Standard Model holds up under scrutiny of measurements
The jolt in the scientific world once seemed imminent. The scientific community was preparing for the foundation of the Standard Model (SM) - the modern theory of particle interactions - to shake as a result of another experimental test.
As a reminder, the Standard Model, laying the groundwork in the early 20th century, allows us to peer deeply into the microcosm of the atom and calculate the interactions of numerous constituent "bricks," including quarks, leptons, and bosons, both appearing and disappearing in the vacuum. Yet, it lacks an explanation for various phenomena such as dark matter (which constitutes 27% of the universe's contents), the gravitational particle, the enigmatic properties of neutrinos, the imbalance between matter and antimatter, and the acceleration of the universe's expansion.
"Although the SM works well, we absolutely know that it is not the final physical theory. It describes how nature functions at the scales and energies we have reached, - explains Ivan Logashenko, the deputy director of the Institute of Nuclear Physics at the Siberian Branch of the Russian Academy of Sciences (IJN primarily responsible for scientific work, head of the Department of Physics of Elementary Particles at Novosibirsk State University) on the institute's website. - When we extend beyond the familiar scales using astrophysical observations and study how the universe was structured during its early moments, we see that the SM can't explain many things. For example, we know that today the universe is filled with dark matter, but in the SM, there is no particle corresponding to it. In other words, something beyond the SM is undeniably present."
The first rumblings about the impending update to the Standard Model began in 2006. At that time, in Brookhaven National Laboratory (USA), during the Muon g-2 experiment involving the precise measurement of the anomalous magnetic moment of the muon - one of the subatomic particles participating in all known physical interactions (there are three known in the microcosm: electromagnetic, strong, and weak; there is a search for the fourth - gravitational) and has a relatively large mass and a relatively long, compared to the microcosm, lifespan (hence, it can be caught and studied).
The magnitude of the anomalous magnetic moment reflects the force of the muon's interaction with its magnetic field, while its anomalous magnetic moment reflects the interaction with short-lived, unobservable, or virtual particles filling the vacuum.
The hunt for calculating the anomalous magnetic moment is proceeding in two directions: theorists seek to improve the accuracy of predictions of this magnitude in the Standard Model, while experimentalists strive to improve the accuracy of its measurement.
If practice does not align with the theory, this means that science is approaching the discovery of a new, yet unpredicted particle, or the fifth force of nature, overturning the entire Standard Model, that is, our perception of the structure of the universe on the microscopic scale.
Then, almost 20 years ago, the results of the experiment for the first time showed a significant discrepancy with theoretical calculations - and this was a major shock to particle physics. Since then, this field of science has spent years in anticipation of a revolution, simultaneously perfecting both theoretical and experimental calculations of the anomalous magnetic moment.
In 2017, the Muon g-2 experiment was rebuilt and continued with a more intense muon beam and a finer tuning magnetic field at Fermilab - the US Department of Energy's laboratory near Chicago. In this project, data was collected 20 times larger. In 2021, after processing a significant portion of the data, preliminary results were announced, causing a sensation.
The difference between experimental and theoretical values of the anomalous magnetic moment amounted to 4.2 standard deviations, a considerable amount: with a confidence level of 5, one could confidently state the need to revise the entire concept of the Standard Model.
Everything was moving towards the fact that the SM in its current form would not withstand a stress test, and science was heading towards New Physics, in which many or even all existing gaps would be filled.
However, in 2023, the first signal was heard that the proclamation of a new era of science, it seems, is premature. It came from Novosibirsk, from the IJN, where scientists are already working on their "infernal machine" - located in a subterranean bunker at the institute's 400-meter collider VEPP-2000 research is being carried out that allows for the correction of predictions of the AMM in the Standard Model based on data obtained at the collision of electrons and positrons.
According to Ivan Logashenko, in the latest calculations, data is being taken into account that was not previously used, which pushes the theory forward. As a result, the calculations on VEPP-2000 were found to be much closer to the Muon g-2 experiment's measurements than previous theoretical predictions.
"The results obtained in Novosibirsk significantly change the perception of the problem of discrepancies in the anomalous magnetic moment of the muon. If before our measurements the physics community was almost ready to announce the discovery of New Physics, now the focus has shifted towards the idea that the Standard Model, as before, remains valid. It is necessary to continue increasing the accuracy of experiments and calculations," - Fedor Ignatov, a senior researcher at IJN, commented on the results at the time.
New Russian data motivated the Muon g-2 Theory Initiative collaboration, consisting of IJN researchers, to perform additional calculations of the anomalous magnetic moment using a different methodology, particle lattice calculations, and they confirmed the validity of the corrections made by our physicists.
It became clear that the longstanding struggle between theory and experiment was nearing its end. The final point in the spiraling saga of the Standard Model update was placed not long ago, after processing all the measurements of the Fermilab: in early June 2025, the Chicago laboratory released its official statement that the experimental results were surprisingly close to the theoretical predictions: no more than one deviation. The precision of the Muon g-2's measurements was 0.000013% - as the authors of the study themselves say, if such delicacy were used to measure the territory of the United States, it would be possible to notice the disappearance of even a single grain of sand.
"At the moment, this is the most detailed measurement of the anomalous magnetic moment of the muon. In the next 10 years, research at the J-PARC accelerator in Japan is planned, which is expected to break the Fermilab record, but this is still very distant future," - says Ivan Logashenko.
It is worth noting that experiments on muon colliders, in terms of precision, still far exceed theoretical calculations by several times, so the race between physicists to set new records for experimental measurements will continue.
"We plan to modernize the detectors at the VEPP-2000 collider and carry out a new round of measurements of the probability of hadron production at the collision of electrons and positrons and hope to break the world record for precision," - said Ivan Logashenko. - Our data will allow us to increase the precision of the AMMM calculation several times and bring it to the level of experimental precision. The higher the precision we achieve, the more closely we examine the Standard Model at smaller and smaller scales, or at higher energies. At the moment, we understand the structure and properties of matter at scales of approximately 1/1000 the size of a proton, that is, about one angstrom. To make further progress, it is necessary to carry out even more precise measurements and calculations."
At first glance, the attempts of scientists to unravel the chaos of particles inside an atom may have no connection with the real chaos of everyday life with its tangible disorder, but this is not the case. In particular, the projects of IJN, with the increase in the complexity of nuclear physics experiments, develop digital radiography technology, create equipment for the selective destruction of malignant cells in cancerous tumors using boron, and develop equipment for the diagnosis of the mechanical behavior of materials under various loads.
One day, the capabilities of science in penetrating inside matter will reach such a level that any macroscopic object, including a person, can be represented as the sum of the interactions of numerous elementary particles - protons, neutrons, electrons, muons, photons, positrons, pions, neutrinos, and many others - with their transformations and movements at speeds close to the speed of light.
- The ongoing search for a more accurate calculation of the anomalous magnetic moment of the muon, a subatomic particle significant to the Standard Model, could potentially reveal a new particle or the fifth force of nature, overturning the current understanding of the structure of the universe.
- Recent findings from the Institute of Nuclear Physics (IJN) in Novosibirsk, including data from the VEPP-2000 collider research, suggest that the Standard Model, currently undergoing updates, may still be valid, necessitating further increases in the accuracy of experiments and calculations to substantiate this theory.
