TALK LIKE A NUCLEAR PHYSICIST

PROTON — a positively charged particle in an atom’s nucleus

NEUTRON — a neutral particle (one that has no charge) in an atom’s nucleus

HADRONS — the collective name for any particle that interacts via the strong nuclear force, e.g., protons and neutrons

ELECTRON — a negatively charged particle orbiting an atom’s nucleus

QUARKS — particles inside hadrons, e.g., protons are made of three quarks

GLUONS — particles that hold quarks together inside of hadrons

PARTONS — the collective name for quarks and gluons

QUANTUM CHROMODYNAMICS — the theory that describes how quarks and gluons interact through the strong nuclear force

SPIN — a quantum-mechanical property of particles, similar to the angular momentum of a sphere rotating on its axis, which has the effect of creating a bar magnet within the particle

Full of exotic-sounding particles, and existing on a scale billions of times smaller than the width of a human hair, the world of nuclear physics can be daunting and bewildering. Until the turn of the 20th century, it was generally believed that everything was made of atoms, and that these atoms were the fundamental units of matter. Indeed, the word atom comes from the ancient Greek word ‘atomos’, meaning ‘indivisible’.

However, thanks to the work of nuclear physicists like J.J. Thomson and Ernest Rutherford, we now know that atoms are composed of even smaller particles. Each atom is comprised of electrons orbiting a very small, dense nucleus made up of protons and neutrons. And these protons and neutrons can then be split into smaller components still, called quarks. Quarks are a type of ‘elementary particle’ that cannot be further subdivided.

This is where nuclear physics starts to get more complex. Quarks come in six different types, or ‘flavours’, – up, down, top, bottom, strange and charm – and they are held together by gluons, another type of elementary particle. Collectively, quarks and gluons are known as partons. The larger particles composed of quarks and gluons (e.g., protons and neutrons) are collectively known as hadrons.

Confused yet? If you are struggling to follow this, do not panic! Even seasoned physicists, like Dr Daniel Pitonyak from Lebanon Valley College, are still working hard to unravel the mysteries of this strange subatomic realm. Daniel’s research lies within the field of high-energy nuclear physics, which investigates how parton interactions and motions influence the internal structure of hadrons.

QUANTUM CHROMODYNAMICS

Some particles, like protons and electrons, carry an electric charge which allows them to interact via electric and magnetic forces by exchanging photons. Quarks and gluons carry another type of charge, known as a colour charge, which allows them to interact via the strong nuclear force. It is these interactions that the theory of quantum chromodynamics sets out to describe. Since hadrons are composed of quarks and gluons, all their properties must arise from and be determined by their constituent partons. For example, we should be able to explain the proton’s spin in terms of the quarks and gluons within it.

WHAT IS SPIN?

Spin is a fundamental property of a particle, like its mass or charge, and is a measure of its intrinsic angular momentum. “The particle is not literally rotating,” explains Daniel. “The spin of a particle has the effect of creating a tiny bar magnet inside it that can respond to a magnetic field.”

The value of a particle’s spin must follow certain rules. It must be either a whole number (1, 2, 3, etc.) or half a whole number (1/2, 3/2, 5/2, etc.) multiple of ℏ (pronounced ‘h-bar’), a fundamental constant that shows up in quantum mechanics. Protons, neutrons, electrons and quarks all have a spin of ℏ/2, while photons and gluons have a spin of ℏ.

WHAT CAUSES A PROTON’S SPIN?

“Typically, spin is measured by observing the direction that a particle is deflected when passing through a magnetic field,” says Daniel. One of the main aims of his research is to determine the source of the proton’s spin. “Since a proton is composed of quarks and gluons, its spin of ℏ/2 must arise from its partons,” he explains.

There are two ways in which the partons (quarks and gluons) contribute to the proton’s spin. Firstly, the partons themselves carry spin and this is transferred to the proton. Secondly, the partons are moving around inside the proton. This creates angular momentum which also contributes to the proton’s spin. The sum of the partons’ spins and the angular momenta created by their movement must add up to ℏ/2. “However, exactly how much comes from each component remains an open question,” says Daniel.

By analysing the results of nuclear physics experiments, it has been determined that 30% of the proton’s spin comes from the quarks’ spins and 40% comes from the gluons’ spins. However, more precise measurements are needed to increase the accuracy of these values. Daniel works on calculating how much spin comes from quarks and gluons that carry only a small fraction of the proton’s momentum, which is difficult to access through current experiments.

WHAT EXPERIMENTS ALLOW DANIEL TO STUDY SUCH TINY PARTICLES?

“Partons exist at length scales that are extremely small,” says Daniel. “We cannot simply place a proton under an ordinary microscope and see the partons moving around inside.” Instead, experimental physicists must smash protons together at incredibly high speeds. These high-energy collisions are conducted in huge particle accelerators, several miles in circumference, that can accelerate protons close to the speed of light. When the protons collide, their partons are ejected, allowing physicists to detect the hadrons they form.

Data from these experiments allow Daniel to better understand the internal structure of hadrons through functions that encode what is going on before and after the collisions. He writes code based on calculations in quantum chromodynamics of how quarks and gluons interact when protons collide, to numerically model the results of these high-energy collisions. He uses algorithms and computational techniques to determine what parameters produce models that best fit the experimental data. From these models, Daniel can discover how the partons are distributed and move within the protons and create images of their 3-dimensional (3D) structure.

WHAT IS THE ELECTRON-ION COLLIDER?

The Electron-Ion Collider (EIC) is a particle accelerator, recently commissioned by the US Department of Energy, which will offer unprecedented insight into the internal structure of hadrons. Thousands of physicists from around the world have been collaborating to design and create the EIC, including Daniel. “I have been involved in analysing how the specifications of the EIC will quantitatively impact our current understanding of the 3D structure of hadrons and how much of the proton’s spin arises from quarks and gluons, to ensure the collider is built in a way that maximises its scientific outcomes,” he explains.

The construction of the EIC will offer nuclear physicists the chance to advance their research. In the meantime, Daniel’s next steps involve creating more complex models that can account for phenomena such as gluon radiation. This will allow him to include additional experimental data in his analyses and make more precise predictions.

WHY IS THIS RESEARCH IMPORTANT?

“This research attempts to understand the most elementary structure of visible matter in our Universe,” says Daniel. “It’s what we are made of at a fundamental level. Moreover, this research involves young scientists who receive the training necessary to branch out into other STEM fields or areas of physics research. Developing and sustaining a scientific workforce is crucial for creating new technologies and solving the critical problems that face our world.”

DR DANIEL PITONYAK
Assistant Professor of Physics, Lebanon Valley College, USA

FIELD OF RESEARCH: Nuclear Physics

RESEARCH PROJECT: Investigating the 3D structure of hadrons and the origin of the proton’s spin

FUNDER: National Science Foundation (NSF) This work is supported by the National Science Foundation, under award number PHY-2011763. The contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Science Foundation.