In some ways, pions seem simple. Pions are the lightest particles to experience the strong force, one of the four forces in nature that give rise to our universe. We also know that pions consist of a quark paired to an antiquark. But until recently scientists did not understand the internal structure of these seemingly simple particles. One reason is that pions are extremely short-lived, making analysis difficult. Now, an advance in supercomputer calculations using lattice Quantum Chromodynamics (QCD) may allow scientists to provide an accurate and precise description of pion structure for the first time.
In addition to being the simplest particles built of quarks, pions are much lighter than any of the other sub-atomic particles that are built of a quark and an antiquark. Scientists have found pions inside the atom’s nucleus, mediating the interactions between protons and neutrons. A better understanding of the internal structure of pions will lead to a better grasp of the inner workings of the subatomic realm. This will advance our grasp of the processes that underlie the standard model of particle physics.
The internal structure of the pion is very difficult to extract from data collected from physical measurements. Theorists can help simplify this process by providing a new, clean description of the pion’s structure from the theory that describes such particles. To do so, nuclear theorists have conducted virtual “experiments” using supercomputers. The current study used two supercomputers to calculate the quark interactions within pions. The study calculated these interactions as if the pions were involved in physical experiments. From that information, theorists then derived the matching formalism, known as factorization. Factorization allows for comparison of the calculated structure of the pion from theory with existing data from physical measurements. Using the new formalism with actual data, theorists extracted preliminary information on how quarks are distributed inside the pion. The first calculations with this method were made on supercomputers at the Oak Ridge Leadership Computing Facility and at Jefferson Lab. The theorists found that the results compared well to current physical measurements and existing theory models. The next step is to apply the same method for a quantity that describes the motion of quarks inside a pion.
This research was supported by the Department of Energy Office of Science, Office of Nuclear Physics within the framework of the TMD Collaboration. The research also used the facilities of the U.S. Lattice Quantum Chromodynamics Collaboration, also funded by the Office of Science, and the resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, a DOE Office of Science user facility. Support was also provided by the Office of Science Graduate Student Research program, the U.K. Science & Technology Facilities Council, and the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge.
Sufian, R., et al., Pion valence quark distribution from matrix element calculated in lattice QCD, Phys. Rev. D 99, 074507. [DOI: 10.1103/PhysRevD.99.074507]