Scientists calculate predictions for meson measurements

Nuclear physics theorists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory demonstrate that complex calculations performed on a supercomputer can accurately predict the charge distribution of mesons, particles made of quarks and antiquarks. did. In a high-energy experiment at the Future Electron-Ion Collider (EIC), a particle collider under construction at Brookhaven Laboratory, scientists have discovered particles made of mesons and quarks, collectively known as hadrons. want to learn more about the class as a whole.

Predictions and measurements at the EIC will reveal how quarks and the gluons that organize quarks into hadrons generate the mass and structure of nearly all visible matter.

“The fundamental scientific goal of the EIC is to understand how the properties of hadrons, including mesons and the more familiar protons and neutrons, arise from the distribution of their constituent quarks and gluons.” said Swagat Mukherjee, a theorist at Brookhaven Institute who led the study.

Pions, the lightest mesons, play an important role in the nuclear strength force that binds protons and neutrons in the atomic nucleus. By exploring the mysteries of pions, protons, and other hadrons, the EIC helps scientists understand how everything made of atoms comes together.

The new predictions, just published in Physical Review Letters, are in good agreement with measurements from low-energy experiments at DOE’s Thomas Jefferson National Accelerator Facility (Jefferson Institute), Brookhaven’s partner in the construction of the EIC. It is also being extended to high-energy areas where In a new facility. These predictions are important because they provide a basis for comparison when the EIC experiment begins in the early 2030s.

However, this finding does more than establish expectations for a single EIC measurement. As described in the paper, the scientists used their predictions in conjunction with additional independent supercomputer calculations to validate a widely used approach for deciphering particle properties. I did. This approach, known as factorization, divides a complex physical process into two components, or factors. Validation of the factorization allows for more EIC predictions and more reliable interpretation of experimental results.

peer into hadrons

To study the internal structure of hadrons, EIC collides high-energy electrons with protons or nuclei. Virtual photons, or particles of light, emitted by electrons help characterize hadrons. It is like a microscope for observing the constituent elements of matter.

Collisions in the EIC allow precise measurements of various physical scattering processes. To convert these precise measurements into high-resolution images of the building blocks of matter within hadrons, scientists rely on factorization. This theoretical approach splits an experimental measurement (for example, the distribution of charge within a meson) into two components, so that scientists can use knowledge of two parts of the process to infer information about the third part. can.

Consider the formula X = Y × Z. A complete value X (experimental measurement) consists of two elements Y and Z. One element, Y, represents how quarks and gluons are distributed within the hadron. Another factor, Z, describes the interaction of these quarks and gluons with the high-energy virtual photons emitted by the colliding electrons.

Calculating the quark/gluon distribution is very difficult because of the strong interaction between quarks and gluons inside hadrons. These calculations involve billions of variables described by the theory of strong interactions known as quantum chromodynamics (QCD). Solving the QCD equations typically requires the use of powerful supercomputers to simulate interactions on an imaginary space-time lattice.

On the other hand, the interaction between quarks and gluons and virtual photons is relatively weak. Theorists can therefore derive their values ​​by doing pen-to-paper calculations. These simple calculations, in combination with experimental measurements (or predicted measurements), and mathematical relationships between those factors, are then used to solve equations regarding the distribution of quarks and gluons within hadrons. You can get an opinion.

“But does separating a phenomenon into these two components actually work?” asked Qi Shi, a visiting graduate student in Brookhaven Institute’s Nuclear Theory Group. “We had to prove it was possible.”

To do this, the scientists performed the factorization in reverse.

“We turned things around,” Shi said.

Shi and Xiang Gao, a postdoctoral researcher in his group, used a supercomputer and space-time lattice simulations to calculate the quark-antiquark distribution of mesons (Y in the equation above). They then used a simpler pen-and-paper calculation of the interaction of a quark/gluon with a photon (Z) to calculate the predicted value of the experimental measurement (X), which is the charge distribution inside the meson. I did it.

Finally, the scientists compared these new predictions to predictions made using another supercomputer’s calculations, one that matched the Jefferson Laboratory’s measurements at low energies. Comparing two predictions, one computed using factorization and one computed independently using a lattice simulation approach, shows that factorization You can test whether it is an effective way to solve the problem.

The inverse factorization calculations perfectly matched the predictions calculated by the supercomputer.

“In this case, you can use the lattice to fully calculate everything,” Shi says. “We chose this particular case because we can use independent computations to compute both the left and right sides of the equation and show that the factorization works.”

Scientists can now use factorization to predict and analyze other EIC observations, even if one side cannot be calculated directly.

“This study shows that the factorization approach works,” said Peter Petretsky, group leader and co-author of the paper. “Scientists will be able to use future EIC data and factorization to infer other, more complex distributions of quarks and gluons within hadrons that cannot be calculated using even the most powerful computers and sophisticated techniques. It became.”