For over a decade, scientists have grappled with a subtle but significant challenge in the realm of particle physics: an imprecise understanding of the proton’s size. Now, a new measurement, published on , offers a level of precision that not only resolves previous discrepancies but also allows for a rigorous test of the Standard Model of particle physics – the prevailing theory describing the fundamental forces and particles of the universe.
The Standard Model has consistently proven remarkably accurate, but its continued validity relies on continually subjecting it to increasingly precise tests. This latest research, appearing in Nature, confirms the theory to within a tenth of a billionth of a percent, a testament to its enduring strength.
The Proton’s Radius: A Long-Standing Puzzle
The proton, along with neutrons and electrons, forms the building blocks of everyday matter. Determining its properties, including its radius, is crucial not only for understanding the fundamental nature of the particle itself but also for refining calculations related to other quantities, such as the mass of the neutron. Discrepancies in previous measurements of the proton’s radius created a bottleneck, hindering the ability to test the Standard Model with the desired level of accuracy.
The research team, led by Lothar Maisenbacher, focused on studying hydrogen atoms. They meticulously measured the frequency of radiation needed to induce transitions between different energy levels within the atom. This frequency is intimately linked to the proton’s radius. However, obtaining an accurate radius requires combining this measurement with other established data.
A Precise Measurement and Resolution of Discrepancies
The new measurement establishes the proton’s radius at approximately 0.84 trillionths of a millimeter. This figure aligns with a growing body of evidence suggesting the proton is smaller than earlier experiments indicated. Crucially, the precision of this measurement is sufficient to rule out the approximately 4 percent larger radius reported by some previous studies. This resolution of conflicting data was a key prerequisite for a meaningful test of the Standard Model.
The team’s success hinged on differentiating between the previously conflicting measurements. Once confident in the smaller radius value, they were able to leverage another measurement, obtained using an exotic form of hydrogen called muonic hydrogen, to further validate their findings. Muonic hydrogen replaces the electron with a heavier particle called a muon, which orbits closer to the proton, making it more sensitive to the proton’s charge radius.
The Role of Penning Traps and Cyclotron Frequencies
While this study focused on hydrogen spectroscopy, other researchers have employed different techniques to precisely measure the proton’s mass. A team led by Sven Sturm at the Max Planck Institute for Nuclear Physics utilized a Penning trap – a device that uses electric and magnetic fields to confine ions – to determine the proton’s mass with unprecedented accuracy. Published in , their work achieved a precision of 32 parts per trillion, a threefold improvement over previous measurements.
The Penning trap method involves rapidly switching between trapping protons and carbon-12 ions. By measuring the ions’ cyclotron frequencies – the rate at which they circle within the magnetic field – and knowing the mass of carbon-12 with extreme precision (defined as exactly 12 atomic mass units), the researchers were able to calculate the proton’s mass. Their result, mp = 1.007276466583 atomic mass units (amu), was slightly lower than the previously accepted value, differing by about 300 parts per trillion.
Implications for Future Research
The improved precision in determining the proton’s mass and radius has broader implications. The proton’s mass is a fundamental constant used in numerous calculations within physics. A more accurate value refines these calculations, potentially shedding light on unresolved questions in the field. For example, the Sturm group’s lower proton mass value may help resolve discrepancies in measurements of the helium-3 mass.
The ability to test the Standard Model with such high precision is a significant achievement. While the theory continues to hold up under scrutiny, these rigorous tests are essential for identifying potential areas where the model may need refinement or extension. The ongoing pursuit of greater precision in particle physics promises to deepen our understanding of the universe and the fundamental laws that govern it.
Recent advancements also extend to the study of atomic nuclei. Scientists have recently reported a record-breaking, watermelon-shaped nucleus, demonstrating the complexity and ongoing discoveries within nuclear physics. Further research into the structure of nuclei, alongside precise measurements of fundamental particles like the proton, will continue to push the boundaries of our knowledge.
