Dr Egle Tomasi-Gustafsson | Dr Simone Pacetti – Probing the Proton: Understanding the Structure of Sub-Atomic Particles

To answer this question, Dr Egle Tomasi-Gustafsson from the University of Paris-Saclay in France and Dr Simone Pacetti from the National Institute of Nuclear Physics in Italy probe the structure of the proton. Through looking at recent experimental results, they are building our knowledge of the dynamics that occur between the components that make up the proton.

In their research, Dr Tomasi-Gustafsson and Dr Pacetti examine data from large-scale particle accelerator experiments. By looking at scattering and annihilation processes, they can learn more about the proton’s so-called FORM FACTORS – observables that describe the electric and magnetic currents inside composite particles – and look at the energy that is transferred throughout the process.

Scattering occurs when an electron and a proton collide in a particle accelerator. By changing the energy of the electron, this in turn changes its wavelength, and allows us to see how the different parts of the proton scatter off the electron.

Annihilation occurs when an electron collides with a positron. The positron is the anti-particle of the electron – it has the same mass as the electron, but opposite charge. Colliding the electron and positron transforms them into a pure energy state of neutral charge.

To explain the changes seen in the energy after scattering or annihilation, Dr Tomasi-Gustafsson and Dr Pacetti consider a ‘quantum vacuum’. In a quantum vacuum, particles and anti-particles can be created. As such, a proton and its antiparticle – the antiproton – can be created. Among the possible final channels, we look to the formation of a proton and antiproton pair.

Through these annihilation and scattering reactions, we see the creation of matter at extremely small time and distance scales, as we see the coexistence of the peculiar quark states that lead to the formation of the proton-antiproton pair. The identical quarks – so for the proton, two up quarks – move away, and one of them is attracted by the third quark of the opposite charge. This two-quark formation is called a diquark. As the system then expands and cools down following the annihilation reaction, the quarks form the proton and antiproton, along with their electric and magnetic properties.

As the proton is a fundamental building block of the atom, increasing our knowledge of its internal structure and dynamics helps us to deepen our understanding of the universe.