The experiments were carried out at the Swiss infrastructure for particle physics CHRISP at PSI.
Photo Credit: © Paul Scherrer Institute/Markus Fischer
Scientific Frontline: Extended "At a Glance" Summary: Muonium Gravity Experiments
The Core Concept: Researchers have generated an intense, cold beam of muonium—an exotic atom composed of an antimuon and an electron—to test whether Earth's gravity acts on second-generation matter exactly as it does on ordinary matter.
Key Distinction/Mechanism: Unlike standard atoms, muonium contains a second-generation antiparticle (an antimuon) instead of a proton, ensuring the neutral charge necessary for gravity tests. By firing antimuons into superfluid helium cooled to near absolute zero (-273°C), scientists utilize chemical potential to eject muonium atoms vertically at uniform, "cold" speeds, which prevents the fast-decaying atoms from scattering and enables precise gravitational measurements.
Origin/History: The breakthrough methodology was detailed in a study published in Nature Physics by a research team at the Paul Scherrer Institute (PSI) and ETH Zurich, building upon the universality of free fall first recognized by Galileo Galilei and Isaac Newton.
Major Frameworks/Components:
- The Equivalence Principle: A cornerstone of Albert Einstein’s theory of gravitation, which posits that the equivalence between gravitational mass and inertial mass causes all bodies to fall at the same rate in a gravitational field.
- Standard Model of Particle Physics: The theoretical framework describing particle generations, where the muon and antimuon exist as heavier, second-generation siblings of the electron.
- Quantum Fluid Dynamics: The application of superfluid helium as a pristine environment devoid of impurities, where individual helium atoms lose their identity, allowing the muonium atoms to move unhindered.
- Interferometry: The planned measurement technique that will rely on the wave properties of atoms to detect tiny shifts in interference patterns caused by gravitational pull.
Branch of Science: Particle Physics, Quantum Physics, and Gravitational Physics.
Future Application: The high-intensity beam technology paves the way for advanced laser spectroscopy, allowing researchers to measure the muon's mass and fundamental physical constants with unprecedented precision, a capability that will be further expanded by the upcoming High Intensity Muon Beams (HIMB) facility.
Why It Matters: Demonstrating any discrepancy in how gravity affects exotic versus ordinary matter would fundamentally challenge our current understanding of physics and could provide the first evidence for a theoretical fifth fundamental force of nature.
Does gravity act equally on all particles in the universe, or are there differences between ordinary and exotic matter? Anna Soter and her team at ETH Zurich and the Paul Scherrer Institute (PSI) are investigating this question. “We have taken an important step toward carrying out an exciting experiment on this topic,” says the physics professor. “We want to measure the gravitational interaction of the muon.”
To do this, the research team uses an exotic atom called muonium. It resembles a hydrogen atom, but its nucleus contains an antimuon instead of a proton.
All matter surrounding us—including ourselves—consists of protons, neutrons, and electrons. Physicists refer to this as the first generation of particles. There are also two further generations comprising heavier particles. The muon, for example, is the heavier sibling of the electron and belongs to the second generation.
At PSI, researchers artificially produce muons and their antiparticles using a large particle accelerator. When a positively charged antimuon combines with a negatively charged electron, a neutral muonium atom is formed.
The Standard Model of particle physics describes the generational structure of matter. “But we physicists do not yet understand why these additional generations exist at all in the first place,” says Soter. “And why are there three in total?” Another particularly fascinating question is whether the increasing masses of the second- and third-generation particles behave in exactly the same way in terms of gravity as the lighter particles of the first generation.
The Enigmatic Principle of Equivalence
In ordinary matter, all bodies fall at the same rate at a given point within a gravitational field. This universality of free fall was recognized as early as the time of Galileo Galilei and Isaac Newton, and, as the equivalence principle between gravitational and inertial mass, it is a cornerstone of Albert Einstein’s theory of gravitation.
This puzzling principle, however, has so far only been demonstrated with ordinary matter or first-generation antimatter. By measuring the gravitational behavior of muonium, the researchers will investigate, for the first time, how a second-generation particle falls. “The exotic muonium is very well suited to this because it is a neutral atom,” explains Soter. “After all, to make something fall, you need something neutral.” In the case of a charged particle, the weak effect of gravity would be masked by stray electromagnetic fields.
However, there are also major difficulties: Muons decay very quickly, after just 2.2 microseconds. Additionally, previous sources of muonium produced atoms that flew off in many directions at different speeds and were therefore unsuitable for precise experiments.
At PSI, at a facility known as the Swiss Research Infrastructure for Particle Physics (CHRISP), researchers have now achieved a breakthrough. “We have managed to produce the muonium atoms in a ‘cold’ state, which is what makes the gravity experiment possible in the first place,” says Soter. “In this case, ‘cold’ means that the atoms propagate at similar speeds, almost parallel to one another.”
This new muonium beam is also exceptionally bright—a feature that is essential for a precise measurement. To measure gravity, scientists watch how atoms fall. Yet the effect of gravity is tiny, so the longer the atoms can fall, the more precise the measurement. “As muonium atoms have such a short lifetime, we need to start with a large number so that enough survive long enough to be measured after falling for a few microseconds,” explains Soter.
Quantum Fluid Providing the Decisive Boost
This was achieved using a new method for generating muonium atoms, as the researchers report in the journal Nature Physics. “In order to achieve this, we used superfluid helium that had been cooled close to absolute zero at minus 273 degrees Celsius,” explains Jesse Zhang, lead author of the study. “Superfluid helium is what is known as a quantum fluid, in which the individual helium atoms lose their identity, and which does not tolerate any impurities within it.”
The method: The antimuons produced in the PSI accelerator are fired into a thin layer of superfluid helium, where they are slowed down. When an antimuon encounters a free electron in the helium, a muonium atom with a positive chemical potential is formed and is effectively forced out of the liquid. When the muon reaches the surface, the chemical potential is converted into kinetic energy. The atom receives a boost and shoots vertically out of the liquid. “So we’re using the chemical potential as an atomic cannon,” explains Zhang.
It is crucial that the muonium atoms can move unhindered through the quantum liquid at a specific speed and without collisions; otherwise, given their extremely short lifetime, they would never reach the surface at all. “For our experiments, we also rely on PSI’s particle accelerator, which generates the world’s most intense, continuous muon beams,” says Soter. “Thanks to this high-quality source, a great many muonium atoms can be produced.”
Gravity Shifts Interference Patterns
As a next step, the researchers are now developing the apparatus they intend to use to measure the effect of gravity on the muonium beam—a so-called interferometer. It utilizes the wave properties of the atoms to generate an interference pattern. The tiny shift in this pattern caused by Earth’s gravitational pull makes it possible to determine the gravitational effect on muons. “We hope to be able to test the method for the first time with the atomic beam this year, and if all goes well, the actual gravity experiment should follow in two or three years,” Soter relates.
The new muonium beam is also expected to enable new experiments in laser spectroscopy. This would allow muonium to be measured much more precisely than was previously possible. This would provide insights into the mass of the muon and fundamental physical constants, which is also a future goal of the research group.
The new muonium beam is set to become even brighter in the future. With the new High-Intensity Muon Beams (HIMB), part of PSI’s IMPACT upgrade, the researchers will be able to create a beam with 100 times more muonium atoms. This will allow Soter and her team to increase the sensitivity of the gravity experiment. “This is a very exciting prospect for precision measurements, since our measurement is statistically limited,” says Soter.
But what if gravity acts differently on this exotic atom than it does on ordinary matter? “That would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force,” Soter outlines. To date, physics has assumed there are four fundamental interactions: gravity, electromagnetism, the strong interaction, and the weak interaction. Although a fifth force has been proposed time and again, it has never been proven.
However, such proof is not Soter’s primary aim. “I am completely open-minded,” she says. “I simply want to measure, for the first time, whether the equivalence between gravitational and inertial mass also applies to the second generation of particles—this alone is quite an inspiring piece of work.”
Funding: This research is supported by the National Center of Competence in Research (NCCR) Muoniverse.
Published in journal: Nature Physics
Authors: J. Zhang, A. Antognini, M. Bartkowiak, D. Goeldi, K. Kirch, A. Knecht, D. Taqqu, R. Waddy, F. Wauters, P. Wegmann, and A. Soter
Source/Credit: Paul Scherrer Institute | ETH Zurich/Barbara Vonarburg
Edited by: Scientific Frontline
Reference Number: phy091526_01