Measurement of electromagnetic transition form factors in two-photon collisions at BESIII

The universe is held together by forces that are not always visible, and one of the most persistent mysteries in modern physics involves a tiny particle called the muon. This particle is a heavier cousin of the electron, and it possesses a magnetic property that makes it wobble as it spins. Scientists have calculated exactly how much this wobble should be based on the Standard Model, the best theory we have for how the universe works. However, when they measure the wobble in the real world, the result is different. This gap between prediction and reality is a sign that something is missing from our understanding, perhaps a new force of nature or a flaw in how we calculate the behavior of the strong nuclear force at low energies. To solve this puzzle, physicists must refine their calculations of how particles interact, specifically looking at how light and matter collide in ways that are difficult to predict with current math.

To get a clearer picture, researchers need precise data on how certain particles respond when struck by two beams of light at once. This process, known as two-photon fusion, is the key to understanding the "hadronic light-by-light" scattering that contributes to the muon's magnetic wobble. Theoretical models struggle to describe this interaction at the low energy levels where the most important effects happen. Consequently, scientists must turn to experiments that can observe these collisions directly. The BESIII experiment, located in Beijing, China, operates at an energy level that makes it uniquely suited to watch these rare events unfold. By colliding electrons and positrons, the machine creates a shower of particles, including neutral pions and other short-lived states, allowing physicists to measure how these particles are formed when hit by two photons.

In this latest work, the BESIII collaboration used a massive collection of collision data to measure the electromagnetic transition form factors of several light particles. A transition form factor is essentially a map that shows how a particle's internal structure changes when it is hit by a photon of a specific energy. The team focused first on the neutral pion, a fundamental particle made of a quark and an antiquark. Using 2.93 fb⁻¹ of collision data, they measured how the pion responds to a single virtual photon—a photon that exists only for a fleeting moment during the collision—across a range of low energies. The result was a highly precise measurement that fills a gap left by previous experiments. When compared to existing theories, the new data provides a stringent test, helping to distinguish between different mathematical models that attempt to describe the strong force. The measurements align well with some theoretical predictions while ruling out others, offering a clearer path toward calculating the muon's magnetic moment.

Beyond the single pion, the researchers also investigated what happens when two photons collide to produce a pair of neutral pions. This process is more complex because it involves intermediate states, or temporary resonances, that act as stepping stones between the initial collision and the final particles. The team analyzed the data to separate out the contributions of specific particles, including the scalar mesons known as f0(500) and f0(980), and the tensor meson f2(1270). By carefully sorting the data based on the energy of the collision and the angle at which the particles emerged, they were able to isolate the signals of these short-lived states. This allowed them to extract the transition form factors for these particles, yielding the first-ever measurement for the f0(500) TFF, alongside the most precise and lowest-energy determinations for the f0(980) and f2(1270) TFFs. The data revealed the internal structure of these particles with a precision that had not been achieved previously, showing how they interact with light at low energies.

The significance of these findings extends beyond just cataloging particle properties. The new measurements provide the critical experimental input needed to sharpen the theoretical predictions for the muon's anomalous magnetic moment. By reducing the uncertainty in the hadronic light-by-light scattering contribution, the work helps to clarify whether the discrepancy between theory and experiment is due to new physics or simply a limitation in our current models. The BESIII team has demonstrated that with large datasets and careful analysis, it is possible to probe the inner workings of these particles with unprecedented accuracy. Looking ahead, the experiment plans to use even larger datasets to measure the form factors of other particles, including the f1(1285), which is expected to be even more important for solving the muon mystery. These efforts represent a steady, methodical step toward resolving one of the most intriguing questions in particle physics, bringing the scientific community closer to understanding the fundamental forces that shape our universe.