An Introduction to Quantum Chromodynamics and the EIC

What if I told you that by obtaining a more detailed understanding of nucleons ( protons and neutrons ) , we could create technology millions of times more efficient than current nanotechnology ? While this speculative dream may be in the distant future and requires a deeper understanding of the quantum world, physicists have already studied this area for decades which has yielded fascinating results. In this short article, I will be exploring some basic concepts in the field of quantum chromodynamics, and how particle accelerators will ameliorate human kind’s understanding of the peculiar world of subatomic particles.

The etymologists among you might suspect that quantum chromodynamics has something to do with colour ( hence “chromo”); you are right ! The term originates from the fact that the terminology for charge involved with the nuclear strong force ( I will explain what this is later) is called “colour”. The strong force itself is one of the 4 fundamental forces of nature ( including the Weak Nuclear Force, Gravity and Electromagnetism ) and holds protons and neutrons together in the nucleus of atoms. Quarks, which make up protons and neutrons, carry this so called “colour” charge and interact with one another by exchanging gluons ( particles which hold quarks together inside of protons and neutrons ). Strangely enough, the way in which these gluons interact with each other is by exchanging other gluons which is what makes quantum chromodynamics ( henceforth QCD) so difficult to model and compute.

How exactly would do scientists aim to discern the structure of nucleons ? This would occur using the Electron-Ion Collider ( EIC ). Unlike existing colliders such as the Large Hadron Collider at CERN or the Relativistic Heavy Ion Collider, which collide composite particles ( those which are composed of two or more elementary particles, so, for example, a proton or an ion ), the EIC would only collide protons & neutrons with electrons.

In order to understand how colliding different particles together can possibly provide any sort of visual conception of the inner structure of nucleons, we must first understand how physicists were able to peer down to the atomic level ( of the order of nanometers (10^-9 m) compared to that of femtometers ( 10^-15 m) required to “see” inside protons ) in the 20th century. The process used was called x-ray diffraction”which consisted of shining a beam of x-rays at a certain material and studying the resultant interference pattern the rays produced. This permitted physicists to observe what is known as the substance’s atomic crystal structure. What permitted this sort of technology to yield results is that the wavelength of an x-ray is comparable to the atomic scale ( nanometers ) which permits the ray to probe inside atoms.

In order to peer into protons themselves, a similar method called deep inelastic scattering ( DIS ) was invented which collided electrons and protons ( or neutrons or nuclei ) rather than shining x rays into certain materials. During DIS, after an electron collides with a proton, something known as a virtual photon is exchanged between said electron and proton. It is known as “virtual” since it, arguably, isn’t real as it pops into and out of existence due to the laws of quantum mechanics, which govern particle interactions at the atomic and subatomic level .

Information can be extracted from this interaction in two ways; firstly by measuring the angle and energy of the recoiling electron and secondly by observing the interference pattern created by the virtual photon ( in the same way that physicists first observed by using x rays ). The higher the energy of this collision between electrons and protons, the smaller the wavelength of the quasi-real virtual photon and the more localised the probe into the nucleon.

So, how can such empirical insights be obtained ? In one word: expensively. To get some perspective, arguably the most famous particle collider in existence, the Large Hadron Collider at CERN, cost $13.25 billion to construct; roughly the GDP of Namibia in 2018. Fortunately, the facilities required to make headway into QCD can be incorporated into already existing colliders making the project considerably less expensive; plans are being made to build an extension to the Brookhaven Lab on Long Island or to the accelerator at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia.

This potential EIC is of utmost priority to the US nuclear science research community and, if approved, would be able to start collecting data from 2030. The key technological advance of the EIC in conducting DIS experiments would be its ability to produce elevated “brightness”: it could produce between 100 and 1,000 more collisions per minute than the Hadron-Electron Ring Accelerator in Hamburg, Germany.

Undertaking this project will undoubtedly be arduous as the process requires particles’ spins to be optimally aligned in a way that can be controlled and manipulated. Though, the vast array of insights which the EIC has the potential to yield not only for the benefit of accelerator physics but also medicine and materials science ultimately makes this pursuit for knowledge worthwhile.

I must acknowledge that the majority of the information found is this post should be attributed to the June 2019 edition of the Scientific American from the article titled ” The Deepest Recesses of the Atom”.

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