Evolving images of the proton: hadron physics over the past 40 years

The quark structure of the proton.

The quark structure of the proton.

Almost 100 years ago Rutherford discovered the proton. Since then great efforts have been made to understand its properties and what it’s made of. It was once thought to be simple, but it turns out to be anything but.

So just what is a proton and how have our perceptions of this particle changed? To answer the question Michael Pennington from Jefferson Lab pieced together the story of the past 40 years for his JPhysG 40th anniversary article:

Once upon a time, the world was simple: the proton contained three quarks, two ups and a down. How these give the proton its mass and its spin seemed obvious. Over the past 40 years the proton has become more complicated, and how even these most obvious of its properties is explained in a universe of quarks, antiquarks and gluons remains a challenge.

Let’s go back just a few years first.  Some 15 years after the proton, the already theorised neutron was discovered by James Chadwick, and shortly afterwards the pion was theorised as a force carrier, meaning that the atom was pretty much complete. Indeed, this level of understanding was certainly enough to drive the start of nuclear fission and transmutation, but it was far from accurate. The picture of the atom and its original proton and neutron was still to be painted in any detail.

So what happened?

…discoveries first from cosmic rays  and then from the first accelerator experiments showed that protons, neutrons and pions were not alone inside the nucleus. Emerging was a whole universe (the femto-universe) inside each nucleus swirling with many strongly interacting particles, we call hadrons.

A proton moves along the light-cone with momentum P+. To an on-coming photon it appears like a pancake.

The parton model: a proton moves with momentum P+. To an on-coming photon it appears like a pancake.

Typically, hadrons are your protons, neutrons and mesons, and (we now know) are made up of quarks. But in physics, things are rarely exactly as they seem; there is always more.  More particles, more interactions and more complex physics than was ever first thought.

The discovery of the quark at SLAC 48 years ago and then the discoveries of the ‘November revolution’ in 1974 marked the start of a major period of discovery in physics; with new experiments and theories rapidly growing and developing our understanding of the subatomic scale. That the proton was made up of three quarks, and pions were not force carriers but mesons (two quarks) was just the beginning, augmenting the already established parton model, which treated hadrons as being made up of point-like constituents, called partons.

Then what of quarks?

Quarks could not just be bits of hadrons: bits with fractional electric charge. They had to have their own uniquely defining property. They carried a strong charge we call colour. While quarks have colour, hadrons are colour neutral: in analogy with electrically neutral atoms built of electrically charged constituents.

This discovery of colour gave birth to the theory of Quantumchromodynamics (QCD). And if quarks had colour, it meant that the force carriers, now called gluons, also have colour, or indeed, anticolour.

After this, there were great steps forward in complexity and understanding. With Pennington’s article we delve deep into the proton, quarks and the sea of particles that make up matter. And we’re well on the way to the big picture, even with the recent discovery of exotic hadrons like pentaquarks.

Forty years ago with the ‘confirmation’ of the quark constituents of hadrons and the ‘revelation’ of partons moving freely inside the femto-universe it seemed that hadron physics would soon be understood. Cold QCD would be a solved problem, and one could move on to search for new states of matter, like the quark-gluon plasma. However, over the past forty years we have discovered that colour confinement works in mysterious ways, bringing a surprising richness to hadron physics even when cold.

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Evolving images of the proton: hadron physics over the past 40 years
Michael R Pennington 2016 J. Phys. G: Nucl. Part. Phys. 43 054001

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Quotes and second image from Evolving images of the proton: hadron physics over the past 40 years Michael R Pennington 2016 J. Phys. G: Nucl. Part. Phys. 43 054001 Copyright IOP Publishing

Title image: By Arpad Horvath – Own work, CC BY-SA 2.5

Categories: Journal of Physics G: Nuclear and Particle Physics

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