For the non-physicist

- In short - 

What happens to matter when you heat it up to 100 000 times the temperature of the sun?
OR
What was the universe like one millionth of a second after the big bang?

AND 
What happens to matter when you compress it to several times the density of normal nuclear matter?

In the last century or so mankind has managed to develop a largely self-consistent mathematical description of all the matter we can see – the normal things that you and I and this computer screen are made of, as well as a few other things we don’t see every day, like anti-matter and the Higgs boson. We call this description the “Standard Model of Particle Physics” and it does a spectacular job, but it isn’t very easy to work with and we still need to do a lot of tests.

One thing it is particularly difficult to use this mathematical description for is the way that many thousands or millions of these particles behave when they’re together – we have neither the mathematical knowledge nor the computing power. This difficulty is similar to the problem of why it is hard to understand how consciousness arises when all we understand about the brain is how individual neurons interact with each other.

I study the way the smallest known pieces of matter – called quarks and gluons – behave when large numbers of them can move around freely. To do this we need to create something called the quark-gluon plasma by colliding the nuclei of very heavy elements like gold or lead at extremely high energies. In such collisions, so much energy is deposited in a very small volume that thousands of quarks and gluons are created, forming the quark-gluon plasma.

My work involves trying to use the mathematics of the Standard Model to predict the behavior of the quark-gluon plasma.

For the general physicist

- In short -

What is the nature of very high density or high temperature matter?

The standard model of particle physics (SM) has been tested extensively. The SM is perfectly tractable and verifiable for interactions between small numbers of particles. The underlying theory governing the matter particles, quantum chromo-dynamics (QCD), is even fairly user-friendly despite its non-Abelian nature and the fact that the phenomenon whereby individual quarks are confined within hadrons like the proton remains unexplained.

However, not all the multi-particle dynamics and emergent phenomena of free quarks can be computed directly. In order to study the collective behavior of quarks, gigantic colliders (the Large Hadron Collider at CERN in Geneva, Switzerland, and the Relativistic Heavy Ion Collider outside New York in the USA) produce a plasma of free quarks and gluons (the quark-gluon plasma, QGP) by colliding the nuclei of heavy elements at relativistic energies. The QGP produced in this way is very short-lived – a lifetime of only a few femtoseconds – and must be studied via the decay products that arrive in massive detectors located around the interaction point.

I have studied the manner in which a highly energetic particle loses energy as it traverses the QGP. If one can accurately predict the nature of the interaction of a particle with the plasma, one might be able to extract properties of the QGP.

I have also thought about the different regimes of the QGP. In a collider experiment, one probes the QGP at very high temperatures, but in the cores of neutron stars one might find a version of the QGP that is at very high densities instead. How might one learn about high-density QGP from collider experiments?

In 2024 I gave a seminar to introduce myself to my new professional associations – the Mandelstam Institute for Theoretical Physics (MITP) and the National Institute for Theoretical and Computational Sciences (NITheCS), which broadly outlines my research interests at a level I believe is accessible to anyone with a BSc. in physics.

For the theoretical high-energy nuclear physicist

- In short -

Why is there no partonic energy loss in small colliding systems?
AND
What is the nature of the fragmentation region?

—– Small Systems —–

The discovery of collective behavior and strangeness enhancement in small colliding systems half a decade ago has prompted the question: “Is there a QGP in small systems?”. At first it seemed obvious that the appropriate check would be the presence of partonic energy loss in small systems. To date, gargantuan efforts to detect such energy loss have resulted in a high level of experimental confidence that the kind of energy loss seen in central AA simply is not present in small systems.

Does this mean there is no QGP in small systems? Do we understand what energy loss in small systems should look like? Are we correctly interpreting the experimental signals of collective behavior? Do we know how to distinguish between “energy loss” and “no energy loss”? Are a pp collision and a PbPb collision at the same multiplicity governed by the same physics? What about OO and PbPb? Most intriguingly: what about pO and pPb? These kinds of questions have driven my research since the beginning of my MSc. and I find it thrilling that we still have very few answers.

—– The Fragmentation Region —–

In recent years though, I have become interested in the properties of the QGP at high baryon density. Such investigations have implications for our understanding of neutron stars and the structure of the phase diagram of the QGP. One may already access some of this physics by considering the fragmentation (or, far forward) region of a heavy-ion collision. In this region, the baryon density is finite and the nature of bremsstrahlung is quite different to the central region. This region then also offers the opportunity for insight into cosmic-ray physics, and yet we understand only very little about it.