‘On clear nights, we would watch the stars from the school rooftop’

When Olaf Massen (27) enrolled at a secondary school with its own observatory, his passion for physics took off. Nowadays, he is a PhD candidate at the Institute for Gravitational and Subatomic Physics at Utrecht University, regularly works at the CERN particle accelerator and mentors the next generation of physicists. In early April, he celebrated his first publication in a scientific journal. Meet a versatile and passionate young researcher in the field of subatomic physics.

Massen enjoys a good book, takes wine courses, and plays, trains, and coaches on the handball court. In his professional life, he dives deep into the mysteries of the quark-gluon plasma — a "primordial soup" that filled the universe just after the Big Bang. "My most-asked question growing up was always ‘Why?’" he says. "I loved science classes at school, and we even had our own observatory. We took astronomy lectures, and on clear nights, we would climb up to the roof to watch the stars. I think that's where my love for physics really began."

Studying collisions

Massen studied physics and chemistry before gradually focussing more on particle physics. Today, he works in the field of heavy-ion physics, which investigates collisions between heavy atomic nuclei at extremely high speeds and energies. The goal is to better understand how matter behaves under extreme conditions — such as those just after the Big Bang. "I love contributing to this field," Massen says enthusiastically. "There’s still so much we don’t understand."

Bridging two worlds

In his research, Massen enjoys working at the intersection of theory and experiment — a rare approach in a field where most researchers specialize in one or the other.
"Particle physics is already so highly developed that people tend to focus deeply on either theory or experiment," he explains. "But it’s crucial to have generalists who can connect the two. The divide can easily cause misunderstandings, while theory and experiment must ultimately work hand in hand. Ideally, you should think about how to test a new theory while you’re developing it."

I love that moment, when you first see confusion in their eyes — and then suddenly you see it click

Shifts at CERN

Massen finds both sides of his research equally fascinating: "Theory is intriguing because it explores the fundamental principles behind how things work. Experimentation is fun because you have to figure out how to actually measure those principles," he says.

From the experimental side, Massen is part of the ALICE experiment at CERN's particle accelerator in Geneva — a massive international collaboration designed to study the quark-gluon plasma. More than a thousand scientists worldwide are involved. Although Massen has only recently started working on the experimental side, he travels to CERN twice a year to take an ALICE shift: a stint in the control room primarily focused on data collection. His next shift takes place at the end of August.

Utrecht’s Trajectum

In the recent study published with colleagues, Massen demonstrated a new method for measuring the temperature of the quark-gluon plasma by analyzing the electromagnetic radiation emitted by heat. The quark-gluon plasma is a phase of matter in which the basic building blocks of atomic nuclei — quarks and gluons — float freely, rather than being bound together. Just after the Big Bang, the entire universe was filled with this exotic state. Gaining a better understanding of its temperature brings scientists one step closer to unraveling the conditions of the early universe.

For his study, Massen used computer simulations of collisions between accelerated lead nuclei — the kind of collisions that create quark-gluon plasmas. The cutting-edge simulation program, developed at Utrecht University, was fittingly named Trajectum. The research was designed in such a way that its theoretical predictions could be tested experimentally with real collisions between lead nuclei. Currently, scientists working on the ALICE experiment are analyzing newly collected data, hoping to detect a signal of the electromagnetic radiation studied in Massen's simulations.

Seeing it click

Massen holds a special appointment at Utrecht University, giving him extra room to focus on teaching. "That was a conscious choice," he says. "I enjoy teaching so much — maybe even more than doing research."
He is now one of the lead lecturers for the Subatomic Physics course in the Bachelor's program in Physics, delivers masterclasses through U-Talent, and helps secondary school students with their final research projects.

Massen also enjoys sharing his passion for physics beyond the university. In March, he visited Group 7 at De Gagel primary school in Utrecht as part of the Meet the Professor initiative, accompanied by his supervisor Raimond Snellings. At the end of last year, he also gave a lecture at the Phoenix Public Observatory in Lochem.
For Massen, it doesn’t really matter who is in front of him. The challenge — and the joy — lies in making complex material accessible. "I love that moment," he says, "when you first see confusion in their eyes — and then suddenly you see it click."