Two PA graduate students aim to resolve the mysteries of the gluons inside the nucleon

  • Feb 28, 2019
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Two PA graduate students, Zhouyou Fan and Adam Anthony, tackled a difficult to calculate problem in particle theory and recently published results in Physical Review Letters (PRL):

photo of Zhouyou Fan and Adam Anthony
PA Graduate Students Zhouyou Fan and Adam Anthony

Nucleons (that is, protons and neutrons) are the building blocks of all ordinary matter, and the study of nucleon structure is a critical part of the mission of the U.S. Department of Energy, being pursued by multiple DOE laboratories, Brookhaven National Laboratory in New York and Jefferson Lab in Virginia, as well as the future planned Electron-Ion Collider (EIC). Gluons and quarks are the underlying degrees of freedom that explain the properties of nucleons, and fully understanding how they contribute to the properties of nucleons (such as its mass or spin structure) helps to decode the last part of the Standard Model that rules our physical world.

In the theory quantum chromodynamics (QCD), a branch of the Standard Model, gluons strongly interact with themselves and with quarks, binding both nucleons and nuclei. However, due to their confinement within these bound states, we cannot single out individual such particles to study them, and the predicted state that is made up of gluons only has yet to be experimentally observed. Therefore, the properties of gluons remain the most mysterious even after decades of experimental efforts to smash up nucleons.

In their recent work, Fan and Anthony, together with group members Dr. Yi-Bo Yang and Prof. Huey-Wen Lin, used the HPCC cluster at iCER to explore gluon operators that probe the probability distribution within nucleon using a theoretical method called “lattice QCD”, which simulates the four-dimensional spacetime of a femtoscale (10−15-meter) world. The team showed that the lattice-QCD results for these matrix elements are consistent with the Fourier transform of the global fit to available experimental data, such as CTEQ-TEA analysis. This work shows, for the very first time, that doing such a calculation with lattice QCD is possible. Future calculations will aim at improving the systematic uncertainties from renormalization and use larger simulation volumes and higher resolution.