In a remarkable advancement that is poised to redefine the precision of nuclear measurements, an international team of physicists led by the Paul Scherrer Institute (PSI) has achieved an unprecedentedly accurate determination of the atomic nucleus radius of muonic helium-3 ions. By leveraging cutting-edge laser spectroscopy techniques in an environment enriched by PSI’s unique proton accelerator facility, the researchers have refined measurements that probe the fundamental structure of matter with blistering precision. This breakthrough not only bolsters the consistency of theoretical nuclear models but also sets a new benchmark for atomic physics experiments worldwide.
Fundamentally, helium-3, a rare isotope of helium, exhibits intriguing features due to its nuclear composition consisting of two protons and just one neutron. Unlike the more abundant helium-4 isotope, which contains two neutrons, helium-3’s comparatively “slimmed-down” nucleus presents physicists with a cleaner and more tractable system for testing the fabric of nuclear forces. The challenge in directly measuring such a nucleus with extreme precision lies in the scale at which nuclear phenomena operate–on the order of femtometres, or quadrillionths of a meter. The current study pinpoints the radius of helium-3’s nucleus at approximately 1.97007 femtometres, a measurement made possible only by innovative experimental strategies.
Central to this precision is the use of muonic helium-3 atoms, which unlike conventional atoms, replace the two orbiting electrons with muons. Muons are elementary particles akin to electrons but about 200 times more massive. This increased mass drastically shrinks the muon’s orbital radius around the nucleus, resulting in a much stronger overlap between the muon’s wave function and the nuclear charge distribution. This enhanced interaction amplifies the sensitivity of spectroscopic measurements to the spatial extent of the nucleus, essentially turning the muon into an exceptionally effective “probe” of nuclear structure.
The formation and study of muonic helium-3 ions require exceptionally specialized infrastructure. At PSI, a proton accelerator generates intense beams of low-energy negative muons, approximately 500 muons per second at energies near one kiloelectron-volt. These slow muons can be captured by helium-3 atoms to form muonic helium-3 ions suitable for precision spectroscopy. PSI’s in-house beamline technology is currently the only facility in the world capable of producing such low-energy, abundant muon beams ideal for this experiment, underscoring the institute’s worldwide scientific prominence.
Laser spectroscopy is the cornerstone technique for determining the nuclear charge radius in this experiment. The team developed a highly specialized, custom-built laser system capable of emitting pulses of light synchronized to the fleeting presence of a muon entering the experimental setup. A thin foil detector first identifies the passage of a muon, triggering an immediate laser pulse. When the laser light frequency resonates perfectly with a specific atomic transition in muonic helium-3, it excites the muon to a higher energy level. This excited state quickly decays within picoseconds, emitting X-ray photons whose characteristics inform the exact nuclear charge radius indirectly but with exquisite precision.
The physical principle underlying these measurements involves comparing the energy levels of the muonic atom to the theoretically predicted values, which depend sensitively on the size of the nuclear charge distribution. By fine-tuning the laser frequency until resonance is achieved, the experimenters can deduce the exact dimensions of the nucleus with unparalleled accuracy. This method has matured substantially in recent years, supported by the successful measurement of muonic hydrogen and helium-4, which preceded this helium-3 investigation.
The results of this latest study dovetail conspicuously well with existing data acquired through alternative measurement techniques, which historically faced larger uncertainties. Intriguingly, the helium-3 measurement from PSI exhibits approximately fifteen times greater precision than earlier experiments, significantly tightening the constraints on nuclear theory. This precision enables theorists employing ab initio nuclear models—calculations rooted directly in the fundamental interactions between protons and neutrons without adjustable parameters—to benchmark and refine their predictions with an unprecedented level of confidence.
Moreover, these precise charge radii carry profound implications beyond nuclear structure alone. Helium ions with more or fewer electrons serve as critical testbeds for validating quantum electrodynamics (QED), the quantum field theory describing electromagnetic interactions. By comparing muonic atom results against data from conventional helium ions and neutral helium atoms—both elegantly complex “few-body” systems—physicists can scrutinize QED predictions under extreme conditions. Discrepancies might hint at new physics, possibly transcending the current Standard Model of particle physics, thereby advancing fundamental science.
A particularly captivating historical backdrop is the earlier measurement of muonic hydrogen conducted by PSI’s team, which initially stunned the community by revealing a proton radius smaller than values obtained by traditional electron-based methods. This “proton radius puzzle” triggered intense debate regarding measurement validity and underlying theory. However, numerous independent confirmations since then have affirmed the accuracy of that result, establishing robust trust in muonic atom spectroscopy as a precise tool for nuclear characterization.
The successful helium-3 measurement was enabled not only by PSI’s unique technical capabilities but also by sophisticated instrumentation innovations developed by the research team. The real-time detection and immediate triggering of the laser pulse required the integration of ultra-thin detectors and state-of-the-art timing electronics. Such technical prowess surmounted the challenge of muons’ extremely short lifetimes and fleeting atomic states, highlighting the convergence of experimental physics ingenuity and advanced engineering.
Looking ahead, Aldo Antognini and collaborators envision expanding these techniques to investigate hyperfine structure in muonic atoms. Hyperfine splitting involves subtle energy separations that arise from interactions between nuclear spin and magnetic moments, unlocking a new dimension of insight into nuclear properties and the interplay of fundamental forces. Experiments probing muonic hydrogen hyperfine transitions are underway, with plans to extend similar studies to muonic helium. Realizing these ambitions demands significant enhancements in laser power and precision, with ongoing developments at PSI and ETH Zurich aiming to meet these stringent requirements.
The wider international scientific community has recognized the impact of this milestone, with complementary follow-up experiments currently progressing at institutions across Amsterdam, Garching, China, and Switzerland. These collaborative efforts underscore the global importance of ultrahigh-precision nuclear measurements to challenges in atomic and nuclear physics that resonate well beyond their apparent scale. As these precision probes evolve, they may ultimately contribute crucial clues in the continuing quest to uncover physics beyond prevailing paradigms.
In sum, this landmark achievement in measuring the nuclear radius of muonic helium-3 epitomizes the fusion of innovative experimental design, sophisticated theoretical frameworks, and relentless international collaboration. By peeling back the veil on atomic nuclei with extraordinary clarity, the study fuels progress toward a deeper understanding of the universe’s fundamental building blocks and inspires new frontiers for quantum science and technology.
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Subject of Research: Not applicable
Article Title: The helion charge radius from laser spectroscopy of muonic helium-3 ions
News Publication Date: 22-May-2025
Image Credits: © Scanderbeg Sauer Photography
Keywords
Muonic helium-3, nuclear charge radius, laser spectroscopy, Paul Scherrer Institute, proton accelerator, muons, atomic nucleus, quantum electrodynamics, ab initio nuclear theory, hyperfine splitting, fundamental physics, precision measurement
Tags: atomic nucleus radius measurementbreakthroughs in nuclear force testingfemtometre scale measurementshelium-3 isotope characteristicsinternational collaboration in physics researchlaser spectroscopy techniques in physicsmuonic helium-3 ions researchnuclear composition of helium-3nuclear physics advancementsprecision in atomic physics experimentsproton accelerator facility innovationstheoretical nuclear models consistency