits at the centre of a water tank (2), viewed by top (3) and bottom (4) PMT arrays and held in a double-walled titanium cryostat (5). Gadolinium-loaded liquid scintillator (6) surrounds it, read out by its own PMTs (7) in the water; xenon-skin PMTs sit in the dome (8), and cabling leaves through the top (9) and bottom (10) conduits.From LZ Collaboration, The Data Acquisition System of the LZ Dark Matter Detector: FADR, arXiv:2405.14732 — Nucl. Instrum. Meth. A 1068, 169712 (2024). Licensed under CC BY 4.0.

Figure 6. The assembled xenon detector (left) and its top PMT array viewed from above (right).Photos by Matthew Kapust, Sanford Underground Research Facility, used with permission. From D.S. Akerib et al., Nucl. Instrum. Methods A 953, 163047 (2020), Figs. 2–3.

With this layered design, LZ has set among the world’s strongest limits on WIMP dark matter across a wide mass range (Figure 7).

LZ exclusion limit on the spin-independent WIMP-nucleon cross-section versus WIMP mass

Figure 7. LZ’s limit on the spin-independent WIMP–nucleon cross-section (WS2022 + WS2024) — among the world’s strongest across a wide range of WIMP masses.Reproduced from Fig. 5 of J. Aalbers et al. (LZ Collaboration), “Dark Matter Search Results from 4.2 Tonne-Years of Exposure of the LUX-ZEPLIN (LZ) Experiment,” Phys. Rev. Lett. 135, 011802 (2025), DOI: 10.1103/4dyc-z8zf; arXiv:2410.17036. Licensed under CC BY 4.0.

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).

A scintillation crystal surrounded by packaged scintillation products

Two vials showing Stokes and anti-Stokes emission under green laser excitation

Scintillating glass optical fibres glowing under ultraviolet illumination

Figure 1. Representative optical materials in radiation detection: a scintillation crystal surrounded by packaged scintillation products (left), wavelength shifting in solution — Stokes emission in red and anti-Stokes emission in blue (centre), and scintillating glass optical fibres (right), where UV-induced fluorescence stands in for scintillation.Left: Scintillation crystal with packaged scintillation products by Saint-Gobain Crystals, via Wikimedia Commons, public domain. Centre: Stokes and anti-Stokes emission by Victor Gray, via Wikimedia Commons, CC BY-SA 4.0. Right: U.S. Department of Energy, via Wikimedia Commons, public domain. Cropped to a common aspect ratio.

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).

Publications

Selected publications

Full list on INSPIRE-HEP and Google Scholar.

Response of wavelength-shifting and scintillating-wavelength-shifting fibers to ionizing radiationLead author W. Bae et al., Journal of Instrumentation 21 (2026) P03053.
DOI arXiv:2510.15913
Optical characterization of wavelength-shifting and scintillating-wavelength-shifting fibersLead author W. Bae et al., Journal of Instrumentation 21 (2026) P01027.
DOI arXiv:2509.20390
First results on the search for lepton-number-violating neutrinoless double beta decay with the LEGEND-200 experiment LEGEND Collaboration, Phys. Rev. Lett. 136 (2026) 022701.
DOI arXiv:2505.10440
The Large Enriched Germanium Experiment for Neutrinoless ββ Decay: LEGEND-1000 preconceptual design report LEGEND Collaboration, arXiv:2107.11462 (2021).
arXiv:2107.11462
Interaction position, time, and energy resolution in organic scintillator bars with dual-ended readout M. Sweany et al. (incl. W. Bae), Nucl. Instrum. Methods Phys. Res. A 927 (2019) 451–462.
DOI

Talks

Selected presentations

Invited talks

LEGEND experiment and fiber studies Eljen Technology (scintillator manufacturer), Texas, USA · 2023, 2024, 2025

Fiber studies for next-generation 0νββ searches in the LEGEND experiment HEAP seminar, University of California, Los Angeles, USA · 2025

Conference talks

Fiber studies to improve the liquid-argon detector for LEGEND-1000 APS Meeting (USA) & KPS Meeting (South Korea), with Eljen Technology representatives · 2025

Fiber studies for the LEGEND experiment LEGEND collaboration meetings · 2023, 2024, 2025, 2026

Notes

Occasional writing and short videos on physics.

Why We Invest in Basic Science?

Essay · September 2026

Basic science is funded mainly by governments — in other words, by taxpayers. In the United States, the federal government spends tens of billions of dollars on basic research each year. In fiscal year 2025, that amounted to about $45 billion—roughly $130 per person.

Public contributions support the search for new knowledge.

That money could be used elsewhere. It could pay for more scholarships for students who cannot afford college, another care center for people with disabilities, or someone's hospital bill. There are many places where a dollar can ease suffering right now. So why do we use this money to try to answer questions that may not seem useful in everyday life? For example, how do stars die? How do unstable subatomic particles decay? What is the faint light left from the universe's first moments?

Immediate needs and open-ended discovery both ask for our care.

I think there are two reasons.

We do not know where discovery will lead

Investing in basic science is like searching for new continents. Some expeditions discover continents rich in iron ore. Others find only empty continents of ice. But centuries later, a huge oil field may be discovered beneath that ice. Even a voyage that finds no valuable land can give us better compasses, better ships, and better maps. These tools help everyone who explores after us.

The value of a discovery may become clear long after the voyage.

The history of basic science shows this pattern again and again. Quantum mechanics began with scientists asking what happens inside an atom. No one expected it to make money. But this curiosity led to the transistor. Transistors led to the semiconductor industry, and that industry gave us modern electronic devices. Today, semiconductors are used in smartphones, cars, satellites, CT scanners, industrial robots, solar power, artificial intelligence, and quantum computers.

Quantum physics leads to transistors, semiconductor chips, and applications including smartphones, cars, satellites, CT scanners, industrial robots, solar power, artificial intelligence, and quantum computers.

The World Wide Web began in the same way. Physicists at CERN built a simple tool to share experimental data. That small tool is now the nervous system of modern society.

Scientific collaboration grows into a way to connect the world.

Space research also brings benefits to life on Earth. Weather and communication satellites are direct uses of space technology. Technologies developed for space research have also helped create or improve products on Earth: technology developed to make spacecraft cameras smaller and more energy-efficient is used in the CMOS image sensors in phone cameras, rocket-engine flow research helped improve heart-assist pumps, and precise GPS technology developed for space research helped tractors steer themselves. These benefits show how knowledge and tools can find uses far beyond their original goals.

Space research and its applications: satellites, a heart-assist pump, a self-steering tractor, and a CMOS camera sensor.

It tells us what we are

But the possibility that basic science may bring economic rewards someday is only part of the answer. It affirms the very value of human existence.

The universe is about 13.8 billion years old. The roots of human civilization go back around 12,000 years, to early farming communities. Compared with the universe, Earth is smaller than a speck of dust. Human life lasts only a moment in the history of the universe — we are here, then gone. Yet humans, who live only briefly on this tiny planet, have measured the age and size of the universe and are coming to understand how it began and took shape.

To see how remarkable this is, reverse the scale. Imagine a tiny, single-celled creature like a paramecium, curious about the world around it. It begins to explore and learn, passing what it learns to the next generation. Each generation adds to that knowledge. Eventually, these creatures discover that Earth spins and travels around the Sun. They understand why the Sun rises and sets, and how Earth’s tilted axis, as Earth moves around the Sun, causes the seasons to change. It may sound impossible, but this is what humanity is doing as we try to understand the universe.

A schematic of 13.8 billion years of cosmic evolution, with a spiral galaxy linked to a magnified Earth and human; alongside a paramecium and Earth’s tilted axis and orbit around the Sun. Not to scale.

What we inherited, and what we will pass on

We inherited our civilization from past generations. We stand on thousands of years of human effort. Now it is our turn to add a little more and pass it on.

Investing in basic science is one way we join this relay. We carry on the aspirations of past generations, move a little further forward in our own lives, and pass them on to the next generation. Human dreams never end.

Each generation receives knowledge and passes it forward.