The new Electron-Ion Collider could solve the mystery of how matter holds together

When the Nobel Prize-winning US physicist Robert Hofstadter and his team fired highly energetic electrons at a small vial of hydrogen at the Stanford Linear Accelerator Center in 1956, they opened the door to a new era of physics. Until then, it was thought that protons and neutrons, which make up an atom’s nucleus, were the most fundamental particles in nature. They were considered to be “dots” in space, lacking physical dimensions. Now it suddenly became clear that these particles were not fundamental at all, and had a size and complex internal structure as well.

What Hofstadter and his team saw was a small deviation in how electrons “scattered”, or bounced, when hitting the hydrogen. This suggested there was more to a nucleus than the dot-like protons and neutrons they had imagined. The experiments that followed around the world at accelerators – machines that propel particles to very high energies – heralded a paradigm shift in our understanding of matter.

Yet there is a lot we still don’t know about the atomic nucleus – as well as the “strong force”, one of four fundamental forces of nature, that holds it together. Now a brand-new accelerator, the Electron-Ion Collider, to be built within the decade at the Brookhaven National Laboratory in Long Island, US, with the help of 1,300 scientists from around the world, could help take our understanding of the nucleus to a new level.

Strong but strange force

After the revelations of the 1950s, it soon became clear that particles called quarks and gluons are the fundamental building blocks of matter. They are the constituents of hadrons, which is the collective name for protons and other particles. Sometimes people imagine that these kinds of particles fit together like Lego, with quarks in a certain configuration making up protons, and then protons and neutrons coupling up to create a nucleus, and the nucleus attracting electrons to build an atom. But quarks and gluons are anything but static building blocks.

A theory called quantum chromodynamics describes how the strong force works between quarks, mediated by gluons, which are force carriers. Yet it cannot help us to analytically calculate the proton’s properties. This isn’t some fault of our theorists or computers — the equations themselves are simply not solvable.

This is why the experimental study of the proton and other hadrons is so crucial: to understand the proton and the force that binds it, one must study it from every angle. For this, the accelerator is our most powerful tool.

Yet when you look at the proton with a collider (a type of accelerator which uses two beams), what we see depends on how deep — and with what — we look: sometimes it appears as three constituent quarks, at other times as an ocean of gluons, or a teeming sea of pairs of quarks and their antiparticles (antiparticles are near identical to particles, but have the opposite charge or other quantum properties).

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