For the first time, the world’s leading conference on heavy-ion physics, Quark Matter, was held at Goethe University from April 6 to 12, 2025. In this interview, Professor Harald Appelshäuser shares the scientific highlights of the event, explains why heavy-ion research is more relevant to our everyday lives than one might think, and what makes Frankfurt the ideal host city.
Professor Appelshäuser, what is Quark Matter all about?
Quark Matter is the largest international conference in the field of relativistic heavy-ion physics. It has been around since 1982, and this year we are proud to host its 31st edition. With around 1,000 participants from around the world, this is also the largest Quark Matter to date.
The central idea is to bring theory and experiment together – especially with regard to the major international facilities such as CERN in Europe or BNL in the USA. In the near future, FAIR in Darmstadt will also become part of this landscape. Our region has been a kind of nucleus for heavy-ion physics from the very beginning and is now recognized worldwide as a center for this field. That makes it all the more exciting for us that Quark Matter is finally taking place in Frankfurt.
What were the hot topics at this year’s Quark Matter?
One major highlight this year is the first results from the third run of the Large Hadron Collider, or LHC, at CERN. Over the course of about ten years, this Run 3 saw substantial upgrades, and for the past two years, experiments have been underway with the new setup. The resulting data were presented here for the very first time – a real breakthrough.
The LHC is the largest and most powerful particle accelerator in the world. Most of the year it operates with protons, but about one month each year is dedicated to heavy-ion physics – when lead ions are collided. That’s the period we are most interested in, because it allows us to reach the highest energy density available anywhere. In these collisions, we recreate temperatures that existed just microseconds after the Big Bang. This enables us to model what the universe looked like in its earliest moments – long before particles such as protons and neutrons even existed.
Another important milestone is the first beam operation of the new sPHENIX experiment in the United States. Initial results from that facility were also presented at the conference – adding to what makes this Quark Matter so exciting.
Why is this research relevant for society?
Basic research is relevant simply because human beings have a natural desire to understand how things work and where the world comes from. Why are we fascinated by dinosaurs? Because we want to understand our origins. In our case, we’re studying the very earliest stages of the universe – the first millionth of a second after the Big Bang. At the same time, we investigate fundamental forces of nature at the smallest scales currently accessible to science.
This kind of research demands major technological effort – and in the process, new technologies are developed that often find applications beyond physics. A particularly strong example is computing: we work with enormous volumes of data, which requires constant innovation. The same is true for detector technologies – many of which later prove useful in fields such as medical imaging or climate research.
Frankfurt is hosting Quark Matter for the first time. What makes the city a good fit?
Frankfurt is ideal in many ways: it’s home to an excellent university and has a long scientific tradition – particularly in the field of heavy-ion physics. The city is also known for its openness and international outlook, which is especially important for a global conference like this. Many of our participants tell us they feel very welcome here.
Science in Frankfurt is not practiced in an ivory tower – it is part of civil society. That’s something we value. And of course, the city’s accessibility plays a role as well: Frankfurt is an international hub. For many people arriving from abroad, it’s hard to believe they can be at their hotel within 30 minutes of landing at the airport.
Then there’s our beautiful Westend campus: it has all the facilities needed for a high-level professional conference, and at the same time it’s open, vibrant, and integrated into both student and city life – which creates a great atmosphere.
You also organized several public events around the conference, including activities for school students. What was the idea behind that?
It’s very important to us to involve the public – to explain our questions and findings in an understandable way and to inspire enthusiasm for physics. We repeatedly see how much interest there is when things are explained clearly. When we talk about what we do, the response is almost always positive. Honestly, I’ve never had anyone say, “That’s completely irrelevant.”
We also want to promote physics – and more generally, the natural sciences. These are crucial fields in which we urgently need young talent. On an even broader level, we’re committed to science as a societal value. Unfortunately, it’s no longer a given that science is universally respected. Many people seem to think we can do without it – or that belief is more important than knowledge. We are convinced: knowledge is better than belief.
What was your personal highlight at this year’s Quark Matter?
What really impressed me is how large and dynamic our community has become. You can clearly feel the growth and the energy. The quality of the new data is also remarkable – especially considering that the experiments haven’t been running for that long. And last but not least, I’ve been very pleased with how well the conference was organized – we’ve received a lot of positive feedback about that.
Phyllis Mania
